Effect of Fluorination on the Photovoltaic Properties of Medium Bandgap Polymers for Polymer Solar Cells

Fluorination of conjugated polymers is one of the effective strategies to tune the molecular energy levels and morphology for high efficient polymer solar cells (PSCs). Herein, two novel donor‐acceptor conjugated polymers, PffBT and PBT, based on bis(3,5‐bis(hexyloxy)phenyl)benzo[1,2‐ b:4,5‐b']dithiophene and benzo[c][1,2,5]thiadiazole (BT) with or without fluorination, respectively, were synthesized, and their photovoltaic properties were compared. The polymer PffBT based on fluorinated BT showed lower frontier energy levels, improved polymer ordering, and a well‐developed fibril structure in the blend with PC₇₁BM. As a result, the PSCs based on PffBT/PC₇₁BM exhibit a superior power conversion efficiency (PCE) of 8.6% versus 4.4% for PBT‐based devices, due to a high space charge limit current (SCLC) hole mobility, mixed orientation of polymer crystals in the active layer, and low bimolecular recombination.     

非富勒烯聚合物太阳电池研究进展

综述了以p-型共轭聚合物为给体、n-型有机半导体为受体的非富勒烯聚合物太阳电池光伏材料最新研究进展,包括n-型共轭聚合物和可溶液加工小分子n-型有机半导体(n-OS)受体光伏材料,以及与之匹配的p-型共轭聚合物给体光伏材料.介绍的n-型共轭聚合物受体光伏材料包括基于苝酰亚胺(BDI)、萘酰亚胺(NDI)以及新型硼氮键连受体单元的D-A共聚物受体光伏材料,目前基于聚合物给体(J51)和聚合物受体(N2200)的全聚合物太阳电池的能量转换效率最高达到8.26%.n-OS小分子受体光伏材料包括基于BDI和NDI单元的有机分子、基于稠环中心给体单元的A-D-A型窄带隙有机小分子受体材料等.给体光伏材料包括基于齐聚噻吩和苯并二噻吩(BDT)给体单元的D-A共聚物,重点介绍与窄带隙A-D-A结构小分子受体吸收互补的、基于噻吩取代BDT单元的中间带隙二维共轭聚合物给体光伏材料.使用中间带隙的p-型共轭聚合物为给体、窄带隙A-D-A结构有机小分子为受体的非富勒烯聚合物太阳电池能量转换效率已经突破12%,展示了光明的前景.最后对非富勒烯聚合物太阳电池将来的发展进行了展望.     

Additive-free non-fullerene organic solar cells with random copolymers as donors over 9% power conversion efficiency

The PBDB-TBT1:ITIC-based device obtains PCE of 9.09%, and is insensitive to additive and thermal annealing, and forms microstructural morphology.     

Synergistic effect of fluorination on both donor and acceptor materials for high performance non-fullerene polymer solar cells with 13.5% efficiency

A high performance polymer solar cells(PSCs) based on polymer donor PM6 containing fluorinated thienyl benzodithiophene unit and n-type organic semiconductor acceptor IT-4 F containing fluorinated end-groups were developed. In addition to complementary absorption spectra(300–830 nm) with IT-4 F, the PM6 also has a deep HOMO(the highest occupied molecular) level(-5.50 e V), which will lower the open-circuit voltage(V_(oc)) sacrifice and reduce the E_(loss) of the IT-4 F-based PSCs. Moreover, the strong crystallinity of PM6 is beneficial to form favorable blend morphology and hence to suppress recombination. As a result, in comparison with the PSCs based on a non-fluorinated D/A pair of PBDB-T:ITIC with a medium PCE of 11.2%, the PM6:IT-4 Fbased PSCs yielded an impressive PCE of 13.5% due to the synergistic effect of fluorination on both donor and acceptor, which is among the highest values recorded in the literatures for PSCs to date. Furthermore, a PCE of 12.2% was remained with the active layer thickness of up to 285 nm and a high PCE of 11.4% was also obtained with a large device area of 1 cm~2. In addition, the devices also showed good storage, thermal and illumination stabilities with respect to the efficiency. These results indicate that fluorination is an effective strategy to improve the photovoltaic performance of materials, as well as the both fluorinated donor and acceptor pair-PM6:IT-4 F is an ideal candidate for the large scale roll-to-roll production of efficient PSCs in the future.     

