Large band-gap copolymers based on a 1,2,5,6-naphthalenediimide unit for polymer solar cells with high open circuit voltages and power conversion efficiencies†
Abstract
In this paper, we developed new types of efficient large band-gap copolymers, PBDTA-INDI and PBDTT-INDI, based on a 1,2,5,6-naphthalenediimide (1,2,5,6-INDI) acceptor block for polymer solar cells (PSCs). PBDTA-INDI and PBDTT-INDI have band-gaps of 2.0 eV and 1.98 eV, which are comparable to that of the widely studied P3HT. Both copolymers also possess low-lying HOMO energy levels below −5.40 eV, which are expected to lead to high Voc values for their PSCs. The XRD analysis indicates their good molecular packing properties. Their photovoltaic properties were evaluated by using conventional devices with a structure of ITO/PEDOT:PSS/copolymer:PC71BM/Ca/Al fabricated under different conditions. The PBDTA-INDI and PBDTT-INDI devices showed a relatively poor performance with PCEs of 2.83% and 4.13% with pure chlorobenzene (CB) solvent by adding 3 vol% DIO. The device performance was largely improved by using a co-solvent of CB and chloroform (CF) with increased PCEs of 3.76% and 5.01%. After employing the thermal annealing (TA) treatment, the PCEs of PBDTA-INDI and PBDTT-INDI devices were finally increased to 5.04% and 6.35%. The gradually increased device performance would be attributed to the gradually optimized morphology of the related active blend films, which significantly induced a higher EQE response and hole mobility, giving rise to higher Jsc and FF values. Our results indicated that 1,2,5,6-naphthalenediimide is a promising acceptor block to build high-performance large band-gap copolymers for potential applications in single-junction and tandem PSCs in the future.