High-performance Polymer Solar Cells Research Articles

High Efficiency Polymer Solar Cells with Efficient Hole Transfer at Zero Highest Occupied Molecular Orbital Offset between Methylated Polymer Donor and Brominated Acceptor.

Achieving efficient charge transfer at small frontier molecular orbital offsets between donor and acceptor is crucial for high performance polymer solar cells (PSCs). Here we synthesize a new wide band gap polymer donor, PTQ11, and a new low band gap acceptor, TPT10, and report a high power conversion efficiency (PCE) PSC (PCE = 16.32%) based on PTQ11-TPT10 with zero HOMO (the highest occupied molecular orbital) offset (ΔEHOMO(D-A)). TPT10 is a derivative of Y6 with monobromine instead of bifluorine substitution, and possesses upshifted lowest unoccupied molecular orbital energy level (ELUMO) of -3.99 eV and EHOMO of -5.52 eV than Y6. PTQ11 is a derivative of low cost polymer donor PTQ10 with methyl substituent on its quinoxaline unit and shows upshifted EHOMO of -5.52 eV, stronger molecular crystallization, and better hole transport capability in comparison with PTQ10. The PSC based on PTQ11-TPT10 shows highly efficient exciton dissociation and hole transfer, so that it demonstrates a high PCE of 16.32% with a higher Voc of 0.88 V, a large Jsc of 24.79 mA cm-2, and a high FF of 74.8%, despite the zero ΔEHOMO(D-A) value between donor PTQ11 and acceptor TPT10. The PCE of 16.32% is one of the highest efficiencies in the PSCs. The results prove the feasibility of efficient hole transfer and high efficiency for the PSCs with zero ΔEHOMO(D-A), which is highly valuable for understanding the charge transfer process and achieving high PCE of PSCs.

Roll-to-roll compatible quinoxaline-based polymers toward high performance polymer solar cells

In this article, two D–A-type quinoxaline-based polymers with multiple fluorine atoms, denoted by PB-QxF and PBF-QxF, were synthesized and tested for polymer solar cells (PSCs).

Regulating active layer thickness and morphology for high performance hot-casted polymer solar cells

Fabrication of high performance polymer solar cells through the hot-casting technique, which modulates the thickness and roughness of the active layer and also the carrier mobility of the solar cell devices.

Narrow bandgap difluorobenzochalcogenadiazole-based polymers for high-performance organic thin-film transistors and polymer solar cells

A series of difluorobenzochalcogenadiazole-bithiophene copolymers are developed for high-performance organic semiconductors.

A naphthodithiophene-based nonfullerene acceptor for high-performance polymer solar cells with a small energy loss

A new non-fullerene acceptor named NTO-4F is developed. The optimal PSC based on PM6:NTO-4F achieves a PCE of 11.5% with simultaneously high open-circuit voltage of 0.99 V and short-circuit current density of 19.1 mA cm−2.

Fluorination of a polymer donor through the trifluoromethyl group for high-performance polymer solar cells

Polymer donor F0 is fluorinated to F1 through converting methyl group to trifluoromethyl group on side chains. F1 exhibits remarkably improved performance in polymer solar cells with a highest PCE of 13.5%.

Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells

Abstract ZnO layer was modified with the addition of Cationic dyes including Crystal Violet (CV)/Ethyl violet (EV) in sol–gel process for an electron transport layer in inverted polymer solar cells (PSCs). X-ray photoelectron spectra showed the presence of CV/EV at the top of ZnO surface. Besides, oxygen defect was significantly reduced by CV/EV modification due to the chloride occupation. Furthermore, the amount of CV/EV decreased progressively from ZnO surface to bottom, being evidenced by depth profile. With modification, the ZnO surface became smoother and more hydrophobic to improve the contact with active layer. Meanwhile, CV/EV participated in the crystallization which resulted in the larger ZnO crystal grain size. Interface dipole after modification would slightly reduce the work function of ZnO and the energy barrier between ZnO and active layer via Ultraviolet Photoelectron Spectroscopy and External Quantum Efficiency analysis. Accordingly, inverted PSCs possessed better morphology, better electron extraction ability with ZnO modified by CV and EV respectively, rendering the power conversion efficiency up to 8.80% and 9.06% in comparison to the pristine ZnO (7.59%). In conclusion, we demonstrate a facile way to improve morphological and electrical properties of ZnO layer by simply adding CV/EV in sol–gel ZnO to fabricate high performance PSCs.

