Light Beam Induced Current Mapping of mc-Si Solar Cells: Influence of Grain Boundaries and Intragrain Defects
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1268-EE01-08
Light Beam Induced Current Mapping of mc-Si Solar Cells: Influence of Grain Boundaries and Intragrain Defects. B. Moralejo1, V. Hortelano1, M.A. González1, J. Jiménez1*, O. Martínez1, V. Parra2, M. Avella3 1
GdS – Optronlab; Dpto. Física de la Materia Condensada; Edificio I+D - Universidad de Valladolid. Campus Miguel Delibes. Paseo de Belén 1, E-47011 Valladolid – Spain 2
Instalaciones Pevafersa S. L. Energías Renovables, Av. Vicente Fernández, Toro (Zamora), Spain 3
Parque Científico de la Universidad de Valladolid, Paseo de Belén 1, E-47011 Valladolid – Spain ABSTRACT The photovoltaic market is currently dominated by multicrystalline silicon. However, this material is characterized by intrinsic structural heterogeneity due to point defects, dislocations and grain boundaries. In order to improve the cell performance the control of the electrical properties of the grain boundaries and dislocations is required. The laser beam induced current technique allows the estimation of the variations of the charge capture rates due to the different trapping centers, and is a powerful tool for the characterization of multicrystalline silicon solar cells. Nevertheless, one has to control the reflected light in order to obtain a reliable estimation of the electrical parameters. INTRODUCTION Multicrystalline (mc-Si) has experienced a noticeable increase in the photovoltaic (PV) industry because of its cost effectiveness, currently accounting for nearly 50% of worldwide production. However, this material is characterized by intrinsic structural heterogeneities, e.g. dislocations, and grain boundaries (GB), which are distributed over the wafer and are related to the growth conditions, inducing changes in the physical properties [1]. In order to improve the conversion efficiency of mc-Si solar cells, increasing the grain size seems to be a key step, even if intragrain defects and impurities also limiting their performance. The diffusion length of minority carriers (Ldiff) gives an indication of the material quality and suitability for solar cell use. The laser beam induced current (LBIC) technique allows the estimation of the local diffusion length from photocurrent data, and is a powerful tool for the characterization of PV cells [2-4]. In order to get reliable diffusion length estimations one must perform an accurate measurement of the reflected light; however, the reflected light in mc-Si is highly inhomogeneous and strongly dependent on the orientation of the grains forming the mc-Si samples. Therefore, a careful monitoring of the reflected light taking account of both the back reflected and the dispersed light is required [5]. We present here the characterization of commercial mc-Si solar cells using an advanced homemade LBIC System. EXPERIMENTAL The LBIC system, Fig. 1, works using three excitation wavelengths, namely a double laser diode (Omicron, 639 and 830 nm) and a second laser diode (785nm), placed perpendicularly, with a beam-splitter that directs the beams into a trinocular microscope, which focuses th
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