Lynn J. Rozanski

Hybrid Graphene‐Metal Oxide Solution Processed Electron Transport Layers for Large Area High‐Performance Organic Photovoltaics

Solution processed core-shell nano-structures of metal oxide-reduced graphene oxide (RGO) are used as improved electron transport layers (ETL), leading to an enhancement in photocurrent charge transport in PCDTBT:PC70 BM for both single cell and module photovoltaic devices. As a result, the power conversion efficiency for the devices with RGO-metal oxides for ETL increases 8% in single cells and 20% in module devices.

Solution processed reduced graphene oxide/metal oxide hybrid electron transport layers for highly efficient polymer solar cells

We report new solution processable electron transport layers for organic photovoltaic devices based on composites of metal oxides and reduced graphene oxides. Low bandgap polymer cells fabricated using these nanohybrid transport layers display power conversion efficiencies in the range of 7.4–7.5% which is observed to be an improvement over conventional metal oxide or thermally evaporated electron transport layers. This efficiency enhancement is driven mainly by improvements in the short circuit current (from ∼14.8 to ∼15.0 mA cm−2) as well as the fill factor (∼65% to ∼68%) upon the inclusion of reduced graphene oxide with the metal oxides. This is attributed to the reduced graphene oxide providing charge transfer pathways between the metal oxide nanoparticles. In addition, the metal oxide/reduced graphene oxide nanohybrids also lead to more balanced electron and hole mobilities which assist in the improvement of the fill factor of the device. The versatile nature of these nanohybrids is increased due to the wrapping of the graphene layers around the metal oxide nanoparticles, which leads to very smooth films with surface roughness of ∼3 nm. The improvement observed in this study upon the incorporation of RGO as well as the solution processable nature of the interfacial layers brings the organic photovoltaic technology a step closer towards realising an all solution processed solar cell.

Graphene oxide hole transport layers for large area, high efficiency organic solar cells

Graphene oxide (GO) is becoming increasingly popular for organic electronic applications. We present large active area (0.64u2009cm2), solution processable, poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]:[6,6]-Phenyl C71 butyric acid methyl ester (PCDTBT:PC70BM) organic photovoltaic (OPV) solar cells, incorporating GO hole transport layers (HTL). The power conversion efficiency (PCE) of ∼5% is the highest reported for OPV using this architecture. A comparative study of solution-processable devices has been undertaken to benchmark GO OPV performance with poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) HTL devices, confirming the viability of GO devices, with comparable PCEs, suitable as high chemical and thermal stability replacements for PEDOT:PSS in OPV.

Solution processed naphthalene diimide derivative as electron transport layers for enhanced brightness and efficient polymer light emitting diodes

Increasing the efficiency and lifetime of polymer light emitting diodes (PLEDs) requires a balanced injection and flow of charges through the device, driving demand for cheap and effective electron transport/hole blocking layers. Some materials, such as conjugated polyelectrolytes, have been identified as potential candidates but the production of these materials requires complex, and hence costly, synthesis routes. We have utilized a soluble small molecule naphthalene diimide derivative (DC18) as a novel electron transport/hole blocking layer in common PLED architectures, and compared its electronic properties to those of the electron transport/hole blocking small molecule bathocuproine (BCP). PLEDs incorporating DC18 as the electron transport layer reduce turn on voltage by 25%; increase brightness over three and a half times; and provide a full five-fold enhancement in efficiencies compared to reference devices. While DC18 has similar properties to the effective conjugated polyelectrolytes used as electron transport layers, it is simpler to synthesise, reducing cost while retaining favourable electron transport properties, and producing a greater degree of efficiency enhancement. The impact on device lifetime is hypothesized to be significant as well, due to the air-stability seen in many naphthalene diimide derivatives.

Hybrid metal grid-polymer-carbon nanotube electrodes for high luminance organic light emitting diodes

Organic light emitting diodes (OLEDs) incorporating grid transparent conducting electrodes (TCEs) with wide grid line spacing suffer from an inability to transfer charge carriers across the gaps in the grids to promote light emission in these areas. High luminance OLEDs fabricated using a hybrid TCE composed of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS PH1000) or regioregular poly(3-hexylthiophene)-wrapped semiconducting single-walled carbon nanotubes (rrP3HT-SWCNT) in combination with a nanometre thin gold grid are reported here. OLEDs fabricated using the hybrid gold grid/PH1000 TCE have a luminance of 18 000 cd m(-2) at 9 V; the same as the reference indium tin oxide (ITO) OLED. The gold grid/rrP3HT-SWCNT OLEDs have a lower luminance of 8260 cd m(-2) at 9 V, which is likely due to a rougher rrP3HT-SWCNT surface. These results demonstrate that the hybrid gold grid/PH1000 TCE is a promising replacement for ITO in future plastic electronics applications including OLEDs and organic photovoltaics. For applications where surface roughness is not critical, e.g. electrochromic devices or discharge of static electricity, the gold grid/rrP3HT-SWCNT hybrid TCE can be employed.

