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Efficient detection of chemical analytes using fluorescence-based sensors necessitates an in-depth understanding of the physical interaction between the analyte molecules and the sensor films. This study explores the interplay between the thermal properties of a series of triphenylamine-centered fluorescent dendrimers with different glass transition temperatures (Tg) for detecting nitroaromatic explosives. When exposed to 4-nitrotoluene (pNT) vapors, biphasic diffusion kinetics were observed for all the dendrimers, corresponding to Super Case II kinetics, suggesting rapid film swelling during initial analyte uptake. The diffusion kinetics were further analyzed using a diffusion–relaxation model, where a strong Tgdependence was observed for both the initial concentration-driven diffusion phase and the slower film relaxation phase. Additionally, a difference in kinetics between analyte uptake and release was observed. The photoluminescence (PL) kinetics also showed a Tgdependence, with more efficient PL recovery observed for films composed of dendrimers that had a lower Tg. Rapid quenching of over 40% with little PL recovery was seen in the dendrimer with the highest Tg(107 °C), while a smaller quench with efficient PL recovery was observed in the dendrimer that had a Tgclose to room temperature. The results highlight the critical role of the thermal properties of sensor films in achieving rapid and sensitive detection.

Luminescence-based sensing has been demonstrated to be a powerful method for rapid trace detection of chemical vapors (analytes). Analyte diffusion has been shown to be the critical factor for real-time luminescence-based detection of explosive analytes via photoinduced electron transfer in amorphous films of conjugated polymers and dendrimers. However, similar studies to determine the critical factors for sensing have not been performed on materials that employ photoinduced hole transfer (PHT) to detect low electron affinity analytes such as illicit drugs. Nor have such studies been performed on semicrystalline sensing films. We have developed a family of perylene diimide-based sensing materials capable of undergoing PHT with amine-group containing analytes. It was found that the choice of branched alkyl chain [1-hexylheptyl (PHH), 2-hexyloctyl (PHO), or 2,2-dihexyloctyl (PDHO)] attached to the nitrogen atoms of the imide moiety strongly affected the solution-processed film morphology. PHHand PHOwere found to contain crystalline phases, whereas PDHOwas essentially amorphous. The degree of crystallinity strongly influenced exciton diffusion, with PHHand PHOexhibiting exciton diffusion coefficients that were 20× and 10× greater than the value of the amorphous PDHO. The degree of film crystallinity was also found to be critical when the films were applied to detect N-methylphenethylamine (MPEA), a simulant of methamphetamine. While PHHhad the largest exciton diffusion coefficient [(1.0 ± 0.2) × 10–2cm2s–1] and analyte uptake (12.3 ± 1.8 ng) it showed the smallest quenching efficiency (2.6% ng–1). In contrast, PHO, which sorbed the least analyte (6.1 ± 0.4 ng) of the three compounds, had the largest quenching efficiency (7.1% ng–1) due to its molecular packing and hence exciton diffusion coefficient [(4.5 ± 1.4) × 10–3cm2s–1] not being affected by sorption of the analyte. These results show that when applying fluorescent films in practical detection scenarios there is a potential trade-off between a high exciton diffusion constant and analyte diffusion for semicrystalline sensing materials and that a high exciton diffusion coefficient in an as-cast film does not necessarily translate into a more efficient fluorescent quenching. The results also show that sensing materials that form semicrystalline films, whose packing is not disrupted by analyte diffusion, provide a route for overcoming these effects and achieving high sensitivity.

We study the effect of (2,3,4,5,6-pentafluorophenyl)alkylamine additives with differing alkyl chain lengths (methyl, ethyl, and n-propyl) on the performance of methylammonium lead triiodide (MAPbI3) perovskite solar cells. The results show that the length of the alkyl chain between the 2,3,4,5,6-pentafluorophenyl group and ammonium moiety has a critical effect on the perovskite film structure and subsequent device performance. The 2,3,4,5,6-pentafluorophenyl ammonium additive with the shortest linking group (a methylene unit), namely (2,3,4,5,6-pentafluorophenyl)methylammonium iodide, was found to be distributed throughout the bulk of the perovskite film with a 2D phase only being observable at high concentrations (>30 mol%). In contrast, the additives with ethyl and n-propyl linking groups phase-separate during solution processing and are found to concentrate at the surface of the perovskite film. Photoluminescence measurements showed that the fluorinated additives passivated the surface defects on the perovskite grains. Of the three additives, inverted devices containing 0.32 mol% of the 2,3,4,5,6-pentafluorophenyl ammonium additive with the methylene linking group achieved a maximum power conversion efficiency of 22.0%, with the device efficiency decreasing with increasing additive concentration. In contrast, the devices composed of the additive with the longest alkyl linker, 3-(2,3,4,5,6-pentafluorophenyl)propylammonium iodide, had the poorest performance, with PCEs less than that of the neat MAPbI3control and decreasing with increasing additive concentration.

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′-N,N′-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.

