Nadja Isabelle Desiree Klipfel
A new and promising technology has emerged to compete with traditional silicon photovoltaic semiconductors. The low material and processing costs and high-power conversion efficiencies of organic-inorganic metal halide perovskites make these a promising technology for the future. Despite record efficiencies for small-area devices, stability remains challenging. Due to rapid progress in the field, avenues for large-area commercial production are being explored. One approach is via thermal evaporation, which is presented in this work. The focus is on the vacuum-deposition method itself, the interface engineering of perovskite layers, and the study of a family of low-cost hole-transporting materials (HTM). To improve reproducibility among research groups in vacuum-deposition processes, a better understanding and control of the crystal formation is essential. Of the experimental parameters, two were investigated that were not yet considered in-depth or combined: deposition speed and underlayer material. It was shown that perovskite growth rate changes affect preferred crystal orientation. In addition, it was found that the final perovskite composition is influenced by its underlayer chemistry. Multi-source vacuum deposition of perovskite layers is complex due to the laborious calibration process necessary for proper stoichiometry. Single-source evaporation of pre-synthesized perovskite powders can reduce effort and time. The feasibility of obtaining pure phase films was demonstrated and confirmed by multiple analysis methods.Interfaces in contact with the perovskite can be a source of defects with a deteriorating effect on stability and efficiency. The charge transport processes at the interface of electron transport material (ETM) and perovskite were investigated. The results showed a synergetic effect between cTiO2 and C60 and the importance of C60 as a charge extraction layer. The perovskite hole transporting material (HTM) interface was also systematically studied. Monitoring of dopants showed, for the first time in literature, TFSI anion migration through grain boundaries to the perovskite surface. It was further shown that non-radiative recombination pathways are reduced, indi-cating self-passivating of crystal defects during migration processes.The state-of-the-art HTM 2,2',7,7'-tetrakis (N,N-di-p-methoxyphenylamine)-9,9'spirofluorene (spiro-OMeTAD) is widely employed in spin-coated PSCs. Devel-oping a less costly manufacturing alternative to this material that retains similar properties is a key challenge. A new class of Zn (II) and Cu (II)-based phthalocyanine HTMs functionalized with butyl- and ethylhexyl- groups in the periphery was investigated. Devices fabricated with the Zn (II)-based HTM (n-butyl functionalization) demonstrated the best performance. The device architecture with the highest stability maintained 80% of its initial power conversion efficiency (PCE) over 20h of illumination. At 20.2%, the perovskite architecture with the highest PCE also was the least stable, indicating a trade-off between these two characteristics for this HTM class.
EPFL2022
