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The conventional picture of photocurrent generation in organic solar cells involves photoexcitation of the electron donor, followed by electron transfer to the acceptor via an interfacial charge-transfer state (Channel I). It has been shown that the mirror-image process of acceptor photoexcitation leading to hole transfer to the donor is also an efficient means to generate photocurrent (Channel II). The donor and acceptor components may have overlapping or distinct absorption characteristics. Hence, different excitation wavelengths may preferentially activate one channel or the other, or indeed both. As such, the internal quantum efficiency (IQE) of the solar cell may likewise depend on the excitation wavelength. We show that several model high-efficiency organic solar cell blends, notably PCDTBT:PC70BM and PCPDTBT:PC60/70BM, exhibit flat IQEs across the visible spectrum, suggesting that charge generation is occurring either via a dominant single channel or via both channels but with comparable efficiencies. In contrast, blends of the narrow optical gap copolymer DPP-DTT with PC70BM show two distinct spectrally flat regions in their IQEs, consistent with the two channels operating at different efficiencies. The observed energy dependence of the IQE can be successfully modeled as two parallel photodiodes, each with its own energetics and exciton dynamics but both having the same extraction efficiency. Hence, an excitation-energy dependence of the IQE in this case can be explained as the interplay between two photocurrent-generating channels, without recourse to hot excitons or other exotic processes.

Complementary skill sets, talent, capabilities, resources, infrastructure, and knowledge available in academic and pharmaceutical organizations have all greatly contributed to the delivery of novel medicines, innovative solutions, and clinical benefits to patients with major and more common diseases (defined as diseases with large patient populations). In sharp contrast, patients with less common diseases (defined as diseases with small and intermediate-size patient populations, including autoimmune diseases such as type 1 diabetes, multiple sclerosis, and celiac disease) have not profited to the same extent. The lack of industry involvement in less common diseases and the absence of critical resources and R&D capabilities in academia have severely hampered progress in this area. Here, the reasons for these greatly different outcomes are analyzed. Strategies that reduce risk, cost, and timelines are proposed to advance novel concepts, implement innovative solutions, and deliver clinical benefits for diseases with small and intermediate-size patient populations.

High efficiency polymer-fullerene bulk heterojunction organic solar cells can generate photocurrent by excitation of the electron donor and acceptor components via Channel I and Channel II processes, respectively. Using a planar Photo-Field-Effect-Transistor (PhotoFET) architecture operated in steady state and quasi-transient modes we have studied the dynamics of charge generation and transport in blends of PCDTBT/70-PCBM (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 methylester). The PhotoFET architecture allows independent measurement of the electron and hole photocurrents and mobilities for charge carriers generated by the Channel I (electron transfer) and Channel II (hole transfer) processes as a function of fullerene content. We find dramatic increases in the photocurrent yield and electron mobility with higher 70-PCBM loading. By analyzing the External Quantum Efficiency (EQE) in n- and p-PhotoFET modes, we estimate that 8...

We have used steady-state and time-resolved neutron reflectometry to study the diffusion of fullerene derivatives into the narrow optical gap polymer poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) to explore the sequential processing of the donor and acceptor for the preparation of efficient organic solar cells. It was found that when [6,6]-phenyl-C61-butyric-acid-methyl-ester (60-PCBM) was deposited onto a thin film of PCDTBT from dichloromethane (DCM), a three-layer structure was formed that was stable below the glass-transition temperature of the polymer. When good solvents for the polymer were used in conjunction with DCM, both 60-PCBM and [6,6]-phenyl-C71-butyric-acid-methyl-ester (70-PCBM) were seen to form films that had a thick fullerene layer containing little polymer and a PCDTBT-rich layer near the interface with the substrate. Devices composed of films prepared by sequential deposition of the polymer and fullerene had efficiencies of up to 5.3%, with those based on 60-PCBM close to optimized bulk heterojunction (BHJ) cells processed in the conventional manner. Sequential deposition of pure components to form the active layer is attractive for large-area device fabrication, and the results demonstrate that this processing method can give efficient solar cells.

A convergent synthesis of a first generation dendrimer with biphenyl-based dendrons and a hexaphenylbenzene core is reported. Suzuki coupling of six first generation biphenyl-dendrons with the rigid hexakis(4-iodophenyl)benzene core furnished the title asterisk-cored dendrimer in 60% yield. Twelve copies of β-(1→6)-linked glucosamine-based oligosaccharides were then efficiently coupled with the acetylenyl surface groups of the dendrimer via a Huisgen 1,3-dipolar cycloaddition reaction to give a series of multivalent carbohydrate constructs that mimic the globular nodules of polysaccharide intercellular adhesin (PIA) produced by Staphylococcus aureus.

