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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]

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]

The strategy of using a bulk-heterojunction light-absorbing layer has led to the most efficient organic solar cells. However, optimising the blend morphology to maximise light absorption, charge generation and extraction can be challenging. Homojunction devices containing a single component have the potential to overcome the challenges associated with bulk heterojunction films. A strategy towards this goal is to increase the dielectric constant of the organic semiconductor to ≈10, which in principle would lead to free charge carrier generation upon photoexcitation. However, the factors that affect the thin film dielectric constants are still not well understood. In this work we report an organic semiconductor material that can be solution processed or vacuum evaporated to form good quality thin films to explore the effect of chromophore structure and film morphology on the dielectric constant and other optoelectronic properties. 2,2′-[(4,4,4′,4′-Tetrakis{2-[2-methoxyethoxy]ethyl}-4H,4′H-{2,2′-bi[cyclo-penta[2,1-b:3,4-b′]dithiophene]}-6,6′-diyl)bis(methaneylylidene)]dimalononitrile [D(CPDT-DCV)] was designed to have high electron-affinity end groups and low ionisation-potential central moieties. It can be processed from solution or be thermally evaporated, with the film morphology changing from face-on to a herringbone arrangement upon solvent or thermal annealing. The glycol solubilising groups led to the static dielectric constant (taken from capacitance measurements) of the films to be between 6 and 7 (independent of processing conditions), while the optical frequency dielectric constant depended on the processing conditions. The less ordered solution processed film was found to have the lowest optical frequency dielectric constant of 3.6 at 2.0 × 1014 Hz, which did not change upon annealing. In contrast, the more ordered evaporated film had an optical frequency dielectric constant 20% higher at 4.2 and thermal annealing further increased it to 4.5, which is amongst the highest reported for an organic semiconductor at that frequency. Finally, the more ordered evaporated films had more balanced charge transport, which did not change upon annealing. [ABSTRACT FROM AUTHOR]

Two light-emitting dendrimers composed of red phosphorescent fac-tris[2-(thiophen-2-yl)-4-(phenyl)quinoline]iridium(III) cores and either blue (BR) or green (GR) thermally activated delayed fluorescence-based (TADF-based) dendrons have been prepared. The dendrimer emission in solution was found to be primarily from the phosphorescent core independent of the dielectric constant of the solvent used. Irrespective of the medium, time-resolved emission spectroscopy (TRES) at room temperature and 77 K provided evidence for the dendrons acting as energy pools for emission from the core. The maximum solution photoluminescence quantum yields (PLQYs) of both dendrimers was around 60%, which was similar to the model core, fac-tris[2-(thiophen-2-yl)-4-(p-tolyl)quinoline]iridium(III) (TQIr). The neat film PLQY values were lower, being 19% and 8% for BR and GR, respectively. However, when blended with tris(4-carbazoyl-9-ylphenyl)amine (TCTA) at low concentration the film PLQY values increased to the value measured for TQIr in solution (≈60%) indicating that the smaller neat film PLQY values was due to intermolecular interchromophore interactions. The external quantum efficiencies (EQEs) for bilayer OLEDs containing the neat emissive layers of BR and GR were 2.6% and 0.9%, respectively, which was consistent with the low PLQY values of the neat films. Finally, the solution processed OLEDs fabricated using 0.1 mol% of BR and GR blended with TCTA resulted in red emission with maximum EQEs of 13.5% and 11.0%, respectively. [ABSTRACT FROM AUTHOR]

Combining light emission and charge carrier transport into one molecule simplifies the manufacturing of organic light-emitting diodes (OLEDs). In this work, phosphorescent iridium(III) complex-emitters are protected by conjugated dendrons to reduce intermolecular interchromophore interactions that lead to concentration quenching of the luminescence in host-free emission layers. When two dendrons are added to each ligand (doubly dendronized) of an iridium(III) complex at the core of the dendrimers (versus one), the core is found to have improved protection, leading to an enhancement of the external quantum efficiency of the OLEDs from 4% to more than 10%. Likewise, OLEDs comprising the doubly dendronized emitters exhibit improved device lifetimes. A detailed study of the degradation mechanisms reveals that exciton-induced ligand and/or dendron detachment promotes interchromophore interactions (aggregation) that lead to quenching sites and hence red electromer emission inside the OLED emissive layer, with the effects being less pronounced for the doubly dendronized emitter. [ABSTRACT FROM AUTHOR]

