Donor Copolymers Research Articles

ANew Diazabenzo[k]fluoranthene-BasedD-A Conjugated Polymer Donor for Efficient Organic Solar Cells.

The development of wide-bandgap polymer donors having complementary absorption and compatible energy levels with near-infrared (NIR) absorbing nonfullerene acceptors is highly important for realizing high-performance organic solar cells (OSCs). Herein, a new thiophene-fused diazabenzo[k]fluoranthene derivative is successfully synthesized as the electron-deficient unit to construct an efficient donor-acceptor (D-A) type alternating copolymer donor, namely, PABF-Cl, using the chlorinated benzo[1,2-b:4,5-b']dithiophene as the copolymerization unit. PABF-Cl exhibits a wide optical bandgap of 1.93eV, a deep highest occupied molecular level of -5.36eV, and efficient hole transport. As a result, OSCs with the best power conversion efficiency of 11.8% are successfully obtained by using PABF-Cl as the donor to blend with a NIR absorbing BTP-eC9 acceptor. This work provides a new design of electron-deficient unit for constructing high-performance D-A type polymer donors.

Chloride side-chain engineered quinoxaline-based D-A copolymer enabling non-fullerene organic solar cells with over 16% efficiency

The simple halogenation strategy of side-chains has been proven an effective approach to boost the photovoltaic performance of organic solar cells (OSCs). Herein, two novel D-A copolymer donors, namely PBDTTS-2FQx and PBDTTS-2ClQx, comprising an alkylthiothiophene benzodithiophene (BDTTS) as donor unit, alkyl substituted thiophene as the π-bridges and an identical molecular framework but alkoxy substituted fluorobenzene (or chlorobenzene) side chains on the quinoxaline (Qx) as acceptor units, are first developed and compared in parallel. The PBDTTS-2ClQx with chlorobenzene side chains on the Qx unit exhibits a distinct redshifted absorption, suppressed energy levels, increased extinction coefficient and electron mobility compared with the counterpart PBDTTS-2FQx bearing fluorobenzene side-chains on the Qx unit. After blended with BTP-eC9 as non-fullerene acceptor (NFA), the blend film of PBDTTS-2ClQx:BTP-eC9 shows higher and balanced hole/electron mobilities, more favorable aggregation, as well as less charge carrier recombination and better molecular order. As a result, the OSCs based on PBDTTS-2ClQx:BTP-eC9 deliver an impressive power conversion efficiency (PCE) of 16.1% with simultaneously increased fundamental parameters, while the PBDTTS-2FQx-based OSCs exhibits only a PCE of 12.2%. The impressive PCE of 16.1% is by far one of the highest values for binary OSCs with the Qx-based copolymer as donors. This work reveals that chlorine side-chain engineering of the Qx-based copolymer donors is a simple and effective approach to further improve their photovoltaic performance.

New Medium Bandgap Donor D-A1 -D-A2 Type Copolymers Based on Anthra[1,2-b: 4,3-b":6,7-c"'] Trithiophene-8,12-dione Groups for High-Efficient Non-Fullerene Polymer Solar Cells.

A new acceptor unit anthra[1,2-b: 4,3-b': 6,7-c'']trithiophene-8,12-dione (А3Т) (A2) is synthesized and used to design D-A1 -D-A2 medium bandgap donor copolymers with same thiophene (D) and A2 units but different A1, i.e., fluorinated benzothiadiazole (F-BTz) and benzothiadiazole (BTz) denoted as P130 and P131, respectively. Their detailed optical and electrochemical properties are examined. The copolymers show good solubility in common organic solvents, broad absorption in the visible spectral region from 300 to 700 nm, and deeper HOMO levels of -5.45 and -5.34 eV for P130 and P131, respectively. Finally, an optimized polymer solar cell (PSC) based on P131 as the donor and narrow bandgap non-fullerene small molecule acceptor Y6 demonstrated a power conversion efficiency (PCE) of >11.13%. To further improve the efficiency of the non-fullerene PSC, the P130 is optimized by introducing a fluorine atom into the BTz unit, F-BTz acceptor unit, and PCE PSC based on P130: Y6 active layer increased to >15.28%, which is higher than that for the non-fluorinated analog P131:Y6. The increase in the PCE for former PSC is attributed to the more crystalline nature and compact π-π stacking distance, leading to more balanced charge transport and reduced charge recombination. These remarkable results demonstrate that A3T-based copolymer P130 with F-BTz as the second acceptor is a promising donor material for high-performance PSCs.

