No abstract

Being at the origin of an ohmic contact, the MoSe interfacial layer at the Mo/Cu(In,Ga)Se interface in CIGS (Cu(In,Ga)Se and related compounds) based solar cells has allowed for very high light-to-electricity conversion efficiencies up to 22.3%. This article gives new insights into the formation and the structural properties of this interfacial layer. Different selenization-steps of a Mo covered glass substrate prior to the CIGS deposition by co-evaporation led to MoSe interfacial layers with varying thickness and orientation, as observed by x-ray diffraction and atomic resolution transmission electron microscopy. A novel model based on the anisotropy of the Se diffusion coefficient in MoSe is proposed to explain the results. While the series resistance of finished CIGS solar cells is found to correlate with the MoSe orientation, the adhesion forces between the CIGS absorber layer and the Mo substrate stay constant. Their counter-intuitive non-correlation with the configuration of the MoSe interfacial layer is discussed and related to work from the literature.

We report on the influence of the substrate temperature during the 2nd and 3rd stage of the Cu(In,Ga)Se2 3-stage co-evaporation process on the in-depth Ga and In concentrations and correlate these with the solar cell parameters and external quantum efficiency of soda-lime glass/Mo/CIGS/CdS/i-ZnO/ZnO:Al devices. An increased homogenization of the [Ga]/[III] fraction ([III] refers to the total concentration of the group 3 elements Ga and In) with temperature is found. In the investigated temperature range, the highest efficiency was measured for the lowest temperature and the steepest Ga-profile. The tendency of the short-circuit current density matches well with the notch-deepness. Surprisingly, the open-circuit voltage decreases for higher substrate temperatures, even though the Ga-concentration in the space-charge region increases. We propose back-grading variations and reduced back-interface recombination to explain this observation. For the highest of the tested temperatures of 540 °C, a homogenization of the Ga and In concentrations close to the surface is found. We explain this by the appearance of a liquid Cu2-xSe phase at the end of stage 2 for this high temperature. Break-off experiments at this point are conducted and reveal morphological and compositional lateral inhomogeneities for Tsub < 540 °C.