Metal halide perovskites have recently emerged as one of the most promising active layers in solar cells for their high power conversion efficiency (>25%) and ease of synthesis and deposition. Spatial heterogeneity is inevitable with current fabrication methods for both monocrystalline and polycrystalline perovskite thin films and crystals. The morphology-dependent imaging of the excited-state dynamics is crucial to the understanding of the conversion efficiency resulting from different fabrication methods. Key processes affecting the performance of the absorber layers include carrier recombination, carrier cooling, and carrier trapping, which mostly happen on the ultrafast timescale (sub-picosecond). To capture all the processes happening across a broad energy landscape, broadband detection is required. In this work, broadband pump-probe spectroscopy and confocal microscopy are combined into one setup, the transient absorption microscope (TAM), to realize hyperspectral imaging of the ultrafast dynamics in metal halide perovskites. Studies on both perovskite thin films and crystals were performed. The first part of the thesis is focused on mapping the morphology-related carrier dynamics in polycrystalline perovskite thin films. Fundamental carrier properties like the Fermi energy level, cooling rate, and defect concentration were extracted from the transient absorption (TA) spectrum acquired at every single pixel. Hundreds of pixels were scanned in one experiment, and the correlations between the carrier properties and the morphological information, for example, grain boundary (GB), were verified using statistical models. The high-dimensional datasets containing spatial, spectral, temporal, and morphological information create the challenge of analyzing gigabytes of correlated data, which typically takes enormous computational resources. To solve this problem, a new global analysis method based on variable projection and subsampling was invented. This method improved the spectrum-fitting speed by a factor of 1500 and was more sensitive to weak spatial and spectral features. The second part of the thesis is focused on the study of monocrystalline perovskite microcrystals or nanocrystals. As an optical microscopic tool, the spatial resolution of the TAM is limited by the light wavelength and objective lenses used. To study the impact of structural change below the diffraction limit, single-particle spectroscopy was performed on monocrystalline perovskite crystals. By varying the cations in the perovskite lattices and organic spacers between perovskite layers, different levels of lattice distortions and multiple stacking geometries were created. The lattice distortion caused by the sterically large organic spacers was found to be the determining factor in the formation of deep trap states detrimental to carrier harvesting.

Creator

• Jiang, Xinyi

DOI

• https://doi.org/10.21985/n2-5da8-f705

Subject

• Physical chemistry

Language

• en

Alternate Identifier

• etdadmin_upload_829756
• http://dissertations.umi.com/northwestern:15621

Keyword

• Carrier cooling
• Transient absorption microscopy
• Trap state
• Metal halide perovskites
• Grain Boundary
• Global analysis

Date created

• 2021-01-01

Resource type

• Dissertation

Rights statement

• In Copyright