This paper presents a brief review of the research that began in the early ‘80s, continued through the ‘90s, and produced a “standard model” for the optical absorption edge of amorphous silicon. The research began as a response to the invention of a-Si:H solar cells by Carlson and Wronski at RCA laboratories in 1976, and the subsequent worldwide interest in the optical characterization of a-Si:H thin films. The immediate need was soon met, but the research continued as an effort to understand the physics of the optical absorption edge in a-Si:H, as well as to understand the differences between, and similarities to, the indirect optical absorption edge of c-Si. In this paper, we highlight the successes of this standard model, and briefly cover its experimental and theoretical development over the last 25 years. We summarize its current status, and suggest some experimental and theoretical opportunities for, and challenges to, what may now be called a standard model for the optical absorption edge of both a-Si:H and a-Ge:H.
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Thanks to all of my colleagues and co-workers, listed in the references, for their contributions over the past 30 years, particularly to B. G. Brooks for the optical absorption measurements, to my friend, advisor, and experimental colleague of 50 years, Ben Abeles, and to my dear wife, friend, and co-parent/grandparent of our 20 member family, Francesca Benson.
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Cody, G.D. Urbach Edge, Disorder, and Absorption On-set in a-Si:H. MRS Online Proceedings Library 862, 13 (2004). https://doi.org/10.1557/PROC-862-A1.3