Chapter Five - Quantitative Luminescence Characterization of Crystalline Silicon Solar Cells

Abstract

Luminescence imaging has become a standard tool for solar cell analysis within the last decade. In order to understand the potential, as well as the limitations of the numerous luminescence characterization approaches, this work provides both the physical background for modeling the luminescence emission from silicon solar cells as well as a review of series resistance imaging, one of the most prominent applications of luminescence imaging. The first part addresses the measurement setup. A specific focus lies on the suitability of different cameras and the optical filters necessary to prevent the detection of reflected excitation light as well as stray light. Detection conditions with respect to capture time and averaging of images are also discussed. In the second part, we derive a general mathematical description of the spectral luminescence emission. We show that the integral over the product of the depth-dependent minority charge carrier profile and the luminescence photon detection profile fully determines the spectral luminescence emission. Moreover, we show that the luminescence photon detection profile can be obtained from the generation profile of minority charge carriers under illumination, for which well-tested expressions can be found in the literature. Based on the mathematical description, we derive a short- and a long-wavelength approximation corresponding to the spectral sensitivity of silicon and indium–gallium–arsenide detectors, widely used for luminescence measurements. While from the short-wavelength approach the local voltage of a solar cell can be determined, the long-wavelength approach yields the local collection length of the device under test. The final part describes the most prominent application of luminescence imaging: the determination of the local series resistance of wafer-based crystalline silicon solar cells. We review a variety of different approaches introduced in the past. We show that all approaches are based on the same general equation, which is a consequence of the underlying independent diode model. Based on numerical circuit simulations, we study the range of applicability of this simple model. Moreover, the most promising series resistance approaches are applied to various silicon solar cells. Resulting local series resistance and local recombination current images are compared among different methods, as well as to global values extracted from the current–voltage characteristics of the solar cell.