Thieno[3,4-c]pyrrole-4,6-dione-Based Donor−Acceptor Conjugated Polymers for Solar Cells

Three new donor−acceptor conjugated polymers incorporating thieno[3,4- c ]pyrrole-4,6-dione (TPD) acceptor and dialkoxybithiophene or cyclopentadithiophene units as donor have been synthesized and explored in bulk heterojunction (BHJ) solar cells and organic field-effect transistors (OFETs). The TPD-containing polymers had optical band gaps of 1.50−1.70 eV and HOMO levels of −4.85 to −5.26 eV. The highly electron-rich character of dialkoxybithiophene in P1 and P2 destabilizes their HOMOs which significantly affects the photovoltaic efficiency. However, polymer P3 containing cyclopentadithiophene donor units results in a deeper HOMO level of −5.26 eV. The field-effect mobility of holes varied from 2 × 10 −4 cm 2 /(V s) in P2 to ∼1 × 10 −2 cm 2 /(V s) in P3- based transistors. BHJ solar cells using polymer P1 or P2 as the electron donor and (6,6)-phenyl-C 71 -butyric acid ester (PC 71 BM) as the electron acceptor exhibit low open circuit voltages ( V oc = 0.40−0.60 V) and power conversion efficiencies below 1.5%. However, BHJ solar cells based on the TPD−cyclopentadithiophene copolymer ( P3 ) achieved a high V oc of ∼0.8 V and power conversation efficiency greater than 3%. These results demonstrate the tuning of the open circuit voltage and thus the photovoltaic efficiency of BHJ solar cells based on copolymers containing thieno[3,4- c ]pyrrole-4,6-dione acceptor.