Microstructurally engineered p-layers for obtaining high open-circuit voltages in a-Si:H n-i-p solar cells

A study was carded out with the goal of obtaining high open circuit voltages (V_oc) in a-Si:H n-i-p solar cells, taking into account the evolutionary nature of the microstructure of the p-layers during growth. It is found that cells with players in the protocrystalline Si:H growth regime give the highest values of V_oc not those with microcrystalline Si:H p-layers. Evidence for this conclusion is presented whereby V_oc is related directly to the microstructure of the p-layers, as characterized using spectroscopic ellipsometry, atomic force microscopy, and electrical measurements. The results clarify the origins of (i) inconsistencies associated with attributing high V_oc in n-i-p cells to the microcrystallinity of the p-layers, as well as (ii) the inability to obtain similarly high values in p-i-n superstrate cells. Because the microstructure of p-type protocrystalline Si:H depends on that of the underlying i-layer, it is not possible to optimize the cell parameters based on an understanding of the process unless detailed characterization of the p-layer in the actual cell configuration is performed.