(a) The absorption coefficients α(ω) of CuSbSe₂, CuSbSeTe, CuSbTe₂, CuBiS₂, CuBiSSe, and CuBiSe₂ on the energy scale ( and with a 0.30 eV Lorentzian broadening. These compounds exhibit much stronger near-gap-energy absorption compared to the traditional absorbers GaAs (E_g~1.41 eV with HSE06) and CIGSe (50% alloying: E_g~1.30 eV). (b) The corresponding maximum solar energy efficiency η_max demonstrates high efficiency of CuSb(Se,Te)₂ and CuBi(S,Se)₂ alloys for very thin films of d< 400 nm (see also Tab. 1).