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A new dialkylthio-substituted naphtho[2,3-c]thiophene-4,9-dione based polymer donor for high-performance polymer solar cells
Abstract
Rational molecular design towards high-performance photovoltaic materials with superior large-area capability is critical but still a challenge for the research field of polymer solar cells (PSCs). In this work, we design and synthesize a new dialkylthio-substituted conjugated polymer donor (namely PBN-S) to address this issue. PBN-S possesses strong absorbance in the wavelength range of 500-700 nm and low-lying HOMO level of -5.48 eV to match with the nonfullerene acceptor IT-4F. The optimized PSCs based on PBN-S:IT-4F demonstrate a high power conversion efficiency (PCE) of 13.10%, benefitted from the efficient charge separation, high and balanced charge mobility, ordered molecular packing and aggregation of the donor and acceptor materials in the active layer. In addition, PCE of the PBN-S based semitransparent PSC reaches 9.83% with an average visible transmittance of 32%. What is more, a remarkable PCE of 10.21% and 10.69% were realized from the spin-coated or blade-coated PSCs with the active area of 100 mm2, respectively, indicating the great potential of PBN-S as donor in high-performance large-area PSC application. More importantly, the PBN-S based devices show excellent shelf-life stability, with over 80% of the initial PCE after 100 days stored in nitrogen-filled glove-box or in air. The results demonstrate the promising capability of PBN-S as donor material for future PSCs applications.
... Further, 2D grazing incidence wide-angle X-ray scattering (2D-GIWAXS) measurements showed stronger π-π stacking after optimization (1% DIO and thermal annealing) than the as-cast blend, resulting in a higher charge mobility and fill factor. The ST-OSC device demonstrated a PCE of 9.83%, with an AVT of 32% and a notable device stability, retaining around 80% PCE after storage in a nitrogen glove-box for 100 days (Figure 4g-p) [69]. dialkylthio-substituted polymer, PBN-S, blended with ITIC-4F. ...
... Further, 2D grazing incidence wide-angle X-ray scattering (2D-GIWAXS) measurements showed stronger π-π stacking after optimization (1% DIO and thermal annealing) than the as-cast blend, resulting in a higher charge mobility and fill factor. The ST-OSC device demonstrated a PCE of 9.83%, with an AVT of 32% and a notable device stability, retaining around 80% PCE after storage in a nitrogen glove-box for 100 days (Figure 4g-p) [69]. In 2017, Li et al. reported a series of narrow-bandgap NFAs by using double-bond π-bridges into ITIC (ITVIC) with monofluorine (ITVfIC) or bifluorine (ITVffIC) substituents on the ITIC end group. ...
The increasing energy demands of the global community can be met with solar energy. Solution-processed organic solar cells have seen great progress in power conversion efficiencies (PCEs). Semitransparent organic solar cells (ST-OSCs) have made enormous progress in recent years and have been considered one of the most promising solar cell technologies for applications in building-integrated windows, agricultural greenhouses, and wearable energy resources. Therefore, through the synergistic efforts of transparent electrodes, engineering in near-infrared photoabsorbent materials, and device engineering, high-performance ST-OSCs have developed, and PCE and average visible transmittance reach over 10% and 40%, respectively. In this review, we present the recent progress in photoabsorbent material engineering and strategies for enhancing the performance of ST-OSCs to help researchers gain a better understanding of structure–property–performance relationships. To conclude, new design concepts in material engineering and outlook are proposed to facilitate the further development of high-performance ST-OSCs.
... The results manifest that both polymers exhibit a ''face-on'' packing tendency, which is helpful for vertical charge transport in OSCs. 22,57,58 By fitting the ''q'' values of the (010) and (100) peaks, the corresponding p-p stacking and lamellar stacking distances were found to be 3.18 Å and 18.78 Å (for P1-2F) and 3.12 Å and 18.58 Å (for P2-2N), respectively. Furthermore, the coherence length (CCL) estimated using Scherrer's equation (L C = 2p/FWHM) for the (100) and (010) peaks were found to be 18.41 Å and 3.22 Å for P1-2F, and 19.03 Å and 3.73 Å for P2-2N, respectively. ...
In organic solar cell research, developing efficient and low-cost photovoltaic materials via insertion of fluorine (F) and nitrogen (N)-substituents has proved as highly successful strategies, thus raising the question of...
... These values are 0.1−0.2 eV deeper than the reported HOMO level measured by cyclic voltammetry; for example, −5.50 to −5.68 eV for ITIC, 53,54 −5.67 to −5.72 eV for IT-4F, 55,56 and −5.62 to −5.70 eV for Y6. 57,58 Nonetheless, we used our HOMO values to ensure the consistency and examine the offset energies of the donor and acceptor (vide infra). ...
... In the D−A copolymer based on the BDTT D-unit and naphtho[2,3-c]thiophene-4,9-dione (NTDO) A-unit, we introduced alkylthio chains on the phenyl ring of the NTDO A-unit for improving the product yield and strengthening the electron-accepting nature of the NTDO unit. 49 The resulting dialkylthio-substituted NTDO-based polymer donor PBN-S (Scheme 3) shows appropriate HOMO level, strong absorption feature matching with low bandgap acceptor IT-4F, and favorable molecular packing, delivering an encouraging PCE of 13.10%, with a notable V oc of 0.891 V, a J sc of 21.03 mA cm −2 , and a FF of 0.699 (Table 2). Particularly, the PBN-S-based device exhibited promising long-term stability, and a PCE of over 10% was achieved in the device with an active area of 100 mm 2 , indicating the attractive potential of PBN-S as a donor material for the practical application of OSCs. ...
Over the past few years, the innovation of narrow bandgap acceptor combined with wide bandgap donor materials significantly promotes the power conversion efficiencies (PCEs) of organic solar cells (OSCs) to exceed 18%. To build a state-of-the-art OSC, absorption spectra, frontier molecular orbital energy levels, molecular packing and crystallinity, and charge carrier mobilities of the photovoltaic materials should be considered in their molecular design. The donor and acceptor materials are the key components determining the photovoltaic performance of the OSCs. The side chain engineering on the conjugated backbone is a critical strategy to optimize the photovoltaic properties of the donor materials. In this Account, we focus on the topic of heteroatom substitution on the molecular backbone of the donor materials for improving their photovoltaic performance, aiming to provide in-depth understanding of the molecular structure optimization for the design of state-of-the-art photovoltaic materials.
... The heteroatoms are widely used in the design of photovoltaic materials to tune the opto-electronic properties [35]. For example, the empty 3d-orbitals of sulfur can accept π-electrons through pπ(C)-dπ(S) orbital overlap between the conjugated backbone and the alkylthio substitution, therefore lowering the highest occupied molecular orbital (HOMO) level [36][37][38][39]. The introduction of strong electron-donating groups such as alkoxy side chains can be used to reduce the optical bandgap via the enhanced intramolecular charge transfer effect and elevate the HOMO levels of organic semiconductors. ...
By altering the number and position of oxygen and sulfur substitutions, four simple non-fused electron acceptors, PTO-4F, PDO-4F, PDS-4F and PTS-4F, were synthesized via feasible two-step reactions. These four acceptors serve as good molecular models to investigate the heteroatom effects on performance of organic solar cells (OSCs) based on their blends with typical polymer donor PBDB-T. The quantity of intramolecular noncovalent bonds, conformation of the molecules and performance of OSCs can be easily adjusted. Gradually increasing oxygen atoms could influence the intramolecular noncovalent (O⋯S, O⋯H) interactions, backbone planarity, film morphology, and electrical and photovoltaic properties significantly. When replacing O atoms with S atoms, the torsional angle of the backbone increases from 3.5° to 97° owing to the reduction of O⋯S attractive coulomb interaction and/or O⋯H hydrogen bonding interaction. With increasing oxygen atom numbers, the absorption is red-shifted gradually and the energy levels are lifted. As a result, the power conversion efficiency of the device increases from 4.06% (PTS-4F) to 6.81% (PTO-4F). This study provides helpful molecular design guideline for the optimization of simple non-fused acceptors and device performances by finely controlling the weak intramolecular noncovalent interactions and molecular conformations.
... Alternating conjugated copolymers incorporating different electronic units have attracted considerable attention over a wide range of opto-electronic and energy transformation applications, such as polymer light-emitting diodes [1][2][3][4] , organic solar cells [5][6][7][8] , organic field-effect transistors [9][10][11] , photocatalytic hydrogen production 12-16 . Many efforts have been devoted to understanding fundamental electronic properties of alternating conjugated copolymers, including ionization potential (IP), electron affinity (EA), optical bandgap [17][18][19] . ...
Alternating conjugated copolymers have been regarded as promising candidates for photocatalytic hydrogen evolution due to the adjustability of their molecular structures and electronic properties. In this work, we developed machine learning (ML) models with Multi-Dimension Fragmentation Descriptor (MDFD) to promote the accurate and universal prediction of electronic properties without any experimental values and then constructed a high-performance prediction classifier model toward photocatalytic hydrogen production of alternating copolymers with high accuracy (real-test accuracy = 0.91). Moreover, photocatalytic dynamic study has been performed as well. Consequently, our work reveals accurate regression and classification models to disclose valuable influencing factors concerning hydrogen evolution rate (HER) of alternating copolymers.
... Å, respectively), they exhibited π-π stacking diffraction peak only along the OOP direction ( Fig. 6b and c). These results clearly emphasize the change of molecular orientation that occurred from mixed in the case of pure PBDT-2FBnT films to typical "face-on" in the blend films, which is advantageous for vertical charge transport in PCSs (Kini et al., 2016;Li, S. et al., 2018;Wu et al., 2019;Yang et al., 2018). Moreover, these blend films also demonstrated a substantially increased and sharper (0 1 0) diffraction peak along the OOP direction together with a lower π-π stacking distance than that of pure , emphasizing that the crystallinities and molecular ordering of the blend components were substantially increased. ...
Although the latest progress in the power conversion efficiency (PCE) of non-fullerene polymer solar cells (NF-PSCs) has proved their potential as next-generation solar technologies, the development of efficient wide-bandgap (WBG) donor polymers having a low-cost and easily scalable molecular design is still lagging in terms of number and diversity. In this contribution, we report a new WBG polymer donor, PBDT-2FBnT, based on alternative benzodithiophene as the donor unit and 2,5-difluoro benzene (2FBn) as the acceptor unit, which was prepared by a facile three-step process with an overall yield over 80% using cheap raw materials. Benefitting from the incorporation of an electron-deficient 2FBn unit, PBDT-2FBnT demonstrated lower frontier energy levels with an optical bandgap of 2.12 eV. Additionally, the combination of 2FBn with an adjacent two thiophene bridge in the polymer backbone significantly minimized the steric hindrance via non-covalent F···S and CH···F inter/intramolecular interactions, thereby promoting highly coplanar geometry for effective molecular packing and charge transport. Consequently, optimized blends of PBDT-2FBnT with an ITIC acceptor delivered complementary panchromatic absorption, well-aligned energy levels, higher charge carrier mobilities, and well-distributed nano-fibrillar morphology, thereby leading to a remarkable efficiency of 9.5% with a good trade-off between corresponding photovoltaic parameters. Thus, the high photovoltaic performance of PBDT-2FBnT together with simple preparation can provide a promising way for the future development of low-cost PSCs, and we can foresee further PCE improvements of the PBDT-2FBnT structure via synergistic variation of donor and acceptor material.
Organic solar cells (OSCs) a class of emerging photovoltaics for potentially disruptive technology have made progress significantly over the past two decades owing to their advantages of being cost-effective, having a low carbon footprint, enabling the production of flexible, semitransparent, and lightweight devices. In this prosperous field, the design and application of new materials had a great impact on the advancement of the performance of the devices. In the majority of works, the optical-active layer of devices comprises a polymer donor and a small molecule acceptor. A polymer donor is designed with a strategy of alternating an electron-donating unit and an electron-accepting unit. Among numerous electron-donating moieties, benzodithiophene-based polymers have exhibited a distinguished photovoltaic performance and break a 19% efficiency barrier in OSC devices. In this review article, we offered an overview of recent developments in benzodithiophene-bearing polymeric materials by giving special attention to modifications of backbones, functional groups, alkyl chains, and their impact on device performance. We purposed to provide a guideline about the chemical structure-property relationship in terms of absorption profile, band gap, energy levels and device performance indexes.
