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

¹ Department of Materials Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden
² Centre for Materials Science and Nanotechnology, University of Oslo, P.O. Box 1048 Blindern, 0316 Oslo, Norway
³ Department of Physics, University of Oslo, P.O. Box 1048 Blindern, 0316 Oslo, Norway

^a e-mail: Rongzhen.Chen@gmail.com

Received: 29 January 2017
Accepted: 4 May 2017
Published online: 16 June 2017

Abstract

We demonstrate that the band-gap energies E_g 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(Se_1−xTe_x)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: m_c ≈ 0.25m₀ for CuSbTe₂. The absorption coefficient α(ω) for CuSb(Se_1−xTe_x)₂ is at ħω = E_g + 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 η_max ≈ 25% even for film thicknesses d ≈ 50 − 150 nm, and the efficiency increases to ∼30% if the Auger effect is diminished.