Revealing Exciton Dynamics in a Small-Molecule Organic Semiconducting Film with Subdomain Transient Absorption Microscopy

Abstract

The ultrafast spectroscopy of single domains of polycrystalline films of TIPS-pentacene, a small-molecule organic semiconductor of interest in electronic and photovoltaic applications, is investigated using transient absorption microscopy. Individual domains are distinguished by their different polarization-dependent linear and nonlinear optical responses. As compared to bulk measurements, we show that the nonlinear response within a given domain can be tied more concretely to specific physical processes that transfer exciton populations between specified electronic states. By use of this approach and a simple kinetic model, the signatures of singlet fission as well as vibrational relaxation of the initially excited singlet state are identified. As such, observing exciton dynamics within and comparing exciton dynamics between different TIPS-pentacene domains reveal the relationship between photophysics and film morphology needed to improve device performance.

Relating the Physical Structure and Optoelectronic Function of Crystalline TIPS‐Pentacene

“…As described elsewhere, [ 26 ] this solution is fi ltered (0.2 µm pore size) and dropcast onto clean glass substrates on top of a hot plate held at ≈60 °C. To control the evaporation rate, samples are covered with a petri dish after deposition, yielding ≈600 nm-thick fi lms with fi ngerlike domains 10−150 µm wide and up to hundreds of µm long.…”

“…Moreover, a spectro-microscopic approach is used to experimentally probe the optical properties of single crystalline domains within polycrystalline fi lms. [ 26 ] Spatially-resolved polarization angle-dependent absorption measurements reveal that individual grains within the polycrystalline fi lm have the same crystal structure but different relative orientation. Our quantitative calculations elucidate the relationship between the polarization-dependence of the absorption and the solid-state structure, allowing the determination of the relative orientation of an arbitrary pair of adjacent grains.…”

Polarization-dependent exciton dynamics in tetracene single crystals

“…The polarization-dependence of singlet signal is in consistence with a recent study on TIPS-pentacene films where the TA signal from single crystalline grains was well characterized. 53 Once the polarization dependences of the singlet and triplet excitons are established, we can assign the spectral profiles of the transitions involved. Roughly speaking, the late-stage signal (~ 3 ns, Fig.…”

Self-Phase-Stabilized Heterodyne Vibrational Sum Frequency Generation Microscopy

“…16 By spatially resolving interfaces and domains, microscopy could obtain domain specific knowledge and mitigate ensemble averaging. [17][18][19][20][21] Yet, in traditional optical microscopy, signals of non-centrosymmetric molecular interfaces and domains are often overwhelmed by those of isotropic bulk materials. Even-order nonlinear optical microscopy, such as second harmonic generation (SHG) microscopy, 22 has been a popular technique to image non-centrosymmetric systems and is widely implemented in biophysics.…”