Lui et al.1 report new ‘perovskite-based’ solar cells having a photon-to-electron-conversion-efficiency (PCE) of a remarkable 15.4%. Traditional photovoltaic technology is dominated by c-Si, with inorganic thin film entrants CdTe, CIS and GIGS making substantial market inroads. Pipeline technologies such as dye-sensitized solar cells (DSSCs) and organic solar cells (OSCs)2 promise ultra-low manufacturing costs as well as light-weight, flexible modules. As yet, the ‘pipeline’ is yet to deliver commercial product with long-term stability and module-scale PCEs being hurdles. The perovskite technology emerged from DSSCs as p-type materials, and then as combinatorial replacements for the dye and hole-transport components.3

Fullerene derivatives are commonly used as electron acceptors in combination with (macro)molecular electron donors in bulk heterojunction (BHJ) organic photovoltaic (OPV) devices. Understanding the BHJ structure at different electron donor/acceptor ratios is critical to the continued improvement and development of OPVs. The high neutron scattering length densities (SLDs) of the fullerenes provide effective contrast for probing the distribution of the fullerene within the blend in a nondestructive way. However, recent neutron scattering studies on BHJ films have reported a wide range of SLDs ((3.6-4.4) × 10(-6) Å(-2)) for the fullerenes 60-PCBM and 70-PCBM, leading to differing interpretations of their distribution in thin films. In this article, we describe an approach for determining more precisely the scattering length densities of the fullerenes within a polymer matrix in order to accurately quantify their distribution within the active layers of OPV devices by neutron scattering techniques.

Polymers containing pendant iridium(III) complexes encapsulated within carbazole-based dendrons were prepared. Their design and photophysical and optoelectronic properties are presented in the context of applications within flexible lighting modules.

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.

We present a novel platinum porphyrin based phosphorescent dendrimer for use as a triplet harvesting dopant in organic light-emitting diodes. Two types of dendritic host materials are used. Through the choice of a common branching architecture around the emissive chromophore unit of both guest and host materials, we are able to achieve excellent miscibility. The relative contribution of guest to host emission is found to depend strongly on the energy level offsets of the two blend materials, indicating strong trapping processes. Under pulsed operation, we observe a striking dependence of the emission spectrum on pulse period, independent of the host material used. This spectral modification is attributed to the quenching of triplet excitations at high excitation densities. We find excellent agreement between our measured data and a model based on bimolecular recombination.

Conjugated dendrimers are ideal materials to study the structure-property relationships in conjugated molecules. We have developed a family of dendrimers that contain t-butyl surface groups, stilbene dendrons, and luminescent cores. For porphyrin and tris(distyrylbenzenyl) amine cored dendrimers cyclic voltammetry (CV) showed that the redox processes occurred at the core. Combining the results from the CV experiments with the device characteristics it was determined that for the dendrimers the cores were held further away from each other with increasing generation. For the amine cored dendrimers it was found that the hole mobility decreased with generation, which is consistent with the cores being further apart. We also found for the amine cored dendrimers that the decrease in hole mobility was matched with increase in device efficiency. (C) 2001 Elsevier Science B.V. All rights reserved.

A novel family of conjugated dendrimers is used as model compounds to explore the effect of intermolecular interactions on photophysical and transport properties. The Figure shows the third generation of the dendrimers. The dendrimer generation controls the degree of chromophore interaction, which leads to a unique correlation between the chemical structure of the molecules and the macroscopic device properties (see also inside front cover).

We report the first use of the Dess–Martin periodinane (DMP) for the oxidation of an arylamine to an α-dione. The methodology is illustrated by the preparation of free-base and metal chelated porphyrin-α-diones in up to 52% yield by oxidation of 2-aminoporphyrins with the DMP. We also found that DMP could be used to oxidise a 2,3-diaminoporphyrin to a porphyrin-α-dione in good yield with other free-base diaminoporphyrin isomers forming a trans-porphyrintetraone in 20% yield.

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.