Introducing Low‐Cost Pyrazine Unit into Terpolymer Enables High‐Performance Polymer Solar Cells with Efficiency of 18.23%

AbstractRecently, a random ternary copolymerization strategy has become a promising and efficient approach to develop high‐performance polymer donors for polymer solar cells (PSCs). In this study, a low‐cost electron‐withdrawing unit, 2,5‐bis(4‐(2‐ethylhexyl)thiophen‐2‐yl)pyrazine (PZ‐T), is incorporated into the polymer backbone of PM6 as the third component, and three D‐A1‐D‐A2 type terpolymers PMZ‐10, PMZ‐20, and PMZ‐30 are synthesized by the random copolymerization strategy, with the PZ‐T proportion of 10%, 20%, and 30%, respectively. The terpolymers exhibit downshifted highest occupied molecular orbital energy levels than PM6, which is beneficial for obtaining higher open‐circuit voltage (Voc) of the PSCs with the polymer as a donor. Importantly, the PSCs based on PMZ‐10:Y6 demonstrate efficient exciton dissociation, higher and balanced electron/hole mobilities, desirable aggregation, and high power conversion efficiency of 18.23%, which is the highest efficiency among the terpolymer‐based PSCs so far. The results indicate that the ternary copolymerization strategy with PZ‐T as the second A‐unit is an efficient approach to further improve the photovoltaic performance and reduce the synthetic cost of the D‐A copolymer donors.

Quinoxaline-Based D-A Copolymers for the Applications as Polymer Donor and Hole Transport Material in Polymer/Perovskite Solar Cells.

Polymer solar cells (PSCs) have achieved great progress recently, benefiting from the rapid development of narrow bandgap small molecule acceptors and wide bandgap conjugated polymer donors. Among the polymer donors, the D-A copolymers with quinoxaline (Qx) as A-unit have received increasing attention since the report of the low-cost and high-performance D-A copolymer donor based on thiophene D-unit and difluoro-quinoxalline A-unit in 2018. In addition, the weak electron-deficient characteristic and the multiple substitution positions of the Qx unit make it an ideal A-unit in constructing the wide bandgap polymer donors with different functional substitutions. In this review article, recent developments of the Qx-based D-A copolymer donors, including synthetic method of the Qx unit, backbone modulation, side chain optimization, and functional substitution of the Qx-based D-A copolymers, are summarized and discussed. Furthermore, the application of the Qx-based D-A copolymers as hole transport material in perovskite solar cells (pero-SCs) is also introduced. The focus mainly on the molecular design strategies and structure-properties relationship of the Qx-based D-A copolymers, aiming to provide a guideline for developing high-performance Qx-based D-A copolymers for the applications as donor in PSCs and as hole transport material in pero-SCs.

