as higher PCEs have been achieved due to lower parasitic absorption losses, [4,5] and their low-temperature processing enables a wider choice of bottom cells.The device stability and PCE of silicon/perovskite tandem solar cells are crucially determined by the quality of the contact layer interfaces, [6,7] as they are found to limit especially the open circuit voltage (V OC ) and fill factor. Charge carrier selective contacts and their interfaces toward MHP absorbers are thus a field of recent and ongoing research. [8][9][10] The charge carrier transport across interfaces depends on two main parameters: The energy level alignment of both materials forming the interface, and the density of defect states at the surfaces and interfaces.The fullerene C 60 has become the standard electron transport layer (ETL) for p-i-n MHP solar cells. Similarly, it has also been applied in silicon/perovskite tandem solar cells with a p-i-n structure for the perovskite top cell, yielding the highest-efficiency devices in their class. [10] However, the obtained open circuit voltage for p-i-n MHP solar cells is found to be limited by the electron selective interface due to non-radiative recombination losses, [11,12] which raises interest in thoroughly studying the energetic formation of the perovskite/C 60 interface, including the energy level alignment and density of gap states. The insertion of an ultra-thin (≈1 nm) LiF interlayer between the perovskite and C 60 has been shown to significantly increase the device V OC by reducing the non-radiative recombination while keeping a highThe fullerene C 60 is commonly applied as the electron transport layer in highefficiency metal halide perovskite solar cells and has been found to limit their open circuit voltage. Through ultra-sensitive near-UV photoelectron spectroscopy in constant final state mode (CFSYS), with an unusually high probing depth of 5-10 nm, the perovskite/C 60 interface energetics and defect formation is investigated. It is demonstrated how to consistently determine the energy level alignment by CFSYS and avoid misinterpretations by accounting for the measurement-induced surface photovoltage in photoactive layer stacks. The energetic offset between the perovskite valence band maximum and the C 60 HOMO-edge is directly determined to be 0.55 eV. Furthermore, the voltage enhancement upon the incorporation of a LiF interlayer at the interface can be attributed to originate from a mild dipole effect and probably the presence of fixed charges, both reducing the hole concentration in the vicinity of the perovskite/C 60 interface. This yields a field effect passivation, which overcompensates the observed enhanced defect density in the first monolayers of C 60 .

We report the complex refractive index of methylammonium lead iodide (CH 3 NH 3 PbI 3) perovskite thin films obtained by means of variable angle spectroscopic ellipsometry and transmittance/reflectance spectrophotometry in the wavelength range of 190 nm to 2500 nm. Film thickness and roughness layer thickness are determined by minimizing a global unbiased estimator in the region where the spectrophotometry and ellipsometry spectra overlap. We then determine the optical bandgap and Urbach energy from the absorption coefficient, by means of a fundamental absorption model based on band fluctuations in direct semiconductors. This model merges both the Urbach tail and the absorption edge regions in a single equation. In this way, we increase the fitting region and extend the conventional (α ω) 2-plot method to obtain accurate bandgap values.

The development of scalable deposition methods for perovskite solar cell materials is critical to enable the commercialization of this nascent technology. Herein, we investigate the use and processing of nanoparticle SnO2 films as electron transport layers in perovskite solar cells and develop deposition methods for ultrasonic spray coating and slot-die coating, leading to photovoltaic device efficiencies over 19%. The effects of postprocessing treatments (thermal annealing, UV ozone, and O2 plasma) are then probed using structural and spectroscopic techniques to characterize the nature of the np-SnO2/perovskite interface. We show that a brief “hot air flow” method can be used to replace extended thermal annealing, confirming that this approach is compatible with high-throughput processing. Our results highlight the importance of interface management to minimize nonradiative losses and provide a deeper understanding of the processing requirements for large-area deposition of nanoparticle metal oxides.