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 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 bis(2-phenylpyridine) (acetylacetonate)iridium(III) [Ir(ppy)2(acac)] and 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) in chloroform at 310 K. The simulations clearly indicate that (a) the system does not undergo uniform phase separation (spinodal decomposition), (b) solute aggregation initiates at the solution–vapor interface, (c) the distribution of solvent in the film is nonhomogeneous, (d) this nonhomogeneous distribution can induce preferential alignment of host molecules, and (e) a portion of the solvent likely remains trapped within the film. The work not only demonstrates the ability to directly model evaporation in atomic detail on the relevant length scales but also shows that atomistic simulations have the potential to shed new light on morphological properties of a wide range of organic semiconductor devices manufactured using solution-processing methods.

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 Drepresents 9,9-di-n-propylfluorene, A1represents benzothiadiazole, and A2represents dicyanovinyl). A large triplet population was observed for the dimer NFA (D-A1-A1-D), whereas for D-A1-Dor 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 S1on 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-Dand D-A1-A2in solution and film, and for D-A1-A1-Din solution. In the film, the singlet excited state of D-A1-A1-Dpredominantly 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 VOCloss 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 ∼10–3cm2/(V s) for solution processed neat films. However, when blended with [6,6]-phenyl-C70-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 ∼5 × 10–4cm2/(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 PCE2.1% for the devices, which was almost three times higher than the cells composed of the parent dendrimer:PC70BM blends.

Poly(dendrimers) are an emerging class of fluorescent organic semiconductors that have been investigated for their ability to detect nitro-based explosives and taggants. In this work, the sensing chromophores were dendritic side chains attached to a norbornenyl-derived polymer backbone. The sensing chromophore of each poly(dendrimer) was based on fluorene and carbazole moieties, both of which have been shown to be sensitive toward nitro-containing explosives. The monomers, and corresponding polymers, were screened against 2,4-dinitrotoluene (DNT) and 2,3-dimethyl-2,3-dinitrobutane (DMNB) vapors and a range of household interferents such as mouthwash and ethanol. An important result of this work was the rapid and sensitive detection of DMNB, a taggant added to commercial plastic explosives, which is usually difficult to detect via fluorescence quenching due to its relatively low electron affinity and spontaneous diffusion out of the sensing film. Testing of the poly(dendrimer)s was also performed with a hand-held prototype detector for explosives. Selectivity was demonstrated with a range of interferents and explosive or taggant vapors, with the interferents giving an increase in fluorescence intensity and the explosive related materials a quenching of the luminescence. The results show that organic semiconductor poly(dendrimer)s are an interesting class of material for the detection of nitro-based explosives and taggants, especially DMNB.

The advent of small molecule non-fullerene acceptor (NFA) materials for organic photovoltaic (OPV) devices has led to a series of breakthroughs in performance and device lifetime. The most efficient OPV devices have a combination of electron donor and acceptor materials that constitute the light absorbing layer in a bulk heterojunction (BHJ) structure. For many BHJ-based devices reported to date, the weight ratio of donor to acceptor is near equal. However, the morphology of such films can be difficult to reproduce and manufacture at scale. There would be an advantage in developing a light harvesting layer for efficient OPV devices that contains only a small amount of either the donor or acceptor. In this work we explore low donor content OPV devices composed of the polymeric donor PM6blended with high performance NFA materials, Y6or ITIC-4F. We found that even when the donor:acceptor weight ratio was only 1:10, the OPV devices still have good photoconversion efficiencies of around 6% and 5% for Y6and ITIC-4F, respectively. It was found that neither charge mobility nor recombination rates had a strong effect on the efficiency of the devices. Rather, the overall efficiency was strongly related to the film absorption coefficient and maintaining adequate interfacial surface area between donor and acceptor molecules/phases for efficient exciton dissociation.

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 C60substrates. The average angle between the long axis of DCV4T-Et2 and the horizontal was 21 ± 1° in the layer deposited on smooth C60, in agreement with experimental measurements of layers deposited on silicon. In the layer deposited on rough C60, the average angle was 25 ± 2°, 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 WSe2and WS2single crystals grown on Au as well as large-area graphene films grown on Cu. As a result of the clean surface, the transferred WSe2retains 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.

Atomistic nonequilibrium molecular dynamics simulations have been used to model the morphology of small-molecule bulk heterojunction films formed by vapor deposition as used in organic photovoltaics. Films comprising C60and 1, 5, 10, and 50 wt % of 1,1-bis[4-bis(4-methylphenyl)aminophenyl]cyclohexane (TAPC) were compared to films of neat C60. The simulations suggest that if holes can hop between donor molecules separated by as little as 1.2–1.5 nm, then a TAPC concentration of 5 wt % is sufficient to form a percolating donor network and facilitate charge extraction. The results provide an explanation for why low donor content organic photovoltaics can still have high efficiencies. In addition, the roughness, porosity, and crystallinity of the films were found to decrease with increasing TAPC content.

