Search

The performance of organic optoelectronic devices, such as organic light-emitting diodes (OLEDs) and organic solar cells (OSCs), is intrinsically related to the molecular-scale morphology of the thin films from which they are composed. However, the experimental characterization of morphology at the molecular level is challenging due to the often amorphous or at best semicrystalline nature of these films. Classical molecular modeling techniques, such as molecular dynamics (MD) simulation, are increasingly used to understand the relationship between morphology and the properties of thin-film devices. PyThinFilm (github.com/ATB-UQ/PyThinFilm) is an open-source Python package which allows fully automated MD simulations of thin film growth to be performed using vacuum and/or solution deposition processes. PyThinFilm utilizes the GROMACS simulation package in combination with interaction parameters from the Automated Topology Builder (atb.uq.edu.au). Here, PyThinFilm is described along with an overview of applications in which PyThinFilm has been used to study the thin films of organic semiconductor materials typically used in OLEDs and OSCs.

We compare the effect of donor strength on the optoelectronic properties of thermally activated delayed fluorescence poly(dendrimer)s and their dendrimer analogues.

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.

We describe the development of an absorption spectroscopy method to monitor the uptake of vapours by PDMS-based pre-concentrator materials and apply it to characterising the uptake of 4-nitrotoluene under continuous flow conditions.

Exciplex or charge transfer blends can be emissive in a similar manner to thermally activated delayed fluorescent materials but without the requirement for covalent linkages. Exciplex blends have been mainly used in organic light-emitting diodes fabricated using thermal evaporation, which is in part due to the poor film forming properties of evaporable materials when processed from solution. In this paper, we report the synthesis of three oxadiazole derivatives designed to have good solubility and compare the properties with commercial 1,3-bis[2-(4-t-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7). The acceptor materials have 4-(2-ethylhexyloxy)phenyl or 9,9-di-n-alkylfluoren-2-yl (where the alkyl groups are either n-propyl or n-hexyl) end groups as opposed to the 4-t-butylphenyl moieties of OXD-7. Neat films of the synthesized oxadiazole derivatives were found to have electron mobilities an order of magnitude higher than that of OXD-7. The oxadiazole derivatives were found to be miscible with the hole transporting 4,4′,4″-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA) with charge transfer emission from solution-processed films. The 4-(2-ethylhexyloxy)phenyl-oxadiazole derivative exhibited the best overall performance when blended with m-MTDATA, with the solution-processed films having a photoluminescence quantum yield of around 21 ± 3% and simple bilayer OLEDs having a maximum external quantum efficiency of 5.3 ± 0.3%.

Balanced charge injection and transport in organic light-emitting diodes (OLEDs) is essential for highly efficient devices with small efficiency roll-off at high luminance. However, there are few reports on the measurement of charge mobility within the emissive blend layer of an OLED. In this paper, we show that photoexcitation in conjunction with Metal–Insulator–Semiconductor Charge Extraction with Linearly Increasing Voltage (photo-MIS-CELIV) can be used to determine the hole and electron mobilities of the emissive blend layer in a single device architecture. We demonstrate the technique by studying the commonly used emissive blend of fac-tris[2-phenylpyridinato-C2,N]iridium(III) [Ir(ppy)3] and tris(4-carbazoyl-9-ylphenyl)amine (TCTA), as well as neat TCTA and Ir(ppy)3 films. It was found that Ir(ppy)3 and its blend films with TCTA have measurable electron mobilities and critically they are of similar magnitude as their hole mobilities, irrespective of the Ir(ppy)3 doping ratio. Such balanced charge mobility suggests that the transport of both holes and electrons occurs mostly on the Ir(ppy)3 guest molecules in the blend. Additionally, we demonstrate that photo-MIS-CELIV can be used to measure the quantum efficiency of exciton dissociation in organic semiconductor thin films.

p-Type inorganic nickel oxide (NiOx) exhibits high transparency, tunable-optoelectronic properties, and a work function (

