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PbS nanocrystals in hybrid systems for solar cell applications
Abstract
Many research efforts are focused toward significant improvements of polymeric solar cells efficiency, energy harvesting range, time, and environmental stability. Particular attention is given to hybrid organic/inorganic composites constituted of polymer/lead chalcogenides (PbS and PbSe) nanocrystals (NCs) to extend the spectral sensitivity of cells to near-infrared wavelengths. In this work we report the synthesis and characterization of PbS nanocrystals with absorption edge at 984 nm. The nanocrystals have a cubic crystal structure and size of about 2 nm as found by X-ray diffraction analysis. They were combined with poly(3-hexylthiophene) (P3HT) polymer to obtain P3HT:PbS blends with different PbS loadings. A post-deposition ligand exchange on PbS NCs by 1,2-ethanedithiol (EDT) allowed a better interaction between polymer and nanocrystals as showed by fluorescence measurements. The P3HT:PbS blends were deposited and treated by a layer by layer spin coating process and used as active layer in solar cells having structure glass/ITO/PEDOT:PSS/blend/Al. The major result obtained for this hybrid system is an increase of PCE by about two orders of magnitude with respect to analogous reported cells where a post-deposition ligand exchange was performed.
... 15 In a typical stack, a P3HT:PbS active layer is sandwiched between a hole transporting poly (3,4ethylenedioxylthiophene):poly(styrenesulfonate) (PEDOT:PSS)) layer on indium doped tin oxide (ITO) and a low work function metal cathode. [16][17][18] But, the cell stability issues caused by the inherent acidic and hygroscopic nature of PEDOT:PSS demanded new structures avoiding its use. An alternative technique emerged then was an inverted architecture employing a cathode buffer layer, for example a zinc oxide (ZnO) layer. ...
An inverted bulk-heterojunction (BHJ) hybrid solar cell having the structure ITO/ZnO/P3HT:PbS/Au was prepared under ambient conditions and the device performance was further enhanced by inserting an interface buffer layer of CdSe quantum dots (QDs) between the ZnO and the P3HT:PbS BHJ active layer. The device performance was optimized by controlling the size of the CdSe QDs and the buffer layer thickness. The buffer layer, with an optimum thickness and QD size, has been found to promote charge extraction and reduces interface recombinations, leading to an increased open-circuit voltage (VOC), short circuit current density (JSC), fill factor (FF) and power conversion efficiency (PCE). About 40% increase in PCE from 1.7% to 2.4% was achieved by the introduction of the CdSe QD buffer layer, whose major contribution comes from a 20% increase of VOC.
... In continuation of our ongoing research in the field of specific probes producing focused optical responses [18][19][20][21][22][23][24], and in particular in the design of emissive complexes of transition metals, we developed a new pyridyl/phenolic/benzothiazole functionalized chemosensor BPAP [25]. The tripodal ligand, acting as a multimetal ion sensor, gave a real-time selective response toward some transition cations. ...
A new complex was obtained starting from a pyridyl/phenolic/benzothiazole functionalized ligand (BPAP)with zinc acetate. The structural and photoluminescence properties of the complex were investigated by X-ray diffraction and confirmed by DFT calculations. By comparing the results with previous studies, we found that the mode of binding of the sensor was influenced by the pH of the medium, changing the zinc coordination sphere. In a basic aqueous solution, the metal coordinates two neutral nitrogen atoms and two negative sites, the phenate oxygen and the deprotonate amide nitrogen and a water molecule. In this condition, the ligand was found to behave as a more sensitive naked eye fluorescence chemosensor towards Zn ²⁺ , showing blue photoluminescence with a lower detection limit of 375 nM.
... Bulk heterojunction (BHJ) of π-conjugated polymer (PCP) and quantum dots were widely used for organic-inorganic hybrid solar cells (HSCs) [1][2][3][4][5][6][7][8][9][10]. Low bandgap lead sulfide quantum dots (PbS QDs) are considered as a promising electron acceptor as a replacer of fullerene due to their tunable and broad absorption range, especially extending to the infrared region [11][12][13][14][15][16] and high electron mobility [17][18][19][20]. A recent study has shown that long alkyl chain ligands, such as the oleic acid (OLA) coating on the core of as-casted PbS QDs, impede efficient charge transfer in hybrid polymer/QD solar cells. ...
We used continuous wave photoinduced absorption (PIA) spectroscopy to investigate long-lived polarons in a blend of PbS quantum dot and regio-regular poly (3-hexylthiophene) (RR-P3HT). The charge transfer from RR-P3HT to PbS as well as from PbS to RR-P3HT were observed after changing the capping ligand of PbS from a long chain molecular to a short one. Therefore, PbS could be used to extend the working spectral range in hybrid solar cells with a proper capping ligand. However, we found that the recombination mechanism in the millisecond time region is dominated by the trap/defects in blended films, while it improves to a bimolecular recombination partially after ligand exchange. Our results suggest that passivating traps of nanocrystals by improving surface ligands will be crucial for relevant solar cell applications.
