Exploring the Early Time Behavior of the Excited States of an Archetype Thermally Activated Delayed Fluorescence Molecule

Optical pump–probe techniques allow for an in-depth study of dark excited states. Here, we utilize them to map and gain insights into the excited states involved in the thermally activated delayed fluorescence (TADF) mechanism of a benchmark TADF emitter DMAC-TRZ. The results identify different electronic excited states involved in the key TADF transitions and their nature by combining pump–probe and photoluminescence measurements. The photoinduced absorption signals are highly dependent on polarity, affecting the transition oscillator strength but not their relative energy positions. In methylcyclohexane, a strong and vibronically structured local triplet excited state absorption (3LE → 3LE n ) is observed, which is quenched in higher polarity solvents as 3CT becomes the lowest triplet state. Furthermore, ultrafast transient absorption (fsTA) confirms the presence of two stable conformers of DMAC-TRZ: (1) quasi-axial (QA) interconverting within 20 ps into (2) quasi-equatorial (QE) in the excited state. Moreover, fsTA highlights how sensitive excited state couplings are to the environment and the molecular conformation

A deep blue B,N-doped heptacene emitter that shows both thermally activated delayed fluorescence and delayed fluorescence by triplet-triplet annihilation

Authors thank the Leverhulme Trust (RPG-2016-047). This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska Curie grant agreement No 838885 (NarrowbandSSL) and 812872 (TADFlife). We thank Umicore for their generous supply of catalysts. S.S. acknowledges support from the Marie Skłodowska-Curie Individual Fellowship. SB acknowledges support from the Bayrisches Staatsministerium für Wissenschaft und Kunst (Stmwk) in the framework of the initiative "SolTech", as well as from the German Science foundation (DFG) (No. 392306670). Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F.R.S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, infrastructure funded by the Walloon Region under the grant agreement n111754.An easy-to-access, near-UV-emitting linearly extended B,N-doped heptacene with high thermal stability is designed and synthesized in good yields. This compound exhibits thermally activated delayed fluorescence (TADF) at ambient temperature from a multiresonant (MR) state and represents a rare example of a non-triangulene-based MR-TADF emitter. At lower temperatures triplet–triplet annihilation dominates. The compound simultaneously possesses narrow, deep-blue emission with CIE coordinates of (0.17, 0.01). While delayed fluorescence results mainly from triplet–triplet annihilation at lower temperatures in THF solution, where aggregates form upon cooling, the TADF mechanism takes over around room temperature in solution when the aggregates dissolve or when the compound is well dispersed in a solid matrix. The potential of our molecular design to trigger TADF in larger acenes is demonstrated through the accurate prediction of ΔEST using correlated wave-function-based calculations. On the basis of these calculations, we predicted dramatically different optoelectronic behavior in terms of both ΔEST and the optical energy gap of two constitutional isomers where only the boron and nitrogen positions change. A comprehensive structural, optoelectronic, and theoretical investigation is presented. In addition, the ability of the achiral molecule to assemble on a Au(111) surface to a highly ordered layer composed of enantiomorphic domains of racemic entities is demonstrated by scanning tunneling microscopy.PostprintPeer reviewe

Improving processability and efficiency of Resonant TADF emitters : a design strategy

This work is funded by the EC through the Horizon 2020 Marie Sklodowska-Curie ITN project TADFlife. The St Andrews team would also like to thank the Leverhulme Trust (RPG-2016- 047) and EPSRC (EP/P010482/1) for financial support. Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F.R.S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, infrastructure funded by the Walloon Region under the grant agreement n1117545. AP acknowledges the financial support from the Marie Curie Fellowship (MILORD project, N°. 748042). DB is a FNRS Research Director. We thank Franck-Julian Kahle for support with data analysis.A new design strategy is introduced to address a persistent weakness with resonance thermally activated delayed fluorescence (R-TADF) emitters to reduce aggregation-caused quenching effects, which we identify as one of the key limiting factors. The emitter Mes3DiKTa shows an improved photoluminescence quantum yield of 80% compared to 75% for the reference DiKTa in 3.5 wt% mCP. Importantly, emission from aggregates, even at high doping concentrations, is eliminated and aggregation-caused quenching is strongly curtailed. For both molecules, triplets are almost quantitatively upconverted into singlets in electroluminescence, despite a significant (~0.21 eV) singlet-triplet energy gap (ΔEST), in line with correlated quantum-chemical calculations, and a slow reverse intersystem crossing. We speculate that the lattice stiffness responsible for the narrow fluorescence and phosphorescence emission spectra also protects the triplets against non-radiative decay. An improved EQEmax of 21.1% for Mes3DIKTa compared to the parent DiKTa (14.7%) and, importantly, reduced efficiency roll- off compared to literature resonance TADF OLEDs, shows the promise of this design strategy for future design of R-TADF emitters for OLED applications.Publisher PDFPeer reviewe

Understanding Method-Dependent Differences in Urbach Energies in Halide Perovskites

