p-n Homojunction perovskite solar cells: effects of ionic density and thickness of the doped layers

The p-n homojunction perovskite solar cells are promising in comparison to planar heterojunction perovskite solar cells. It is observed that the p-n homojunction provides an efficient built-in electric field for the photo-generated electron-hole pairs to dissociate into free charge-carriers and orient them to transport to the respective electrodes, thereby reducing the overall losses due to recombination processes. However, the existence of p-n homojunction, while in operation, is currently undergoing experimental tests. In this study, we simulated the current-voltage characteristics of both planar heterojunction and p-n homojunction perovskite solar cells. For simulation, we utilised a one-dimensional drift-diffusion equation. The current-voltage characteristics show a high fill factor for the p-n homojunction devices indicating better power conversion efficiency as compared to that of planar heterojunction devices. In addition, we report the spatial distribution of electron- and hole-density with variations in (i) the mobile ionic density, inherently present in the perovskite material and (ii) the thickness of p- and n-type perovskite layers. It is observed that the role of p-n homojunction ceases when the ionic density is higher than 10(17) cm(-3) or when one of the doped layers is substantially thicker as compared to the other. We correlate the cease of p-n homojunction to (i) the ionic effect which screens the built-in field, and (ii) the predominance of the effect of one doped layer over the other. The study provides impetus to the development of the p-n homojunction perovskite solar cells.