Non-technical Abstract: This project addresses the properties of a promising kind of semiconductor material, called hybrid organic-inorganic perovskites, upon interaction with light, in the context of a variety of envisioned technologies such as solar cells, light-emitting diodes, and lasers. The atoms that constitute these materials produce substantially "shaky" crystal lattices, in which at ambient conditions the motion of interconnected atoms is vigorous. In spite of this highly dynamic environment, particles that arise from interaction with light, so-called excitons, interact amongst each other strongly and are stable. An analogy would be that of a perfectly choreographed set of multiple dancers in perfect choreography during a high-magnitude earthquake. The details on how and for how long these particles interact is of central importance in this project. To study these details, researchers at Georgia Tech will develop experimental methods based on lasers producing very short bursts of light (femtoseconds, shorter than amillionth of a billionth of a second) to study selectively their mutual interactions as a function of time. In parallel, researchers at the University of Houston will develop theoretical techniques to model the experiments. These collaborative activities will result in new understanding of the fundamental materials physics of these exciton semiconductors, which will facilitate the next significant breakthroughs in optoelectronic devices for solid-state lighting and lasers.
This collaborative project will develop a coherent two-dimensional (2D) photoexcitation spectroscopy optimized to probe multi-particle correlations in semiconductor materials implemented in functional optoelectronic devices. The technique is based on phase modulation of a sequence of four variably delayed ultrashort laser pulses, and phase-sensitive detection of the photocurrent induced by that pulse sequence. This permits acquisition of 2D coherent spectra by isolating specific nonlinear contributions to the photocurrent excitation signal. This methodology has been previously implemented by several researchers. Recently it was demonstrated that an incoherent contribution to the measured line shape, arising from nonlinear population dynamics over the entire photoexcitation lifetime, generates a similar line shape to the expected 2D coherent spectra in many condensed-phase systems, in which photoexcitations are sufficiently mobile such that inter-particle interactions are important on any time scale, including those long compared with the 2D coherent experiment. The co-investigators propose a heterodyne detection scheme that surmounts this limitation, and will exploit the technique to study multi-particle correlations involving excitons and charge-carriers in two-dimensional, quantum-well-like hybrid organic-inorganic metal-halide perovskites. The specific objectives are (i) to develop a heterodyne detection protocol for photocurrent-detected 2D coherent multiexcitation spectroscopy and to demonstrate it in a highly efficient GaAs photodetector, in an optimized methylammonium lead-iodide perovskite solar cell, and in a phenylethylammonium lead-iodide 2D perovskite light-emitting diode; (ii) to develop a multiquantum 2D variant based on the same detection scheme and to study multi-excitons and trions in the 2D perovskite device above, and (iii) to develop an accessible, general modeling protocol for the photocurrent-detected 2D coherent excitation spectra based on parametrized lattice Hamiltonians, which connects early-time non-Markovian dynamics with the time-integrated response.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
