Femtosecond Transient Absorption Microscopy of Singlet Exciton Motion in Side-Chain Engineered Perylene-Diimide Thin Films

Abstract

We present a statistical analysis of femtosecond transient absorption microscopy applied to four different organic semiconductor thin films based on perylene-diimide (PDI). By achieving a temporal resolution of 12 fs with simultaneous sub-10 nm spatial precision, we directly probe the underlying exciton transport characteristics within 3 ps after photoexcitation free of model assumptions. Our study reveals sub-picosecond coherent exciton transport (12-45 cm² s^-1) followed by a diffusive phase of exciton transport (3-17 cm² s^-1). A comparison between the different films suggests that the exciton transport in the studied materials is intricately linked to their nanoscale morphology, with PDI films that form large crystalline domains exhibiting the largest diffusion coefficients and transport lengths. Our study demonstrates the advantages of directly studying ultrafast transport properties at the nanometer length scale and highlights the need to examine nanoscale morphology when investigating exciton transport in organic as well as inorganic semiconductors.

Figure 1

Implementations of transient absorption microscopy. (a)–(c) Confocal-like microscopy based on point-scanning and high-sensitivity point detection with an avalanche photodiode (APD). (d)–(f) Wide-field microscopy based on free-space probe focusing and two-dimensional image detection. BS – beam splitter, S – sample, BP – band-pass filter, TL – tube lens, M – mirror, CM – curved focusing mirror, F_O – objective focal length, F_TL – tube lens focal length.

Figure 2

Characterization of evaporated PDI thin films measured in this work. (a) Chemical structure of PDI with various imide substituents termed PDI 1–4. (b) Absorption spectra and (c) X-ray diffractograms of PDI 1–4. The degree of crystallinity increases in the order from PDI 1 to PDI 4, which is assessed by the broadening of any diffraction peaks and AFM characterization of the domain size (Supporting Information, section 1 and section 5).

Figure 3

Femtosecond transient absorption microscopy results for the studied PDI films. (a)−(d) Normalized transient absorption kinetics (top) and associated spatial dynamics (middle) recorded at 740 nm retrieved from a 2D Gaussian fit analysis. (bottom) Resulting time-dependent mean-square displacement curves (MSD). Individual colors indicate experiments conducted at different sample locations. All pixels contributing to the ΔT/T signal were averaged to obtain the kinetics.

Figure 4

Retrieved spatiotemporal parameters for measured PDI films. (a) Box-plot representation of the fast component of the transient absorption kinetics. (b, c) Box-plot representation of the retrieved diffusion coefficient in different time segments according to eq 6 for PDI films that show transport (see main text for temporal segmentation). p-values according to a two-tailed Student t-test are indicated between each data set (gray).