The effect of intermolecular interactions on the properties of organic semiconductors is investigated using a family of conjugated dendrimers as model systems. Increasing the amount of branching, or generation number, of these molecules reduces the degree of interaction between the chromophores. The effect of this on both photophysical and charge transporting properties is reported. It is found that an increase in generation gives rise to a reduction in the red tail emission of the dendrimer. Time of flight measurements show a slowing of charge transport with increasing generation, which is found to be related to the films becoming more insulating. The results show that dendrimer generation provides an elegant way of controlling intermolecular interactions.

We have studied the electrical fields in single-layer polymer light-emitting diodes (LEDs) using electroabsorption (EA) spectroscopy. The materials under investigation were PPV derivatives, namely poly(2,5-dialkoxy- para -phenylene vinylene) and a precursor route poly(2-methoxy-5-ethyl(2′-hexyloxy)- para -phenylene vinylene). The applied bias generates an opposing electric field that decays over time. The magnitude of the reversed field increases as the applied DC bias is increased and can be as high as one-third of the applied external field. The development of the internal field is less pronounced in vacuum than in air and it vanishes for devices prepared and tested under inert conditions.

Integrated three-pulse stimulated echo peak shift data are compared for N,N-bis-dimethylphenyl-2,4,6,8perylenetetracarbonyl diamide and poly@2-(28-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene# ~MEHPPV! in toluene solvent. These two molecules represent a model probe of solvation dynamics and a prototypical soluble, electroluminescent conjugated polymer, respectively. The results indicate that it is inappropriate to describe the linear absorption spectrum of MEH-PPV as being primarily inhomogeneously broadened. Conformational disorder along the polymer backbone gives rise to an ensemble of polyene electronic oscillators that are strongly coupled to each other. As a consequence, fluctuations in the electronic energy gap on a time-scale of 50-fs derive primarily from bath-mediated exciton scattering. The data reported here provide an explanation for the broad, structureless electronic absorption of MEH-PPV. This interpretation provides a valuable insight into the nature of the initial photoexcited state, and the efficient population of the emissive state.

We have successfully synthesised exo and endo diphenylacetylene linked bis-porphyrin dimers. We found that the exo-bis-porphyrin dimer could be easily prepared in good yield whilst the endo-dimer was harder to form due to the steric constraints of the meso-substituents. Electrochemical studies indicated that the porphyrin centred reductions of the bis-porphyrin dimers were dominated by the bis-porphyrin moieties. We found that the first four one-electron reductions of the exo- and endo-bis-porphyrin dimers occurred at similar potentials. Finally, at more negative potentials the bis-porphyrin dimers were reduced further which significantly changed the subsequent oxidation processes. This is thought to be due to a conformational change in the molecule which causes a change in orbital density in the “excited state”.

A strategy for the synthesis of asymmetrically substituted tetraazaanthracene linked bis-porphyrins in which the two porphyrin rings contain differences in their peripheral substituents has been developed. The method is illustrated by the preparation of bis-porphyrins with a single meso-halophenyl and seven meso-3,5-di-tert-butylphenyl substituents. The bis-porphyrins were prepared by condensation of a porphyrin-α-dione with benzene-1,2,4,5-tetraamine to form a porphyrin diaminoquinoxaline intermediate which was subsequently condensed with a second different porphyrin-α-dione. The key issue in the synthesis was the separation of the desired asymmetrically substituted bis-porphyrin from the symmetric bis-porphyrin by-products of similar polarities. Enhanced separation of the bis-porphyrin products was achieved by chelation of a metal into one of the porphyrin rings, the metal being introduced at the porphyrin-α-dione stage. Copper was successfully used when chelated into the less polar porphyrin-α-dione but the use of zinc in the more polar porphyrin-α-dione to enhance bis-porphyrin separation was unsuccessful as the pyridinium hydrochloride produced in the reaction was found to demetallate the porphyrins.

We have prepared poly[2-(2′-ethylhexyloxy)-5-phenylethynyl-1,4-phenylenevinylene] (PAPPV) 13via, S-methyl xanthate 10, chloro 11, and O-ethyl xanthate 12 precursor polymers. All three precursor polymers could be thermally converted to give films of 13. We found that the UV–VIS and photoluminescence (PL) spectra, and PL quantum yields (11–14%) were similar when 13 was prepared from either the S-methyl xanthate or chloro precursor polymers. In contrast, the UV–VIS and PL spectra of 13 prepared via the O-ethyl xanthate precursor polymer were observed to be blue-shifted and the PL quantum yield was much lower (2%).

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%.

