Yttrium doped CuInSe₂ solar cells: higher conversion efficiency and promising candidate for top cell of tandem cells

To explore material solutions for improving the power conversion efficiency (PCE) of copper based thin film solar cells, we systematically study the CuInSe₂ doped by Y element containing shell-unfilled 4d electrons. The crystal and electronic structures are investigated by the first-principles calculations and the device performance with Y-doped CuInSe₂ as absorber is simulated by employing SCAPS-1D software. And the matching between different buffer layers and absorber is also examined. The result shows that the substitution of Y for In can increase the band gap and make the valence bands flatting, which increases the density of states at Fermi level and effective masses of electrons and holes while decreases the carrier mobility. Two doping cases with the band gap of 1.262 and 1.465 eV are considered promising candidates for the absorber and exhibit the PCE higher than 23.6% of Cu(In,Ga)(S,Se)₂. CdS, SnS₂, ZnMgO, and Zn(S,O) can all serve as buffer layers for Y-doped CuInSe₂ devices and the impact of their thickness on conversion efficiency is analyzed. From the perspective of achieving the same efficiency, the thickness of SnS₂ buffer layer can be reduced to 30 nm. The highest conversion efficiency of device based on Cu(In,Y)Se₂ absorber with the band gap of 1.465 eV reaches 25.32%, which demonstrates excellent performance of broadband gap photovoltaic cells using as a top cell of tandem cells. The results suggest that, by doping yttrium and adjusting thickness of buffer and absorber layers, the conversion efficiency of copper based thin film solar cells is able to further be improved.