Robert P. Haley

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.

Structure optimization for a high efficiency CIGS solar cell

This paper uses numerical simulation to study the effects of Ga concentration profile on the performance of CuIn1−xGaxSe2 (CIGS) solar cell, including the effects of acceptor type Cu antisite defects whose concentration depends on Ga composition. These defects are the dominant deep traps in the CIGS material system. The concentration and spatial distribution of these traps affect the solar cell performance. The trap density model used in this work follows experimental reports in the literature. The trap concentration is 4.3×1015 cm−3 for CIS (x=0) and decreases to 1.2×1014 cm−3 when the Ga mole fraction, x, reaches 0.24. The trap concentration increases exponentially above x=0.30. Applying this model to solar cells with uniform composition absorber layer predicts that the power conversion efficiency reaches a maximum value of 14.6%, at x=0.24 and decreases with increasing Ga content above x=0.30, in good agreement with experimental results. When this model is used to simulate a solar cell where the Ga composition in the absorber layer is graded, the electric field produced by compositional grading improves the efficiency because of the reduced recombination rate. However compositions where x is higher than 0.45 lead to a drop in performance due to the high trap density and shorter lifetime. Both grading from the CdS/CIGS interface (forward grading), and back grading where the Ga concentration increases from the junction into the CIGS film were studied. In forward grading, the maximum efficiency is achieved when the Ga concentration is graded such that x decreases from 0.35 at the surface to 0.24 at 0.4 µm into the CIGS film. In back grading, the maximum efficiency is achieved when x increases from 0.45 at the surface to 0.5 at 0.4 µm into the CIGS film.

Optimization of electrical and optical properties in multilayer TCO thin film structures

Multilayer oxide thin film stacks of aluminum doped zinc oxide (AZO) and tin doped indium oxide (ITO) have been sequentially deposited on soda lime glass substrates by RF sputtering of AZO and ITO ceramic targets at a substrate temperature of 150 °C. The ratio of the AZO thickness to the ITO thickness is varied while keeping the total thickness of the stack constant. The electrical and optical properties of the multilayer stacks have been investigated as a function of this ratio and the number of interfaces. The experimental results are compared and their impact on device performance is demonstrated by simulations with validated AMPS-1D models. XRD and microscopy measurements have been carried out to understand the microstructure of the multilayer system and to establish its correlation with the opto- electric properties. The results have been evaluated for use of these multilayer TCO stacks as potential window layers for the photovoltaic solar cell applications.

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.