We study the effect of (2,3,4,5,6-pentafluorophenyl)alkylamine additives with differing alkyl chain lengths (methyl, ethyl, and

A common feature of fluorescent sensing materials for detecting chemical warfare agents (CWAs) and simulants is the presence of nitrogen-based groups designed to nucleophilically displace a phosphorus atom substituent, with the reaction causing a measurable fluorescence change. However, such groups are also basic and so sensitive to acid. In this study we show it is critical to disentangle the response of a candidate sensing material to acid and CWA simulant. We report that pyridyl-containing sensing materials designed to react with a CWA gave a strong and rapid increase in fluorescence when exposed to Sarin, which is known to contain hydrofluoric acid. However, when tested against acid-free diethylchlorophosphate and di-iso-propylfluorophosphate, simulants typically used for evaluating novel G-series CWA sensors, there was no change in the fluorescence. In contrast, simulants that had been stored or tested under a standard laboratory conditions all led to strong changes in fluorescence, due to acid impurities. Thus the results provide strong evidence that care needs to be taken when interpreting the results of fluorescence-based solid-state sensing studies of G-series CWAs and their simulants. There are also implications for the application of these pyridyl-based fluorescence and other nucleophilic/basic sensing systems to real-world CWA detection. Chemical warfare agents and simulants are commonly detected with fluorescent sensing materials containing nitrogen-based groups, however these groups’ basicity can cause false positives in the presence of acids. Here, the authors disentangle the response of pyridyl-containing sensing materials to acid-containing and acid-free Sarin and simulants.

Graphene has shown tremendous potential as a transparent conductive electrode (TCE) for flexible organic solar cells (OSCs). However, the trade-off between electrical conductance and transparency as well as surface roughness of the graphene TCE with increasing layer number limits power conversion efficiency (PCE) enhancement and its use for large-area OSCs. Here, we use a 300 nm-thick poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]thiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester blend as the photoactive layer and a benzimidazole (BI)-doped graphene as the transparent anode to demonstrate efficient OSCs with good flexibility. It is found that 3 layer (L) graphene had the best balance between sheet resistance, optical transmittance and surface roughness for optimized cell design. A 0.2 cm2 cell with a 3L BI-doped graphene anode had a PCE of 6.85%, which is one of the highest PCE values reported so far for flexible graphene anode-based OSCs. The flexible cells were mechanically robust, showing only a small performance degradation during up to 250 flexing cycles. Moreover, the combination of the thick photoactive layer with the optimized 3L BI-doped graphene TCE enabled production of 1.6 cm2 flexible OSCs with a PCE of 1.8%. Our work illustrates the importance of graphene TCE development for flexible OSCs as well as other wearable optoelectronic devices.

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.

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.

We report the first example of a thermally activated delayed fluorescence (TADF) poly(dendrimer), composed of a norbornenyl-derived polymer backbone and dendritic side-chain chromophores comprising 2,3-dicyanopyrazino as the electron acceptor and a first-generation fluorenylcarbazole derivative as the electron donor. The TADF poly(dendrimer) homopolymer, with one dendritic side chain attached to each monomer unit, emitted deep-red light. The emission of the poly(dendrimer) was found to be red-shifted relative to the nonpolymeric doubly dendronized emitter composed of the same components. The simple dendrimer was found to have a solution photoluminescence quantum yield (PLQY) of around 70%. In contrast, the poly(dendrimer) had a PLQY of 9%, which was attributed to intramolecular interchromophore interactions. An interesting feature of the poly(dendrimer) was that oxygen did not quench the TADF emission. We found that the PLQY of the simple dendrimer decreased markedly in neat films, whereas that of the poly(dendrimer) did not, with both having a solid-state PLQY of around 10%. The results suggest that intrapolymer chromophore-chromophore interactions observed in solution for the poly(dendrimer) were similar to the intermolecular chromophore-chromophore interactions of the dendrimer in the solid state. Simple two-layer organic light-emitting diodes comprising nondoped films of the materials and an electron transport layer showed red emission with CIE coordinates of (x > 0.66, y < 0.34). The dendrimer-based device had a maximum external quantum efficiency of 2.4%, which is among the best for solution-processed deep-red emissive TADF-based OLEDs but in a simpler device architecture.

