Defect characterization of UMG mc-Si solar cells using LBIC and luminescence imaging techniques
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.366
Defect characterization of UMG mc-Si solar cells using LBIC and luminescence imaging techniques L.A. Sánchez1, A. Moretón1, M. Guada1, S. Rodríguez-Conde1, O. Martínez1 and J. Jiménez1 1
GdS-Optronlab, Dpto. Física de la Materia Condensada, Universidad de Valladolid, Edificio LUCIA, Paseo de Belén 19, 47011 Valladolid, Spain.
ABSTRACT
Upgraded metallurgical-grade silicon (UMG Si) solar cells with different ranges of efficiencies were characterized through electroluminescence imaging (ELi) and light-beam induced current (LBIC) measurements. The results showed a good correlation between the EL intensity and the efficiency of the solar cells. ELi images gave a bright contrast at the defects, grain boundaries and intragrain defects, and dark contrast inside the grain bodies. Metallic impurities are much more present in some cells due to the directional solidification of the Si ingot. Local short-circuit current mapping with LBIC measurements revealed a bright zone in the neighborhoods of the defects due to the depletion of impurities. Internal quantum efficiencies (IQE) and effective diffusion lengths (Leff) were calculated using different excitation wavelengths. High resolution LBIC measurements revealed micrometric clusters of impurities around intragrain defects.
INTRODUCTION Photovoltaic (PV) industry is currently dominated by multicrystalline silicon (mc-Si) due to its cost-efficiency ratio. The traditional purification method of metallurgical grade silicon is called Siemens process. This process uses chemical vapor deposition (CVD) to obtain high-quality solar-grade Silicon (SoG-Si) but it results to a be a very energy-intensive mechanism [1]. SoG-Si produced through metallurgical purification processes is called upgraded metallurgical-grade silicon (UMG Si), and it appears as an alternative method to Siemens process due to reduced production cost and time. Directional solidification is one of the steps of this method, exploiting the small segregation coefficient of many impurities between the solid and liquid silicon [2]. Most of the impurities removed by this solidification process are metals, therefore elements like boron (B) and phosphorus (P) need to be removed by other processes. These impurities act like shallow acceptors and donors and reduce the minority carrier diffusion length. UMG Si is not the main material in PV industry; however, efficiencies of 20% have been achieved [3], which makes this material an attractive field of research for the future PV market. Characterization of the electrical activity in UMG Si solar cells is essential to know the spatial distribution of impurities and how these affect the efficiency. Electroluminescence imaging (ELi) and light-beam induced current (LBIC) are powerful
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