We have used absorption and electroabsorption spectroscopy to investigate the electronic structure of poly(para-phenylene vinylene) (PPV) and poly (2-methoxy, 5-(2'-(ethyl)hexyloxy)-p-phenylene vinylene) (MEH-PPV). In particular we examine the often used assumption that the electronic structure of PPV and its dialkoxy substituted derivatives are essentially the same. The absorption spectrum of PPV consists of three peaks, while that of MEH-PPV has four peaks. We discuss the controversial origin of the extra peak as well as evidence for Davydov splitting effects in the absorption spectrum of PPV. The analysis of the nonlinear spectra shows further differences between the two materials. First, the binding energy of the 1B(u) exciton for PPV is some 0.1 eV higher than for MEH-PPV. Second, the peak value of Im{chi((3))(-omega;0,0,omega)} for PPV is approximately 40 times higher than that of MEH-PPV. We also found that the sum-over-states modeling of the electroabsorption spectra indicates that the transition dipole moment between the mA(g) and nB(u) states is of opposite sign in the two polymers. [S0163-1829(99)02523-0].

Conjugated dendrimers are an interesting class of materials for light emitting diodes, and can be used either as light-emitting or charge-transporting layers. We report a study of the use of a solution-processible dendrimer with distyrylbenzene core and stilbene dendrons. We find that its use in bilayer LEDs with polypyridine gives efficiencies ten times greater than for single layer polypyridine LEDs. The light emission can be either from the polypyridine or the dendrimer layer, depending on the layer thicknesses. This offers a way of tuning the colour of bilayer LEDs.

meso-Functionalised porphyrin-α-diones have been prepared as the basic building blocks for bis-porphyrin arrays. meso-Halophenyl porphyrins 4 and 7 were prepared by condensation of 3,5-di-tert-butylbenzaldehyde with dipyrromethanes 1, 2, and 3. Conversion of 4 and 7 to the corresponding meso-halophenylporphyrin-α-diones was achieved in five steps in overall yields of up to 58%. The reaction sequence consisted of chelation of the porphyrin with copper(II), nitration, hydroxylation with the benzaldoximate anion, demetallation, followed by oxidation with the Dess–Martin periodinane. The key hydroxylation step was found to proceed chemoselectively, displacing the β-pyrrolic nitro group whilst leaving the meso-halophenyl group intact.

The synthesis of poly[2-(2′-ethylhexyloxy)-1,4-phenylenevinylene] 10via chloro 7, S-methyl xanthate 8, and O-ethyl xanthate 9 precursor polymers has been investigated. We found that the chloro and O-ethyl xanthate precursor polymers could be easily formed whilst the S-methyl xanthate gave only low molecular weight material. The observed molecular weights of the chloro and O-ethyl xanthate precursor polymers were found to decrease with decreased polymer concentration when measured by gel permeation chromatography (GPC) with the decrease probably being due to the dissociation of polymer aggregates or physical networks. In contrast, no decrease of molecular weight was observed by GPC for the S-methyl xanthate precursor polymer on dilution. The chloro, S-methyl xanthate, and O-ethyl xanthate precursor polymers were thermally converted to the conjugated polymer 10. The UV-visible absorption and photoluminescence quantum yield of 10 was determined to be strongly dependent on the precursor route used. For 10 formed from the chloro precursor 7 the PL quantum yield was found to be 55±5%.

We have synthesized and investigated the electronic properties of poly[2-(2‘-ethylhexyloxy)-5-phenyl-1,4-phenylenevinylene], 12, and its methylsulfonyl derivative poly[2-(2‘-ethylhexyloxy)-5-(4‘ ‘-methylsulfonylphenyl)-1,4-phenylenevinylene], 13. Polymer 13 contains a second-order nonlinear optic chromophore which is in conjugation with the polymer backbone. Polymers 12 and 13 have been used as the electroluminescent (EL) layers in single layer light-emitting diodes (LEDs) (ITO/polymer/Al). We have found that when 12 was used, the LEDs had an external EL quantum efficiency of 0.01%. Attachment of the methylsulfonyl moiety to 12 to give 13 in an attempt to increase the electron affinity of the parent polymer was found to make no difference to the efficiency of the single layer EL devices and appeared qualitatively to reduce their lifetimes.

