Regular conjugated D–A copolymer containing two benzotriazole and benzothiadiazole acceptors and dithienosilole donor units for photovoltaic application

Abstract

Herein, we report the synthesis and characterization of a regular D₁–A₁–D₁–A₂–D₂–A₂ conjugated copolymer with an optical bandgap of 1.53 eV, denoted as PTBTfBTzSi consisting of two acceptors i.e. benzothiadizole (A₁) flanked with two thiophene donors (D₁) and fluorinated benzotriazole (A₂) with one more donor dithienosilole (D₂) and investigated its optical and electrochemical properties. The highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of PTBTfBTzSi are estimated from cyclic voltammetry measurements and are −5.38 eV and −3.67 eV, respectively. We have used this copolymer as a donor along with a PC₇₁BM as acceptor for the fabrication of solution processed BHJ polymer solar cells (PSCs). The PSC based on the optimized PTBTfBTzSi:PC₇₁BM active layer (weight ratio 1 : 1.5 and 3 v% 1,8-diiodooctance (DIO)) as a solvent additive in chloroform, showed overall PCE of 8.91% (short circuit current of 14.36 mA cm⁻², open circuit voltage of 0.94 V and fill factor of 0.66) which is higher than that of PSC based on a chloroform cast active layer (4.95% with a short circuit current of 9.53 mA cm⁻², open circuit voltage of 0.98 V and fill factor of 0.53). The higher PCE is attributed to the balanced charge transport, elevated light harvesting efficiency and more favorable nanoscale morphology and enhanced crystallinity of the active layer processed with the solvent additive. Moreover, the energy loss in our polymer solar cell is 0.59 eV which is one of the lowest values among the most of the efficient polymer solar cells reported so far based on a fullerene based acceptor, to the best of our knowledge.