This work proposes gallium oxide grown by plasma‐enhanced atomic layer deposition, as a surface passivation material at the CdS buffer interface of Cu(In,Ga)Se2 (CIGS) solar cells. In preliminary experiments, a metal‐insulator‐semiconductor (MIS) structure is used to compare aluminium oxide, gallium oxide, and hafnium oxide as passivation layers at the CIGS‐CdS interface. The findings suggest that gallium oxide on CIGS may show a density of positive charges and qualitatively, the least interface trap density. Subsequent solar cell results with an estimated 0.5 nm passivation layer show an substantial absolute improvement of 56 mV in open‐circuit voltage (VOC), 1 mA cm−2 in short‐circuit current density (JSC), and 2.6% in overall efficiency as compared to a reference (with the reference showing 8.5% under AM 1.5G).

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.

Copper indium gallium selenide/sulfide (CIGS) and copper zinc tin selenide/sulfide (CZTS) are two thin film photovoltaic materials with many similar properties. Therefore, three new processing steps – which are well-known to be beneficial for CIGS solar cell processing – are developed, optimized and implemented in CZTS solar cells. For all these novel processing steps an increase in minority carrier lifetime and cell conversion efficiency is measured, as compared to standard CZTS processing. The scientific explanation of these effects is very similar to its CIGS equivalent: the incorporation of alkali metals, ammonium sulfide surface cleaning, and Al2O3 surface passivation leads to electrical enhancement of the CZTS bulk, front surface and reduced front interface recombination, respectively.