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The aim of this research is to fabricate high efficiency a-Si/lc-Si tandem solar cell modules on flexible (polymer) superstrates using the Helianthos concept. As a first step we began by depositing the top cell which contains an amorphous silicon (a-Si:H) i-layer of $350 nm made by VHF PECVD at 50 MHz in a high vacuum multichamber system called ASTER, with hydrogen to silane gas flow ratio of 1:1. Such amorphous cells on-foil showed an initial active area (0.912 cm 2 ) efficiency of 7.69% (V oc = 0.834 V, FF = 0.71). These cells were light soaked with white light at a controlled temperature of 50°C. The efficiency degradation was predominantly due to degradation of FF that amounted to only 11% after 1000 h of light soaking. The cell-on-foil data prove that thin film silicon modules of high stability on cheap plastics can be made at a reasonable efficiency within 30 min of deposition time. A minimodule of 8 Â 7.5 cm 2 area (consisting of 8 cells interconnected in series) with the same single junction a-Si:H p-i-n structure had an initial efficiency of 6.7% (V oc = 6.32 V, FF = 0.65).

a b s t r a c tIn order to increase industrial viability and to find niche markets, high deposition rate and low temperature depositions compared to standard deposition conditions are two recent trends in research areas concerning thin film silicon. In situ diagnostic tools to monitor gas phase conditions are useful in quick optimization processes of deposition parameters without going into time consuming material characterizations. Optical emission spectroscopy is an efficient technique to monitor/predict growth rate and phase of the material (amorphous or nanocrystalline). However, at high growth rate conditions which are generally achieved at high chamber pressures (p), the simple correlation breaks down. We see that at high pressure condition a higher H ␣ /Si * is needed for the onset of crystallinity than that is found at lower pressure conditions. Additional methods such as estimating the silane depletion from the experiment and the flux ratio of atomic hydrogen to deposited silicon atoms from simulations can be used for fine-tuning the amorphous to nanocrystalline transition regime. On the other hand, intensity of Si * line loses its character as a monitoring tool for deposition rate. Moreover, the plasma changes its character when the pressure is varied, even when the pd product is kept constant. In situ diagnosis of the ion energy distribution function by a retarding field ion energy analyzer has thrown new lights on the role of hydrogen dilution for depositions at low substrate temperature conditions, namely to compensate the loss in ion energy due to lower gas temperature.

Thiscontributions describes the successful implementation of a novel method for in-line continuous deposition of a-Si:H p-i-n solar cells by Hot Wire Chemical Vapour Deposition (HWCVD). As HWCVD does not use RF power supplies, there are no high frequency electromagnetic fields, and thus, scaling up is not hampered by finite wavelength effects or rigorous requirements to avoid inhomogeneous electrical fields. The hot catalytic wire in HWCVD is a linear source of radicals mounted perpendicular to the transport direction. We demonstrate the homogeneous in-line deposition of HWCVD solar cells on upward-facing substrates. The downward, dust-free deposition of thin film silicon greatly simplifies the mounting of the substrates, which can either be rigid or a flexible web, for in-line manufacturing at reduced cost. Amorphous (protocrystalline) as well as nanocrystalline silicon thin films with device-quality properties have been achieved on moving substrates. The local deposition rate is relatively high, at 1 nm/s, and a linear speed up to 30 cm/min has been used to produce device-quality i-layers with protocrystalline nature. We have recently further improved our thus deposited p-i-n solar cells to 8.3% efficiency (despite air breaks after the p-layer and before the n-layer). The potential for high deposition rate and fast roll-to-roll deposition is also discussed.

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