Pyrrolo[3,4-g]quinoxaline-6,8-dione-based conjugated copolymers for bulk heterojunction solar cells with high photovoltages

Xiaofeng Xu; Chuanfei Wang; Olof Bäcke; David I. James; Kim Bini; Eva Olsson; Mats R. Andersson; Mats Fahlman; Ergang Wang

doi:10.1039/C5PY00394F

Pyrrolo[3,4-g]quinoxaline-6,8-dione-based conjugated copolymers for bulk heterojunction solar cells with high photovoltages†

Xiaofeng Xu,‡^a Chuanfei Wang,‡^b Olof Bäcke,^c David I. James,^a Kim Bini,^a Eva Olsson,^c Mats R. Andersson,^ad Mats Fahlman^b and Ergang Wang*^a

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* Corresponding authors

^a Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden
E-mail: ergang@chalmers.se

^b Division of Surface Physics and Chemistry, IFM, Linköping University, SE-581 83 Linköping, Sweden

^c Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden

^d Ian Wark Research Institute, University of South Australia, Mawson Lakes, South Australia 5095, Australia

Abstract

A new electron-deficient building block 5,9-di(thiophen-2-yl)-6H-pyrrolo[3,4-g]quinoxaline-6,8(7H)-dione (PQD) was synthesized via functionalizing the 6- and 7-positions of quinoxaline (Qx) with a dicarboxylic imide moiety. Side chain substitution on the PQD unit leads to good solubility which enables very high molecular weight copolymers to be attained. The fusion of two strong electron-withdrawing groups (Qx and dicarboxylic imide) makes the PQD unit a stronger electron-deficient moiety than if the unit had just one electron-withdrawing group, thus enhancing the intramolecular charge transfer between electron-rich and deficient units of the copolymer. Four PQD-based polymers were synthesized which feature deep-lying highest occupied molecular orbital (HOMO) levels and bathochromic absorption spectra when compared to PBDT–Qx and PBDT–TPD analogues. The copolymers incorporated with benzo[1,2-b:4,5-b‘]dithiophene (BDT) units show that the 1D and 2D structural variations of the side groups on the BDT unit are correlated with the device performance. As a result, the corresponding solar cells (ITO/PEDOT:PSS/polymer:PC₇₁BM/LiF/Al) based on the four copolymers feature very high open-circuit voltages (V_oc) of around 1.0 V. The copolymer PBDT-PQD1 attains the best power conversion efficiency of 4.9%, owing to its relatively high absorption intensity and suitable film morphology. The structure–property correlation demonstrates that the new PQD unit is a promising electron-deficient building block for efficient photovoltaic materials with high V_oc.

Polymer Chemistry

Pyrrolo[3,4-g]quinoxaline-6,8-dione-based conjugated copolymers for bulk heterojunction solar cells with high photovoltages†

Abstract

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Pyrrolo[3,4-g]quinoxaline-6,8-dione-based conjugated copolymers for bulk heterojunction solar cells with high photovoltages

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