Adjustment of alkali element incorporations in Cu(In,Ga)Se2 thin films with wet chemistry Mo oxide as a hosting reservoir

•Low cost non-vacuum H2O2 soaking is employed to modify the Mo surface morphology and oxygen content.•Mo oxide layer as a reservoir can adjust alkali element contents in Cu(in,Ga)Se2 films.•Narrowing down of grain boundaries by Mo oxidation suppress sodium excess incorporation.•The enrichment effect on K by MoO3 leads to increased K content in CIGS absorber.•Properties of Cu(In,Ga)Se2 films have been improved with the proper sodium content.

Thin film Cu(In,Ga)Se2 solar cells remain one of the most promising devices for commercial photovoltaic panels. Aimed at high quality absorber film, alkali elements such as sodium and potassium incorporation can improve the crystal structure and electrical properties of the Cu(In,Ga)Se2 film. However, the precise control on sodium content as well as the incorporation mechanisms is yet comprehensively understood. We propose a facile wet chemistry oxidation approach for large area homogeneous Mo oxidation and allow for uniform sodium incorporation in a controllable manner. Eventually the oxidized Mo substrate in an optimal condition leads to a relative increase of 8% of conversion efficiency in the final solar device compared with the non-oxidized sample. We thoroughly examine the surface chemistry of the Mo substrate with X-ray photoelectron spectroscopy after a systematic H2O2 oxidation treatment. A qualitative analysis show that the predominant Mo trioxides as a reservoir can adjust alkali elements contents in Cu(in,Ga)Se2 films. Upon oxidation, the narrowing down of Mo grain boundaries has now suppressed the over sufficient sodium incorporation compared to uncontrolled diffusion from soda lime glass. Also, the emergence of potassium element in the thin film improves the electrical properties. Thermodynamic calculation indicate that the K2MoO4 as an intermediate from K2O and MoO3 reaction is a preferable reaction path compared the Na2O reaction and thus enhanced potassium ion diffusion. We have correlate the sodium contents by simply adjusting the H2O2 treatment time to improved Cu(InGa)Se2 film qualities, e.g., crystal structures, grain sizes, charge carrier concentrations and defect densities.

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