Metal halide perovskite semiconductors hold great promise for future photovoltaic applications. The efficiency of metal halide perovskites solar cells (PSCs) is determined by several parameters, one of them is the open circuit voltage (V_OC). In order to achieve high efficiency solar cells, it is important to reach a high V_OC. Consequently, we want to understand the loss mechanisms limiting the V_OC. It is known that the V_OC loss is dominated by non-radiative recombination occurring e.g. in the bulk of the perovskite, or at the interfaces. Measuring the V_OC of a PSC in order to understand what exactly causes the non-radiative recombination and where in PSC, will not provide the answer, because it is just the overall result. Therefore, we try to understand the V_OC loss in perovskite solar cells via absolute photoluminescence intensity analysis. With this analysis, the quasi Fermi level splitting (QFLS) of different components in a PSC can be determined_. Since the QFLS is related to the maximum V_OC achievable for a photovoltaic device, this relation can be used for predicting device properties.

In this work, the effect of passivation on defects and the non-radiative recombination of charge carriers in mid to wide bandgap perovskite solar cells at the perovskite and electron transport layer interface is examined with absolute PL to understand the detrimental processes happening in the PSCs. We begin by analyzing the voltage loss for p-i-n devices and by describing the passivation of surface defects at the interface of the mid-bandgap perovskite with the electron transport layer using a solution-processed quaternary ammonium halide (choline chloride, CCl) or an evaporated ultrathin lithium fluoride (LiF) layer, that translates to solar cells with a high V_OC. We extend this passivation evaluation to an array of wide-bandgap perovskite devices. After this analysis, we demonstrate that non-radiative recombination is primarily located in the film and that it increases with increasing bandgap. The other prominent loss is located near the interface of the perovskite with the electron transport layer and is passivated equally for both passivation techniques and for all bandgaps investigated in this study. This implies that the surface treatment with two different materials result in the same gain. To further understand the working principle of the passivation strategies, we extend the passivation study by increasing the amount of passivation material in between the perovskite-ETL interface. It is found that upon application of increasing thicknesses of LiF or increasing concentration of CCl in between the perovskite/ETL interface, the gain in QFLS and V_OC starts plateauing and an optimum thickness or concentration should be used.

In summary, by combining the absolute photoluminescence spectroscopy results, it is found that by using a LiF or CCl a significant reduction of the non-radiative recombination losses that currently limit wide-bandgap PSCs is achieved. And that this gain is primarily due to minimizing the loss at the perovskite-ETL interface. These results contribute to solving the main challenges of the wide-bandgap perovskite in achieving a high V­_OC.