Effect of different alkali (Li, Na, K, Rb, Cs) metals on Cu₂ZnSnSe₄ solar cells

It is well established that the addition of sodium (Na) to chalcopyrite or kesterite based solar cells markedly increases the solar cell performance. In this work, we explore the effect of Na and other alkali metals like potassium (K), rubidium, caesium and lithium (Li) – on pure selenide Cu₂ZnSnSe₄ (CZTSe) solar cells. We demonstrate the deposition of alkali metals using spin coating on e-beam evaporated metal precursors. The stack of metal precursors with alkali layer was then selenised at high temperatures to obtain a good quality CZTSe absorber. The diffusion of alkali metals into the absorber layer was confirmed using glow discharge optical emission spectroscopy. Samples doped with Na or K have shown improvement in the open circuit voltage. A maximum power conversion efficiency of 8.3% (without anti-reflection coating) and a top open circuit voltage 430 mV was achieved for combination of K and Na. Amongst the rest of alkali metals, Li looks the most promising dopant as far as optoelectronic properties are concerned. - Highlights: • Effect of Li, Na, K, Rb and Cs doping investigated • Spin coating employed to deposit alkali elements on evaporated metal precursor stack. • Doping the samples with alkali metals improves minority carrier lifetime. • Combination of Na and K showed efficiency of 8.3% (without ARC) and V_oc of 430 mV. • Li is most promising dopant apart from Na and K in terms of optoelectronic properties.