Characterization of CIGS/Oxide interface passivation for Cu(In,Ga)Se2 solar cell applications

In this thesis, we characterize the passivation effects of three different dielectric materials (Al2O3, HfO2 and TiO2) at the interface with Cu(In,Ga)Se2 so as to determine their eligibility for the production of front passivated CIGS solar cells. Using simulations and experiments on Metal-Insulator-Semiconductor study structures, the effectiveness of these oxide passivation layers is both qualitatively and quantitatively described along with the influence of diverse parameters, mainly surface pre-cleaning and post-deposition annealing. In particular, we show that Al2O3 and HfO2 both exhibit effective chemical passivation with densities of interface traps Dit of the order of 10^11cm^−2eV^−1, on average slightly lower for Al2O3, and further reduced by annealing treatments at 300°C in N2 environment. On the contrary, we observe negative densities of fixed charges Qf of the order of 10^12cm^−2 in both materials, again moderately lower for Al2O3, which result in misdirected fieldeffect potentially limiting the overall improvements induced by passivation effects. The negative values found for Qf can be reduced by activating positive charges through similar annealing treatments as for chemical passivation. We highlight the suspected presence of other phenomena such as series resistance, secondary phases and complex defects whose behaviours are affected by CIGS surface pre-cleaning along with chemical passivation. The superposition of Al2O3 and HfO2 in a multi-layer passivation stack is investigated so as to solve the issue of processing compatibility in complete solar cells. For that innovative design, we show effective chemical passivation (Dit ~ 10^11cm^−2eV^−1) and negative Qf similarly to both individual materials. The application of Ammonia pre-cleaning and 300°C Nitrogen annealing on those samples lead to our optimal design for front interface passivation. Eventually, we discuss the need for further investigations and adapted analysis techniques taking into account the peculiar properties of TiO2 as compared to Al2O3 and HfO2. Our preliminary qualitative analysis suggests potential passivation effects of titania at the interface with CIGS.