Investigation of post-thermal annealing-induced enhancement in photovoltaic performance for squaraine-based organic solar cells

In this work, we have systematically investigated the post-thermal annealing-induced enhancement in photovoltaic performance of a 2,4-bis[4-(N, N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (DIBSQ)/C₆₀ planar heterojunction (PHJ) organic solar cells (OSCs). An increased power conversion efficiency (PCE) of 3.28% has been realized from a DIBSQ/C₆₀ device with thermal annealing at 100 °C for 4 min, which is about 33% enhancement compared with that of the as-cast device. The improvement of the device performance may be mainly ascribed to the crystallinity of the DIBSQ film with post-thermal annealing, which will change the DIBSQ donor and C₆₀ acceptor interface from PHJ to hybrid planar-mixed heterojunction. This new donor–acceptor heterojunction structure will significantly improve the charge separation and charge collection efficiency, as well as the open circuit voltage (V_oc) of the device, leading to an enhanced PCE. This work provides an effective strategy to improve the photovoltaic performance of SQ-based OSCs.