We report thermally activated delayed fluorescence (TADF) poly(dendrimer)s, composed of a norbornenyl-derived polymer backbone and dendritic side-chain chromophores comprising benzonitrile as the electron acceptor and two differently substituted first-generation 3,6-disubstituted carbazoles as electron donors. In particular, we compare the effect of the donor strength on the properties of the poly(dendrimer)s and their dendrimer analogues. The groups attached to the carbazolyl moieties to form the donor units were either 9,9-di-n-propylfluorene or diphenylamine. All four compounds were found to have TADF emission with neat film photoluminescence quantum yields (PLQYs) of up to 27%. The di-n-propylfluorenyl groups were found to afford a higher degree of protection from intrapolymer interchromophore interactions along the poly(dendrimer) backbone, and the solution and neat film PLQYs were similar. Photoluminescence studies performed at 77 K revealed that the triplet excitons can migrate along the poly(dendrimer) chain until they reach a chromophore that has the configuration required for charge transfer emission. Also, neat films of the poly(dendrimer)s were found to transport holes and electrons with reasonably balanced mobilities, whilst the dendrimers only had measurable hole mobilities. Organic light-emitting diodes with neat and blended emissive layers were found to have modest performance. [ABSTRACT FROM AUTHOR]

The photophysical and charge transport properties of neat and blend films of fac-tris[2-phenylpyridinato-C2,N]iridium(III) [Ir(ppy)3]-cored light-emitting dendrimers and tris(4-carbazoyl-9-ylphenyl)amine (TCTA)-based hosts, with each combination having the same first generation dendrons were measured and compared. The first generation dendrons were composed of bis(fluorenyl)carbazolyl with n-propyl surface groups, or biphenyl with 2-ethylhexyloxy or t-butyl surface groups. Dendronisation of the emitter and host was found to decrease the energy transfer efficiency from the host to the guest relative to an evaporated Ir(ppy)3:TCTA blend film, with the blend composed of materials with biphenyl dendrons and t-butyl surface groups having the largest decrease. The hole mobilities of the solution processed neat and blend layers were found to be 1–2 orders of magnitude lower than those of the equivalent evaporated films. The blend film containing the host and dendrimer with first generation biphenyl dendrons and 2-ethylhexyloxy surface groups had the highest photoluminescence quantum yield (PLQY), comparable to evaporated Ir(ppy)3:TCTA-based films but the lowest hole mobility (≈10−8 cm2 V−1 s−1). In contrast, the blend with the dendrimers composed of the bis(fluorenyl)carbazolyl dendrons with n-propyl surface groups had a low PLQY but higher hole mobility. It was found for the combinations of these solution processable materials that the hole mobility of the blend film was the limiting factor in OLED performance. Devices containing an emissive layer of the materials with the bis(fluorenyl)carbazolyl dendrons with n-propyl surface groups (11 mole percent of the emitter) had a PLQY of 40.8% but the highest external quantum efficiency of 10.0 ± 0.4%, reaching a maximum luminance of almost 10 000 cd m−2. [ABSTRACT FROM AUTHOR]

We report the first example of a solution-processable dendronised bis-tridentate iridium(III) complex composed of a bis(imidazolyl)phenyl ligand with a first-generation biphenyl dendron containing t-butyl surface groups and a 2-pyrazolyl-6-phenylpyridine co-ligand. The non-dendronised analogue and the dendrimer were found to emit green light. Both complexes were found to have a solution photoluminescence quantum yield [PLQY (Φ)] of ∼70% in toluene. The PLQY of both complexes was reduced in the neat film due to intermolecular quenching. However, the dendrimer retained a higher portion of its PLQY (Φ = 22% versus 9%) in the neat film due to the dendrons partially shielding the emissive iridium(III) complex at its core. The solid-state intermolecular interchromophore interactions of the dendronised complex were suppressed by blending with tris(4-carbazoyl-9-ylphenyl)amine (TCTA) and the PLQY increased to 65%. The non-dendronised material had insufficient solubility to enable it to be solution processed to form films of sufficient thickness and quality to be used as the emissive material in an organic light-emitting diode (OLED). In contrast, the dendronised complex was amenable to solution coating techniques and simple two-layer OLEDs comprising a neat dendrimer film or films with 10 wt% of the dendrimer in TCTA and an electron transport layer showed green emission with maximum external quantum efficiency of 4.5% and 10.7%, respectively. [ABSTRACT FROM AUTHOR]