All‐Polymer Solar Cells: Recent Progress,Challenges, and Prospects

For over two decades bulk‐heterojunction polymer solar cell (BHJ‐PSC) research was dominated by donor:acceptor BHJ blends based on polymer donors and fullerene molecular acceptors. This situation has changed recently, with non‐fullerene PSCs developing very rapidly. The power conversion efficiencies of non‐fullerene PSCs have now reached over 15 %, which is far above the most efficient fullerene‐based PSCs. Among the various non‐fullerene PSCs, all‐polymer solar cells (APSCs) based on polymer donor‐polymer acceptor BHJs have attracted growing attention, due to the following attractions: 1) large and tunable light absorption of the polymer donor/polymer acceptor pair; 2) robustness of the BHJ film morphology; 3) compatibility with large scale/large area manufacturing; 4) long‐term stability of the cell to external environmental and mechanical stresses. This Minireview highlights the opportunities offered by APSCs, selected polymer families suitable for these devices with optimization to enhance the performance further, and discusses the challenges facing APSC development for commercial applications.     

基于一种新型聚噻吩衍生物为给体的非富勒烯聚合物太阳能电池

With the development of non-fullerene small-molecule acceptors, non-fullerene polymer solar cells (PSCs) have garnered increased attention due to their high performance. While photons are absorbed and converted to free charge carriers in the active layer, the donor and acceptor materials both play a critical role in determining the performance of PSCs. Among the various conjugated-polymer donor materials, polythiophene (PT) derivatives such as poly(3-hexylthiophene), have attracted considerable interest due to their high hole mobility and simple synthesis. However, there are limited studies on the applications of PT derivatives in non-fullerene PSCs. Fabrication of highly efficient non-fullerene PSCs utilizing PT derivatives as the donor is a challenging topic. In this study, a new PT derivative, poly[5, 5′-4, 4′-bis(2-butyloctylsulphanyl)-2, 2′-bithiophene-alt-5, 5′-4, 4′-difluoro-2, 2′-bithiophene] (PBSBT-2F), with alkylthio groups and fluorination was synthesized for use as the donor in non-fullerene PSC applications. The absorption spectra, electrochemical properties, molecular packing, and photovoltaic properties of PBSBT-2F were investigated and compared with those of poly(3-hexylthiophene) (P3HT). The polymer exhibited a wide bandgap of 1.82 eV, a deep highest occupied molecular orbital (HOMO) of -5.02 eV, and an ordered molecular packing structure. Following this observation, PSCs based on a blend of PBSBT-2F as the donor and 3, 9-bis(2-methylene-(3-(1, 1-dicyanomethylene)-indanone)-5, 5, 11, 11-tetrakis(4-hexylphenyl)-dithieno-[2, 3-d:2′, 3′-d′]-s-indaceno[1, 2-b:5, 6-b′]dithiophene (ITIC) as the acceptor were fabricated. The absorption spectra were collected and the energy levels were found to be well matched. These devices exhibited a power conversion efficiency (PCE) of 6.7% with an open-circuit voltage (V_OC) of 0.75 V, a short-circuit current density (J_SC) of 13.5 mA·cm^-2, and a fill factor (FF) of 66.6%. These properties were superior to those of P3HT (1.2%) under the optimal conditions. This result indicates that PBSBT-2F is a promising donor material for non-fullerene PSCs.     