Low-weight-percent donor content fullerene-based organic solar cells have been reported to have good efficiencies although the exact reason for their performance is still a matter of discussion. We use a solution processable hole-transporting poly(dendrimer) with minimal absorption in the visible region to study the blend-ratio dependency of energy-loss mechanisms in fullerene low-donor content organic solar cells in which [6,6]-phenyl-C71-butyric acid methyl ester is used as the primary absorber. The photocurrent losses for each of the steps that govern the device performance have been assessed, with the optimized device performance achieved when the donor was at a concentration of 6 wt %. The 6 wt % donor device had balanced charge transport, and transient absorption spectra revealed that although the low-donor content devices suffered from a low-exciton-quenching rate, the dissociation efficiency of the resulting charge-transfer states was almost unity. That is, essentially all of the formed charge-transfer states led to charge-separated states. A gradual increase in the open-circuit voltage was observed as the donor ratio decreased from 50 to 6 wt %. Internal quantum-efficiency measurements indicated efficient formation of charge-separated states from the intermediate charge-transfer states and high charge-collection efficiency at low-donor content resulting in an overall higher external quantum efficiency despite the lower interfacial area. Overall, these measurements highlight the competing effects of exciton dissociation (favored by the high surface area associated with large donor content) and charge-transfer state dissociation, which we observe to be more efficient when the donor content is low.

PNNThas been prepared as a polymeric electron acceptor for organic solar cells. The polymer has an A–A′–A acceptor motif linked alternatively with thiophene and vinyl moieties. The A′-unit is a naphthalene diimide, while the A groups are thiazoles. PNNTfilms were found to have an estimated electron affinity of ≈4.3 eV and an electron mobility of the order of 10–4cm2V–1s–1. Its relatively low solubility in common chlorinated solvents at ambient temperature allowed the manufacture of sequentially deposited (SD) devices, which were found to have significantly higher efficiency than that of bulk heterojunction (BHJ) solar cells containing the same materials. Grazing-incidence wide-angle X-ray scattering measurements indicated that the SD films retained the ordering of the individual polymers to a greater extent compared to the BHJ films. The best SD devices were found to have a power conversion efficiency of up to 4.5%, with stable performance under thermal stress.

Engineering the interface between the perovskite absorber and the charge-transporting layers has become an important method for improving the charge extraction and open-circuit voltage (VOC) of hybrid perovskite solar cells. Conjugated polymers are particularly suited to form the hole-transporting layer, but their hydrophobicity renders it difficult to solution-process the perovskite absorber on top. Herein, oxygen plasma treatment is introduced as a simple means to change the surface energy and work function of hydrophobic polymer interlayers for use as p-contacts in perovskite solar cells. We find that upon oxygen plasma treatment, the hydrophobic surfaces of different prototypical p-type polymers became sufficiently hydrophilic to enable subsequent perovskite junction processing. In addition, the oxygen plasma treatment also increased the ionization potential of the polymer such that it became closer to the valance band energy of the perovskite. It was also found that the oxygen plasma treatment could increase the electrical conductivity of the p-type polymers, facilitating more efficient charge extraction. On the basis of this concept, inverted MAPbI3perovskite devices with different oxygen plasma-treated polymers such as P3HT, P3OT, polyTPD, or PTAA were fabricated with power conversion efficiencies of up to 19%.

Organic light-emitting devices containing solution-processed emissive dendrimers can be highly efficient. The most efficient devices contain a blend of the light-emitting dendrimer in a host and one or more charge-transporting layers. Using neutron reflectometry measurements with in situ photoluminescence, we have investigated the structure of the as-formed film as well as the changes in film structure and dendrimer emission under thermal stress. It was found that the as-formed film stacks comprising poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/host:dendrimer/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (where the host was deuterated 4,4′-N,N′-di(carbazolyl)biphenyl or tris(4-carbazol-9-ylphenyl)amine, the host:dendrimer layer was solution-processed, and the 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene evaporated) had well-defined interfaces, indicating good wetting of each of the layers by the subsequently deposited layer. Upon thermal annealing, there was no change in the poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/host:dendrimer interface, but once the temperature reached above the Tgof the host:dendrimer layer, it became a supercooled liquid into which 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene dissolved. When the film stacks were held at a temperature just above the onset of the diffusion process, they underwent an initial relatively fast diffusion process before reaching a quasi-stable state at that temperature.

Nanoparticle dispersions open up an ecofriendly route toward printable organic solar cells. They can be formed from a variety of organic semiconductors by using miniemulsions that employ surfactants to stabilize the nanoparticles in dispersion and to prevent aggregation. However, whenever surfactant-based nanoparticle dispersions have been used to fabricate solar cells, the reported performances remain moderate. In contrast, solar cells from nanoparticle dispersions formed by precipitation (without surfactants) can exhibit power conversion efficiencies close to those of state-of-the-art solar cells processed from blend solutions using chlorinated solvents. In this work, we use small-angle neutron scattering measurements and transient absorption spectroscopy to investigate why surfactant-free nanoparticles give rise to efficient organic solar cells. We show that surfactant-free nanoparticles comprise a uniform distribution of small semiconductor domains, similar to that of bulk-heterojunction films formed using traditional solvent processing. This observation differs from surfactant-based miniemulsion nanoparticles that typically exhibit core–shell structures. Hence, the surfactant-free nanoparticles already possess the optimum morphology for efficient energy conversion before they are assembled into the photoactive layer of a solar cell. This structural property underpins the superior performance of the solar cells containing surfactant-free nanoparticles and is an important design criterion for future nanoparticle inks.