The design of nonfullerene acceptors (NFAs), which comprise donor and acceptor moieties, has led to significant improvement in organic photovoltaic (OPV) device performance over the past few years. Although there have been many NFAs synthesized and device-optimized, there have been fewer studies on the excited-state photophysics of donor-acceptor-containing NFAs. Using a combination of femtosecond transient absorption spectroscopy and time-resolved photoluminescence (PL), we have investigated the excited-state photophysical processes in three donor-acceptor-containing NFAs, where the chemical structure of the NFAs are systematically modified to allow direct comparison (D-A1-A1-D, D-A1-D, and D-A1-A2, where D represents 9,9-di-n-propylfluorene, A1 represents benzothiadiazole, and A2 represents dicyanovinyl). A large triplet population was observed for the dimer NFA (D-A1-A1-D), whereas for D-A1-D or D-A1-A2, the nonradiative decay rate was significantly decreased, and higher PL quantum yields observed. Furthermore, a singlet exciton relaxation process between S1∗ and S1 on the picosecond timescale was observed for each of the NFAs, although the mechanism was not always the same. Torsional relaxation was found to dominate the picosecond singlet relaxation for D-A1-D and D-A1-A2 in solution and film, and for D-A1-A1-D in solution. In the film, the singlet excited state of D-A1-A1-D predominantly relaxed via resonant excitonic energy transfer. The observation of singlet relaxation on the tens of picosecond timescale also provides a plausible explanation to the reduced VOC loss commonly associated with NFA-based OPV devices.

Multication metal-halide perovskites exhibit desirable performance and stability, compared to their monocation counterparts. However, the study of the photophysical properties and the nature of defect states in these materials is still a challenging and ongoing task. Here, we study bulk and interfacial energy loss mechanisms in solution-processed MAPbI3 (MAPI) and (CsPbI3)0.05[(FAPbI3)0.83(MAPbBr3)0.17]0.95 (triple cation) perovskite solar cells using absolute photoluminescence (PL) measurements. In neat MAPI films, we find a significantly smaller quasi-Fermi level splitting than for the triple cation perovskite absorbers, which defines the open-circuit voltage of the MAPI cells. PL measurements at low temperatures (∼20 K) on MAPI films demonstrate that emissive subgap states can be effectively reduced using different passivating agents, which lowers the nonradiative recombination loss at room temperature. We conclude that while triple cation perovskite cells are limited by interfacial recombination, the passivation of surface trap states within the MAPI films is the primary consideration for device optimization.

Recent work on bulk-heterojunction organic solar cells has shown that photoexcitation of the electron acceptor followed by photoinduced hole transfer can play a significant role in photocurrent generation. To establish a clear understanding of the role of the donor in the photoinduced hole transfer process, we have synthesized a series of triphenylamine-based hole-transporting poly(dendrimer)s with mechanically flexible nonconjugated backbones via ring-opening metathesis polymerization and used them in low donor content solar cells. The poly(dendrimer)s were found to retain the hole transporting properties of the parent dendrimer, with hole mobilities of similar to 10(-3) cm(2)/(V s) for solution processed neat films. However, when blended with [6,6]-phenyl-C-70-butyric acid methyl ester (PC70BM), the best performing poly(dendrimer) was found to form films that had balanced and relatively high hole/electron mobilities of similar to 5 x 10(-4) cm(2) /(V s). In contrast, at the same concentration the parent dendrimer:PC70BM blend was found to have a hole mobility of 4 orders of magnitude less than the electron mobility. The balanced hole and electron mobilities for the 6 wt % poly(dendrimer):PC70BM blend led to an absence of second-order bimolecular recombination losses at the maximum power point and resulted in a fill factor of 0.65 and a PCE 2.1% for the devices, which was almost three times higher than the cells composed of the parent dendrimer:PC70BM blends.

