Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV23)
Publication date: 30th March 2023
Organic-inorganic halide perovskites (OHPs) are well known for their defect tolerance.1 Nevertheless, the non-radiative recombination caused by defects is still the dominant factor limiting the performance of photovoltaic devices. In order to study defect states in OHPs, we have to be able to increase their density. The addition of Bismuth introduces optically active defects which make Bi-doped perovskites the perfect system to study the properties of deep defect states in OHPs.
In our work, photothermal deflection spectroscopy (PDS) is used to study the absorption spectra of MAPbBr3 single crystals. Bismuth is used to introduce bulk defects and exposure to visible light surface defects. PDS is a pump-probe type of method. It uses a mechanically chopped light beam illuminating the front side of the sample and a probe laser beam propagating along the sample's surface. Here, the probe beam detects the thermal signal generated inside the sample. The chopping frequency of the pump beam dictates probed depth, i.e., the depth out of which the thermal signal is able to reach the surface and be detected. Measurements taken at different chopping frequencies allow us to probe OHPs properties from different depths. Additional information can be obtained from the phase shift between the pump beam and the detected signal. This shift increases when the signal is generated in greater depths. Using this approach, we are able to distinguish between defects localized at the surface and defects distributed homogeneously within the bulk of OHP SC. That is crucial information when designing a specific passivation strategy for those materials.
By analysing the PDS phase shift dependence on excitation energy, we are able to localise defect states spatially. This result will be taken into account during the optimization of passivation strategies.
The authors acknowledge the support of the Operational Programme Research, Development, and Education financed by the European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (Project No.: LUASK 22202, and Czech NanoLab Research Infrastructure LM2018110).
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