Improved photovoltaic properties of copolymer donors by regulating alkyl and alkylsilyl side chains

By changing the ratio of alkyl and tripropylsilyl side chain of the donor unit, a series of copolymer donor materials, named PBT1, PBT10Si, PBT20Si, PBT30Si and PBDB-Si, are synthesized and applied in polymer solar cells (PSCs). All copolymers exhibit complementary absorption spectra and matched energy levels with small molecular acceptor Y6. Grazing incidence wide angle X-ray scattering (GIWAXS) measurements demonstrate that all copolymer pure films and blend films show distinct face-on orientations. Whereas, the blend film based on PBT20Si:Y6 manifests appropriate phase separation and less bimolecular recombination, which is in favor of effective exciton disintegration and charge transfer. As a result, the PBT20Si-based device exhibits a relatively high hole mobility (μh) value of 4.99 × 10−4 cm2 V−1 s−1, and obtains the highest power conversion efficiency (PCE) of 13.06%. These results indicate that the moderate introduction of tripropylsilyl side chains into polymer donors can improve the molecular aggregation and suppress bimolecular recombination of the photoactive layer, which is an effective strategy worthy of further research to promote the device performances.

Binary and Ternary Polymer Solar Cells Based on a Wide Bandgap D-A Copolymer Donor and Two Nonfullerene Acceptors with Complementary Absorption Spectral.

A new wide-bandgap conjugated D-A polymer denoted as P106 with a medium acceptor dithieno [2,3-e;3'2'-g]isoindole-7,9 (8H) (DTID) unit and strong 2-dodecylbenzo[1,2-b:3,4-b':6,5-b"]trithiophene (3TB) donor units shows an optical bandgap of 2.04 and highest occupied molecular orbital energy level of -5.56 eV. P106 is used as the donor and two nonfullerene acceptors-medium bandgap DBTBT-IC and narrow band Y18-DMO-are used as acceptors for the construction of binary and ternary bulk heterojunction polymer solar cells. The optimized polymer solar cells based on P106 : DBTBT-IC and P106 : Y18-DMO exhibit power conversion efficiencies of 11.76 % and 14.07 %, respectively. The short-circuit current density (22.78 mA cm-2 ) for the P106 : Y18-DMO device is higher than that for P106 : DBTBT-IC (18.56 mA cm-2 ) one, which could be attributed to the more photon harvesting efficiency of the P106 : Y18-DMO active layer. In light of the high short-circuit current densities and fill factors for the Y18-DMO based device and the high value of open circuit voltage of the DBTBT-IC based device, ternary polymer solar cells are fabricated by using ternary active layer (P106 : DBTBT-IC : Y18-DMO) and achieve a power conversion efficiency of 16.49 % with low energy loss of 0.47 eV.

Low‐cost and efficient organic solar cells based on polythiophene‐ and poly(thiophene vinylene)‐related donors

AbstractAt present, most of state‐of‐the‐art power conversion efficiencies (PCEs) of organic solar cells (OSCs) are achieved from the photoactive materials involving donor–acceptor (D–A) copolymer donors. It is well known that the complicated molecular structure of D–A copolymers means the tedious synthesis, which brings about severe cost issue and poor scalability for the industrial production. Therefore, to develop application‐oriented OSCs, considerable attention should be paid on simplifying the chemical structures of polymer donors. Polythiophene (PT) and poly(thiophene vinylene) (PTV) derivatives should be among the simplest polymer donors, and OSCs based on them have made some breakthroughs in past 2 years. Here, we briefly introduce the recent advances of OSCs based on low‐cost polymers including poly(3‐hexylthiophene) (P3HT), PT derivatives, and PTV derivatives, respectively, and emphasize the importance of modulating energy levels, preaggregation effect, and D/A miscibility for the past progress as well as the future development. At last, we also propose some challenges demanding prompt solution for realizing practical application of OSCs, aiming at providing guidance and stimulating new ideas for further research.

