Thin-film (amorphous and microcrystalline) silicon

The Japanese had effectively already delivered the coup de grace to Cu2S/CdS technology by the early 1980s by commercialising small amorphous hydrogenated silicon (a-Si:H) PV panels of modest but sufficient (3-4%) efficiency to power small consumer goods such as watches and calculators. Amorphous silicon of good quality (with sufficiently few mid-gap states to be dopable either n – or p- type) had earlier been made by Spear and Le Comber (1975) in Dundee, Scotland. Independently, David Carlson and Chris Wronski, then both at RCA Laboratories in Princeton, New Jersey, made several square-centimetre sized n-i -p and p-i – n cells of ~2% efficiency (Carlson and Wronski, 1976), and smaller area MIS cells of 5.5% efficiency. The p-i – n configuration was to be the forerunner of modern a – Si:H photovoltaic technology. The Staebler-Wronski effect, which is the ~ 10-30% diminution of efficiency that occurs on the first prolonged exposure of a cell to light, was discovered soon afterwards, in 1977. Unwelcome as this was, ways to reduce its impact by using thin cells (in which the higher built-in field reduces this volume recombination effect) have been developed.

Single-junction amorphous silicon modules now achieve stabilised efficien­cies of 6-8%. However, most commercial product now has the so-called micro- morph configuration, consisting of a two-junction (tandem) cell with an a-Si:H p-i-n top cell and a microcrystalline (^c-Si:H) p-i-n bottom cell which utilises the red and near-IR light that passes through the a-Si:H top cell. These cells, orig­inally developed by Meier et al. (1994) at IMT Neuchatel with manufacturing technology licensed to the Swiss company Oerlikon Solar in 2003, have achieved stabilised module efficiencies of 11.9% (Bailat et al., 2010) at a reported cost of EUR0.5 Wp-1. Triple-junction cells (a-Si:H/a-SiGe:H/^c-Si:H) of 1.5% stabilised efficiency have recently been reported (Soderstrom et al., 2012).

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