Assessment of Bulk and Interface Quality for Liquid Phase Crystallized Silicon on Glass

This paper reports on the electrical quality of liquid phase crystallized silicon (LPC-Si) on glass for thin-film solar cell applications. Spatially resolved methods such as light beam induced current (LBIC), microwave photoconductance decay (MWPCD) mapping, and electron backscatter diffraction were used to access the overall material quality, intra-grain quality, surface passivation, and grain boundary (GB) properties. LBIC line scans across GBs were fitted with a model to characterize the recombination behavior of GBs. According to MWPCD measurement, intra-grain bulk carrier lifetimes were estimated to be larger than 4.5 μs for n-type LPC-Si with a doping concentration in the order of 10
¹⁶
cm
^-3
. Low-angle GBs were found to be strongly recombination active and identified as highly defect-rich regions which spatially extend over a range of 40-60 μm and show a diffusion length of 0.4 μm. Based on absorber quality characterization, the influence of intra-grain quality, heterojunction interface, and GBs/dislocations on the cell performance were separately clarified based on two-dimensional (2-D)-device simulation and a diode model. High back surface recombination velocities of several 10
⁵
cm/s are needed to get the best match between simulated and measured open circuit voltage (V
_oc
), indicating back surface passivation problem. The results showed that V
_oc
losses are not only because of poor back surface passivation but also because of crystal defects such as GBs and dislocation.