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. (C) 2016 Elsevier B.V. All rights reserved.

The influence of the duration of the KCN etching step on the efficiency of Cu2ZnSnSe4 (CZTSe) solar cells and Post deposition annealing (PDA) has been explored. CZTSe thin film absorbers prepared by selenization at 450 degrees C were etched by 5 wt% KCN/KOH from 30s up to 360 s before solar cell processing. KCN etching times above 120 s resulted in poor efficiencies. The fill factor (FF) and short circuit current density Jsc) of these devices were affected severely. After annealing the solar cells at 200 degrees C in N-2 atmosphere the best devices degraded and poor devices improved. Combined physical and optoelectronic characterization of the solar cells showed that PDA modifies the bulk defect density and also surface composition which reflects in the solar cell performance. (C) 2016 Elsevier B.V. All rights reserved.

In this contribution, we report on the impact of ammonium sulfide ((NH4)(2)S-x) chemical treatments done at room temperature on the surface properties of Cu2ZnSnSe4 (CZTSe) thin film. The first approach is based on the immersion of the absorber layer in a (NH4)(2)S-x solution (20 wt.%). This method results in the preferential etching of Se, Sn and Zn atoms from the absorber surface. After a treatment time of 1 min, the performances of the solar cells are found to be improved. The second approach consists in the exposure of the CZTSe layer to ammonium sulfide vapors. In this case, it was found that the progressive sulfurization of the surface of the absorber leads to the decomposition of the CZTSe phase into Sn(S,Se)(x) and Se-0 secondary phases. As a consequence, the short circuit current of the corresponding solar cells is reduced. (C) 2016 Elsevier B.V. All rights reserved.

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 mu m thickness in which a low doping density of the order of 10(15) cm(-3) was measured, leading to a wide space charge region of about 300 nm. (C) 2016 Elsevier B.V. All rights reserved.