Search

Materials with chiral electronic structures are of great interest. We report a triazatruxene, TAT, molecular semiconductor with chiral alkyl side chains that crystallises from solution to form chirally-stacked columns with a helical pitch of 6 TATs (2.3 nm). These crystals show strong circularly polarised, CP, green photoluminescence, with dissymmetry of 24%. Electronic structure calculations using the full crystal structure, show that this chiral stacking associates angular momentum to the valence and conduction states and thus gives rise to the observed CP luminescence. Free-standing crystals are not useful for active semiconductor devices, but we have discovered that co-sublimation of TAT as the guest in a structurally mismatched host enables the fabrication of thin films where the chiral crystallization is achieved in-situ by thermally-triggered nano-phase segregation of dopant and host whilst preserving the integrity of the film. This enables fabrication of bright (green) organic light-emitting diodes with unexpectedly high external quantum efficiencies of up to 16% and electroluminescence dissymmetries above 10%. These materials and this process method offer significant application potential in spintronics, optical displays and multidimensional optoelectronics.

Optical control and read-out of the ground state spin structure has been demonstrated for defect states in crystalline semiconductors, including the diamond NV- center, and these are promising systems for quantum technologies. Molecular organic semiconductors offer synthetic control of spin placement, in contrast to current limitations in these crystalline systems. Here we report the discovery of spin-optical addressability in a diradical molecule that comprises two trityl radical groups coupled via a fluorene bridge. We demonstrate the three important properties that enable operation as a spin-photon interface: (i) triplet and singlet spin states show photoluminescence peaked at 640 and 700 nm respectively; this allows easy optical measurement of ground state spin. (ii) the ground state spin exchange is small (~60 {\mu}eV) that allows preparation of ground state spin population. This can be achieved by spin-selective excited state intersystem crossing, and we report up to 8% microwave-driven contrast in photoluminescence. (iii) both singlet and triplet manifolds have near-unity photoluminescence quantum yield, which is in contrast to the near-zero quantum yields in prior reports of molecular diradicals. Our results establish these tuneable open-shell organic molecules as a platform to engineer tailor-made spin-optical interfaces.

Defect tolerance is a critical enabling factor for efficient lead-halide perovskite materials, but the current understanding is primarily on band-edge (cold) carriers, with significant debate over whether hot carriers (HCs) can also exhibit defect tolerance. Here, this important gap in the field is addressed by investigating how internationally-introduced traps affect HC relaxation in CsPbX3 nanocrystals (X = Br, I, or mixture). Using femtosecond interband and intraband spectroscopy, along with energy-dependent photoluminescence measurements and kinetic modelling, it is found that HCs are not universally defect tolerant in CsPbX3, but are strongly correlated to the defect tolerance of cold carriers, requiring shallow traps to be present (as in CsPbI3). It is found that HCs are directly captured by traps, instead of going through an intermediate cold carrier, and deeper traps cause faster HC cooling, reducing the effects of the hot phonon bottleneck and Auger reheating. This work provides important insights into how defects influence HCs, which will be important for designing materials for hot carrier solar cells, multiexciton generation, and optical gain media., Comment: 18 pages, 5 figures

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 figures

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.

The family of hybrid organic-inorganic lead-halide perovskites are the subject of intense interest for optoelectronic applications, from light-emitting diodes to photovoltaics to X-ray detectors. Due to the inert nature of most organic molecules, the inorganic sublattice generally dominates the electronic structure and therefore optoelectronic properties of perovskites. Here, we use optically and electronically active carbazole-based Cz-Ci molecules, where Ci indicates an alkylammonium chain and i indicates the number of CH2 units in the chain, varying from 3-5, as cations in the 2D perovskite structure. By investigating the photophysics and charge transport characteristics of (Cz-Ci)2PbI4, we demonstrate a tunable electronic coupling between the inorganic lead-halide and organic layers. The strongest interlayer electronic coupling was found for (Cz-C3)2PbI4, where photothermal deflection spectroscopy results remarkably demonstrate an organic-inorganic charge transfer state. Ultrafast transient absorption spectroscopy measurements demonstrate ultrafast hole transfer from the photoexcited lead-halide layer to the Cz-Ci molecules, the efficiency of which increases by varying the chain length from i=5 to i=3. The charge transfer results in long-lived carriers (10-100 ns) and quenched emission, in stark contrast with the fast (sub-ns) and efficient radiative decay of bound excitons in the more conventional 2D perovskite (PEA)2PbI4, in which phenylethylammonium (PEA) acts as an inert spacer. Electrical charge transport measurements further support enhanced interlayer coupling, showing increased out-of-plane carrier mobility from i=5 to i=3. This study paves the way for the rational design of 2D perovskites with combined inorganic-organic electronic proper-ties through the wide range of functionalities available in the world of organics., Comment: 16 main text pages + 20 supplemental text pages, 5 main text figures + 19 supplemental figures