Abstract
Reducing the absorber layer thickness below 1 μm for a regular copper indium gallium di-selenide (CIGS) solar cell lowers the minimum quality requirements for the absorber layer due to shorter electron diffusion length. Additionally, it reduces material costs and production time. Yet, having such a thin absorber reduces the cell efficiency significantly. This is due to incomplete light absorption and high Molybdenum/CIGS rear-surface recombination [1]. The aim of this research is to implement some innovative rear surface modifications on a 430 nm thick CIGS absorber layer to reduce both these affects: an aluminium oxide passivation layer to reduce the back-surface recombination and point contact openings using nano-particles for electrical contact. The impact of the implementation of all these rear-surface modifications on the opto-electrical properties of the CIGS solar cell will be discussed and analyzed in this paper.
Highlights
Power conversion efficiencies of about 22.9 % were obtained for small-area copper indium gallium di-selenide (CIGS) solar cells, they were achieved with absorbers thicker than 1 μm [2]
The substrate was immersed into the chemical bath deposition (CBD) solution after X minutes of reaction time i.e. the time taken for the formation of CdS nano-particles
Recently, power conversion efficiencies of about 22.9 % were obtained for small-area copper indium gallium di-selenide (CIGS) solar cells, they were achieved with absorbers thicker than 1 μm [2]
Summary
Power conversion efficiencies of about 22.9 % were obtained for small-area copper indium gallium di-selenide (CIGS) solar cells, they were achieved with absorbers thicker than 1 μm [2]. For sub-micrometer CIGS solar cells, issues surrounding a highly recombinative rear interface (the electron-hole pair is generated in the vicinity of the back contact) and incomplete absorption of incident solar spectrum (partly due to low reflection at the Molybdenum (Mo) /CIGS interface) limit its usage. The use of an Al2O3 layer is justified as 1) first principle calculations by Hsu et al [4] estimate a 35 % reduction in the interface trap density (Mo/CIGS) 2) a built-in field is created due to a high density of fixed negative charge which shields the minority charge carriers from getting recombined in the rear. By implementing an Al2O3 layer, the surface recombination velocity can be brought down to 100 cm/s (estimated from Solar Cell Capacitance Simulator [5]), leading to an enhanced VOC [5]
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