a-Si:H/a-Si:H stacked cell from VHF-deposition in a single chamber reactor with 9% stabilized efficiency

In the present paper we present results on a-Si:H/a-Si:H stacked cells deposited in a single-chamber reactor by the very high frequency-glow discharge (VHF-GD) deposition technique at 70 MHz. Hydrogen dilution of the i-layer yields more stable amorphous p−i−n solar cells, similar to what is observed for RF deposition. High dilution ratios of the i-layer are found to enhance contaminations. This is, for the single-chamber reactor, due to the etching effect of the highly reactive H₂-plasma. Additionally, oxygen incorporation into the i-layer is favored by the high hydrogen dilution. Different means to suppress these contaminations are employed and discussed. Regarding the stacked cell design, we show by experiment and simulation that it is important to carefully adjust the current mismatch between the component cells such as to obtain a slight top-cell-limited behavior after degradation. We present an a-Si:H/a-Si:H stacked cell with an initial efficiency of 9.8% showing only 8% relative degradation which results in a stabilized efficiency of 9%. The deposition rate of the employed H₂-diluted i-layer material is 4 Å/s. It is therefore demonstrated that it is possible to make highly efficient stacked cells showing good stability also in a single-chamber system and employing the VHF technique to obtain higher rates.