Professor Paul Dastoor

A new design paradigm is presented for the production of low bandgap (Eg < 1.5 eV) polymers which takes advantage of the dual nature of building blocks that can simultaneously exhibit both strong donor and strong acceptor properties. By replacing the donor unit of commonly applied donor-acceptor (D-A) frameworks with such ambipolar units, the resulting ambipolar-acceptor framework maintains the classical D-A backbone, while effectively adding a second acceptor. As illustrative examples of this approach, the synthesis of a series of alternating copolymers are reported that pair the ambipolar unit thieno[3,4-b]pyrazine (TP) with various traditional acceptors, including 2,1,3-benzothiadiazole, quinoxaline, and 2H-benzotriazole, successfully producing soluble, processible materials with band gaps of 1.02-1.12 eV. Analysis of these materials reveals the tunability of this approach and provides convincing evidence that the LUMOs of the TP and traditional acceptor hybridize to give a delocalized and suitably stabilized polymer LUMO, thus contributing to the low bandgaps of the resulting polymers. To demonstrate the potential of these materials, initial application to bulk heterojunction photonic devices are also reported, revealing photoresponse out to beyond 1100 nm, with initial NIR photodetectors providing detectivity (D*) values as high as 8.9 × 1011 Jones at 815 nm.

The effects of branched vs. linear alkyl sidechains were investigated in poly(2,3-dialkylthieno[3,4-b]pyrazine)s via GRIM polymerization, focusing on molecular weight, processibility, and device performance. New 2-ethylhexyl derivatives exhibited enhancement in nearly all aspects, with photovoltaic devices exhibiting photoresponse out to 1300 nm and competitive specific detectivity values for NIR photodetectors.

The prospect of large-scale production of low-cost electronic devices is a driving factor behind the recent interest in printed organic electronics. However, the upscaling of laboratory organic electronic devices is extremely challenging since it requires the adaptation of materials and fabrication processes optimized for the small scale to industrial manufacturing techniques, such as roll-to-roll printing. Here, we demonstrate the fabrication of all-printed organic biosensors at the pilot production scale for use in the detection of glucose. By translating device architecture and operation, as well as electrode design and ink formulations of previously reported laboratory-scale glucose sensors to industrial printing and coating processes, we demonstrate sub-millimolar sensitivity to glucose in fully printed devices in a process which is now scalable to commercial production quantities. This Letter highlights the significant challenges associated with developing upscaled printed organic electronic biosensors and the approaches needed to address them.

Resolution is a key parameter for microscopy, but methods for standardizing its definition are often poorly defined. For a developing technique such as scanning helium microscopy, it is critical that a consensus-based protocol for determining instrument resolution is prepared as a written standard to allow both comparable quantitative measurements of surface topography and direct comparisons between different instruments. In this paper we assess a range of quantitative methods for determining instrument resolution and determine their relative merits when applied to the specific case of the scanning helium microscope (SHeM). Consequently, we present a preliminary protocol for measuring the resolution in scanning helium microscopy based upon utilizing appropriate test samples with sets of slits of well-defined dimensions to establish the quantitative resolution of any similar instrument.

