A figure of merit for efficiency roll-off in TADF-based organic LEDs.

Organic light-emitting diodes (OLEDs) are a revolutionary light-emitting display technology that has been successfully commercialized in mobile phones and televisions1,2. The injected charges form both singlet and triplet excitons, and for high efficiency it is important to enable triplets as well as singlets to emit light. At present, materials that harvest triplets by thermally activated delayed fluorescence (TADF) are a very active field of research as an alternative to phosphorescent emitters that usually use heavy metal atoms3,4. Although excellent progress has been made, in most TADF OLEDs there is a severe decrease of efficiency as the drive current is increased, known as efficiency roll-off. So far, much of the literature suggests that efficiency roll-off should be reduced by minimizing the energy difference between singlet and triplet excited states (ΔEST) to maximize the rate of conversion of triplets to singlets by means of reverse intersystem crossing (kRISC)5-20. We analyse the efficiency roll-off in a wide range of TADF OLEDs and find that neither of these parameters fully accounts for the reported efficiency roll-off. By considering the dynamic equilibrium between singlets and triplets in TADF materials, we propose a figure of merit for materials design to reduce efficiency roll-off and discuss its correlation with reported data of TADF OLEDs. Our new figure of merit will guide the design and development of TADF materials that can reduce efficiency roll-off. It will help improve the efficiency of TADF OLEDs at realistic display operating conditions and expand the use of TADF materials to applications that require high brightness, such as lighting, augmented reality and lasing.

Distinguishing Electron Diffusion and Extraction in Methylammonium Lead Iodide.

Charge diffusion and extraction are crucial steps in the operation of solar cells. Here we show that time-resolved photoluminescence can be used to study electron diffusion in hybrid perovskite films and subsequent transfer to the adjacent electron extraction layer. As diffusion and transfer to the extraction layer are consecutive processes, they can be hard to distinguish, but by exciting from each side of the sample we can separate them and identify which process limits charge extraction. We find that the introduction of a fullerene monolayer between the methylammonium lead iodide (MAPbI3) and the electron-transporting SnO2 layers greatly increases the electron transfer velocity between them to the extent that electron diffusion limits the rate of electron extraction. Our results suggest that increasing the electron diffusion coefficient in MAPbI3 would further enhance the electron extraction rate, which could result in more efficient n-i-p type solar cells.

Fluorescence and thermal imaging of non-melanoma skin cancers before and during photodynamic therapy.

BACKGROUND: Photodynamic therapy (PDT) has been shown to be less effective on the extremities. Protoporphyrin-IX (PpIX) fluorescence and skin surface temperature are variables that have been implicated in the differences in efficacy with body site, but objective studies have not been undertaken. OBJECTIVES: To further investigate observations from our previous study that temperature and fluorescence during pro-drug incubation are correlated, through a prospective objective investigation of the relationships between fluorescence and skin surface temperature before and during PDT and relationships with body site and efficacy. METHODS: Eighteen patients with Bowen's disease or basal cell carcinoma, who had been referred for PDT, were recruited to this study. PpIX fluorescence and thermal measurements were recorded at intervals during the pro-drug incubation and irradiation phases of PDT. Pain immediately after irradiation, and outcome at 3- and 12-months were recorded. RESULTS: Temperature and PpIX fluorescence were higher on the trunk than lower leg immediately before treatment (median temperature 32.7 °C vs. 27.8 °C, p < 0.05 and median fluorescence 16.5 vs. 6.7, p < 0.05). Higher pain levels were reported during PDT on the extremities (median 5.7 vs. 2.2, p < 0.05). Clearance rates at 12-months were 80 %. CONCLUSIONS: The study supports a correlation between temperature and PpIX fluorescence during PDT, providing robust objective data to support our previous hypothesis and observations. The higher pain levels, lower PpIX fluorescence on the lower leg, and the high efficacy rates at all body sites irrespective of temperature and fluorescence indicates that relationships between PDT treatment conditions and parameters is likely to be multifactorial.

