CRYSTALLINE SILICON SURFACE PASSIVATION BY PECV-DEPOSITED HYDROGENATED AMORPHOUS SILICON OXIDE FILMS a-SiO_x:H

摘要

In the research field of crystalline silicon (c-Si) solar cells, electronic surface passivation has been recognized as a crucial step to achieve high conversion efficiencies. The main issue of this article is to analyze the surface passivation properties of both, n-type and p-type crystalline silicon wafers by hydrogenated amorphous silicon sub oxide [a-SiO_x:H] films the for use in hetero-junction (a-Si/c-Si) solar cells. A window layer is obtained with a certain fraction of oxygen in the a-SiO_x:H layers. The a-SiO_x:H films were deposited by decomposition of silane, carbon dioxide and hydrogen as source gases using plasma enhanced chemical vapor deposition (PECVD). Films with varying deposition parameters such as gas flow ratio (oxygen fraction) and plasma frequency (13.56, 70.0 and 110.0 MHz) are compared. To determine the passivation quality of the a-SiO_x:H films, microwave-detected photo conductance decay (μ-PCD) provides a contactless measurement of the effective recombination lifetime of free carriers. The film compositions and also the changes in the microscopic structure of the amorphous network upon thermal annealing are studied using Raman spectroscopy and optical profiling techniques. The Raman spectra reveal the generation of Si-(OH)_x and Si-O-Si bonds after thermal annealing in the layers, leading to a higher effective lifetime, as it reduces the defect absorption of the sub oxides. For n-type FZ material, lifetime values as high as 1650 μs are obtained, resulting in a surface recombination velocity Seff < 9.5 cm/s.