Enhancement of charge transfer in thermally-expanded and strain-stabilized TIPS-pentacene thin films

Abstract

We present an extensive study of the optical and electronic properties of TIPS-pentacene thin films utilizing in situ x-ray diffraction, polarized optical spectroscopy, and ab initio density functional theory. The influence of molecular packing on the properties are reported for thin films deposited in the temperature range from 25 • C to 140 • C and for films that are strain stabilized at their as-deposited lattice spacings after cooling to room temperature. Anisotropic thermal expansion causes relative displacement of neighboring molecules while maintaining a nearly constant stacking distance. This leads to a large blueshift in the absorption spectrum as the temperature increases. The blueshift largely reverses a redshift at room temperature compared to the solution absorption spectrum. A reduction in the ratio of the first two vibronic peaks relative to the solution spectrum is also observed. This combination of electronic and vibronic effects is a signature of charge transfer excitonic coupling with a positive coupling constant J CT , which depends sensitively on the alignment of the nodes of the frontier molecular orbitals with those on neighboring molecules. These effects are also correlated with the sign and magnitude of electron and hole charge transfer integrals t e and t h calculated from density functional theory that provide additional evidence for charge transfer mediated coupling, as well as insight into the origin of an experimentally observed enhancement of the field-effect transistor mobility in strain-stabilized thin films. The results suggest approaches to improve carrier mobility in strained thin films and for optical monitoring of electronic changes.

Exciton Polaritons Reveal “Hidden” Populations in Functionalized Pentacene Films

“…In contrast, spectra of the more concentrated TIPS-Pn:PMMA blend F 1 ( B ) featured an ∼0.14 eV (∼50 nm) red-shift of the 0–0 energy (1.78 eV) and significant peak broadeningfeatures similar to those in pristine polycrystalline TIPS-Pn films F ( P ) (Figure , Figures S7 and S8). These spectral changes are due to intermolecular interactions, and they are consistent with previous studies of TIPS-Pn films and single crystals. ,,,, For example, the 0–0 red-shifts of 45–70 nm in polycrystalline TIPS-Pn films have been previously observed depending on morphology, and the large 0–0 solution–crystal red-shifts in TIPS-Pn crystals were attributed to efficient mixing of the Frenkel and charge transfer (CT) states . Furthermore, GW/BSE calculations confirmed that the lowest-energy exciton in crystalline TIPS-Pn possesses a charge transfer (CT) character and is delocalized over ∼3 nm in the a–b plane of the crystal .…”

“…However, the 0– m peak ratios and the peak widths were different from those in dilute blends (Table ), which suggests that intermolecular interactions are not negligible in these films even though the lowest-energy exciton does not show spectral signatures of Frenkel–CT mixing and delocalization in these amorphous films A ( P ) . We note that, in contrast to dilute blends and amorphous films, the 0– m transitions in crystalline films are not a simple vibronic progression due to a complicated mixture of various states of Frenkel and CT nature contributing to each peak. ,, Nevertheless, in the following discussions of the crystalline films (e.g., F 1 ( B ) and F ( P ) ), the notation “0– m ” is used to differentiate between different peaks contributing to the absorption spectra in the 1.8–2.4 eV energy region for easier comparison to other films.…”

“…With the N agg and N M notations introduced above, the scenario for the data in pristine TIPS-Pn films in Figure a would be represented as , where with the N agg determined by the molecular density N tot (which is a constant for a film with a given molecular concentration) and N M increasing with m for 0– m vibronic excitons in pristine TIPS-Pn cavities due to enhanced exciton delocalization (Figure b­(ii)),which is equivalent of increasing a fraction of the molecules α that participate in the coupling to the cavity. This delocalization could either arise directly from larger coherence lengths for the Frenkel exciton states or indirectly by mixing of the Frenkel excitons with CT states. ,, In either case, an increased delocalization would require the short-ranged intermolecular interactions of aggregates, and so the dilute TIPS-Pn:PMMA films (such as F 2 ( B ) in Figure a) would not exhibit this effect (as schematically shown in Figure b­(i)) and follow the observed reduced scaling relationship .…”

Quantitative Prediction of the Electro‐Mechanical Response in Organic Crystals

“…increase device performances, [6] as already demonstrated in the past for inorganic semiconductors, [7,8] where increase of hole and electron mobility (μ) higher than 100% have been reported for silicon transistors under strain. Finally, comparing relative trends in charge mobility as a function of the applied mechanical deformation represents a unique opportunity to validate the models describing charge transport in organic materials, since systematic errors of theories and experiments on the absolute value of the mobility become unimportant.…”

“…Indeed, many advanced applications require either materials with a controlled yet marked electro‐mechanical response (e.g., pressure sensors [ 1,2 ] and e‐skin, a bionic device that can mimic the skin of human beings [ 3 ] ) or, conversely, materials preserving their electrical performances under mechanical deformation, for example, in flexible displays or in bioelectronic applications, [ 2,4 ] such as bio‐integrated circuits. [ 4,5 ] Moreover, inducing strain is a potential mechanism to increase device performances, [ 6 ] as already demonstrated in the past for inorganic semiconductors, [ 7,8 ] where increase of hole and electron mobility (μ) higher than 100% have been reported for silicon transistors under strain. Finally, comparing relative trends in charge mobility as a function of the applied mechanical deformation represents a unique opportunity to validate the models describing charge transport in organic materials, since systematic errors of theories and experiments on the absolute value of the mobility become unimportant.…”