The power conversion efficiencies of solar cells incorporating non-fullerene donor–acceptor blends now approach those of fullerene-based devices in the best performing examples. However, the lack of clear structure–property relationships means that the poor device performance of many novel non-fullerene systems cannot be readily explained. We report a series of non-fullerene acceptors, comprising essentially the same chromophore, that differ in terms of their shape and number of chromophores. To understand the impact of these structural differences on charge generation, we have employed transient absorption spectroscopy to investigate the photophysical properties of the acceptors in blends with the conjugated polymer PTB7. To minimize the impact of morphology, we employed a broad range of acceptor concentrations and compared the results with those of blends containing the fullerene derivative PC70BM. In terms of singlet exciton harvesting, the non-fullerene acceptors exhibited similar performance to PC70BM in blends with PTB7. The rate of singlet exciton quenching as a function of acceptor concentration was consistent with exciton diffusion mediated quenching with a diffusion length of 4–5 nm for the PTB7 singlet exciton. The polaron generation efficiency of the non-fullerene acceptors was comparable to that of PC70BM although the fraction of polarons that subsequently underwent geminate recombination was much greater in the non-fullerene blends. Furthermore, the photophysical properties of the non-fullerene acceptors were not influenced by the shape of the acceptor, the chromophore number, or the donor–acceptor ratio, which indicates that the higher geminate recombination is related to the structure of the acceptor chromophore and the interface formed with the donor. The implication of these results is that despite appropriate energetics and optical absorption, the performance of some non-fullerene donor–acceptor blends will be intrinsically limited by the choice of acceptor chromophore and the nature of the interface it forms with the donor material.

Organic light-emitting diodes (OLEDs) are subjected to varying conditions of temperature during their lifetime. Using a combination of neutron reflectometry and in situ photoluminescence spectroscopy we show that layers typically found in OLEDs can interdiffuse. The diffusion is temperature dependent and it is found that the iridium(III) complexes can be remarkably mobile, even with relatively bulky alkyl groups attached to the ligand.

Organic light-emitting diodes (OLEDs) are subject to thermal stress from Joule heating and the external environment. In this work, neutron reflectometry (NR) was used to probe the effect of heat on the morphology of thin three-layer organic films comprising materials typically found in OLEDs. It was found that layers within the films began to mix when heated to approximately 20 °C above the glass-transition temperature (Tg) of the material with the lowest Tg. Diffusion occurred when the material with the lowest Tgformed a supercooled liquid, with the rates of interdiffusion of the materials depending on the relative Tg’s. If the supercooled liquid formed at a temperature significantly lower than the Tgof the higher-Tgmaterial in the adjacent layer, then pseudo-Fickian diffusion occurred. If the two Tg’s were similar, then the two materials can interdiffuse at similar rates. The type and extent of diffusion observed can provide insight into and a partial explanation for the “burn in” often observed for OLEDs. Photoluminescence measurements performed simultaneously with the NR measurements showed that interdiffusion of the materials from the different layers had a strong effect on the emission of the film, with quenching generally observed. These results emphasize the importance of using thermally stable materials in OLED devices to avoid film morphology changes.

Organohalide lead perovskite solar cells have emerged as a promising next-generation thin-film photovoltaic technology. It has been clearly recognized that interfacial engineering plays a critical role in cell performance. It has been also proposed that the open-circuit voltage is dependent on the ionization potential of the hole transport layer at the anode. In this communication, we report a simple modification of the anode with a triarylamine-based small molecule (1), which avoids the need to use standard hole transport materials and delivers a relatively high open-circuit voltage of 1.08 V and a power conversion efficiency of 16.5% in a simple planar architecture.

A family of first-generation dendrimers containing 3,5-bis(carbazolyl)phenyl dendrons attached to a green emissive fac-tris(2-phenylpyridyl)iridium(III) core were prepared. The solubility of the dendrimers was imparted by the attachment of tert-butyl surface groups to the carbazole moieties. The dendrimers differed in the number of dendrons attached to each ligand (one or two dendrons) as well as the degree of rotational restriction within the dendrons. The densities of the films containing the doubly dendronized materials were higher than those of their mono-dendronized counterparts, with the dendrimer containing two rotationally constrained dendrons per ligand having the highest density at 1.12 ± 0.04 g cm–3. The dendrimers were found to have high photoluminescence quantum yields (PLQYs) in solution of between 80 and 90%, with the doubly dendronized materials having the lower values and a red-shifted emission. The neat film PLQY values of the dendrimers were less than those measured in solution although the relative decrease was smaller for the doubly dendronized materials. The dendrimers were incorporated into solution-processed bilayer organic light-emitting diodes (OLEDs) composed of neat or blend emissive layers and an electron transport layer. The best-performing devices had the dendrimers blended with a host material and external quantum efficiencies as high as 14.0%, which is higher than previously reported results for carbazole-incorporating emissive dendrimers. A feature of the devices containing blends of the doubly dendronized materials was that the maximum efficiency was relatively insensitive to the concentration in the host between 1 and 7 mol %.

