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The absorption spectra of PTB7-Th in CB, CB/DIO (97/3, v/v) and CB/PX/DIO (87/10/3, v/v/v) (a). The spectra fitted with Gaussian shaped single-band absorption spectra representing different vibrational states in ordered polymer structures (b). Exciton bandwidths W corresponding to PTB7-Th in different solvents (c)
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The current understanding of the active layer morphology in ternary organic solar cells (OSCs) is superficial owing to more variables and complexity compared to that of binary OSCs. The PTB7-Th:PF12TBT:PC71BM ternary system with complementary polymer absorption spectra and efficient energy transfer from PF12TBT to PTB7-Th was anticipated to have an...
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... The results are reported in Table 2. The surface energy of PTB7 is 33.1 mN m −1 , in agreement with the typical values (20-35 mN m −1 ) reported in the literature for other conjugated polymers using OPV applications, with a very low polar contribution [58][59][60]. ...
... It is important to remark that a controlled separation of organic solvents from the aqueous suspension can be integrated into a proper circular industrial plant equipped with condenser systems to recover and reuse the organic solvents. The surface energy of PTB7 is 33.1 mN m −1 , in agreement with the typical values (20-35 mN m −1 ) reported in the literature for other conjugated polymers using OPV applications, with a very low polar contribution [58][59][60]. ...
We synthetized a new rod-coil block copolymer (BCP) based on the semiconducting polymerpoly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) and poly-4-vinylpyridine (P4VP), tailored to produce water-processable nanoparticles (WPNPs) in blend with phenyl-C71-butyric acid methyl ester (PC71BM). The copolymer PTB7-b-P4VP was completely characterized by means of two-dimensional nuclear magnetic resonance (2D-NMR), matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry (MS), size-exclusion chromatography (SEC), and differential scanning calorimetry (DSC) to confirm the molecular structure. The WPNPs were prepared through an adapted miniemulsion approach without any surfactants. Transmission electron microscopy (TEM) images reveal the nano-segregation of two active materials inside the WPNPs. The nanostructures appear spherical with a Janus-like inner morphology. PTB7 segregated to one side of the nanoparticle, while PC71BM segregated to the other side. This morphology was consistent with the value of the surface energy obtained for the two active materials PTB7-b-P4VP and PC71BM. The WPNPs obtained were deposited as an active layer of organic solar cells (OSCs). The films obtained were characterized by UV-Visible Spectroscopy (UV-vis), atomic force microscopy (AFM), and grazing incidence X-ray diffraction (GIXRD). J-V characteristics of the WPNP-based devices were measured by obtaining a power conversion efficiency of 0.85%. Noticeably, the efficiency of the WPNP-based devices was higher than that achieved for the devices fabricated with the PTB7-based BCP dissolved in chlorinated organic solvent.
... For further insight into the crystallization mechanism, changes in crystallinity and crystal size were calculated via equations (1) and (2), respectively. The change in crystallinity can be represented by C, and its expression is as follows [34,35]: ...
The crystal structure of thin π-conjugated polymer films is critical to their performance. Here, we utilized regio-regular poly (3-hexylthiophene-2,5-diyl) (P3HT) films as a model system with grazing incident X-ray diffraction (GIXD) characterization. We systematically studied the effects of solvent polarity on the control of the crystal structure of thin π-conjugated polymer films in mixed solvent immersion. The polarity of poor solvents plays an important role in the crystallization behavior of the films, and the polarity of good solvents slightly influences the crystallization behavior of the thin P3HT films. Non-polar poor solvents increase the crystallinity of thin π-conjugated P3HT films by producing new crystalline nuclei in the thin polymer films; however, polar poor solvents can improve crystallinity via crystal growth. The results of this work offer a better understanding of the crystallization behavior of thin π-conjugated polymer films induced by liquid mixtures. The results offer important insight into solvent immersion control technologies.
... These two main peaks of the absorption spectrum of PTB7-Th can be related to the A 0−1 and A 0−0 transition peaks, respectively. 53,54 The relative decrease of the intensity of the A 0−0 transition in the blends compared to the neat donor and acceptor films indicates a reduction of the size of the PTB7-Th aggregates due to the presence of the small molecule acceptor O-IDTBR. 54 However, within the investigated series of blend films, the A 0−0 transition becomes again more pronounced with higher amounts of the acceptor indicating an enhanced aggregation of the polymer in the blend films with a higher O-IDTBR content. ...