Nanoscale mobility mapping in semiconducting polymer films.

Local electrical properties of thin films of the polymer PTB7 are studied by conductive atomic force microscopy (C-AFM). Non-uniform nanoscale current distribution in the neat PTB7 film is revealed and connected with the existence of ordered PTB7 crystallites. The shape of local I-V curves is explained by the presence of space charge limited current. We modify an existing semi-empirical model for estimation of the nanoscale hole mobility from our experimental C-AFM measurements. The procedure of nanoscale charge mobility estimation was described and applied to the PTB7 films. The calculated average C-AFM hole mobility is in good agreement with macroscopic values reported for this material. Mapping of nanoscale hole mobility was achieved using the described procedure. Local mobility values, influenced by nanoscale structure, vary more than two times in value and have a root-mean-square value 0.22 × 10-8 m2/(Vs), which is almost 20% from average hole mobility.

An investigation of the role acceptor side chains play in the processibility and efficiency of organic solar cells fabricated from small molecular donors featuring 3,4-ethylenedioxythiophene cores.

Organic photovoltaic devices fabricated from small molecular donors continue to receive significant interest due to their desirable properties such as convenient synthesis, purification and batch-to-batch reproducibility. In this study, we have synthesized two small molecules based on an alternating A-D-A structure, utilizing a central EDOT donor moiety and either 2-ethylhexyl cyanoacetate (SAM-72) or N-(2-ethylhexyl)cyanoacetamide (SAM-80) units as acceptor termini. The small molecules were incorporated into bulk heterojunction solar cells with PC71BM. Our investigations have shown that the side chains utilized for SAM-80 only allow for solution processing using volatile solvents, such as chloroform, which limits the reproducibility of device fabrication. However, SAM-72 displays better solubility and devices fabricated using a SAM-72:PC71BM active layer reached average power conversion efficiencies of 1.9%, with fill factors reaching 60%. Post-processing methods such as thermal and solvent vapor annealing were found to significantly increase the stability of devices, but were not able to improve overall device performance.

Transfer-printed micro-LED and polymer-based transceiver for visible light communications.

Visible light communications (VLC) is an emerging technology that uses LEDs, such as found in lighting fixtures and displays, to transmit data wirelessly. Research has so far focused on LED transmitters and on photoreceivers as separate, discrete components. Combining both types of devices into a single transceiver format will enable bi-directional VLC and offer flexibility for the development of future advanced VLC systems. Here, a proof of concept for an integrated optical transceiver is demonstrated by transfer printing a microsize LED, the transmitter, directly onto a fluorescent optical concentrator edge-coupled to a photodiode, the receiver. This integrated device can simultaneously receive (downlink) and transmit (uplink) data at rates of 416 Mbps and 165 Mbps, respectively. Its capability to operate in optical relay mode at 337 Mbps is experimentally demonstrated.

Green Perovskite Distributed Feedback Lasers.

A visible perovskite distributed feedback laser is fabricated for the first time. Through the use of nanocrystal pinning, highly luminescent methylammonium lead bromide films are used to produce stable lasers emitting at 550 nm, with a low threshold of 6 µJcm-2. The lasers were able to support multiple polarisations, and could be switched between transverse magnetic and transverse electric mode operation through simple tuning of the distributed feedback grating period.

Probing the energy levels of perovskite solar cells via Kelvin probe and UV ambient pressure photoemission spectroscopy.

The field of organo-lead halide perovskite solar cells has been rapidly growing since their discovery in 2009. State of the art devices are now achieving efficiencies comparable to much older technologies like silicon, while utilising simple manufacturing processes and starting materials. A key parameter to consider when optimising solar cell devices or when designing new materials is the position and effects of the energy levels in the materials. We present here a comprehensive study of the energy levels present in a common structure of perovskite solar cell using an advanced macroscopic Kelvin probe and UV air photoemission setup. By constructing a detailed map of the energy levels in the system we are able to predict the importance of each layer to the open circuit voltage of the solar cell, which we then back up through measurements of the surface photovoltage of the cell under white illumination. Our results demonstrate the effectiveness of air photoemission and Kelvin probe contact potential difference measurements as a method of identifying the factors contributing to the open circuit voltage in a solar cell, as well as being an excellent way of probing the physics of new materials.

