Effect of side chain modification on edge-on oriented dithienobenzodithiophene-based non-fullerene acceptors for organic solar cells

Two non-fullerene acceptors (NFAs), DTBDT-ICN and DTBDT-SEH, based on dithienobenzodithiophene (DTBDT) and a 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (IC) with different side chains of alkylthienyl and alkylthio-thienyl, respectively, were designed and used as electron acceptors in organic solar cells (OSCs). Both NFAs provide suitable energy level configurations that ensure efficient charge transfer with the donor polymer PBDB-T, as confirmed by significant photoluminescence reduction in the blend films. However, due to the high planarity together with strong π-π stacking interactions, the DTBDT-ICN presented significant aggregation and phase separation in the blend films, leading to suboptimal charge generation. In addition, grazing incidence wide-angle x-ray scattering measurements revealed a predominance of edge-on molecular orientations, which are unfavorable for vertical charge transport. On the other hand, DTBDT-SEH exhibited less pronounced molecular aggregation and edge-on orientation properties compared to DTBDT-ICN, resulting in improved carrier mobility (μ_e of 3.86 × 10⁻⁶ compared to 7.59 × 10⁻⁷) and mitigated recombination losses (1.19 kT/q compared to 1.21 kT/q) in OSC devices. The improved morphological features of PBDB-T:DTBDT-SEH led to a high power conversion efficiency of 3.31%, which is three times higher than that of PBDB-T:DTBDT-ICN-based devices (1.55%). Furthermore, paired with the high performance polymer PM6, PM6:DTBDT-SEH demonstrated an enhanced efficiency, reaching 7.03%.