Variation of the Defect Density in the Absorber Layer of a-Si:H and μc-Si:H Solar Cells over Two Orders of Magnitude: Influence on Solar Cell Performance

To investigate the limiting role of electronic defects in the absorber layer of amorphous and microcrystalline (a-Si:H, µc-Si:H) solar cells we have been able to vary the defect density in the i-layer of solar cells and in corresponding ESR samples by exposing both sample groups simultaneously to 2 MeV electron bombardment and successive stepwise annealing. Cell parameters were analyzed then as a function of NS measured in the ESR samples. Cells with a-Si:H and µc-Si:H absorber layers of different thickness illuminated through p- and n-layers (p-i-n and n-i-p configurations) were studied. Comparison of p-and n-side illuminated solar cells is performed to detect possible asymmetry in the collection of electrons and holes at increased NS. Variation of the defect density over 2 orders of magnitude provided sufficient dynamic range for observation of clear trends and further verification of device operation models. Experimental results of the dependence of the diode I-V characteristics and the quantum efficiency on the defect density were accompanied by device modeling using the ASA simulation program. In a-Si:H cells the open circuit voltage (VOC) and short circuit current (JSC) decrease only moderately up to NS of 5x1016 cm-3 but drop steeply at higher NS. On the other hand the fill factor (FF) showed continuous decrease with an increase in NS. Generally a stronger degradation was observed in cells with thicker a-Si:H absorber and in cells illuminated through n-layer. However at NS < 1017 cm-3 both p-side and n-side illuminated cells showed very similar performance. In µc-Si:H cells a much more continuous decrease of both VOC and JSC is observed already at low NS values. The FF continuously decreased as NS was increasing up to 1017 cm-3 and shows considerable scatter at NS>1017 cm-3. At NS above 1016 cm-3 I-V parameters, especially JSC, of n-side illuminated µc-Si:H solar cells were much more sensitive to an NS increase than parameters of the p-side illuminated device. The difference was remarkably stronger compared to the difference between p- and n-side illuminated a-Si:H cells. Comparison of QE of µc-Si:H cells illuminated from p- and n-side showed that while electrons are easily collected even at the highest achieved NS=1018 cm-3, hole collection is strongly suppressed by NS increase. In the simulation with the ASA program this asymmetry in carrier collection could be reproduced by placing the defect density peak 0.2 eV above the midgap of µc-Si:H i-layer. We conclude that the bulk defect density in a-Si:H cells is less critical for state of the art material and improvement at lower NS could be achieved mostly in FF. It suggests, for instance, that interfaces or doped layers may limit VOC, rather than defects in i-layer. For µc-Si:H absorber, contrary to a-Si:H, bulk NS in the range of 1015 - 1016 cm-3 (state of the art material) seems to still limit the solar cell performance. Therefore here we speculatively expect improvements in cell performance given NS of µc-Si:H absorber is eventually reduced