Phosphorescence quenching of fac-tris(2-phenylpyridyl)iridium(iii) complexes in thin films on dielectric surfaces.

We study the influence of the film thickness on the time-resolved phosphorescence and the luminescence quantum yield of fac-tris(2-phenylpyridyl)iridium(iii) [Ir(ppy)3]-cored dendrimers deposited on dielectric substrates. A correlation is observed between the surface quenching velocity and the quenching rate by intermolecular interactions in the bulk film, which suggests that both processes are controlled by dipole-dipole interactions between Ir(ppy)3 complexes at the core of the dendrimers. It is also found that the surface quenching velocity decreases as the refractive index of the substrate is increased. This can be explained by partial screening of dipole-dipole interactions by the dielectric environment.

Photophysical and structural characterisation of in situ formed quantum dots.

Conjugated polymer-semiconductor quantum dot (QD) composites are attracting increasing attention due to the complementary properties of the two classes of materials. We report a convenient method for in situ formation of QDs, and explore the conditions required for light emission of nanocomposite blends. In particular we explore the properties of nanocomposites of the blue emitting polymer poly[9,9-bis(3,5-di-tert-butylphenyl)-9H-fluorene] together with cadmium sulphide (CdS) and cadmium selenide (CdSe) precursors. We show the formation of emissive quantum dots of CdSe from thermally decomposed precursor. The dots are formed inside the polymer matrix and have a photoluminescence quantum yield of 7.5%. Our results show the importance of appropriate energy level alignment, and are relevant to the application of organic-inorganic systems in optoelectronic devices.

High power efficiency phosphorescent poly(dendrimer) OLEDs.

We show that it is possible to produce an efficient solution-processable phosphorescent poly(dendrimer) OLED with a 32 lm/W power efficiency at 100 cd/m2 without using a charge transporting host or any improvements in light extraction. This is achieved by using the dendrimer architecture to control inter-chromophore interactions. The effects of using 4,4',4″-tris(N-carbazolyl)triphenylamine (TCTA) as a charge transporting host and using a double dendron structure to further reduce inter-chromophore interactions are also reported.

Photophysical properties of 9,10-disubstituted anthracene derivatives in solution and films.

We have carried out absorption, time-resolved fluorescence, and fluorescence quantum yield measurements of four new soluble anthracene derivatives. They show natural radiative lifetimes in the range of 2.5-4.4 ns, which is 5-10 times shorter than those reported for unsubstituted anthracene. The 9,10-bis(phenylethynyl)anthracene (BPEA) derivatives show the largest fluorescence transition dipoles, which is attributed to extended π-conjugation between anthracene and phenyls through acetylene linkages. Spin-cast films of the BPEA derivatives show strong fluorescence quenching by weakly emitting low energy excitations, which is attributed to excimer-like traps. Quenching is significantly reduced when bulky dendrons are attached so that they give maximum coverage of the emitting chromophore and prevent their aggregation. The results show that anthracene derivatives can be developed into efficient solution-processable fluorescent emitters for the blue and green spectral regions.

A hybrid organic semiconductor/silicon photodiode for efficient ultraviolet photodetection.

A method employing conjugated polymer thin film blends is shown to provide a simple and convenient way of greatly enhancing the ultraviolet response of silicon photodetectors. Hybrid organic semiconductor/silicon photodetectors are demonstrated using fluorene copolymers and give a quantum efficiency of 60% at 200 nm. The quantum efficiency is greater than 34% over the entire 200-620 nm range. These devices show promise for use in high sensitivity, low cost UV-visible photodetection and imaging applications.

Bipolaron formation in organic solar cells observed by pulsed electrically detected magnetic resonance.