Nanoengineered, eco-friendly, solution-processable electroactive materials are in demand for the growing field of printed electronics, and these material requirements can be achieved by the development of waterborne colloidal dispersions. Functionality in these composite materials can be tuned by thermodynamically modifying the material nanomorphology, often by creation of kinetically stabilized aqueous nanoparticle dispersions. In this work we demonstrate that the internal structure of organic nanoparticles is controlled by the surface energy difference between the polymeric donor material and the non-fullerene acceptor (NFA) material. Nanoparticles of the following donor-acceptor combinations, suitable for printed organic photovoltaics, have been synthesized: TQ1:N2200, TQ1:PNDIT10, P3HT:N2200, P3HT:o-IDTBR and P3HT:eh-IDTBR. Advanced synchrotron-based X-ray spectroscopy and microscopy are used to correlate the formation of core-shell nanoparticle morphology to the material surface energy. We subsequently present a viable avenue for customizing the blended nanoparticle structure into (i) core-shell, (ii) molecularly intermixed, or (iii) inverted shell-core structures. Our results showed that TQ1:PNDIT10 and P3HT:o-IDTBR nanoparticles were comprised of a donor-rich shell and an NFA-rich core, however, interestingly we show a reversal to the inverse NFA shell/donor core structure for TQ1:N2200, P3HT:N2200 and P3HT:eh-IDTBR nanoparticles, driven by the low surface energy of N2200 (23.7 mJ m-2) and eh-IDTBR (18.3 mJ m-2). This article is the first report of a flipped nanoparticle core-shell morphology comprising an NFA-rich shell for the miniemulsion synthesis route. The composition of the shells and cores was able to be controlled by the differential mismatch in the surface energy of the donor and acceptor materials, with ?Gsurface > 0, ?Gsurface = 0, and ?Gsurface < 0 for acceptor core-donor shell, molecularly intermixed, and acceptor shell-donor core, respectively. Accordingly, we introduce an entirely overlooked new figure of merit (FoM) for customizing nanoparticulate colloidal inks: tunable surface energy of non-fullerene-based semiconductors. The establishment of this FoM opens up electroactive material design to a wide range of functional printing applications with varying device and ink structure requirements, thereby reshaping the nanoengineering toolkit for waterborne colloidal dispersions and hence printed electronics.

The use of nanostructured materials for targeted and controlled delivery of bioactive molecules is an attractive alternative to conventional drug administration protocols, enabling selective targeting of diseased cells, lower administered dosages, and reduced systemic side effects. Although a variety of nanocarriers have been investigated in recent years, electroactive organic polymer nanoparticles present several exciting advantages. Here we demonstrate that thin films created from nanoparticles synthesized from violanthrone-79, an n-type semiconducting organic material, can incorporate and release dexamethasone in vitro in a highly controlled manner. By systematically altering the nanoparticle formation chemistry, we successfully tailored the size of the nanoparticles between 30 and 145 nm to control the initial amount of drug loaded into the organic particles. The biocompatibility of the different particles was tested using live/dead assays of dorsal root ganglion neurons isolated and cultured from mice, revealing that elevated levels of the sodium dodecyl sulfate surfactant used to create the smaller nanoparticles are cytotoxic; however, cell survival rates in nanoparticles larger than 45 nm exceed 86% and promote neurite growth and elongation. By manipulating the electrical stimulus applied to the electroactive nanoparticle films, we show an accelerated rate of drug release in comparison to passive release in aqueous media. Furthermore, pulsing the electrical stimulus was successfully used to selectively switch the accelerated release rate on and off. By combining the tuning of drug loading (through tailored nanoparticle synthesis) and drug release rate (through electrical stimulus protocols), we demonstrate a highly advanced control of drug delivery dosage in a biocompatible delivery vehicle. This work highlights the significant potential of electroactive organic nanoparticles for implantable devices that can deliver corticosteroids directly to the nervous system for the treatment of inflammation associated with neurological disorders, presenting a translatable pathway toward precision nanomedicine approaches for other drugs and diseases.

Understanding doping in polymer semiconductors has important implications for the development of organic electronic devices. This study reports a detailed investigation of the doping of the poly(3-hexylthiophene) (P3HT)/Nafion bilayer interfaces commonly used in organic biosensors. A combination of UV-visible spectroscopy, dynamic secondary ion mass spectrometry (d-SIMS), dynamic mechanical thermal analysis, and electrical device characterization reveals that the doping of P3HT increases with annealing temperature, and this increase is associated with thermally activated interdiffusion of the P3HT and Nafion. First-principles modeling of d-SIMS depth profiling data demonstrates that the diffusivity coefficient is a strong function of the molar concentration, resulting in a discrete intermixed region at the P3HT/Nafion interface that grows with increasing annealing temperature. Correlating the electrical conductance measurements with the diffusion model provides a detailed model for the temperature-modulated doping that occurs in P3HT/Nafion bilayers. Point-of-care testing has created a market for low-cost sensor technology, with printed organic electronic sensors well positioned to meet this demand, and this article constitutes a detailed study of the doping mechanism underlying such future platforms for the development of sensing technologies based on organic semiconductors.

