Carrier Dynamics of Nanopillar Textured Ultrathin Si Film/PEDOT:PSS Heterojunction Solar Cell

Computational study of nanopillar Si/poly (3,4-ethylene dioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) heterojunction solar cell is performed by using finite element method to solve Poisson equation, drift-diffusion equations, and current continuity equations. The depletion width around the edge of the Si nanopillar is almost the same as that of planar junction, while the depletion width around the middle of the Si nanopillar is much wider than that of planar junction. As a result, besides the enhanced light absorption due to light trapping induced by nanopillar array as in previous studies, larger depletion region, which leads to higher electron-hole separation efficiency under light illumination, is another reason for short-circuit current increase of nanopillar solar cell. The reason for less open-circuit voltage of nanopillar solar cell is also clarified by explaining the reason for large dark current of the nanopillar solar cell compared to its planar structure counterpart as follows. The kink of the potential profile along the lines around the edge nanopillar leads to much sharper potential drop in the following depletion region, and results in greater hole density slope when the solar cells are forwarded biased. Since the Si/PEDOT:PSS behaves like a pn junction rather than a Schottky junction as clarified in the previous study and the PEDOT:PSS is much highly doped, nanopillar heterojunction has larger dark current than planar junction due to the larger hole diffusion current around Si nanopillar edge.