Oligomeric donor with appropriate crystallinity for organic solar cells

Abstract

Improving the performance of all-small-molecule organic solar cells (ASM-OSCs) largely depends on the design and application of novel donors with appropriate crystallinity. Extending molecular conjugation is an effective method for regulating molecular stacking and crystallinity. In this work, we successfully designed and synthesized two novel acceptor-donor-donor-donor-acceptor (A-D-D-D-A) type oligomeric donors with three dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b']dithiophene (DTBDT) as the central unit, named as 3DTBDT-Cl and 3DTBDT, depending on with and without chlorine substitution on the thiophene side chains. We found that the introduction of chlorine atoms makes the blend films display stronger crystallinity but with large-scale phase separation morphology and more defects, which eventually leads to a power conversion efficiency (PCE) of only 10.83%, whereas the blend films based 3DTBDT with appropriate crystallinity achieved 13.74% PCE. Compared with 3DTBDT-Cl/L8-BO, the 3DTBDT/L8-BO films exhibited a nanoscale bi-continuous interpenetrating network morphology with a smaller domain size and more suitable crystallinity, which guarantees the corresponding devices obtained more efficient exciton dissociation, efficient charge transport, reduced bimolecular recombination, and performed more balanced carrier mobility. These results demonstrated that regulating the crystallinity of oligomeric donors to obtain the desired phase separation morphology in the blend films could facilitate further improving the performance of ASM-OSCs.