Sodium tungsten bronze (NaxWO3) is a promising alternative plasmonic material to nanoparticulate gold due to its strong plasmonic resonances in both the visible and near-infrared (NIR) regions. Additional benefits include its simple production either as a bulk or a nanoparticle material at a relatively low cost. In this work, plasmonic NaxWO3nanoparticles were introduced and mixed into the nanoparticulate zinc oxide electron transport layer of a water processed poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) nanoparticle (NP) based organic photovoltaic device (NP-OPV). The power conversion efficiency of NP-OPV devices with NaxWO3NPs added was found to improve by around 35% compared to the control devices, attributed to improved light absorption, resulting in an enhanced short circuit current and fill factor.

This work investigated the photophysical pathways for light absorption, charge generation, and charge separation in donor-acceptor nanoparticle blends of poly(3-hexylthiophene) and indene-C60-bisadduct. Optical modeling combined with steady-state and time-resolved optoelectronic characterization revealed that the nanoparticle blends experience a photocurrent limited to 60% of a bulk solution mixture. This discrepancy resulted from imperfect free charge generation inside the nanoparticles. High-resolution transmission electron microscopy and chemically resolved X-ray mapping showed that enhanced miscibility of materials did improve the donor-acceptor blending at the center of the nanoparticles; however, a residual shell of almost pure donor still restricted energy generation from these nanoparticles.

The history of organic photovoltaics has been characterised by the complex interplay between fundamental research, large scale manufacture and commercialization activities. In addition, the field is highly interdisciplinary; ranging across physics, chemistry and engineering. This environment has resulted in a frontier character to the field, with researchers constantly expanding into new areas and confronting new challenges as the area has developed. This article seeks to chart the developments in organic photovoltaic research, with emphasis on the last two decades, to provide some historical context to current status of the field.

A new type of lead-free, formamidinium (FA)-based halide perovskites, FASnI2Br, are investigated as light-harvesting materials for low-temperature processed p¿i¿n heterojunction solar cells with different configurations. The FASnI2Br perovskite, with a band-gap of 1.68 eV, exhibits optimal photovoltaic performance after low-temperature annealing at 75 °C. By using C60 as electron-transport layer, the device yields a hysteresis-less power conversion efficiency of 1.72%. The possible use of an inorganic MoOx film as a new type of independent hole-transport layer for the present tin-based perovskite solar cells is also demonstrated. [Figure not available: see fulltext.]

The phase segregation in P3HT:PCBM blend films has been investigated from an experimental and theoretical viewpoint. Optical microscopy, atomic force microscopy, scanning electron microscopy and X-ray diffraction show that thermal annealing of P3HT:PCBM blend films leads to the formation of PCBM aggregates. These aggregates are composed of dense randomly packed ~50 nm PCBM crystallites with an overall aggregate density of ~0.85 g cm-3. By applying the critical radius of nucleation for PCBM and the Stokes-Einstein equation for mobility of PCBM in a P3HT matrix, a model is developed which explains the formation of both crystallites and aggregates.

In order for OPV devices to transition from the laboratory to the industrial scale, accurate measurements of device operating stability and lifetime are crucial. This paper compares the degradation of ITO/PEDOT:PSS/P3HT:ICBA/Ca/Al and ITO/MoO3/P3HT:ICBA/Ca/Al devices using the three main ISOS standard testing protocols: (a) ISOS-D-1, (b) ISOS-O-1 and (c) ISOS-L-1. We show that: (1) ITO/MoO3/P3HT:ICBA/Ca/Al devices are more stable than their PEDOT counterparts under the ISOS-D-1 protocol, as has been reported previously. (2) Under the ISOS-O-1 protocol, unencapsulated MoO3 based devices are more stable than the equivalent PEDOT device but, when encapsulated, the degradation rates of the MoO3 and PEDOT devices are the same. (3) By contrast, when measured under the ISOS-L protocol, the MoO3 based devices are either equivalent to (unencapsulated devices) or, indeed, actually degrade faster (encapsulated devices) that their PEDOT counterparts. We demonstrate that these differences arise from the dominant degradation mode changing under the different protocols. As such, this paper highlights that the choice of testing protocol significantly influences the reported stability of OPV devices. In particular, the ISOS-D and ISOS-L protocols do not necessary reflect OPV device performance under actual operating conditions and thus stability measurements using these protocols should be treated with caution.

