Hydrogen passivation of polycrystalline Si thin film solar cells

Hydrogen passivation is a key process step in the fabrication of polycrystalline Si (poly-Si) thin film solar cells. In this work a parallel plate rf plasma setup was used for the hydrogen passivation treatment. The main topics that have been investigated are (i) the role of plasma parameters (like hydrogen pressure, electrode gap and plasma power), (ii) the dynamics of the hydrogen treatment and (iii) passivation of poly-Si with different material properties. Passivation was characterized by measuring the open-circuit voltage VOC of poly-Si reference samples. Optimum passivation conditions were found by measurements of the breakdown voltage Vbrk of the plasma for different pressures p and electrode gaps d. For each pressure, the best passivation was achieved at a gap d that corresponded to the minimum in Vbrk. Plasma simulations were carried out, which indicate that best VOC corresponds to a minimum in ion energy. VOC was not improved by a larger H flux. Investigations of the passivation dynamic showed that a plasma treatment in the lower temperature range (≤400 C) is slow and takes several hours for the VOC to saturate. Fast passivation can be successfully achieved at elevated temperatures around 500 C to 600 C with a plateau time of 10 min. It was found that prolonged hydrogenation leads to a loss in VOC, which is less pronounced within the observed optimum temperature range (500 C-600 C). Electron beam evaporation has been investigated as an alternative method to fabricate poly-Si absorbers. The material properties have been tuned by alteration of substrate temperature Tdep=200-700 C and were characterized by Raman, ESR and VOC measurements. Largest grains were obtained after solid phase crystallization (SPC) of a-Si, deposited in the temperature range of 300 C. The defect concentration of Si dangling bonds was lowered by passivation by about one order of magnitude. The lowest dangling bond concentration of 2.5.1016 cm-3 after passivation was found for poly-Si with largest grains and coincides with best solar cell results, obtained after rapid thermal annealing and hydrogen passivation. Hydrogen passivation of poly-Si films was successfully achieved with a parallel plate rf H plasma treatment at elevated temperatures around 500 C to 600 C. Yet it seems that treatment induced defect generation causes a loss in VOC with prolonged passivation time and should be minimized. In order to achieve high open circuit voltages larger than 450 mV, in addition to hydrogen passivation, low recombination at the interfaces becomes more and more important. (orig.)