Ultrafast Tracking of Exciton and Charge Carrier Transport in Optoelectronic Materials on the Nanometer Scale

Abstract

We present a novel optical transient absorption and reflection microscope based on a diffraction-limited pump pulse in combination with a wide-field probe pulse, for the spatiotemporal investigation of ultrafast population transport in thin films. The microscope achieves a temporal resolution down to 12 fs and simultaneously provides sub-10 nm spatial accuracy. We demonstrate the capabilities of the microscope by revealing an ultrafast excited-state exciton population transport of up to 32 nm in a thin film of pentacene and by tracking the carrier motion in p-doped silicon. The use of few-cycle optical excitation pulses enables impulsive stimulated Raman microspectroscopy, which is used for in situ verification of the chemical identity in the 100–2000 cm–1 spectral window. Our methodology bridges the gap between optical microscopy and spectroscopy, allowing for the study of ultrafast transport properties down to the nanometer length scale.

Flickering Polarons Extending over Ten Nanometres Mediate Charge Transport in High‐Mobility Organic Crystals

“…A similar relaxation dynamics was found by Schnedermann et al. [ 38 ] by using optical transient absorption spectroscopy in the context of exciton and charge transport in a thin pentacene film. As we describe further below the polaron is a highly dynamical species constantly changing shape and extension.…”

Non-Equilibrium Carrier Transport in Strongly Coupled Quantum Dot Solids and Heterostructures

“…A near-diffraction-limited pump pulse, centred at 580 nm (Supplementary Fig. 4), was used to excite the sample, generating a gaussian shaped excited carrier distribution 25 . A time-delayed wide-field probe pulse was applied to spatially resolve the transient response and therefore monitor the distribution of carrier population as a function of time.…”

Observation of an Ultrafast Exciton Transport Regime at Early Times in Quantum Dot Solids

“…Interestingly, two distinct transport regimes seem to exist in the MSD profiles: a very fast initial expansion of the exciton distribution within the first ~300 fs upon photoexcitation, followed by a much slower expansion (up to 4 ps time window of measurement). To test whether it is a two-step diffusion process or an anomalous diffusion process, we fitted the MSD evolution with the power law equation as MSD = 2Dt α , where D is the diffusivity and α is the diffusion exponent 7,14,15,18 . We noticed that the MSD profiles cannot be fitted by a single power law equation, due to the abrupt transition from the fast to the slow expansion stage (Supplementary Discussion 1 and Supplementary Fig.…”

“…2b as an example. Within the range of t0 < t < tfast, the dynamics can be well described by a power law equation where α = 1, signifying that the initial exciton transport is diffusive 7,8,14,15,18 . The corresponding diffusivity (Dfast) can therefore be extracted from the slope of the fitted line.…”

“…We emphasize that such an early-time process of exciton transport in QD solids is revealed here experimentally for the first time. It has been well demonstrated that the exciton diffusivity of disordered systems such as QD and organic semiconductor solids decrease over time 7,12,13,18 . Therefore higher diffusivities in such an earlytime range (< 300 fs) can be expected.…”