High-throughput approaches to optimization of crystal silicon surface passivation and heterojunction solar cells

We use a high-throughput (combinatorial) hot-wire chemical vapor deposition system to passivate the crystal silicon surface and to grow heterojunction silicon solar cells. We study the effectiveness of crystal surface treatments by atomic H or/and NH_x radicals, followed by the growth of thin hydrogenated amorphous silicon (a-Si:H) films. Treatment and layer properties such as times, thicknesses and gas mixtures can be continuously graded, creating a two-dimensional sample with each variable varying in one direction. This results in high-throughput optimization of the processes. Effective carrier lifetime is measured by photoconductive decay to evaluate the effectiveness of the surface passivation by surface treatments. The effective carrier lifetime increases from about 5 μs without passivation to about 24 μs with an optimized surface treatment and thickness a-Si:H on single-sided c-Si. Transmission electron microscopy reveals that a-Si:H, a mixed phase, or epitaxial growth of thin-film Si depending upon the surface treatment. Improvement in effective carrier lifetime correlates to with an immediate a-Si:H growth on c-Si, rather than a mixed phase and epitaxial Si growth. We have obtained an efficiency of 13.4% on a non-textured single-sided heterojunction solar cell on a p-type CZ-Si processed with optimized surface treatment.