P3HT-Based Polymer Solar Cells with 8.25% Efficiency Enabled by a Matched Molecular Acceptor and Smart Green-Solvent Processing Technology.

A novel molecular acceptor of TrBTIC (2,7,12-tris((2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile-7-benzothiadiazole-2-)truxene) is designed by attaching the 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile-benzothiadiazole (BTIC) electron-deficient unit to an electron-rich truxene core. TrBTIC has excellent solubility in common solvents and features good energy level matching with poly(3-hexylthiophene) (P3HT). Interestingly, P3HT can be readily dissolved in warm 1,2,4-trimethylbenzene (TMB), a green solvent, but crystallizes slowly with long-term aging in TMB at room temperature. A prephase separation can thus occur before active blend film deposition, and the separation degree can be easily controlled by varying the aging time. After 40 min of aging, the resulting active blend has the most appropriate phase separation with uniform nanowires, which forms favorable interpenetrating networks for exciton dissociation and charge transport. As a result, the device performance is improved from 6.62% to 8.25%. Excitingly, 8.25% is a new record for P3HT-based solar cells. The study not only provides an efficient nonfullerene acceptor for matching P3HT donors but also develops a promising processing technology to realize high-performance P3HT-based polymer solar cells with an efficiency over 8%.

Positive effects of side-chain fluorination and polymer additive SBS on the enhanced performance of asymmetric-indenothiophene-based polymer solar cells

Two new D-A type photovoltaic polymers, namely PITPh-DfQx and PITPhf-DfQx, based on asymmetric indenothiophene (IT) donor units with alkoxyphenyl or fluoroalkoxyphenyl substitutes were designed and synthesized. Effects of the fluorine substitution in the asymmetric IT donor units on the electronic structure, ordering structure, photovoltaic properties, and charge generation and recombination dynamics were investigated. It is found that side-chain fluorination in the asymmetric donor units of the D-A polymers endowed the relative polymers with a deeper HOMO level, higher and more balanced charge mobilites, increased charge dissociation efficiency and reduced bimolecular recombination. As a result, the bulk heterojunction solar cell based on the blend film of PITPhf-DfQx and PC71BM demonstrated an efficiency of 6.10%, whereas the cell efficiency based on PITPh-DfQx was only 3.00%. In addition, a triblock copolymer, poly(styrene-block-butadiene-block-styrene) (SBS), was employed for the first time as a polymer additive into the active layers based on PITPh-DfQx/PC71BM and PITPhf-DfQx/PC71BM devices to promote donor crystallization and tune the extent of phase separation between the donor and acceptor. The presence of SBS obviously improved the molecule packing and induced the crystallization of the two polymers, giving rise to a better phase separation due to enhanced aggregation effect of photovoltaic polymers. Therefore, with a small addition of SBS, the optimal PCE was further increased from 6.10% to 6.60% for PITPhf-DfQx based device and from 3.00% to 5.50% for PITPh-DfQx based device. The positive effects of SBS additive on the performance of photovoltaic polymer/fullerene BHJ solar cells provide a new strategy for developing high performance polymer solar cells.

Unsymmetric Side Chains of Indacenodithiophene Copolymers Lead to Improved Packing and Device Performance

Two conjugated polymers (PuIDTBD and PuIDTQ) with unsymmetric side chains have been prepared for polymer solar cells using two other polymers (PIDTBD and PIDTQ) with symmetric side chains as control compounds. The combination of methyl and 4-hexylphenyl side chains on the same bridged carbon can ensure good solubility, decrease π-π stacking distances, and bring proper miscibility with PC71BM simultaneously. Therefore, the corresponding polymer solar cells (PSCs) based on donor polymers with unsymmetric side chains exhibited enhanced short-circuit current density (JSC) and power conversion efficiency (PCE) compared with those of control polymers. The PIDTBD and PIDTQ based devices possessed low PCE of 2.13% and 1.48%, while PCEs of devices based on PuIDTBD and PuIDTQ were improved to 3.93% and 4.12%, respectively. The results demonstrate that unsymmetric side chain engineering of conjugated polymers is an effective approach to achieve high performance PSCs.