... The IPCE in the range of 300 to 1100 nm is a combined contribution of both the PbS QDs and the P3HT polymer. Similar observations have been reported by M. Nam et al. and C. Borriello et al. [15,32]. Incorporation of PbS QDs in the P3HT matrix extends the IPCE beyond 650 nm as is clearly seen. ...
... This is an indication that no appreciable change occurs in the polymer chain after this procedure. Similar results have been achieved for PbS nanocrystals/P3HT blends [45]. ...
The use of inorganic nanoparticles as electron acceptors in hybrid solar cells has been increased. Some advantages compared to electron acceptors based on organic materials, are better electron transfer and lower production costs. We report the synthesis of β-HgS nanoparticles by a combination of solution method using polyvinylpyrrolidone (PVP) as stabilizing agent, and hydrothermal treatment to diminish nanoparticles size and improve size distribution. The combination of a bottom-up and a top-down technique proved to be an appropriate method to obtain HgS nanoparticles with sizes in the order of the Bohr radius, essential for charge carrier generation in the solar cell. Additionally, PVP was replaced by hexadecanethiol (HDT). This exchange allowed the spin coating of a thin film of β-HgS dispersed in poly(3-hexylthiophene) (P3HT) onto a glass substrate. Characterizations showed that the HgS nanoparticles and the polymer were homogeneously mixed, but several aggregated HgS nanoparticles were still present. The lowering in the photoluminescence signal of the layer is an indicative of charge carrier transfer between the nanoparticles and P3HT. Our results are encouraging for the development of hybrid solar cells using β-HgS nanoparticles and P3HT. To our knowledge, these results are the first obtained for β-HgS nanoparticles, using a bottom-up/top-down approach. It is also the first time a nano-β-HgS/P3HT layer with promising properties as hybrid solar cell is reported.
The photoconductivity of condensates of lead-sulfide quantum dots (QDs)—QD solids—with various organic ligands is studied. It is demonstrated that the QD solid photoconductivity increases exponentially with a reduction in length of ligand molecules and does not depend on their chemical structures, since it is governed by hopping transport of charge carriers. In contrast, the photocurrent in photovoltaic ITO/PEDOT: PSS/PbS/ZnO/Al elements depends on the ligand structure, since this structure sets the positions of QD energy levels and thus affects the efficiency of charge carrier transfer to electrodes. The difference between mechanisms of generation of photoconductivity and photovoltaic currents is discussed.
Organic solar cells are among the most promising devices for low-cost solar energy conversion. The classical device consists of a bulk heterojunction of a conjugated polymer/fullerene network. Many research groups have focused on the replacement of the fullerene derivative with other materials, especially inorganic nanoparticles, due to their easily tunable properties, such as size/shape, absorption/emission, and charge carrier transport. In this chapter, the progress achieved on the incorporation of inorganic semiconductor nanoparticles and metal nanoparticles into organic solar cells is highlighted. The role of such nanoparticles in the improvement of current, voltage, and efficiency is discussed and a critical view is presented, particularly considering their effects on the morphology of the systems.
Recent advances in the fields of application of the composites based on quantum dots (QDs) as optical converters for the light emitting devices, solar cells and biofunctional nanoprobes for detection of markers and medical diagnostics are considered. The possibilities of application of various QD—ligand—polymer combinations depending on desired photophysical properties in the final composite are analyzed. An attempt is made to predict the key future trends in the fabrication and application of hybrid nanocomposites for biomedicine and optoelectronics.
The field of photovoltaics is undergoing a surge of interest following the recent discovery of the lead hybrid perovskites as a remarkably efficient class of solar absorber. Of these, methylammonium lead iodide (MAPI) has garnered significant attention due to its record breaking efficiencies, however, there are growing concerns surrounding its long-term stability. Many of the excellent properties seen in hybrid perovskites are thought to derive from the 6s(2) electronic configuration of lead, a configuration seen in a range of post-transition metal compounds. In this review we look beyond MAPI to other ns(2) solar absorbers, with the aim of identifying those materials likely to achieve high efficiencies. The ideal properties essential to produce highly efficient solar cells are discussed and used as a framework to assess the broad range of compounds this field encompasses. Bringing together the lessons learned from this wide-ranging collection of materials will be essential as attention turns toward producing the next generation of solar absorbers.
