Stable, high deposition rate, wide gap silicon for the rather thick top cells in thin film silicon multibandgap solar cells

Abstract

In thin film silicon multibandgap tandem cells, also called 'micromorph' or 'hybrid' solar cells, it is of great importance that the amorphous component cell is stable against light-induced defect creation. There are two main reasons for this: the high current generation capability of the muc-Si:H bottom cell necessitates a rather thick top cell, which is inherently more susceptible to performance degradation than a thin one. The second reason is that for outdoor application, in certain locations a top cell limited design offers higher daily and annual yield than a bottom cell limited tandem cell. This is due to the fact that under practical outdoor circumstances the operation temperature and light intensity are correlated. This correlation is advantageous for the daily and annual energy yield of the top cell but adverse for the bottom cell. If the top cell is designed to be current limiting it becomes even more important that it has a stable fill factor. We present the development of high deposition rate (up to 3.5 nm/s) protocrystalline Si:H from undiluted silane by Hot Wire Chemical Vapor Deposition (HWCVD, or Catalytic CVD). Highly stable single junction cells showing only very small light-induced fill factor changes and top-cell limited multibandgap tandem devices with only 4 % relative efficiency degradation have been obtained