Surface Modification of Al-Doped ZnO Transparent Conducive Thin Films with Polycrystalline Zinc Molybdenum Oxide

High-work function (WE) transparent conductive thin films improve the performance of solar cells and organic light-emitting diodes by facilitating interfacial charge carrier transport. Al-doped ZnO (AZO) becomes a very promising transparent conductive material because of nontoxicity, abundant material resources, and low cost. To increase the WF of AZO without enhancing the series resistance of the device, a high-WF and low-resistance surface modifier of polycrystalline zinc molybdenum oxide (ZMO) was developed by utilizing thermal evaporation of MoO3 on the surface of AZO and a subsequent two-step annealing treatment. The first step of air annealing causes the formation of monoclinic ZnMoO4 nanocrystals in the ZMO modifier. This improves the WF of AZO from 3.83 to 4.86 eV by increasing the group electronegativity and cation oxidation state. Furthermore, the second step of N-2 annealing decreases the resistivity of the polycrystalline ZMO by increasing the donor states of oxygen vacancies. The surface modification effect is verified by applying the ZMO-modified AZO to the front electrode of hydrogenated amorphous silicon thinfilm solar cells. The low-resistance polycrystalline ZMO modifier not only increases light harvesting in the solar cells by improving interfacial refractive index matching but also improves the open-circuit voltage by modifying the interfacial band alignment. In particular, the modifier increases the fill factor by ca. 13% by reducing the series resistance of the device. These enable a gain of ca. 23% in photoelectric conversion efficiency compared to the unmodified AZO. The results suggest the feasibility to tune the WF and conductivity of a material independently.