A Donor Polymer with a Good Compromise between Efficiency and Sustainability for Organic Solar Cells

Impressive advancements in solution‐processed bulk heterojunction (BHJ) solar cells have been driven to a large extent by the rational design of conjugated polymers as photoactive donors. These achievements have been obtained without paying much attention to green metrics parameters (such as E factor) and to the increasing synthetic complexity (SC), which is a bottleneck for industrial scalability. In this context, a novel donor copolymer (PBDT3T) based on benzodithiophene and terthiophene building blocks with ester functionalities is synthesized. The 26‐fold‐reduced E factor for the benzodithiophene monomer synthesis and the SC index of 24 for PBDT3T, much lower compared with benchmark donor polymers, provide a hint for good scalability, sustainability, and low costs. PBDT3T features a relatively wide optical bandgap and a deep highest occupied molecular orbital (HOMO), meeting the requirements for suitable donor material in both fullerene and nonfullerene‐based solar cells. PBDT3T is studied in binary and ternary blend, using PC71BM and ITIC as acceptors. The best performances are obtained in the ternary blend devices, reaching power conversion efficiency (PCE) values of 7.14%. Trade‐off considerations between PCE and SC make PBDT3T promising on an industrial perspective.

Design of simple-structure wide-bandgap conjugated polymers based on BDT for efficient non-fullerene solar cells

The recent evolution of non-fullerene acceptors yields brilliant progress for organic solar cells (OSCs). However, high-performance copolymer donors with simple chemical structures for new acceptors are a few. In this work, a series of donors based on BDT are developed with ester-substituted trithiophene as a repeating acceptor unit. Copolymerized polymers exhibit slightly red-shifted absorption and down-shifted the highest occupied molecular orbital (HOMO) as the side chains turning from alkyl (3TCO-R) and oxyalkyl (3TCO-OR) to thienyl (3TCO-Th) and chlorine-substituted thienyl (3TCO-Th-Cl), respectively. As a result, the corresponding power conversion efficiency (PCE) of 3TCO-Th-Cl based devices shows the most excellent performance (PCE = 7.70%) especially in open-circuit voltage (VOC = 1 V) when blending with ITIC and its PCE makes a breakthrough with Y6 (PCE = 10.30%). Moreover, the fabricated semitransparent OSCs exhibit a rational PCE of 8.16% with average visible transparency of 16.40%.

Triisopropylsilyl‐Substituted Benzo[1,2‐b:4,5‐c′]dithiophene‐4,8‐dione‐Containing Copolymers with More Than 17% Efficiency in Organic Solar Cells

AbstractConsidering the special functions of fused‐ring aromatic building blocks and Si‐atom in high‐performance donor–acceptor‐conjugated materials at the same time, herein the synthesis of a novel fused‐ring tricyclic heterocycle, triisopropylsilyl‐substituted benzo[1,2‐b:4,5‐c′]dithiophene‐4,8‐dione (iBDD‐Si), an isomer of well‐known benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione is presented. The iBDD‐Si‐based copolymer series (PM6, PM6‐5Si, PM6‐10Si, and PM6‐15Si) is synthesized via Stille polymerization, revealing fine‐tuned optical and electrochemical properties, and molecular packing with varying iBDD‐Si contents in the backbone. Organic solar cells are fabricated by pairing the copolymer donors with nonfullerene acceptor N3 and characterized. High power conversion efficiency of more than 17% is achieved using the PM6‐5Si‐based solar cell, which is attributed to the balanced charge transport, enhanced charge generation/dissociation kinetics, and minimized total energy and recombination losses. It is demonstrated that iBDD‐Si is a promising backbone toolbox for various high‐performance conjugated materials.

Effect of Fused Thiophene Bridges on the Efficiency of Non-Fullerene Polymer Solar Cells made with Conjugated Donor Copolymers Containing Alkyl Thiophene-3-Carboxylate

Effect of Fused Thiophene Bridges on the Efficiency of Non-Fullerene Polymer Solar Cells made with Conjugated Donor Copolymers Containing Alkyl Thiophene-3-Carboxylate

Fluorine functionalized asymmetric indo [2,3-b]quinoxaline framework based D-A copolymer for fullerene polymer solar cells

