Low band-gap polymers based on easily synthesized thioester-substituted thieno[3,4-b]thiophene for polymer solar cells

Abstract

A new acceptor S-alkyl thieno[3,4-b]thiophene-2-carbothioate-based acceptor (TTS) was firstly developed via easy of synthesis and applied in the construction of donor–acceptor (D–A) type conjugated polymers, by replacing the alkyl chain of ketone-substituted thieno[3,4-b]thiophene with an alkylthio side chain. Then, two new TTS-based polymers PBDTT-TTSO and PBDTT-TTSE were synthesized by Stille coupling reaction. The TTS acceptor moieties made the polymers exhibit a lower band gap (∼1.5 eV) and appropriate HOMO and LUMO energy levels relative to the fullerene acceptors, which could make the polymers perform well in photovoltaic devices. Solar cells were fabricated with the structure ITO/PEDOT:PSS/polymer:PC₇₁BM/Ca/Al. The polymer PBDTT-TTSO device exhibits a power conversion efficiency (PCE) of 4.7% with an open circuit voltage (V_OC) of 0.68 V, a short circuit current density (J_SC) of 12.7 mA cm⁻², and a fill factor (FF) of 54.6% while the PBDTT-TTSE device yields a higher PCE of 5.8% with a V_OC of 0.70 V, a J_SC of 14.6 mA cm⁻², and a FF of 56.7%. The results indicate that thioester-substituted thieno[3,4-b]thiophene (TTS) is a promising building block for further design of high performance photovoltaic polymers.