Recently, the power conversion efficiency (PCE) of organic solar cells (OSCs) has increased dramatically, making a big step toward the industrial application of OSCs. Among numerous OSCs, benzodithiophene (BDT)‐based OSCs stand out in achieving efficient PCE. Notably, single‐junction OSCs using BDT‐based polymers as donor materials have completed a PCE of over 19%, indicating a dramatic potential for preparing high‐performance large‐scale OSCs. This paper reviews the recent progress of OSCs based on BDT polymer donor materials (PDMs). The development of BDT‐based OSCs is concisely summarized. Meanwhile, the relationship between the structure of PDMs and the performance of OSCs is further described in this review. Besides, the development and prospect of single junction OSCs are also discussed.
Semitransparent photovoltaic (STPV) solar cells offer an immense opportunity to expand the scope of photovoltaics to special applications such as windows, facades, skylights, and so on. These new opportunities have encouraged researchers to develop STPVs using traditional thin‐film solar cell technologies (amorphous‐Si, CdTe, and CIGS or emerging solar cells (organic, perovskites, and dye‐sensitized). There are considerable improvements in both power conversion efficiency (PCE) and semitransparency of these STPV devices. This review studies the device structure of state‐of‐the‐art STPV devices and thereby analyzes the different approaches toward maximizing the product of PCE and average visible transmittance. The origins of PCE losses during the opaque‐to‐semitransparent transition in the different STPV technologies are discussed. In addition, critical practical aspects relevant to all STPV devices, such as compatibility of the top transparent electrode with the device structure, buffer layer optimization, light management engineering, scale‐up, and stability, are also reported. This overview is expected to facilitate researchers across different technologies to identify and overcome the challenges toward achieving higher light utilization efficiencies in STPVs.
The elaborate balance between the open‐circuit voltage (VOC) and the short‐circuit current density (JSC) is critical to ensure efficient organic solar cells (OSCs). Herein, the chalcogen containing branched chain engineering is employed to address this dilemma. Three novel nonfullerene acceptors (NFAs), named BTP‐2O, BTP‐O‐S, and BTP‐2S, featuring different peripheral chalcogen containing branched chains are synthesized. Compared with symmetric BTP‐2O and BTP‐2S grafting two alkoxy or alkylthio branched chains, the asymmetric BTP‐O‐S grafting one alkoxy and one alkylthio branched chains shows mediate absorption range, applicable miscibility, and favorable crystallinity. Benefiting from the enhanced π–π stacking and charge transport, an optimal power conversion efficiency (PCE) of 17.3% is obtained for the PM6:BTP‐O‐S‐based devices, with a good balance between VOC (0.912 V) and JSC (24.5 mA cm⁻²), and a high fill factor (FF) of 0.775, which is much higher than those of BTP‐2O (16.1%) and BTP‐2S‐based (16.4%) devices. Such a result represents one of the highest efficiencies among the binary OSCs with VOC surpassing 0.9 V. Moreover, the BTP‐O‐S‐based devices fabricated by using green solvent yield a satisfactory PCE of 17.1%. This work highlights the synergistic effect of alkoxy and alkylthio branched chains for high‐performance OSCs by alleviating voltage loss and enhancing FF.
The current power conversion efficiencies of laboratory‐sized organic solar cells (OSCs), based on the spin‐coating process with halogenated solvents, have exceeded 19%. Environmentally friendly printing is needed to bridge the gap between laboratory and industrialization by being compatible with roll‐to‐roll large‐area production. Here, the molecular design rules are revealed for enhancing the green printing potential of the state‐of‐the‐art photovoltaic martial systems by investigating the detailed structure formation dynamic and the key determining factors. By comparing two model systems based on D18:Y6 and D18:BTP‐eC9, it is found that disordered preaggregation in liquid state can result in over‐sized domains with reduced crystallinity and disordered molecular orientation, which significantly limits device performance. By systematically tuning the length of the inner alkyl side chains with multiple Y‐series materials, the authors demonstrate that molecular side‐chain engineering can effectively supress the detrimental disordered preaggregation in liquid state during environmentally friendly printing process, leading to enhanced crystallization with preferential faceon molecular orientation, more efficient exciton dissociation and charge carrier transport, and finally high upscaling potential. The work provides deeper insights into molecular engineering and structure formation dynamics toward environmentally friendly production of OSCs.
Developing the slot-die coated efficient all-polymer solar cells (all-PSCs) using eco-friendly solvent is extremely crucial for the further large-scale production. However, how to gain control over the balanced phase separation...
Two halogen-free donor polymers PCN1 (zigzag-shape backbone) and PCN2 (linear-shape backbone) were constructed with dicyanobenzotriazole as the acceptor (A) block. Their absorption, energy levels, charge transport properties, and applications as donors in organic solar cells (OSCs) were thoroughly investigated. PCN2 with strong absorption at 400−700 nm, maximum extinction coefficient of up to 8.67 × 10⁴ M⁻¹ cm⁻¹, low highest occupied molecular orbital (-5.50 eV), and the high hole mobility (1.42 × 10⁻³ cm² V⁻¹ s⁻¹) is an excellent donor polymer. The OSCs based on PCN2:Y6 reached a high PCE of 15.20% without the addition of any additives and any post-treatment, with an open-circuit voltage (Voc) of 0.862 V, which is by far the highest PCE among OSCs with donor polymers based on benzotriazole derivatives, and it is also one of the highest Voc reported for binary OSCs matched to Y6 in halogen-free donor polymers. The measurements of atomic force microscopy, transmission electron microscopy, and grazing-incidence wide-angle X-ray scattering show that PCN2 has a more orderly arrangement and stronger π-π stacking, and the PCN2:Y6 device exhibits better charge transport, higher and more balanced carrier mobility, less exciton recombination loss, suitable phase separation size, which lead to its higher photovoltaic performance.
For utilizing the organic solar cells for commercial applications, reducing the overall cost of the photo absorbent materials is also very crucial, along with the realization of high power conversion efficiency (PCE) and excellent stability. Herein, we tried to address such challenge by synergistically controlling the amount of fluorine (F)‐substituents (n = 2, 4) on easily scalable, low‐cost wide‐bandgap molecular design involving alternate fluorinated‐thienyl benzodithiophene donor and 2,5‐difluoro benzene (2FBn) or 2,3,5,6 tetrafluorobenzene (4FBn) to form two new polymer donors PBDT‐2FBn and PBDT‐4FBn, respectively. As expected, sequential fluorination causes lowering of the frontier energy levels and planarization of polymer backbone via F···S and C‐H···F noncovalent molecular locks, which results in more pronounced molecular packing and enhanced crystallinity from PBDT‐2FBn to PBDT‐4FBn. By mixing with IT‐4F acceptor, PBDT‐2FBn:IT‐4F‐based blend demonstrated favorable molecular orientation with shorter π–π stacking distance, higher carrier mobilities with good trade‐off ratio and desirable nanoscale morphology, hence delivered good PCE of 9.3% than PBDT‐2FBn:IT‐4F counterpart (8.6%). Furthermore, pairing PBDT‐2FBn with BTP‐BO‐4Cl acceptor further improved absorption range and promoted privileged morphology with ideal domain sizes for efficient exciton dissociation and charge transport, resulting in further improvement of PCE to 10.2% with remarkably low energy loss of 0.46 eV, which is seldomly reported in NF‐OSCs. Consequently, this study provides valuable guidelines for designing efficient and low‐cost polymer donors for organic solar cell applications. This article is protected by copyright. All rights reserved
Silicon based inorganic semiconductors were preferred for solar cell fabrication until scalability and actual commercialization of inorganic photovoltaics at reasonable costs became a problem. The coming of organic semiconductor-based technologies proved beneficial as the fabrication of unique optoelectronic devices were achieved at relatively lower costs and new device functionalities like improved optical transparency, enhanced mechanical flexibilities became a possibility. The usage of organic polymers as electron donors and acceptors multiplied the benefits of synthesizing organic photovoltaics by several folds, although only a power conversion efficiency of over 18% has been achieved so far. Putting together various inferences made through the years, this review aims at establishing a comprehensive understanding of organic photovoltaics and the science of bulk heterojunction solar cells. The need for low-bandgap photoactive materials and the different ways to synthesize them has been elaborated and a detailed review of the various donor and acceptor semiconducting polymers has been done. This paper also provides a comprehension of the specific strategies that might improve the industrial scalability of organic photovoltaics, following which the challenges and the future of organic photovoltaics-based research have also been highlighted.
Multiple strategies have been developed to improve the photovoltaic performance of organic solar cells (OSCs) by fine-tuning the morphology of the active layers, such as introducing functional third components, taking post-treatments and so on. In this work, a series of non-volatile solid additives BT, BTOR, BT2F, BTNO2(2 + 6), and BTNO2(6) were synthesized, by introducing substituent groups into benzothiadiazole units, and the thermal stability, optical and electrochemical properties were systematically studied. These additives were applied to further investigate the effect on the photovoltaic performance of OSCs. The results show that these additives could effectively promote exciton dissociation and charge transfer, reduce bimolecular recombination, and optimize the morphology of the active layer. Significantly, the PBDB-T:ITIC-based devices processed with BT achieved a power conversion efficiency (PCE) of 11.57%, increased by 17% compared to the as-cast device due to the high and balanced carrier mobility. Enhancement of PCE proved the feasibility of the application of non-volatile solid additives in the non-fullerene-based binary system.
Organic solar cells (OSCs) have drawn tremendous interest because of their potential for low-cost solution processing and color tunability. OSCs with bulk-heterojunction structures offer an attractive pathway to efficiently utilize solar energy and have the potential to make transparent and flexible devices. The design of new materials and optimized device fabrications with visible-light transparency make this technology attractive for various applications, including greenhouses, windows, building structures, etc. In this review, we highlight the developments of fullerene-free acceptors (FFAs) for semitransparent OSCs (ST-OSCs) and device architectures, including transparent electrodes, giving special emphasis to the power conversion efficiency (PCE) and the average visible transparency (AVT).
Semi-transparent organic cells (STOSCs) make the industrialization of power-generating windows a reality with promising average visible transmittance (AVT) and rapidly increased power conversion efficiencies (PCEs). To achieve the ultimate goal, improving the efficiency of STOSCs without degrading the transparency in visible range is critical. In this work, a non-fused ring acceptor (BDC-4F-C8) by alkyl chain engineering is used as third component for ternary blend active layer to enhance the electrical and optical properties of STOSCs. With optimizing the proportion of third component, STOSCs with ternary active layer exhibit improved open-circuit voltage, short-circuit current and fill factor, which leads to the best PCE of 13.19% with AVT of 24.56%, while the PCE of opaque one is17.02%. This work paves an effective strategy to achieve high-performance ternary STOSCs with high AVT employing a low-cost non-fused ring electron acceptor material, and manifests how the third component affects the optical features of ternary active layer to achieve efficient semi-transparent devices.
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We designed and synthesized two benzotrithiophene (BTT)-based copolymers, poly(2,5-bis(2-butyloctyl)-3-(5-(4,6-dioctylbenzo[1,2-b:6,5-b′:3,4-c″]trithiophen-2-yl)thiophen-2-yl)-6-(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione) (P1) and poly(4-(5-(4,6-bis(2-octyldodecyl)benzo[1,2-b:6,5-b’:3,4-c”]trithiophen-2-yl)thiophen-2-yl)-5,6-difluoro-7-(thiophen-2-yl)benzo[c][1,2,5]thiazole) (P2) by engineering the spectral range adopting diketopyrrolopyrrole (DPP) and difluorobenzothiadiazole (DFBT) as acceptors. The absorption spectra of P1 and P2 display an absorption maximum (λmax) at 799 nm and at 610 nm as a film, respectively. The highest occupied molecular orbital (HOMO) levels of P1 and P2 were found to be -5.45 and -5.73, respectively, by optical and electrochemical measurements. Two inverted organic photovoltaic (OPV) devices of P1/ITIC-4F and P2/ITIC-4F were fabricated based on traditional non-fullerene architecture using 3,9-bis(1-oxo-2-methylene-3-(1,1-dicyanomethylene)-5,6-difluoro-indanone)-5,5,11,11-tetrakis(4-n-hexylphenyl)-dithieno[2,3d:2′,3′d′]-s-indaceno[1,2b:5,6b′]dithiophene (ITIC-4F) as an acceptor. After optimizing by polymer concentration, additive concentration, annealing, and film thickness, the best power conversion efficiencies (PCEs) of the devices fabricated from P1 and P2 were measured as 1.62% and 1.24%, respectively. Atomic force microscopy (AFM) study revealed that addition of 1,8-diiodooctane (DIO) to the blend film of P1 changed its morphology, from a big granular structure to a smooth and nano-scale phase separation, leading to great improvement of the PCE. However, DIO addition to P2 did not have a positive effect on the blend film morphologies, which became more rough and granular, leading to inferior PCEs. These AFM results proved the close correlations between the polymer blend morphologies and the corresponding device performances.Graphic abstract
Three polymer donors named Qx-8F, Qx-10F, and Qx-12F, with similar chemical structures, was synthesized. The energy level of these donors is manipulated by precisely controlling the fluorination sites. We demonstrate...
