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The electronic doping mechanism of the Spiro-OMeTAD (2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene) with LiTFSI (lithium bis(trifluoromethylsulfonyl)imide) and tBP (4-tert-Butylpyridine) has not been fully understood. Spiro-OMeTAD is a highly studied hole transport layer in solar cells and, especially, perovskite solar cells (PSCs). Its p-doping not only improves the hole transport characteristics and thus the photocurrent but also increases the band bending at the interface with the perovskite absorber and thus the cell photovoltage. A better understanding of the p-doping of Spiro-OMeTAD and its interface energetics with MAPI would contribute to optimization strategies for a better overall efficiency of the PSC. Until now, doping mechanisms were studied separately for liquid and thermal evaporation processing. It was impossible to pinpoint the similarities and differences between the two approaches. Using an integrated ultra-high vacuum cluster tool (DAISY-SOL), we prepared LiTFSI-doped Spiro-OMeTAD samples by spin-coating under an inert atmosphere and by thermal co-evaporation in a vacuum. Without contact with the atmosphere, the thin films were then characterized by photoelectron spectroscopy to deduce their electronic properties. It has been evidenced that with thermal co-evaporation, LiTFSI alone dopes the Spiro-OMeTAD layer, while for spin-coating, the additive tBP is required to prevent LiTFSI precipitation. Both processes give a more p-doped Spiro-OMeTAD layer by shifting its Fermi level toward the highest occupied molecular orbital (HOMO) at best by 0.65 eV. This results in a final energy difference between the Fermi level and the HOMO onset of 0.4 eV for the most p-doped sample. The co-evaporation process of LiTFSI and Spiro-OMeTAD was then used to perform interface experiments of doped Spiro-OMeTAD on MAPI and of Au on doped Spiro-OMeTAD to study a simplified solar cell structure SnO2| MAPI | Spiro-OMeTAD | Au. Results confirm that the band alignment is suitable for electron blocking and hole extraction. Under light, a surface photovoltage of at least 0.65 eV is measured at the MAPI | Spiro-OMeTAD interface, making it the functional key interface. These interface experiments provide a not only detailed but also quantitative picture of the band alignment both in the dark and in operation under illumination under open circuit conditions.

One of the great challenges of hybrid organic–inorganic perovskite photovoltaics is the material’s stability at elevated temperatures. Over the past years, significant progress has been achieved in the field by compositional engineering of perovskite semiconductors, e.g., using multiple-cation perovskites. However, given the large variety of device architectures and nonstandardized measurement protocols, a conclusive comparison of the intrinsic thermal stability of different perovskite compositions is missing. In this work, we systematically investigate the role of cation composition on the thermal stability of perovskite thin films. The cations in focus of this study are methylammonium (MA), formamidinium (FA), cesium, and the most common mixtures thereof. We compare the thermal degradation of these perovskite thin films in terms of decomposition, optical losses, and optoelectronic changes when stressed at 85 °C for a prolonged time. Finally, we demonstrate the effect of thermal stress on perovskite thin films with respect to their performance in solar cells. We show that all investigated perovskite thin films show signs of degradation under thermal stress, though the decomposition is more pronounced in methylammonium-based perovskite thin films, whereas the stoichiometry in methylammonium-free formamidinium lead iodide (FAPbI3) and formamidinium cesium lead iodide (FACsPbI3) thin films is much more stable. We identify compositions of formamidinium and cesium to result in the most stable perovskite compositions with respect to thermal stress, demonstrating remarkable stability with no decline in power conversion efficiency when stressed at 85 °C for 1000 h. Thereby, our study contributes to the ongoing quest of identifying the most stable perovskite compositions for commercial application.

The surface, interface, and bulk properties are a few of the most critical factors that influence the performance of perovskite solar cells. The photoelectron spectroscopy (PES) is used as a technique to analyze these properties. However, the information depth of PES is limited to 10–20 nm, which makes it not suitable to study the complete devices, which have a thickness of ∼1 μm. Here, we introduce a novel and simple technique of PES on a tapered cross section (TCS-PES). It provides both lateral and vertical resolutions compared to the conventional PES so that it is suitable to study a complete perovskite solar cell. It offers many benefits over conventional PES methods such as the chemical composition in the micrometer scale from the surface to the bulk and the electronic properties at the multiple interfaces. The chemical natures of different layers of the perovskite-based solar cells [(FAPbI3)0.85(MAPbBr3)0.15] can be identified precisely for the first time using the TCS-PES method. We found that the perovskite layer has higher iodine concentration at the Spiro/perovskite interface and higher bromine concentration at the TiO2/perovskite interface. UPS measurements on the tapered cross section revealed that the perovskite is n-type, and the solar cell studied here is a p-n-n structure type device. The unique possibilities to analyze the complete solar cell by XPS and UPS allow us to estimate the band bending in a working solar cell. Moreover, this technique can further be used to study the device under operating conditions, and it can be applied in other solid-state devices like solid electrolyte Li-ion batteries, LEDs, or photoelectrodes.

Using the one‐step flash evaporation technique, it is possible to deposit a film of methylammonium tin iodide (MASI, CH3NH3SnI3) from the solid perovskite powder that is prepared by a mechanochemical synthesis, without the use of any solvents. The source material and the film are characterized by X‐ray photoelectron spectroscopy (XPS) and X‐ray diffraction (XRD). The XPS measurements show that the MASI film is stoichiometric and is a p‐type material with the Fermi level of 0.4 eV above the valence band maximum (VBM) and a bandgap of 1.3 eV. The XRD pattern of the film reveals the formation of MASI perovskite of high purity, crystallizing with pseudo‐cubic symmetry, having the lattice parameters a≈ c= 6.239(8) Å. Using the one‐step flash evaporation technique, it is possible to deposit a film of methylammonium tin iodide (MASI, CH3NH3SnI3) from the solid perovskite powder that is prepared by a mechanochemical synthesis, without the use of any solvents.