Optoelectronic methods for characterizing uniformity in CIGS thin film solar cells

Abstract

The areal uniformity of electrical characteristics for photovoltaic materials is crucial to achieving maximum efficiency in any solar cell technology. Various techniques have been demonstrated to quantify the planar materials properties for CIGS thin film solar cells. Here we discuss modifications to traditional laser beam induced current (LBIC) and spectral photoluminescence (PL) to quantify the level of macroscopic and microscopic non-uniformity in the device properties. Subsequently, the impact of heterogeneity is analyzed and correlated to both simulated and empirical device performance. It is demonstrated that the application of a DC bias voltage coupled to a typical AC small signal LBIC measurement enables the detection of subtle fluctuations in the JV characteristics across the cell. Thus, regions with lower fundamental Voc will begin to forward bias before the remainder of the device resulting in a fill factor reduction (i.e. weak diode). The bias dependant LBIC technique clearly identifies these lower Voc regions enabling detailed analysis of the mechanism resulting in the variable voltage. High resolution PL has also been used to study the effects of non-uniformity on length scales approaching 1 µm. These results show that the spectral granularity can be significant at this resolution. Comparison of the high resolution spectral PL maps with large area spectrum demonstrates how the non-uniformity must be accounted for when interpreting the intrinsic spectral structure of the absorber layer. Additionally, a SPICE lumped element distributed network simulation is incorporated into the analysis to facilitate the interpretation of the impact on the overall performance.