Innovating molecular structure of copolymer donor materials is still one of the prominent approach to obtain high-performance polymer solar cells (PSCs). In this paper, two novel wide bandgap (WBG) copolymers, namely PBDTTS-IQ and PBDTTS-DFIQ, based on asymmetric planar aromatic core indo [( Li et al., 2012; Wang et al., 2020) 2,32,3-b]quinoxaline (IQ) as acceptor unit through tuning side chains with fluorine (F) atom engineering and exemplary alkylthio-thienyl substituted benzodithiophene (BDTTS) donor group, are synthesized and finally employed as the photovoltaic donor materials for fullerene polymer solar cells (PSCs). After blending with PC 71 BM acceptor, the PBDTTS-DFIQ:PC 71 BM blend film presented better efficient exciton dissociation and charge extraction, more balanced electron/hole mobility ( μ h / μ e ), and nice morphology in comparison with PBDTTS-IQ:PC 71 BM blend film. Encouragingly, the PBDTTS-DFIQ:PC 71 BM based PSCs exhibits a higher power conversion efficiency (PCE) of 7.4% than that of the device based on the PBDTTS-IQ:PC 71 BM blend with a PCE of 4.96%, which thanks to an enhancement of open-circuit voltage ( V oc ) of 0.84 V, short current density ( J sc ) of 13.26 mA cm −2 and fill factor (FF) of 66.00% simultaneously. These results demonstrate that this asymmetric IQ framework is a wonderful acceptor moiety to build light-harvesting copolymers for highly efficient PSCs. Two novel NBG D-A copolymers, namely PBDTTS-IQ and PBDTTS-DFIQ, based on asymmetric indo [2,3-b]quinoxaline (IQ) unit as acceptor unit through tuning the side chains with halogen atom fluorine (F) engineering, aiming to achieve novel copolymer donors with good photovoltaic performance. • Both narrow bandgap copolymers were developed based on asymmetric planar aromatic backbone indo [2,3-b]quinoxaline as an acceptor unit. • The fluorinated PBDTTS-DFIQ exhibits a lower HOMO energy level, better planarity and higher dipole moment in comparison with PBDTTS-IQ. • The PBDTTS-DFIQ:PC 71 BM-based device presents a maximum PCE of 7.40%. • Fluorinated asymmetric indo [2,3-b]quinoxaline framework is a wonderful acceptor moiety to build light-harvesting copolymers for efficient PSCs.

New Dithiazole Side Chain Benzodithiophene Containing D–A Copolymers for Highly Efficient Nonfullerene Solar Cells

AbstractTo examine the effects of pendent alkyl chain on the benzodithiophenedione (BDD) acceptor in the donor–acceptor (D–A) conjugated copolymers, two D–A copolymers with same thiazole ring substituted benzo[1,2‐b:4,5‐b′]dithiophene (BDTTz) and different acceptors BDD with two pendent alkyl units PBDTTz‐BDD and one pendent alkyl unit in benzo[1,2‐b:4,5‐c′]dithiophene‐4,8‐dione (BDTD) PBDTTz‐BDTD are synthesized. There is small effect on the optical properties but significant effect on the intermolecular π–π stacking, charge transport, and photovoltaic performance. PBTTz‐BDTD (with one pendent alkyl chain) on the BDD exhibits smaller π–π stacking distance as compared to PBDTTz‐BDD (with two pendent alkyl chains). The balanced charge carrier mobilities for the PBDTTZ‐BDTD based blends than those for PBDTTz‐BDD based blends are beneficial for suppressing the recombination processes, resulting in higher short circuit current and fill factor for the polymer solar cells (PSCs) based on PBDTTz‐BDTD. Combined with BThIND or Y6 as acceptor, PBDTTZ‐BDTD based PSCs show power conversion efficiency of 12.85% and 14.08%, respectively, which are larger than that for PBDTTz‐BDD counterparts (10.46% and 10.90% for BThIND and Y6, respectively). Thus, the photovoltaic performance of the D–A copolymer donors can be regulated through the slight variations of the pendent alkyl chains in the BDD acceptor unit.

