Analysis of Surface Passivation and Laser Firing on Thin-Film Silicon Solar Cells Via Light-Beam Induced Current

Abstract

Liquid phase crystallized silicon solar cells on glass have recently demonstrated 15.1% efficiency using a heterojunction interdigitated back contact cell architecture and an absorber thickness of 14 $\mu$ m. One of the key enabling developments was a new method to first passivate electron contact fingers with a-Si:H(i) and then locally laser fire them to maintain a low contact resistance. In this work, high resolution, light-beam induced current measurements (LBIC) were used to analyze this approach. Charge collection was observed to have increased from 0.13 ${\text{mAcm}}^{-2}$ to 0.9 ${\text{mAcm}}^{-2}$ under the electron contact which is a sevenfold increase. Using 520, 642, 932, and 1067 nm wavelengths of incident light, external quantum efficiency was mapped in regions including grain boundaries, dislocation defects, shunts, defect-free regions, and laser fired spots. Reduction of charge collection in the laser fired spots was limited to diameters of 50–20 $\mu$ m, depending on whether electrical recombination or optical losses dominated. Effective minority carrier diffusion length under the majority carrier contacts was obtained by fitting of LBIC measurements. It was observed to have improved from 20.5 $\mu$ m to 22.7–40.4 $\mu$ m and up to 89.0 $\mu$ m in the best case. Based on this, wider contact fingers and improved surface passivation at the electron contact is encouraged in the near future to achieve efficiencies $\geq$ 16%.