Organic-inorganic halide perovskite (OIHP) nanocrystals (NCs) are emerging materials that show significant promise for applications as optoelectronic emitters. Their narrow, tunable emission spectrum and high photoluminescence quantum yields, coupled with a facile, solution-based synthesis, makes OIHP NCs ideal for applications such as LEDs, lasers, and optical sensors. The surface-area-to-volume ratio of NCs results in device performance that is largely dependent on surface properties. A complete understanding of the relationship between surface defects and electronic structure is unknown; notably, the mechanism by which surface states become bound by ligands during growth is poorly understood. Under-coordinated atoms at the NC surface can trap charge carriers, promoting non-radiative recombination. This process is typically studied using time-resolved, non-linear optical spectroscopies such as transient absorption (TA), but these techniques require long measurement timescales, sometimes on the order of hours. OIHP NC syntheses, such as hot-injection, are complete within a few minutes, and the growing NCs are highly unstable. These factors preclude in situ investigation of excited state dynamics with standard time-resolved spectroscopies. Here, we have developed a single-shot TA (SSTA) spectrometer that spatially encodes the pump-probe time with a tilted-pulse geometry. The reduced timescale for data acquisition permits collection of TA spectra, with a 60 ps time delay range and broadband wavelength resolution, within seconds. This novel instrument, combined with a solvation-mediated NC synthesis, has permitted us to report the first ever measurement of evolving dynamics in growing OIHP NCs. SSTA spectra of nascent NCs display a unique spectral lineshape that decreases in amplitude as the NCs grow. This is attributed to a Stark shift arising from spatially trapped carriers at the NC surface, indicating young NCs possess surface traps that become passivated near the end of the synthesis. This ephemeral lineshape makes possible a complete exploration of the reaction parameter space, allowing us to adjust conditions and directly observe the effect on surface passivation kinetics. Our experiments permit a much deeper understanding of OIHP NC formation and can yield fundamental insights into the processes of nucleation and growth. This mechanistic understanding will reveal novel reaction pathways that allow for greater synthetic control of NC properties.

Investigating perovskite nanocrystal growth via single shot transient absorption (SSTA) spectroscopy

ACS Spring Meeting 2021

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