Porphyrin-α-diones have been used for the preparation of extended rigidly oriented porphyrin systems. Copper 2, free-base 3, and zinc 4 2-hydroxyporphyrins have been prepared by an improved method and treated with a range of oxidants to form the corresponding porphyrin-α-diones, 5, 6, and 7 in yields of up to 94%. We have found that the free-base 2-hydroxyporphyrin 3 was easily oxidised and usually gave the best yields of porphyrin-α-dione. In this study the most effective general oxidant was determined to be the Dess–Martin periodinane which oxidised all three 2-hydroxyporphyrins to their respective porphyrin-α-diones in good yields.

We report the results of an electroabsorption (EA) spectroscopy study of the electronic structure of the conjugated polymer PPV and its soluble derivative MEH-PPV. We find that the EA response of both polymers is dominated by a Stark shift of the 1Bu exciton. Comparison of the EA spectrum with the Third Harmonic Generation (THG) and two-photon fluorescence spectra for PPV allows us to address the question of the exciton binding energy in these materials. We estimate the binding energy to be 0.8(±0.15) eV for PPV and 0.7(±0.15) eV for MEH-PPV. We discuss the difference between the optically determined binding energy and that estimated via indirect electrical methods. We also present data for the EA response of higher lying transitions and discuss these in terms of current theoretical models.

Improved performance of light-emitting devices is reported for devices containing the newly synthesized, triazole-containing polymer TRIDSB (see Figure). This fully conjugated material has a high solid-state photoluminescence quantum efficiency, unlike other triazole polymers investigated previously, and emits blue light. TRIDSB holds promise as an electron-transporting material.

Both soluble and insoluble conjugated polymers have advantages and disadvantages for the construction of light-emitting diodes (LEDs). A route is reported for the formation of optical quality films of insoluble (I) MEHPPV, a poly(phenylenevinylene) derivative whose soluble form is widely used in single-layer devices. The incorporation of I-MEHPPV into a multilayer device is described, in which the electron-transport/hole-blocking layer was deposited on top of the I-MEHPPV layer from a solvent in which the standard soluble MEHPPV would dissolve.

We report the in-plane microwave surface impedance of a high-quality single crystal of $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu[N(CN)${}_{2}$]Br. In the superconducting state, we find three independent signatures of $d$-wave pairing: (i) a strong, linear temperature dependence of superfluid density; (ii) deep in the superconducting state the quasiparticle scattering rate $\ensuremath{\Gamma}\ensuremath{\sim}{T}^{3}$; and (iii) no BCS coherence peak is observed in the quasiparticle conductivity. Above ${T}_{c}$, the Kadowaki-Woods ratio and the temperature dependence of the in-plane conductivity show that the normal state is a Fermi liquid below $\ensuremath{\simeq}23$ K, yet resilient quasiparticles dominate the transport up to $\ensuremath{\simeq}50$ K.

The photophysical properties in solution of three generations of carbazole-based dendrons and dendrimers with fluorenyl surface groups were studied using steady-state, time-resolved femtosecond transient absorption and anisotropy, and coherent two-dimensional ultraviolet spectroscopy. It was found that increasing the generation caused a switch in the nature of the emissive state between the first-generation compounds and the second- and third-generation dendrimers. Time-resolved anisotropy measurements revealed low initial anisotropies that decreased with increasing dendrimer generation consistent with increasing intradendrimer interchromophore coupling. Two-dimensional UV spectroscopy showed that the signal from the second- and third-generation dendrimers is the product of multiple chromophores interacting. The maximum number of interacting chromophores is reached by the second generation.

We report on the effects of screening of the electric field by doping-induced mobile charges on photocurrent collection in operational organic solar cells. Charge transport and recombination were studied using double injection (DI) and charge extraction by linearly increasing voltage (CELIV) transient techniques in bulk-heterojunction solar cells made from acceptor-donor blends of poly(3-n-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PC60BM). It is shown that the screening of the built-in field in operational solar cells can be controlled by an external voltage while the influence on charge transport and recombination is measured. An analytical theory to extract the bimolecular recombination coefficient as a function of electric field from the injection current is also reported. The results demonstrate that the suppressed (non-Langevin) bimolecular recombination rate and charge collection are not strongly affected by native doping levels in this materials combination. Hence, it is not necessary to reduce the level of doping further to improve the device performance of P3HT-based solar cells.

An innovative design strategy for light emitting field effect transistors (LEFETs) to harvest higher luminance and switching is presented. The strategy uses a non-planar electrode geometry in tri-layer LEFETs for simultaneous enhancement of the key parameters of quantum efficiency, brightness, switching, and mobility across the RGB color gamut.

