Lifetime Enhancement During Processing of Porous Silicon Cells M. Hilali, B. Damiani, and A. Rohatgi University Center of Excellence for Photovoltaics Research and Education, School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0250 ABSTRACT Porous silicon (PS) induced lifetime enhancement was investigated in dendritic web and string ribbon silicon. As-grown lifetime in web was <1 μs, which improved by a factor of 3.27 due to 860°C/2 min heat treatment. The same heat treatment after PS formation increased the lifetime by a factor of 8.33, indicating that PS alone is quite effective in raising the lifetime of web. Al gettering by itself raised the lifetime of as-grown web by a factor of 15, but the simultaneous annealing heat treatment increased the lifetime by a factor of 42, indicating a positive synergistic interaction between PS and Al. Finally, simultaneous phosphorus and Al diffusion, after PS formation, raised the as-grown web lifetime by a factor of 58. String ribbon Si, which has a low oxygen content and responds more favorably to hydrogenation, did not show much improvement due to PS+Al. Instead it showed greater response to SiN+Al treatment, suggesting that Al+PS induced lifetime enhancement is due to gettering and not hydrogenation. Screen-printed PS coated 100 μm web cells were formed by a single step firing and back diffusion with efficiency of 12 %. Cell analysis showed that process optimization can raise that efficiency by 2-3 % absolute. 1 INTRODUCTION Porous silicon has certain properties that make it very attractive for solar cell applications. Porous silicon has been proven to be a good antireflection (AR) coating for Si solar cells [1]. Moreover, due to the porous nature of the surface, it can be used for surface texturing of single or multicrystalline Si materials [2]. Porous silicon can also act as a surface passivating layer, since it has a larger bandgap than silicon (>1.40 eV). It has also been shown that porous silicon could be used as an extrinsic gettering method on metallurgical-grade silicon [3]. Most of the work reported on porous silicon has been through the use of electrochemical etching to form the porous silicon layer that is not as suitable for industrial applications. In this paper, porous silicon is formed using a cheap and simple wet chemical etching process that takes only few seconds. Gettering and hydrogenation ability of PS is determined through a systematic study of lifetime enhancement in dendritic web silicon, which has a high oxygen content, and low-oxygen string ribbon which responds strongly to hydrogenation. In addition, solar cells are fabricated with SiN and PS AR coating.