The formation of excitons in OLEDs is spin dependent and can be controlled by electron‐paramagnetic resonance, affecting device resistance and electroluminescence yield. We explore electrically detected magnetic resonance in the regime of very low magnetic fields (, Perdeuteration of a common OLED emitter allows the detection of paramagnetic resonances at field strengths comparable to Earth's. The competition between narrow resonance spectra and a quasistatic zero‐field feature resulting from reduced hyperfine coupling points at a low‐field limit for detecting paramagnetic resonances in radical‐pair‐based systems.

Insoluble glycol-derivatized poly(1,4-phenylenevinylene)s (PPVs) have been prepared by in situ thermal conversion of solution-processable xanthate precursor polymers and used as interlayers in inverted perovskite solar cells. The insolubility of the PPVs enabled the perovskite active layer to be deposited on top without dissolution, and the glycol chains provided a suitable surface energy for the formation of the perovskite films. It was found that the surface of the films became more hydrophilic with increasing glycol side-chain length. The energy levels of the PPVs indicated they were appropriate for hole transport and electron blocking with respect to the perovskite layer. The performance of the perovskite cells was found to be dependent on the length of PPV glycol side-chain with the optimized planar p-i-n perovskite devices incorporating an MeO-PPV/PFN-P2 hole-extracting interlayer exhibiting a champion power conversion efficiency of 12.1%.

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.

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.

Doubly dendronized blue phosphorescent emitters promote more efficient and stable OLEDs than their singly dendronized counterparts. Degradation occurs as red electromer emission attributable to the detachment of the ligands and/or dendrons.

We report an all-polymer photodiode comprising a new electron-transporting polymer, PNNTH, that contains naphthalene diimide and thiazole moieties. PNNTH has strong absorption at around 670 nm and an electron mobility of ≈10−4 cm2 V−1 s−1. Bulk heterojunction films composed of PNNTH blended with the donor polymer, PBDTT-FTTE, in a weight ratio of 1:2 were found to have electron and hole mobilities of ≈10−5 cm2 V−1 s−1 and ≈10−4 cm2 V−1 s−1, respectively. The photoresponse of conventional and inverted organic photodiodes containing the blend could be tuned from broadband (400–800 nm) to narrowband (50 nm full-width-at-half-maximum) simply by changing the thickness of the all-polymer blend. The narrowband response was achieved using the charge collection narrowing mechanism, which was enhanced by the unbalanced charge mobility. Transfer matrix-based optical modelling confirmed the wavelength dependence of the photoresponse. For both the broadband and narrowband photodiodes, the specific detectivity was greater than 1011 Jones.

Atomistic nonequilibrium molecular dynamics simulations have been used to model the orientation of a side-chain substituted dicyanovinyl oligothiophene, DCV4T-Et2, in thin films formed by vacuum deposition as used in organic photovoltaics. The orientation of the DCV4T-Et2 molecules was analyzed in neat layers deposited onto smooth or rough C60 substrates. The average angle between the long axis of DCV4T-Et2 and the horizontal was 21 +/- 1 degrees in the layer deposited on smooth C-60, in agreement with experimental measurements of layers deposited on silicon. In the layer deposited on rough C-60, the average angle was 25 +/- 2 degrees, which could lead to a decrease of 6% in the efficiency of light absorption compared to the layer on the smooth substrate. Thus, the simulations suggest that surface roughness must be considered in the design of model systems in order to maintain relevance to devices. Importantly, the simulations provide the ability to analyze the orientation of each individual molecule as opposed to experiments that provide an average across a film. The orientation of the thiophene rings of DCV4T-Et2 and the variation in orientation with respect to distance from the substrate were also analyzed and found to be similar for films on smooth and rough substrates. In addition, we show that X-ray diffraction patterns of the layers can be constructed from the simulations.

