Influence of oxygen content on the properties of In₂(O_xS_1-x)₃ used as buffer material in Cu(In,Ga)Se₂ solar cells

We investigate magnetron-sputtered In-2(OxS1-x)(3) compounds acting as an alternative buffer system to the solution-grown CdS or Zn(O,S) buffer layers in Cu(In,Ga)Se-2 (CIGS) thin-film solar cells. The influence of the oxygen content on the solar cell performance, microstructure of the mixed systems, bandgap, and band offsets to CIGS is investigated experimentally and also characterized by calculations based on density functional theory. Samples in a series with different chemical compositions ranging from In2S3 to In2O3 are either directly deposited from ceramic targets or from a pure In2S3 target by reactive sputtering by adding O-2 in the Ar sputtering gas. The binary compounds In2S3 and In2O3 sputtered at 220 degrees C substrate temperature from ceramic targets exhibit a crystalline structure, whereas the ternary In-2(O,S)(3) compounds are either nanocrystalline in the case of In-2(O0.25S0.75)(3) or amorphous for In-2(O0.5S0.5)(3) and In-2(O0.75S0.25)(3). For [O]/([O] + [S]) ratios above 0.25, the cell efficiencies decrease drastically, mainly due to lower open-circuit voltages (V-OC). This behavior can be explained by an increase of the negative conduction band offset between the CIGS absorber and the oxygen-rich In-2(OxS1-x)(3) or In2O3 buffer, resulting in pronounced V-OC losses. Adding oxygen to In2S3 with optical bandgap energies of around 2 eV results in a bowing of the values to below 2 eV and finally reaching values of around 2.7 eV for In2O3 if an indirect band transition is assumed. In summary, our results reveal that pronounced oxygen incorporation in In2S3 is not beneficial in terms of CIGS device efficiency because oxygen is electronically inactive and poorly miscible.