Robert T. Nilsson

Examination of lifetime-limiting failure mechanisms in CIGSS-based PV minimodules under environmental stress

In our study, Shell Solar Industries (SSI) minimodules were subjected to dry heat (85°C), damp heat (85°C/100% RH), and anaerobic/aerobic 85°C water baths. After 168 hrs exposure to moisture-containing environments, the SSI power generation decreased by over 50% of that of the original state. Analytical characterization performed before and after the exposure identified degradation of the Al:ZnO and Mo layers as likely device failure routes. To elucidate the observed degradation mechanism, individual Al:ZnO and Mo films were sputtered onto borosilicate glass and exposed to both 85°C/100% RH and a room temperature water bath. After 24 hrs the resistivity and optical transmission of the Al:ZnO films increased significantly following both exposure methods. XPS surface analysis of the films revealed changes in the O to Zn bonding ratio suggesting film hydration may have occurred. In addition, after 48 hours by both exposure methods the Mo films corroded, and the film resistivities increased. Our results show Al:ZnO layer degradation limits the lifetime of CIGSS based PV devices, whereas Mo degradation is considered a non-lifetime-limiting failure.

Exploration of binary & ternary photosensitive thin film silver selenides: Prediction, preparation, and characterization

The solar power industry is enjoying rapid growth, but challenges remain to produce new photoactive materials with the proper balance of cost and efficiency. Much of the fundamental science concerning alternative materials has not yet been elucidated. Using ab initio density functional theory to predict band structure and densities of state for 3-D unit cells, four binary and ternary silver selenides have been identified as prime candidates for use as an absorber layer in photovoltaic cells. Thin films of these silver selenides were grown, and morphology, stoichiometry, and structure were analyzed using SEM, EDS, and XRD. Optoelectronic characterization of the films was performed using UV-Vis, Hall Effect, and photoluminescence measurements. These data were used to judge the quality of the materials and to confirm the accuracy of the predictive capability of calculations.