Efficient wide-band-gap copolymer donors for organic solar cells with perpendicularly placed benzodithiophene units

The working horses for the drastically improved power conversion efficiency (PCE) of organic solar cells (OSCs) were the small-band-gap molecular non-fullerene acceptors. This development not only removed the low performance obstacle for OSCs but also raised needs for wide-band-gap polymers for donor/acceptor mix with complimentary absorptions. Benzodithiophene (BDT) is one of the mostly used unit for constructing donor polymers. Here, we introduced one type of donor polymer with two BDT units placed perpendicularly. This exploration is aimed to reduce the complexity of molecule design, potentially to reduce the cost of donor materials. We also explored the halogenation of BDT side group to optimize the performances of the copolymers. The chlorinated donor polymer PvBDT-Cl delivered a decent PCE of 8.23% with Y6-T. This work explored the possibility of using only BDT unit to construct high performance donor polymers, which is a crucial insight for future low-cost donor polymer design.

A Quinoxaline-Based D-A Copolymer Donor Achieving 17.62% Efficiency of Organic Solar Cells.

Side-chain engineering has been an effective strategy in tuning electronic energy levels, intermolecular interaction, and aggregation morphology of organic photovoltaic materials, which is very important for improving the power conversion efficiency (PCE) of organic solar cells (OSCs). In this work, two D-A copolymers, PBQ5 and PBQ6, are designed and synthesized based on bithienyl-benzodithiophene (BDTT) as the donor (D) unit, difluoroquinoxaline (DFQ) with different side chains as the acceptor (A) unit, and thiophene as the π-bridges. PBQ6 with two alkyl-substituted fluorothiophene side chains on the DFQ units possesses redshifted absorption, stronger intermolecular interaction, and higher hole mobility than PBQ5 with two alkyl side chains on the DFQ units. The blend film of the PBQ6 donor with the Y6 acceptor shows higher and balanced hole/electron mobilities, less charge carrier recombination, and more favorable aggregation morphology. Therefore, the OSC based on PBQ6:Y6 achieves a PCE as high as 17.62% with a high fill factor of 77.91%, which is significantly higher than the PCE (15.55%) of the PBQ5:Y6-based OSC. The PCE of 17.62% is by far one of the highest efficiencies for the binary OSCs with polymer donor and Y6 acceptor.

Novel efficient accptor1-acceptor2 type copolymer donors: Vinyl induced planar geometry and high performance organic solar cells

Accptor1-acceptor2 (A1-A2) type polymer donors are attractive to down-shift the energy levels, due to the double acceptor units with strong electron affinity. The fruitful A units provide a plenty of opportunities to enrich the types of polymer donors. Nevertheless, the reported A1-A2 type polymer donors always suffer from the low short-circuit current, ascribing to their unsatisfactory spectral absorption. In this work, two novel A1-A2 type wide bandgap copolymer donors, PV2TC-BDD and PV2TC-FTAZ, were designed and synthesized by a new acceptor unit A1, 2-butyl-1-octyl carboxylate substituted thiophene-vinyl-thiophene (V2TC) with two different acceptor units A2 (BDD and FTAZ), respectively. The small and simple vinyl unit can effectively reduce the steric hindrance and improve the planarity of molecules, deepen the HOMO energy level, enhance the light absorption and facilitate charge transport. The two copolymer donors show a deep-lying HOMO level of −5.50 eV for PV2TC-BDD and −5.35 eV for PV2TC-FTAZ. The absorption bands of the copolymers are observed to distinctly red-shift than those of the reported A1-A2 type copolymers based on carboxylate substituted thiophene. As a result, when blended with IT-4F, both of A1-A2 type copolymers obtain an improved Jsc of 18.83 mA cm−1 for PV2TC-BDD and 20.49 mA cm−1 for PV2TC-FTAZ, which is the highest value for the A1-A2 type copolymer-based devices. The PV2TC-BDD-based OSC also achieves a high power conversion efficiency (PCE) of 11.50%, which is an outstanding performance of the devices based on A1-A2 type polymers. These results demonstrate that introducing the new acceptor unit V2TC to construct high efficiency A1-A2 type copolymers is an effective strategy to develop high performance polymer donors for OSCs.