We demonstrate a pathway for fully roll-to-roll (R2R) prepared organic solar cells in a normal geometry with a R2R sputtered aluminium top electrode. Initial attempts utilizing a stack geometry without an electron transport layer (ETL) failed to obtain working devices. By applying aluminium zinc oxide (AZO) as an ETL, and optimizing the AZO thickness, working printed OPV devices with an efficiency of 0.58% were obtained. Further optimization of the donor:acceptor ratio in the active layer increased the efficiency to 0.90%. This work demonstrates that normal geometry organic solar cells using a metal top contact can be produced using large scale production techniques.

Here we report the application of a conjugated copolymer based on thiophene and quinoxaline units, namely poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1), to nanoparticle organic photovoltaics (NP-OPVs). TQ1 exhibits more desirable material properties for NP-OPV fabrication and operation, particularly a high glass transition temperature (Tg) and amorphous nature, compared to the commonly applied semicrystalline polymer poly(3-hexylthiophene) (P3HT). This study reports the optimisation of TQ1:PC71BM (phenyl C71 butyric acid methyl ester) NP-OPV device performance by the application of mild thermal annealing treatments in the range of the Tg (sub-Tg and post-Tg), both in the active layer drying stage and post-cathode deposition annealing stage of device fabrication, and an in-depth study of the effect of these treatments on nanoparticle film morphology. In addition, we report a type of morphological evolution in nanoparticle films for OPV active layers that has not previously been observed, that of PC71BM nano-pathway formation between dispersed PC71BM-rich nanoparticle cores, which have the benefit of making the bulk film more conducive to charge percolation and extraction.

Delicate structures (such as biological samples, organic films for polymer electronics and adsorbate layers) suffer degradation under the energetic probes of traditional microscopies. Furthermore, the charged nature of these probes presents difficulties when imaging with electric or magnetic fields, or for insulating materials where the addition of a conductive coating is not desirable. Scanning helium microscopy is able to image such structures completely non-destructively by taking advantage of a neutral helium beam as a chemically, electrically and magnetically inert probe of the sample surface. Here we present scanning helium micrographs demonstrating image contrast arising from a range of mechanisms including, for the first time, chemical contrast observed from a series of metal-semiconductor interfaces. The ability of scanning helium microscopy to distinguish between materials without the risk of damage makes it ideal for investigating a wide range of systems.

Single layer graphene has been successfully grown via chemical vapour deposition (CVD) at low temperature by using chlorobenzene trapped in a PMMA polymer matrix as the carbon source. By varying the carbon source temperature, we are able to vary the dominant carbon source from just chlorobenzene to PMMA. Raman spectroscopy and atomic force microscopy (AFM) have been used to characterize the as-grown graphene layer, while scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been used to characterize film quality and growth dynamics. Lower source temperatures (corresponding to a chlorobenzene carbon source) result in high quality single layer graphene whereas higher source temperatures (PMMA carbon source) produce disordered multilayered graphene films. SEM imaging reveals that a preferential surface mediated edge growth mechanism for single layer graphene is observed as a function of growth time. This development offers a new methodology for graphene synthesis at low temperatures with implications for the development of printed graphene structures.

The potential for organic electronic technologies to produce low-cost energy at large scales is often cited as the most attractive feature of these materials. Such aspirations depend on the ability of materials to be printed from solution at high speeds across large areas using roll-to-roll (R2R) processing. However, progressing the technology from the laboratory environment into the industrial manufacturing arena is highly challenging. Closing the gap between exciting laboratory scale insights and the industrial scale potential requires a new focus on upscaling existing technology. Some recent progress in this area is discussed, concentrating on the need to pursue research across several different scales simultaneously in order to most effectively optimize large-scale fabrication efforts. These discussions are placed in the context of a design philosophy that combines printing, coating, and vacuum-based procedures. The challenges associated with selecting, and subsequently synthesizing, the optimal materials for device construction at large scales are considered. Case histories that highlight the unique challenges encountered during printing, coating, and sputtering at the R2R scale are presented. Developing testing and characterization procedures that can interrogate organic photovoltaic device (OPV) structures in real time is also discussed, and the opportunity for new tools to probe device photophysics is highlighted. The collection of innovative approaches to R2R fabrication challenges discussed here highlights the exciting progress toward efficient OPV modules becoming a commercial reality.