Constructing a Strongly Absorbing Low‐Bandgap Polymer Acceptor for High‐Performance All‐Polymer Solar Cells

All‐polymer solar cells (all‐PSCs) offer unique morphology stability for the application as flexible devices, but the lack of high‐performance polymer acceptors limits their power conversion efficiency (PCE) to a value lower than those of the PSCs based on fullerene derivative or organic small molecule acceptors. We herein demonstrate a strategy to synthesize a high‐performance polymer acceptor PZ1 by embedding an acceptor–donor–acceptor building block into the polymer main chain. PZ1 possesses broad absorption with a low band gap of 1.55 eV and high absorption coefficient (1.3×10⁵ cm⁻¹). The all‐PSCs with the wide‐band‐gap polymer PBDB‐T as donor and PZ1 as acceptor showed a record‐high PCE of 9.19 % for the all‐PSCs. The success of our polymerization strategy can provide a new way to develop efficient polymer acceptors for all‐PSCs.     

Reduced‐bandgap triphenylamine‐alt‐benzo[1,2‐b:4,5‐b′]dithiophene copolymers pending benzothiadiazole or diketopyrrolopyrrole units for efficient polymer solar cells

Two new side‐chain donor–acceptor (D‐A)‐based triphenylamine‐alt‐benzo[1,2‐b:4,5‐b′]dithiophene (TPA‐alt‐BDT) copolymers ( P1 and P2 ) with pendant benzothiadiazole (BT)/diketopyrrolopyrrole (DPP) in TPA unit were synthesized by Stille coupling polymerization. Their thermal, photophysical, electrochemical, blend film morphology and photovoltaic properties were investigated. Efficient bulk heterojunction polymer solar cells (PSCs) were obtained by solution process using both copolymers as donor materials and PC₇₁BM as acceptor. The maximum power conversion efficiency (PCE) of 3.17% with a highest open‐circuit voltage (V_oc) of 0.86V was observed in the P1 ‐based PSCs, while the maximum short‐circuit current (J_sc) of 10.77 mA cm^?2 was exhibited in the P2 ‐based PSCs under the illumination of AM 1.5, 100 mW cm^?2. The alternating binary donor units and pending acceptor groups played a significant role in tuning photovoltaic properties for this class of the side‐chain D–A‐based copolymers. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4103–4110     

Polymer Acceptor Based on B←N Units with Enhanced Electron Mobility for Efficient All‐Polymer Solar Cells

We demonstrate that polymer electron acceptors with excellent all‐polymer solar‐cell (all‐PSC) device performance can be developed from polymer electron donors by using B←N units. By alleviating the steric hindrance effect of the bulky pendant moieties on the conjugated polymers that contain B←N units, the π–π stacking distance of polymer backbones is decreased and the electron mobility is consequently enhanced by nearly two orders of magnitude. As a result, the power conversion efficiency of all‐PSCs with the polymer acting as the electron acceptor is greatly improved from 0.12 % to 5.04 %. This PCE value is comparable to that of the best all‐PSCs with state‐of‐the‐art polymer acceptors.     

Synthesis of Poly(benzothiadiazole‐co‐dithienobenzodithiophenes) and Effect of Thiophene Insertion for High‐Performance Polymer Solar Cells

We describe herein the synthesis of novel donor–acceptor conjugated polymers with dithienobenzodithiophenes (DTBDT) as the electron donor and 2,1,3‐benzothiadiazole as the electron acceptor for high‐performance organic photovoltaics (OPVs). We studied the effects of strategically inserting thiophene into the DTBDT as a substituent on the skeletal structure on the opto‐electronic performances of fabricated devices. From UV/Vis absorption, electrochemical, and field‐effect transistor analyses, we found that the thiophene‐containing DTBDT derivative can substantially increase the orbital overlap area between adjacent conjugated chains and thus dramatically enhance charge‐carrier mobility up to 0.55 cm² V^?1 s^?1. The outstanding charge‐transport characteristics of this polymer allowed the realization of high‐performance organic solar cells with a power conversion efficiency (PCE) of 5.1 %. Detailed studies on the morphological factors that enable the maximum PCE of the polymer solar cells are discussed along with a hole/electron mobility analysis based on the space‐charge‐limited current model.     