We report the observation of a spin-dependent dark transport current, exhibiting spin coherence at room temperature, in a π-conjugated polymer-fullerene blend using pulsed electrically detected magnetic resonance. The resonance at g = 2.0028(3) is due to polarons in the polymer, and exhibits spin locking at high microwave fields. The presence of an excess of fullerene, and the operating voltage (1 V) used, suppresses negative polaron formation in the polymer. It is concluded that spin-dependent transport is due to the formation of positive bipolarons.

An open pilot study of ambulatory photodynamic therapy using a wearable low-irradiance organic light-emitting diode light source in the treatment of nonmelanoma skin cancer.

BACKGROUND: Photodynamic therapy (PDT) is a popular treatment for nonmelanoma skin cancer with clearance rates of between 70% and 100%. Although reported to have a superior cosmetic outcome, the inconvenience of hospital visits and discomfort during therapy are considered drawbacks. OBJECTIVES: To present an open pilot study of a low-irradiance, potentially disposable, lightweight, organic light-emitting diode (OLED), which is an area-emitting light source (2 cm diameter), suitable for ambulatory PDT. METHODS: Twelve patients with Bowen's disease (eight) and superficial basal cell carcinoma (four) < 2 cm in diameter were recruited into the study following histological confirmation of the diagnosis. Two treatments (45-60 J cm(-2) red light, 550-750 nm, peak 620 nm, irradiance 5 mW cm(-2)) were administered 1 month apart following application of aminolaevulinic acid for 4 h. RESULTS: At the 12-month follow-up, seven of the 12 patients remained clear, with four of the nonresponders demonstrating peripheral margin failure. Patients were scored for pain during and immediately after treatment using the numerical rating scale (NRS; 1-10). All 12 subjects scored pain as < 2 using the NRS (median score 1). In contrast, a similar cohort of 50 consecutive patients from our routine PDT clinic (Aktilite inorganic LED source; 75 J cm(-2), irradiance 80 mW cm(-2)) scored a median of 6 on the NRS. CONCLUSIONS: Pain and inconvenience are practical barriers to the use of conventional PDT. This pilot study suggests that OLED-PDT is less painful than conventional PDT with the added advantage of being lightweight, and therefore has the potential for more convenient 'home PDT'. These results need to be validated in larger studies.

Influence of the dendron chemical structure on the photophysical properties of bisfluorene-cored dendrimers.

A detailed study of the photophysics of a family of bisfluorene-cored dendrimers is reported. Polarized time-resolved fluorescence, singlet-singlet exciton annihilation and fluorescence quantum yield measurements were performed and used to understand how the dendron structure affects the light-emitting properties of the materials. The exciton diffusion rate is similar in all films studied. An increase in the nonradiative deactivation rate by nearly one order of magnitude is observed in films of dendrimers with stilbenyl and carbazolyl based dendrons as compared to solutions, whereas the dendrimers with biphenyl and diphenylethylenyl dendrons showed highly efficient emission (photoluminescence quantum yields of 90%) in both solution and the solid state. The results of the materials that show fluorescence quenching can be explained by the presence of quenching sites at a concentration of just a fraction of a percent of all macromolecules. A possible explanation of this quenching is hole transfer from the emissive chromophore to the dendron in a face-to-face geometry. These results are important for the design of efficient blue emitters for optoelectronic applications.

Triplet exciton diffusion and phosphorescence quenching in iridium(III)-centered dendrimers.

A study of triplet-triplet exciton annihilation and nonradiative decay in films of iridium(III)-centered phosphorescent dendrimers is reported. The average separation of the chromophore was tuned by the molecular structure and also by blending with a host material. It was found that triplet exciton hopping is controlled by electron exchange interactions and can be over 600 times faster than phosphorescence quenching. Nonradiative decay occurs by weak dipole-dipole interactions and is independent of exciton diffusion, except in very thin films (<20 nm) where surface quenching dominates the decay.