Pre-concentrators are used to improve the detection of analytes that are at low concentration, particularly in air. Polymeric materials are the basis for most pre-concentrators and a strategy to enhance the pre-concentration capability is to add moieties that enhance the polymer matrix–analyte interaction. In this study, the sorption capability of poly(dimethylsiloxane) (PDMS)-based pre-concentrator films towards nitro-based explosives was improved by incorporating a 1,4-divinylbenzene (DVB) derived component. It was found that the mode of DVB incorporation strongly influenced the structure and pre-concentration properties of the resulting film. Films formed from blends of PDMS and DVB (PDMS:DVB) were found to have similar sorption properties as PDMS. The copolymerised (PDMS-co-DVB) film was found to be comprised of voids and spheroidal features, which were retained even after thermal treatment. The copolymer film offered the greatest sorption and retention of analyte vapor, which we attribute to increased material surface area and π–π interactions between the DVB-derived units and the nitroaromatic analytes. The copolymerised film was able to increase the effective concentration of TNT vapor by 3 orders of magnitude relative to the headspace vapor concentration. [ABSTRACT FROM AUTHOR]

Food wastage due to spoiling is a global economic issue and contributes to over-farming and overfishing with real environmental consequences. Smart food packaging is a promising solution to this problem, the concept of which is to incorporate responsive sensors that allow direct monitoring of the gasses released from the decay of food into a label that provides a visual indicator to the consumer as to whether the food is fresh or has spoiled. Here we report two dicyanovinyl-fluorene-benzothiadiazole-based fluorescent compounds, K12 and K12b, both of which showed rapid response to biogenic amines via two independent mechanisms. When primary alkyl amines were present in solution, they underwent an aza-Michael addition with the dicyanovinyl group of the sensing material, resulting in a rapid colour change. The reaction products of K12 and K12b with primary amines showed a decrease and increase in the fluorescence quantum yield, respectively, enabling a unique dual-sensor array with turn-off/turn-on response. In addition, fluorescence quenching via photoinduced hole transfer was observed in the solid-state sensor films with a wide range of primary, secondary and tertiary amines, enabling rapid and sensitive detection of amine vapours. Finally, we show as proof-of-concept that the fluorescence quenching response of sensing films was observable at cadaverine concentrations relevant to the food industry. [ABSTRACT FROM AUTHOR]

Control of the effective local hyperfine fields in a conjugated polymer, poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), by isotopic engineering is reported. These fields, evident as a frequency-independent line broadening mechanism in electrically detected magnetic resonance (EDMR) spectroscopy, originate from the unresolved hyperfine coupling between the electronic spin of charge carrier pairs and the nuclear spins of surrounding hydrogen isotopes. The room temperature study of effects caused by complete deuteration of this polymer through magnetoresistance, magnetoelectroluminescence, coherent pulsed and multi-frequency EDMR, as well as inverse spin-Hall effect measurements, confirm the weak hyperfine broadening of charge-carrier magnetic resonance lines. As a consequence, we can resolve coherent charge-carrier spin-beating, allowing for direct measurements of the magnitude of electronic spin–spin interactions. In addition, the weak hyperfine coupling allows us to resolve substantial spin–orbit coupling effects in the EDMR spectra, even at low magnetic field strengths. These results illustrate the dramatic influence of hyperfine fields on the spin physics of organic light-emitting diode (OLED) materials at room temperature, and point to routes to reaching exotic ultra-strong resonant-drive regimes in the study of light-matter interactions. [ABSTRACT FROM AUTHOR]