For decades, solution-processable, light absorbing semiconductor materials of quantum dots (QDs) and conjugated polymers have been extensively studied in third-generation photovoltaic (PV) devices, targeting both high efficiency and low cost, due to their various advantages. It is promising to fabricate PV devices by further combining near infrared (NIR) QDs and conjugated polymers, which can potentially merge the respective advantages of both polymers and inorganic nanomaterials in mechanical flexibility, specific weight, light absorption and photostability. In this review article, we first present the application of NIR QDs in organic solar cells built upon traditional polymers. Exciton dissociation and charge carrier transport are described, and strategies for improving the performance of these devices are analyzed. Then, the developments of NIR QDs-polymer hybrid solar cells based on recently developed low-bandgap polymers are summarized. Finally, future perspectives are discussed.
In this work, FETs are used as a research tool to study charge carrier mobilities in PbS nanocrystals (NCs) thin-films employed as semiconducting layer in bottom-gate bottom-contact (BGBC) field-effect transistors (FETs). The as-synthesised NCs are surrounded by long alkyl chain ligands which act as electrical insulators. Therefore, a ligand exchange process with shorter molecules is necessary to enhance the free charges generation and transport. We used two different ligands: 1,2 ethandithiol (EDT) and 1,2,3,4-tetrabutylammo-nium iodide (TBAI) and studied the charge mobility of PbS NCs comparing the electrical characteristics of FETs made by exchanged NCs. We analysed also the contemporary presence of both exchanged nanocrystals on the device. All the transistors showed p-type transport behaviour, enhanced by an annealing process at 100 8C for 10 min. After this, only the TBAI-treated NCs devices showed a n-type transport, resulting in an ambipolar behaviour. Inkjet printing deposition techniques was also successfully used to deposit PbS-(TBAI) NCs and ambipolar devices were obtained. In addition, for printed devices it was found that it is possible to modulate the charge transport properties by applying surface treatment to the substrate with a pentafluorothiophenol (PFTP). Indeed in this case, the p-type transport was suppressed while n-type behaviour was induced.
Different approaches of surface modification of the quantum dots (QDs), namely, solution-phase (octylamine, octanethiol) and post-deposition (acetic acid, 1,4-benzenedithiol) ligand exchange were used in the fabrication of hybrid bulk heterojunction solar cell containing poly (3-hexylthiophene) (P3HT) and small (2.4 nm) PbS QDs. We show that replacing oleic acid by shorter chain ligands improves the figures of merit of the solar cells. This can possibly be attributed to a combination of a reduced thickness of the barrier for electron transfer and an optimized phase separation. The best results were obtained for post-deposition ligand exchange by 1,4-benzenedithiol, which improves the power conversion efficiency of solar cells based on a bulk heterojunction of lead sulfide (PbS) QDs and P3HT up to two orders of magnitude over previously reported hybrid cells based on a bulk heterojunction of P3HT:PbS QDs, where the QDs are capped by acetic acid ligands. The optimal performance was obtained for solar cells with 69 wt. % PbS QDs. Besides the ligand effects, the improvement was attributed to the formation of an energetically favorable bulk heterojunction with P3HT, when small size (2.4 nm) PbS QDs were used. Dark current density-voltage (J-V) measurements carried out on the device provided insight into the working mechanism: the comparison between the dark J-V characteristics of the bench mark system P3HT:PCBM and the P3HT:PbS blends allows us to conclude that a larger leakage current and a more efficient recombination are the major factors responsible for the larger losses in the hybrid system.
Hybrid heterojunctions of conjugated polymers and inorganic nanomaterials are a promising combination for obtaining high performance solar cells (SC). In this work we have explored new possible uses of the WS2 nanotubes (NTs) both as the only acceptor material blended with a polymer and in ternary systems mixed with a polymer and quantum dots (QDs). In particular we have spectroscopically investigated binary blends of poly(3-hexylthiophene) (P3HT) and WS2 NTs, P3HT and CdSe QDs, and ternary blends of P3HT, CdSe QDs and WS2 NTs. We report fluorescence quenching effects of the QD signal in the P3HT-CdSe-WS2 system with the increase of NT concentration. Static and time-resolved fluorescence studies reveal efficient resonant energy transfer from the QDs to the NTs upon photoexcitation. The evidence of energetic interaction between WS2 NTs and QDs opens new fields of application of WS2 NTs and holds very promising potential for improving charge transfer phenomena in the active layer of hybrid solar cells.
Inorganic/organic hybrid solar cells have attracted a lot of interest due to their potential in combining the advantages of both components. To understand the key issues in association with photoinduced charge separation/transportation processes and to improve overall power conversion efficiency, various combinations with nanostructures of hybrid systems have been investigated. Here, we briefly review the structures of hybrid nanocomposites studied so far, and attempt to associate the power conversion efficiency with these nanostructures. Subsequently, we are then able to summarize the factors for optimizing the performance of inorganic/organic hybrid solar cells.