Clean transfer of two-dimensional (2D) materials grown by chemical vapor deposition is critical for their application in electronics and optoelectronics. Although rosin can be used as a support layer for the clean transfer of graphene grown on Cu, it has not been usable for the transfer of 2D materials grown on noble metals or for large-area transfer. Here, we report a poly(methyl methacrylate) (PMMA)/rosin double support layer that enables facile ultraclean transfer of large-area 2D materials grown on different metals. The bottom rosin layer ensures clean transfer, whereas the top PMMA layer not only screens the rosin from the transfer conditions but also improves the strength of the transfer layer to make the transfer easier and more robust. We demonstrate the transfer of monolayer WSe2 and WS2 single crystals grown on Au as well as large-area graphene films grown on Cu. As a result of the clean surface, the transferred WSe2 retains the intrinsic optical properties of the as-grown sample. Moreover, it does not require annealing to form good ohmic contacts with metal electrodes, enabling high-performance field effect transistors with mobility and ON/OFF ratio ∼10 times higher than those made by PMMA-transferred WSe2. The ultraclean graphene film is found to be a good anode for flexible organic photovoltaic cells with a high power conversion efficiency of ∼6.4% achieved.

We have synthesised new materials comprised of 9,9′-bifluorenylidene and dithienodiketopyrrolopyrrole units. While 9,9′-bifluorenylidene has been primarily used in non-fullerene acceptors, when used in combination with the diketopyrrolopyrrole moiety it can form donor materials that can be used in conjunction with fullerene acceptors. The compounds differ in the substituents on the 9,9′-bifluorenylidene (protonated = 1A and dimethoxy = 1B) moiety. The structure of both the neat and blend films with PC70BM were found to be strongly dependent on the processing solvent used. In particular, Grazing Incidence Wide Angle X-ray Scattering measurements of films of 1A or 1B blended with PC70BM prepared from chloroform or chloroform with o-dichlorobenzene as an additive showed that the donor material had no particular ordering. However, when 1,8-diiodooctane was added to the processing solvent the 1A blends showed liquid crystalline ordering while 1B formed well-defined crystallites with three-dimensional ordering. The difference in film structure had a profound effect on the device properties. For 1A the optimised blend ratio with PC70BM was 1:4, but when 1,8-diiodooctane was used as the additive the best ratio of 1A to fullerene was 1:1. The films containing the well-defined crystallites of 1B all performed poorly, which was ascribed to a lack of a percolation pathway for hole extraction. The best performing device was comprised of a 1:1 blend of 1A and PC70BM, which had a power conversion efficiency of 2.6%.

Fluorenylboronate ester chromophore-based thin films were investigated for the detection of triacetone triperoxide (TATP) vapors via the decomposition product, hydrogen peroxide. Sensing with a high level of sensitivity was achieved using a fluorescence "turn-on" mechanism based on the significant shifts in the absorption and photoluminescence spectra that occurs when the boronate esters were converted to phenoxides by hydrogen peroxide under basic conditions. The addition of an organic base was found to be critical for achieving fast conversion reactions and the formation of the phenoxide anions. Addition of a nitrile group to the fluorenyl boronate ester moiety improved the stability of the material to photooxidation, increased the photoluminescence quantum yields, and enhanced the absorption and emission shifts to longer wavelengths. In real-time sensing measurements, films comprising the cyanofluorenyl boronate ester moiety and tetra- n-butylammonium hydroxide had a response time to acid-decomposed TATP vapor of seconds and a limit of detection of 40 ppb in 60 s.

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).

