Fine Tuning the Work\nFunction of ZnO Cathode Buffer\nLayers in Organic Solar Cells by Phenanthroline Coordination

Zinc oxide (ZnO) is widely used as a cathode buffer layer\n(CBL)\nin inverted organic solar cells (OSCs). Performance enhancement of\nOSCs by work function (WF) reduction of the ZnO CBL is a prominent\narea of research. Here, we report the role of three phenanthroline\nligands, 1,10-phenanthroline (Phen-A), 4,7-phenanthroline (Phen-B),\nand 1,7-phenanthroline (Phen-C), in reducing the WF of ZnO. Phen-A\nfunctionalized ZnO has the lowest WF, which can be attributed to the\neffective donation of nitrogen lone pairs to the Zn center thereby\neffectively raising the Fermi energy of the system. Significant improvements\nin efficiency and stability have been experimentally demonstrated\nby using functionalized ZnO thin films as the CBLs in PTB7:PC<sub>70</sub>BM-based OSCs. The X-ray photoelectron spectroscopy analysis\nrevealed the formation of a Zn–N bond and a significant reduction\nin oxygen deficiency defects due to the functionalization of the ZnO\nsurface with phenanthroline ligands. The density functional theory\nresults confirmed the formation of strong N–Zn bonding with\nadsorption energies −2.05, −1.77, and −1.33 eV\nfor Phen-A, Phen-B, and Phen-C, respectively. The improved interfacial\nproperties due to functionalization of the ZnO surface resulted in\n13.2, 7.8, and 6.7% enhancement in power conversion efficiency for\nPhen-A, Phen-B, and Phen-C, respectively.