Alternating copolymers of electron‐rich arylamine and electron‐deficient 2,1,3‐benzothiadiazole: Synthesis,characterization and photovoltaic properties

A series of alternating copolymers of electron‐rich arylamine and electron‐deficient 2,1,3‐benzothiadiazole (BT), PV‐BT, DP‐BT, and TP‐BT, were synthesized by Heck coupling reaction. UV–vis absorption and fluorescence spectra show that the copolymerization of electron‐rich diphenylamine (DP), triphenylamine (TP), MEH‐PV (PV), and electron‐deficient BT results in low‐bandgap conjugated polymers. Within the three copolymers of PV‐BT, DP‐BT, and TP‐BT, TP‐BT possesses the highest hole mobility of 4.68 × 10^{? 5} cm²/V, as determined from the space charge limited current (SCLC) model. The bulk heterojunction‐typed polymer solar cells (PSCs) were fabricated with the blend of the copolymers and PCBM as the photosensitive layer. The power conversion efficiencies (PCE) of the PSCs based on PV‐BT, DP‐BT, and TP‐BT reached 0.26%, 0.39%, and 0.52%, respectively, under the illumination of AM 1.5, 100 mW/cm². The results indicate that TP‐BT is a promising photovoltaic polymer for PSCs. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3861–3871, 2007     

Highly Flexible and Efficient All‐Polymer Solar Cells with High‐Viscosity Processing Polymer Additive toward Potential of Stretchable Devices

Considering the potential applications of all‐polymer solar cells (all‐PSCs) as wearable power generators, there is an urgent need to develop photoactive layers that possess intrinsic mechanical endurance, while maintaining a high power‐conversion efficiency (PCE).Herein a strategy is demonstrated to simultaneously control the intercalation behavior and nanocrystallite size in the polymer–polymer blend by using a newly developed, high‐viscosity polymeric additive, poly(dimethylsiloxane‐co‐methyl phenethylsiloxane) (PDPS), into the TQ‐F:N2200 all‐PSC matrix. A mechanically robust 10wt% PDPS blend film with a great toughness was obtained. Our results provide a feasible route for producing high‐performance ductile all‐PSCs, which can potentially be used to realize stretchable all‐PSCs as a linchpin of next‐generation electronics.     

Random copolymers based on 3‐hexylthiophene and benzothiadiazole with induced π‐conjugation length and enhanced open‐circuit voltage property for organic photovoltaics

A series of donor‐acceptor low‐bandgap conjugated polymers, that is, HThmBT (m = 3, 6, 9, 12, 15), composed of regioregular 3‐hexylthiophene segments and 2,1,3‐benzothiadiazole units, were synthesized through the Stille coupling polymerization to optimize the π‐conjugation length of the polymer and the intramolecular charge transfer (ICT) effect in the polymer backbone. The polymers had relatively low optical bandgaps ranging from 1.6 to 1.72 eV. Among these polymers, HTh6BT exhibited the best device performance with a power conversion efficiency (PCE) of 1.6%. Moreover, despite being based on thiophene, HTh6BT exhibited a high‐open circuit voltage (V_OC) of over 0.8 V because of its low high occupied molecular orbital (HOMO) energy level. These results provided an effective strategy for designing and synthesizing low‐bandgap conjugated polymers with broad absorption ranges and well‐balanced energy levels. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Planar Conjugated Polymers Containing 9,10‐Disubstituted Phenanthrene Units for Efficient Polymer Solar Cells

Four novel conjugated polymers ( P1‐4 ) with 9,10‐disubstituted phenanthrene (PhA) as the donor unit and 5,6‐bis(octyloxy)benzothiadiazole as the acceptor unit are synthesized and characterized. These polymers are of medium bandgaps (2.0 eV), low‐lying HOMO energy levels (below −5.3 eV), and high hole mobilities (in the range of 3.6 × 10⁻³ to 0.02 cm² V⁻¹ s⁻¹). Bulk heterojunction (BHJ) polymer solar cells (PSCs) with P1‐4 :PC₇₁BM blends as the active layer and an alcohol‐soluble fullerene derivative (FN‐C60) as the interfacial layer between the active layer and cathode give the best power conversion efficiency (PCE) of 4.24%, indicating that 9,10‐disubstituted PhA are potential donor materials for high‐efficiency BHJ PSCs.

