Effect of Sn/Zn/Cu precursor stack thickness on two-step processed kesterite solar cells

We have fabricated Cu₂ZnSnSe₄ (CZTSe) solar cells with different absorber layer thickness. Absorber layers with different thicknesses were fabricated by changing the thickness of e-beam evaporated Sn/Zn/Cu precursor stacks and then selenization in a rapid thermal processing system. Scanning electron microscopy revealed that by increasing the thickness the morphology of CZTSe films improves substantially and energy dispersive spectrometry measurements showed that the Cu to Sn ratio increased with increasing the film thickness, despite a similar Cu to Sn ratio in the starting layers. A longer minority carrier lifetime and higher open circuit voltage were achieved for solar cells with thicker absorber layers. A maximum conversion efficiency of 7.8% (without anti reflection coating) was achieved for a solar cell with 1.7 μm thickness in which a low doping density of the order of 10¹⁵ cm^− 3 was measured, leading to a wide space charge region of about 300 nm.