Evidence and Influence of Copper Vacancies in p‑Type CuGaO₂ Mesoporous Films

Delafossite CuGaO₂ nanocrystals were hydrothermally synthesized and characterized spectroscopically and electrochemically as mesoporous thin films. The nanocrystals demonstrate a preferred orientation within the film structure, as shown by enhancement of the (00l) peaks via two-dimensional powder X-ray diffraction. Annealing conditions of low and high temperature (i.e., 100–300 °C), with oxygen and/or argon atmospheres, were investigated, and the resulting effect on the thin film electrochemistry was measured. Cyclic voltammetry showed an increase in non-faradaic current with higher annealing temperatures and demonstrated a quasi-reversible redox feature (E_1/2 = 0.1 V vs Fc^+1/0). This feature is assigned to a Cu^II/Cu^I redox couple associated with surface defects. X-ray photoelectron and energy dispersive spectroscopies provide evidence for Cu^II surface defects and copper vacancies. Electrochemical impedance spectroscopy revealed that CuGaO₂ films were highly conductive with σ ∼ 10^–5 Ω^–1 cm^–1, consistent with a large density of hole carriers induced by copper vacancies. The significance of synthesis, film preparation, and annealing conditions on the presence of surface defects and large hole densities is discussed. The prevalence of such defects in delafossite CuGaO₂ is expected to have a large impact on the use of this material as a hole transport layer in solar cell architectures.