Influence on cell performance of bulk defect density in microcrystalline silicon grown by VHF PECVD

Microcrystalline silicon based single junction solar cells have been deposited on Asahi U-type SnO2:F and texture-etched ZnO by means of very high-frequency plasma-enhanced chemical vapor deposition (VHF PECVD) using a showerhead cathode at high pressures in depletion conditions. The defect densities in the intrinsic layers (in the solar cell configuration) have been estimated by means of Fourier-transform photocurrent spectroscopy. An Urbach tail parameter of 35 meV and a defect density of smaller than 1015 cm−3 show the high quality of the material. It was found that the cell performance of our cells is dominated by the bulk defect density of the i-layers: the fill factor and open circuit voltage of solar cells deposited at various conditions show a consistent decrease with increasing defect density. It was found that the bulk defect density increases one order of magnitude upon an increase in deposition rate from 0.45 to 4.5 nm/s. We show that this increase in defect density can be reduced when the ion bombardment energy is reduced by artificially lowering the plasma potential in the deposition process using an external DC bias. On texture-etched ZnO:Al, at a deposition rate of 0.45 nm/s, a stabilized conversion efficiency of 10.0% (4 mm × 4 mm cell area) is obtained for a single junction solar cell with a μc-Si i-layer of 1.5 μm thick. This belongs to the highest efficiencies reported for microcrystalline silicon solar cells in the superstrate (p–i–n) configuration. Upon increasing the deposition rate to 4.5 nm/s, an efficiency of 6.4% (initial) was obtained. It is observed that the performance of solar cells that are deposited at high deposition rates improves under light soaking conditions at 50 °C, even though these cells are deposited at the amorphous to crystalline transition regime.