Amorphous solar cells, the micromorph concept and the role of VHF-GD deposition technique

During the last two decades, the Institute of Microtechnology (IMT) has contributed in two important fields to future thin-film silicon solar cell processing and design: (1) In 1987, IMT introduced the so-called "very high frequency glow discharge (VHF-GD)" technique, a method that leads to a considerable enhancement in the deposition rate of amorphous and microcrystalline silicon layers. As a direct consequence of reduced plasma impedances at higher plasma excitation frequencies, silane dissociation is enhanced and the maximum energy of ions bombarding the growing surface is reduced. Due to softer ion bombardment on the growing surface, the VHF process also favours the formation of microcrystalline silicon. Based on these beneficial properties of VHF plasmas, for the growth of thin silicon films, plasma excitation frequencies fexc in the range 30-300 MHz, i.e. clearly higher than the standard 13.56 MHz, are indeed scheduled to play an important role in future production equipment. (2) In 1994, IMT pioneered a novel thin-film solar cell, the microcrystalline silicon solar cell. This new type of thin-film absorber material - a form of crystalline silicon - opens up the way for a new concept, the so-called "micromorph" tandem solar cell concept. This term stands for the combination of a microcrystalline silicon bottom cell and an amorphous silicon top cell. Thanks to the lower band gap and to the stability of microcrystalline silicon solar cells, a better use of the full solar spectrum is possible, leading, thereby, to higher efficiencies than those obtained with solar cells based solely on amorphous silicon. Both the VHF-GD deposition technique and the "micromorph" tandem solar cell concept are considered to be essential for future thin-film PV modules, as they bear the potential for combining high-efficiency devices with low-cost manufacturing processes. ©2004 Elsevier Ltd. All rights reserved.