Fermi Energy Limitation at β-CuGaO₂Interfaces Induced by Electrochemical Oxidation/Reduction of Cu

β-CuGaO2 possesses a direct band gap of 1.5 eV and is therefore expected as a suitable light absorber layer for thin-film solar cells. To evaluate the potential of β-CuGaO2 as a light absorber, the energy difference between the achievable highest and lowest Fermi energy of β-CuGaO2 and its polymorph α-CuGaO2 for comparison are investigated at the interfaces with a low work function material (ZnO) and high work function materials (RuO2 and Co3O4). X-ray photoelectron spectroscopic analysis indicates that the highest and lowest Fermi levels of β-CuGaO2 are intrinsically limited by pinning levels at ∼0.9 and ∼0.3 eV above the valence band maximum, respectively, which are almost consistent to those of α-CuGaO2. The most likely origins of these pinning levels are intrinsic defects in β- and α-CuGaO2 induced by an electrochemical oxidation/reduction of Cu. Such a narrow energy range in which the Fermi level of β-CuGaO2 can be tuned is unfavorable, and making full use of its band gap as an absorber layer of a solar cell is difficult. A way to solve this problem and increase the potential photovoltage of the solar cell involving β-CuGaO2 is discussed based on the results obtained in terms of the electronic structures.