The prospect of exploiting quantum dots (QDs) properties (tunable absorption spectrum, multiple exciton generation) while maintaining the flexible structure of polymer systems opens new possibilities in the photovoltaic field. Although charge transport dynamics in pristine polymer and QDs systems have been quite well established lately, a complete understanding of the charge transfer process between QDs and polymers when they are in blends is still lacking. In this work we used static and ultrafast fluorescence spectroscopy together with Atomic force Microscopy (AFM) to study the exciton dynamics in polymer/QDs films. Specifically we used poly(3-hexylthiophene) (P3HT) as the hole conducting donor material and the core shell CdSe(ZnS) QDs as the electron acceptor material. The QDs surface has been treated with two different capping ligands treatments: one based on the use of pyridine and the other one on hexanoic acid. The influence of the two different methods on the exciton dynamics and on the morphology will also be discussed. Blends containing differently treated P3HT/CdSe(ZnS) wt% ratios have been prepared producing films having uniform morphology and good intermixing, as proved by AFM measurements. Ultrafast fluorescence decays allowed us to compare the exciton dynamics in the polymer pristine respect to the treated P3HT/CdSe(ZnS) films. Efficient fluorescence quenching has been shown by both kind of blends respect to the pure polymer.
For the last decade, researchers have attempted to construct photovoltaic (PV) devices using a mixture of inorganic nanoparticles and conjugated polymers. The goal is to construct layers that use the best properties of each material e.g., flexibility from the polymer and high charge mobility from the nanoparticles or blue absorbance from the polymer complementing red absorbance from the nanoparticles. This critical review discusses the main obstacles to efficient hybrid organic/inorganic PV device design in terms of contributions to the external and internal quantum efficiencies. We discuss in particular the role that ligands on the nanoparticles play for mutual solubility and electronic processes at the nanoscale. After a decade of work to control the separation distance between unlike domains and the connectivity between like domains at the nanoscale, hybrid PV device layers are gaining in efficiency, but the goal of using the best properties of two mixed materials is still elusive.
Solution-processed hybrid solar cells, a blend of conjugated polymers and semiconducting nanocrystals, are a promising candidate for next-generation energy-conversion devices. The renaissance of this field in recent years has yielded a much deeper understanding of optoelectronic interactions in organic–inorganic hybrid systems. In this article, we review the state-of-the-art progress in hybrid bulk heterojunction solar cells, covering new materials design, interfacial interaction, and processing control. Furthermore, critical challenges that determine photovoltaic performance and prospects for future directions are discussed.
We have demonstrated efficient hybrid solar cells based on lead sulfide (PbS) nanocrystals and a narrow band gap polymer, poly[{2,5-bis(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl}-alt-{[2,2′-(1,4-phenylene)bis-thiophene]-5,5′-diyl}], (PDPPTPT). An opportune mixing of the two materials led to the formation of an energetically favorable bulk hetero-junction with a broad spectral response. Using a basic device structure, we reached a power conversion efficiency of 3%, which is one of the highest values reported for this class of solar cells. Photo-physical measurements carried out on the device provided insights into the working mechanism: the comparison between the time decay of the pristine polymer and the polymer–PbS blend allows us to conclude that efficient charge transfer is taking place in this hybrid system.
We report a comparison of photoconductive performance of PbS nanocrystal/polymer composite devices containing either oleic acid-capped or octylamine capped nanocrystals (NCs). The octylamine-capped NCs allow over two orders of magnitude more photocurrent under −1 V bias; they also show an infrared photovoltaic response, while devices using oleic acid-capped NCs do not. Further improvement in the photovoltaic performance of films made with octylamine-capped NCs occurs upon thermally annealing the composite layer at 220 °C for 1 h. The procedure leads to a 200-fold increase in short circuit current, a 600-fold increase in maximum power output, and an order of magnitude faster response time.
We report on bulk-heterojunction hybrid solar cells based on blends of non-ligand-exchanged CdSe quantum dots (QDs) and the conjugated polymer poly(3-hexylthiophene) with improved power conversion efficiencies of about 2% under AM1.5G illumination after spectral mismatch correction. This is the highest reported value for a spherical CdSe QD based photovoltaic device. After synthesis, the CdSe QDs are treated by a simple and fast acid-assisted washing procedure, which has been identified as a crucial factor in enhancing the device performance. A simple model of a reduced ligand sphere is proposed explaining the power conversion efficiency improvement.
