Comprehensive modelling study of singlet exciton diffusion in donor-acceptor dyads:When small changes in chemical structure matter

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Efficient near-infrared organic light-emitting diodes with emission from spin doublet excitons

The development of luminescent organic radicals has resulted in materials with excellent optical properties for near-infrared (NIR) emission. Applications of light generation in this range span from bioimaging to surveillance. Whilst the unpaired electron arrangements of radicals enable efficient radiative transitions within the doublet-spin manifold in organic light-emitting diodes (OLEDs), their performance is limited by non-radiative pathways introduced in electroluminescence. Here, we present a host:guest design for OLEDs that exploits energy transfer with demonstration of up to 9.6% external quantum efficiency (EQE) for 800 nm emission. The tris(2,4,6-trichlorophenyl)methyl-triphenylamine (TTM-TPA) radical guest is energy-matched to the triplet state in a charge-transporting anthracene-derivative host. We show from optical spectroscopy and quantum-chemical modelling that reversible host-guest triplet-doublet energy transfer allows efficient harvesting of host triplet excitons

On the Role of Charge Transfer Excitations in Non-Fullerene Acceptors for Organic Photovoltaics

Through the development of new non-fullerene electron acceptor (NFA) materials, such as Y6 and its molecular derivatives, the power conversion efficiencies of organic photovoltaics (OPVs) have now exceeded 19%. However, despite this rapid progress, our fundamental understanding of the unique optical and electronic properties of these Y-series NFAs is lacking, and this currently limits progress in material design. In this work, we provide a detailed computational-experimental characterisation of the archetypal NFA, Y6. To explain the significant broadening and red shift of the absorption spectrum observed when moving from the solution phase to the solid state, we first rule out more typical causes, such as J-aggregation. Instead, by considering the role of charge transfer (CT) excitations and their mixing with Frenkel exciton (FE) states, we can computationally reproduce the experimental absorption spectra of Y6 with excellent accuracy. Using transient absorption spectroscopy, we provide evidence for this dense manifold of FE-CT hybrid electronic excitations in Y6 through the prominent sub-picosecond relaxation events following supra band gap excitation. Furthermore, through sub band gap excitation, we also find states with polaronic character in Y6 that are in a dynamic equilibrium with the FE-CT hybrid states. Magnetic resonance spectroscopies reveal that these polaronic states are polaron pairs, most likely located on neighbouring Y6 molecules, not free charge carriers, as has been previously suggested. Thus, this new understanding of how the solid-state packing motif directly controls the optical and electronic properties of Y-series NFAs opens the door to intelligently design NFA materials to further increase OPV performance.Comment: 31 pages, 7 figure

Triphenylamine/Tetracyanobutadiene-Based π-Conjugated Push–Pull Molecules End-Capped with Arene Platforms:Synthesis, Photophysics, and Photovoltaic Response

π-Conjugated push–pull molecules based on triphenylamine and 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) have been functionalized with different terminal arene units. In solution, these highly TCBD-twisted systems showed a strong internal charge transfer band in the visible spectrum and no detectable photoluminescence (PL). Photophysical and theoretical investigations revealed very short singlet excited state deactivation time of ≈10 ps resulting from significant conformational changes of the TCBD-arene moiety upon photoexcitation, opening a pathway for non-radiative decay. The PL was recovered in vacuum-processed films or when the molecules were dispersed in a PMMA matrix leading to a significant increase of the excited state deactivation time. As shown by cyclic voltammetry, these molecules can act as electron donors compared to C 60. Hence, vacuum-processed planar heterojunction organic solar cells were fabricated leading to a maximum power conversion efficiency of ca. 1.9 % which decreases with the increase of the arene size

Reversible spin-optical interface in luminescent organic radicals

Molecules present a versatile platform for quantum information science, and are candidates for sensing and computation applications. Robust spin-optical interfaces are key to harnessing the quantum resources of materials. To date, carbon-based candidates have been non-luminescent, which prevents optical read-out. Here we report the first organic molecules displaying both efficient luminescence and near-unity generation yield of high-spin multiplicity excited states. This is achieved by designing an energy resonance between emissive doublet and triplet levels, here on covalently coupled tris(2,4,6-trichlorophenyl) methyl-carbazole radicals (TTM-1Cz) and anthracene. We observe the doublet photoexcitation delocalise onto the linked acene within a few picoseconds and subsequently evolve to a pure high spin state (quartet for monoradicals, quintet for biradical) of mixed radical-triplet character near 1.8 eV. These high-spin states are coherently addressable with microwaves even at 295 K, with optical read-out enabled by intersystem crossing to emissive states. Furthermore, for the biradical, on return to the ground state the previously uncorrelated radical spins either side of the anthracene show strong spin correlation. Our approach simultaneously supports a high efficiency of initialisation, spin manipulations and light-based read-out at room temperature. The integration of luminescence and high-spin states creates an organic materials platform for emerging quantum technologies

Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors.