We report the fabrication and charge transport characterization of carbazole dendrimer-based organic field-effect transistors (OFETs) for the sensing of explosive vapors. After exposure to para-nitrotoluene (pNT) vapor, the OFET channel carrier mobility decreases due to trapping induced by the absorbed pNT. The influence of trap states on transport in devices before and after exposure to pNT vapor has been determined using temperature-dependent measurements of the field-effect mobility. These data clearly show that the absorption of pNT vapor into the dendrimer active layer results in the formation of additional trap states. Such states inhibit charge transport by decreasing the density of conducting states.

Most charge generation studies on organic solar cells focus on the conventional mode of photocurrent generation derived from light absorption in the electron donor component (so called channel I). In contrast, relatively little attention has been paid to the alternate generation pathway: light absorption in the electron acceptor followed by photo-induced hole transfer (channel II). By using the narrow optical gap polymer poly(3,6-dithieno[3,2-b]thiophen-2-yl)-2,5-bis(2-octyldodecyl)-pyrrolo-[3,4-c]pyrrole-1,4-dione-5′,5″-diyl-alt-4,8-bis(dodecyloxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl with two complimentary fullerene absorbers; phenyl-C61-butyric acid methyl ester, and phenyl-C71-butyric acid methyl ester (70-PCBM), we have been able to quantify the photocurrent generated each of the mechanisms and find a significant fraction (>30%), which is derived in particular from 70-PCBM light absorption.

Control over the structure of donor/acceptor blends is essential for the development of solution processable organic solar cells (OSCs). We have used time-resolved neutron reflectometry (NR) and in situ annealing to investigate the nanoscale structure and interdiffusion of sequentially spin-coated thin films of poly(3-n-hexylthiophene-2,5-diyl) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and correlated the evolving structure with the device performance. While the as-prepared film shows a clear two-layer structure it is evident that (19 wt%) PCBM has percolated throughout the lower P3HT layer. Upon heating, analysis of time-resolved NR data shows that the diffusion process is dependent on the annealing temperature. At temperatures up to 110 °C, the two-layer structure is retained and only a small amount of PCBM diffuses from the interface into the lower layer, increasing the total PCBM content throughout the P3HT layer to 26 wt%. Significantly, this small change in acceptor content leads to a profound increase in device performance; with the power conversion efficiency (PCE) of the OSCs increasing from 0.47% (unannealed, 19 wt% PCBM) to 3.23% (annealed, 26 wt% PCBM) with the latter showing a similar efficiency to devices prepared from a blend containing 50 wt% PCBM. Further annealing at 120 and 130 °C sees rapid interdiffusion between the two layers, along with an overall expansion in the thickness of the bilayer film. Despite the complete intermixing of the PCBM and P3HT to form a structure resembling a bulk heterojunction, essentially no improvement in device performance was observed for annealing at temperatures above 110 °C.

We have found that by the inclusion of a conjugated spacer it is possible to synthesise a poly(l,4-phenylenevinylene) derivative with an electron-withdrawing group attached directly from the substituted monomer. We describe a route for the synthesis of an electronically asymmetric styryl-derivatised poly(l,4-phenylenevinylene). The preparation was achieved in two steps from the nitrostyryl-substituted arylenedimethylene bisbromo monomer 13. In the first step, base-induced polymerisation of 13 afforded the soluble and processible precursor polymer 14. The precursor polymer 14 could then be converted in the second step to the insoluble and unprocessible conjugated polymer 15 by thermal treatment in the solid state.

Low donor content solar cells containing polymeric and non-polymeric donors blended with fullerenes have been reported to give rise to efficient devices. In this letter, we report that a dendrimeric donor can also be used in solution-processed low donor content devices when blended with a fullerene. A third generation dendrimer containing 42 thiophene units (42T) was found to give power conversion efficiencies of up to 3.5% when blended with PC70BM in optimized devices. The best efficiency was measured with 10 mole percent (mol. %) of 42T in PC70BM and X-ray reflectometry showed that the blends were uniform. Importantly, while 42T comprised 10 mol. % of the film, it made up 31% of the film by volume. Finally, it was found that solvent annealing was required to achieve the largest open circuit voltage and highest device efficiencies.

