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.
Highlights
Organic semiconductors are of interest for a large variety of light-emitting[1,2] and light-harvesting applications.[3,4] These materials can be readily synthesized, integrated into devices,[5] display a high degree of electronic tunability,[6] and have high absorption cross sections.[7]
The time resolution of current transient absorption microscopy setups, is typically restricted to ∼100−200 fs, preventing direct insight into the ultrafast dynamics occurring directly after photoexcitation.[19−25] To overcome this shortcoming, we recently developed an ultrafast extension of transient absorption microscopy, which can access sub-100 fs exciton transport behavior in real time and space.[26−28]
By carefully considering the microstructure of the grains and the inherent crystallinity, we identified two transport regimes occurring on the sub-3 ps time scale
Summary
Organic semiconductors are of interest for a large variety of light-emitting[1,2] and light-harvesting applications.[3,4] These materials can be readily synthesized, integrated into devices,[5] display a high degree of electronic tunability,[6] and have high absorption cross sections.[7]. Ultrafast pump−probe spectroscopy has been used to measure exciton diffusion lengths in a wide variety of organic and inorganic materials, with transport lengths of up to 1 μm reported in some cases.[9−15] Such studies, require careful modeling of the observed photoresponse, which involves several approximations that can greatly affect the extracted transport parameters. Ensemble-level spectroscopic studies average out the effect of micromorphologies due to the large sample volume probed simultaneously, further complicating accurate modeling of the transport parameters.[16−18]
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