Oleksandr V. Mikhnenko

Temperature Dependence of Exciton Diffusion in Conjugated Polymers

The temperature dependence of the exciton dynamics in a conjugated polymer is studied using time-resolved spectroscopy. Photoluminescence decays were measured in heterostructured samples containing a sharp polymer-fullerene interface, which acts as an exciton quenching wall. Using a 1D diffusion model, the exciton diffusion length and diffusion coefficient were extracted in the temperature range of 4-293 K. The exciton dynamics reveal two temperature regimes: in the range of 4-150 K, the exciton diffusion length (coefficient) of approximately 3 nm (approximately 1.5 x 10 (-4) cm2/s) is nearly temperature independent. Increasing the temperature up to 293 K leads to a gradual growth up to 4.5 nm (approximately 3.2 x 10 (-4) cm2/ s). This demonstrates that exciton diffusion in conjugated polymers is governed by two processes: an initial downhill migration toward lower energy states in the inhomogenously broadened density of states, followed by temperature activated hopping. The latter process is switched off below 150 K.

Trap-Limited Exciton Diffusion in Organic Semiconductors

Excited states in organic semiconductors, imposed either by electroor by photoexcitation, are referred to as excitons, and are known to migrate throughout the organic material. [ 1 ] In an organic semiconductor singlet excitons migrate between conjugated segments via Förster energy transfer. [ 2 ] Due to the disordered nature of the polymers such hopping can be regarded as diffusion. The exciton diffusion length, the average distance an excitation can migrate in a material during its lifetime, is generally used as a quantitative characterization of this process. The exciton diffusion length has been determined using a variety of measurement techniques such as fl uorescence quenching in thin fi lms of organic semiconductors, in which one or both interfaces act as an exciton quenching wall, [ 3–13 ] exciton density modulation due to light absorption, [ 14–16 ] exciton-exciton annihilation, [ 17–19 ] photocurrent modeling in solar cells, [ 20–24 ] and fl uorescence quenching in thin fi lms with randomly distributed quenchers. [ 25–31 ] For a large number of (disordered) organic semiconductors, exciton diffusion lengths of typically 5–10 nm have been reported. The dynamics of exciton migration in organic semiconductors is governed by the exciton diffusion coeffi cient. Knowledge of the exciton diffusion coeffi cient is therefore required in order to describe the spatialand temporal evolution of the exciton population in organic semiconductors. Experimentally, it can be determined by modeling the photoluminescence (PL) decay curves. [ 5,25,30,32,33 ]

Effect of thermal annealing on exciton diffusion in a diketopyrrolopyrrole derivative

We show that the method we have developed for measuring the singlet exciton diffusion length in blends with [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) can be applied not only to polymeric materials, but also to small molecule organic semiconductors. Small organic molecules have a large potential for molecular re-organization upon thermal annealing. Here we show that the exciton diffusion length is decreased upon annealing from 9 to 3 nm in a thin film of a diketopyrrolopyrrole derivative. Such a variation is attributed to exciton delocalization effects in the crystalline domains that are formed during the annealing process as well as to the exciton quenching at grain boundaries.

Cyclometalated red iridium(III) complexes containing carbazolyl-acetylacetonate ligands : efficiency enhancement in polymer LED devices

The design, synthesis, photophysical and significantly improved electrooptical properties of a series of red emitting cyclometalated iridium(iii) complexes containing carbazolyl-acetylacetonate ligands are described.

Suppression of roll-off characteristics of organic light-emitting diodes by narrowing current injection/transport area to 50 nm

Using e-beam nanolithography, the current injection/transport area in organic light-emitting diodes (OLEDs) was confined into a narrow linear structure with a minimum width of 50 nm. This caused suppression of Joule heating and partial separation of polarons and excitons, so the charge density where the electroluminescent efficiency decays to the half of the initial value (J0) was significantly improved. A device with a narrow current injection width of 50 nm exhibited a J0 that was almost two orders of magnitude higher compared with that of the unpatterned OLED.

