Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
pubs.acs.org/Macromolecules
Backbone Conformation Tuning of Carboxylate-Functionalized Wide Band Gap Polymers for Efficient Non-Fullerene Organic Solar Cells Jianhua Chen,†,§ Lei Wang,† Jie Yang,† Kun Yang,†,§ Mohammad Afsar Uddin,‡ Yumin Tang,† Xin Zhou,† Qiaogan Liao,† Jianwei Yu,† Bin Liu,† Han Young Woo,*,‡ and Xugang Guo*,† †
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Department of Materials Science and Engineering, Southern University of Science and Technology (SUSTech), No. 1088, Xueyuan Road, Shenzhen, Guangdong 518055, China ‡ Department of Chemistry, Korea University, Seoul 136-713, Republic of Korea § Key Laboratory of Functional Polymer Materials and State Key Laboratory of Medicinal Chemical Biology, The Co-Innovation Center of Chemistry and Chemical Engineering of Tianjin, Institute of Polymer Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China S Supporting Information *
ABSTRACT: Two carboxylate-functionalized wide band gap polymers, 2TC-TT-BDTFT and 2T-TTC-BDTFT, which feature a fluorinated benzodithiophene (BDTFT)-alt-2,5-di(thiophen-2-yl)thieno[3,2-b]thiophene (2T-TT) backbone having different carboxylate attaching positions, were designed and synthesized. By variation of the substitution position of carboxylate groups on the 2T-TT unit, the backbone conformation of the designed building blocks 2TC-TT and 2T-TTC and their corresponding donor−acceptor polymers was finetuned as demonstrated by single crystal study and DFT calculation, thus yielding a large device performance difference in organic solar cells. As a result of the relatively higher planarity of the 2T-TTC unit in which the two carboxylate groups were attached on the inner thieno[3,2-b]thiophene moiety, the 2T-TTC-BDTFT polymer exhibited a red-shifted UV−vis absorption, stronger aggregation, and improved charge transport property than its polymer analogue 2TC-TT-BDTFT, in which the two outer thiophene rings were functionalized with carboxylate groups. Benefiting from the improved exciton dissociation and charge collection efficiency, better film morphology, and higher photoresponse, non-fullerene organic solar cells based on 2T-TTC-BDTFT:m-ITIC achieved a power conversion efficiency (PCE) of 11.15% with a fill factor (FF) of ∼70%, while the 2TC-TT-BDTFT:m-ITIC cells showed a relatively lower PCE of 9.65% and FF of 59.31%. The much higher FF of 2T-TTC-BDTFT-based solar cells reflects the great merit of the carboxylation on thienothiophene moiety rather than the outer thiophene counterpart. Therefore, the modulation of the carboxylate position on polymer backbones is an efficient strategy to tune the backbone conformation, interchain packing, film morphology, and the resulting optical, electrical, and photovoltaic properties. Moreover, both the 2T-TTC-BDTFT:m-ITIC and 2TC-TTBDTFT:m-ITIC solar cells showed excellent stability during annealing and long-term storage. These results demonstrate that carboxylate-functionalized 2T-TTC and 2TC-TT have great potentials as a weak electron-accepting building block for wide band gap polymers for high-performance non-fullerene organic solar cells, and the carboxylate position on the polymer backbones is critical for performance improvement of organic photovoltaic devices.
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INTRODUCTION
infrared region as well as morphology and device performance degradation caused by the low photostability of fullerene structures, which seriously limit the further development and application of fullerene-based OSCs.9−15 Non-fullerene acceptors, i.e., small-molecule fused-ring electron acceptors (FREAs)16−19 and low band gap polymer acceptors,20−22 have emerged recently as new classes of highly promising alternatives to fullerene-type acceptors because of their widely tunable energy levels of the frontier molecular orbitals (FMOs), broad and intensified absorption, and high
Over the past two decades, fullerene-type acceptors such as [6,6]-phenyl-C71-butyric acid methyl ester, [6,6]-phenyl-C61butyric acid methyl ester, and indene-C60 bis-adducts have been the dominant electron-accepting materials in bulk heterojunction organic solar cells (OSCs) because of their advantages of strong electron-accepting ability, high electron mobility, and isotropic electron transport properties.1−6 The power conversion efficiencies (PCEs) of OSCs based on fullerene-type acceptors blended with polymers or smallmolecule donors have reached 11.7%7 and 11.5%,8 respectively. However, fullerene-type acceptors suffer from several drawbacks, such as limited structural modification, difficult purification, and poor light absorption in the visible to near© XXXX American Chemical Society
Received: November 4, 2018 Revised: December 15, 2018
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DOI: 10.1021/acs.macromol.8b02360 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. Molecular structure of carboxylate-functionalized building blocks reported in the literature and new building blocks in this work.