Sequential deposition of monolayers, composed of nanoparticles with varied donor-acceptor concentration ratios, has allowed the fabrication of organic photovoltaic (OPV) active layers with engineered vertical morphology. The performance of polymer-polymer poly(9,9-dioctylfluorene-co-bis-N,N-phenyl-1,4-phenylenediamine):poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (PFB:F8BT) and polymer-fullerene poly(3-hexylthiophene):phenyl C61 butyric acid methyl ester (P3HT:PCBM) nanoparticulate (NP), graded nanoparticulate (GNP) and bulk heterojunction (BHJ) OPV devices have been studied. For both material systems the highest device VOC is observed for the graded structure. Furthermore, thermal treatments can be used to alleviate parasitic series resistance in the GNP devices, thus improving device JSC and efficiency. Overall, this work shows that the nanoparticle approach provides a new experimental lever for morphology control in OPV devices.

The kinetics and thermodynamics of PCBM phase segregation and aggregation in P3HT:PCBM blends has been studied. We develop a thermodynamic model for PCBM phase segregation in P3HT:PCBM blends which explains the formation of nanoscale crystallites which subsequently diffuse and coalesce into larger PCBM aggregates. We show that the formation of nanoscale crystallites during the film making process prevents spinodal decomposition of the P3HT:PCBM blends even at PCBM weight fractions above the spinodal decomposition boundary for the system. Finally, we demonstrate that the observed aggregate morphology can be understood in terms of a kinetic model based on the diffusional flux lines of PCBM crystallite which, in turn, govern the evolution of the macroscopic growth front.

The synthesis and performance of a cost-effective mixed fullerene at the 100+ g scale with a reaction yield of 85% is demonstrated. The cost to convert a fullerene such as C60 into the mixed acceptor blend is less than $1 g-1. The photovoltaic performance of the mixed acceptor is demonstrated in both small scale and roll-to-roll printed devices.

We investigate the suitability of four different inorganic materials (chromium oxide (CrOX), titanium oxide (TiOX), aluminium doped zinc oxide (AZO) and zinc oxide (ZnO)) as electrode transport layers in fully roll-to-roll (R2R) fabricated P3HT:ICxA organic solar cells. CrOX and TiOX were found to be unsuitable, as the CrOX devices did not exhibit rectifying behaviour while the TiOX devices did not withstand the annealing conditions. Of the last two ETLs, ZnO showed by far the most promise with devices demonstrating an average efficiency of 2.2%, which is the highest reported value for R2R devices in normal geometry, and a significantly extended lifetime compared with AZO devices under ISOS-L-2 conditions.

Near-field scanning optical lithography (NSOL) has been used to produce ordered structures of the semi-conducting polymer poly(p-phenylene vinylene) at sizes comparable to optical wavelengths. Structures on this scale are of interest for integrated optical devices. Here we study the effect of precursor film thickness upon the size and shape of lithographically produced features. We show that the discrepancies between the predicted and measured structures arise from two key factors: (1) the inherent change in the refractive index of the precursor polymer that occurs during lithography and (2) the considerable length of a precursor polymer relative to the characteristic NSOL feature dimension. Suggestions are made for the improvement of modelling accuracy.

Abstract Scanning transmission X-ray microscopy (STXM) compositional mapping has been used to probe the mesomorphology of nanoparticles (NPs) synthesized from two very different polymer:fullerene blends: poly(3-hexylthiophene) (P3HT): phenyl-C61-butyric acid methyl ester (PCBM) and poly[4,8-bis(2-ethylhexyloxy)benzo(1,2-b:4,5-b')dithiophene-alt-5, 6-bis(octyloxy)-4,7-di(thiophen-2-yl)(2,1,3-benzothiadiazole)-5,5'-diyl] (PSBTBT): PCBM. The STXM data shows that both blends form core-shell NP structures with similar shell compositions, but with different polymer:fullerene ratios in the core regions. P3HT:PCBM and PSBTBT:PCBM NP organic photovoltaic (OPV) devices have been fabricated and exhibit similar device efficiencies, despite the PSBTBT being a much higher performing low band gap material. By comparing the measured NP shell and core compositions with the optimized bulk hetero-junction (BHJ) compositions, we show that the relatively higher performance of the P3HT:PCBM NP device arises from the fact that its shell composition is much closer to the optimal BHJ value than that of the PSBTBT:PCBM NP device.