In bulk-heterojunction solar cells, the device performance strongly depends on the donor and acceptor properties, the phase separation in the absorber layer, and the formation of a bicontinuous network. While this phase separation is well explored for polymer:fullerene solar cells, only little is known for polymer:nonfullerene acceptor solar cells. The main hurdle in this regard is often the chemical similarity of the conjugated polymer donor and the organic nonfullerene acceptor (NFA), which makes the analysis of the phase separation via atomic force microscopic (AFM) phase images or conventional transmission electron microscopy difficult. In this work, we use the donor polymer PTB7-Th and the small molecule acceptor O-IDTBR as the model system and visualized the phase separation in PTB7-Th:O-IDTBR bulk-heterojunctions with different donor:acceptor ratios via scanning transmission electron microscopy (STEM) high-angle annular dark-field (HAADF) images and electron energy loss spectroscopy (EELS) based elemental mapping, which resulted in a good contrast between the donor and the acceptor despite very low differences in the chemical composition. AFM as well as grazing-incidence wide-angle X-ray scattering (GIWAXS) investigations support the electron microscopic data. Furthermore, we elucidate the implications of the phase separation on the device performance as well as charge carrier mobilities in the bulk-heterojunction layers, and a high performance of the solar cells was found over a relatively broad range of polymer domain sizes. This can be related to the larger domain sizes of the acceptor phase with higher amounts of O-IDTBR in the blend, while the polymer donor phase still forms continuous pathways to the electrode, which keeps the hole mobility at a relatively constant level.
... Therefore, to harvest most of this energy, the absorbance of an ideal donor material should cover regions of 450 -800 nm. Since the solar simulator output irradiance is in the region of 300 -900 nm any photocurrent generation analysis on the absorption spectrum has to be done for ≥300 nm. Figure 5.3 shows the absorption profile of pristine PBDTTT-EFT thin film with its two main peaks identified at 700 and 635 nm associated with backbone conjugation length and π-π stacking [212], [213]. It was also observed that the onset point of absorption is about 782 nm, which corresponds to 1.57 eV using Plank's equation. ...
Organic solar cells (OSCs) based on PBDTTT-EFT copolymer as the electron donor
material have shown promising power conversion efficiencies (PCE) when blended
with fullerene derivative acceptor materials in bulk heterojunction structures. The
properties of blended donor and acceptor material and their compatibility plays a
critical role in the device performance of OSCs. Therefore, to optimise OSCs devices
based on PBDTTT-EFT, it is necessary to investigate the behaviour of the donor and
acceptor materials in their pristine and blended form. Based on the literature, the role
of donor: acceptor [D:A] blend ratio is amongst the critical aspects for optimising the
active layer morphology. However, for OSC devices based on PBDTTT-EFT:
PC71BM, the significant correlation between device performance and device physics
for different D:A ratios has not yet been fully understood.
Therefore, in this research project, one of the major case studies investigated, will
be on devices fabricated from PBDTTT-EFT: PC71BM blends with different PC71BM
percentage weight ratios. Results obtained from combined optical absorption
spectroscopy, structural, morphological and electrical characterisation, indicates that
the best device performance belongs to D:A ratio of 1:2, achieving an average PCE of
9.56%. This is believed to stem from the change in preferred molecular orientation of
PBDTTT-EFT molecules from Face-on to Edge-on, as a result of increased PC71BM
content within the blend. For the first time, the existence of PBDTTT-EFT molecules
in Edge-on orientation was identified using an OOP GIXRD. Also, further evidence
of the GIXRD analysis indicates that the highest vertical stacking of PC71BM
molecules occurs at D:A blend ratio of 1:2, which is favourable for charge transport
and extraction.
The other major investigation on PBDTTT-EFT based OSC devices is on the
impact of different fullerene derivative materials to be used as the acceptor, such as
PC61BM and IC61BA. IC61BA, in particular, was chosen, as it has higher levels of
LUMO compared to PC71BM. Therefore, the Voc of devices fabricated from PBDTTTEFT: IC61BA is expected to be higher than PBDTTT-EFT: IC61BA. Although device
enhancement of some OSCs has been reported by employing IC61BA instead of PCBM
ii
[1], using IC61BA as the acceptor material in a blend with BDT-based low bandgap
polymers has shown the opposite effect [2]. As an example, it has been reported that
devices fabricated using blend of PBDTTT-C-T: IC61BA will show poor device
performance compared to PBDTTT-C-T: PC61BM, despite achieving higher Voc
values when employing IC61BA as the acceptor [2]. It was suggested that poor
‘network’ formation between the donor and acceptor material within the blend causes
the problem in reduced device performance, however very little detail is provided as
to what is the exact issue. Therefore, investigating PBDTTT-EFT: IC61BA devices has
also been the focus of this PhD project. Devices fabricated from PBDTTT-EFT:
IC61BA blends resulted in averaged PCE of 5.86%, with Voc value just under 1V.