     

Novel alkoxyanthracene donor and benzothiadiazole acceptor for organic thin film transistor and bulk heterojunction organic photovoltaic cells

Novel alkoxy anthracene (ODA)‐based polymeric semiconductors were designed for polymer solar cell applications. Alkoxyanthracene, which contains many π electrons and electron donating group, was easily synthesized. The copolymers, poly(alkoxy anthracene‐alt‐thiophene benzothiadiazole thiophene) poly(ODA‐TBT) and poly(alkoxy anthracene‐alt‐benzothiadiazole) poly(ODA‐BT), have been obtained by Suzuki coupling polymerization. Both polymers have ODA unit as a donor and benzothiadiazole as an acceptor. ODA‐TBT has thiophene linkages between ODA and benzothiadiazole. The optical, thermal, and electrochemical properties have been investigated by UV–visible absorption, thermal gravimetric analysis, differential scanning calorimetry, and CV. Organic thin‐film transistor using polymers showed that the hole mobility of poly(ODA‐alt‐TBT) was around 3.6 × 10^?3 cm²/Vs with on/off ratio of 9.91 × 10⁵ while that of poly(ODA‐alt‐BT) was around 1.21 × 10^?2 cm²/Vs with on/off ratio of 2.64 × 10⁶. Organic photovoltaic performance based on polymers were evaluated with a configuration of ITO/PEDOT:PSS/active layer/LiF/Al. Poly(ODA‐TBT) exhibits a short circuit current (J_sc) of 3.9 mA/cm² and power conversion efficiency (PCE) of 1.4%, and poly(ODA‐BT) exhibits the J_sc of 6.4 mA/cm² and PCE of 2.2%. The better device performance of poly(ODA‐BT) is attributed to its charge transfer ability and enhanced mobility and crystallinity although poly(ODA‐BT) does not have extended π‐conjugation due to twisted structure compared with poly(ODA‐TBT). © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1306–1314     

An Electron‐Deficient Building Block Based on the B←N Unit: An Electron Acceptor for All‐Polymer Solar Cells

A double B←N bridged bipyridyl (BNBP) is a novel electron‐deficient building block for polymer electron acceptors in all‐polymer solar cells. The B←N bridging units endow BNBP with fixed planar configuration and low‐lying LUMO/HOMO energy levels. As a result, the polymer based on BNBP units (P‐BNBP‐T) exhibits high electron mobility, low‐lying LUMO/HOMO energy levels, and strong absorbance in the visible region, which is desirable for polymer electron acceptors. Preliminary all‐polymer solar cell (all‐PSC) devices with P‐BNBP‐T as the electron acceptor and PTB7 as the electron donor exhibit a power conversion efficiency (PCE) of 3.38 %, which is among the highest values of all‐PSCs with PTB7 as the electron donor.     

Recent Progress in All‐Polymer Solar Cells Based on Wide‐Bandgap p‐Type Polymers

All‐polymer solar cells (all‐PSCs), with the photoactive layer exclusively composed of polymers as both donor and acceptor, have attracted growing attention due to their unique merits in optical, thermal and mechanical durability. Through the combined strategies in materials design and device engineering, recently the power conversion efficiencies of single‐junction all‐PSCs have been boosted up to 11 %. This review focuses on the recent progress of all‐PSCs comprising of wide band‐gap p‐type polymers, especially those based on the units of thieno[3,4‐c]pyrrole‐4,6(5H)‐dione], fluorinated benzotriazole, benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione, and pyrrolo[3,4‐f]benzotriazole‐5,7(6H)‐dione. Meantime, several categories of n‐type polymers used to match with these polymer donors are also reviewed. Finally, a brief summary of the strategies of molecular design and morphology optimization is given, and strategies toward further improving performance of all‐PSCs are outlined.     

