Comprehensive study of microcrystalline silicon solar cells deposited at high rate using 13.56 MHz plasma-enhanced chemical vapor deposition

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Abstract
In this article we present a comprehensive study of microcrystalline silicon (μc-Si:H) p-i-n solar cells prepared by using plasma-enhanced chemical vapor deposition (PECVD) at 13.56 MHz excitation frequency. In the first step the cell development was performed in a small area PECVD reactor showing the relationship between the deposition process parameters and the resulting solar cell performance. Subsequent up-scaling to a substrate area of 30×30 cm2 confirmed the scalability of optimized deposition parameters to large area reactors. We investigated the deposition regime of high rf power Prf (0.25–0.7 W/cm2) and high deposition pressure pdep (1–11 Torr) for the μc-Si:H i layer. Furthermore, the influence of silane concentration and deposition temperature was studied. A transition between amorphous and microcrystalline growth could be achieved by a variation of either deposition pressure, plasma power, or silane concentration. The best microcrystalline silicon solar cells were prepared close to the transition to amorphous growth. A high deposition pressure was a prerequisite for obtaining high quality material at a high growth rate. The best solar cell efficiencies achieved so far are 8.1% and 6.6% at i-layer growth rates of 5 and 10 Å/s, respectively, for μc-Si:H single junction cells. Applied in a-Si:H/μc-Si:H tandem cells a stabilized efficiency of 10.0% was achieved.