However, the Jsc and FF parameters are reduced when compared to devices
fabricated from PCBM (C61 or C71). From the analysed GIXRD results, it was evident
that IC61BA has a very low vertical segregation and stacking, which is detrimental for
electron charge transport and extraction. Also, it has been noticed that IC61BA
molecules had the least impact on the change in molecular orientation of the PBDTTTEFT molecule. The mobility measurements further provide evidence to unbalance
charge transport mechanism in PBDTTT-EFT: IC61BA devices compared to
PBDTTT-EFT: PC71BM devices.
Photoluminescence spectroscopy indicates that thin films composed of PBDTTTEFT: IC61BA have a higher recombination rate compared to PBDTTT-EFT: PC71BM
blend. Thus, when compiled with the analysis of GIXRD results from the blend films,
it is concluded that the morphology between PBDTTT-EFT molecules and IC61BA is
poor, and their phase domain separation is significant. The recombination analysis
elucidates that the inadequate vertical segregation of IC61BA molecules (due to the
nature of the molecule shape) results in weak charge dissociation and transport.
Therefore, high recombination rates will take place within the active layer, mainly
governed by the trap-assisted mechanism.
... It is observed, in both patterns, the (010) plane formation indicating the p-p stacking with a face-on orientation [52,53]. Both polymer films show the maximum peak between 22°a nd 23°, which is in good agreement with previous literature [53][54][55]. The higher intensity for the PTB7-Th pattern could indicate that has a better organized structure (packing of the chains) and higher co-planarity than the PTB7 film [37,53]. ...
In this work is reported a comparison of the film morphology, film molecular ordering and X-ray diffraction pattern between two of the most common and efficient donor polymers used in organic photovoltaic (OPV) cells: PTB7 and PTB7-Th. These comparisons indicate that PTB7-Th film chains are somewhat thicker and less spaced than those for PTB7; also, PTB7-Th films have a slightly better organized structure and higher co-planarity, which could provide a possible better electrical charge transport. On the order hand, an analysis of the external/internal quantum efficiency (EQE/IQE) of OPVs, based on PTB7-Th, as a function of the active layer thickness ranging from 40 to 165 nm was carried out. It was used the bulk heterojunction architecture to fabricate OPVs cells under the configuration glass/ITO/PEDOT:PSS/PTB7-Th:PC71BM/PFN/FM (Field’s Metal: eutectic alloy, composed by 32.5% Bi, 51% In and 16.5% Sn by weight that melts at 62 °C). IQE spectra were determined by using the active layer absorption calculated through the transfer matrix method (TMM). Our results show a significant reduction of IQE when increasing the active layer thickness above 120 nm. IQE decreases, and consequently EQE and PCE, mainly due to the reduction in charge carriers collection probability. On the reversed side, when the active layer is very thin (< 70 nm), there exists also a decrease in the IQE values. A comparison between the experimental measurements and theoretical simulations (by TMM) is discussed in order to have better understanding of the OPVs performance.
... DPPTPTA has a broad absorption wavelength, a favorable absorption intensity, high charge mobility, and a terphenylbased conjugated π-bridge that can efficiently inhibit molecular aggregation, induce optimal morphology for the multicomponent system, and confer a number of sites for reactivity with fullerene derivatives. 41 After incorporating DPPTPTA into the active layer, we observed an enhancement in device performance and thermal stability. The DPPTPTA-containing PTB7-Th:PC 61 BM device exhibited a PCE that was 8.2−22% greater than that of the control device. ...
We developed a star-shaped diketopyrrolopyrrole (DPP)–based additive as efficient morphology fixing agent for organic photovoltaics (OPVs). This conjugated small molecule, DPPTPTA, has four arms, with two terphenyl units and four alkyl azide groups. We tested the behavior of DPPTPTA after incorporating it into an active layer comprising poly[4,8-bis(5-(2-ethylhexyl)thien-2-yl)benzo[1,2-b;4,5-b´]dithiophene-2,6-diyl–alt–(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2-6-diyl)] (PTB7-Th) and the fullerene PC61BM. Atomic force microscopy, UV–Vis spectroscopy, optical microscopy, photoluminescence (PL) spectroscopy, and an X-ray photoelectron spectroscopy (XPS) depth profile revealed the effects of the resulting morphological change on the device performance and thermal stability. Compared with the PTB7-Th:PC61BM device prepared without DPPTPTA, the device incorporating this additive exhibited an increase in power conversion efficiency (from 6.7 to 8.2%) and improved thermal stability. DPPTPTA served as a multifunctional additive, providing ladder-like energy levels for efficient charge separation, altering the morphology of the blend film for improved performance, and suppressing the large-scale crystallization of PCBM (only a few fullerene crystals appeared in the active layer after heating the blend film at 150 °C for 18 h) by constructing local borders, ensuring long-term thermal stability.