The out-coupling of light from an organic light-emitting diode, and thus its efficiency, strongly depends on the orientation of the transition dipole moment (TDM) of the emitting molecules with respect to the substrate surface. Despite the importance of this quantity, theoretical investigations of the direction of the TDM of phosphorescent emitters based on iridium(iii) complexes remain limited. One challenge is to find an appropriate level of theory able to accurately predict the direction of the TDM. Here, we report relativistic time-dependent density functional theory (TDDFT) calculations of the TDM, emission energies and lifetimes for both the ground-state (S0) and triplet (T1) excited-state geometries of fac-tris(2-phenylpyridyl)iridium(iii) (Ir(ppy)3), using the two-component zero-order regular approximation (ZORA) or including spin–orbit coupling (SOC) perturbatively using the simpler one-component (scalar) formulation. We show that the one- and two-component approaches give similar emission energies and overall radiative lifetimes for each individual geometry. Use of the S0 geometry leads to two of the excited triplet substates being degenerate, with the degeneracy lifted for the T1 geometry, with the latter matching experiment. Two-component calculations using the T1 geometry give results for the direction of the TDM more consistent with experiment than calculations using the S0 geometry. Finally, we show that adding a dielectric medium does not affect the direction of TDM significantly, but leads to better agreement with the experimentally measured radiative lifetime. [ABSTRACT FROM AUTHOR]

Saturated red emitting dendrimers have been prepared for use in organic light-emitting diodes (OLEDs). The dendrimers consist of a homoleptic core iridium(iii) complex comprising a 2-(thiophen-2-yl)quinoline ligand and fluorenylcarbazolyl-based first generation dendrons with n-propyl surface groups. The dendrimers differ in the position the dendron is attached to the ligand. It was found that the dendrimers had solution photoluminescence quantum yields of up to 86 ± 9%, which is amongst the highest for saturated red phosphorescent emitters. The dendrons and surface groups were found to provide sufficient processing ability to enable good quality films when blended with 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP). The dendrimer with the dendron attached to the quinoline unit of the ligand via a phenyl ring was found to have a film PLQY of 87 ± 10% when blended with CBP at a concentration of 10 wt%. A simple two-layer device comprising an emissive layer containing 10 wt% of the dendrimer in CBP and an electron transport layer was found to have a maximum EQE of 12% with CIE co-ordinates of (0.65, 0.34). [ABSTRACT FROM AUTHOR]

A new class of twisted rigid biphenyl-based dendrons with t-butyl surface groups have been prepared and attached to a green emissive fac-tris(2-phenylpyridyl)iridium(iii) core. Both singly (one dendron per ligand) and doubly (two dendrons per ligand) dendronised first generation dendrimers were prepared. In both cases the solution photoluminescence quantum yields (PLQYs) were over 10% higher than the equivalent dendrimers comprised of non-rigid biphenyl-dendrons and the same core, with the doubly dendronised compound having an almost quantitative PLQY. However, for both materials there was significant reduction in the PLQY in the solid-state indicating that while the t-butyl groups can provide a degree of solubility they do not shield the emissive core as well as surface groups such as 2-ethylhexyloxy, which have greater length. When the doubly dendronised material was blended with 4,4′,4′′-tris(carbazol-9-yl)triphenylamine the film PLQY approached that of the solution measurement and simple two-layer devices comprised of the emissive layer and an electron transport/hole-blocking layer of 1,3,5-tris(3-pyridyl-3-phenyl)benzene reached a maximum brightness of ≈10 000 cd m−2 and external quantum efficiency of 16.3%, corresponding to an out-coupling efficiency of around 20%. [ABSTRACT FROM AUTHOR]

The dielectric properties of three pairs of organic semiconductors that contain increasing numbers of cyclopentadithiophene-co-benzothiadiazole moieties (monomer, dimer and polymer) were studied and compared. The materials in each pair differed in the nature of the ‘solubilizing groups’, which are either alkyl- or glycol-based. At low frequencies (＜MHz), dielectric constants of up to ∼9 were obtained for the glycolated materials. In addition, the optical- (high-) frequency dielectric constants for the glycolated dimer and polymer were 4.6 and 4.2 respectively, which are the highest values reported thus far for non-ionic organic semiconductors. The external and internal quantum efficiencies (EQE and IQE) of homojunction (i.e., single component) solar cells comprising the dimer and polymer glycolated materials both showed measurable improvements at wavelengths close to their optical gap when compared with the alkylated equivalents. The improvement is suggestive of an increase in the charge generation efficiency, potentially facilitated by the high optical-frequency dielectric constant. [ABSTRACT FROM AUTHOR]

