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

Abstract

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 f exc 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 thinfilm 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.

In situ silicon oxide based intermediate reflector for thin-film silicon micromorph solar cells

“…8 The p-i-n micromorph cells are either deposited in a KAI-S deposition reactor or in a smaller laboratory system by PECVD on a rough front ZnO layer, deposited by low-pressure chemical vapor deposition. 9 The intrinsic layer thicknesses are between 250 and 270 nm for the a-Si: H and 1800 nm for the c-Si: H cell. The SiO-based layers incorporated in the micromorph solar cell are deposited with gas flow ratios of PH 3 / SiH 4 = 0.024 and CO 2 / SiH 4 = 2.7. n-type doping by phosphorous is chosen since n-type doping is usually more efficient than p-type doping.…”

Asymmetric intermediate reflector for tandem micromorph thin film silicon solar cells

“…2 shows that c-Si: H smoothens the interface with the IR and that the surface structure is not suited for inducing any light trapping in the a-Si: H top cell. The AIR restores a random roughness, which creates favorable light scattering for a-Si: H solar cells 17 since light is both scattered and reflected. A part of the weakly absorbed light that is reflected by the AIR is also out-coupled from the device.…”

Geometrical optimization and electrical performance comparison of thin-film tandem structures based on pm-Si:H andμc-Si:H using computer simulation

“…The first defect category, even if it is known to be of amphoteric type [15,16], can be modelled by two Gaussian continuous distributions of monovalent states [17]:…”