Atomic Layer Deposition of ZnO and Doped ZnO As Alternative Transparent Conducting Oxides for Photovoltaics

Organometallic halide perovskite solar cells have gained considerable interest in recent times, with low cost and high power conversion efficiencies [1]. However, such cells are not without problems such as, stability, scalability and the need for a low thermal budget in the fabrication of tandem structures. In this work, we examine the transparent conducting electrode where the current materials of choice are FTO and ITO which require a higher process temperature (>450oC).

Zinc oxide thin films have attracted significant attention as transparent conducting oxides due to their diverse electrical and optical properties. ZnO is an n-type semiconductor with a direct band gap of 3.3 eV. It is optically transparent in the UV/visible and infrared spectra and exhibits promising electrical properties.

ALD has distinct characteristics that make it a prospective deposition method of ZnO and doped ZnO, low temperature, conformal and large surface area coating capabilities. The deposition of ZnO and doped ZnO has previously been reported. In this work, an ALD process has been developed to deposit ZnO as a potential TCO in solar cells.

ZnO was deposited using low temperature atomic layer deposition (ALD). Subsequently, zinc oxide was doped with Al, Ti and Hf and the electrical, optical and structural properties were analyzed to determine the effect of doping on ZnO. The presented results revealed that Ti-doped ZnO was the suitable TCO candidate with a sheet resistance value of 41 Ω/□ and electron mobility of 12.8 cm2/Vs. In addition to promising electrical properties, Ti-doped ZnO also exhibited high optical transparency of >80% in the UV/ visible and IR ranges. We demonstrate that ALD of Ti-doped ZnO could be a viable alternative to ITO and FTO for TCO materials in photovoltaics. Preliminary data from perovskite solar cells fabricated with a ZnO based transparent conducting electrode are presented and discussed.