electron mobilities.23−27 Because ITIC, the first FREA, was reported by Zhan et al. in 2015, the state-of-the-art PCEs of non-fullerene organic solar cells (NF-OSCs) have been increasing rapidly and have currently reached a remarkable value of over 14%.28−33 Small-molecule FREAs typically feature an acceptor−donor−acceptor (A−D−A) backbone motif with an electron-donating fused core and two strong electron-withdrawing termini. FREAs usually have strong absorption in the visible and near-infrared region from 600 to 800 nm,34,35 which arises from their strong intramolecular charge transfer characteristics but poor absorption in the shortwavelength range from 300 to 600 nm. Therefore, developing wide band gap (WBG) polymers36 with strong light harvesting at short wavelengths is critical to realizing complementary light absorption with small molecule FREAs. Moreover, the deeplying highest occupied molecular orbital (HOMO) energy level of WBG polymers can maximize the energy offset between the HOMO of polymer donors and the lowest unoccupied molecular orbital (LUMO) of FREAs, resulting in a large open-circuit voltage (Voc) in the OSCs. Combining the complementary light absorption and enlarged HOMOdonor− LUMOacceptor offset is expected to yield simultaneous improvement in the short-circuit current (Jsc) and Voc, which will in turn enhance the photovoltaic performance of WBG polymer:FREA-based OSCs. Therefore, the NF-OSC device performance can be improved not only by developing novel FREAs with red-shifted absorption at long wavelengths34,37 but also by devising WBG donor polymers with strong absorption at short wavelengths and deep-lying HOMO levels.38−40 Most of the reported high-performance WBG polymers are constructed with a D−A motif, in which a two-dimensional benzodithiophene (BDT) derivative is typically used as the D unit and is copolymerized with a weak A unit such as benzotriazole,41−51 benzodithiophenedione,28,52−56 bithiazole,57−59 benzothiadiazole,60−62 or quinoxaline.63−66 The resulting D−A polymer semiconductors have strongly suppressed intramolecular charge transfer characteristics, thus affording a WBG and low-lying HOMO level. However, very limited WBG polymers are working nicely (having wellmatched absorption and FMO levels) with the FREA nonfullerene acceptors, and the developments of new weak A building blocks to construct high-performance WBG polymers are strongly required.10,60 Carboxylate-functionalized thiophene derivatives were developed recently as a new type of
building block for constructing D−A type WBG polymers because of their advantages of easy and straightforward synthesis as well as relatively high device stability.33,67−73 For instance, Hou et al. reported a polythiophene derivative, PDCBT, prepared by a facile four-step synthetic protocol with two symmetric carboxylate groups located at the 4,4′-positions of the 2,2′-bithiophene moiety (DCBT-2, Figure 1). Compared to those based on P3HT, PDCBT-based NFOSCs exhibited significant improvement in Voc (from 0.52 to 0.94 V) and PCE (from 1.25% to 10.16%).68 The WBG polymer P3BT was further obtained, which is based on a dicarboxylate-substituted terthiophene moiety (3TC) as the A unit (Figure 1) with a BDT derivative as the D counit via a five-step synthetic route. The P3BT-based NF-OSC achieved a remarkable PCE of 11.9% with a Voc of 1.0 V.69 The PCE was further improved to 14.2% with a Voc of 0.9 V and a fill factor (FF) of 0.76 after a minor structural modification via introduction of a fluorine atom combined with side-chain engineering.33 Chen et al. also reported a series of BDT-altbithiophene polymers, PBDT-2TC and PBDT-S-2TC, based on a monocarboxylate-substituted asymmetric 2TC building block (Figure 1) synthesized via a three-step procedure. The PCE of PBDT-S-2TC-based NF-OSCs reached 10.12%.70 Choi and co-workers reported a regiorandom BDT-alt-methyl thiophene carboxylate polymer, 3MT-Th, which exhibited an excellent PCE of 9.73% with a Voc of 0.95 V in NF-OSCs and long-term operation stability with a shelf life of >1000 h under continuous light illumination.71 Although promising photovoltaic performance has been reported for OSCs fabricated with carboxylate-functionalized thiophene-derivative-based WBG polymers, the FFs of these cells are relatively low, typically
