Room temperature triplet state spectroscopy of organic semiconductors

Organic light-emitting devices and solar cells are devices that create, manipulate, and convert excited states in organic semiconductors. It is crucial to characterize these excited states, or excitons, to optimize device performance in applications like displays and solar energy harvesting. This is complicated if the excited state is a triplet because the electronic transition is ‘dark’ with a vanishing oscillator strength. As a consequence, triplet state spectroscopy must usually be performed at cryogenic temperatures to reduce competition from non-radiative rates. Here, we control non-radiative rates by engineering a solid-state host matrix containing the target molecule, allowing the observation of phosphorescence at room temperature and alleviating constraints of cryogenic experiments. We test these techniques on a wide range of materials with functionalities spanning multi-exciton generation (singlet exciton fission), organic light emitting device host materials, and thermally activated delayed fluorescence type emitters. Control of non-radiative modes in the matrix surrounding a target molecule may also have broader applications in light-emitting and photovoltaic devices.United States. Dept. of Energy. Center for Excitonics (Award DE-SC0001088

[[alternative]]苯基替代的階梯型poly(para-phenylene)在壓力下的光調致光譜

[[abstract]]The pressure-dependence photomodulation spectrum of a semiconducting conjugated polymer, phenyl-substituted ladder-type poly(para-phenylene) (Ph-LPPP) with trace-concentrations of metallic impurities, was studied. The so called photo-induced absorption gave the information about the triplet-triplet (TT) transitions and bleaching (PB) of the singlet state in this material. Planarization, therefore, stronger interchain interaction, under pressure was observed. The photo-modulation spectra coupled with Raman scattering and photoluminescence spectra of Ph-LPPP revealed the influence of electronic structure due to pressure on singlet and triplet excitons. As a result, the localized character of triplet state could be changed. The lifetimes of the triplet excitions and the photobleaching were also observed. It was found that the triplet state was less stable than the singlet state.[[notice]]補正完

Plasmonic nanomeshes: their ambivalent role as transparent electrodes in organic solar cells

In this contribution, the optical losses and gains attributed to periodic nanohole array electrodes in polymer solar cells are systematically studied. For this, thin gold nanomeshes with hexagonally ordered holes and periodicities (P) ranging from 202 nm to 2560 nm are prepared by colloidal lithography. In combination with two different active layer materials (P3HT:PC(61)BM and PTB7:PC(71)BM), the optical properties are correlated with the power conversion efficiency (PCE) of the solar cells. A cavity mode is identified at the absorption edge of the active layer material. The resonance wavelength of this cavity mode is hardly defined by the nanomesh periodicity but rather by the absorption of the photoactive layer. This constitutes a fundamental dilemma when using nanomeshes as ITO replacement. The highest plasmonic enhancement requires small periodicities. This is accompanied by an overall low transmittance and high parasitic absorption losses. Consequently, larger periodicities with a less efficient cavity mode, yet lower absorptive losses were found to yield the highest PCE. Nevertheless, ITO-free solar cells reaching ~77% PCE compared to ITO reference devices are fabricated. Concomitantly, the benefits and drawbacks of this transparent nanomesh electrode are identified, which is of high relevance for future ITO replacement strategies