Enhanced Photovoltaic Performance of Amorphous Donor–Acceptor Copolymers Based on Fluorine‐Substituted Benzodioxocyclohexene‐Annelated Thiophene

Abstract

and acceptor (n-type) materials. For the p-type materials, it is well established that the donor-acceptor (D-A) architecture is essential for efficient photovoltaic application, since it enables managing of light-harvesting properties as well as energy-level alignment. [7,8] Furthermore, the blend-film investigation of BHJ active layers has revealed that formation of a nanometer-scale phase-separated structure for favorable carrier (hole and electron) transport is a key requisite for achieving high power conversion efficiencies (PCEs) of more than 10%. Thus, the development of D-A π-conjugated copolymers with crystalline behavior has become a rational direction in recent years to acquire high carrier mobility in the blend films. [9][10][11][12][13][14][15][16][17] On the other hand, organic thin-film devices based on amorphous active layers have been utilized as an alternative approach to create novel applications. [32,33] In this context, the realization of high-performance OPVs with amorphous active layers has been expected to have advantages in largescale printing, flexibility, and stretchable usages. [34,35] However, OPV devices consisting of amorphous D-A copolymers with a PCE over 8% are still limited. [36] We previously reported that D-A copolymers bearing naphtho[2,3-c]thiophene-4,9-dione as an acceptor unit showed a PCE of 7.85% in combination with [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) as an n-type material, [37][38][39] even though both the copolymer pristine films and blend films Donor-acceptor (D-A) type π-conjugated copolymers with crystalline behavior have been extensively investigated as donor semiconductors in organic photovoltaics (OPVs). On the other hand, the development of highperformance amorphous donor materials is still behind. The amorphous donor copolymer DTS-C 0 (F 2 ) consisting of dithieno[3,2-b:2′,3′-d]silole (DTS) donor unit and the recently developed fluorine-substituted naphtho[2,3-c]thiophene-4,9-dione (C 0 (F 2 )) acceptor unit shows moderate photovoltaic performance upon blending with PC 71 BM. In this work, to enhance the hole-transporting characteristics, a 3-hexylthiophene (HT) spacer unit is integrated into the conjugated backbone, resulting in a new amorphous copolymer DTS-HT-C 0 (F 2 ). The strong electron-accepting nature of C 0 (F 2 ) allows the introduction of the HT spacer without affecting the frontier orbital energies and thus the D-A character. Without using solvent additives and thermal annealing, OPVs based on DTS-HT-C 0 (F 2 ) and [6,6]-phenyl-C71butyric acid methyl ester PC 71 BM show an improved power conversion efficiency of 9.12%. Investigation of the device physics unambiguously reveals that the hole mobility of the copolymer in the blend is increased by an order of magnitude by the introduction of HT, while keeping an amorphous film nature, leading to higher short-circuit current density and fill factor. These results demonstrate the realization of high-performance OPVs based on amorphous active layers. Amorphous Semiconductors

Synthesis, Properties, and Photovoltaic Characteristics of Donor-Acceptor Copolymers Based on Tetrafluoro-Substituted Benzodioxocyclohexene-Annelated Thiophene

“…Even for the best-performance DTS-T 6 -C 0 (F 4 ):PC 71 BM film, no obvious diffraction signals could be observed, indicating the amorphous behaviour. The similar amorphous character was observed for the DTS-T 6 -C 0 (F 2 ):PC 71 BM and DTS-C 0 (F 4 ):PC 71 BM blend films [16,17]. These results indicate that variation in the chemical structure of the acceptor and spacer units in copolymers with DTS has little influence on the crystallinity.…”

“…The HOMO energy level (E HOMO ), obtained from the onset value of PESA measurements of the films, was estimated as −5.37 eV for all the copolymers (Fig. 3), verifying that the HOMO level was deeper than that in DTS-T 6 -C 0 (F 2 ) (−5.29 eV) [17]. From these values, the LUMO energy levels (E LUMO ) of DTS-T 6 -C 0 (F 4 ), DTS-T 10 -C 0 (F 4 ), and DTS-T EH -C 0 (F 4 ) were determined as −3.81, −3.81, and −3.78 eV, respectively.…”

“…Compared to the solution spectra, those of the films are shifted to the longer-wavelength side, suggesting the occurrence of intermolecular interactions. The onset of DTS-T 6 -C 0 (F 4 ) (795 nm) is higher than that of DTS-T 6 -C 0 (F 2 ) (778 nm), indicating a greater contribution from the D-A properties of DTS-T 6 -C 0 (F 4 ) [17]. In addition, both absorption onsets are lower than that of DTS-C 0 (F 4 ) (816 nm) [16], which is attributed to the presence of introduced thiophene spacers.…”

“…We recently developed fluorine-substituted benzodioxocyclohexene-annelated thiophene units (C 0 (F 2 ) and C 0 (F 4 )) as new acceptor units (Fig. 1) [16,17]. A D-A copolymer using C 0 (F 4 ) and dithienosilole (DTS) [18][19][20] as donor units exhibited low photovoltaic characteristics under the combination with [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) as an acceptor [16].…”

“…In contrast, OSCs based on DTS-C 0 (F 2 ) and PC 71 BM exhibited good power conversion efficiency (PCE) of 7.30% [16]. Furthermore, the donor polymer of DTS-T 6 -C 0 (F 2 ), incorporating a 3hexylthiophene (T 6 ) spacer to improve the hole transport properties, showed the high PCE of 9.12% [17]. From these results, we expected that the incorporation of thiophene spacer to DTS-C 0 (F 4 ) may increase the energy levels suitable for a donor material.…”