Orientation tuning by modulating end group of small-molecule donors boosts efficiency of all-small-molecule organic solar cells

Abstract

In the active layer of all-small-molecule organic solar cells, the small molecule donors (SMDs) should ideally assume a face-on orientation in keeping with the non-fullerene acceptors. This well-matched interfacial molecular orientation in blends ensures heavy overlapping in the π-orbitals of donors and acceptors to further improve the manifestation of excitons and charges. However, there have been few studies on the effect of different end groups in the A-π-D-π-A-type structure. To explore this issue, we designed two f-DTBDT-based A-π-D-π-A-type-type SMDs, DR-3T and DE-3T, ending with different electron-withdrawing groups (rhodanine and 2-ethylhexyl cyanoacetate, respectively). To match the energy levels and absorption band of these two SMDs, Y6-BO was chosen as the acceptor in the blends. For DR-3T-based devices, space charge limited current measurement confirmed a low mobility in the vertical direction, which goes against restraining non-radiative recombination and energy loss, resulting in a low V_oc of 0.64 V and PCE of 5.20 % for the device. Compared to DR-3T, the film based on DE-3T showed the desired face-on orientation, which facilitated charge transport. As a result, the corresponding device delivered higher performance with PCE = 8.78 %, V_oc = 0.77 V, J_sc = 17.68 mA cm⁻², and FF = 64.50 %. This result demonstrates that the end group can significantly affect the orientation of the molecular backbone, further modulating the efficiency of organic solar cell devices.