The design and synthesis of a stable and efficient hole-transport material (HTM) for perovskite solar cells (PSCs) are one of the most demanding research areas. At present, 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-MeOTAD) is a commonly used HTM in the fabrication of high-efficiency PSCs; however, its complicated synthesis, addition of a dopant in order to realize the best efficiency, and high cost are major challenges for the further development of PSCs. Herein, various diketopyrrolopyrrole-based small molecules were synthesized with the same backbone but distinct alkyl side-chain substituents (i.e., 2-ethylhexyl-, n-hexyl-, ((methoxyethoxy)ethoxy)ethyl-, and (2-((2-methoxyethoxy)ethoxy)ethyl)acetamide, designated as D-1, D-2, D-3, and D-4, respectively) as HTMs. The variation in the alkyl chain has shown obvious effects on the optical and electrochemical properties as well as on the molecular packing and film-forming ability. Consequently, the power conversion efficiency (PCE) of the PSC under one sun illumination (100 mW cm-2) is shown to increase in the order of D-1 (8.32%) < D-2 (11.12%) < D-3 (12.05%) < D-4 (17.64%). Various characterization techniques reveal that the superior performance of D-4 can be ascribed to the well-aligned highest occupied molecular orbital energy level with the counter electrode, the more compact π-π stacking with a higher coherence length, and the excellent hole mobility of 1.09 × 10-3 cm2 V-1 s-1, thus providing excellent energetics for effective charge transport with minimal charge-carrier recombination. Furthermore, the addition of the dopant Li-TFSI in D-4 is shown to deliver a remarkable PCE of 20.19%, along with a short-circuit current density (JSC), open-circuit voltage (VOC), and fill factor (FF) of 22.94 mA cm-2, 1.14 V, and 73.87%, respectively, and superior stability compared to that of other HTMs. These results demonstrate the effectiveness of side-chain engineering for tailoring the properties of HTMs, thus offering new design tactics to fabricate for the synthesis of highly efficient and stable HTMs for PSCs.
Semi‐transparent organic photovoltaics (ST‐OPVs) are promising solar windows for building integration. Improving the light‐absorbing selectivity, that is, transmitting the visible photons while absorbing the invisible ones, is a key step toward high‐performance ST‐OPV. To achieve this goal, the optical properties of the active layer, transparent electrode, and capping layer are comprehensively tailored, and a highly efficient ST‐OPV with good absorbing selectivity is demonstrated. First, a numerical method is established to quantify the absorbing selectivity of materials and devices, based on which, an infrared absorbing non‐fullerene acceptor, that is, H3, is selected among a large pool of photo‐active materials. Second, an ultra‐smooth transparent thin Ag layer with small granule size is developed via polyethylenimine wetting, which alleviates light scattering and improves the electric properties for ST‐OPV. Finally, as guided by optical simulation, a TeO2 capping layer is deposited on top of the ultra‐thin Ag to further improve the light‐absorbing selectivity. As a result, the light utilization efficiency is significantly improved to 3.95 ± 0.02% (best ≈4.06%), with a good color rendering index of 76.85. These results make it one of the best among color‐neutral ST‐OPVs. This work stresses the importance of manipulating the light‐absorbing selectivity for high‐performance ST‐OPVs.
Semi‐transparent organic solar cells (ST‐OSCs) have revolutionized the field of photovoltaics (PVs) due to their unique abilities, such as transparency and color tunability, and have transformed normal power‐harvesting OSC devices into multifunctional devices, such as building‐integrated photovoltaics, agrivoltaics, floating photovoltaics, and wearable electronics. Very recently, ST‐OSCs have seen remarkable progress, with a rapid increase in power conversion efficiency from below 7% to 12–14%, with an average visible transparency of 9–25%, especially due to the use of low bandgap semiconductors including polymer donors and non‐fullerene acceptors that exhibit absorption in the near‐infrared region as photoabsorbent materials. From this perspective, the latest developments in ST‐OSCs stemming from the innovations in photovoltaic materials that delivered multifunctional ST‐OSCs with top‐of‐the‐line power conversion efficiencies are discussed to shed light on the structure‐property relationship between molecular design and current challenges in this cutting‐edge research field. Finally, personal perspectives, including research directions for the future use of ST‐OSCs in multifunctional applications, are also proposed. The significant advances in efficient photoabsorbent materials have been instrumental in the performance enhancement of semitransparent organic solar cells (ST‐OSCs) from <7% to 12–14% (with good visible transmittance) only in the last 3 years. This study reviews the progress of photoabsorbent materials for ST‐OSCs, and discusses the structure–property relationships and future perspectives for the development of multifunctional ST‐OSCs.
Polymer solar cells based on fullerene acceptors have reached in recent years power conversion efficiencies (PCEs) approaching 13%. The advent of non-fullerene acceptors (NFAs) with the advantages of synthetic versatility, a strong absorption ability and high thermal stability has resulted in impressive PCEs of over 18% in single junction devices. The insertion of interlayers between the active components and electrodes plays a key role in charge collection, boosts the efficiency and improves the device stability. However, the mechanisms regulating the interaction between interlayer materials and active layers based on NFAs are not yet completely rationalized. This review article summarizes organic, inorganic and hybrid materials used as anode and cathode interlayers in conventional and inverted fullerene-free solar cells. Particular attention is paid to the distinctive features of the interlayers when used in non-fullerene solar cells. We will also comment on the fabrication processes with an emphasis on the transition from small area, lab devices to large area modules and on possible mechanisms which are behind.
Semitransparent organic solar cells (OSCs) have great potential for vehicle-integrated and building-integrated photovoltaics with their rapidly increased power conversion efficiencies (PCEs). For these purposes, the efficiency of semitransparent OSCs at an adequate transparency should be further increased. In this work, a ladder-type dithienonaphthalene-based acceptor (DTNIF) with a high-lying lowest unoccupied molecular orbital energy level is used as a third component material for ternary OSCs which show increased PCEs with enlarged values in open-circuit voltage, short-circuit current and fill factor. Consequently, outstanding PCEs of 16.73% and 13.49% are achieved for the corresponding opaque and semitransparent ternary OSCs, respectively. The efficiency of 13.49% for the semitransparent OSCs at an average visible transmittance (AVT) of 22.58% is the highest reported to date, to the best of our knowledge. This work provides an effective strategy to fabricate high-performance semitransparent OSCs at high AVT values by using an excellent third component acceptor material.
In the past few years, the power conversion efficiency (PCE) of organic solar cells (OSCs) has improved rapidly with the milestone value exceeding 18%, primarily owing to the development of novel non-fullerene acceptors (NFAs) as well as the matching polymer donors. The molecular structure of a polymer donor fundamentally determines its molecular packing (crystal structure and morphology) and optoelectronic properties, which influence the photovoltaic processes and the ultimate PCE of the OSC device. The structure-property-cell performance relationships of polymer donors with respect to the spefcific accetor are very complex, involving numerous parameters, but are extremely important towards the development of high-performance polymer donors to achieve high PCE. This review provides a timely analysis of the top-performing wide bandgap (WBG) polymer donors that have been developed to match three most representative narrow bandgap NFAs, ITIC, IT-4F, and Y6, in terms of their structural design, fine-tuning of their optoelectronic properties, as well as control of the morphology and crystallinity of their blends with NFAs. We hope this article provides a deeper insight into the structure-property-cell performance relationships of polymer donors and a collection of useful guidelines and strategies for the design and processing of novel polymer donors for matching with NFAs for achieving ultrahigh performance OSCs.
Development of high-performance small-molecular acceptors (SMAs) and an eco-friendly and simple device fabrication procedure is very crucial for scalable production of polymer solar cells (PSCs) in the future. Here, we designed and synthesized two new nonacyclic SMAs (IPYT-IC and IPYT-ICF) featuring an A-DA′D-A-type molecular configuration, in which an electron-deficient 6,12-dihydro-diindolo[1,2-b:10,20-e]pyrazine (IPY) moiety was used as the additional acceptor unit (A′) and fused with the electron-rich diarylcyclopentadienylthiophene segment (D) to form a DA′D-type central core (IPYT), and 3-(dicyanomethylidene)indol-1-one (IC) or 5,6-difluoro-3-(dicyanomethylene)indol-1-one (ICF) acted as the terminal acceptor group (A). The rigid coplanar DA′D-type core containing a weakly electron-deficient IPY unit is beneficial to broaden the absorption range, improve light-harvesting ability, reduce the band gap, upshift the lowest unoccupied molecular orbital (LUMO) energy level, and enhance the charge transport of the resultant SMAs. Meanwhile, with respect to IPYT-IC, fluorinated IPYT-ICF exhibits a stronger absorption with a narrower band gap, higher electron mobility, and lower-lying highest occupied molecular orbital/LUMO energy levels. The as-cast PSCs based on IPYT-ICF using the polymer PTB7-Th as an electron donor achieve a power conversion efficiency of up to 7.00% with eco-friendly o-xylene (XY) as the processing solvent without any additive and post-treatment, which is higher than that of devices based on IPYT-IC (4.50%) mainly originating from the larger Jsc and FF because of the higher carrier mobilities, better charge transport and collection properties, weaker charge recombination, and superior film morphology. However, IPYT-IC-based devices present an outstanding Voc of up to 0.98 V because the weakly electron-deficient A′ unit (IPY) upshifts LUMO levels of these SMAs. Our results illustrate that the weakly electron-deficient IPY can be a promising A′ unit to develop efficient A-DA′D-A-type SMAs for additive-free and eco-friendly as-cast PSCs.
Designing and synthesizing new organic photovoltaic materials is an inexhaustible driving force for the development of polymer solar cells (PSCs). For an excellent molecular structure with a certain conjugated skeleton, studying its analogues is of considerable significance for fully exploiting its photovoltaic potential. In this work, naphtho[2,3-c]thiophene-4,9-dione (NTD), an analogue of benzo [1,2-c:4,5-c’]dithiophene-4,8-dione (BDD), was designed and synthesized. Based on this building block, alkoxy side chains and alkylthio side chains were grafted on NTD to build NTD-O and NTD-S unit respectively, and then two new D-A conjugated donor polymers, PBNO and PBNS, were synthesized with NTD-O and NTD-S units as acceptor units and asymmetric benzodithiophene unit (asy-BDTBP) as donor units. Later, the photoelectric properties of these two materials in fullerene PSCs were studied. Compared with BDD based benzodithiophene copolymer (asy-BDTBP-BDD), PBNO and PBNS show similar LUMO energy levels, −3.59 eV and −3.63 eV, respectively, PBNO and PBNS show higher HOMO energy levels, which are −5.25 eV and −5.37 eV, respectively. PBNO blended with fullerene acceptor PC71BM exhibited similar power conversion efficiency (PCE) compared to the copolymer asy-BDTBP-BDD (4.64%). The PCE is 4.71% with short-circuit current density (JSC) of 9.85 mA cm⁻², open-circuit voltage (VOC) of 0.784 V and fill factor (FF) of 60.98%. Encouragingly, PBNS presented an obviously enhanced PCE in PC71BM system PSCs, and PCE boosted to 7.58% (JSC, 12.40 mA cm⁻²; VOC, 0.865 V; FF, 70.83%). This work shows that analogues of excellent molecules have huge photovoltaic potential, and that side-chain modification has a significant impact on improving the photovoltaic performance of PSCs.
