High absorption coefficients of the CuSb(Se,Te)2 and CuBi(S,Se)2 alloys enable high-efficient 100 nm thin-film photovoltaics

Rongzhen Chen,Clas Persson,J. F. Guillemoles

doi:10.1051/epjpv/2017005

Rongzhen Chen, Clas Persson + Show 1 more

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https://doi.org/10.1051/epjpv/2017005

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Abstract

We demonstrate that the band-gap energies Eg of CuSb(Se,Te)2 and CuBi(S,Se)2 can be optimized for high energy conversion in very thin photovoltaic devices, and that the alloys then exhibit excellent optical properties, especially for tellurium rich CuSb(Se1−xTex)2. This is explained by multi-valley band structure with flat energy dispersions, mainly due to the localized character of the Sb/Bi p-like conduction band states. Still the effective electron mass is reasonable small: mc ≈ 0.25m0 for CuSbTe2. The absorption coefficient α(ω) for CuSb(Se1−xTex)2 is at ħω = Eg + 1 eV as much as 5–7 times larger than α(ω) for traditional thin-film absorber materials. Auger recombination does limit the efficiency if the carrier concentration becomes too high, and this effect needs to be suppressed. However with high absorptivity, the alloys can be utilized for extremely thin inorganic solar cells with the maximum efficiency ηmax ≈ 25% even for film thicknesses d ≈ 50 − 150 nm, and the efficiency increases to ∼30% if the Auger effect is diminished.

Highlights

No experimental gap is available for CuBiSe2, but the calculated gap energy is ∼1.13 eV [4, 11]
The conversion efficiency is around 3% for devices based on CuSbS2 or CuSbSe2 [15, 16], so further investigations of the materials are required as well as development of the devices
The calculations of the electronic structure and the optical properties were based on a ground-state density functional theory (DFT), employing the projector augmented wave basis set of the VASP package [17,18,19] and a hybrid functional (HSE06) [20]

Summary

Introduction

To the rather flat lowest conduction band (CB) [4, 5, 11]. The experimental values of the band-gaps energies are ∼1.5 eV [12], ∼1.65 eV [13], and ∼1.09 eV [14] for CuSbS2, CuBiS2, and CuSbSe2, respectively. No experimental gap is available for CuBiSe2, but the calculated gap energy is ∼1.13 eV [4, 11]. The conversion efficiency is around 3% for devices based on CuSbS2 or CuSbSe2 [15, 16], so further investigations of the materials are required as well as development of the devices

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Results

Conclusion