Traditional artist-in-science-residency schemes have tended to focus on artists using scientific tools and technology as a medium for their art. What kind and quality of work might occur, however, between scientists working on cutting-edge solar energy research and a visual artist (a sculptor) when they are integrated in a truly collaborative environment? Is it good for the art? Is it good for the science? The authors describe a new methodology for art-science interactions whereby they have integrated arts practice within a scientific environment. A critical aspect of the methodology for the residency was the development of an interaction framework that ensured that both artist and scientist had equal voice in discussions involving the art and science of the project within an environment of mutual respect. The integration led to the development of outcomes that would not have occurred otherwise.

The effect of device architecture upon the response of printable enzymatic glucose sensors based on poly(3-hexythiophene) (P3HT) organic thin film transistors is presented. The change in drain current is used as the basis for glucose detection and we show that significant improvements in drain current response time can be achieved by modifying the design of the sensor structure. In particular, we show that eliminating the dielectric layer and reducing the thickness of the active layer reduce the device response time considerably. The results are in good agreement with a diffusion based model of device operation, where an initial rapid dedoping process is followed by a slower doping of the P3HT layer from protons that are enzymatically generated by glucose oxidase (GOX) at the Nafion gate electrode. The fitted diffusion data are consistent with a P3HT doping region that is close to the source-drain electrodes rather than located at the P3HT:[Nafion:GOX] interface. Finally, we demonstrate that further improvements in sensor structure and morphology can be achieved by inkjet-printing the GOX layer, offering a pathway to low-cost printed biosensors for the detection of glucose in saliva.

The diffusion of PCBM in P3HT:PCBM blend films has been investigated using multi-wavelength scanning absorption microscopy (MWSAM). By studying the depletion of PCBM in the vicinity of the phase segregated PCBM-rich regions that form upon thermal annealing, we are able to measure the diffusion constant and activation energy for PCBM diffusion in P3HT:PCBM blend films. The measured kinetic parameters are consistent with the diffusion of nanoscale PCBM crystallites rather than molecular PCBM. We show that the presence of two distinct diffusion processes in these blend materials provides an explanation for the large differences that have been reported for PCBM diffusion in P3HT:PCBM blends. This insight allows us to develop a unified model for PCBM mass transport in these materials.

We present a scanning helium microscope equipped to make use of the unique contrast mechanisms, surface sensitivity, and zero damage imaging the technique affords. The new design delivers an order of magnitude increase in the available helium signal, yielding a higher contrast and signal-to-noise ratio. These improvements allow the microscope to produce high quality, intuitive images of samples using topological contrast, while setting the stage for investigations into further contrast mechanisms.

Here, we report the development of an organic thin film transistor (OTFT) based on printable solution processed polymers and employing a quantum tunnelling composite material as a sensor to convert the pressure wave output from detonation transmission tubing (shock tube) into an inherently amplified electronic signal for explosives initiation. The organic electronic detector allows detection of the signal in a low voltage operating range, an essential feature for sites employing live ordinances that is not provided by conventional electronic devices. We show that a 30-fold change in detector response is possible using the presented detector assembly. Degradation of the OTFT response with both time and repeated voltage scans was characterised, and device lifetime is shown to be consistent with the requirements for on-site printing and usage. The integration of a low cost organic electronic detector with inexpensive shock tube transmission fuse presents attractive avenues for the development of cheap and simple assemblies for precisely timed initiation of explosive chains.

