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A10.3.1

INFLUENCE OF PRESSURE AND PLASMA POTENTIAL ON HIGH GROWTH RATE MICROCRYSTALLINE SILICON GROWN BY VHF PECVD A. Gordijn1, J. Francke1, L. Hodakova2, J.K. Rath1*, and R.E.I. Schropp1 1

Utrecht University, Debye Institute, SID-Physics of Devices, PO box 80.000, 3508 TA Utrecht, The Netherlands Dept. Optical Crystals, Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, CZ16253 Prague 6, Czech Republic

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ABSTRACT Microcrystalline silicon (µc-Si) based single junction solar cells are deposited by VHF PECVD using a showerhead cathode at high pressures in depletion conditions. At a deposition rate of 4.5 nm/s, a stabilized conversion efficiency of 6.7 % is obtained for a single junction solar cell with a µc-Si i-layer of 1 µm. The i-layer is made near the transition from amorphous to crystalline. In order to control the material properties in the growth direction, the hydrogen dilution of silane in the gas phase is graded following different profiles with a parabolic shape. It is observed that the performance of solar cells deposited at high rate improves under light soaking conditions at 50 ºC, which we attribute to post deposition equilibration of a fast deposited transition material. The performance is lower at higher rates due to poorer i-layer quality (higher defect density), which may be attributed to smaller relaxation times for growth precursors at the growth surface and the higher energy ion bombardment at higher plasma power. High process pressures can be used to reduce the ion energy by decreasing the mean free path. We have introduced an additional method to limit the ion energy by controlling the DC self bias voltage using an external power source. In this way the quality of the µc-Si layers and the performance of the solar cells is further improved. INTRODUCTION Microcrystalline silicon based thin-film solar cells (single junction µc-Si:H and “micromorph” tandem a-Si:H/µc-Si:H) are advantageous compared to cells solely based on a-Si:H, because µcSi:H has a lower band gap, due to which the optical absorption in the infrared region is higher. Transition-type µc-Si:H (deposited close to the regime of a-Si:H growth) has promising optoelectronic properties for absorber layers applications. However, inhomogeneous growth and small but significant light induced degradation for cells with these types of materials limit their ultimate efficiency. This paper addresses these issues, especially for materials made at high deposition rate. Single junction µc-Si:H and tandem µc-Si:H/a-Si:H solar cells are being developed by several groups [1-5] using different types of deposition configurations and plasma regimes. Here, we combine the approaches of VHF PECVD [4-8] at 60 MHz, high pressure depletion (HPD) conditions [7-10], and a shower head gas inlet [2,10], that have separately been studied by other groups. EXPERIMENTAL Transition-type µc-Si i-layers have been made at an RF frequency of 60 MHz, at pressures of 5 – 10 mbar, at an estimated substrate temperature of 180 ºC, using an ele