Kinetic Monte Carlo simulation of transport in amorphous silicon passivation layers in silicon heterojunction solar cells

Silicon heterojunction solar cell device structures use carrier-selective contacts to maximize collection of photogenerated carriers. The carrier-selective contact structure consists of doped hydrogenated amorphous silicon and intrinsic hydrogenated amorphous silicon [a-Si:H(i)]. In this structure, the a-Si:H(i) layer plays a crucial role as it passivates the heterointerface between the doped hydrogenated amorphous silicon and the crystalline silicon enabling the solar cell to achieve high device efficiencies. However, the a-Si:H(i) layer also creates a potential barrier to photogenerated carriers which obstructs them from getting collected. Previously, experimental studies in the literature have predicted that the photogenerated carriers cross the barrier by defect-assisted transport (hopping). Traditionally, theoretical models that are employed to study the electrical characteristics of silicon heterojunction solar cells do not provide any great insight into the transport of carriers via defects. In this paper, we present an in-house developed kinetic Monte Carlo that simulates the transport of photogenerated holes through the band tail defect states in the a-Si:H(i) layer. This is done primarily by defining transition rates associated with carrier-defect interactions. We conduct simulations to understand the impact of the properties (optical phonon energy, defect density, etc.) of the a-Si:H(i) layer on transport of photogenerated holes. Our simulations indicate that multi-phonon injection and hopping processes assist photogenerated holes to cross the a-Si:H(i) layer, which is in agreement with experimental findings.