The conventional active layer structure for lead halide-based perovskite solar cells consists of an electron transporting layer (ETL), the perovskite layer, and a hole transporting layer (HTL). The ambipolar charge transport nature of the perovskite layer requires the presence of both the ETL and the HTL for efficient charge extraction. In this presentation, we report a systematic analysis of charge carrier transport in perovskite hybrid solar cells based on deterministic charge carrier transport models. The models describe the transport, i.e., diffusion and drift, of a single type of charge carrier in the ETL and the HTL. In the perovskite layer, the dynamics of both electron and hole transport and of charge generation, in conjunction with the kinetics of bimolecular recombination, are accounted for. In each layer, the charge transport equations are coupled self-consistently with Poisson’s equation for the electrostatic potential.
Our models predict the influence on the photovoltaic device performance of the charge mobilities, the active layer thicknesses, as well as the energy barriers for charge extraction at the ETL/perovskite interface, the perovskite/HTL interface, and the active layer/electrode interfaces. The computed charge density and electric field distributions provide valuable insights into the multi-physical processes that govern charge transport in perovskite hybrid solar cells. Using the model predictions for photocurrent-voltage (I-V) relations to fit I-V experimental measurements for devices with PCBM and PEDOT:PSS layers used as ETL and HTL layers, respectively, we find that the electron-hole bimolecular recombination rate is lower by orders of magnitude than that predicted by Langevin recombination theory. We have also demonstrated that the overall device performance is improved by effectively incorporating multi-walled carbon nanotubes (MWCNTs) into the perovskite layer due to the reduction of the bulk recombination rate and the resultant increase in the open-circuit voltage. We have found that the device efficiency is maximized for a certain (optimal) concentration of MWCNTs in the active layer. The model predictions, in full agreement with experimental measurements, provide valuable input toward optimal design of perovskite-based hybrid solar cells for next-generation photovoltaics.