Solution-processible Cd-doped ZnO nanoparticles as an electron transport layer to achieve high performance polymer solar cells through improve conductivity and light transmittance

In this work, electron transport layers (ETLs) with high charge transfer ability were prepared by doping ZnO nanoparticles with different concentrations of cadmium(Cd). The inverted polymer solar cell based on PTB7-Th: PC71BM as active layer and various concentrations Cd-doped ZnO (CZO) as ETLs were fabricated. The PCE of the device with optimized Cd content in the ZnO film was about 14.7% larger than that of the pure ZnO-based cells. The cadmium-doped ZnO(CZO) is a good candidate to be used as a high-quality transparent electrode in solar cell applications.

Random Polymer Donor for High-Performance Polymer Solar Cells with Efficiency over 14.

Constructing random copolymers has been regarded as an easy and effective approach to design polymer donors for state-of-the-art polymer solar cells (PSCs). In this work, we develop a naphtho[2,3-c]thiophene-4,9-dione-based copolymer PBN-Cl as a donor material for PSCs, and a moderate power conversion efficiency (PCE) of 11.21% is achieved with a relatively low fill factor (FF) of 0.615. We then incorporate a similar acceptor unit benzo[1,2-c:4,5-c']dithiophene-4,8-dione (BDD) into the polymeric backbone of PBN-Cl to tune its photovoltaic performance, and a significantly higher PCE of 14.05% is achieved from the random polymer PBN-Cl-B80 containing 80% BDD unit. The enhanced PCE of the PBN-Cl-B80-based device mainly relies on the higher FF value, resulting from the improved charge mobility properties, reduced bimolecular and trap-assisted recombination, and more appropriate phase separation. The results demonstrate a feasible strategy to tune the photovoltaic performance of polymer donors by constructing a random polymer with a compatible component.

Rationally pairing photoactive materials for high-performance polymer solar cells with efficiency of 16.53%

The emergence of non-fullerene acceptors (NFA) offers a promising opportunity to develop high-performance donor/acceptor pairs with high power conversion efficiency, as NFAs offer tunable energy levels, broad absorption and suitable aggregation property. In order to enhance light-harvesting capability of active layers, we choose a wide bandgap polymer PTQ10 as the donor to blend with a narrow bandgap NFA Y6 as the acceptor. In comparison with PTQ10:IDIC blend, ~130 nm red-shifted absorption spectrum is observed in the PTQ10:Y6 blend, which potentially enhance the short-circuit current density ( J sc) for the PSCs. In addition, the optimal PTQ10:Y6 blend shows higher photoluminescence quenching efficiency and more efficient charge separation, higher charge mobilities, as well as weaker bimolecular recombination over the PTQ10:IDIC blend, which leads to an outstanding power conversion efficiency (PCE) of 16.53%, with a notable J sc of 26.65 mA cm−2 and fill factor (FF) of 0.751.

Efficient Interface Engineering Enhances Photovoltaic Performance of a Bulk-Heterojunction PCDTBT:PC71BM System

High-performance conventional polymer solar cells based on a poly[N-9''-heptadecanyl-2,7-carbazole-alt- 5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT): [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) system are achieved via cathodic interfacial engineering by a bispyridinium salt small molecule (FPyBr). The bispyridinium salt can form a directed dipole at the cathode interface and thus decrease the cathode work function, leading to an improved built-in potential and open-circuit voltage. The good energy level alignment and hole-blocking ability of FPyBr at the cathode may suppress the charge recombination and promote the electron extraction process to improve the device performance. A smooth and uniform FPyBr film can be formed on the active layer, resulting in a good interfacial contact. As a result, a power conversion efficiency of 7.68% can be realized with FPyBr, which is significantly higher than for bare Al, solvent-treated and poly[(9,9-bis(3'-(N,N-dimethylamino)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]-modified devices. To the best of our knowledge, this result is one of the highest photovoltaic performance based on PCDTBT:PC71BM system. Therefore, FPyBr is a promising cathode modifier for high-performance polymer solar cells.