The binding strength of the carboxylic acid group (-COOH) with different divalent metal ions displays considerable variation in arachidic acid (AA) thin films. It is considered that in AA thin films the metal ions straddle the hydrophilic regions of the stacked bilayers of AA molecules via formation of carboxylates. In this study first the uptake of different divalent cations in films of AA is estimated by atomic absorption spectroscopy (AAS). Through the amount of cation uptake, it is found that the strength of binding of different cations varies as Ca2+>Co2+>Pb2+>Cd2+. Variation in the binding strength of different ions is also manifested in experiments where AA thin films are exposed to metal ion mixtures. The higher binding strength of AA with certain metal ions when exposed individually, as well as the preference over the other metal ions when exposed to mixtures, reveal some interesting deviation from the expected behavior based on considerations of ionic radii. For example, Pb2+ is always found to bind to AA much more strongly than Cd2+ even though the latter has smaller ionic radius, indicating that other factors also play an important role in governing the binding strength trends apart from the effects of ionic radii. Then, to get a more meaningful knowledge regarding the binding capability, first-principles calculations based on density functional theory have been applied to study the interaction of different cations with the simplest carboxylic acid, acetic acid, that can result in formation of metal diacetates. Their electronic and molecular structures, cohesive energies, and stiffness of the local potential energy well at the cation (M) site are determined and attempts are made to understand the diversity in geometry and the properties of binding of different metal ions with -COOH group. We find that the calculated M-O bond energies depend sensitively on the chemistry of M atom and follow the experimentally observed trends quite accurately. The trends in M-O bond energies and hence the total M-acetate binding energy trends can actually be related to their molecular structures that fall into different categories: Ca and Cd have tetrahedral coordination; Fe, Ni, and Co exhibit planar 4-fold coordination; and Pb is off-centered from the planar structure (forming pyramidal structure) due to its stereochemically active lone pair of electrons.
The electronic properties of colloidal quantum dots (QDs) are critically dependent on both QD size and surface chemistry. Modification of quantum confinement provides control of the QD bandgap, while ligand-induced surface dipoles present a hitherto-underutilized means of control over the absolute energy levels of QDs within electronic devices. Here we show that the energy levels of lead sulfide QDs, measured by ultraviolet photoelectron spectroscopy, shift by up to 0.9 eV between different chemical ligand treatments. The directions of these energy shifts match the results of atomistic density functional theory simulations and scale with the ligand dipole moment. Trends in the performance of photovoltaic devices employing ligand-modified QD films are consistent with the measured energy level shifts. These results identify surface-chemistry-mediated energy level shifts as a means of predictably controlling the electronic properties of colloidal QD films and as a versatile adjustable parameter in the performance optimization of QD optoelectronic devices.
Polymer/inorganic-nanocrystals bulk heterojunction solar cells, where inorganic semiconductor nanocrystals such as CdSe, CdS, CdTe, ZnO, TiO2, and silicon, replace the fullerene molecules as the electron acceptors, typically exhibit a power conversion efficiency (PCE) below 3% even after tremendous engineering efforts to optimize the nanocrystal size, shape, and nanoscale morphology. One promising feature of polymer hybrid solar cells is the ability to sensitize conjugated polymers, which on their own absorb only in the visible part of solar spectrum, into the infrared spectral range using infrared-active lead salt nanocrystal quantum dots (NQDs). Here we observed for the first time hole transfer from PbS NQDs to polymers as evidenced by the quenching of the PbS photoluminescence (PL), a sign of the presence of charge separating type II heterojunction. The type II band-offset at the NQD/polymer heterojunction enables efficient hole extraction from NQDs and leads to a record PCE of 3.80%, realized in a planar junction configuration under simulated air mass 1.5 global (AM 1.5G) irradiation of 100 mW/cm2. The photocurrent has an extended spectral range spanning from the ultraviolet (UV) to the infrared (IR). Contributions from the polymer and PbS to the photocurrent were identified. Infrared photons (>700 nm) contribute about 30% of the photocurrent and yield a high external quantum efficiency (EQE) of 20% at 1050 nm.
Organic materials have recently become of great interest for photovoltaic applications, due to their potential to utilise high throughput, solution phase processing, which will lead to low cost electricity production. Hybrid solar cells combine organic and inorganic materials with the aim of utilising the low cost cell production of organic photovoltaics (OPV) as well as obtaining other advantages, such as tuneable absorption spectra, from the inorganic component. Whilst hybrid solar cells have the potential to achieve high power conversion efficiencies (PCE), currently obtained efficiencies are quite low. The design of the inorganic material used as the electron acceptor in hybrid solar cells, particularly the electronic structure, is crucial to the performance of the device. There exists an optimal electronic structure design for an inorganic acceptor. To date, four major material types have been investigated, being cadmium compounds, silicon, metal oxide nanoparticles and low band gap nanoparticles. Currently, Cadmium Sulfide (CdS) quantum dots represent the state of the art, yielding a PCE of greater than 4%. This review compares the electronic structure of these materials with the optimal design components of an inorganic material and also explores possible limitations to the PCE of these devices, such as nanomorphology control and nanoparticle surface chemistry. This report provides the reader with a concise synthesis of the current state of the art for bulk heterojunction organic—inorganic hybrid solar cells. Additionally, it highlights key research areas which require attention to allow for the commercialisation of this technology.
Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) capped PbS quantum dots about 3–6nm in diameter were synthesized with a novel method. Unlike the synthesis of oleic acid capped PbS quantum dots, the reactions were carried out in solution at room temperature, with the presence of a capping ligand species, MDMO-PPV. The quantum dots were used to fabricate bulk heterojunction solar cells with an indium tin oxide (ITO)/polyethylenedioxythiophene/polystyrenesulphonate (PEDOT: PSS)/MDMO-PPV: PbS/Al structure. Current density–voltage characterization of the devices showed that after the addition of the MDMO-PPV capped PbS quantum dots to MDMO-PPV film, the performance was dramatically improved compared with pristine MDMO-PPV solar cells.
Photovoltaic devices based on PbS nanoparticles remained inactive in the near-IR region due to a not-so-favorable energy band-diagram that does not allow dissociation of excitons generated in PbS. In this work, with the introduction of TiO2 nanostructures in the PbS-based hybrid system, we show an enhancement of photovoltaic performance in both visible and near-IR regions. The addition of TiO2 increases the power conversion efficiency from 0.006% to 0.12%. With the aid of energy band-diagram, we show that excitons generated in PbS even in the near-IR range can now become dissociated to yield photocurrent in the external circuit.
Emerging alternative photovoltaic technologies such as dye sensitized solar cells (DSSCs) and organic solar cells (OSCs) have recently gained much attention as well as maturity and are on the step of being commercialized. Bulk heterojunction hybrid solar cells containing inorganic nanoparticles and semiconducting polymers are still lagging behind in respect of device performance although they have theoretically the potential to exhibit better performances than devices containing solely organic compounds. In this article we review the recent state of the art development of bulk heterojunction hybrid solar cells. Critical factors limiting the solar cell device performance are highlighted and strategies for further device improvement are demonstrated by giving recent examples from literature.
We use photoinduced absorption (PIA) spectroscopy to study charge generation and recombination in a series of bulk heterojunction blends relevant to organic and hybrid solar cells. We compare both organic and inorganic electron acceptors, including the fullerene, phenyl-C61-butyric acid methyl ester (PCBM), oxides such as ZnO and TiO2, and colloidal quantum dots, including CdSe and PbS nanocrystals. We use a variety of donor host polymers, including poly(3-hexylthiophene) (P3HT), poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylenevinylene] (MDMO-PPV), and poly(2,3-bis(2-hexyldecyl)quinoxaline-5,8-diyl-alt-N-(2-hexyldecyl)-dithieno[3,2-b:2′,3′-d]pyrrole) (PDTPQx-HD). In every case, we measure longer average carrier lifetimes in blends with the inorganic acceptors as compared to blends with PCBM. The PIA data also suggest that the internal electric fields are attenuated in the inorganic blends, consistent with increased screening between the photogenerated carriers due to the higher dielectric constants of the inorganic nanoparticles. Using ligand exchange experiments, we further demonstrate that surface electron trapping on the inorganic colloids contributes to at least part of the increased lifetime in the PDTPQx-HD/PbS blends and that ligand exchange to remove traps can be used to improve the performance of these polymer/low-band-gap quantum dot hybrid photovoltaics.
Tapping mode atomic force microscopy (AFM) was used to probe the surface and internal structures of bulk heterojunction poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) films. AFM images obtained from the surface of the P3HT:PCBM film and from the cross section reveal for the first time the nanoscale three-dimensional P3HT and PCBM networks. Two different measurement modes of conducting atomic force microscopy (C-AFM) were used to study the nanoscale charge transport in P3HT:PCBM films. In C-AFM, the local current map at a constant bias and the surface topography are obtained simultaneously. Hole and electron current images can be used to determine the composition of the phase segregation domains. Alternatively, local currentvoltage curves can be measured that can be used to calculate charge carrier mobilities. Hole current images and mobilities were obtained using Pt-coated silicon probes, whereas Mg-coated silicon probes were used to measure nanoscale electron mobilit
We have shown recently that the use of high-aspect-ratio inorganic nanorods in conjunction with conjugated polymers is a route to obtaining efficient solar cells processed from solution. Here, we demonstrate that the use of binary solvent mixtures in which one of the components is a ligand for the nanocrystals is effective in controlling the dispersion of nanocrystals in a polymer. By varying the concentration of the solvent mixture, phase separation between the nanocrystal and polymer could be tuned from micrometer scale to nanometer scale. In addition, we can achieve nanocrystal surfaces that are free of surfactant through the use of weak binding ligands that can be removed through heating. When combined, the control of film morphology together with surfactant removal result in nanorod–polymer blend photovoltaic cells with a high external quantum efficiency of 59 % under 0.1 mW cm–2 illumination at 450 nm.
