Benzothiadiazole-pyrrolo[3,4-b]dithieno[2,3-f:3′,2′-h]quinoxalindione-based random terpolymer incorporating strong and weak electron accepting [1,2,5]thiadiazolo[3,4g]quinoxalinefor polymer solar cells

Abstract

We report the synthesis of a D-A random terpolymer denoted as P2 consists of one thiophene donor unit and three acceptor benzothiadiazole (BT), pyrrolodithienoquinoxalinedione (PDQD) and thiadiazoloquinoxaline (TDQ) units by Stille-coupling reaction and investigated its optical and electrochemical properties. We have compared its properties with the parent copolymer P1. The P2 exhibits bandgap of about 1.18 eV which is lower than that of P1 (1.50 eV), indicating strength of accepting units controls both the optical and electrochemical bandgap. We have used terpolymer P2 as electron donor along with [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as electron acceptor for the fabrication of solution processed bulk heterojunction polymer solar cells (PSCs). PSC based on an optimized P2:PC71BM (1:2 by weight) active layer processed with 3v % DIO/DCB solution, displayed a power conversion efficiency (PCE) of 7.22%, which is higher than that for P1 based polymer solar cell (PCE = 6.56%) processed under same conditions. The higher value of PCE for P2:PC71BM may be related to more favorable phase separated morphology of active layer as compared to P1:PC71BM, beneficial for the exciton dissociation and charge transport, as evidenced from the larger hole mobility.