Molecular engineering of conjugated polymers for solar cells and field‐effect transistors: Side‐chain versus main‐chain electron acceptors

Two model polymers, containing fluorene as an electron‐donating moiety and benzothiadiazole (BT) as an electron‐accepting moiety, have been synthesized by Suzuki coupling reaction. Both polymers are composed of the same chemical composition, but the BT acceptor can be either at a side‐chain (i.e., S‐polymer) or along the polymer main chain (i.e., M‐polymer). Their optical, electrochemical, and photovoltaic properties, together with the field‐effect transistor (FET) characteristics, have been investigated experimentally and theoretically. The FET carrier mobilities were estimated to be 5.20 × 10^?5 and 3.12 × 10^?4 cm² V^?1 s^?1 for the S‐polymer and M‐polymer, respectively. Furthermore, polymeric solar cells (PSCs) with the ITO/PEDOT:PSS/S‐polymer or M‐polymer:PC₇₁BM(1:4)/Al structure were constructed and demonstrated to show a power conversion efficiency of 0.82 and 1.24% for the S‐polymer and M‐polymer, respectively. The observed superior device performances for the M‐polymer in both FET and PSCs are attributable to its relatively low band‐gap and close molecular packing for efficient solar light harvesting and charge transport. This study provides important insights into the design of ideal structure–property relationships for conjugate polymers in FETs and PSCs. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

基于无规三元共聚物的非卤溶液加工型高效聚合物太阳能电池

在本工作中,我们以烷硫基噻吩基取代的苯并二噻吩(BDTT-S)为给体单元、5, 6-二氟取代苯并三唑(FBTz)和噻唑并噻唑(TTz)为弱吸收电子受体单元,设计合成了一系列宽带隙的无规三元共聚物给体材料。通过改变两个受体单元FBTz和TTz在聚合物中的摩尔比,有效调节了聚合物的光学、电化学、分子排列以及电荷传输性能。最终,使用非卤溶剂为加工溶剂,以三元共聚物PSBTZ-60为给体、ITIC为非富勒烯受体的聚合物太阳能电池(PSCs)获得了10.3%的能量转换效率(PCE),其中开路电压为0.91 V,短路电流为18.0 mA·cm⁻²,填充因子为62.7%;与之相比,在相同的器件制备条件下,基于PSTZ:ITIC的PSCs仅获得8.5%的PCE,基于PSBZ:ITIC的PSCs也仅获得8.1%的PCE。这些结果表明：三元无规共聚能够作为一种简单且实用的策略去设计、合成高性能聚合物光伏材料。     

The Crucial Role of Chlorinated Thiophene Orientation in Conjugated Polymers for Photovoltaic Devices

Chlorinated conjugated polymers not only show great potential for the realization of highly efficient polymer solar cells (PSCs) but also have simple and high‐yield synthetic routes and low‐cost raw materials available for their preparation. However, the study of the structure–property relationship of chlorinated polymers is lagging. Now two chlorinated conjugated polymers, PCl(3)BDB‐T and PCl(4)BDB‐T are investigated. When the polymers were used to fabricate PSCs with the nonfullerene acceptor (IT‐4F), surprisingly, the PCl(3)BDB‐T:IT‐4F‐based device exhibited a negligible power conversion efficiency (PCE) of 0.18 %, while the PCl(4)BDB‐T:IT‐4F‐based device showed an outstanding PCE of 12.33 %. These results provide new insight for the rational design and synthesis of novel chlorinated polymer donors for further improving the photovoltaic efficiencies of PSCs.