Analysis and optimization of the performance of polymorphous silicon solar cells: Experimental characterization and computer modeling

Hydrogenated polymorphous silicon (pm-Si:H) is a nanostructured silicon thin film produced by plasma enhanced chemical vapor deposition under conditions close to powder formation. It has a lower initial and stabilized density of states, and a hole mobility considerably higher than state-of-the-art a-Si:H, which makes this material an interesting candidate for solar cell applications. In this article, we present experimental studies in conjunction with computer modeling to analyze and explain the relative performances of solar cells in which either a-Si:H or pm-Si:H is used as the intrinsic layer. Our results reveal large differences in the transport and metastability behavior of the two types of solar cells. Moreover, we observe a more damaged p/i interface for the pm-Si:H cells, although the p and n layers have been deposited under identical conditions. As a consequence, the cells fabricated initially with pm-Si:H did not perform better than standard a-Si:H based cells, despite the fact that the model confirmed the better transport properties of pm-Si:H films with respect to a-Si:H. Using insight gained from modeling, the deposition parameters were optimized to ultimately yield pm-Si:H based solar cells with conversion efficiencies higher than a-Si:H based cells in both the annealed and the light stabilized states.