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.

Tuning the Coherent Propagation of Organic Exciton‐Polaritons through Dark State Delocalization

“…We exploit these spectral signatures and the dynamical insights of our Q‐ factor dependence to interrogate the nature of polariton transport in the initial coherent regime using transient absorption microscopy (fs‐TAM). [ 58 , 59 , 60 ] In this technique, a pump pulse focused to the diffraction limit ( σ ≈ 140 nm on SiO 2 ) locally excites a region of the sample. A wide‐field counter‐propagating probe pulse then reads out the spatial pump‐probe signal generated, as a function of time.…”

“…This figure arises from our ability to resolve even subtly different Gaussian spatial pump‐probe signals, thanks to the high signal‐to‐noise of our experiment and calibration in previous studies. [ 58 , 60 , 61 ] In order to achieve ultrafast 2D spatial imaging we must sacrifice spectral resolution, so for each sample we characterize the band at 640 nm integrated over a 10 nm window, which was established above as a signature of the polariton population. This technique enables direct imaging of the lateral propagation of the polariton population, [ 34 ] a phenomenon much less studied than polariton‐mediated energy transfer between molecules separated along the cavity thickness.…”

Exciton Diffusion in Highly-Ordered One Dimensional Conjugated Polymers: Effects of Back-Bone Torsion, Electronic Symmetry, Phonons and Annihilation

“…In order to extract the exciton diffusion characteristics of “blue” and “red” PDA we performed widefield femtosecond transient absorption microscopy (fs-TAM) with a similar 10 fs pump pulse centered at 560 nm. ,, In fs-TAM a pump pulse is focused via a high-numerical aperture (NA = 1.1) objective to near the diffraction limit Gaussian spot (σ = 143 nm), which locally creates excitons within the sample. After a variable time delay, a broadband, 7 fs probe pulse centered at 780 nm is loosely focused on the sample to image the pump-induced signal changes in the material.…”

“…The femtosecond wide-field detected transient absorption microscope has been described in detail previously. , Briefly, a Yb:KGW amplifier system (LightConversion, Pharos, 5 W, 180 fs, 1030 nm, 200 kHz) was used to seed two white-light stages for pump and probe generation. The pump white-light (3 mm Sapphire) was spectrally adjusted with a 650 nm short-pass filter (FESH650, Thorlabs), and compressed to 10 fs for all optical elements with two pairs of third-order compensated chirped mirrors and a wedge-prism pair (Layertec).…”

Exciton transport in molecular organic semiconductors boosted by transient quantum delocalization

“…20 DCVSN5 is of interest because of the high exciton diffusion constant predicted for this material 30 and because it was used as an electron donor in fullerene-based heterojunctions with moderate power conversion efficiency (about 6.5%). 30,33 Finally, we study two non-fullerene acceptor materials 34 , PDI as a representative of the chemically highly tunable class of perylene diamides 35 , as well as Y6 26,36 .…”