Charge carrier and exciton dynamics in organic-inorganic perovskite compounds
African, American and Oceania Studies
Physical Chemistry of Solids and Surfaces, Material Characterisation
Final Report Abstract
Following groundbreaking experiments in 2009, the organic-inorganic lead halide perovskite CH3 NH3 PbI3 has become the probably most promising candidate material for efficient, easily processable, and affordable new thin film solar cell technology. Within the research project, I performed pump-probe experiments on thin films of CH3 NH3 PbI3 to investigate their reaction to light on an ultrafast timescale. Within a pump-probe experiment, the material is exposed to a short laser pulse (”pump”) to mimic the exposure to sunlight. After an adjustable time delay, a second laser pulse (”probe”) probes changes in the physical properties of the thin film, like its absorption or the energy of the electrons excited by the pump pulse. By varying the pump-probe delay, a video is recorded, which follows the dynamics of the photoexcitation with a temporal resolution of femtoseconds. These measurements aim at creating a basic understanding of the underlying microscopic physical processes giving rise to the outstanding performance of lead halide perovskite solar cells. This understanding may be used to optimize the solar cell architecture, in particular interfaces to electric contacts. It may help to develop alternative, more stable and less toxic materials which provide similar functionality. The main findings are: (1) Defects like structural defects, which limit the performance of devices, are predominantly located at surface of lead halide perovskites. Interfaces, in particular to electrodes, need to be designed carefully to minimize these defects. Followup experiments on the interface between perovskites and potential electrode materials are on the way. (2) Illumination of the perovskites creates singleparticle excitations. This means that the current created by illumination with light is carried by electrons which can move almost freely through the material, possibly distorting the crystal slightly. The interaction between electrons, in particular exciton formation, is negligible at room temperature. (3) Those electrons which interact with a blue, high-energy photon, keep a high energy for an unusually long time (> 100 ps). This finding represents the main result of the project. In any other known crystalline semiconductor material, high-energy electrons loose most of their energy to heat before they can be harvested as electric power. This process ultimately limits the efficiency of nowadays’ solar cells. Harvesting the high energy stored in the electrons in CH3 NH3 PbI3 would create a new type of solar cell of much higher maximum efficiency.
Publications
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Manybody interactions in photo-excited lead iodide perovskite. Journal of Materials Chemistry A 17, 9285 (2015)
M. Tuan Trinh, Xiaoxi Wu, Daniel Niesner, and X.-Y. Zhu
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Trap States in Lead Iodide Perovskites. Journal of the American Chemical Society 137, 2089 (2015)
Xiaoxi Wu, M. Tuan Trinh, Daniel Niesner, Haiming Zhu, Zachariah Norman, Jonathan S. Owen, Omer Yaffe, Bryan J. Kudisch, and X.-Y. Zhu