The optimization of bulk heterojunction morphology is one of the most challenging topics in all‐small‐molecule organic solar cells. Herein, three small molecular donors based on dithieno[2,3‐d;2′,3′‐d′]benzo[1,2‐b;4,5‐b′]dithiophene (DTBDT) unit by systematically moving the branching point of the alkyl chain have been designed, synthesized, and applied in organic solar cells. Modifying the branching points enables the properties of the aggregation state to be tuned, and an efficient nanofiber‐based hierarchical morphology is successfully demonstrated by combining with different nonfullerene acceptors. The molecules with far branching points can form nanofibers in active layers, and theses nanofibers help the charge separation and charge transport in a large donor‐rich or acceptor‐rich domain of approximately 100 nm. Using nonfullerrene Y6 as an acceptor, the highest power conversion efficiency of 14.78% is obtained, which is one of the highest efficiencies in all‐small‐molecule organic solar cells. The strategy of modification of alkyl side chain branching points can be a practical way to actualize crystallinity control and active layer morphology for improving the performance of all‐small‐molecule organic solar cells. Three new dithieno[2,3‐d;2ʹ,3ʹ‐dʹ]benzo[1,2‐b;4,5‐bʹ]dithiophene (DTBDT) based small‐molecule donors, with different branching points for alkyl side chains are designed and synthesized for all small molecular organic solar cells. Modifying the branching points tunes the properties in the aggregation state, and an optimal nanofiber‐based hierarchical morphology for efficient charge separation and transport is successfully demonstrated.
Perovskite solar cells (PSCs) with advantages of exceptional photovoltaic performance and facile solution‐processed fabrication have shown great potential in future scalable application. After about a decade of rapid development, this new PSCs technology demonstrates over 25% efficiency, a comparable performance with traditional silicon solar cells. Further, the development of PSCs in the direction of scalable production still highly relies on designing innovative materials with low cost and high efficiency. Recently, a great number of functional organic molecules as hole transport materials (HTMs) have been designed, synthesized, and studied in PSCs, including molecules with planar structure, 3D geometry, or different core units. Discovering the correlation between their chemical structures and physicochemical properties plays a fundamental role in supervising future molecular design and synthesis. Herein, recent advances in organic molecular HTMs with various structures in typical and reverse PSCs device configuration are summarized, including doped and doping‐free materials. By evaluating the structural modification and analyzing their effects on photovoltaic performance, the goal is to generate universal strategies for preparing low‐cost and efficient HTMs, paving the way for future scalable application of PSCs. Herein, the correlation between chemical structures and physicochemical properties of organic hole transporting materials, which plays fundamental roles in supervising future molecular design and synthesis for perovskite solar cells, is outlined. To move in the practical line of perovskite solar marketing, interfacial contact materials are key parts in case of stability and efficiency.
Semitransparent organic solar cells (ST-OSCs) exhibit great potential in building-integrated photovoltaics (BIPV) due to their low cost large area manufacturing process manufacturing process and tunable vivid colors for power-generating glass. However, the contradiction of achieving high power conversion efficiency (PCE) whilst keeping rational average visible transmittance (AVT) leads to the development of ST-OSCs lagging behind that of traditional opaque OSCs. In this work, chemically precipitated SnO2 colloidal particles are used as an electron collection interlayer in ST-OSCs for the first time. Due to the excellent transparency and high reflective index, the SnO2 layer can effectively tune the light-distribution of the incident light within the whole multilayered ST-OSCs. Guided by finite-difference time-domain (FDTD) and optical transfer matrix formalism (TMF) simulation, we successfully solved the contradiction between PCE and AVT, and achieved multicolored ST-OSCs with record high efficiency. The deep blue device shows the highest PCE of 12.88%, AVT of 25.60% (from 370 nm to 740 nm) and color rendering index (CRI) of 97.6, which are the best values for the state-of-the-art ST-OSCs. Our findings indicate that interfacial engineering and optical coupling are effective approaches to achieve high performance ST-OSCs with vivid colors, remarkable transparency and high efficiency.
This work was inspired by a previous report [Janjua, M. R. S. A. Inorg. Chem. 2012, 51, 11306−11314] in which the optoelectronic properties were improved with an acceptor bearing heteroaromatic rings. Herein, we have designed four novel Y-series non-fullerene acceptors (NFAs) by end-capped acceptor modifications of a recently synthesized 15% efficient Y21 molecule for better optoelectronic properties and their potential use in solar cell applications. Density functional theory (DFT) along with time-dependent density functional theory (TDDFT) at the B3LYP/6-31G(d,p) level of theory is used to calculate the band gap, exciton binding energy along with transition density matrix (TDM) analysis, reorganizational energy of electrons and holes, and absorption maxima and open-circuit voltage of investigated molecules. In addition, the PM6:YA1 complex is also studied to understand the charge shifting from the donor polymer PM6 to the NFA blend. Results of all parameters suggest that the DA'D electron-deficient core and effective end-capped acceptors in YA1−YA4 molecules form a perfect combination for effective tuning of optoelectronic properties by lowering frontier molecular orbital (FMO) energy levels, reorganization energy, and binding energy and increasing the absorption maximum and open-circuit voltage values in selected molecules (YA1−YA4). The combination of extended conjugation and excellent electron-withdrawing capability of the end-capped acceptor moiety in YA1 makes YA1 an excellent organic solar cell (OSC) candidate owing to promising photovoltaic properties including the lowest energy gap (1.924 eV), smallest electron mobility (λ e = 0.0073 eV) and hole mobility (λ h = 0.0083 eV), highest λ max values (783.36 nm (in gas) and 715.20 nm (in chloroform) with lowest transition energy values (E x) of 1.58 and 1.73 eV, respectively), and fine open-circuit voltage (V oc = 1.17 V) with respect to HOMO PM6 −LUMO acceptor. Moreover, selected molecules are observed to have better photovoltaic properties than Y21, thus paving the way for experimentalists to look for future developments of Y-series-based highly efficient solar cells.
To date, the power conversion efficiency (PCE) of lab‐scale organic solar cells (OSCs) has exceeded 17%, which heralds the bright future for commercial applications of OSCs. High‐performance OSCs with thick active layers are essential for large‐scale production. First, the relatively thick active layers should be more compatible with the roll‐to‐roll (R2R) large‐area processing, which is conducive to forming uniform and defect‐free active layers in the process of high‐speed, mass production. Second, the thick active layers can absorb more incident light in their spectral range, which helps thick‐film OSCs to obtain relatively high short‐circuit current density (JSC). So far, relatively little attention has been paid to thick‐film OSCs, and the PCE of thick‐film OSCs lags far behind its thin‐film analogues. In this review, we highlight the recent development of thick‐film OSCs and point out the critical limit factors on the PCE of thick‐film OSCs. Some strategies are highlighted to improve the efficiency of thick‐film OSCs. This review article will be helpful to the researchers engaging in the development of efficient thick‐film OSCs.
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The synthesis of donor (D)-acceptor (A) polymers using structurally elaborated monomers is an active research field. Some of the challenges with the use of alternating D-A polymers for photovoltaic applications are the relatively narrow absorption widths, the presence an absorption valleys in the visible region, unoptimized molecular energy levels and even lack of compatibility of the polymers with the common acceptors. The synthesis and characterization of polymers consisting of multiple chromophores (random and regular terpolymers) with complementing properties is currently gaining momentum in order to delicately optimize properties of polymers. A random terpolymer can either be of a system composed of one donor and two acceptors [(D-A1)-ran-(D-A2)] or a one acceptor and two donor segments [(D1-A)-ran-(D2-A)] incorporated in the polymer backbone. By varying the composition of the monomers in the feed of the polymerization reaction, the properties of the resulting terpolymers can be carefully optimized. Using this strategy, many materials with desired properties have been developed and power conversion efficiency (PCE) surpassing 14% in a single layer bulk heterojunction (BHJ) solar cell device have been reported. This review summarizes the most recent advances made in the development of electron donor terpolymers for organic photovoltaics (OPVs). The properties of the terpolymers are compared with their respective reference polymer.
The field of bulk heterojunction (BHJ) organic photovoltaics (OPVs) or solar cells (OSCs) has experienced a dramatic advance toward a competitive technology reflecting the introduction of new materials, tuning of materials combinations, and optimization of the device architecture. Thus, binary BHJ OSCs with power conversion efficiencies surpassing 18% have been demonstrated. In this review we discuss recent developments in the area of π-conjugated small-molecule and polymeric semiconductors for organic BHJ-OSCs focusing on both electron-donor (hole-transporting) and electron-acceptor (electron-transporting) semiconductors developed during the past three years. Thus, several families of semiconductor materials including donor-acceptor (D-A) polymers, fullerene, and non-fullerene acceptors (NFAs) are reviewed including their combination for polymer-fullerene, donor polymer-NFA, all-small molecule, and all-polymer solar cells.
The innovation of photoactive layer materials is crucial for improving power conversion efficiency (PCE) of polymer solar cells (PSCs). Herein, we report two polymer donors (PBTN-o and PBTN-p), which only differ on alkyl chains substituted on sites (6,7- or 5,8-) of naphtho[2,3-c]thiophene-4,9-dione (NTD) unit. The single crystals of both NTD monomers demonstrate that the NTD with alkyl chains at 6,7-sites has a planar NTD skeleton, Surpringly, the NTD with alkyl chains at 5, 8-sites produces a bent NTD skeleton. The bent NTD-based polymer (PBTN-p) has a more twisted conjugated backbone than that of PBTN-o. Our comparative studies show that PBTN-p possesses a suitable aggregation property that can optimize photoactive layer morphology in NF PSCs. In PSCs, the optimal PBTN-o:BO-4Cl-based device shows a PCE of 11.85% with a VOC of 0.84 V, JSC of 22.41 mA cm-2, and FF of 0.63, in contrast, the optimal PBTN-p:BO-4Cl-based device gives a better PCE of 14.10% with the same VOC of 0.84 V, and enhanced JSC of 24.67 mA cm-2, and FF of 0.68. This work provides a new insight that BHJ morphology can be optimized by side chian induced polymer main chain twist in PSCs.
Semitransparent organic solar cells (ST-OSCs) have drawn great attention due to their wide application, such as smart windows, greenhouses and building integrated photovoltaics. The ternary strategy is often considered an useful way to enhance device performance due to improve the attracting or retracting morphology. Here, non-fullerene small molecule (Y8) with electron-deficient core as the center unit and a novel PBDTTT-type low-bandgap polymer (PCE10) were chosen to fabricate ST-OSCs according to the principle of more closely matched energy levels. With the introduction of indene-C60bisadduct (ICBA) as third component, opaque organic solar cells with a power conversion efficiency (PCE) of 12.86% and ST-OSCs with a PCE of 10.46% were obtained by using spin coating after optimization. The average visible transmittance of ST-OSCs is up to 26.56%. By the blade coating technique, 9.52% PCE is achieved for ST-OSCs, which is the highest value reported to date. These results show that it's higher efficiency to improve the performance of ST-OSCs through ternary strategy. And the blade coating technique has a promising application prospect for mass manufacturing ST-OSCs.
As a new generation of solid-state film cells, the organic solar cells (OSCs) have become the research focus in the field of renewable energy sources, and the reported power conversion efficiencies (PCEs) have been boosted to 18%. The hole transport layer (HTL) materials, a critical component of the OSCs, exert a tremendous impact on the PCE and stability of the OSCs. At present, the HTL materials used in OSCs can be divided into two main categories, which are inorganic HTL materials and organic HTL materials. Although, the OSCs with inorganic HTL materials can achieve satisfied PCE, they are not suitable for the large-scale commercial roll-to-roll production due to the unavoidable process of high-temperature vacuum evaporation. Recently, a great number of organic HTL materials have been designed, synthesized, and successfully applied in the OSCs. Herein, we review the recent advances of organic HTL materials in single-junction OSCs and systematically discuss the relationships between the structure and properties of various HTL materials, and highlight the designing rules of HTL materials to toward highly efficient and stable OSCs.
