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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., A low singlet-triplet energy gap, necessary for delayed fluorescence organic semiconductors, results in a small radiative rate that limits performance in OLEDs. Here, the authors show that it is possible to reconcile these conflicting requirements in materials that can access both high oscillator strength intramolecular excitations and intermolecular charge transfer states.

Thermally-activated delayed fluorescence (TADF) organic molecules undergo more efficient light emission than traditional organic fluorescent emitters, making them attractive materials for OLEDs. In TADF OLEDs, indirect emission from dark triplet states via bright singlet excitons is activated and leads to boosted electroluminescence efficiencies. Most studies of TADF use optical spectroscopies that can examine the photophysics and interconversion rates, but do not shed light on the critical spin physics. Here we use transient electron spin resonance spectroscopy to study triplet states involved in TADF and the role of hyperfine and spin-vibronic couplings on the critical singlet-triplet intersystem crossing for efficient TADF.

Harnessing the near-infrared (NIR) region of the electromagnetic spectrum is exceedingly important for photovoltaics, telecommunications, and the biomedical sciences. While thermally activated delayed fluorescent (TADF) materials have attracted much interest due to their intense luminescence and narrow exchange energies (ΔEST), they are still greatly inferior to conventional fluorescent dyes in the NIR, which precludes their application. This is because securing a sufficiently strong donor-acceptor (D-A) interaction for NIR emission alongside the narrow ΔEST required for TADF is highly challenging. Here, we demonstrate that by abandoning the common polydonor model in favor of a D-A dyad structure, a sufficiently strong D-A interaction can be obtained to realize a TADF emitter capable of photoluminescence (PL) close to 1000 nm. Electroluminescence (EL) at a peak wavelength of 904 nm is also reported. This strategy is both conceptually and synthetically simple and offers a new approach to the development of future NIR TADF materials.

Current materials leaders in OLED technology are largely based on phosphorescent iridium complexes and Thermally Activated Delayed Fluorescence (TADF) materials which emit by harvesting light from all excited states ensuring nearly 100% internal quantum efficiency (IQE). Although, high efficiency red, green and blue OLEDs were realized, very short operating stability remains a fundamental challenge for blue OLEDs. Here we present our materials design strategy. We have recently designed numerous linear coinage metal complexes with efficient photo- and electroluminescent properties.[1,2] Our materials are composed of the donor and acceptor ligands which are linked by a coinage metal atom. Linear geometry of coinage metal complexes enables rotational flexibility. Rotation about the metal-ligand bond allowed us to tune the energy gap between singlet and triplet excited states. When the gap is close to zero, facile intersystem crossing and reversed intersystem crossing are possible which enables efficient singlet and triplet excited state harvesting. Depending on the value of the energy gap we have designed various functional materials with phosphorescent or delayed fluorescence properties. As a proof of concept, we fabricated OLED devices with exceptionally high external quantum efficiencies (>28% EQE) in both solution-processed and vacuum-deposited OLEDs.[3] Power and current efficiency are comparable to or exceeding state-of-the-art phosphorescent OLEDs and quantum dot LEDs. Our materials possess short excited state lifetime (100-300 ns) for the delayed emission which is highly important for the fabrication of the long-lived OLEDs. [1] A.S. Romanov, D. Di, L. Yang, J. Fernandez-Cestau, C.R. Becker, C.E. James, B. Zhu, M. Linnolahti, D. Credgington, M. Bochmann, Chem. Commun., 52, 6379 (2016) [2] A.S. Romanov, C.R. Becker, C.E. James, D. Di, D. Credgington, M. Linnolahti, M. Bochmann, Chem. Eur. J., 23, 4625 (2017). [3] D. Di, A.S. Romanov, L. Yang, J.M. Richter, J.P.H. Rivett, S. Jones, T.H. Thomas, M.A. Jalebi, R.H. Friend, M. Linnolahti, M. Bochmann, D. Credgington, Science, 356, 159 (2017)

Non-fullerene acceptors (NFAs) have recently outperformed their fullerene counterparts in binary bulk-heterojunction (BHJ) organic solar cells (OSCs). Further development of NFA OSCs may benefit other novel OSC device structures that alter or extend the standard BHJ concept. Here, we report such a new processing route that forms a BHJ-like morphology between sequentially processed polymer donor and NFA with high power conversion efficiencies in excess of 10%. Both devices show similar charge generation and recombination behaviours, supporting formation of similar BHJ active layers. We correlate the approximate to 30 meV smaller open-circuit voltage in sq-BHJ devices to more substantial non-radiative recombination by voltage loss analysis. We also determine the exciton diffusion length of benchmark polymer PBDB-T to be 10 +/- 3 nm. Our results demonstrate high-efficiency OSC devices using sequential deposition method and provide new opportunities to further improve performance of state-of-the-art OSCs. Funding Agencies|Engineering and Physical Sciences Research Council; Science and Technology Facilities Council [RB1800095]; China Scholarship Council [201503170255]; SPIE Optics and Photonics Education Scholarship; NSFC [91633301]; MOST of China [2014CB643502]; [2016-02051]

Efficient vacuum-processed organic light-emitting diodes (OLEDs) have been fabricated using the carbene-metal-amide material CMA1. Electroluminescence external quantum efficiencies of greater than 24 % have been achieved with host-guest emissive layers utilizing a 1,3-bis(N-carbazolyl)benzene (mCP) host as well as in host-free emissive layers comprising pure CMA1. Furthermore, an external quantum efficiency greater than 26 % has been achieved by substitution of the emissive layer host for a symmetrical, non-polar variant. Electroluminescence spectra are found to depend on both emissive layer doping concentration and the choice of host material, enabling tuning of emission colour from mid-green (Commission Internationale de l’Eclairage co-ordinates [0.24, 0.46]) to sky blue ([0.22 0.35]) without changing dopant. This tuning is achieved without compromising luminescence efficiency (> 80 %) and maintaining a short radiative lifetime of triplets (< 1 µs).