... electron-accepting material to demonstrate a PCE as high as 10%. [15][16][17][18][19][20][21][22][23][24][25][26][27][28] Two intrinsically compatible fullerene derivatives form a well-mixed alloy with anticipation of superior acceptor performance. A ternary blend of polymer with two such fullerene derivatives in a D:A:A configuration has demonstrated superior device performance courtesy of balanced charge mobility and optimized morphology of photoactivity leading to improved device performance as compared with the fullerene-based binary device. ...
Non-fullerene acceptors (NFAs) with vibrant superiority over fullerene derivatives have proved advantageous as a guest component in a ternary blend signifying improved photon harvesting. However, heterogeneity of NFA-based ternary blend morphology is very complex, and the optimization requires simultaneous considerations of molecular miscibility and electronic properties. We discuss the role of guest NFA for an eased optimization of ternary blend photoactive layer in four different ways. Our four-model prospect of component selection is driven by the well-matched intrinsic miscibility, electronic, and optical characteristics of organic materials. We analyze each model, modulating photoactive layer morphology toward efficient transfer, transport, and collection of charges along with their balanced dissociation and drifting. We believe a model-based selection of NFA and its organization as a guest in a complex ternary blend will serve as a guide for researchers to optimize thin film nanostructures, a route toward efficient larger-area ternary devices.
It was shown that the chain length of the aliphatic substituent in the alkyl derivative of 3-(1-indenyl)propionic acid of C60 and C70 fullerenes affects the efficiency of photovoltaic cells with...
In this work, a series of C 60 /C 70 fullerene derivatives bearing non-aromatic and mono-, bi-, tri- and tetracyclic aryl-substituents was synthesized according to the modified Bingel method and characterized using spectral methods: ESI-MS, ¹ H NMR, ¹³ C NMR, UV–Vis, FT-IR and cyclic voltammetry (CV). HOMO and LUMO energy levels and the band gaps for optimized structures of reported fullerene derivatives were determined according to the DFT functionals, B3LYP 6-31G(d) and PBE/6-311G(d,p). Results obtained from CV and UV–Vis measurements, which showed very good agreement, were compared to calculated theoretical values also revealing satisfactory level of compliance of the obtained results. We have studied the impact of presence and number of aromatic rings in malonate substituent of C 60 /C 70 fullerene derivatives employed as acceptor materials on performance of BHJ solar cells prepared using PTB7-Th as donor material. We successfully prepared solar cells based on all the synthesized compounds, and the highest performance of the obtained photovoltaic devices was observed for fullerene derivatives bearing monocyclic and bicyclic aromatic moieties. The obtained voltage was about 0.8 V and current density was above 10 mA/cm ² . Optical studies showed absorption edges at 1.9 eV and 1.8 eV for C 60 and C 70 derivatives, respectively, with absorption coefficients comparable to C 60 /C 70 PCBM samples. Photocurrent spectroscopy showed 20–40% quantum efficiency. Long-term ageing measurements showed T80 time between 105 days and 115 days for derivatives with phenyl and naphthalene substituents. Taking into account that synthesis of reported fullerene derivatives is very convenient, the reported compounds are very promising materials for construction of BHJ solar cells.
Recently, the ternary active layer‐based organic solar cells have shown remarkable increment in the power conversion efficiency by utilizing the synergistic effect of complementary absorption, band alignment, and nano‐scale morphology enhancement. Non‐fullerene acceptors are an important class of functional materials for the improved performance of ternary organic solar cells due to their role in improvising the light absorption and the morphology of the active layer. Here, the non‐fullerene (NF) molecule, NAI‐FN‐NAI (BO), is used as the third component in the bulk heterojunction of PTB7‐Th: PC71BM to fabricate the ternary organic solar cell. The magical number of the composition i.e., 20% by weight of the NF in the ternary active layer resulted in the power conversion efficiency (PCE) of 8.1% devices which is almost 35% higher efficiency than PTB7‐Th: PC71BM binary devices having the PCE of 6%. The enhanced efficiency is observed even though the lessened effect of complementary absorption and band alignment factors of the NF. We attribute such an improved efficiency in ternary devices to the nano‐scale morphology enhancement. It could pave the way to realize the best possible bulk heterojunction blend to attain high power conversion efficiency close to inorganic counterparts. This article is protected by copyright. All rights reserved.