We demonstrate a relative improvement in power conversion efficiency of polymer nanocomposite photovoltaic cells consisting of poly(3-hexylthiophene) (P3HT) functionalized CdSe nanocrystals. Thermal deprotection processing of the tert-buthoxycarbonyl moiety in the carbamate ligand surrounding the surface of CdSe nanocrystal significantly shortened the length of the ligand between nanocrystals and between the nanocrystal and the polymer matrix. The resulting device performance was investigated as a function of the composition ratio of P3HT/CdSe and the heating temperature. This simple and straightforward ligand deprotection strategy resulted in a significant increase in current density due to improvement of charge transport between the constituent materials.
The nature of adsorbed calcium carboxylates (oleate and stearate) has been investigated by in situ/ex situ Fourier transform infrared internal reflection spectroscopy (FTIR/IRS). Experimental results confirm that chemisorbed carboxylate is characterized by a singlet band near 1550 cm−1 while physisorbed calcium dicarboxylate salt is characterized by a doublet band near 1540 and 1575 cm−1. The singlet IR band at 1550 cm−1 was attributed to the carboxylate groups associated with calcium ions in a bridging mode. On the other hand, the doublet IR absorbance band at 1540 and 1575 cm−1, which is observed for adsorbed carboxylate groups at multilayer coverage and for bulk-precipitated calcium dicarboxylate salts, was assigned for the carboxylate groups associated with calcium ions in unidentate and bidentate modes, respectively. In this regard, the IR singlet at 1550 cm−1 and the doublet at 1540 and 1575 cm−1 are correlated to the coordination structure of carboxylate groups with calcium ions. Constrained carboxylate groups at a surface such as a chemisorbed layer at a calcium semisoluble mineral surface (carboxylate bridging coordination at the fluorite surface, for example) and/or a transferred Langmuir–Blodgett (LB) layer at a high-transfer pressure give rise to the IR singlet at 1550 cm−1. Carboxylate groups associated with calcium ions in three-dimensional seven- or eight-fold coordination, which may contain carboxylate groups coordinated with calcium ions in different modes (i.e., unidentate and bidentate modes) are responsible for the IR doublet at 1540 and 1575 cm−1. This assignment is in agreement with the observed IR doublet for bulk-precipitated calcium dicarboxylate salts, adsorbed/surface precipitated calcium carboxylate species at multilayer coverage, and transferred calcium distearate at a low-transfer pressure because under these conditions formation of the three-dimensional structure is possible.
A facile approach to make an efficient hybrid bulk heterojunction photovoltaic device with lead sulfide nanocrystals and a low-bandgap polymer is demonstarted, resulting in a power conversion efficiency of about 2-3%.
A derivative of a known trinuclear iron(III) stearate cluster was prepared from ferrous sulfate and sodium stearate in ethanol/water solvent with air oxidation. It was also prepared by increasing the pH of a solution of ferrous sulfate stirred with stearic acid. The formula for the trimer as prepared here is [Fe3O(St)6(H2O)3]+St- (St- = stearate, n-C17H35CO2-. The reaction of ferric citrate with stearate to form the trimer does not go to completion. The true composition of most iron stearates has not been well established previously. Two iron stearate compounds are available commercially; of these, only "ferrous stearate" appears to be a pure compound, albeit not a simple mononuclear one. The "ferric stearate" of commerce appears to be a mixture of stearic acid and a compound similar to the iron trimer prepared here. References exist to "iron distearate hydroxide" in the literature. The physical and spectroscopic properties of this compound are very similar to those of the iron trimer prepared here. Spectral data (mid- and far-IR, 300 MHz 1H and 75 MHz 13C NMR) and TGA/DSC data are reported for stearic acid, "ferrous stearate," "ferric stearate," and [Fe3O(St)6(H2O)3][St].
We describe bulk heterojunction (BHJ) solar cells containing blends of colloidal PbS nanocrystal quantum dots with several new donor-acceptor conjugated polymers. Using photoinduced absorption spectroscopy we found that blends of PbS quantum dots with one polymer, poly(2,3-didecyl-quinoxaline-5,8-diyl-alt-N-octyldithieno[3,2-b:2',3'-d]pyrrole) (PDTPQx), produce significantly more photoinduced charge than blends of PbS with the other donor-acceptor polymers or with traditionally studied polymers like [2-methoxy-5-(3',7'-dimethyloctyloxy)-para-phenylene vinylene] (MDMO-PPV) and poly(3-hexylthiophene) (P3HT). Photovoltaic devices made with PDTPQx/PbS blends exhibit power conversion efficiencies 10-100 times larger than previously reported BHJ blends made with IR-absorbing quantum dots.
Improvement in power conversion efficiency has been observed in cadmium selenide nanorods/poly(3-hexylthiophene) hybrid solar cells through benzene-1,3-dithiol chemical vapor annealing. Phosphor NMR studies of the nanorods and TEM/AFM characterizations of the morphology of the blended film showed that the ligand exchange reaction and related phase separation happening during the chemical vapor annealing are responsible for the performance enhancement.