Correction for ‘Efficient non-fullerene organic solar cells employing sequentially deposited donor–acceptor layers’ by Jiangbin Zhang et al., J. Mater. Chem. A, 2018, 6, 18225–18233.

The Eastern Hindukush forms part of an elongate belt (“Central Domain”, collage of Cimmerian microcontinents) that encircles the northern part of the Indian subcontinent. A Gondwanan origin is commonly assumed for this belt, but a “Laurasian” origin for the Chitral region has been argued on palaeontological (Talent and Mawson, 1979) and palaeomagnetic (Klootwijk and Conaghan, 1979) grounds. The “Laurasian” view was based on a pilot study we undertook of Upper Devonian pisolitic ironstones from a thrust sheet at Kurāgh Spur in Chitral. Preliminary results showed a characteristic magnetization component [D = 318°, I= −6.5°, N = 7 (block samples), k = 14, α95 = 16.5°] indicating an equatorial palaeoposition. This component was thought to be of primary origin and was interpreted in terms of a Late Devonian “Laurasian” affinity of the Kurāgh Spur rocks. This controversial conclusion has been tested in the present more comprehensive study of the thrust pile of sedimentary rocks in the Reshūn-Kurāgh-Būni region of Chitral and the primary origin of the characteristic magnetization component refuted. Thermal demagnetization of 333 block samples from Middle to Upper Devonian variegated sediments, Permian quartz flysch, Permo-Triassic carbonates, and mid-Cretaceous redbeds showed two interpretable components. A softer component of recent origin (A); and a harder characteristic component (B) of both normal and reverse polarity whose mean direction [D = 314.1°, I = 6.0°, N = 4 (thrust sheets), k = 198.2, α95 = 6.5°] is comparable to the characteristic component observed in our preliminary study. However, the universal presence of this component throughout the thrust pile proves its overprint origin, which we attribute to initial India-Asia contact. Palaeomagnetic information pertinent to the controversy of a “Laurasian” versus a Gondwanan origin of the Chitral region has not been obtained in this further study because primary magnetizations could not be identified beyond doubt. Hence, we retract herewith our original conclusion of a Late Devonian “Laurasian” affinity of the Chitral region on the basis of the palaeomagnetic evidence. The secondary component (B) comprises a suite of secondary magnetizations, acquired at equatorial-to-low-northern palaeolatitudes, and is attributed to initial contact between Greater India and southern Asia. Component B has been observed previously in the Himalayan-Tibetan region, both north and south of the Indus-Tsangpo Suture zone. Identification is herein extended to the Hindukush region north of the Northern Kohistan (or Shyok) Suture zone, which is a western continuation of the Indus-Tsangpo Suture. Comparison of this suite of collision-at-tributed equatorial palaeolatitude data from the India-Asia convergence zone with new palaeolatitude constraints from the Ninetyeast Ridge on the northward movement of the Indian plate, constrained additionally by a recent minimal estimate of the palaeogeographic northern extent of Greater India, indicates that initial contact between northwestern Greater India and southern Asia was established at, or before, the Cretaceous-Tertiary boundary. The overprint origin of component B at about this time is further supported by observations by Zeitler (1985) on rocks from the sampled area in Chitral of partially reset zircon fission-track ages around 68-55 Ma. The NW-SE declination axis of component B indicates a 60–70° counterclockwise rotation of the sampled thrust pile with respect to Eurasia and a counterclockwise rotation between 10 and 30° with respect to India. Some of the recent field components (A) show a comparable rotation and indicate that the tectonic activity that led to the formation of the Hindukush-Pamir-Karakorum syntaxial zone has continued into recent times.

Comparison of the Late Permian northeastern Sydney Basin with the Pliocene-Quaternary “Purari recess” of the Papuan Basin on the basis of gross lithology and lithologic sequence, sediment provenance, sedimentary environment and basin architecture confirms its tectonic context as an orogenic recess of closely similar character. This comparison, together with comparison to the Late Carboniferous Appalachian Basin of Pennsylvania, suggests that, under favourable climatic circumstances, orogenic recesses are optimum sites for thick coal accumulation.

Late Permian and Triassic successions of the Sydney Basin, exposed in cliffs of the north coast, south coast and Blue Mountains, include sandstones in which quartz content increases up‐sequence and palaeocurrents swing from southwesterly through southeasterly to northeasterly. These basin‐wide phenomena are interpreted to have resulted from the northeasterly migration of a drainage net, in which northeasterly‐flowing tributaries bearing quartz sand from the craton met southwesterly‐flowing tributaries bearing labile sediment from an arcuate rim‐orogen, and blended in a southeasterly‐flowing trunk stream down the axis of a foredeep. Migration of the drainage net records the retreat of an arc‐derived clastic wedge in the latter part of a megacycle which extends from the top of the mid‐Permian Nowra Sandstone to the top of the mid‐Triassic Hawkesbury Sandstone. The same megacycle occurs in the Bowen Basin, and may also occur in the Nilsen‐Mackay Basin of Antarctica.