David G. Lidzey

Space-charge limited conduction with traps in poly(phenylene vinylene) light emitting diodes

Current–voltage, impedance, and transient conductance measurements have been carried out on indium-tin-oxide/poly(phenylene vinylene)/Al light emitting diodes. In these devices injection and transport is expected to be dominated by positive carriers. Fowler–Nordheim tunneling theory cannot account for the temperature dependence, the thickness dependence, or the current magnitude of the current–voltage characteristics. Space-charge limited current theory with an exponential distribution of traps is however in extremely good agreement with all of the recorded current–voltage results in the higher applied bias regime (approximately 0.7⩽V/d⩽1.6×106 V cm−1). This gives a trap density Ht of 5(±2)×1017 cm−3 and the product of μNHOMO of between 1014 and 5×1012 cm−1 V−1 s−1. Assuming NHOMO is 1020 cm−3 gives an effective positive carrier mobility between 10−6 and 5×10−8 cm2 V−1 s−1. The characteristic energy Et of the exponential trap distribution is 0.15 eV at higher temperatures (190⩽T⩽290 K), but this decreases...

Strong exciton-photon coupling in an organic semiconductor microcavity

The modification and control of exciton–photon interactions in semiconductors is of both fundamental and practical interest, being of direct relevance to the design of improved light-emitting diodes, photodetectors and lasers. In a semiconductor microcavity, the confined electromagnetic field modifies the optical transitions of the material. Two distinct types of interaction are possible: weak and strong coupling. In the former perturbative regime, the spectral and spatial distribution of the emission is modified but exciton dynamics are little altered. In the latter case, however, mixing of exciton and photon states occurs leading to strongly modified dynamics. Both types of effect have been observed in planar microcavity structures in inorganic semiconductor quantum wells and bulk layers. But organic semiconductor microcavities have been studied only in the weak-coupling regime. Here we report an organic semiconductor microcavity that operates in the strong-coupling regime. We see characteristic mixing of the exciton and photon modes (anti-crossing), and a room-temperature vacuum Rabi splitting (an indicator of interaction strength) that is an order of magnitude larger than the previously reported highest values for inorganic semiconductors. Our results may lead to new structures and device concepts incorporating hybrid states of organic and inorganic excitons, and suggest that polariton lasing may be possible.

Efficient planar heterojunction mixed-halide perovskite solar cells deposited via spray-deposition

We report the use of ultra-sonic spray-coating under ambient conditions as a deposition technique for the fabrication of planar heterojunction CH3NH3PbI3−xClx perovskite solar cells. We make a first optimization of processing parameter space using this deposition technique, and explore the role of the temperature of the substrate during spray-casting, the volatility of the casting solvent and the post deposition anneal on determining the efficiency of the resultant solar cells. We find that maximum device efficiency is correlated with the creation of dense films having a surface coverage above 85%. When such films are incorporated into a solar cell device, power conversion efficiencies of up to 11% are realized. These results demonstrate that spray-coating can be used in the large-area, low-cost manufacture of high efficiency solution-processed perovskite solar cells.

Use of poly(phenyl quinoxaline) as an electron transport material in polymer light‐emitting diodes

We report the use of a poly(phenyl quinoxaline) (PPQ) as an electron transporting conjugated polymer for electroluminescence (EL) applications. Single‐layer PPQ devices with ITO anode and aluminium cathode show unipolar electron transport with current densities up to 60 mA/cm2 but no emission. Two‐layer structures combining PPQ as electron transport material with the hole transporting poly(2,5‐dialkoxy‐p‐phenylene vinylene) (PDAOPV) show strong emission from the PDAOPV with brightnesses up to 250 cd/m2. These two‐layer structures have a maximum EL quantum efficiency of 0.35% which is ten‐fold enhanced compared with the corresponding single‐layer PDAOPV devices.

Depletion of PCBM at the Cathode Interface in P3HT/PCBM Thin Films as Quantified via Neutron Reflectivity Measurements

[*] Dr. A. J. Parnell, A. J. Pearson, Dr. P. A. Staniec, A. J. C. Dennison, Dr. H. Hamamatsu, Prof. D. G. Lidzey, Prof. R. A. L. Jones Department of Physics and Astronomy, The University of Sheffield Hicks Building, Hounsfield Road, Sheffield, S3 7RH (UK) E-mail: a.j.parnell@sheffield.ac.uk; d.g.lidzey@sheffield.ac.uk Dr. A. D. F. Dunbar Department of Chemical and Process Engineering, The University of Sheffield Sir Robert Hadfield Building, Mappin St, Sheffield S1 3JD (UK)

