Researchers at Loughborough University in the United Kingdom have designed a cadmium telluride (CdTe) solar cell with a buffer layer made of nickel oxide (NiO) deposited without oxygen, which reportedly improves considerably the device's open-circuit voltage.
They explained that CdTe cells generally suffer from low open-circuit voltage levels, as these devices rely on low minority carrier lifetime, low carrier density and non-ohmic back contacts. To solve this issue, they used NiO, which is an efficient electron reflector due to the large conduction band offset, as a back buffer layer to form an ohmic back contact.
“Optical transmission and band gaps are affected by oxygen input, typically reducing in both aspects with increasing oxygen,” the scientists explained. “NiO begins to exhibit structural changes when introducing oxygen, forming nickel vacancies. These characteristic changes suggest that there is a trade-off point that should be studied with regards to its use as a back buffer layer for CdTe solar cells.”
The research group used the SCAPS-1D solar cell capacitance software, developed by the University of Ghent, to simulate a novel CdTe cell configuration. The 0.25 cm2 device was based on a substrate made of glass and fluorine-doped tin oxide (FTO), a tin oxide (SnO2) electron transport layer (ETL), a CdTe absorber, the Nio buffer layer with a thickness of 100 nm, and gold (Au) metal contacts.
In the proposed cell architecture, the NiO back buffer layer increases device efficiency by reducing the barrier height at the Au back contact and improving the valence band offset at the CdTe/NiO interface.
“The NiO layer also leads to a large conduction band offset at the CdTe/NiO interface, therefore forming an efficient electron reflector to reduce interface recombination and increase open-circuit voltage,” the team explained, noting that the conduction band offset between NiO and CdTe is large.
“Where between 0–20% oxygen, the value lies between 2.65 eV and 2.86 eV and between 40–60% it is reduced to 2.18 eV and 2.21 eV,” the academics further explained. “It increases again at 80% to 2.62 eV. The large conduction band offset would directly improve the open-circuit voltage by being efficient electron reflectors, which prevents accumulation of electrons at the interface and reduces the interface recombination.”
The new solar cell concept was introduced in the study “The effect of oxygen on NiO as a back buffer layer in CdTe solar cells,” published in the Royal Society of Chemistry. Looking forward, the team said it plans to further improve the NiO layer by optimizing substrate temperature during sputtering.
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