Effects of hydrogen dilution grading in active layer on performance of nanocrystalline single junction bottom component and corresponding a-Si:H based triple junction solar cells

Using hydrogen dilution grading in nanocrystalline silicon (nc-Si:H) intrinsic layer, considerably high spectral response has been achieved in the longer wavelength (650nm-1000nm) of solar spectrum from single junction solar cells, to be used as bottom junction in monolithic triple junction a-Si:H based thin film solar cells. The intrinsic layers have been deposited with a high deposition rate of 8 Aring/s, using VHP PECVD deposition technique from Si₂H₆ diluted with H₂. In the present work, using R (R=[H₂]/[Si ₂H₆]) from an initial value 50 to different final values viz. 40, 37.5 and 35, single junction nc-Si:H cells have been fabricated and the same single junction components have been applied as bottom junction to fabricate a-Si:H/a-SiGe:H/nc-Si:H triple junction structure where the properties of top and middle junctions were kept fixed. The short circuit current of the triple junction cells is shown to be limited by the bottom junction and the fill factor has been improved in triple junction cells compared to the same obtained from the corresponding single junction cells used as nc-Si:H bottom junction. Comparison of the fill factors under red (>630nm) illumination and that under blue (<510nm) illumination for these single junction nc-Si:H cells indicated structural quality of p/i interface and bulk of the i-layer of nc-Si:H solar cells. A conversion efficiency of 11.2% has been achieved from triple junction solar cell. The light induced degradation studies are currently in process for triple junction cells under a 100 mW/cm² white light illumination. Using spectroscopic ellipsometry, evaluation of crystalline volume fraction at different level of growth is undergoing, which will enable fine-tuning of grading in hydrogen dilution