Disentangling “Bright” and “Dark” Interactions in Ordered Assemblies of Organic Semiconductors

Abstract

We report on spatially correlated wavelength-resolved photoluminescence and Kelvin probe force microscopy to probe ground state charge-transfer coupling and its correlation with pi-stacking order in nanoscale assemblies of a small molecule n-type organic semiconductor, tetraazaterrylene (TAT). We find a distinct upshift in surface potential contrast (SPC) corresponding to a decrease in work function in TAT in the transition from disordered spun-cast films to ordered crystalline nanowire assemblies, accompanied by a nanowire size dependence in the SPC shift suggesting that the shift depends on both ground state charge transfer interaction and a size (volume)-dependent intrinsic doping associated with the nitrogen substitutions. For the smallest nanowires studied (surface height ≈ 10-15 nm), the SPC shift with respect to disordered films is +110 meV, in close agreement with recent theoretical calculations. These results illustrate how "dark" (ground-state) interactions in organic semiconductors can be distinguished from "bright" (excited-state) exciton coupling typically assessed by spectral measurements alone.

Perylene Diimide-Based Hj- and hJ-Aggregates: The Prospect of Exciton Band Shape Engineering in Organic Materials

“…For applications such as OLEDs requiring large exciton bandwidths and large emission crosssections, JJ-aggregates are desirable, whereas for organic photovoltaic applications, HH-aggregates would be preferable since efficient solar cells require rapid excitation transport with minimal radiative loss, with the added potential benefit of reduced exciton−exciton annihilation. 30 Thus far, the destructive interference characteristic of HJ-aggregation has been observed in 7,8,15,16-tetraazaterrylene (TAT), 26,28,31,32 which forms π-stacked nanopillars when deposited on a graphitic surface. 26,31,32 The most dramatic example of short-and long-range coupling interference occurs in the so-called "integrated" null aggregate where the cancellation is nearly complete, resulting in a flat exciton dispersion band and monomer-like photophysical properties.…”

“…30 Thus far, the destructive interference characteristic of HJ-aggregation has been observed in 7,8,15,16-tetraazaterrylene (TAT), 26,28,31,32 which forms π-stacked nanopillars when deposited on a graphitic surface. 26,31,32 The most dramatic example of short-and long-range coupling interference occurs in the so-called "integrated" null aggregate where the cancellation is nearly complete, resulting in a flat exciton dispersion band and monomer-like photophysical properties. 29 Recently, null-dimers have been discovered in perylene diimide foldamers.…”

Probing the Evolution of Molecular Packing Underlying HJ-Aggregate Transition in Organic Semiconductors Using Solvent Vapor Annealing

“…The ∼0.1 eV blue shift of the origin transition energy from the monomer transition energy is quite surprising, given that one would expect that exciton coupling should generally produce strongly red-shifted emission . We surmise that the dominant effect here is a dielectric effect associated with isolated clusters in the polymer matrix; we have observed similar J-like spectral signatures at the ends of TAT nanowire crystals in axially resolved spectra without the accompanying blue shift . The mean value of 2.1 for the intensity ratio of the J-aggregate clusters is tempting to interpret in terms of a molecular dimer structure whose theoretical 0–0/–0–1intensity ratio should be twice that of the monomer (∼2.8).…”

“…8 We surmise that the dominant effect here is a dielectric effect associated with isolated clusters in the polymer matrix; we have observed similar J-like spectral signatures at the ends of TAT nanowire crystals in axially resolved spectra without the accompanying blue shift. 40 The mean value of 2.1 for the intensity ratio of the J-aggregate clusters is tempting to interpret in terms of a molecular dimer structure whose theoretical 0−0/−0− 1intensity ratio should be twice that of the monomer (∼2.8). Aggregate disorder, most likely caused by chromophore misalignment, 41 is the likely origin of the softening in intensity ratio relative to ideal dimer structures.…”

Kelvin Probe Microscopy Investigation of Poly-Octylthiophene Aggregates

“…However, very little information about the relative strength of these interactions is gained from optical measurements [ 10 ], which, in addition, determine the electronic structure of the ground state changing the HOMO and LUMO energies and therefore the charge transport properties [ 6 , 7 ]. In fact, polymer aggregates with similar optical spectra but different work functions or aggregates with different optical signatures but almost similar work functions may be found [ 11 , 12 , 13 ]. Therefore, the characterization of the SCP aggregates’ ground state is key to gain control over the relationship between structure, morphology and electronic properties for the design of SCP assemblies.…”