A common feature of organic light-emitting diodes is their stacked multilayer structure, which is critical for efficient charge injection and transport, and light emission. In this study, it is found that a blended layer of the hole-transport material tris(4-carbazol-9-ylphenyl)amine with 6 wt% fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)(3)] readily undergoes interdiffusion with adjacent layers of typical charge transport materials: bathocuproine; 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene; N,N-bis(3-methylphenyl)-N,N-diphenylbenzidine; and N,N-bis(naphthalen-1-yl)-N,N-diphenylbenzidine. This process is followed using combined neutron reflectometry and in situ photoluminescence measurements. The temperature at which diffusion occurred is found to correlate with the glass transition temperature of the materials. Importantly, the layer of the material with the lowest T-g is the material that acts as a diffusive host for the adjacent layer, which has a higher T-g. That is, a high T-g material does not necessarily act as a blocking layer for diffusion. Furthermore, the results show that the order of structural change within a film can be predicted on the basis of the thermal properties of the materials. These results confirm the necessity of using materials with high glass transition temperatures throughout the device to minimize performance degradation by layer interdiffusion.

The electronic structure of a conjugated polymer of current interest in organic LED's, poly(p-phenylenevinylene), or PPV, has been studied by ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy. The focus of this work is on the nature of the pi-electronic band structure nearest the Fermi level and the physical influence of finite torsion angles, the geometry of the polymer backbone, on the electronic properties of the system. Details of the ct-electronic bands, as reflected in the associated density-of-states, are observed clearly in the spectra, from which some underlying geometrical details of the polymer system can be deduced. The experimental spectra have been analyzed theoretically using band structure calculations based upon the valence effective Hamiltonian (VEH) model. In addition, in order to control the band structure, three ring-substituted derivatives of PPV, each of which induces a different bonding geometry in the backbone, have been studied. The changes in the experimental results can be explained on the basis of both physical and chemical interactions of the substituents with the backbone, which lead to geometrical changes along the backbone, which influence the ct-bandwidths and contribute to differences in both the optical absorption threshold and the binding energy of the valence band edge.

We demonstrate a method for achieving polarized organic electroluminescence for liquid crystalline conjugated polymers that allows the polymer to be deposited directly onto the anode. The technique utilizes a top-down alignment approach whereby the predeposited polymer was aligned from above using a rubbed polyimide master and a smectic liquid crystal transfer layer. The liquid crystal/polyimide master bilayer was sandwiched with the liquid crystalline polymer that had been deposited onto the electrode. The sandwiched layers were then heated to achieve alignment before the removal of the polyimide master and liquid crystal transfer layer. Using this method, poly[2,7-{9,9-di(2-ethylhexyl)}fluorene] (PF2-6) was aligned to give an anisotropic polymer film. Light emitted from single layer light-emitting diodes containing the aligned PF2-6 had integrated dichroic ratios of up to 9.7. At 100 cd/m(2), the single layer devices had external quantum and power efficiencies of 0.08% and 0.05 lm/W, respectively. Bilayer devices containing an electron transport layer between the PF2-6 and the cathode gave emitted light with good dichroic ratios and with the external quantum and power efficiencies at 100 cd/m(2) being increased to 2.2% and 1.1 lm/W. (C) 2003 American Institute of Physics.

We demonstrate that neutron reflection (NR) can be used to characterize polymer films deposited on indium tin oxide (ITO). When the chloro precursor to poly[2-(2′-ethylhexyloxy)5-methoxy-1,4-phenylenevinylene] (MEHPPV) was spin-coated onto ITO NR revealed that between the ITO and the uniform polymer layer was a 20 A thick low contact zone. We found that the conversion of the chloro precursor to MEHPPV at 180 °C under vacuum gave a uniform film of MEHPPV with the layer of low contact between the polymer and ITO remaining. Finally, the NR profile suggests that the blueshift in the MEHPPV absorption spectrum on ITO when compared to quartz is due to polymer morphology and not incomplete conversion.

Poly-N-acetylglucosamine (PNAG) saccharides are an important constituent of bacterial biofilms, such as those produced by Staphylococcus aureus. We have developed a simple two-step iterative method for the synthesis of β-(1→6)-glucosamine oligosaccharides that are structurally similar to PNAG. We illustrate the method with the formation of a pentasaccharide. The key building block is an orthogonally protected N-trifluoroacetamido thioglycoside donor that was added in succession to a glycosyl acceptor, enabling efficient glycosylation of the growing chain. In the second step of the iterative cycle, this building block is quantitatively deprotected at the C-6-hydroxyl position, ready for the next saccharide addition. Building from an azido-functionalised GlcNAc monosaccharide acceptor, the pentasaccharide was synthesised in seven steps in an overall yield of 25%.

