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 V_oc 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 J_sc 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.