Generally, molecular optimization is widely used to fine-tune the absorption features and energy levels of photovoltaic materials to improve their photovoltaic performance for polymer solar cells (PSCs). In this work, we demonstrate an example that the morphological properties can be effectively optimized by conjugated side-chains engineering on benzo[1,2-b:4,5-b']dithiophene (BDT) unit. The polymer donors PBNT-S with alkylthio-thienyl substitution and PBNP-S with alkylthio-phenyl substitution have identical absorption spectra and energy levels, while exhibit significantly different morphological properties when blended with nonfullerene acceptor Y6. The PBNT-S:Y6 blend shows obviously over crystallinity with excessive domain sizes, while the PBNP-S:Y6 blend realizes better nanoscale phase separation. As a result, a notable power conversion efficiency (PCE) of 14.31% with a high fill factor (FF) of 0.694 is achieved in the PBNP-S:Y6-based device, while the PBNT-S:Y6-based device yields a moderate PCE of 11.10% and a relatively low FF of 0.605. Additionally, PBNP-S shows promising potential in semitransparent PSCs, that the PBNP-S:Y6-based semitransparent PSC achieves an outstanding PCE of 11.86%, with an average visible transmittance of 26.4%. The results demonstrate a feasible strategy to manipulate the morphological properties of blend film via rational molecular optimization to improve the photovoltaic performance.
Besides high short-circuit current density (Jsc) and fill factor (FF), high open-circuit voltage (Voc) is urgently necessary for obtaining high overall efficiencies of polymer solar cells (PSCs). In order to produce high Voc PSCs, herein, we developed three wide-bandgap donor–acceptor (D-A) alternate copolymers (PBDTO-TPTI, PBDTT-TPTI, and PBDTS-TPTI) of benzodithiophene (BDT) and thienopyridothieno-isoquinoline-5,11(4H,10H)-dione (TPTI) moieties. These copolymers possess a uniform BDT-TPTI framework, but various side chains (alkoxyl, alkylthienyl, alkylthiothienyl) on the BDT unit. The resultant data convincingly reveal that the spectral absorption, optical bandgap (Egopt), aggregation characteristic, energy levels, charge transport properties and active layer morphology of the D–A copolymers can be effectively manipulated via side-chain engineering on the BDT segment. The gradually increased Egopt (1.92–1.95 and then to 1.97 eV) and gradually decreased HOMO/LUMO levels (−5.43/−3.47 to −5.54/−3.53 and then to −5.56/−3.76 eV) are found while the side group on the BDT unit is varied from alkoxyl (PBDTO-TPTI) to alkylthieyl (PBDTT-TPTI) and then to alkylthiothienyl (PBDTS-TPTI). Importantly, the geometric and optoelectronic properties of these polymers are supported by theoretical predictions. PSCs based on all the three copolymers with a fullerene-based acceptor (PC71BM) exhibit power conversion efficiencies (PCEs) exceeding 5% and a Voc over 0.93 V. Notably, PBDTS-TPTI-based PSC achieves the highest PCE of 5.35% accompanied with the highest Voc as far as 0.99 V and Jsc up to 12.60 mA cm⁻². This work indicates side-chain engineering on polymers is an impactful and feasible approach to realize high Voc PSCs by manipulating electronic levels of D–A copolymers.
Herein, a new “Y-series” non-fullerene acceptor, Y21, bearing an asymmetric electron-deficient-core (DA’D) and fluorinated dicyanomethylene derivatives as flanking groups, was designed and synthesized for organic solar cell applications. Rather than being perfectly C2 symmetric manner of the traditional “Y-series” acceptor, Y21 possesses an electron-withdrawing unit (A’) shifted from the center of DA’D, turning into an asymmetric molecular geometry. Photovoltaic devices based on PM6:Y21 can realize a high Jsc of 24.9 mA cm-2, and a PCE of 15.4 %. Our work demonstrates a new way to tune the photoelectronic property of the “Y-series” NFAs.
Both the efficiency and stability of low cost organic solar cells are central components to meeting the requirements of commercialization for organic photovoltaics (OPV). Furthermore, the relationship between chemical structure of active material and morphology and its effects on efficiency and stability is still largely undetermined. Additionally, both the kinetic and thermodynamic morphology states of active layer can have a large impact on efficiency and stability, even when the chemical structures of materials applied in the active layer are especially same or similar. Here, using two series of acceptor-donor-acceptor (A-D-A) type small molecule acceptors (SMAs) with the similar backbone structure, we demonstrate the relevance of fine-tuned chemical structures with their solution and solid-state properties, further leading to significantly different behavior in terms of both device efficiency and stability. This is also partially due to the different morphology states caused by such fine chemical structure tuning. Our results indicate that a delicate balance of molecular aggregation and ordered stacking morphology is not only required to achieve but also could lead to both high efficiency and stability. Thus, among the two series of molecules, UF-EH-2F with both optimal length and steric hindrance of side-chains achieves the preponderant morphology in its corresponding device, where its morphology “Efficient State” and “Stable State” are almost overlapped and thus lead to both the highest efficiency (PCE = 13.56%) and best stability. Our results indicate that it is highly possible to achieve the morphology state required for both high efficiency and stability simultaneously by fine-tuning the chemical structure of active materials for organic solar cells.
Although organic solar cells have surpassed the 17% power conversion efficiency threshold, commercial modules efficiencies are only around 4‐5%. One of the reason, is the lack of effective solution processable hole transport materials, that are a key element for the scale up on roll‐to‐roll printing equipments and the commercial development. In this work, we have developed a class of novel vanadium and molybdenum polyoxometallate salts that, alone or in combination with a traditional poly(ethylene‐3,4‐dioxytiophene):poly(styrene sulfonate) (PEDOT:PSS) layer, can be employed as anodic buffer layer in inverted polymer solar cells. These materials exhibit work function values around 5.8 eV that match well with HOMO energies of typical polymer donors. They have been tested with different widely used active systems, including PTB7:PC71BM, PV2000:PCBM and PffBT4T:PC71BM. Vanadium and molybdenum polyoxometallate can be deposited from solutions and, contrary to PEDOT:PSS used alone, do not cause a drop of performances compared to evaporated molybdenum oxide (e‐MoOx); on the contrary, in the best cases they achieve similar performances to e‐MoOx. Slot‐die coated PV2000:PCBM solar cells on flexible substrate achieve a remarkable power conversion efficiency of almost 7.6%. This article is protected by copyright. All rights reserved.
The field of non-fullerene organic solar cells has experienced rapid development during the past few years, mainly driven by the development of novel non-fullerene acceptors and matching donor semiconductors. However, organic solar cell material development has progressed via a trial-and-error approach with limited understanding of the materials’ structure-property relationships and the underlying device physics of non-fullerene devices. In addition, the availability of hundreds of donor and acceptor semiconductors creates an extremely large pool of possible donor-acceptor combinations, which poses a daunting challenge for rational material screening and matching. This Review describes several important conceptual aspects of the emerging non-fullerene devices by highlighting key contributions that provided fundamental insights regarding rational material design, donor-acceptor pair matching, blend morphology control and the reduced voltage losses in non-fullerene organic solar cells. We also discuss the key challenges that need to be addressed to develop more-efficient non-fullerene organic solar cells.
Fluorine‐contained polymers, which have been widely used in highly efficient polymer solar cells (PSCs), are rather costly due to their complicated synthesis and low yields in the preparation of components. Here, the feasibility of replacing the critical fluorine substituents in high‐performance photovoltaic polymer donors with chlorine is demonstrated, and two polymeric donors, PBDB‐T‐2F and PBDB‐T‐2Cl, are synthesized and compared in parallel. The synthesis of PBDB‐T‐2Cl is much simpler than that of PBDB‐T‐2F. The two polymers have very similar optoelectronic and morphological properties, except the chlorinated polymer possess lower molecular energy levels than the fluorinated one. As a result, the PBDB‐T‐2Cl‐based PSCs exhibit higher open circuit voltage (Voc) than the PBDB‐T‐2F‐based devices, leading to an outstanding power conversion efficiency of over 14%. This work establishes a more economical design paradigm of replacing fluorine with chlorine for preparing highly efficient polymer donors.
Over the past three years, a particularly exciting and active area of research within the field of organic photovoltaics has been the use of non-fullerene acceptors (NFAs). Compared with fullerene acceptors, NFAs possess significant advantages including tunability of bandgaps, energy levels, planarity and crystallinity. To date, NFA solar cells have not only achieved impressive power conversion efficiencies of ~13–14%, but have also shown excellent stability compared with traditional fullerene acceptor solar cells. This Review highlights recent progress on single-junction and tandem NFA solar cells and research directions to achieve even higher efficiencies of 15–20% using NFA-based organic photovoltaics are also proposed. This Review describes how non-fullerene electron acceptor materials are bringing improvements in the power conversion efficiency and stability of organic solar cells.
The application of polymer solar cells requires the realization of high efficiency, high stability, and low cost devices. Here we demonstrate a low-cost polymer donor poly[(thiophene)-alt-(6,7-difluoro-2-(2-hexyldecyloxy)quinoxaline)] (PTQ10), which is synthesized with high overall yield of 87.4% via only two-step reactions from cheap raw materials. More importantly, an impressive efficiency of 12.70% is obtained for the devices with PTQ10 as donor, and the efficiency of the inverted structured PTQ10-based device also reaches 12.13% (certificated to be 12.0%). Furthermore, the as-cast devices also demonstrate a high efficiency of 10.41% and the devices exhibit insensitivity of active layer thickness from 100 nm to 300 nm, which is conductive to the large area fabrication of the devices. In considering the advantages of low cost and high efficiency with thickness insensitivity, we believe that PTQ10 will be a promising polymer donor for commercial application of polymer solar cells.
Non-fullerene acceptors (NFAs) are currently a major focus of research in the development of bulk-heterojunction organic solar cells (OSCs). In contrast to the widely used fullerene acceptors (FAs), the optical properties and electronic energy levels of NFAs can be readily tuned. NFA-based OSCs can also achieve greater thermal stability and photochemical stability, as well as longer device lifetimes, than their FA-based counterparts. Historically, the performance of NFA OSCs has lagged behind that of fullerene devices. However, recent developments have led to a rapid increase in power conversion efficiencies for NFA OSCs, with values now exceeding 13%, demonstrating the viability of using NFAs to replace FAs in next-generation high-performance OSCs. This Review discusses the important work that has led to this remarkable progress, focusing on the two most promising NFA classes to date: rylene diimide-based materials and materials based on fused aromatic cores with strong electron-accepting end groups. The key structure–property relationships, donor–acceptor matching criteria and aspects of device physics are discussed. Finally, we consider the remaining challenges and promising future directions for the NFA OSCs field.
A new synthetic route, to prepare an alkylated indacenodithieno[3,2-b]thiophene-
based nonfullerene acceptor (C8-ITIC), is reported. Compared to the
reported ITIC with phenylalkyl side chains, the new acceptor C8-ITIC exhibits
a reduction in the optical band gap, higher absorptivity, and an increased
propensity to crystallize. Accordingly, blends with the donor polymer PBDB-T
exhibit a power conversion efficiency (PCE) up to 12.4%. Further improvements
in efficiency are found upon backbone fluorination of the donor polymer
to afford the novel material PFBDB-T. The resulting blend with C8-ITIC shows
an impressive PCE up to 13.2% as a result of the higher open-circuit voltage.
Electroluminescence studies demonstrate that backbone fluorination reduces
the energy loss of the blends, with PFBDB-T/C8-ITIC-based cells exhibiting a
small energy loss of 0.6 eV combined with a high JSC of 19.6 mA cm−2.
A fused-undecacyclic electron acceptor IUIC has been designed, synthesized and applied in organic solar cells (OSCs) and semitransparent organic solar cells (ST-OSCs). In comparison with its counterpart, fused-heptacyclic ITIC4, IUIC with a larger π-conjugation and a stronger electron-donating core exhibits a higher LUMO level (IUIC: −3. 87 eV vs. ITIC4: –3.97 eV), 82 nm-redshifted absorption with larger extinction coefficient and smaller optical bandgap, and higher electron mobility. Thus, IUIC-based OSCs show higher values in open-circuit voltage, short-circuit current density and thereby much higher power conversion efficiency (PCE) than those of the ITIC4-based counterpart. The as-cast OSCs based on PTB7-Th: IUIC without any extra treatment yield PCEs of up to 11.2%, higher than that of the control devices based on PTB7-Th: ITIC4 (8.18%). The as-cast ST-OSCs based on PTB7-Th: IUIC without any extra treatment afford PCEs of up to 10.2% with an average visible transmittance (AVT) of 31%, higher than those of the con-trol devices based on PTB7-Th: ITIC4 (PCE = 6.42%, AVT = 28%).