Engineering a low singlet-triplet energy gap (ΔEST) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a strong optical absorption (absorption coefficient = 3.8 × 105 cm-1) and a relatively large ΔEST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (delayed lifetime = 260 μs), but in aggregated films, BF2 generates intermolecular charge transfer (inter-CT) states on picosecond timescales. In contrast to the microsecond intramolecular rISC that is promoted by spin-orbit interactions in most isolated DF molecules, photoluminescence-detected magnetic resonance shows that these inter-CT states undergo rISC mediated by hyperfine interactions on a ~24 ns timescale and have an average electron-hole separation of ≥1.5 nm. Transfer back to the emissive singlet exciton then enables efficient DF and LED operation. Thus, access to these inter-CT states, which is possible even at low BF2 doping concentrations of 4 wt%, resolves the conflicting requirements of fast radiative emission and low ΔEST in organic DF emitters

The nature of carotenoid S* state and its role in the nonphotochemical quenching of plants

Abstract In plants, light-harvesting complexes serve as antennas to collect and transfer the absorbed energy to reaction centers, but also regulate energy transport by dissipating the excitation energy of chlorophylls. This process, known as nonphotochemical quenching, seems to be activated by conformational changes within the light-harvesting complex, but the quenching mechanisms remain elusive. Recent spectroscopic measurements suggest the carotenoid S* dark state as the quencher of chlorophylls’ excitation. By investigating lutein embedded in different conformations of CP29 (a minor antenna in plants) via nonadiabatic excited state dynamics simulations, we reveal that different conformations of the complex differently stabilize the lutein s-trans conformer with respect to the dominant s-cis one. We show that the s-trans conformer presents the spectroscopic signatures of the S* state and rationalize its ability to accept energy from the closest excited chlorophylls, providing thus a relationship between the complex’s conformation and the nonphotochemical quenching

Molecular Quadrupole Moments Promote Ground‐State Charge Generation in Doped Organic Semiconductors

International audienceThe role of local environmental interactions on the generation of free charge carriers in doped organic layers is investigated. Via a classical micro-elec-trostatic model, a dual effect of molecular quadrupole moments of host and dopant molecules on doping is demonstrated. Namely, electrostatic interactions ease ionization of the dopant by altering the energy level alignment between the host and the dopant and reduce the barrier for charge dissocia-tion by flattening the energy landscape around the ionized dopants. These results indicate that tailoring molecular quadrupole moments of the host and/or dopant are an attractive strategy toward improved doping efficiency in organic semiconductors. The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202004600. steps. [1b,4,5] The first step involves either a hybridization of the host-dopant frontier molecular orbitals or a ground-state integer charge transfer (CT) process between the host and the dopant. [1a,6] For p-type doping, the latter is favored when the energy difference between the ioni-zation potential (IP) of the host and the electron affinity (EA) of the dopant (i.e., the host-dopant gap Γ hd = IP host − EA dop) is smaller in magnitude than the Cou-lomb binding energy between an electron and its geminate hole sitting on nearest neighbor molecules (V eh). [7] In such conditions , a bound CT state with an electron added to the dopant and a hole left on the host molecule is generated. [7] This is the ionization step. The second step, namely, the charge dissociation, then consists of the spatial migration of the hole (for p-doping) away from the ionized acceptor that requires overcoming the Coulomb binding of the CT pair, which usually amounts to several hundreds of meV. [4a,8] The energetic and kinetic aspects of both steps need to be concomi-tantly optimized in order to maximize the overall charge generation efficiency, which calls for a control of the interactions between the molecules at the microscopic level. [3] Tuning these microscopic interactions can be achieved by molecular and material engineering. For instance, Warren et al. demonstrated that the energy levels in an organic semiconductor and the Fermi energy can be simultaneously tuned by taking advantage of molecular quadrupolar interactions. [1c] This result was achieved by co-evaporating a host mixture between zinc-phthalocyanine (ZnPc) and its eight times fluori-nated counterpart (F 8 ZnPc) together with the p-type dopant F6-TCNNQ. Such mixtures are very interesting because they provide an effective and practical approach to finely control the p-doping efficiency of F6-TCNNQ upon tuning the host energy levels, which are a function of the ZnPc:F 8 ZnPc molar ratio. [1c,9] Here, in the wake of the very recent experimental work reported by Warren et al., [1c] we investigate the role of environmental interactions on the charge generation mechanism in doped binary (ZnPc:F6-TCNNQ and F 8 ZnPc:F6-TCNNQ) and ternary (ZnPc:F 8 ZnPc:F6-TCNNQ) blends. Based on an in-depth atomistic modeling of electrostatic and dielectric phenomena in molecular solids, [10] we show that charge-quadrupole interactions affect both the ionization step (by reshuffling the energy levels of the dopant and the host) and the charge dissociation step (by creating a favorable energy pathway for the hole). Interestingly, in addition to long-range electrostatics phenomena characterizing the energy landscape of ordered molecular films, [10] we observe that the substitutio

Fate of Low-Lying Charge-Transfer Excited States in a Donor:Acceptor Blend with a Large Energy Offset

International audienceIn an effort to gain a comprehensive picture of the interfacial states in bulk heterojunction solar cells, we provide a combined experimental−theoretical analysis of the energetics and dynamics of low-lying electronic charge-transfer (CT) states in donor:acceptor blends with a large frontier orbital energy offset. By varying the blend composition and temperature, we unravel the static and dynamic contributions to the disordered density of states (DOS) of the CT-state manifold and assess their recombination to the ground state. Namely, we find that static disorder (conformational and electrostatic) shapes the CT DOS and that fast nonradiative recombination crops the low-energy tail of the distribution probed by external quantum efficiency (EQE) measurements (thereby largely contributing to voltage losses). Our results then question the standard practice of extracting microscopic parameters such as exciton energy and energetic disorder from EQE