Small energy loss and high open circuit voltage in conventional structure polymer solar cells with the mixture thin films of polyethylenimine ethoxylated and TiOX as the electron extraction layer and Ag as the cathode

One of the challenges facing the fabrication of high-performance conventional structure polymer solar cells (PSCs) is the development of new interface materials that have better air stability and charge carrier collection and transport ability. In this study, the mixture thin films of polyethylenimine ethoxylated (PEIE) and TiOX are developed as the electron extraction layer (EEL) and Ag as the cathode for the application in PSCs, replacing the Ca/Al composite cathode. Ultraviolet photoelectron spectroscopy (UPS) and space-charge-limited current (SCLC) measurements demonstrate that PEIE:TiOX/Ag shows appropriate energy levels, substantially reduced barrier potential, weakened charge carrier recombination and enhanced electron mobility. As a result, the PEIE:TiOX-based PSCs demonstrate not only an enhanced power conversion efficiency (PCE) of 10.94% but also the photovoltaic performance insensitive to the storage time, yielding an aged PCE that is 92% of the fresh PCE. This result can be further explained by the dependence of the charge carrier mobility on storage time. Our work suggests exploiting the PEIE:TiOX/Ag composite thin films as the composite cathode replacing Ca/Al thin films for successfully enhancing the photovoltaic performance and improving the stability of PSCs.

Förster resonance energy transfer and improved charge mobility for high performance and low-cost ternary polymer solar cells

In this study, the enhanced performance of a conventional bulk heterojunction (BHJ) system based on poly({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):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) is realized by adding a high charge mobility and low-cost small-molecular material of alpha-hexathienyl (α-6T) as an electron donor. Both the short circuit current (JSC) and fill factor (FF) are facilitated by adding 2 wt% α-6T to form a ternary system, resulting in a 22.2% improvement of power conversion efficiency (PCE) compared with the control binary device. The state-of-art BHJ layer was characterized by UV–Vis absorption, steady-state photoluminescence, space-charge-limited current, atomic force microscopy, and impedance spectra, and the enhanced device performance is mainly attributed to the Förster resonance energy transfer effect from α-6T to PTB7 and the increased charge mobility. The result also showed that the blend morphology can be modulated by the introduction of α-6T electron donor. Moreover, the incorporation of α-6T can contribute to the formation of more potential pathways for charge transportation in the active layer, and decrease the resistance of device.

Impact of the Siloxane-Terminated Side Chain on Photovoltaic Performances of the Dithienylbenzodithiophene-Difluorobenzotriazole-Based Wide Band Gap Polymer Donor in Non-Fullerene Polymer Solar Cells.

To thoroughly disclose the role of the siloxane-terminated side chain with different substituent positions, three difluorobenzotriazole-dithienylbenzodithiophene (FTAZ-BDTT)-based polymers PBZ-1Si, PBZ-2Si, and PBZ-3Si with the siloxane-terminated side chain on the FTAZ unit (PBZ-1Si), on the BDTT unit (PBZ-2Si), and both on BDTT and FTAZ units (PBZ-3Si), respectively, were synthesized. The different side chain substitutions have slight influences on absorption behavior, thermal stability, and frontier molecular orbitals but have shown a great effect on the aggregation of the polymers. Grazing-incidence wide-angle X-ray scattering measurements reveal that, relative to PBZ-1Si with branched alkyl on the BDTT unit, polymers PBZ-2Si and PBZ-3Si, bearing the siloxane-terminated side chains on the BDTT unit, exhibit smaller π-π stacking distances and larger crystal coherence lengths, suggesting that adopting the siloxane-terminated side chain on the BDTT unit can promote the interchain π-π interaction and the ordering of molecular packing. With IT-M as the non-fullerene acceptor, among the three polymers, the PBZ-2Si-based active layer possesses the highest ordered crystals for both polymers and IT-M as well as the purest domain, which affords efficient exciton dissociation, the most balanced hole-electron transport, and reduced recombination, leading to the highest short-circuit current density (Jsc) and fill factor (FF) and then the highest power conversion efficiency (PCE) of 11.14%. In contrast, PBZ-1Si- and PBZ-3Si-based devices show lower PCEs of 8.98 and 9.92%, respectively. Moreover, PBZ-2Si:IT-M also exhibits good thickness tolerance, and its thick active layer of 240 nm shows the most limited decrease of efficiency after 77 days of storage, supplying good potential for mass fabrication. Our work suggests that the fine pairing of a siloxane-terminated side chain and an alkyl side chain is beneficial for the optimizing of a conjugated polymer donor toward high-performance non-fullerene polymer solar cells.

