Opto-electrical, charge transport and photovoltaic property modulation of 2,5-di(2-thienyl)pyrrole-based polymers via the incorporation of alkyl, aryl and cyano groups on the pyrrole unit

To investigate the property modulation of 2,5-di(2-thienyl)pyrrole (TPT)-based polymers via the incorporation of alkyl, aryl or cyano groups on the TPT unit, four new polymers namely P(BDT-AlTPTH), P(BDT-AlTPTCN), P(BDT-ArTPTH) and P(BDT-ArTPTCN) containing 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene (BDT) and TPT derivatives were prepared and their opto-electrical, charge transport and photovoltaic properties were studied. The opto-electrical studies suggest that the N-aryl TPT-based polymers such as P(BDT-ArTPTH) and P(BDT-ArTPTCN) displayed redshifted absorption maxima and higher highest occupied molecular orbital (HOMO) energy levels compared to those of their corresponding N-alkyl TPT-based polymers such as P(BDT-AlTPTH) and P(BDT-AlTPTCN). On the other hand, polymers P(BDT-AlTPTCN) and P(BDT-ArTPTCN) incorporating electron-accepting cyano group on TPT backbone showed blueshifted absorption maxima and deeper HOMO energy levels compared to those of polymers P(BDT-AlTPTH) and P(BDT-ArTPTH) containing hydrogen on TPT backbone. The organic field-effect transistors (OFETs) made from the synthesized polymers suggest that the TPT-based polymers containing hydrogen on TPT backbone exhibit quite similar mobility, but the incorporation of cyano group on TPT backbone notably decrease the hole mobility. The photovoltaic studies suggest that the polymer solar cells (PSCs) made from the TPT-based polymers containing hydrogen on TPT backbone give higher power conversion efficiency (PCE) than those made from their corresponding polymers incorporating cyano group on TPT backbone.