While the performance of laboratory-scale organic solar cells (OSCs) continues to grow, development of high efficiency large area OSCs remains a big challenge. Although a few attempts to produce large area organic solar cells (OSCs) have been reported, there are still challenges on the way to realizing efficient module devices, such as the low compatibility of the thickness-sensitive active layer with large area coating techniques, the frequent need for toxic solvents and tedious optimization processes used during device fabrication. In this work, highly efficient thickness-insensitive OSCs based on PTB7-Th:PC71BM that processed with single-component green solvent 2-methylanisole are presented, in which both junction thickness limitation and solvent toxicity issues are simultaneously addressed. Careful investigation reveals that this green solvent prevents the evolution of PC71BM into large area clusters resulting in reduced charge carrier recombination, and largely eliminates trapping centers, and thus improves the thickness tolerance of the films. These findings enable us to address the scalability and solvent toxicity issues and to fabricate a 16 cm² OSC with doctor-blade coating with a state-of-the-art power conversion efficiency of 7.5% using green solvent.
Flexible and semitransparent organic solar cells (OSCs) have been regarded as the most promising photovoltaic devices for the application of OSCs in wearable energy resources and building-integrated photovoltaics. Therefore, the flexible and semitransparent OSCs have developed rapidly in recent years through the synergistic efforts in developing novel flexible bottom or top transparent electrodes, designing and synthesizing high performance photoactive layer and low temperature processed electrode buffer layer materials, and device architecture engineering. To date, the highest power conversion efficiencies have reached over 10% of the flexible OSCs and 7.7% with average visible transmittance of 37% for the semitransparent OSCs. Here, a comprehensive overview of recent research progresses and perspectives on the related materials and devices of the flexible and semitransparent OSCs is provided.
The absence of near-infrared (NIR) solar cells with high open circuit voltage (Voc) and external quantum efficiency (EQE) has impeded progress toward achieving organic photovoltaic (OPV) power conversion efficiency PCE > 15%. Here we report a small energy gap (1.3 eV), chlorinated nonfullerene acceptor-based solar cell with PCE = 11.2 ± 0.4%, short circuit current of 22.5 ± 0.6 mA cm–2, Voc = 0.70 ± 0.01 V and fill factor of 0.71 ± 0.02, which is the highest performance reported to date for NIR single junction OPVs. Importantly, the EQE of this NIR solar cell reaches 75%, between 650 and 850 nm while leaving a transparency window between 400 and 600 nm. The semitransparent OPV using an ultrathin (10 nm) Ag cathode shows PCE = 7.1 ± 0.1%, with an average visible transmittance of 43 ± 2%, Commission d’Eclairage chromaticity coordinates of (0.29, 0.32) and a color rendering index of 91 for simulated AM1.5 illumination transmitted through the cell.
Nowadays, most of the solution-processed high-efficiency polymer solar cell (PSC) devices are fabricated by halogenated solvents (such as chlorobenzene, 1,2- dichlorobenzene, and chloroform etc) which are harmful to people and the environment. Therefore, it is essential to develop high-efficiency PSC devices processed by environmental-friendly solvent-processing for their industrialization. In this regard, we report a new alkylthio chain-based conjugated polymer PBDB-TS as donor material for environmental-friendly solvent-processed PSCs. PBDB-TS possesses a low-lying HOMO energy level at -5.42 eV and a good solubility in toluene and o-xylene. By using o-xylene and 1% N-methylpyrrolidone as processing solvent, following by the thermal annealing treatment for PBDB-TS:ITIC blend films, well-developed morphological features and balanced charge transport properties are observed, leading to a high power conversion efficiency (PCE) of 9.85%, which is higher than that of the device cast from halogenated solvent (PCE = 9.65%). The results suggest that PBDB-TS is an attractive donor material for non-halogen solvents-processing PSCs.
Two novel narrow bandgap π-conjugated polymers based on naphtho[1,2-c:5,6-c′]bis([1,2,5]thiadiazole) (NT) unit are developed, which contain the thiophene or benzodithiophene flanked with alkylthiophene as the electron-donating segment. Both copolymers exhibit strong aggregations both in solution and as thin films. The resulting copolymers with higher molecular weight show higher photovoltaic performance by virtue of the enhanced short-circuit current densities and fill factors, which can be attributed to their higher absorptivity and formation of more favorable film morphologies. Polymer solar cells (PSCs) fabricated with the copolymer PNTT achieve remarkable power conversion efficiencies (PCEs) > 11% based on both conventional and inverted structures at the photoactive layer thickness of 280 nm, which is the highest value so far observed from NT-based copolymers. Of particular interest is that the device performances are insensitive to the thickness of the photoactive layer, for which the PCEs > 10% can be achieved with film thickness ranging from 150 to 660 nm, and the PCE remains >9% at the thickness over 1 µm. These findings demonstrate that these NT-based copolymers can be promising candidates for the construction of thick film PSCs toward low-cost roll-to-roll processing technology.
A new polymer donor (PBDB-T-SF) and a new small molecule acceptor (IT-4F) for fullerene-free organic solar cells (OSCs) were designed and synthesized. The influences of fluorination on the absorption spectra, molecular energy levels and charge mobilities of the donor and acceptor were systematically studied. The PBDB-T-SF:IT-4F-based OSC device showed a record high efficiency of 13.1%, and an efficiency of over 12% can be obtained with a thickness of 100–200 nm, suggesting the promise of fullerene-free OSCs in practical applications.
To achieve efficient non-fullerene organic solar cells, it is important to reduce the voltage loss from the optical bandgap to the open-circuit voltage of the cell. Here we report a highly efficient non-fullerene organic solar cell with a high open-circuit voltage of 1.08 V and a small voltage loss of 0.55 V. The high performance was enabled by a novel wide-bandgap (2.05 eV) donor polymer paired with a narrow-bandgap (1.63 eV) small-molecular acceptor (SMA). Our morphology characterizations show that both the polymer and the SMA can maintain high crystallinity in the blend film, resulting in crystalline and small domains. As a result, our non-fullerene organic solar cells realize an efficiency of 11.6%, which is the best performance for a non-fullerene organic solar cell with such a small voltage loss.
Inspired by the remarkable promotion of power conversion efficiency (PCE), commercial applications of organic photovoltaics (OPVs) can be foreseen in near future. One of the most promising applications is semitransparent (ST) solar cells that can be utilized in value-added applications such as energy-harvesting windows. However, the single-junction STOPVs utilizing fullerene acceptors show relatively low PCEs of 4%-6% due to the limited sunlight absorption because it is a dilemma that more photons need to be harvested in UV-vis-near-infrared (NIR) region to generate high photocurrent, which leads to the significant reduction of device transparency. This study describes the development of a new small-bandgap electron-acceptor material ATT-2, which shows a strong NIR absorption between 600 and 940 nm with an Eg(opt) of 1.32 eV. By combining with PTB7-Th, the as-cast OPVs yield PCEs of up to 9.58% with a fill factor of 0.63, an open-circuit voltage of 0.73 V, and a very high short-circuit current of 20.75 mA cm(-2) . Owing to the favorable complementary absorption of low-bangap PTB7-Th and small-bandgap ATT-2 in NIR region, the proof-of-concept STOPVs show the highest PCE of 7.7% so far reported for single-junction STOPVs with a high transparency of 37%.
Simutaneously high open circuit voltage and high short circuit current density is a big challenge for achieving high efficiency polymer solar cells due to the excitonic nature of organic semdonductors. Herein, we developed a trialkylsilyl substituted 2D-conjugated polymer with the highest occupied molecular orbital level down-shifted by Si–C bond interaction. The polymer solar cells obtained by pairing this polymer with a non-fullerene acceptor demonstrated a high power conversion efficiency of 11.41% with both high open circuit voltage of 0.94 V and high short circuit current density of 17.32 mA cm−2 benefitted from the complementary absorption of the donor and acceptor, and the high hole transfer efficiency from acceptor to donor although the highest occupied molecular orbital level difference between the donor and acceptor is only 0.11 eV. The results indicate that the alkylsilyl substitution is an effective way in designing high performance conjugated polymer photovoltaic materials.
Solution-processed organic photovoltaics (OPV) offer the attractive prospect of low-cost, light-weight and environmentally benign solar energy production. The highest efficiency OPV at present use low-bandgap donor polymers, many of which suffer from problems with stability and synthetic scalability. They also rely on fullerene-based acceptors, which themselves have issues with cost, stability and limited spectral absorption. Here we present a new non-fullerene acceptor that has been specifically designed to give improved performance alongside the wide bandgap donor poly(3-hexylthiophene), a polymer with significantly better prospects for commercial OPV due to its relative scalability and stability. Thanks to the well-matched optoelectronic and morphological properties of these materials, efficiencies of 6.4% are achieved which is the highest reported for fullerene-free P3HT devices. In addition, dramatically improved air stability is demonstrated relative to other high-efficiency OPV, showing the excellent potential of this new material combination for future technological applications.
The active layer in a solution processed organic photovoltaic device comprises a light absorbing electron donor semiconductor, typically a polymer, and an electron accepting fullerene acceptor. Although there has been huge effort targeted to optimize the absorbing, energetic, and transport properties of the donor material, fullerenes remain as the exclusive electron acceptor in all high performance devices. Very recently, some new non-fullerene acceptors have been demonstrated to outperform fullerenes in comparative devices. This Account describes this progress, discussing molecular design considerations and the structure–property relationships that are emerging.
Organic solar cells now exceed 10% efficiency igniting interest not only in the fundamental molecular design of the photoactive semiconducting materials, but also in overlapping fields such as green chemistry, large-scale processing and thin film stability. For these devices to be commercially useful, they must have lifetimes in excess of 10 years. One source of potential instability, is that the two bicontinuous phases of electron donor and acceptor materials in the photoactive thin film bulk heterojunction, change in dimensions over time. Photocrosslinking of the π-conjugated semiconducting donor polymers allows the thin film morphology to be ‘locked’ affording patterned and stable blends with suppressed fullerene acceptor crystallization. This article reviews the performance of crosslinkable polymers, fullerenes and additives used to-date, identifying the most promising.
Low bandgap and two-dimensional (2D)-conjugated copolymers based on benzo[1,2-b:4,5-b′]dithiophene with conjugated thiophene side chains (BDTT) and thieno[3,4-b]thiophene with electron-withdrawing substituents (TT) are attractive high efficiency polymer donor materials in polymer solar cells (PSCs). In this work, we introduced an alkylthio substituent on the thiophene side chain in the polymer and synthesized a new low bandgap 2D-conjugated polymer PBDTT-S-TT. The alkylthio substituent increased the hole mobility of the polymer to 4.08 × 10−3 cm2 V−1 s−1 and down-shifted the HOMO energy level of the polymer by 0.11 eV with absorption of the polymer film red-shifted slightly. The PSCs based on PBDTT-S-TT as a donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) as an acceptor without solvent additive treatment demonstrated a high open-circuit voltage (Voc) of 0.84 V, leading to a high power conversion efficiency (PCE) of 8.42%, under the illumination of AM 1.5 G 100 mW cm−2. For comparison, the Voc and PCE of the devices based on the corresponding parent polymer PBDTT-TT with the device optimization of 3% DIO additive treatment are 0.77 V and 7.38%, respectively. The enhanced Voc value of 0.84 V for the PSC based on PBDTT-S-TT should be benefited from the down-shifted HOMO energy level of the polymer. The results indicate that the alkylthio substitution is an effective way to further improve the photovoltaic performance of the 2D-conjugated polymer donor materials in PSCs.
Photovoltaic devices based on organic semiconductors (OPVs) hold great promise as a cost-effective renewable energy platform because they can be processed from solution and deposited on flexible plastics using roll-to-roll processing. Despite important progress and reported power conversion efficiencies of more than 10% the rather limited stability of this type of devices raises concerns towards future commercialization. The tandem concept allows for both absorbing a broader range of the solar spectrum and reducing thermalization losses. We designed an organic tandem solar cell with an inverted device geometry comprising environmentally stable active and charge-selecting layers. Under continuous white light irradiation, we demonstrate an extrapolated, operating lifetime in excess of one decade. We elucidate that for the current generation of organic tandem cells one critical requirement for long operating lifetimes consists of periodic UV light treatment. These results suggest that new material approaches towards UV-resilient active and interfacial layers may enable efficient organic tandem solar cells with lifetimes competitive with traditional inorganic photovoltaics.
