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We report the development of light-emitting dendrimers with heteroleptic or homoleptic iridium(III) complexes at the core, with one, two, or three first- or second-generation biphenyl-based dendrons attached to the 2-phenylpyridyl ligand that is involved in the emission of light. The 5-phenyl-1-methyl3-n-propyl-1H-1,2,4-triazole co-ligand was not dendronised and was not involved in the emission process. The dendrimers with two emissive ligands were found to have the highest solution photoluminescence quantum yields (PLQYs) which were greater than 90% independent of the dendrimer generation. The second-generation dendrimers were all found to have higher neat film PLQYs than their first-generation counterparts due to the increased shielding of the emissive core by the dendrons and surface groups. The maximum external quantum efficiencies of devices made from the neat film and a guest:host blend of the second-generation dendrimer with two emissive ligands were found to be the highest (7.5% and 15.8%, respectively), which was attributed to a balance between the interactions required for charge transport and minimising those that lead to non-radiative processes. Thus, we have demonstrated that combining dendrimer generation and number of emissive dendronised ligands is a viable strategy for optimising light-emitting dendrimer OLED performance. [ABSTRACT FROM AUTHOR]

The performance of electronic and semiconductor devices is critically dependent on the distribution of guest molecules or atoms in a host matrix. One prominent example is that of organic light‐emitting diode (OLED) displays containing phosphorescent emitters, now ubiquitous in handheld devices and high‐end televisions. In such OLEDs the phosphorescent guest [normally an iridium(III)‐based complex] is typically blended into a host matrix, and charge injection and transport, exciton formation and decay, and hence overall device performance are governed by the distribution of the emissive guest in the host. Here high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) is used with depth sectioning to reconstruct the 3D distribution of emissive iridium(III) complexes, fac‐tris(2‐phenylpyridine)iridium(III) [Ir(ppy)3], blended into the amorphous host material, tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA), by resolving the position of each single iridium(III) ion. It is found that most Ir(ppy)3 complexes are clustered with at least one other, even at low concentrations, and that for films of 20 wt.% Ir(ppy)3 essentially all the complexes are interconnected. The results validate the morphology of blend films created using molecular dynamics simulations which mimic the evaporation film‐forming process and are also consistent with the experimentally measured charge transport and photophysical properties. [ABSTRACT FROM AUTHOR]

Controlled functionalization of 2D black phosphorus (BP) nanosheets provides unique opportunities to tune their chemical, physical, and electronic properties. Herein, the preparation of single‐atom nickel‐doped BP (Ni–BP) sheets using a simple solution‐based strategy is reported. Using the Ni–BP sheets as a passivation layer on top of a perovskite film leads to standard perovskite solar cells (PSCs) with improved performance. The standard n–i–p PSCs with the Ni–BP interlayer achieve maximum power conversion efficiencies of over 22%, with negligible hysteresis and a modest improvement in stability when subjected to different testing conditions. The perovskite films prepared with Ni–BP sheets‐based passivation are found to have reduced defect densities as well as improved charge‐transfer properties and carrier lifetimes. Density‐functional theory calculations support the experimental results through showing that the atomic Ni‐doping increases the work function of the BP interlayer, enabling better hole extraction, as well as increasing the surface hydrophobicity of the BP layer, hence reducing water sorption into the perovskite film. [ABSTRACT FROM AUTHOR]

Singlet fission (SF) is a potential avenue for augmenting the performance of silicon photovoltaics, but the scarcity of SF materials energy‐matched to silicon represents a barrier to the commercial realization of this technology. In this work, a molecular engineering approach is described to increase the energy of the S1 and T1 energy levels of diketopyrrolopyrrole derivatives such that the energy‐level requirements for exothermic SF and energy‐transfer to silicon are met. Time‐resolved photoluminescence studies show that the silicon‐matched materials are SF active in the solid state, forming a correlated triplet pair 1(TT) – a crucial intermediate in the SF process – as observed through Herzberg‐Teller emission from 1(TT) at both 77 K and room temperature. Transient electron paramagnetic resonance studies show that the correlated triplet pair does not readily separate into the unbound triplets, which is a requirement for energy harvesting by silicon. The fact that the triplet pair do not separate into free triplets is attributed to the intermolecular crystal packing within the thin films. Nevertheless, these results demonstrate a promising route for energy‐tuning silicon‐matched SF materials. [ABSTRACT FROM AUTHOR]

In‐field rapid and reliable identification of nerve agents is critical for the protection of Defence and National Security personnel as well as communities. Fluorescence‐based detectors can be portable and provide rapid detection of chemical threats. However, most current approaches cannot differentiate between dilute vapors of nerve agent classes and are susceptible to false positives due to the presence of common acids. Here a fluorescence‐based method is shown for rapid differentiation between the V‐series and phosphonofluoridate G‐series nerve agents and avoids false positives due to common acids. Differentiation is achieved through harnessing two different mechanisms. Detection of the V‐series is achieved using photoinduced hole transfer whereby the fluorescence of the sensing material is quenched in the presence of the V‐series agent. The G‐series is detected using a turn‐on mechanism in which a silylated excited state intramolecular proton transfer sensing molecule is selectively deprotected by hydrogen fluoride, which is typically found as a contaminant and/or breakdown product in G‐series agents such as sarin. The strategy provided discrimination between classes, as the sensor for the G‐series agent class is insensitive to the V‐series agent, and vice versa, and neither responded to common acids. [ABSTRACT FROM AUTHOR]

