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 x 10(-4) cm(2)/(V s) in P2 to similar to 1 x 10(-2) cm(2)/(V s) in P3-based transistors. BHJ solar cells using polymer PI or P2 as the electron donor and (6,6)-phenyl-C-71-butyric acid ester (PC7IBM) 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 similar to 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.