Fluorescence-based detection of explosive analytes requires an understanding of the nature of the excited state responsible for the luminescence response of a sensing material. Many measurements are carried out to elucidate the fundamental photophysical properties of an emissive material in solution. However, simple transfer of the understanding gained from the solution measurements to the solid-state can lead to errors. This is in part due to the absence of inter-molecular interactions of the chromophores in solution, which are present in the solid-state. To understand the role of inter-molecular interactions on the detection of explosive analytes we have chosen dendrimers from two different families, D1 and D2, which allow facile control of the inter-molecular interactions through the choice of dendrons and emissive chromophores. Using ultrafast transient absorption spectroscopy we find that the solution photoinduced absorption (PA) for both materials can be explained in terms of the generation of singlet excitons, which decay to the ground state, or intersystem cross (ISC) to form a triplet exciton. In neat films however, we observe different photophysical behaviours; first, ISC to the triplet state does not occur, and second, depending on the chromophore, charge transfer and charge separated states are formed. Furthermore, we find that when either dendrimer is interfaced with analyte vapour, the singlet state is strongly quenched, generating a charge transfer state that undergoes geminate recombination. [ABSTRACT FROM AUTHOR]

Bulk heterojunction (BHJ) organic solar cells and photodiodes require optimised active layer structures for both charge carrier photo-generation and extraction to occur efficiently. The most efficient polymer:fullerene devices comprising narrow optical gap donor polymers invariably have a high fullerene fraction. In such circumstances photo-generated carrier extraction is compromised, as charge transport is often unbalanced with electron transport in the fullerene dominating. In this current work we study the high efficiency blend system poly(2,5-{2-octyldodecyl}-3,6-diketopyrrolopyrrole-alt-5,5-{2′,5′-di[thien-2-yl]thieno[3,2-b]thiophene}) (DPP–DTT):phenyl-C71-butyric acid methyl ester (PC70BM). We show that by manipulating the packing of the (DPP–DTT) chains, large hole mobilities ＞0.02 cm2 V−1 s−1 can be obtained in a simple polymer-only diode. More importantly, when blended with PC70BM in a 1 : 3 ratio the hole mobility was found to increase to 0.2 cm2 V−1 s−1. This is the first case of a polymer:fullerene solar cell where the charge carrier transport is limited by the electron mobility of the fullerene rather than the hole mobility of the donor material. Photophysical measurements reveal that the charge generation quantum yield is also enhanced when the DPP–DTT chains are optimally packed. The simultaneous improvement of the charge transport properties and charge generation quantum yield delivers an order of magnitude increase in the power conversion efficiency. [ABSTRACT FROM AUTHOR]

A method of introducing pulses of analyte vapours has been developed to study the interactions of nitro-containing analytes with fluorescent sensing films. The quenching of the photoluminescence of thin dendrimer films by exposure to sub-saturation concentration vapour pulses of nitro-containing analytes is quantified and the reversibility of quenching interactions qualitatively examined. The analysis reveals a linear dependence of the initial/quenched fluorescence ratio (F0/F) with quencher concentration akin to Stern–Volmer behaviour routinely used to quantify quenching efficiency in solution. Detection limits and quenching efficiencies were calculated in the sub-saturation regime for each of the analytes and trace-level detection of 1,4-dinitrobenzene, 2,4-dinitrotoluene, 4-nitrotoluene, and nitromethane vapours was observed. It was found that the sensitivity of first generation bifluorene- and carbazole-cored dendrimers towards vapours of the nitroaliphatic taggant 2,3-dimethyl-2,3-dinitrobutane was significantly enhanced by changing the solubilising chains on the core from alkyl to oligo(ethylene oxide) groups. The responses of the films were observed to be independent of film exposure history making these dendrimers interesting candidates for deployment in thin film vapour sensors for explosives. [ABSTRACT FROM AUTHOR]

The low dielectric constants of conventional organic semiconductors leads to poor charge carrier photogeneration in homojunction organic solar cells due to large exciton binding energies. Increasing the dielectric constant can potentially enhance the spontaneous exciton dissociation rate at room temperature in homojunction cells, and decrease the charge carrier recombination in heterojunction solar cells comprising blends of electron donors and acceptors. We report that substituting the ubiquitous alkyl solubilizing groups with short glycol chains can give non-fullerene electron acceptors with a static dielectric constant of up to 9.8. [ABSTRACT FROM AUTHOR]

