Abstract
Heterojunction Cu2O solar cells are an important class of Earth-abundant photovoltaics that can be synthesized by a variety of techniques, including electrochemical deposition (ECD) and thermal oxidation (TO). The latter gives the most efficient solar cells of up to 8.1% reported in the literature, but is limited by low external quantum efficiencies (EQE) in the long wavelength range (490–600 nm). By contrast, ECD Cu2O gives higher short wavelength EQEs of up to 90%. We elucidate the cause of this difference by characterizing and comparing ECD and TO films using impedance spectroscopy and fitting with a lumped circuit model to determine the trap density, followed by simulations. The data indicates that TO Cu2O has a higher density of interface defects, located approximately 0.5 eV above the valence band maximum (NV), and lower bulk defect density thus explaining the lower short wavelength EQEs and higher long wavelength EQEs. This work shows that a route to further efficiency increases of TO Cu2O is to reduce the density of interface defect states.
Highlights
Heterojunction solar cells with p-type Cu2O are appealing because they are non-toxic, composed of Earth-abundant elements, and can be synthesized by a variety of techniques [1,2,3,4,5,6]
For the Cu2O/Zn0.8Mg0.2O HJ in the current study, the circuit is comprised of two types of junctions: (1) the p-n junction and (2) two metal-semiconductor junctions, Ag/ITO/Al doped ZnO (AZO)/Zn0.8Mg0.2O and Cu2O/Au
We have analyzed defects in Cu2O made by thermal oxidation (TO) and electrochemical deposition (ECD) by developing a lumped circuit model in impedance spectroscopy measurements
Summary
Heterojunction solar cells with p-type Cu2O (with a direct forbidden bandgap of 2.1 eV) are appealing because they are non-toxic, composed of Earth-abundant elements, and can be synthesized by a variety of techniques [1,2,3,4,5,6] These techniques include electrochemical deposition (ECD) and thermal oxidation (TO) [6,7,8,9,10,11,12,13]. We have found AP-CVD to be highly advantageous for rapidly depositing pinhole-free, thin (10–200 nm) oxide buffer layers for both ECD and TO Cu2O solar cells [6,9] We characterized these devices by impedance spectroscopy and developed an equivalent lumped circuit model to analyze and compare differences in interfacial and bulk traps. We determine that further efficiency improvements to TO Cu2O hetero junctional solar cells could come about by improving the interface with less interface defect recombination
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