Temperature Dependence of Exciton Diffusion in a Small‐Molecule Organic Semiconductor Processed With and Without Additive

The temperature dependence of exciton diffusion in a small-molecule organic semiconductor processed with and without additive is investigated. As-cast and 1,8-diiodooctane-processed films yield exciton diffusion lengths of 6.8 and 4.9 nm, respectively. Using a Monte Carlo simulation, it is shown that processing with 1,8-diiodooctane increases the excitonic trap density, which directly reduces the exciton diffusion length.

Rectifying Electrical Noise with an Ionic‐Organic Ratchet

Electronic ratchets can rectify AC signals that are extracted from unpredictable energy fluctuations. A device is presented with ratchet-like current-voltage characteristics, which delivers record high electrical currents of 2.6 and 1.7 μA when driven with an AC signal of square wave and random amplitude, respectively. The device is based on a poly(3-hexylthiophene-2,5-diyl):salt blend, which acquires rectification properties after a voltage stress in a transistor configuration.

Influence of Energetic Disorder on Exciton Lifetime and Photoluminescence Efficiency in Conjugated Polymers

Using time-resolved photoluminescence (TRPL) spectroscopy the exciton lifetime in a range of conjugated polymers is investigated. For poly(p-phenylenevinylene) (PPV)-based derivatives and a polyspirobifluorene copolymer (PSBF) we find that the exciton lifetime is correlated with the energetic disorder. Better ordered polymers exhibit a single exponential PL decay with exciton lifetimes of a few hundred picoseconds, whereas polymers with a larger degree of disorder show multiexponential PL decays with exciton lifetimes in the nanosecond regime. These observations are consistent with diffusion-limited exciton quenching at nonradiative recombination centers. The measured PL decay time reflects the time that excitons need to diffuse toward these quenching sites. Conjugated polymers with large energetic disorder and thus longer exciton lifetime also exhibit a higher photoluminescence quantum yield due to the slower exciton diffusion toward nonradiative quenching sites.

Sensitive triplet exciton detection in polyfluorene using Pd-coordinated porphyrin

We developed a sensitive spectroscopic method to probe triplet concentration in thin films of polyfluorene (PF) at room temperature. The energy of photoexcited triplet excitons is transferred to the guest metal-organic complex, meso-tetratolylporphyrin-Pd (PdTPP), and detected as phosphorescent emission. The phosphorescence intensity of PdTPP-PF blends is proportional to the independently measured triplet concentration using photoinduced absorption experiments. The high sensitivity of this method allows room temperature detection of triplet excitons in spin-coated polymer films as thin as 10 nm. We found that the triplet lifetime is independent of PdTPP concentration and therefore this method is nearly non-perturbing for the triplet population.

On the nature of the singlet and triplet excitations mediating thermally activated delayed fluorescence (Conference Presentation)

Thermally Activated Delayed Fluorescence (TADF) process is the new paradigm for Organic Light-Emitting Diodes (OLEDs). Despite all the efforts, a complete mechanistic understanding of TADF materials has not been fully uncovered yet. Part of the complexity arises from the apparent dichotomy between the need for small energy difference between the lowest singlet and triplet excited states (EST) which has to carry a significant charge transfer (CT) character; and for a significant spin-orbit coupling which according to El-Sayed rules requires the involved singlet and triplet excited states to have very different natures. In this contribution, we will show: (i) How the nature of these excited can be characterized and how this nature can be tuned by varying the nature of the electron donating (D) or accepting (A) units in D-A(-D) compounds. (ii) How this dichotomy can be resolved once accounting in a fully atomistic model of reference carbazole derivatives for thermal fluctuations of the molecular conformations and discrete electronic polarization effects in amorphous films. For both topics, we will demonstrate that, electronic excitations involved in the TADF process have a mixed CT-locally excited character being dynamically tuned by torsional vibrational modes and that overall, the conversion of triplet-to-singlet is a dynamic process gated by conformational fluctuations.