Understanding and control over defects in halide perovskites through defect chemical studies combined with in situ optical characterization and detailed optical spectroscopy

• Applicants: Professorin Dr. Anna Köhler; Professor Dr.-Ing. Ralf Moos
• Subject Area: Synthesis and Properties of Functional Materials
• Term: since 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 324052211

Due to the excellent optoelectronic properties of halide perovskites, their use as an efficient semiconductor material in many applications such as solar cells, light-emitting diodes, or in detectors for light, X-rays and gamma radiation is being discussed. A remarkable feature of halide perovskites in this context is their high tolerance and self-healing ability towards intrinsic ionic point defects. Nevertheless, these defects can affect the optoelectronic properties and, for example, cause ions to move through the halide perovskite when a potential difference is applied, leading to efficiency losses and degradation of the properties of optoelectronic devices based on halide perovskites. Therefore, the goal of this project is to gain a deep understanding of how various point defects in halide perovskites affect their optoelectronic properties and to understand how point defect concentrations can be tuned through controlled modifications of the perovskites. Key to achieving these goals is the combination of defect chemistry studies with optical measurements on halide perovskite samples with well-defined properties. The basic investigations involve the precise setting of an iodine partial pressure that leads to a thermodynamically defined interaction of the perovskites with iodine and thus to an equilibrium concentration of point defects that depends on the iodine partial pressure and the temperature. Such an approach is standard for oxide perovskites. In the previous project, it was also successfully demonstrated for methylammonium lead iodide that such an approach can work. In the proposed continuation project, the resulting point defect concentrations will be measured via the electrical conductivity, while the effects of these point defects on the optoelectronic properties will be studied directly via in-situ optical characterization, i.e., via the measurement of absorption and emission during film formation. In addition, doping will be used to target the defect concentrations in order to refine the defect chemical model and to determine quantitatively formation enthalpies. In order to make statements about the nature of the excited states and their relaxation, to identify defects and impurities, and to determine the location of defects in the band gap, powerful optical ex-situ measurement methods, such as transient absorption, temperature-dependent and time-dependent measurement of photoluminescence and thermally stimulated luminescence, are used. Overall, with the combination of in-situ and ex-situ investigations, a comprehensive defect chemical model will be established, which will allow to target defect concentrations, to understand the effects on optoelectronic properties, and to improve optoelectronic devices.

• DFG Programme: Research Grants