Photoconductivity Stability Improvement in Hydrogenated Amorphous Silicon by Ultraviolet Illumination
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Photoconductivity Stability Improvement in Hydrogenated Amorphous Silicon by Ultraviolet Illumination Howard M. Branz, Yueqin Xu, Stephan Heck, Qi Wang, Wei Gao*, Richard S. Crandall and Brent P. Nelson National Renewable Energy Laboratory Golden, CO 80401 USA *present address: Sharp Corporation, Camas, WA USA 98607
ABSTRACT We observe improved photoconductivity stability against light-soaking in hydrogenated amorphous silicon thin films as a result of an ultraviolet (UV) illumination and etch treatment. UV-etch treated samples begins with red-light photoconductivities inferior to that of a control sample which is only etched. After less than an hour of 1 sun red light-soaking, the photoconductivity of the etched-only control falls below that of the UV-etch treated sample. After 2 to 3 days light soaking, the UV-etch films can have a photosensitivity 20 to 38% above their control. We observe no corresponding improvement of defect optical absorption by constant photocurrent method spectroscopy. The UV-etch treatment also produces small improvements in the stabilized open-circuit voltage of Schottky barier solar cells. We speculate that mobile hydrogen produced during UV illumination is penetrating the film and improving stability.
INTRODUCTION Metastable carrier- and light-induced degradation of hydrogenated amorphous silicon (a-Si:H) [1] continues to limit application of a-Si:H although significant advances have been made toward understanding and controlling the problem [2]. A decade ago, several experiments revealed several unusual effects of ultraviolet (UV) exposure of a-Si:H, but there was little follow-up. In this paper, we describe experiments showing that pre-treatment of a-Si:H with UV light and top-surface etching can improve the stability of photoconductivity and of Schottky solar cell open-circuit voltage against subsequent light soaking. In 1989, Ganguly et al. demonstrated that UV exposure could reverse the metastable lightinduced photoconductivity degradation [3]. Follow-up work revealed that the light-induced bulk deep defect density increases were also reversed by UV post-treatment [4]. These changes extended much further into the bulk of thick a-Si:H films than either the UV absorption depth (10 to 20 nm) or the ambipolar carrier diffusion length (about 100 nm [5]). Shortly thereafter, Lee et al. found that UV illumination post-treatment of the top surface annealed away biasinduced metastable degradation in thin-film transistor channel layers buried 150 nm deep [6]. Also in 1989, Nevin et al. found that pre-treatment of pin/pin tandem a-Si:H solar cells with pulses of UV-rich Xe lamp light could improve their stability against subsequent light-induced degradation [7]. The H collision model of metastability degradation in a-Si:H suggested that illumination could produce mobile H atoms by exciting H from Si-H bonds [8], and we suspected the findings A19.11.1 Downloaded from https://www.cambridge.org/core. Univ of Michigan Law Library, on 21 Nov 2018 at 06:21:50, subject to the Cambridge Core terms o
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