The Urbach energy as a measure of energetic disorder is an important characteristic of semiconductors to evaluate their optoelectronic functionality. However, discrepancies occur in Urbach energy values EU determined via different measurement and analysis methods, whose origin of a profound understanding is still missing. To reliably analyze the origin of such discrepancies, we recorded quasi-simultaneously temperature-dependent absorption and photoluminescence (PL) spectra of halide perovskite (MAPbI3) thin-film and single-crystal samples. Performing profound Urbach analyses in an extended energy range down to 0.2 eV below the bandgap, we find energy-range-dependent exaggeration effects on Urbach energy values to be only present in the near bandgap region (∼0.02 eV below the bandgap), where non-Urbach absorption states start to contribute. Besides that, generally lower EU values and a lower temperature-dependence of EU are obtained from PL than from absorption, which originates from the sensitivity of PL for sites with lower energetic disorder and/or higher phonon energies in the excited-state geometry. Thus, our work is sensitized to proper interpretation and comparison of EU values and contributes to developing a more fundamental understanding of semiconductor materials

A Combined Theoretical and Experimental Study of Dissociation of Charge Transfer States at the Donor–Acceptor Interface of Organic Solar Cells

The observation that in efficient organic solar cells almost all electron–hole pairs generated at the donor–acceptor interface escape from their mutual coulomb potential remains to be a conceptual challenge. It has been argued that it is the excess energy dissipated in the course of electron or hole transfer at the interface that assists this escape process. The current work demonstrates that this concept is unnecessary to explain the field dependence of electron–hole dissociation. It is based upon the formalism developed by Arkhipov and co-workers as well as Baranovskii and co-workers. The key idea is that the binding energy of the dissociating “cold” charge-transfer state is reduced by delocalization of the hole along the polymer chain, quantified in terms of an “effective mass”, as well as the fractional strength of dipoles existent at the interface in the dark. By covering a broad parameter space, we determine the conditions for efficient electron–hole dissociation. Spectroscopy of the charge-transfer state on bilayer solar cells as well as measurements of the field dependence of the dissociation yield over a broad temperature range support the theoretical predictions

Influence of the Excited-State Charge-Transfer Character on the Exciton Dissociation in Donor–Acceptor Copolymers

We synthesize a polytriphenylamine homopolymer and two donor–acceptor copolymers (D–A-copolymers) based on triphenylamine (TPA) as donor in combination with two different acceptor moieties to study the effect of the acceptor unit on the excited-state charge-transfer characteristics (CT-characteristics) and charge separation. The two acceptor moieties are a dicyanovinyl group in the side chain and a thieno­[3,4-<i>b</i>]­thiophene carboxylate in the main chain. Absorption and photoluminescence studies show new CT-bands for both of the D–A-copolymers. Field-dependent charge extraction studies in bilayer solar cells indicate a stronger CT-character for the copolymer in which the acceptor group is less conjugated with the copolymer backbone. The D–A-copolymer carrying the acceptor unit in the main chain exhibits smaller excitonic CT-character and good conjugation leading to less-bound electron–hole pairs and a better charge separation. This fundamental study gives insight into the interdependence of conjugation, charge carrier mobility, and solar cell performance for two different D–A-copolymers

To Hop or Not to Hop? Understanding the Temperature Dependence of Spectral Diffusion in Organic Semiconductors

In disordered organic semiconductors, excited states and charges move by hopping in an inhomogeneously broadened density of states, thereby relaxing energetically (“spectral diffusion”). At low temperatures, transport can become kinetically frustrated and consequently dispersive. Experimentally, this is observed predominantly for triplet excitations and charges, and has not been reported for singlet excitations. We have addressed the origin of this phenomenon by simulating the temperature dependent spectral diffusion using a lattice Monte Carlo approach with either Miller–Abrahams or Förster type transfer rates. Our simulations are in agreement with recent fluorescence and phosphorescence experimental results. We show that frustrated and thus dispersive diffusion appears when the number of available hopping sites is limited. This is frequently the case for triplets that transfer by a short-range interaction, yet may also occur for singlets in restricted geometries or dilute systems. Frustration is lifted when more hopping sites become available, e.g., for triplets as a result of an increased conjugation in some amorphous polymer films

The Impact of Polydispersity and Molecular Weight on the Order–Disorder Transition in Poly(3-hexylthiophene)

Conjugated poly­(3-hexylthiophene) (P3HT) chains are known to exist at least in two distinct conformations: a coiled phase and a better ordered aggregated phase. Employing steady state absorption and fluorescence spectroscopy, we measure the course of aggregation of P3HT in tetrahydrofuran (THF) solution within a temperature range of 300 K to 170 K. We show that aggregation is a temperature controlled process, driven by a thermodynamic order–disorder transition. The transition temperature increases with the molecular weight of the chains and can be rationalized in the theory of Sanchez. This implies a smearing out of the phase transition in samples with increasing polydispersity and erodes the signature of a first order phase transition. The detection of a hysteresis when undergoing cooling/heating cycles further substantiates this reasoning