Storage Lifetime of Polymer-Carbon Nanotube Inks for Use as Charge Transport Layers in Organic Light Emitting Diodes

The long-term stability of multiwall carbon nanotubes (MWCNTs) mixed with the hole-transport polymer Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been examined. These surfactant stabilized solutions, used as transport layers in organic light-emitting diodes (OLEDs), are shown to be stable for periods of up to 15 months and show no signs of degrading soon after this time. In comparison, nonstabilized aqueous MWCNT solutions have been shown to aggregate within 30 min of production, and, although these aggregates can be redispersed, the solution displays an increase in smaller aggregates over time which cannot subsequently be redispersed by manual agitation. The stable MWCNT/PEDOT:PSS solutions have been used in ink-jet printing and as composite MWCNT/PEDOT:PSS films suitable as charge transport layers in spin-coated OLEDs.

High efficiency air stable organic photovoltaics with an aqueous inorganic contact

We report a ZnO interfacial layer based on an environmentally friendly aqueous precursor for organic photovoltaics. Inverted PCDTBT devices based on this precursor show power conversion efficiencies of 6.8-7%. Unencapsulated devices stored in air display prolonged lifetimes extending over 200 hours with less than 20% drop in efficiency compared to devices based on the standard architecture.

Hybrid and Nano-composite Carbon Sensing Platforms

Carbon nanomaterials offer a number of possibilities for sensing platforms. The ability to chemically functionalize the surfaces of the nano-carbon, using hybrid or nano-composite structures, can further enhance the material properties. Complementary to the addition of any requisite chemical or biochemical functionality, such enhancements can take the form of improved electrical, optical or morphological properties which improve the transduction capabilities of the carbon nano-material, or facilitate detection of the transduced signal, for example by improving charge transfer to detection electronics. Here we review the methods of producing hybrid and nano-composite carbon structures for sensing systems, highlighting the advantages of the functionalization in each case and benchmark their performance against existing carbon-only devices. Finally, we detail some of the recent applications of hybrid and nano-composite carbon technologies in a wide variety of sensor technologies.

Equivalent Circuit Modeling for a High-Performance Large-Area Organic Photovoltaic Module

For organic photovoltaics (OPVs) to contribute significantly to energy generation, they need to be scaled to large areas, much like all organic electronics. Therefore, there is a need for the development of a specific model that describes the electrical properties related to the size effects and cell interconnections. We report here on the equivalent circuit models for a high-performance series-connected OPV module based on a polymer:fullerene bulkheterojunction formulation. We examine the validity of the effective single cell methodology in the conventional framework and suggest a modified model that includes the net series resistance and additional parasitic leakage conductions. The photocurrent is found to follow the diffusion-limited voltage dependence, for which an empirical treatment enables an improved reproduction of the measurement near the short-circuit point.

Filtration properties of hierarchical carbon nanostructures deposited on carbon fibre fabrics

Hierarchical carbon nanostructures have been produced and examined for their use in liquid filtration experiments. The nanostructures are based on carbon nanotube growth and graphite oxide sponge deposition on the surface of commercially available carbon fibre fabrics. The hierarchical nanomaterial construction on the carbon fibre fabric is made possible due to the chemical vapour deposited carbon nanotubes which act as anchoring sites for the solution deposited sponge nanomaterial. The nanomaterials show a high capacity for Rhodamine B filtration, with the carbon fibre—carbon nanotube—graphite oxide sponge fabric showing filtering performance comparable to a commercial activated carbon filter. After 40 successive filtrations of 10 mg ml−1 Rhodamine B solution, the filtrate of dual modified fabrics returned an increase in transparency of 94% when measured at approx. 550 nm compared to 72% for the commercial carbon filter. When normalised with respect to the areal density of the commercial filter, the increase in optical transparency of the filtrate from the dual modified fabrics reduces to 65%. The Rhodamine B is found to deposit in the carbon nanomaterials via a nucleation, growth and saturation mechanism.