Impact of highly crystalline, isoindigo-based small-molecular additives for enhancing the performance of all-polymer solar cells

We have developed a simple yet versatile approach for enhancing the performance of all-polymer solar cells (all-PSCs) using a highly crystalline small-molecular additive, 6,6 '-dithiopheneisoindigo (DTI). The DTI additive in a blend of PTB7-Th donor and P(NDI2HD-T) acceptor enhances the power conversion efficiency of all-PSCs from 5.9 to 6.8%. Based on the analyses of the electrical, optical, and structural properties of all-PSCs, it is suggested that DTI additives promote tighter pi-pi packing and larger sizes of crystalline domains. Such morphological changes of the PTB7-Th: P(NDI2HD-T) blend films upon the addition of DTI enhance the exciton lifetime and diffusion length, which are proved by static and femtosecond-transient absorption spectroscopies and time-resolved photoluminescence. As a result of improved exciton dissociation probability and charge transport, the short-circuit current density of all-PSCs is greatly increased. Importantly, we also show that the DTI additive can be applied to other all-PSC systems with different polymer donors and naphthalene diimide (NDI)-based polymer acceptors, where the efficiencies of all-PSCs are enhanced by 10-20%.