Band offsets of n-type electron-selective contacts on cuprous oxide (Cu2O) for photovoltaics

Abstract

The development of cuprous oxide (Cu2O) photovoltaics (PVs) is limited by low device open-circuit voltages. A strong contributing factor to this underperformance is the conduction-band offset between Cu2O and its n-type heterojunction partner or electron-selective contact. In the present work, a broad range of possible n-type materials is surveyed, including ZnO, ZnS, Zn(O,S), (Mg,Zn)O, TiO2, CdS, and Ga2O3. Band offsets are determined through X-ray photoelectron spectroscopy and optical bandgap measurements. A majority of these materials is identified as having a negative conduction-band offset with respect to Cu2O; the detrimental impact of this on open-circuit voltage (VOC) is evaluated through 1-D device simulation. These results suggest that doping density of the n-type material is important as well, and that a poorly optimized heterojunction can easily mask changes in bulk minority carrier lifetime. Promising heterojunction candidates identified here include Zn(O,S) with [S]/[Zn] ratios >70%, and Ga2O3, which both demonstrate slightly positive conduction-band offsets and high VOC potential. This experimental protocol and modeling may be generalized to evaluate the efficiency potential of candidate heterojunction partners for other PV absorbers, and the materials identified herein may be promising for other absorbers with low electron affinities.

Improved Heterojunction Quality in Cu₂O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited <br />Zn_1-xMg_xO

“…While no grey film could be detected visually on the etched substrate, some CuO was still present on its surface, as the PDS measurements suggests. The presence of a very thin CuO film on the surface of Cu 2 O substrates was also confirmed with x-ray photoelectron spectroscopy (XPS) 19,25 . Cupric oxide present on the Cu 2 O surface introduces deep level trap states (Cu 2+ ) 18 at the heterojunction interface that can act as recombination centers and, therefore, CuO presence at the p-n junction is undesirable.…”

“…Incorporating Mg into ZnO allows the conduction band to be tuned, which is important for reducing losses due to band-tail thermalization 13 and interfacial recombination. 18,19 Here we show how tuning the conditions for depositing zinc oxide and zinc magnesium oxide films on thermally oxidized cuprous oxide substrates allowed for improved interface quality and hence better solar cell performance to be obtained. This improvement was made possible through the identification of the major limiting factor in Cu 2 O based solar cells: recombination at the heterojunction interface due to an excessive formation of cupric oxide (CuO) on the Cu 2 O surface.…”

Piezopotential‐Induced Schottky Behavior of Zn1−xSnO3 Nanowire Arrays and Piezophotocatalytic Applications

“…UV‐vis spectrometry was employed for measuring the band gap of the sample by using the Tauc equation:where α is the absorption coefficient; h represents Plank's constant; ν is the irradiation frequency; A denotes the proportionality constant; E g is the band gap; and m represents a constant (0.5 for a direct band gap, and 2 for an indirect band gap). ZTO is a direct band gap semiconductor; m = 0.5 was applied.…”

Tandem Solar Cells Based on Cu2O and c-Si Subcells in Parallel Configuration: Numerical Simulation

“…A four-terminal device has the subcells electrically decoupled and independently controllable that can be connected in parallel, ensuring a maximum output power at all times [18]. The tandem efficiency is estimated to reach values above 30% [1,2,6,19]. A possible four-terminal configuration is presented in Figure 1 [20].…”