In this study, we present an optoelectronic simulation study that enhances the understanding of interface engineering of electron and hole transport layers (ETL, HTL) and their impact on the performance of perovskite-based tandem cells. The complexity of the structures, the metastability and the intrinsic perovskite bulk make it difficult to understand and quantify electrical losses of perovskite-based tandem solar cells without simulation. However, in recent years, the predictive power of simulation models has significantly improved through the improvements of models and multiple comparisons with experimental perovskite results [1–4].

At the ETL side, in this case C₆₀/perovskite, we address two important parameters influencing device performance: the quality of chemical passivation, indicated by the surface recombination velocity (S_0,ETL), and the conduction band offset (ΔE_C,ETL) at the perovskite/C₆₀ interface (Fig 1A), which can be optimized experimentally by dipole engineering at the ETL interface. We show that a reduction of the conduction band offset results in an increased ratio of majority to minority charge carriers, suppressing recombination losses at the interface and minimizing selectivity losses (defined as difference between internal and external voltage), within the device (Fig 1B). Notably, we demonstrate that the reduction of the ETL/perovskite conduction band offset not only enhances so-called field-effect passivation, thereby suppressing interface recombination, but also results in an electron accumulation throughout the perovskite absorber, thereby enhancing conductivity and reducing transport losses, which are crucial for improving fill factor (FF) and therefore efficiency.

For the HTL side, we show state-of-the-art modelling of self-assembled monolayers (SAMs) on transparent conductive oxides (TCOs). The perovskite/SAM/TCO interface is investigated and the influence of the SAM dipole moment, TCO doping concentration and TCO electron affinity on the device performance starting from the baseline TCO (here indium tin oxide (ITO)) are studied (Fig 1C). We showcase that the ITO properties cause selectivity losses reflected in a Fermi level gradient towards the ITO/SAM stack which results in an open-circuit voltage of 1245 mV that is significantly lower than the high internal voltage 1323 mV (Fig 1D). We could show both by simulation and experimentally, that by replacing ITO with zinc tin oxide (ZTO) the performance of the perovskite top cell is enhanced, due to lower electron concentration and higher effective work function of the ZTO. This results in an increase of both internal voltage (from 1323 mV to 1339 mV) and external voltage (from 1245 mV to 1302 mV), whereby the selectivity losses are significantly reduced. The effect of field-effect passivation of a C₆₀ or TCO strongly depends on the perovskite bandgaps in the perovskite-based tandem cell. In the conference presentation, the interplay of perovskite bandgap and transport layer stack will be further discussed and the losses quantified.

Overall, this study contributes to an enhanced understanding of interface physics and supports the optimization of perovskite-based tandem solar cells.