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

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 (similar to 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.