Control of the preferred orientations of Cu(In,Ga)Se₂ films and the photovoltaic conversion efficiency using a surface-functionalized molybdenum back contact

The surface microstructures of molybdenum (Mo) back contacts were shown to play a crucial role in the preferred orientations of Cu(In,Ga)Se₂ (CIGS) films. The lower surface density of Mo tends to drive the growth of CIGS films toward favoring a (220)/(204) orientation, attributed to the higher likelihood of a MoSe₂ reaction. This work showed that the presence of a very thin layer on a Mo bilayer facilitated the tuning of the CIGS grain orientations from strongly favoring (112) to strongly favoring (220)/(204) without sacrificing the electrode conductivity. The efficiency of Na-doped CIGS cells was increased toward decreasing Mo surface density, that is, increasing (220)/(204) CIGS orientation. Although slight changes in Na doping found between different Mo surface properties could contribute in part, the comparison with Na-reduced CIGS cells showed that it was more likely due to the (220)/(204) orientation-related enhancement of CdS/CIGS junction characteristics, which were possibly attributed to a favorable CdS reaction and a reduction in the defect metastabilities.