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0910-A26-01

Highly Efficient Microcrystalline Silicon Solar Cells Deposited from a Pure SiH4 Flow M.N. van den Donker1, B. Rech1, R. Schmitz1, J. Klomfass1, G. Dingemans1, F. Finger1, L. Houben2, W.M.M. Kessels3, and M.C.M. van de Sanden3 1 IPV, Forschungszentrum Juelich GmbH, Juelich, 52425, Germany 2 IFF, Forschungszentrum Juelich GmbH, Juelich, 52425, Germany 3 Applied Physics, TU Eindhoven, PO Box 513, Eindhoven, 5600MB, Netherlands

ABSTRACT The effect of process parameters on the deposition of µc-Si:H solar cells is reviewed. Then, our approach to solar cell optimization is presented, in which in situ diagnostics are used to study the process stability. We compared a highly H2-diluted condition with a pure SiH4 flow condition, using optical emission spectroscopy (OES) to characterize the plasma and Raman spectroscopy to characterize the film. In low dilution conditions we measured a sharp drop of SiH emission in the first 90 s after plasma ignition, indicating uncontrolled deposition for the first ~50 nm of film. This instability could be prevented by filling the chamber with pure H2 and switching on the SiH4 flow only shortly before plasma ignition. In the steady-state deposition phase following the transient depletion phase the H2 flow hardly effected plasma emission and deposited film crystallinity. This is explained by a depletion of the SiH4. The pure SiH4 flow deposition was applied into a µc-Si:H solar cell and yielded a solar energy conversion efficiency of 9.5 %. In the discussion an updated view on the role of the process parameters total flow and dilution ratio is presented.

INTRODUCTION Microcrystalline silicon (µc-Si:H) provides a promising route towards solar cells with high stabilized efficiency and cost-effective large-area production [1-2]. Fig. 1 shows a TEM micrograph of a single junction µc-Si:H solar cell, consisting of glass, textured ZnO, µc-Si:H p-in junction, and back contact. Deposition of the intrinsic µc-Si:H film takes place in vacuum processes involving a silicon-containing source gas. The deposition process receives considerable scientific interest since the deposited material quality and the deposition rate at which it is obtained will be crucial to the performance and cost price of commercially produced solar modules. A commonly used source gas is SiH4. Reactive radicals are formed from this gas by e.g. thermal dissociation using a hot wire or electron-impact dissociation using a plasma. Used plasma sources are e.g. an expanding thermal plasma [3], an inductively-coupled plasma [4] and a microwave plasma [5]. In this contribution we restrict ourselves to the parallel plate plasma source. This plasma source is used commonly in research and industry. It has shown the best solar cell performance at high rate so far. The variable process parameters are the plasma power

η (%)

FF (%)

70 60 50

2

Figure 1. TEM micrograph of a single junction µc-Si:H solar cell. Glass coated with textureetched ZnO:Al functions as substrate. The µcSi:H p-i-n device is deposited on top of it. Typi