Organophosphorus ligand-coordinated ZnO cathode buffer layers in organic solar cells—Work function reduction and performance enhancement

Functionalization of the ZnO cathode buffer layer (CBL) has been considered as a promising strategy to improve the performance of organic solar cells (OSCs). In the present study, three tricoordinated organophosphorus ligands, triphenylphosphine (TPP), methyldiphenylphosphine (MDPP) and dimethylphenylphosphine (DMPP), were successfully employed to functionalize ZnO CBL in OSCs based on the PTB7:PC70BM active layer. The effect of substituents on the device performance was systematically studied. Density functional theory calculations confirmed the formation of a strong Zn–P bond with adsorption energies of −1.85, −1.97, and −2.09 eV for TPP, MDPP, and DMPP, respectively. The electron transfer from the molecules to ZnO has significantly reduced the work function (WF) of CBL. The ZnO/DMPP CBL exhibited the lowest WF, which was attributed to the positive inductive effect of methyl groups. The OSCs with functionalized ZnO CBL exhibited improved exciton dissociation and charge collection efficiencies due to reduced bimolecular and trap-assisted recombination, as revealed by intensity-dependent J–V and impedance measurements. As a result, the devices fabricated with the functionalized ZnO as CBL exhibited superior photovoltaic performance, with ZnO/DMPP showing the highest power conversion efficiency of 7.24%, followed by ZnO/MDPP (6.91%) and ZnO/TPP (6.73%) compared to the reference device (6.29%).