A methodology for achieving versatile and facile ligand exchange by post-fabrication chemical treatment in PbS nanocrystal:poly(3-hexylthiophene) (P3HT) hybrid composite photovoltaic devices is demonstrated. We report a considerable improvement of the photovoltaic performance after post-fabrication chemical treatment using acetic acid to produce PbS nanocrystals surrounded by short-length ligands. Annealing induced morphological and photovoltaic performance changes in the resulting composite devices were investigated as a function of the annealing time.
We study the processes of charge separation and transport in composite materials formed by mixing cadmium selenide or cadmium sulfide nanocrystals with the conjugated polymer poly(2-methoxy,5-(2{prime}-ethyl)-hexyloxy-{ital p}-phenylenevinylene) (MEH-PPV). When the surface of the nanocrystals is treated so as to remove the surface ligand, we find that the polymer photoluminescence is quenched, consistent with rapid charge separation at the polymer/nanocrystal interface. Transmission electron microscopy of these quantum-dot/conjugated-polymer composites shows clear evidence for phase segregation with length scales in the range 10{endash}200 nm, providing a large area of interface for charge separation to occur. Thin-film photovoltaic devices using the composite materials show quantum efficiencies that are significantly improved over those for pure polymer devices, consistent with improved charge separation. At high concentrations of nanocrystals, where both the nanocrystal and polymer components provide continuous pathways to the electrodes, we find quantum efficiencies of up to 12{percent}. We describe a simple model to explain the recombination in these devices, and show how the absorption, charge separation, and transport properties of the composites can be controlled by changing the size, material, and surface ligands of the nanocrystals. {copyright} {ital 1996 The American Physical Society.}
Nanocrystals or quantum dots of the IV-VI semiconductors PbS, PbSe, and PbTe provide unique properties for investigating the effects of strong confinement on electrons and phonons. The degree of confinement of charge carriers can be many times stronger than in most II-VI and III-V semiconductors, and lead salt nanostructures may be the only materials in which the electronic energies are determined primarily by quantum confinement. This Account briefly reviews recent research on lead salt quantum dots.
In recent years, the search to develop large-area solar cells at low cost has led to research on photovoltaic (PV) systems based on nanocomposites containing conjugated polymers. These composite films can be synthesized and processed at lower costs and with greater versatility than the solid state inorganic semiconductors that comprise today's solar cells. However, the best nanocomposite solar cells are based on a complex architecture, consisting of a fine blend of interpenetrating and percolating donor and acceptor materials. Cell performance is strongly dependent on blend morphology, and solution-based fabrication techniques often result in uncontrolled and irreproducible blends, whose composite morphologies are difficult to characterize accurately. Here we incorporate three-dimensional hyperbranched colloidal semiconductor nanocrystals in solution-processed hybrid organic-inorganic solar cells, yielding reproducible and controlled nanoscale morphology.
The need to develop and deploy large-scale, cost-effective, renewable energy is becoming increasingly important. In recent years photovoltaic (PV) cells based on nanoparticles blended with semiconducting polymers have achieved good power conversion efficiencies (PCE). All the nanoparticle types used in these PV cells can be considered as colloids. These include spherical, rod-like or branched organic or inorganic nanoparticles. Nanoparticle-polymer PV cells have the long-term potential to provide low cost, high-efficiency renewable energy. The maximum PCE achieved to date is about 5.5%. This value should rise as recently reported theoretical predictions suggest 10% is achievable. However, there are a number of challenges that remain to be overcome. In this review two general types of nanoparticle-polymer PV cells are considered and compared in detail. The organic nanoparticle-polymer PV cells contain fullerene derivatives (e.g., phenyl C61-butyric acid methyl ester, PCBM) or single-walled nanotubes as the nanoparticle phase. The second type is hybrid inorganic nanoparticle-polymer PV cells. These contain semiconducting nanoparticles that include CdSe, ZnO or PbS. The structure-property relationships that apply to both the polymer and nanoparticle phases are considered. The principles underlying nanoparticle-polymer PV cell operation are also discussed. An outcome of consideration of the literature in both areas are two sets of assembly conditions that are suggested for constructing PCBM-P3HT (P3HT is poly(3-hexylthiophene)) or CdSe-P3HT PV cells with reasonable power conversion efficiency. The maximum PCE reported for organic nanoparticle PV cells is about twice that for inorganic nanoparticle-polymer PV cells. This appears to be related to morphological differences between the respective photoactive layers. The morphological differences are attributed to differences in the colloidal stability of the nanoparticle/polymer/solvent mixtures used to prepare the photoactive layers. The principles controlling the colloid stability of the nanoparticle/polymer/solvent mixtures are discussed.