A roll-to-roll compatible, high throughput process is reported for the production of highly conductive, transparent planar electrode comprising an interwoven network of silver nanowires and single walled carbon nanotubes imbedded into poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). The planar electrode has a sheet resistance of between 4 and 7 Ω□−1 and a transmission of ＞86% between 800 and 400 nm with a figure of merit of between 344 and 400 Ω−1. The nanocomposite electrode is highly flexible and retains a low sheet resistance after bending at a radius of 5 mm for up to 500 times without loss. Organic photovoltaic devices containing the planar nanocomposite electrodes had efficiencies of ∼90% of control devices that used indium tin oxide as the transparent conducting electrode. [ABSTRACT FROM AUTHOR]

Target-tilted room temperature sputtering of aluminium doped zinc oxide (AZO) provides transparent conducting electrodes with sheet resistances of ＜10 Ω□−1 and average transmittance in the visible region of up to 84%. The properties of the AZO electrode are found to be strongly dependent on the target-tilting angle and film thickness. The AZO electrodes showed comparable performance to commercial indium tin oxide (ITO) electrodes in organic photovoltaic (OPV) devices. OPV devices containing a bulk heterojunction active layer comprised of poly(3-n-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) and an AZO transparent conducting electrode had a power conversion efficiency (PCE) of up to 2.5% with those containing ITO giving a PCE of 2.6%. These results demonstrate that AZO films are a good alternative to ITO for transparent conducting electrodes. [ABSTRACT FROM AUTHOR]

A new family of Ir(iii) complexes were synthesised and employed as light-induced hydrogen-production photosensitisers in aqueous systems, where hydrogen evolution was observed only when the PS* was reduced by the sacrificial agent, NEt3, signifying that a minimum potential difference of ＞0.2 V between E(PS*/PS−) and E(NEt3+/NEt3) is required for efficient hydrogen production [i.e., E(PS*/PS−) ＞1.19 V versus NHE]. The analytical method developed here is demonstrated to be useful for screening new photosensitisers for light-driven hydrogen generation. [ABSTRACT FROM AUTHOR]

The synthesis of three pyrrolo[3,2-b]pyrrole-1,4-dione (isoDPP) derivatives is described, namely 1,3,4,6-tetraphenylpyrrolo[3,2-b]pyrrole-2,5(1H,4H)-dione 2, 1,4-diphenyl-3,6-di(thiophen-2-yl)pyrrolo[3,2-b]pyrrole-2,5(1H,4H)-dione 3, and 1,4-bis(4-(hexyloxy)phenyl)-3,6-di(thiophen-2-yl)pyrrolo[3,2-b]pyrrole-2,5(1H,4H)-dione 7 in which the molecular structures differ in the aromatic ring (phenyl or thiophene) attached to the nitrogen atom. Thin films of 2, 3, and 7 could be formed by evaporation under vacuum. In the case of 2 and 3 GIWAXS measurements showed that the film structural ordering was similar to that measured in single crystals. In contrast GIWAXS showed that 7 had features associated with liquid crystalline materials. Time dependent density functional theory (TDDFT) calculations predicted that the transition between the lowest energy singlet excitation (S1) and the ground state (S0) would be optically forbidden due to the centrosymmetric geometries of compounds. Photophysical measurements showed that the compounds were weakly luminescent, with low radiative rates in solution of order 106 s−1, which are consistent with the TDDFT predictions. Furthermore, photoinduced absorption (PIA) spectroscopy showed that there is a long-lived low energy state, which has been assigned as a triplet and provides a further non-radiative decay pathway for the excited state. [ABSTRACT FROM AUTHOR]

Film structure plays a critical role in defining the performance of all organic optoelectronic devices, with the importance clearly illustrated in the development of organic acceptor–donor bulk heterojunction (BHJ) photovoltaic (OPV) devices where solvent and/or thermal annealing of the deposited active layer affect solar cell output. Herein we report that the polymer–polymer interactions in solution, which are dependent on the thermal history of the solution, are a first order parameter in controlling the properties of the final active layer and hence device performance. We illustrate the key role played by organic semiconductor interactions in solution with the high efficiency donor–acceptor co-polymer, poly[N-9′′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), and its blends with [6,6]-phenyl C71-butyric acid methyl ester (PC71BM). Differences in the cooling rate of the casting solution after dissolution can lead to a 20% variation in the ultimate efficiency of cells with identical active layer thicknesses with slow-cooled solutions giving rise to poorer devices. The oft-ignored intermolecular (polymer–polymer) interactions that occur in solution are manifest by dramatic differences in viscosity and are a function of concentration and molecular weight. Hence solution thermal history represents a critical new dimension in the processing landscape for organic polymer semiconductors. [ABSTRACT FROM AUTHOR]