The impact of a calcium interface layer in combination with a thermal annealing treatment on the performance of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-buteric acid methylester (PCBM) nanoparticle photovoltaic devices is investigated. Annealing is found to disrupt the microstructure of the nanoparticle active layer leading to a reduction in fill factor. However, X-ray photoelectron spectroscopy measurements show that the calcium interface layer causes PCBM to preferentially migrate to the cathode interface upon annealing, resulting in better charge extraction from the PCBM moiety, an increase in the built-in voltage, open-circuit voltage, and power conversion efficiency. Moreover, the annealing trends could be completely explained by the observed PCBM migration. Unlike P3HT:PCBM bulk heterojunction devices, the P3HT:PCBM nanoparticle devices showed a remarkable thermal stability up to 120°C. As such, OPVs fabricated from aqueous nanoparticle inks provide an attractive alternative to conventional organic solvent based bulk heterojunction devices.

Nanoparticulate zinc oxide can be prepared at low temperatures from solution processable zinc acetylacetonate. The use of this material as a cathode interfacial layer in nanoparticulate organic photovoltaic devices results in comparable performances to those based on reactive calcium layers. Importantly, the enhanced degradation stability and full solution processability make zinc oxide a more desirable material for the fabrication of large area printed devices. © 2014 AIP Publishing LLC.

We present a dynamic Monte Carlo (DMC) study of s-shaped current-voltage (I-V) behaviour in organic solar cells. This anomalous behaviour causes a substantial decrease in fill factor and thus power conversion efficiency. We show that this s-shaped behaviour is induced by charge traps that are located at the electrode interface rather than in the bulk of the active layer, and that the anomaly becomes more pronounced with increasing trap depth or density. Furthermore, the s-shape anomaly is correlated with interface recombination, but not bulk recombination, thus highlighting the importance of controlling the electrode interface. While thermal annealing is known to remove the s-shape anomaly, the reason has been not clear, since these treatments induce multiple simultaneous changes to the organic solar cell structure. The DMC modelling indicates that it is the removal of aluminium clusters at the electrode, which act as charge traps, that removes the anomalous I-V behaviour. Finally, this work shows that the s-shape becomes less pronounced with increasing electron-hole recombination rate; suggesting that efficient organic photovoltaic material systems are more susceptible to these electrode interface effects.

Poly (3-hexylthiophene) (P3HT), at six molecular weights varying from 5 kDa to 72 kDa (Mw), was used to prepare P3HT: phenyl C61 butyric acid methyl ester (PCBM) nanoparticulate organic photovoltaic (NP OPV) devices and the effect of this variation on device performance is reported. Power conversion efficiency (PCE) is observed to peak for the mid-range of molecular weights tested, this behaviour varies from the trend generally observed with bulk heterojunction (BHJ) devices, where high molecular weight polymers deliver the highest PCEs. Here we demonstrate that polymer molecular weight affects the electronic, morphological and compositional structure of the nanoparticulate film. Significantly, it is the domain composition that is most highly correlated with device performance and this composition is driven by the PCBM mobility and aggregation within the nanoparticulate structure. © 2014 Elsevier B.V.

The high-temperature structure of the ¿ = 5 bicrystalline interface of B2 NiAl with a large boundary plane is investigated by molecular dynamics simulations on parallel computers. It is observed that the atoms in the grain boundary region tend to form clusters in a thermal structural disorder transition, which is initiated at a temperature well below the thermodynamic melting point Tm (~0.52 Tm). The number and size of the clusters are monitored over a wide temperature range including Tm. Below Tm, the number and size of the clusters increase continuously with increasing temperature. When the temperature is up to Tm, the abrupt growth of the clusters induces melting. Once above Tm, the number and size of the clusters decrease significantly upon raising the temperature. The calculations of the potential energy also indicate that the thermal disorder transition is a continuous process, in contrast to the first-order melting transform ation. Copyright © 2001 John Wiley & Sons, Ltd.

Conductive transparent carbon thin film electrodes have been grown on a copper foil using poly(methyl methacrylates) as a carbon source at a temperature below 450 °C, in contrast to the preparation temperature above 800 °C in the previously reported chemical vapour deposition method. Raman and UV-Vis absorption spectroscopy have been used to identify their graphene based composition. Scanning electron microscopy and Transmission electron microscopy have been used to characterize the film quality. Conductivity and transmittance of the thin films have been evaluated. Using the conductive transparent electrodes, organic solar cells have been successfully fabricated. This work paves a potential pathway for an easier and cheaper production of organic solar cells. © 2013 IEEE.