D-(π-A)3 type low bandgap star-shaped fused-ring electron acceptor with alkoxy-substituted thiophene as π-bridge

Abstract As a strong electron-donating π-bridge, alkoxy-substituted thiophene has been widely used in low bandgap fused-ring electron acceptors (FREAs), while its application in star-shaped FREAs has never been explored. Herein, a donor-(π-acceptor)3 type low bandgap star-shaped FREA with ethylhexyloxy-thiophene π-bridge, namely TITT-OT-ICF, is synthesized. The enhanced intramolecular charge transfer of TITT-OT-ICF leads to a high-lying HOMO level, and thus a narrower optical band gap of 1.47 eV relative to its counterpart TITT-T-ICF with ethylhexyl-thiophene as π-bridge (1.61 eV). When blending with a wide bandgap polymer donor PBDB-T, EQE spectrum of TITT-OT-ICF-based organic solar cell (OSC) device is extended to 880 nm, while that of PBDB-T: TITT-T-ICF-based device is limited to 800 nm. In spite of similar phase morphologies in both active layers, the broader photoresponse of PBDB-T: TITT-OT-ICF-based device leads to a higher short-circuit current density (JSC) of 16.95 mA cm-2, yielding a higher power conversion efficiency (PCE) of 8.23% in comparison with PBDB-T: TITT-T-ICF-based device (JSC = 14.33 mA cm-2, PCE = 7.53%). These results demonstrate a rational design of low bandgap star-shaped FREAs based on alkoxy-substituted thiophene as the π-bridge for achieving relatively high JSC and PCE.