Inorganic semiconductor-based broadband photodetectors are ubiquitous in imaging technologies such as digital cameras and photometers. Herein a broadband organic photodiode (OPD) that has performance metrics comparable or superior to inorganic photodiodes over the same spectral range is reported. The photodiode with an active layer comprised of a poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester bulk heterojunction blend had a dark current < 1 nA/cm2, specific detectivity of ∼1013 Jones, reverse bias −3 dB frequency response of 100 kHz to 1 MHz, and state-of-the-art Linear Dynamic Range for organic photodiodes of nine orders of magnitude (180 dB). The key to these performance metrics was the use of a thick junction (700 nm), which flattened the spectral response, reduced the dark current and decreased performance variations. The strategy also provides a route to large area defect free “monolithic” structures for low noise integrated photo-sensing, position determination, or contact, non-focal imaging.
In bulk heterojunction organic photovoltaics, electron-donating and electron-accepting materials form a distributed network of heterointerfaces in the photoactive layer, where critical photo-physical processes occur. However, little is known about the structural properties of these interfaces due to their complex three-dimensional arrangement and the lack of techniques to measure local order. Here, we report that molecular orientation relative to donor/acceptor heterojunctions is an important parameter in realizing high-performance fullerene-based, bulk heterojunction solar cells. Using resonant soft X-ray scattering, we characterize the degree of molecular orientation, an order parameter that describes face-on (+1) or edge-on (-1) orientations relative to these heterointerfaces. By manipulating the degree of molecular orientation through the choice of molecular chemistry and the characteristics of the processing solvent, we are able to show the importance of this structural parameter on the performance of bulk heterojunction organic photovoltaic devices featuring the electron-donating polymers PNDT-DTBT, PBnDT-DTBT or PBnDT-TAZ.
Organic photovoltaic (OPV) cells represent an exciting class of renewable energy technology; they are lightweight and flexible, and have a low production cost. Over the last two decades, the efficiency of these devices has improved significantly, in particular through the development of solution-processed bulk heterojunction (BHJ) OPV cells. While fullerenes have been the most intensively studied acceptor materials in BHJ OPVs, research is currently underway in several groups investigating non-fullerene molecular acceptors. In this review, initial breakthroughs and recent progress in the development of polymer donor-polymer acceptor (all-polymer) BHJ OPVs are highlighted.
Two donor–acceptor (D–A) alternative copolymers of benzodithiophene (BDT) donor unit and an acceptor unit of naphtho[2,3-c]thiophene-4,9-dione (NTDO) with different alkyl side chains, PBDTNTDO-1 and PBDTNTDO-2, were synthesized for application as donor materials in polymer solar cells (PSCs). The copolymers show good solubility in common organic solvents, broad visible absorption from 350 nm to 670 nm, and relatively lower HOMO energy levels at −5.14 eV for PBDTNTDO-1 and −5.19 eV for PBDTNTDO-2. The PSCs based on PBDTNTDO-2 as donor and PC70BM as acceptor demonstrated power conversion efficiency of 1.52% with an open circuit voltage of 0.88 V and a short circuit current of 5.67 mA cm−2, under the illumination of AM1.5, 100 mW cm−2.
The open-circuit voltage Voc of polymer:fullerene bulk heterojunction solar cells is investigated as a function of light intensity for different temperatures. Devices consisted of a blend of a poly(p-phenylene vinylene) derivative as the hole conductor and 6,6-phenyl C61-butyric acid methyl ester as the electron conductor. The observed photogenerated current and Voc are at variance with classical p–n junction-based models. The influence of light intensity and recombination strength on Voc is consistently explained by a model based on the notion that the quasi-Fermi levels are constant throughout the device, including both drift and diffusion of charge carriers.
Bimolecular recombination in organic semiconductors is known to follow the Langevin expression, i.e., the rate of recombination depends on the sum of the mobilities of both carriers. We show that this does not hold for polymer/fullerene bulk heterojunction solar cells. The voltage dependence of the photocurrent reveals that the recombination rate in these blends is determined by the slowest charge carrier only, as a consequence of the confinement of both types of carriers to two different phases.
Recent progress in the development of polymer solar cells has improved
power-conversion efficiencies from 3% to almost 9%. Based on
semiconducting polymers, these solar cells are fabricated from
solution-processing techniques and have unique prospects for achieving
low-cost solar energy harvesting, owing to their material and
manufacturing advantages. The potential applications of polymer solar
cells are broad, ranging from flexible solar modules and semitransparent
solar cells in windows, to building applications and even photon
recycling in liquid-crystal displays. This Review covers the scientific
origins and basic properties of polymer solar cell technology, material
requirements and device operation mechanisms, while also providing a
synopsis of major achievements in the field over the past few years.
Potential future developments and the applications of this technology
are also briefly discussed.
To simultaneously achieve the low photon energy loss (Eloss) and the broad spectral response, the molecular design of the efficient wide band-gap (WBG) donor polymer with a deep HOMO level is of critical importance in fullerene-free polymer solar cells (PSCs). Herein, we developed a new benzodithiophene unit, i.e., DTBDT-EF and conducted systematic investigations on a WBG DTBDT-EF-based donor polymer, namely, PDTB-EF-T. Due to the synergistic electron-withdrawing effect of the fluorine atom and ester group, PDTB-EF-T exhibits higher oxidation potential, i.e. deeper HOMO level (ca. -5.5 eV) than most well-known donor polymers. Hence, a high open-circuit voltage of 0.90 V was obtained when paired with a fluorinated small molecule acceptor (IT-4F), corresponding to a low Eloss of 0.62 eV. Furthermore, side-chain engineering demonstrated that fine side-chain modulation of the ester greatly influences the aggregation effects and molecular packing of polymer PDTB-EF-T. Benefiting from the stronger interchain π-π interaction, the improved ordering structure, and thus the highest hole mobility, the most symmetric charge transport and reduced recombination are achieved for the linear decyl-substituted PDTB-EF-T (P2)-based PSCs, leading to the highest short-circuit current density and fill factor (FF). Due to the high Flory-Huggins interaction parameter (χ), surface-directed phase separation occurs in the P2:IT-4F blend, which is supported by X-ray photoemission spectroscopy results and cross-sectional transmission electron microscope images. By taking advantage of the vertical phase distribution of the P2:IT-4F blend, a remarkably high power conversion efficiency (PCE) of 14.2% with an outstanding FF of 0.76 was recorded for inverted devices. These results demonstrate the great potential of DTBDT-EF unit for future organic photovoltaic applications.
We have prepared an efficient ternary polymer solar cell incorporating a poly(indacenodithiophene)-based conjugated polymer (PIDTBT), [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2´,3´-d´]-s-indaceno[1,2-b:5,6-b´]-dithiophene (ITIC). Grazing-incidence wide-angle X-ray scattering (GIWAXS), atomic force microscopy (AFM), and photoluminescence (PL) spectroscopy revealed that intermolecular interactions between the PC71BM and ITIC units disrupted the formation of large ITIC crystals. The ITIC and PC71BM components formed compatible domains that dispersed well within the PIDTBT matrix, providing optimized ternary blends for efficient carrier transport and led to greater photon-to-electron conversion efficiency. Compared with the pre-optimized PC71BM binary device, the ternary device displayed an improvement in short circuit current density (Jsc) from 12.0 ± 0.3 to 14.2 ± 0.5 mA cm–2, due primarily to complementary light harvesting in the visible and near-infrared regions; as a result, the device performance improved by 20%—from 5.5 ± 0.2 to 6.6 ± 0.1% under AM 1.5G (100 mW cm–2) irradiation. Furthermore, the ternary cell exhibited outstanding long-term stability, with its performance remaining high (at 6.8%) after storage for 410 days in a glove box (ISOS-D-1 (shelf lifetime)).
The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within the past five years, there have been over 100 nonfullerene acceptor molecules synthesized, and the power conversion efficiency of nonfullerene organic solar cells has increased dramatically, from ∼2% in 2012 to >13% in 2017. This review summarizes this progress, aiming to describe the molecular design strategy, to provide insight into the structure–property relationship, and to highlight the challenges the field is facing, with emphasis placed on most recent nonfullerene acceptors that demonstrated top-of-the-line photovoltaic performances. We also provide perspectives from a device point of view, wherein topics including ternary blend device, multijunction device, device stability, active layer morphology, and device physics are discussed.
A high performance polymer solar cells (PSCs) based on polymer donor PM6 containing fluorinated thienyl benzodithiophene unit and n-type organic semiconductor acceptor IT-4F containing fluorinated end-groups were developed. In addition to complementary absorption spectra (300–830 nm) with IT-4F, the PM6 also has a deep HOMO (the highest occupied molecular) level (−5.50 eV), which will lower the open-circuit voltage (V oc) sacrifice and reduce the V loss of the IT-4F-based PSCs. Moreover, the strong crystallinity of PM6 is beneficial to form favorable blend morphology and hence to suppress recombination. As a result, in comparison with the PSCs based on a non-fluorinated D/A pair of PBDB-T:ITIC with a medium PCE of 11.2%, the PM6:IT-4Fbased PSCs yielded an impressive PCE of 13.5% due to the synergistic effect of fluorination on both donor and acceptor, which is among the highest values recorded in the literatures for PSCs to date. Furthermore, a PCE of 12.2% was remained with the active layer thickness of up to 285 nm and a high PCE of 11.4% was also obtained with a large device area of 1 cm². In addition, the devices also showed good storage, thermal and illumination stabilities with respect to the efficiency. These results indicate that fluorination is an effective strategy to improve the photovoltaic performance of materials, as well as the both fluorinated donor and acceptor pair-PM6:IT-4F is an ideal candidate for the large scale roll-to-roll production of efficient PSCs in the future.
Organic solar cells (OSCs) have been dominated by donor:acceptor blends based on fullerene acceptors for over two decades. This situation has changed recently, with non-fullerene (NF) OSCs developing very quickly. The power conversion efficiencies of NF OSCs have now reached a value of over 13%, which is higher than the best fullerene-based OSCs. NF acceptors show great tunability in absorption spectra and electron energy levels, providing a wide range of new opportunities. The coexistence of low voltage losses and high current generation indicates that new regimes of device physics and photophysics are reached in these systems. This Review highlights these opportunities made possible by NF acceptors, and also discuss the challenges facing the development of NF OSCs for practical applications.
Two novel wide-bandgap copolymers, PBDT-TDZ and PBDTS-TDZ, are developed based on 1,3,4-thiadiazole (TDZ) and benzo[1,2-b:4,5-b']dithiophene (BDT) building blocks. These copolymers exhibit wide bandgaps over 2.07 eV and low-lying highest occupied molecular orbital (HOMO) levels below -5.35 eV, which match well with the typical low-bandgap acceptor of ITIC, resulting in a good complementary absorption from 300 to 900 nm and a low HOMO level offset (≤0.13 eV). Compared to PBDT-TDZ, PBDTS-TDZ with alkylthio side chains exhibits the stronger optical absorption, lower-lying HOMO level, and higher crystallinity. By using a single green solvent of o-xylene, PBDTS-TDZ:ITIC devices exhibit a large open-circuit voltage (Voc ) up to 1.10 eV and an extremely low energy loss (Eloss ) of 0.48 eV. At the same time, the desirable high short-circuit current density (Jsc ) of 17.78 mA cm-2 and fill factor of 65.4% are also obtained, giving rise to a high power conversion efficiency (PCE) of 12.80% without any additive and post-treatment. When adopting a homotandem device architecture, the PCE is further improved to 13.35% (certified as 13.19%) with a much larger Voc of 2.13 V, which is the best value for any type of homotandem organic solar cells reported so far.
Bulk heterojunction (BHJ) organic solar cells (OSCs) have attracted intensive research attention over the past two decades owing to their unique advantages including mechanical flexibility, light weight, large area, and low-cost fabrications. To date, OSC devices have achieved power conversion efficiencies (PCEs) exceeding 12%. Much of the progress was enabled by the development of high-performance donor polymers with favorable morphological, electronic, and optical properties. A key problem in morphology control of OSCs is the trade-off between achieving small domain size and high polymer crystallinity, which is especially important for the realization of efficient thick-film devices with high fill factors. For example, the thickness of OSC blends containing state-of-the-art PTB7 family donor polymers are restricted to ∼100 nm due to their relatively low hole mobility and impure polymer domains. To further improve the device performance and promote commercialization of OSCs, there is a strong demand for the design of new donor polymers that can achieve an optimal blend morphology containing highly crystalline yet reasonably small domains.
