  Ternary chalcostibite copper antimony selenide (CuSbSe2) is a promising
absorber material for next generation thin film solar cells due to the
non-toxic nature, earth-abundance, low-cost fabrication technique, optimum
bandgap and high optical absorption coefficient of CuSbSe2. Conventional single
heterojunction CuSbSe2 solar cells suffer from high recombination rate at the
interfaces and the presence of a Schottky barrier at the back contact, which
limit their power conversion efficiencies (PCEs). In this study, we propose a
dual-heterojunction n-ZnSe/p-CuSbSe2/p+-CGS solar cell, having copper gallium
selenide (CGS) as the back surface field (BSF) layer. The BSF layer absorbs
longer wavelength photons through a tail-states-assisted (TSA) two-step
upconversion process, leading to enhanced conversion efficiency. Numerical
simulations were carried out using SCAPS-1D to investigate the performance of
the proposed solar cell with respect to absorber layer thickness, doping
concentrations and defect densities. The simulation results exhibit PCE as high
as 43.77% for the dual-heterojunction solar cell as compared to 27.74% for the
single heterojunction n-ZnSe/p-CuSbSe2 counterpart. The dual-heterojunction
structure has, therefore, the potential to approach the Shockley-Queisser (SQ)
detailed balance limit and can lead to extremely high PCEs in emerging thin
film solar cells.
