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 ∼ × 10^-2 cm²/(V s) in P3-based transistors. BHJ solar cells using polymer P1 or P2 as the electron donor and (6,6)-phenyl-C₇₁-butyric acid ester (PC₇₁BM) 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.