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Amorphous Silicon and Silicon Germanium Alloy Solar Cells Deposited by VHF at High Rates Jeffrey Yang, Baojie Yan, Jozef Smeets*, and Subhendu Guha United Solar Systems Corp., 1100 West Maple Road, Troy, Michigan 48084 * N.V. Bekaert S.A., Bekaertstraat, 2, B-8550 Zwevegem, Belgium

ABSTRACT A modified very high frequency (MVHF) glow discharge technique is used to deposit amorphous silicon (a-Si) and amorphous silicon-germanium (a-SiGe) alloy solar cells at high deposition rates. High quality a-Si alloy solar cells have been obtained by using MVHF at deposition rates up to ~10 Å/s. The cells show good initial and stabilized efficiencies comparable to those obtained from conventional radio-frequency (RF) glow discharge deposition at low rates (~1 Å/s). However, high quality a-SiGe alloy solar cells are more difficult to achieve at high deposition rates. In this paper, we present the progress made on a-SiGe alloy solar cells by incorporating bandgap profiling and appropriate buffer layers. Using the improved a-SiGe alloy solar cells, a-Si/a-SiGe tandem configurations are made and results presented.

INTRODUCTION The very high frequency (VHF) glow discharge technique has been widely used in the deposition of amorphous silicon (a-Si) alloy and microcrystalline silicon (µc-Si) materials and devices, such as solar cells and thin film transistors [1-4]. Compared to radio frequency (RF) glow discharge, VHF has the advantage of depositing a-Si alloy and µc-Si at high deposition rates. It is well known that a-Si alloys made with RF glow discharge at high rates exhibit poor quality. The materials contain a high density of defects, microvoids, and dihydride structures. Solar cells made by RF at high rates also show low initial efficiency and poor stability. In production, the deposition rate is usually limited to 2-3 Å/s. In order to reduce the production cost by increasing the deposition rate, new deposition techniques are required. VHF glow discharge is one of the promising techniques. We previously reported that VHF-deposited a-Si alloy solar cells showed good initial efficiency and stability; the performance is largely independent of the deposition rate up to ~10 Å/s [3,4]. We also found that the ion energy is lower and the ion flux is higher for VHF plasma than RF [4]. The high flux of ions with low energies in the VHF plasma is believed to improve the quality of the material. Recently, Takai et al. found that the electron density increases and the electron temperature decreases with the increase of excitation frequency [5]. High temperature electrons in the plasma are associated with the formation of dihydride structures and degrade the stability of the solar cells. High quality a-SiGe alloy solar cells are usually more difficult to obtain at high deposition rates using RF glow discharge than at low rates. The same problem appears with VHF glow discharge. We reported that the difference between RF low rate cells and VHF high rate cells becomes larger as the germanium content is increased [4]. In this paper, we present our