Three-dimensional (3D) perovskite solar cells (PSCs) containing additives capable of forming two-dimensional (2D) structures in neat films have attracted attention due to their ability to enhance power conversion efficiency (PCE) in combination with improved operational stability. Herein, a newly designed fluorinated ammonium salt, 2-(perfluorophenyl)ethanaminium bromide:chloride50:50(FEABr:Cl50:50), is introduced into CsMAFAPbI3-based PSCs with a standard n–i–p architecture. FEABr:Cl50:50was used as an additive in the tin(IV) oxide (SnO2) electron transporting layer (ETL) as well as a surface treatment for the perovskite film. Used in this dual way, the additive was found to passivate charge-trapping defects within the SnO2ETL and regulate the crystal growth of the perovskite layer. When FEABr:Cl50:50was deposited onto the surface of the 3D perovskite film, it formed a thin hydrophobic 2D capping layer. Adopting this dual strategy led to the perovskite film having larger grain sizes, improved quality, and overall better device performance. As a result, the best-performing device exhibited a PCE of over 23% with negligible hysteresis in an n–i–p device architecture with an area of 0.2 cm2. Furthermore, unencapsulated devices with the hydrophobic 2D capping layer showed improved stability compared to the control device when measured under continuous light irradiation at a maximum power point (MPP) at 80 ± 5 °C in a humid (≈50%) environment.

We investigate the role of metal–ligand bond fission in the nonradiative decay of excited states in iridium(III) complexes with applications in blue organic light-emitting diodes (OLEDs). We report density functional theory (DFT) calculations of the potential energy surfaces upon lengthening an iridium–nitrogen (Ir–N) bond. In all cases we find that for bond lengths comparable to those of the ground state the lowest energy state is a triplet with significant metal-to-ligand change transfer character (3MLCT). But, as the Ir–N bond is lengthened there is a sudden transition to a regime where the lowest excited state is a triplet with significant metal centered character (3MC). Time-dependent DFT relativistic calculations including spin–orbit coupling perturbatively show that the radiative decay rate from the 3MC state is orders of magnitude slower than that from the 3MLCT state. The calculated barrier height between the 3MLCT and 3MC regimes is clearly correlated with previously measured nonradiative decay rates, suggesting that thermal population of the 3MC state is the dominant nonradiative decay process at ambient temperature. In particular, fluorination both drives the emission of these complexes to a deeper blue color and lowers the 3MLCT–3MC barrier. If the Ir–N bond is shortened in the 3MC state another N atom is pushed away from the Ir, resulting in the breaking of this bond, suggesting that once the Ir–N bond breaks the damage to the complex is permanentthis will have important implications for the lifetimes of devices using this type of complex as the active material. The consequences of these results for the design of more efficient blue phosphors for OLED applications are discussed.

Decoupling charge generation and extraction is critical to understanding loss mechanisms in polymer:fullerene organic solar cells and photodiodes but has thus far proven to be a challenging task. Using steady-state and time-resolved light intensity dependent photocurrent (iPC) measurements in combination with transient photovoltage, we estimate the total charge inside a typical device during steady-state photoconduction, which is defined by the trapped, doping-induced, and mobile charge populations. Our results show that nongeminate recombination of any order can be avoided as long as this charge is much less than that capable of being stored on the electrodesa criterion that is typically met in the linear iPC regime in donor:fullerene systems even with low, imbalanced mobilities. Knowing the conditions under which nongeminate recombination is essentially absent is an important device and materials design consideration. Our work also demonstrates that the technique of iPC is not only useful to assess the charge extraction efficiency but can also be used to estimate the efficiency of free carrier generation in fully operational devices.

It is commonly thought that inhomogeneities in the bulk heterojunction thickness of donor:acceptor organic solar cells are a major efficiency‐limiting factor in scaling to modules. This is primarily related to degradation of the charge collection efficiency associated with the thicker segments of the active layer and/or loss of fill factor from thin segments where shorting occurs. We study the former effect in two model polymer–fullerene organic solar cell blend systems with very different charge transport properties: poly(3‐n‐hexylthiophene):[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PC60BM) and poly[N‐9”‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)]:[6,6]‐phenyl‐C71‐butyric acid methyl ester (PCDTBT:PC70BM). We find that junction thickness inhomogeneities do not cause substantial power losses in large area cells (5 × 5 cm2monolithic submodules) when the bimolecular recombination dynamics are suppressed, i.e., the system is non‐Langevin. This insight into the scaling physics of organic solar cells suggests that non‐Langevin materials will deliver more robust and viable large area manufacturing opportunities.

To date, nonfullerene acceptors for organic solar cells have yet to reach the overall device performance achieved with fullerene derivatives. Power conversion efficiencies are low, even when combined with narrow optical gap polymers, with the processing conditions and blend microstructure difficult to optimize. To understand the potential origins of the decreased performance, we performed a photophysical study on a model system of blends of poly(3-n-hexylthiophene) (P3HT) with the planar small molecule electron acceptor 2-[{7-(9,9-di-n-propyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile (K12), focusing, in particular, on the effects of the crystallinity of the K12 phase on exciton dissociation and charge generation. Our results show that the microstructure of the blends can be manipulated by the processing conditions to give amorphous through to (semi)crystalline films. The amorphous blends show strong quenching of the photoluminescence, which indicates that there is a fine mixture of P3HT and K12. After annealing, the blends all showed increased photoluminescence and signs of phase separation with the formation of large-scale crystalline K12 domains. Photoinduced absorption spectroscopy confirmed the presence of positive polarons in the P3HT and revealed triplet excitons in blends containing crystalline K12 domains, indicating that exciton harvesting from the K12 phase was inefficient. However, charge generation in the 1:2 blend (the best for devices) was enhanced in the (semi)crystalline films, which suggests that aggregation of the P3HT and K12 aids charge separation. Hence, the complex phase behavior of K12 results in a trade-off between charge generation and collection against exciton harvesting from the K12 phase.