Dendrimers have proved to be successful materials for solution-processable phosphorescent organic light emitting diodes (OLEDs). Recently we have been investigating poly(dendrimer)s with improved viscosity for ink-jet printing applications. In this work we present three phosphorescent side-chain copolymers comprised of a poly(styrene) backbone, carbazole charge transporting units, and iridium(iii) complexes containing no dendrons (simple complex), a single dendron attached to two of the ligands, or doubly dendronised complexes with two dendrons added to the ligands. In addition to their charge transporting capability the copolymer structure incorporating carbazole units can also prevent aggregation of the chromophores, as evidenced by there being only one emissive environment in time-resolved luminescence. We report high external quantum efficiencies of 11.0% (37.3 cd A−1) at 100 cd m−2 and 8.3 V for devices made with the neat copolymers. The best performance is achieved by blending the doubly dendronised copolymer with 50 wt% of 4,4′-bis(N-dicarbazolyl)biphenyl (CBP) which results in 14.7% EQE (48.3 cd A−1) at 100 cd m−2 and 9.3V. [ABSTRACT FROM AUTHOR]

We have prepared a poly(styrene) [poly(Irppy2acac)] that has a bis(2-phenylpyridyl)iridium(iii) acetylacetonate complex attached to every ‘monomer’ viaa methylene unit. The polymer is solution processable and can be spin-coated to give good quality neat thin films. We show that despite the close proximity of the phosphorescent chromophores along the polymer backbone there is only a modest reduction in the solution photoluminescence quantum yield (PLQY) relative to a monomer model compound. The PLQY of the 4-ethylbenzyl substituted bis(2-phenylpyridyl)iridium(iii) acetylacetonate complex was 52% while the poly(Irppy2acac) had a PLQY of 34%. Time-resolved photoluminescence measurements showed that the photoluminescence decay of the poly(Irppy2acac) in solution was bi-exponential indicating the presence of more than one emissive species, which was attributed to intra-polymer chromophore interactions. In contrast to the solution measurements the solid-state PLQY was very low (<1%) due to quenching arising from inter-chromophore interactions. When 6 wt% of the poly(Irppy2acac) was blended with the small molecule host 4,4′-bis(N-carbazolyl)-2,2′-biphenyl (CBP), which is a ratio similar to that used for small molecule iridium(iii) complexes, significant emission from the CBP was observed suggesting that there was not an even distribution of the polymer in the film. In contrast a 19 wt% blend showed almost complete energy transfer [the CBP peak was 100 times smaller than that of the poly(Irppy2acac)] and there was an increase in the PLQY relative to the neat films. [ABSTRACT FROM AUTHOR]

New high triplet energy dendrons based on 1,2-diphenylethylene with and without 2-ethylhexyloxy surface groups have been developed for deep blue phosphorescent iridium(III) dendrimers. The fac-tris[1-methyl-5-(4-fluoro)phenyl-3-n-propyl-1H-[1,2,4]triazolyl]iridium(III)-cored dendrimers, bearing first generation 1,2-diphenylethylene dendrons on the ligand triazolyl and phenyl rings, were prepared in excellent yields, employing Sonogashira cross-couplings and palladium catalysed hydrogenation as the key synthetic steps. Both dendrimers showed good thermal stability although the flexible nature of the dendrons led to the materials having low glass transition temperatures. Dendrimer 15(without the surface groups) emitted good blue phosphorescence with a solution photoluminescence quantum yield (PLQY) of 46%, Commission Internationale de l'Eclairage (CIE) co-ordinates of (0.15, 0.14) and photoluminescence peaks at 441 and 468 nm. The solution PLQY was 50% higher than the parent iridium(III) complex showing that the high triplet energy of the diphenylethylene dendrons does not quench the luminescence of the iridium(III) complex core. Dendrimer 34, which has the surface groups, had a film PLQY of 49% and CIE co-ordinates of (0.16, 0.19) with PL peaks at 441 and 469 nm. [ABSTRACT FROM AUTHOR]

Correction for ‘Dielectric constant enhancement of non-fullerene acceptors via side-chain modification’ by Jenny E. Donaghey et al., Chem. Commun., 2015, 51, 14115–14118. [ABSTRACT FROM AUTHOR]