Process control of high rate microcrystalline silicon based solar cell deposition by optical emission spectroscopy

Silicon thin-film solar cells based on microcrystalline silicon (mu c-Si:H) were prepared in a 30 x 30 cm(2) plasma-enhanced chemical vapor deposition reactor using 13.56 or 40.68 MHz plasma excitation frequency. Plasma emission was recorded by optical emission spectroscopy during mu c-Si:H absorber layer deposition at deposition rates between 0.5 and 2.5 nm/s. The time course of SiH* and H-beta emission indicated strong drifts in the process conditions particularly at low total gas flows. By actively controlling the SiH4 gas flow, the observed process drifts were successfully suppressed resulting in a more homogeneous i-layer crystallinity along the growth direction. In a deposition regime with efficient usage of the process gas, the mu c-Si:H solar cell efficiency was enhanced from 7.9% up to 8.8% by applying process control. (C) 2007 Elsevier B.V. All rights reserved.