Sensing of TATP vapours via the decomposition product, hydrogen peroxide, was achieved using a fluorescence “turn-on” mechanism through conversion of boronate esters to phenoxides under basic conditions in solid-state films. High sensitivity was achieved with two new fluorenylboronate esters comprising either 2,4-difluorophenyl or 4-(trifluoromethyl)phenyl substituents. The key to the sensitivity was the fact that the phenoxide anion products from the hydrogen peroxide oxidation absorbed at longer wavelengths than the starting boronate esters. Selective excitation of the phenoxide anions avoided the background fluorescence from the corresponding boronate esters. The use of the electron withdrawing substituents also led to greater photostability. The derivative containing the 4-(trifluoromethyl)phenyl moiety was found to give the most stable phenoxide, and demonstrated fast fluorescence “turn-on” kinetics with a lower limit of detection of ≈2.5 ppb in 60 s.

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.

Efficient organic light-emitting diodes (OLEDs) consist of an emissive layer comprising a blend of a light-emitting and host material in contact with one or more charge transporting layers. The distribution of the active material in the guest-host emissive layer blend and the changes that may occur upon thermal annealing are two important factors in determining the stability and efficiency of OLEDs. We have combined neutron reflectometry and photoluminescence measurements to investigate the structures of films comprising an emissive layer containing a phosphorescent poly(dendrimer) material blended with 4,4'-,'-di(carbazolyl)biphenyl. This combination has been shown to give rise to highly efficient OLEDs. Here, we show that the emissive poly(dendrimer) material was not uniformly distributed in the host, but formed a concentration gradient within the emissive layer. Upon heating, the adjacent electron transport layer was found to intermix with the emissive layer, accompanied by changes in the material distribution in the emissive layer. The intermixing of the materials led to a decrease in the photoluminescence from the poly(dendrimer) within the film. The decrease in the photoluminescence was ascribed to an increase in interchromophore interactions that could arise from a conformational change of the poly(dendrimer) or phase separation leading to aggregation. The results indicate that, while uniform mixing of the guest and host is not essential for efficiency, the thermal stabilities of both host and charge transport materials are important for device durability.

Emissive layers in phosphorescent organic light-emitting diodes commonly make use of guest-host blends such as Ir(ppy)

Weakly spin-orbit coupled electron and hole spins in organic light-emitting diodes (OLEDs) constitute near-perfect two-level systems to explore the interaction of light and matter in the ultrastrong-drive regime. Under such highly non-perturbative conditions, the frequency at which the spin oscillates between states, the Rabi frequency, becomes comparable to its natural resonance frequency, the Larmor frequency. For such conditions, we develop an intuitive understanding of the emergence of hybrid light-matter states, illustrating how dipole-forbidden multiple-quantum transitions at integer and fractional g-factors arise. A rigorous theoretical treatment of the phenomena comes from a Floquet-style solution to the time-dependent Hamiltonian of the electron-hole spin pair under resonant drive. To probe these phenomena experimentally requires both the development of a magnetic-resonance setup capable of supporting oscillating driving fields comparable in magnitude to the static field defining the Zeeman splitting; and an organic semiconductor which is characterized by minimal inhomogeneous broadening so as to allow the non-linear light-matter interactions to be resolved. The predicted exotic resonance features associated with the Floquet states are indeed found experimentally in measurements of spin-dependent steady-state OLED current under resonant drive, demonstrating that complex hybrid light-matter spin excitations can be formed and probed at room temperature. The spin-Dicke state arising under strong drive is insensitive to power broadening so that the Bloch-Siegert shift of the resonance becomes apparent, implying long coherence times of the dressed spin state with potential applicability for quantum sensing., Comment: Manuscript: 26 pages, 3 figures; supplementary information: 28 pages, 7 figures

Efficient and stable perovskite solar cells with a simple active layer are desirable for manufacturing. Three-dimensional perovskite solar cells are most efficient but need to have improved environmental stability. Inclusion of larger ammonium salts has led to a trade-off between improved stability and efficiency, which is attributed to the perovskite films containing a two-dimensional component. Here, we show that addition of 0.3 mole percent of a fluorinated lead salt into the three-dimensional methylammonium lead iodide perovskite enables low temperature fabrication of simple inverted solar cells with a maximum power conversion efficiency of 21.1%. The perovskite layer has no detectable two-dimensional component at salt concentrations of up to 5 mole percent. The high concentration of fluorinated material found at the film-air interface provides greater hydrophobicity, increased size and orientation of the surface perovskite crystals, and unencapsulated devices with increased stability to high humidity., Efficient and stable perovskite solar cells with simple active layers are desirable for manufacturing, yet formation of a two-dimensional component in the perovskite film compromises the performance. Here, the authors report low temperature fabrication of highly efficient and stable inverted solar cells by adding a fluorinated lead salt.

