Fermi Energy Limitation at β-CuGaO₂ Interfaces Induced by Electrochemical Oxidation/Reduction of Cu

Abstract

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.

P-Type α-CuGaO2 Electronics: A Study on Gap States and Fermi Level Pinning and a Solution to Schottky to Ohmic Transition for Electronic Applications

“…Formation of small polarons at the interface manifests itself as an internal oxidation process of Cu + to Cu 2+ . Suzuki et al have investigated experimentally the oxidation of Cu + to Cu 2+ at the CGO interface . In this process, a free hole (which already exists in the p-type CGO material) is trapped by Cu + at the interface and forms Cu 2+ .…”

“…Suzuki et al have investigated experimentally the oxidation of Cu + to Cu 2+ at the CGO interface. 25 In this process, a free hole (which already exists in the p-type CGO material) is trapped by Cu + at the interface and forms Cu 2+ . In other words, we have a bounded hole at the interface that cannot move and therefore creates a surface charge density.…”

“…By investigating the XPS of α-CGO and β-CGO, Suzuki et al have examined the Fermi level restrictions for the polymorphs of this material in detail. 25 By deposition of low work function materials such as ZnO and high work function materials such as RuO 2 and Co 3 O 4 on CGO, they have concluded that the highest and lowest positions of the Fermi level for α-CGO are 0.9 and 0.25 eV above the valence band edge, respectively, and the Fermi level is partially pinned. They reported that the cause of this FLP was the internal oxidation of Cu + to Cu 2+ ions on the surface.…”

Investigating various metal contacts for p-type delafossite α-CuGaO2 to fabricate ultraviolet photodetector

“…The phase β, which has a wurtzite structure, is composed of vertex-sharing GaO 4 and CuO 4 tetrahedra and demonstrates a 1.47 eV bandgap 1 . Suzuki et al, point out that β-CGO is an appropriate option for manufacturing solar cells due to the high absorption coefficient and suitable direct bandgap 15 . CGO in α phase has a delafossite structure with symmetry, in which the Cu atoms form a linear arrangement with O as O–Cu–O, while the Ga atoms create edge-sharing octahedra with O atoms.…”

Avoiding Fermi Level Pinning at the SnS Interface for High Open-Circuit Voltage

“…18 This mechanism is very effective in constraining the Fermi energy within a material, as the respective concentration of the defects is equal to that of the atoms. The oxidation/reduction of Cu has recently been demonstrated to limit Fermi shifts in β-CuGaO 2 to ∼0.8 eV, 19 which is significantly smaller than its band gap of 1.5 eV. Additionally, the limitations of Fermi-level shifts have been observed in Cu(In,Ga)Se 2 .…”