We use a combination of low temperature, high field magnetic circular dichroism, absorption, and emission spectroscopy with relativistic time-dependent density functional calculations to reveal a subtle interplay between the effects of chemical substitution and spin-orbit coupling (SOC) in a family of iridium(III) complexes. Fluorination at the ortho and para positions of the phenyl group of fac-tris(1-methyl-5-phenyl-3-n-propyl-[1,2,4]triazolyl)iridium(III) cause changes that are independent of whether the other position is fluorinated or protonated. This is demonstrated by a simple linear relationship found for a range of measured and calculated properties of these complexes. Further, we show that the phosphorescent radiative rate, k(r), is determined by the degree to which SOC is able to hybridize T(1) to S(3) and that k(r) is proportional to the inverse fourth power of the energy gap between these excitations. We show that fluorination in the para position leads to a much larger increase of the energy gap than fluorination at the ortho position. Theory is used to trace this back to the fact that fluorination at the para position increases the difference in electron density between the phenyl and triazolyl groups, which distorts the complex further from octahedral symmetry, and increases the energy separation between the highest occupied molecular orbital (HOMO) and the HOMO-1. This provides a new design criterion for phosphorescent iridium(III) complexes for organic optoelectronic applications. In contrast, the nonradiative rate is greatly enhanced by fluorination at the ortho position. This may be connected to a significant redistribution of spectral weight. We also show that the lowest energy excitation, 1A, has almost no oscillator strength; therefore, the second lowest excitation, 2E, is the dominant emissive state at room temperature. Nevertheless the mirror image rule between absorption and emission is obeyed, as 2E is responsible for both absorption and emission at all but very low (10 K) temperatures.

Although efficiencies of > 10% have recently been achieved in laboratory-scale organic solar cells, these competitive performance figures are yet to be translat ed to large active areas and geometries relevant for viable manufacturing. One of the factors hindering scale-up is a lack of knowledge of device physics at the sub-module level, particularly cell architecture, electrode geometry and current collection pathways. A more in depth understanding of how photocurrent and photovoltage extraction can be optimised over large active areas is urgently needed. Another key factor suppressing conversion efficiencies in large area cells is the relatively high sheet resistance of the transparent conducting anode - typically indium tin oxide. Hence, to replace ITO with altern ative transparent conducting anod es is also a high priority on the pathway to viable module-level organic solar cells. In our paper we will focus on large area devices relevant to sub-module scales – 5 cm × 5 cm monolithic geometry. We have applied a range of experimental techniques to create a more comprehensive understanding of the true device physics that could help make large area, monolithic organic solar cells more viable. By employing this knowledge, a novel transparent anode consisting of molybdenum oxide (MoO

We report the fabrication and electrical characteristics of structured-gate organic field-effect transistors consisting of a gate electrode patterned with three-dimensional pillars. The pillar gate electrode was over-coated with a gate dielectric (SiO2) and solution processed organic semiconductors producing both unipolar p-type and bipolar behaviour. We show that this new structured-gate architecture delivers higher source–drain currents, higher gate capacitance per unit equivalent linear channel area, and enhanced charge injection (electrons and/or holes) versus the conventional planar structure in all modes of operation. For the bipolar field-effect transistor (FET) the maximum source–drain current enhancements in p- and n-channel mode were >600% and 28%, respectively, leading to p and n charge mobilities with the same order of magnitude. Thus, we have demonstrated that it is possible to use the FET architecture to manipulate and match carrier mobilities of material combinations where one charge carrier is normally dominant. Mobility matching is advantageous for creating organic logic circuit elements such as inverters and amplifiers. Hence, the method represents a facile and generic strategy for improving the performance of standard organic semiconductors as well as new materials and blends.

A transparent top anode consisting of molybdenum oxide (MoO ) and silver (Ag) is employed in an "inverted-illumination" geometry in conjunction with a thick aluminium (100 nm) bottom contact. The optimised MoO -Ag-MoO stack (MAMS) has an optical transmission of 80% at the wavelength of 520 nm, sheet resistance of 5 ohm sq and yields conversion efficiencies in 5 cm × 5 cm photoactive-area "sub-modules" of 3.17% - outperforming the equivalent conventional ITO/PEDOT:PSS devices.