We report three generations of dendrimers incorporating either a fluorene or spirobifluorene core with carbazole dendrons and fluorene surface groups that are effective sensing materials for the detection of nitrated explosives by fluorescence quenching. The photophysical properties of the dendrimers were investigated with a combination of steady-state absorption and photoluminescence and time-resolved photoluminescence. We show that the first-generation dendrimers behave as single chromophores while the higher-generation dendrimers contain multiple chromophores that interact to give excimer-like emissive states. Stern–Volmer measurements with nitrated analytes show that the quenching efficiency decreases with generation for the planar fluorene-cored dendrimers and increases with generation for the more three-dimensional spirobifluorene-cored dendrimers. These contrasting trends are shown to be caused primarily by changes in the quenching efficiency of static interactions with the nitrated analytes, which is a consequence of the choice of core. Our results highlight the potential for exploiting such excimer-like states for chemical sensing, particularly in the case of nitrated explosives.

The charge transporting properties of a phosphorescent iridium(III) complex‐cored dendrimer, with two dendrons attached to each ligand of the core are reported. The results show that the high photoluminescence quantum yield of this material is obtained without compromising charge transport. The hole mobility values are reported over a wide range of temperatures and electric fields using the charge‐generation layer time‐of‐flight technique. The results are analysed using the Gaussian disorder model (GDM), the correlated disorder model, the polaronic correlated disorder model, and the short‐range correlated Gaussian disorder model. It is found that the GDM model gives the most comprehensive description of hole transport in this material. In spite of its larger size, the hole mobility of the doubly dendronised material compares favourably with that of a smaller singly dendronised material, and its spherical shape leads to low energetic disorder and clearly non‐dispersive charge transport. This shows how molecular shape can be used to combine favourable photoluminescence and charge‐transporting properties.

Poly(dendrimers) comprised of a poly(styrene) backbone with dendrimer side chains containing an iridium(III) complex core, first-generation biphenyl dendrons, and (2-ethylhexyl)oxy surface groups show increased viscosity compared to their individual dendrimer components, which is important for inkjet printing processes. However, intrachain interchromophore interactions lead to lower photoluminescence quantum yields even in solution relative to the simple isolated dendrimers. We demonstrate that the phosphorescence efficiency of a polymer can be enhanced by incorporating the dendrimer monomer unit(s) into a copolymer with poly(styrene) spacer units. The poly(styrene) spacer units remove the intrachain interchromophore interactions between the chromophores in solution. The copolymer gives a >50% increase in solution photoluminescence quantum yields (to 94%) and an improved organic light-emitting diode performance with an external quantum efficiency of 6.7% at 100 cd/m2at 11 V when compared to the homopolymer with the same dendrimer side chain.

We compare two dendrimers, which contain the same luminescent chromophores but differ in dimensionality, for the detection of an explosive analyte via PL quenching. Each dendrimer has first generation biphenyl dendrons with 2-ethylhexyloxy surface groups but differ in the core units. One dendrimer has a bifluorene core and hence has a “planar” structure, whereas the second has four bifluorene units tetrahedrally arranged around an adamantyl center and hence has a “three-dimensional” structure. Solution Stern–Volmer measurements have previously been reported to show that the three-dimensional dendrimer has a higher binding constant than that of the more planar compound. Films of the dendrimers rapidly detect 2,4-dinitrotoluene (DNT) with thinner films (∼25 nm) being more responsive than thicker films (∼85 nm). Neutron reflectometry measurements show that the analyte can diffuse completely through the films with the three-dimensional dendrimer absorbing more of the analyte. The rate of recovery of the PL was faster for the planar dendrimer than the three-dimensional material showing that large binding constants are not necessary for reversible detection of analytes.

In this work, the synthesis of an oligothiophene having a donor–acceptor–donor (D–A–D) chromophore with hydrogen bonding groups is described. The D–A–D molecule was demonstrated to self-organize via intermolecular H-bonding between barbituric acid units. Interactions between the oligothiophene subunits were also found to be important, affording nanoribbons that could be observed by atomic force and transmission electron microscopy. The applicability of the oligothiophene for organic electronic applications was investigated by fabricating organic field-effect transistors (OFETs) and organic photovoltaic devices. The OFET measurements yielded p-type mobility of 7 × 10–7cm2/(V s), and when blended with C60-PCBM, the photovoltaic efficiency was observed to be 0.18%.