Side-chain engineering of copolymers based on benzotriazole (BTA) and dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophenes (DTBDT) enables a high PCE of 14.6%

Dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophenes (DTBDT) is a kind of prospective candidate for constructing donor-π-acceptor (D-π-A) copolymer donors applied in organic solar cells but is restricted due to its relatively poor photovoltaic performance compared with benzo[1,2-b;4,5-b′]dithiophenes (BDT)-based analog. Herein, three conjugated polymers (PE51, PE52 and PE53)-based DTBDT and benzo[d][1,2,3]triazole (BTA) bearing different lengths of alkyl side chain were designed and synthesized. The change in alkyl chain length can obviously affect the energy level distribution, molecular stacking, miscibility and morphology with the non-fullerene acceptor of Y6. Polymer PE52 with a moderate alkyl chain realized the highest short-current density (J SC) and fill factor (FF) of 25.36 mA cm−2 and 71.94%, respectively. Compared with BDT-based analog J52-Cl, the significantly enhanced crystallinity and intermolecular interaction of PE52 had effectively boosted the charge transport characteristic and optimized the surface morphology, thereby increasing the power conversion efficiency from 12.3% to an impressive 14.6%, which is the highest value among DTBDT-based and BTA-based polymers. Our results show that not only could high efficiency be achieved via using DTBDT as a D unit, but the length of the alkyl chain on BTA has a significant impact on the photovoltaic performance.

Fluorination strategy enables greatly improved performance for organic solar cells based on polythiophene derivatives

The power conversion efficiencies (PCEs) of organic solar cells (OSCs) have reached 18% recently, which have already met the demand of practical application. However, these outstanding results were generally achieved with donor-acceptor (D-A) type copolymer donors, which can hardly fulfill the low-cost large-scale production due to their complicated synthesis processes. Therefore, developing polymer donors with simple chemical structures is urgent for realizing low-cost OSCs. Polythiophene (PT) derivatives are currently regarded as promising candidates for such kind of donor materials, which has been illustrated in many works. In this work, two new alkylthio substituted PT derivatives, P301 and P302, were synthesized and tested as donors in the OSCs using Y5 as the acceptor. In comparison, the introduction of fluorine atoms on the backbone of P302 can not only downshift the energy levels, but also greatly improve the phase separation morphologies of the active layers, which is ascribed to the enhanced aggregation effect and the reduced miscibility with the non-fullerene acceptor. As a result, the P302:Y5-based OSC exhibits a significantly improved PCE of 9.65% than that of P301:Y5-based one, indicating the important role of fluorination in the construction of efficient PT derivative donors.

Backbone regulation of a bithiazole-based wide bandgap polymer donor by introducing thiophene bridges towards efficient polymer solar cells

Abstract Miscibility and morphology of the active layers have significant influence on the photovoltaic performance of polymer solar cells (PSCs). Chemical strategies, especially molecular structure design, have been proven to be crucial for polymer donor materials. In this work, two wide bandgap D-A copolymer donors composed of tripropylsilyl substituted bithienyl-benzodithiophene as donor (D) unit and dialkyl substituted bithiazole as acceptor (A) unit were designed and synthesized. By introducing thiophene π-bridges into the backbone, the miscibility and morphological properties of the materials are effectively tuned, leading to tremendous progress in power conversion efficiency from 0.95% to 10.73% with m-ITIC as the acceptor. The results demonstrate that manipulating molecular distortion can be an effective strategy to regulate molecular self-assembly behavior of the polymer donors and achieve excellent aggregation properties, blend miscibility, and photovoltaic performance of the PSCs.