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Role of nanoscale compositional inhomogeneities in limiting the open circuit voltage in Cu(In,Ga)S2 solar cells

Published version
Peer-reviewed

Repository DOI


Type

Article

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Abstract

jats:pAs Si-based solar cell technologies approach their theoretical efficiency limits, alternative photovoltaic systems, such as tandem solar cells, are gathering increased attention due to their potential to reach higher efficiencies by better use of the solar spectrum. Cu(In,Ga)S2 (CIGS) is a promising material for the top cell due to its large, tunable bandgap energy (Eg), stability, and already established high efficiencies. However, the deficit in open circuit voltage is still large; therefore, an improved understanding of the efficiency losses is required. Scanning electron microscopy cathodoluminescence was used to study the role of the polycrystalline nature for radiative recombination in CIGS samples of varying Cu-content. Considerable differences between neighboring grains were observed in the emission energy and the emission intensity, with significant drops in emission energy at the grain boundaries. Lateral homogeneity in the near band edge (NBE) energy was found to reduce for samples with Cu-poor compositions, with its standard deviation halving (σNBE ∼ 20 meV) compared to the more stoichiometric films (σNBE ∼ 50 meV), which corresponds to an open circuit voltage loss contribution that is nearly an order of magnitude lower. Such inhomogeneities can be attributed mainly to local variations of the Ga concentration. Hence, the differences between the samples could be explained by the different deposition times at elevated temperature allowing for different extents of homogeneity. Thus, Cu-poor films are not only favorable because of lower concentrations of deep defects but also because of reduced bandgap variations.</jats:p>

Description

Keywords

40 Engineering, 4016 Materials Engineering, 51 Physical Sciences, 7 Affordable and Clean Energy

Journal Title

APL Energy

Conference Name

Journal ISSN

2770-9000
2770-9000

Volume Title

1

Publisher

AIP Publishing
Sponsorship
Engineering and Physical Sciences Research Council (EP/R025193/1)
EPSRC (EP/V029231/1)
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