Stable film morphology is critical for long‐term high performance organic light‐emitting diodes (OLEDs). Neutron reflectometry (NR) is used to study the out‐of‐plane structure of blended thin films and multilayer structures comprising evaporated small molecules. It is found that as‐prepared blended films of fac‐tris(2‐phenylpyridyl)iridium(III) [Ir(ppy)3] in 4,4′‐bis(N‐carbazolyl)biphenyl (CBP) are uniformly mixed, but the occurrence of phase separation upon thermal annealing is dependent on the blend ratio. Films comprised of the ratio of 6 wt% of Ir(ppy)3in CBP typically used in OLEDs are found to phase separate with moderate heating while a higher weight percent mixture (12 wt%) is found to be stable. Furthermore, it is found that thermal annealing of a multilayer film comprised of typical layers found in efficient devices ([tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA)/Ir(ppy)3:CBP/bathocuproine (BCP)]) causes the BCP layer to become mixed with the emissive blend layer, whereas the TCTA interface remains unchanged. This significant structural change causes no appreciable difference in the photo­luminescence of the stack although such a change would have a dramatic effect on the charge transport through the device, leading to changes in performance. These results demonstrate the effect of thermal stress on the delicate interplay between the chemical composition and morphology of OLED films.

We report a new class of thiophene-catechol light harvesting molecules for dye-sensitized solar cells. The catechol-titania combination has been described as a Type II injection system where it is thought charge injection occurs directly from the highest occupied molecular orbital of the dye to states near the titania conduction band edge. We show that the optical absorption of the catechol-titania complex moves to longer wavelengths with extension of the conjugation (1, 2, or 3 thiophene units). Binding of the dyes is via a bidentate catechol-titania bridge with the strength of the binding following the acidity of the catechol: the more acidic the stronger the binding. The binding strength was found to increase with increasing chromophore length. We have optimized device performance based upon dye loading and find open circuit voltage and short circuit current density are consistent with the observed improvements in light harvesting efficiency with extended conjugation. However, the maximum open circuit voltage is observed at low dye loadings, and dark current−voltage and open circuit voltage decay measurements indicate that recombination plays a significant role in suppressing device performance as dye loading increases. The interplay between open circuit voltage and short circuit current as a function of dye uptake means that efficiencies in these catechol-thiophene sensitizers are limited. It is clear that these factors need to be independently controlled if Type II systems are to challenge Type I injection systems for high performance dye-sensitized solar cells.

The article describes activities on dendrimers which can be optimised to given desired opto-electronic and processing properties. Phosphorescent dendrimers have been developed and led to efficient organic light-emitting diodes (OLEDs) with two-layered device architecture. Particular focus is given to the processing and emission properties of dendrimers, as well as the intermolecular interaction in OLEDs. Light-emitting dendrimers are considered a unique class of macromolecules, consisting of molecular components that allow property optimisation.

The article discusses the aspects of research at the Centre for Organic Photonics and Electronics (COPE) at the University of Queensland. Organic light-emitting diodes and solar cells are some of the applications of opto-electronic materials. Particular focus is given to the main classes of organic semiconducting materials. Being able to control the intermolecular interactions of the active materials at the molecular level is the key to the success of developing organic materials for applications. The members of the research team at COPE are also listed.

p-Type inorganic nickel oxide (NiOx) exhibits high transparency, tunable-optoelectronic properties, and a work function (WF) that is potentially suitable for hole extraction in inverted perovskite solar cells (PSCs). However, NiOx films possess surface defects that lead to high interfacial recombination and an energy offset with the ionization potential of the perovskite. Herein, we show that fluorinated 3-(2,3,4,5,6-pentafluorophenyl)propan-1-aminium iodide (FPAI) can be used to modify the electronic properties of the NiOx anode interlayer. The FPAI modification led to good perovskite crystal growth and films with reduced surface defects. The FPAI modification also increased the WFof NiOx and improved charge extraction. These improvements led to an increased Vocvalue compared with control devices without FPAI modification, 1.05 V versus 1.00 V, and a higher short-circuit current and larger fill factor. As a result, the best PSCs with FPAI-modified NiOx had a power conversion efficiency of 19.3%. Finally, the PSCs with the FPAI-modified NiOx layer were found to have improved stability.

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.

Here, the charge transporting properties of a family of highly phosphorescent iridium(III) complex‐cored carbazole dendrimers designed to have improved charge transport by incorporating carbazole units into the dendrons are studied. Firstly, the effect of the dendrimer generation and the role of dendron for materials with one dendron per ligand of the core are considered. It is shown, in contrast to previously reported light‐emitting dendrimers, that in this case the carbazolyl‐based dendrons have an active role in charge transport. Next, the effect on the charge transport of attaching two dendrons per ligand to the dendrimer core is explored. In this latter case, for the so called “double dendron” material a highly non‐dispersive charge transport behavior is observed, together with a time‐of‐flight mobility of the order of 10−3cm2V−1s−1. Furthermore the lowest energetic disorder parameter (σ) ever reported for a solution‐processed conjugated organic material is found, σ <  20 meV.

