Fine Tuning the Work Function of ZnO Cathode Buffer Layers in Organic Solar Cells by Phenanthroline Coordination

Zinc oxide (ZnO) is widely used as a cathode buffer layer (CBL) in inverted organic solar cells (OSCs). Performance enhancement of OSCs by work function (WF) reduction of the ZnO CBL is a prominent area of research. Here, we report the role of three phenanthroline ligands, 1,10-phenanthroline (Phen-A), 4,7-phenanthroline (Phen-B), and 1,7-phenanthroline (Phen-C), in reducing the WF of ZnO. Phen-A functionalized ZnO has the lowest WF, which can be attributed to the effective donation of nitrogen lone pairs to the Zn center thereby effectively raising the Fermi energy of the system. Significant improvements in efficiency and stability have been experimentally demonstrated by using functionalized ZnO thin films as the CBLs in PTB7:PC70BM-based OSCs. The X-ray photoelectron spectroscopy analysis revealed the formation of a Zn-N bond and a significant reduction in oxygen deficiency defects due to the functionalization of the ZnO surface with phenanthroline ligands. The density functional theory results confirmed the formation of strong N-Zn bonding with adsorption energies -2.05, -1.77, and -1.33 eV for Phen-A, Phen-B, and Phen-C, respectively. The improved interfacial properties due to functionalization of the ZnO surface resulted in 13.2, 7.8, and 6.7% enhancement in power conversion efficiency for Phen-A, Phen-B, and Phen-C, respectively.