Temperature-Modulated Optimization of High-Performance Polymer Solar Cells Based on Benzodithiophene–Difluorodialkylthienyl–Benzothiadiazole Copolymers: Aggregation Effect

A novel series of low band gap donor–acceptor copolymers derived from 4,5-bis(2-ethylhexyloxy)-benzo[2,1-b:3,4-b′]dithiophene (BDT) and 5,6-difluoro-4,7-bis(4-alkylthien-2-yl)benzo[c][1,2,5]thiadiazole bearing various alkyl side chains, such as PffBB-n (n = 10, 12, 14, and 16), were developed for high-performance bulk-heterojunction (BHJ) polymer solar cells (PSCs). PffBB-n exhibited not only strong and wide absorption but also controllable aggregation behavior in solution and thin films, in which aggregation behavior was greatly influenced by the length of alkyl side chains attached, temperature applied, and solvent used. Aggregation-induced spectral broadening further extended the absorption cut-off to ∼780 nm in thin films, leading to a narrow optical band gap of ∼1.6 eV. Because of the strong aggregation strength, PffBB-n equipped with long alkyl side chains shows enhanced ordered molecular packing and good crystallinity as revealed by X-ray diffraction studies. In addition, temperature-dependent aggr...

Low-Temperature Processed TiOx/Zn1-xCdxS Nanocomposite for Efficient MAPbIxCl1-x Perovskite and PCDTBT:PC70BM Polymer Solar Cells.

The majority of high-performance perovskite and polymer solar cells consist of a TiO2 electron transport layer (ETL) processed at a high temperature (>450 °C). Here, we demonstrate that low-temperature (80 °C) ETL thin film of TiOx:Zn1−xCdxS can be used as an effective ETL and its band energy can be tuned by varying the TiOx:Zn1−xCdxS ratio. At the optimal ratio of 50:50 (vol%), the MAPbIxCl1−x perovskite and PCBTBT:PC70BM polymer solar cells achieved 9.79% and 4.95%, respectively. Morphological and optoelectronic analyses showed that tailoring band edges and homogeneous distribution of the local surface charges could improve the solar cells efficiency by more than 2%. We proposed a plausible mechanism to rationalize the variation in morphology and band energy of the ETL.

RETRACTED: Study of dielectric and optical constants of Benzobis(thiazole)-based copolymer thin films for designing optoelectronic devices

Abstract A new series of benzobisthiadiazole (BBT)-based donor–acceptor copolymers, namely, PBBT-FT, PBBT-T-FT, and PBBT-Tz-FT, with different π-conjugated bridges have been developed for polymer thin film transistors (TFTs). It was found that inserting different π-conjugated bridges into the backbone of the polymer allowed tailoring of opto-electrical properties, molecular organizations, and accordingly, ambipolar transport of TFTs. Copolymer PBB-T with benzo[1,2-d:4,5-d’]bis(thiazole) (BBT) as the accepting unit and benzodithiophene (BDT) as the donor unit is a promising candidate for high-performance non-fullerene polymer solar cells (PSCs). As much as we know the optical and dielectric constants of the PBB-T are not fully known. PBB-T was synthesized and subsequently thin films were properly prepared. By using the Kramers-Kronig relations and the transmission spectra of these thin films, the dielectric and optical constants were calculated and analyzed, including the absorption coefficient, dielectric constant, extinction coefficient, and refractive index of the PBB-T thin films. The results show that these dielectric and optical constants of the PBB-T thin films are quasi-independent on the thicknesses of the thin films, indicating our results are reliable. The features of the dielectric and optical constants show the PBB-T thin films are very promising candidates for high-performance non-fullerene PSCs and potential cut-off filter only permitting red and near-infrared light pass. These results are significant for designing optoelectronic devices related to the PBB-T thin films.