Solution‐processible saturated blue phosphorescence is an important goal for organic light‐emitting diodes (OLEDs). Fac‐tris(5‐aryltriazolyl)iridium(III) complexes can emit blue phosphorescence at room temperature. Mono‐ and doubly dendronized fac‐tris(1‐methyl‐5‐phenyl‐3‐n‐propyl‐1H‐[1,2,4]triazolyl)iridium(III) 1and fac‐tris{1‐methyl‐5‐(4‐fluorophenyl)‐3‐n‐propyl‐1H‐[1,2,4]triazolyl}iridium(III) 4with first generation biphenyl‐based dendrons were prepared. The dendrimers emitted blue light at room temperature and could be solution processed to form thin films. The doubly dendronized 3had a film photoluminescence quantum yield of 67% and Commission Internationale de l'Eclairage (CIE) coordinates of (0.17, 0.33). OLEDs comprised of a neat film of dendrimer 3and an electron transport layer achieved a brightness of 142 cd m−2at 3.8 V with an external quantum efficiency of 7.9%, and CIE coordinates of (0.18, 0.35). Attachment of the fluorine atom to the emissive core had the effect of moving the luminescence to shorter wavelengths but also quenched the luminescence of the mono‐ and doubly dendronized dendrimers.

We report methodology for the preparation of symmetric and asymmetric solution processable phosphorescent dendrimers that are comprised of 2-ethylhexyloxy surface groups, biphenyl based dendrons, and iridium(iii) complex cores. The symmetric dendrimer has three dendritic 2-benzo[b]thiophene-2'-ylpyridyl (BTP) ligands with the dendritic ligands responsible for red emission. The asymmetric dendrimer has two dendritic 2-phenylpyridyl ligands and one unsubstituted BTP ligand. Iridium(iii) complexes comprised of 2-phenylpyridyl ligands are normally associated with green emission whereas those containing BTP ligands emit red light. Red emission is observed from the asymmetric dendrimer demonstrating that emission occurs primarily from the metal-to-ligand charge transfer state associated with the ligand with the lowest HOMO–LUMO energy gap. The photoluminescence quantum yields (PLQYs) of the symmetric and asymmetric dendrimers were strongly dependent on the dendrimer structure. In solution the PLQYs of the asymmetric and symmetric dendrimers were 47 ± 5% and 29 ± 3% respectively. The photoluminescence lifetime of the emissive state of both dendrimers in solution was 7.3 ± 0.1 µs. In the solid state the comparative PLQYs were reversed with the symmetric dendrimer having a PLQY of 10 ± 1% and the asymmetric dendrimer a PLQY of 7 ± 1%. The comparatively larger decrease in PLQY for the asymmetric dendrimer in the solid state is attributed to increased core–core interactions. The intermolecular interactions are greater in the asymmetric dendrimer because there is no dendron on the BTP ligand. Electrochemical analysis shows that charge is injected directly into the cores of the dendrimers.

We have synthesised a new family of dendrimers with stilbene dendrons attached to a porphyrin core via a stilbene unit and compared their properties with a family of dendrimers with the same core and dendrons but with the dendrons attached via a phenyl unit. The oxidation and reduction half potentials of the two dendrimer families were found to be the same and independent of generation indicating that the dendrons were not creating a micro-environment for the core. However, the rate of heterogeneous electron transfer was found to be strongly dependent on link type and generation. The photoluminescence quantum yield (PLQY) of the dendrimers was also found to be strongly dependent on the method of attachment of the core. In solution the dendrimers with the stilbene link between core and dendrons had PLQYs 1.5 times higher than their phenyl counterparts but in the solid state the trend was reversed with the phenyl linked dendrimers generally having a higher PLQY. The difference in properties has been assigned to the comparative openness of the dendrimer architectures and the effect of the dendrons on the shape of the porphyrin core.

We have prepared poly(2,5-dimethoxy-1,4-phenylenevinylene) (DMEOPPV) and poly(2,5-thienylenevinylene) (PTV) via xanthate precursor polymer routes. The respective precursor polymers can be thermally converted to their corresponding conjugated polymers without the need for acid catalysis. For both DMEOPPV and PTV the UV-visible absorption and infrared spectra were similar to the polymers when prepared via alkoxy precursors. Films of DMEOPPV and blends of DMEOPPV and PPV were found to be luminescent with energy being transferred from the PPV host to the DMEOPPV guest. Films of PTV or PTV–PPV blends were found not to be luminescent.

Poly[2-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (EHPPV) was prepared via chloro, S-methyl xanthate and O-ethyl xanthate precursor polymers. The choice of leaving group used was found to control the optical and device properties of the EHPPV with the variations arising from the differing chemical interactions of the leaving groups with indium tin oxide (ITO). The leaving group–ITO interactions were found to dominate the properties of EHPPV when formed from the commonly used chloro precursor polymer and this problem could be solved by use of xanthate leaving groups which were found not to react with the contact.

We have synthesised a new distyrylbenzene cored dendrimer which utilises triazine rings to link the surface groups. It was found that methoxy surface groups were sufficient to give the dendrimer good solubility and good quality thin films could be prepared by spin-coating from solution. The dendrimer was luminescent both in solution and the solid state with the emission occurring from the core. The photoluminescence quantum yield was determined to be 31 ± 3% in the solid state. The dendrimer was incorporated into a single layer light-emitting diode (ITO/dendrimer/Ca) and was found to emit blue light with an external quantum efficiency of 0.003%.