The effect of morphology on the temperature-dependent photoluminescence quantum efficiency of the conjugated polymer poly(9, 9-dioctylfluorene)

We have measured the temperature-dependent photoluminescence quantum yields (PLQYs) of poly(9, 9-dioctylfluorene) (PFO) films with four morphologies, namely as-spin-coated (SC) glass, quenched nematic glass, crystalline, and vapour-treated SC glass containing a fraction of 21 helix conformation (β-phase) chains. We find that the room temperature PLQYs of the as-SC, crystalline, and quenched films all increase as the temperature is reduced. However, the PLQY of the film containing β-phase chains decreases at temperatures below 150 K. Via temperature-dependent photoinduced absorption measurements, we show that the polaron population in films containing β-phase PFO chains grows as the temperature is reduced, and is significantly larger than in films with any of the other morphologies. Because of the smaller HOMO–LUMO (highest occupied molecular orbital–lowest unoccupied molecular orbital) energy gap of the β-phase chains compared to chains in the surrounding glassy PFO matrix, they act as recombination sites for excitons, and as traps for polarons. Hence at low temperatures, the polarons become strongly localized on these chains, where they quench the singlet excitons and reduce the PLQY.

Photoprocessed and micropatterned conjugated polymer LEDs

Abstract We present a detailed study of the effect of fabricating light-emitting diodes (LEDs) containing conjugated polymers, using photolithographic processing; a technique more conventionally used in inorganic semiconductor device manufacture. It is shown that for the specific poly (2,5-dialkoxy- p -phenylenevinylene) used here, the photoprocessing procedure chemically modifies the polymer, resulting in an increase in the degree of conjugation and a concomitant reduction in photoluminescence quantum efficiency to 35% compared to an unprocessed film. A similar reduction in electroluminescence quantum efficiency indicates that this is also the dominant effect in photoprocessed LEDs. LED device characteristics show an increase in threshold field for the photoprocessed devices suggestive of the formation of a barrier layer at the polymer/cathode interface, resulting in a further partial reduction in device power efficiency. There are however no catastrophic effects on device performance, showing that standard photoprocessing is a viable approach to fabrication of polymer LED structures. It is anticipated that optimization of the procedures will allow much less degradation in device performance. In addition it is shown that photoprocessing can be readily applied to the fabrication of arrays of micron-sized LEDs, demonstrating advanced applications of this combination of technologies.

Bulk limited conduction in electroluminescent polymer devices

The current–voltage (J–V) characteristics of ITO/polymer film/Al or Au structures of poly(phenylene vinylene) (PPV) and a dialkoxy PPV copolymer have been recorded for a range of different film thickness d and temperatures T. At high applied bias all the characteristics can be fitted over a given range to a power law J=KVm, where m increases with decreasing T, log(K) is proportional to m, and K is proportional to d−α m, where α∼2 (ITO/polymer film/Al devices) and ∼1 (ITO/polymer film/Au devices). Different single carrier space charge limited conduction theories have been used to try and explain this behavior. The analytical theory in which the carrier density is decreased by an exponential trap distribution lying below effectively isoelectronic transport states is in good agreement, but cannot explain the thickness dependence of the ITO/polymer film/Au devices and can be criticized as being physically unreasonable. A numerical analysis in which the mobility has the field and temperature dependence found f...

The role of dynamic measurements in correlating structure with optoelectronic properties in polymer:fullerene bulk-heterojunction solar cells

The characterization of morphology in blend thin-films of conjugated polymers and functionalized fullerenes is a critical aspect in organic photovoltaic (OPV) device research. Understanding the links between thin-film processing conditions, film nanostructure and photocurrent generation efficiency is necessary in order to develop this technology for commercial viability. Here, we review recent developments of experimental studies that probe sample nanostructure formation and modification during the processing steps commonly used in OPV device fabrication, potentially offering a deeper insight and more rational understating of these conditions.

Strong exciton–photon coupling in a low-Q all-metal mirror microcavity

We report the experimental observation of strong exciton–photon coupling in a planar microcavity composed of an organic semiconductor positioned between two metallic (silver) mirrors. Via transmission and reflectivity measurements, we observe a very large, room temperature Rabi splitting in excess of 300 meV. We show that the Rabi-splitting is enhanced in all-metal microcavities by a factor of more than 2 compared to an organic film positioned between a silver mirror and a dielectric mirror. This enhancement results from the significantly larger optical fields that are confined within all-metal microcavities.