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Spatially Resolving the Coexistence of Photoexcitations in Complex Materials

Multidimensional transient absorption microscopy data shows regions with either excitons (red) or free charge carriers (blue), but the majority of the image contains a mixture. Isolating the signatures of these distinct photoexcitations enables quantitative unmixing of their respective contributions across the entire image as shown in the two maps (on the right) of individual photoexcitation species.


Spatial localization of distinct photoexcited species were identified in light harvesting perovskite based materials using ultrafast transient absorption microscopy (TAM). 

Organometal halide perovskite-based solar cells represent a major breakthrough in emerging photovoltaic technology as the reported power conversion efficiency has extended beyond 20%. However, the fundamental photophysics underlying this remarkable performance was, until now, poorly understood and a topic hotly debated. A key fundamental question limiting our basic understanding of hybrid perovskite photophysics pertains to the nature and distribution of the elementary photoexcitations that are generated immediately after the absorption of light, which can be either free charge carriers or bound electron-hole pairs (i.e., excitons). The intrinsic differences between these species leads to vastly different descriptions of excited state processes and energy flow inside a solar cell.

In this work, the distributions of free charge carriers and excitons are spatially resolved on a microscopic level. Using femtosecond transient absorption microscopy (TAM), we were able to acquire ultrafast snapshots of electronic excitations, which describe the spatially-dependent relaxation dynamics evolving on the femtosecond and picosecond timescales. However, analysis of the multi-dimensional TAM data set is generally challenging, especially for those systems with high spatial heterogeneity and complicated electronic excited-state dynamics. In this paper, we demonstrate a combined approach that allows us to separate the contributions of both excitons and free charge carriers in the observed transient absorption response from a composite organometallic lead halide perovskite film on a microscopic level. Quantitative decomposition of the transient absorption response curves further enables us to estimate the relative contribution of each photoexcitation to the measured response at spatially distinct locations in the thin film sample. This advance in mapping the spatially dependent photoexcited species that are formed upon absorption of light may prove to be a valuable tool for improving material synthesis and fabrication techniques aimed at achieving higher power conversion efficiencies.  



M. J. Simpson, B. Doughty, B. Yang, K. Xiao, Y.-Z. Ma, Separation of Distinct Photoexcitation Species in Femtosecond Transient Absorption Microscopy. ACS Photonics 2016, DOI: 10.1021/acsphotonics.5b00638