Electron paramagnetic resonance (EPR) is a versatile tool to probe spin physics in organic semiconductor materials. A common method used to detect the spin-�� paramagnetic resonance in organic light-emitting diodes (OLEDs) is to measure the device resistance under EPR conditions, i.e., to record electrically detected magnetic resonance (EDMR). Here, we present ultralow-frequency EDMR experiments on OLEDs that exhibit a qualitatively new line shape because of a quasistatic magnetic field effect: the modulation of the static ultrasmall field-effect magnetoresistance arising from the magnetic field amplitude B1 of the radio frequency (rf) radiation. The disappearance of spin-�� Zeeman resonances of individual charge carriers in the OLED, i.e., the resonances at magnetic fields where the Zeeman splitting matches the photon energy of the incident rf radiation, coincides with the emergence of the quasistatic effect. We discuss the origin of this quasistatic magnetic field effect, its characteristic line shape in terms of the magnetic field dependence, the influence of experimental parameters, and the application potential with regards to EDMR experiments. The EDMR line shape can be inferred numerically from the magnetoresistance measurements. This approach enables a unique means of determining the drive-field strength B1 in EDMR under driving conditions where alternative methods employing an analysis of the Zeeman resonance���such as power broadening and Rabi flopping���are not applicable.

We introduce two novel solution-processable electron acceptors based on an isomeric core of the much explored diketopyrrolopyrrole (DPP) moiety, namely pyrrolo[3,2-b]pyrrole-1,4-dione (IsoDPP). The newly designed and synthesized compounds, 6,6&prime, [(1,4-bis{4-decylphenyl}-2,5-dioxo-1,2,4,5-tetrahydropyrrolo[3,2-b]pyrrole-3,6-diyl)bis(thiophene-5,2-diyl)]bis[2-(2-butyloctyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione] (NAI-IsoDPP-NAI) and 5,5&prime, [(1,4-bis{4-decylphenyl}-2,5-dioxo-1,2,4,5-tetrahydropyrrolo[3,2-b]pyrrole-3,6-diyl)bis(thiophene-5,2-diyl)]bis[2-(2-butyloctyl)isoindoline-1,3-dione] (PI-IsoDPP-PI) have been synthesized via Suzuki couplings using IsoDPP as a central building block and napthalimide or phthalimide as end-capping groups. The materials both exhibit good solubility in a wide range of organic solvents including chloroform (CF), dichloromethane (DCM), and tetrahydrofuran (THF), and have a high thermal stability. The new materials absorb in the wavelength range of 300&ndash, 600 nm and both compounds have similar electron affinities, with the electron affinities that are compatible with their use as acceptors in donor-acceptor bulk heterojunction (BHJ) organic solar cells. BHJ devices comprising the NAI-IsoDPP-NAI acceptor with poly(3-n-hexylthiophene) (P3HT) as the donor were found to have a better performance than the PI-IsoDPP-PI containing cells, with the best device having a VOC of 0.92 V, a JSC of 1.7 mAcm&minus, 2, a FF of 63%, and a PCE of 0.97%.

The crucial role played by the solution-vapor interface in determining the growth and morphology of an organic semiconductor thin film formed by solvent evaporation has been examined in atomic detail. Specifically, how the loss of individual solvent molecules from the surface of the solution induces solute assembly has been studied using molecular dynamics simulations. The system consisted of