Solution-processing of organic light-emitting diode films has potential advantages in terms of cost and scalability over vacuum-deposition for large area applications. However, solution processed small molecule films can have lower overall device performance. Here, novel molecular dynamics techniques are developed to enable faster simulation of solvent evaporation that occurs during solution processing and give films of thicknesses relevant to real devices. All-atom molecular dynamics simulations are then used in combination with kinetic Monte Carlo transport modeling to examine how differences in morphology stemming from solution or vacuum film deposition affect charge transport and exciton dynamics in films consisting of light-emitting bis(2-phenylpyridine)(acetylacetonate)iridium(III) [Ir(ppy)2(acac)] guest molecules in a 4,4′-bis(N-carbazolyl)biphenyl host. While the structures of the films deposited from vacuum and solution were found to differ, critically, only minor variations in the transport properties were predicted by the simulations even if trapped solvent was present. [ABSTRACT FROM AUTHOR]

Bulk heterojunction organic solar cells have undergone significant improvements in power conversion efficiency and stability over the past decade. However, translation from laboratory-scale devices to commercial panels is complicated by the sensitivity of the efficiency to morphological changes, which requires delicate control of the processing conditions. A homojunction architecture, with only a single chromophoric material in the light absorbing layer, could provide a pathway to overcoming problems associated with controlling complex blend morphologies. However, to achieve that goal the organic semiconductor would need to have a dielectric constant of ≈10 such that free charge carriers are formed directly upon photoexcitation. In this study we explore the effect of fluorination on the low and high frequency dielectric constants of two sets of materials with monomeric (A′–A–D) or dimeric (A′–A–D–D–A–A′) structures, where A′ is an aldehyde, A is a protonated or fluorinated benzothiadiazole, and D is a glycolated 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) donor unit. Fluorination was found to have only a small effect on the film absorption properties (small blue shift and decreased coefficient), but led to films with increased density relative to the protonated material (1.44 g cm−3versus 1.32 g cm−3), and dispersive unbalanced electron (≈10−6 cm2 V−1 s−1) and hole transport (≈10−5 cm2 V−1 s−1). Critically, while the low frequency dielectric constant of the fluorinated dimeric material was larger than that of the protonated (8.3 versus 7.3), the optical frequency dielectric constant that is thought to be critical for exciton dissociation was smaller (3.3 versus 3.6). [ABSTRACT FROM AUTHOR]

A series of solution-processable green emitting dendronised bis-tridentate iridium(III) complexes composed of [2,6-diimidazolylidene]benzene and the dianionic ligand, 2-[3-(trifluoromethyl)-1H-pyrazol-5-yl]-6-[4-(trifluoromethyl)phenyl]pyridine, have been synthesised to investigate the role of surface groups (t-butyl versus 2-ethylhexyloxy) and dendron generation (first versus second) on the photoluminescent (PL) and electroluminescent (EL) properties of the dendrimers. The dendrimers exhibited similar solution photoluminescent quantum yields (PLQYs) in toluene (60–70%) independent of the surface groups and dendrimer generation. In contrast, the PLQY values of the second-generation dendrimers were significantly higher than those with the first-generation dendrons in the neat film due to the higher generation dendrons providing more effective encapsulation of the emissive cores. When the dendrimers were blended at a concentration of 2 mol% in tris[4-carbazoyl-9-ylphenyl]amine (TCTA) the PLQYs increased but were still below the values measured in solution suggesting that the dendrimers were not evenly distributed throughout the host. Emission from the guest and host in the 2 mol% blends provided evidence for uneven distribution of the guest and inefficient energy transfer from the host to the dendrimers. The difference in encapsulation of the cores by the dendrons also affected charge transport in the neat films. In the case of the second-generation dendrimer with 2-ethylhexyloxy groups, the hole mobility was <10−8 cm2 V−1 s−1, which was an order of magnitude lower than for the less sterically encumbered dendrimers. Simple two-layer organic light-emitting diodes (OLEDs) with a solution-processed light-emitting layer were prepared. The best of the host-free devices exhibited a maximum external quantum efficiency (EQE) of 13.4%. The devices with the dendrimers blended with TCTA had more uniform EQEs of 12.2% to 13.5%, which are amongst the highest reported for solution-processed OLEDs containing a bis-tridentate iridium(III) complex emitter. Finally, it was found that devices containing the dendrimer with the t-butyl surface groups had greater stability. [ABSTRACT FROM AUTHOR]

Emissive layers in phosphorescent organic light-emitting diodes commonly make use of guest–host blends such as Ir(ppy)3:CBP to achieve high external quantum efficiencies. However, while the Ir(ppy)3:CBP blend has been studied experimentally, crucial questions remain regarding how exciton diffusion is dependent on the distribution of the guest in the host, which can currently only be addressed at the atomic level via computational modeling. In this work, kinetic Monte Carlo simulations are utilized to gain insight into exciton diffusion in Ir(ppy)3:CBP blend films. The effects of both guest concentration and exciton density on various system properties are analyzed, including the probability of singlet excitons being converted to triplets, and the probability of those triplets decaying radiatively. Significantly, these simulations suggest that triplet diffusion occurs almost exclusively via guest–guest Dexter transfer and that concentration quenching of triplets induced by guest–guest intermolecular dipole-dipole interactions has a negligible effect at high exciton densities due to the prevalence of triplet–triplet annihilation. Furthermore, results for vacuum deposited morphologies derived from molecular dynamics simulations are compared to the results obtained using a simple cubic lattice approximation with randomly distributed guest molecules. We show that while differences in host-based processes such as singlet diffusion are observed, overall, the results on the fate of the excitons are in good agreement for the two morphology types, particularly for guest-based processes at low guest concentrations where guest clustering is limited. [ABSTRACT FROM AUTHOR]