Three small molecules with different substituents on bithienyl-benzo[1,2-b:4,5-b']dithiophene (BDTT) units, BDTT-TR (meta-alkyl side chain), BDTT-O-TR (meta-alkoxy), and BDTT-S-TR (meta-alkylthio), are designed and synthesized for systematically elucidating their structure-property relationship in solution-processed bulk heterojunction organic solar cells. Although all three molecules show similar molecular structures, thermal properties and optical band gaps, the introduction of meta-alkylthio-BDTT as the central unit in the molecular backbone substantially results in a higher absorption coefficient, slightly lower highest occupied molecular orbital level and significantly more efficient and balanced charge transport property. The bridging atom in the meta-position to the side chain is found to impact the microstructure formation which is a subtle but decisive way: carrier recombination is suppressed due to a more balanced carrier mobility and BDTT based devices with the meta-alkylthio side chain (BDTT-S-TR) show a higher power conversion efficiency (PCE of 9.20%) as compared to the meta-alkoxy (PCE of 7.44% for BDTT-TR) and meta-alkyl spacer (PCE of 6.50% for BDTT-O-TR). Density functional density calculations suggest only small variations in the torsion angle of the side chains, but the nature of the side chain linkage is further found to impact the thermal as well as the photostability of corresponding devices. The aim is to provide comprehensive insight into fine-tuning the structure-property interrelationship of the BDTT material class as a function of side chain engineering.
Non-fullerene polymer solar cells (PSCs) with solution-processable n-type organic semiconductor (n-OS) as acceptor have seen rapid progress recently owing to the synthesis of new low bandgap n-OS, such as ITIC. To further increase power conversion efficiency (PCE) of the devices, it is of a great challenge to develop suitable polymer donor material that matches well with the low bandgap n-OS acceptors thus providing complementary absorption, nanoscaled blend morphology as well as suppressed recombination and minimized energy loss. To address this challenge, we synthesized three medium bandgap 2D-conjugated bithienyl-benzodithiophene-alt-fluorobenzotriazole copolymers J52, J60 and J61 for the application as donor in the PSCs with low bandgap n-OS ITIC as acceptor. The three polymers were designed with branched alkyl (J52), branched alkylthio (J60) and linear alkylthio (J61) substituent on the thiophene conjugated side chain of the benzodithiophene (BDT) units for studying effect of the substituents on the photovoltaic performance of the polymers. The alkylthio side-chain red-shifted absorption, down-shifted the highest occupied molecular orbital (HOMO) level and improved crystallinity of the 2D conjugated polymers. With linear alkylthio side-chain, the tailored polymer J61 exhibits an enhanced JSC of 17.43 mA/cm2, a high VOC of 0.89 V and a PCE of 9.53% in the best non-fullerene PSCs with the polymer as donor and ITIC as acceptor. To the best of our knowledge, the PCE of 9.53% is one of the highest values reported in literatures to date for the non-fullerene PSCs. The results indicate that J61 is a promising medium bandgap polymer donor in non-fullerene PSCs.
Two new two-dimension (2D)-conjugated copolymers (PBDTT-S-TT-CF and PBDTT-O-TT-CF) were designed and synthesized for the application of donor materials in polymer solar cells (PSCs) and for further investigation of the effect of side chain engineering on the photovoltaic performance of the polymers. The two copolymers were prepared by copolymerization of alkylthio- or alkoxy-thienyl-benzodithiophene (BDTT-S or BDTT-O) and thienothiophene with carbonyl and fluorine substituents (TT-CF), and they demonstrated strong and broad absorption spectra in the wavelength region from 450 nm to ca. 800 nm. The HOMO energy level of PBDTT-S-TT-CF was further down-shifted to -5.44 eV by alkylthio substituent on thiophene conjugated side chain of BDT unit and the carbonyl and fluorine substitution on TT unit. The inverted-structured PSCs based on PBDTT-S-TT-CF:PC70BM exhibited a high PCE of 9.58% with a remarkable high Voc of 0.89 V and a high FF of 71.0%. The PCE of the PSCs based on PBDTT-O-TT-CF also reached a high value of 8.68% with a Voc = 0.78 V and a higher Jsc = 16.5 mA cm-2 which is benefited from the broad absorption of PBDTT-O-TT-CF. The results further confirm the unique advantages of alkylthio side chain in the design of state-of-the-art polymer donor materials for high performance PSCs with high Voc
Side chains play a considerable role not only in improving the solubility of polymers for solution-processed device fabrication, but also in affecting the molecular packing, electron affinity and thus the device performance. In particular, electron-donating side chains show unique properties when employed to tune the electronic character of conjugated polymers in many cases. Therefore, rational electron-donating side chain engineering can improve the photovoltaic properties of the resulting polymer donors to some extent. Here, a survey of some representative examples which use electron-donating alkylthio and alkoxy side chains in conjugated organic polymers for polymer solar cell applications will be presented. It is envisioned that an analysis of the effect of such electron-donating side chains in polymer donors would contribute to a better understanding of this kind of side chain behavior in solution-processed conjugated organic polymers for polymer solar cells.
A new 2D-conjugated small-molecule with alkylthio-thienyl-conjugated side chains (BDTT-S-TR) is synthesized for application as a donor material in organic solar cells (OSCs). The OSCs based on BDTT-S-TR/PC70 BM demonstrate a power conversion efficiency (PCE) of 9.20% without extra treatments. Moreover, an encouraging PCE of 6.68% is achieved from the device with active area of 144 mm(2) .
Improving the power conversion efficiency of polymer-based bulk-heterojunction solar cells is a critical issue. Here, we show that high efficiencies of 1/410% can be obtained using the crystalline polymer PNTz4T in single-junction inverted cells with a thick active layer having a thickness of 1/4300...nm. The improved performance is probably due to the large population of polymer crystallites with a face-on orientation and the favourable' distribution of edge-on and face-on crystallites along the film thickness (revealed by in-depth studies of the blend films using grazing-incidence wide-angle X-ray diffraction), which results in a reduction in charge recombination and efficient charge transport. These results underscore the great promise of polymer solar cells and raise the hope of achieving even higher efficiencies by means of materials development and control of molecular ordering.
A novel non-fullerene electron acceptor (ITIC) that overcomes some of the shortcomings, for example, weak absorption in the visible spectral region and limited energy level variability, of fullerene acceptors is designed and synthesized. Fullerene-free polymer solar cells (PSCs) based on the ITIC acceptor are demonstrated to exhibit power conversion efficiencies of up to 6.8%, a record for fullerene-free PSCs.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Inline printing and coating methods have been demonstrated to enable a high technical yield of fully roll-to-roll processed polymer tandem solar cell modules. We demonstrate generality by employing different material sets and also describe how the ink systems must be carefully co-developed in order to reach the ambitious objective of a fully printed and coated 14-layer flexible tandem solar cell stack. The roll-to-roll methodologies involved are flexographic printing, rotary screen printing, slot-die coating, X-ray scattering, electrical testing and UV-lamination. Their combination enables the manufacture of completely functional devices in exceptionally high yields. Critical to the ink and process development is a carefully chosen technology transfer to industry method where first a roll coater is employed enabling contactless stack build up, followed by a small roll-to-roll coater fitted to an X-ray machine enabling in situ studies of wet ink deposition and drying mechanisms, ultimately elucidating h
A solar park based on polymer solar cells is described and analyzed with respect to performance, practicality, installation speed, and energy payback time. It is found that a high voltage installation where solar cells are all printed in series enables an installation rate in Watts installed per minute that far exceed any other PV technology in existence. The energy payback time for the practical installation of polymer solar cell foil on a wooden 250 square meter platform in its present form is 277 days when operated in Denmark and 180 days when operated in southern Spain. The installation and de-installation rate is above 100 m min−1, which, with the present performance and web width, implies installation of >200 W min−1. In comparison, this also exceeds the overall manufacturing speed of the polymer solar cell foil with a width of 305 mm which is currently 1 m min−1 for complete encapsulated and tested foil. It is also significant that simultaneous installation and de-installation which enables efficient schemes for decommissioning and recycling is possible. It is highlighted where research efforts should most rationally be invested in order to make grid electricity from OPV a reality (and it is within reach).
Organic photovoltaic (OPV) technology has been developed and improved
from a fancy concept with less than 1% power conversion effi ciency (PCE)
to over 10% PCE, particularly through the efforts in the last decade. The
signifi cant progress is the result of multidisciplinary research ranging from
chemistry, material science, physics, and engineering. These efforts include
the design and synthesis of novel compounds, understanding and controlling
the fi lm morphology, elucidating the device mechanisms, developing
new device architectures, and improving large-scale manufacture. All of these
achievements catalyzed the rapid growth of the OPV technology. This review
article takes a retrospective look at the research and development of OPV,
and focuses on recent advances of solution-processed materials and devices
during the last decade, particular the polymer version of the materials and
devices. The work in this fi eld is exciting and OPV technology is a promising
candidate for future thin fi lm solar cells.
We present highly transparent and conductive silver thin films in a thermally evaporated dielectric/metal/dielectric (DMD) multilayer architecture as top electrode for efficient small molecule organic solar cells. DMD electrodes are frequently used for optoelectronic devices and exhibit excellent optical and electrical properties. Here, we show that ultrathin seed layers such as calcium, aluminum, and gold of only 1 nm thickness strongly influence the morphology of the subsequently deposited silver layer used as electrode. The wetting of silver on the substrate is significantly improved with increasing surface energy of the seed material resulting in enhanced optical and electrical properties. Typically thermally evaporated silver on a dielectric material forms rough and granular layers which are not closed and not conductive below thicknesses of 10 nm. With gold acting as seed layer, the silver electrode forms a continuous, smooth, conductive layer down to a silver thickness of 3 nm. At 7 nm silver thickness such an electrode exhibits a sheet resistance of 19 Ω/□ and a peak transmittance of 83% at 580 nm wavelength, both superior compared to silver electrodes without seed layer and even to indium tin oxide (ITO). Top-illuminated solar cells using gold/silver double layer electrodes achieve power conversion efficiencies of 4.7%, which is equal to 4.6% observed in bottom-illuminated reference devices employing conventional ITO. The top electrodes investigated here exhibit promising properties for semitransparent solar cells or devices fabricated on opaque substrates.
Procedures for testing organic solar cell devices and modules with respect to stability and operational lifetime are described. The descriptions represent a consensus of the discussion and conclusions reached during the first 3 years of the international summit on OPV stability (ISOS). The procedures include directions for shelf life testing, outdoor testing, laboratory weathering testing and thermal cycling testing, as well as guidelines for reporting data. These procedures are not meant to be qualification tests, but rather generally agreed test conditions and practices to allow ready comparison between laboratories and to help improving the reliability of reported values. Failure mechanisms and detailed degradation mechanisms are not covered in this report.
Over the last five years, organic photovoltaic devices have emerged as a new competitor to silicon-based solar cells. In particular, the bulk heterojunction architecture (BHJ), in which the photoactive layer consists of a bicontinuous blend of an electron donor and an electron acceptor, has allowed power conversion efficiencies around 8%. We will present in this review the latest conjugated polymers used in such BHJ solar cells. We will mainly focus on electron-donating (p-type) polymers based on thiophenes, 1,3,2-benzodiathiazoles, pyrrolo[3,4-c]pyrrole-1,4-diones, benzo[1,2-b;3,4-b]dithiophenes, and few other materials with more exotic structures. This review should be helpful to evaluate which are the most promising materials and where this research field is going in the years to come.
A professional network for the chemical sciences, hosted by the RSC MyRSC is leading the way in providing an international networking hub for advancing the chemical sciences. Connect, engage and interact with scientists across the globe regardless of location, career stage or interest. • Personalise your profile to reflect your specific interests • Grow your network with people who share common interests, research or workplace • Join groups and communities based in your field • Exchange knowledge and expertise with fellow scientists or to get help and ask questions • Follow blogs and discussion forums -carry on the debate The present review rationalizes the information spread in the literature concerning the use and role of buffer layers in polymer solar cells. Usual device structures include buffer layers, both at the anode and at the cathode interface, mainly to favour charge collection and extraction, but also to improve the device's overall performance. Buffer layers are actually essential for achieving highly efficient polymer solar cells and can no more be considered as ''optional'', thus the need and importance of understanding their properties and role. The aim of this review is to give the reader an overview of this topic and to provide a practical and useful tool for the daily activities of researchers in the field of polymer photovoltaics.