Michail J. Beliatis

Role of the Exposed Polar Facets in the Performance of Thermally and UV Activated ZnO Nanostructured Gas Sensors.

ZnO nanostructures with different morphologies (nanowires, nanodisks, and nanostars) were synthesized hydrothermally. Gas sensing properties of the as-grown nanostructures were investigated under thermal and UV activation. The performance of the ZnO nanodisk gas sensor was found to be superior to that of other nanostructures (Sg ∼ 3700% to 300 ppm ethanol and response time and recovery time of 8 and 13 s). The enhancement in sensitivity is attributed to the surface polarities of the different structures on the nanoscale. Furthermore, the selectivity of the gas sensors can be achieved by controlling the UV intensity used to activate these sensors. The highest sensitivity value for ethanol, isopropanol, acetone, and toluene are recorded at the optimal UV intensity of 1.6, 2.4, 3.2, and 4 mW/cm2, respectively. Finally, the UV activation mechanism for metal oxide gas sensors is compared with the thermal activation process. The UV activation of analytes based on solution processed ZnO structures pave the way for better quality gas sensors.

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.

Roll-coating fabrication of flexible organic solar cells: comparison of fullerene and fullerene-free systems

Flexible organic solar cells (OSCs) based on a blend of low-bandgap polymer donor PTB7-TH and non-fullerene small molecule acceptor IEIC were fabricated via a roll-coating process under ambient atmosphere. Both an indium tin oxide (ITO)-free substrate and a flexible ITO substrate were employed in these inverted OSCs. OSCs with flexible ITO and ITO-free substrates exhibited power conversion efficiencies (PCEs) up to 2.26% and 1.79%, respectively, which were comparable to those of the reference devices based on fullerene acceptors under the same conditions. This is the first example for all roll-coating fabrication procedures for flexible OSCs based on non-fullerene acceptors with the PCE exceeding 2%. The fullerene-free OSCs exhibited better dark storage stability than the fullerene-based control devices.

Engineering the plasmon resonance of large area bimetallic nanoparticle films by laser nanostructuring for chemical sensors

Large area fabrication of metal alloy nanoparticles with tunable surface plasmon resonances on low-cost substrates is reported. A UV excimer laser was used to anneal 5 nm thick Ag Au bilayer films deposited with different composition ratios to create alloy nanoparticles. These engineered surfaces are used to investigate how the wavelength of the surface plasmon resonance affects the optical detection capability of chemical species by surface-enhanced Raman spectroscopy.

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 (0.64 cm2), 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.

Flexible ITO-free organic solar cells applying aqueous solution-processed V2O5 hole transport layer: An outdoor stability study

Solution processable semiconductor oxides have opened a new paradigm for the enhancement of the lifetime of thin film solar cells. Their fabrication by low-cost and environmentally friendly solution-processable methods makes them ideal barrier (hole and electron) transport layers. In this work, we fabricate flexible ITO-free organic solar cells (OPV) by printing methods applying an aqueous solution-processed V2O5 as the hole transport layer (HTL) and compared them to devices applying PEDOT:PSS. The transparent conducting electrode was PET/Ag/PEDOT/ZnO, and the OPV configuration was PET/Ag/PEDOT/ZnO/P3HT:PC60BM/HTL/Ag. Outdoor stability analyses carried out for more than 900 h revealed higher stability for devices fabricated with the aqueous solution-processed V2O5.

Laser Ablation Direct Writing of Metal Nanoparticles for Hydrogen and Humidity Sensors

A UV pulsed laser writing technique to fabricate metal nanoparticle patterns on low-cost substrates is demonstrated. We use this process to directly write nanoparticle gas sensors, which operate via quantum tunnelling of electrons at room temperature across the device. The advantages of this method are no lithography requirements, high precision nanoparticle placement, and room temperature processing in atmospheric conditions. Palladium-based nanoparticle sensors are tested for the detection of water vapor and hydrogen within controlled environmental chambers. The electrical conduction mechanism responsible for the very high sensitivity of the devices is discussed with regard to the interparticle capacitance and the tunnelling resistance.

Hierarchically designed ZnO nanostructure based high performance gas sensors

Rationally controlled multistage hydrothermal methods have been developed to prepare different types of hierarchical zinc oxide (ZnO) nanostructures with high surface-to-volume ratios and more exposed polar facets. Four types of hierarchical ZnO nanostructures, i.e. nanobrushes (ZNBs), nanoleaves (ZNLs), hierarchical nanodisks (HNDs) and nanoflakes (ZNFs), assembled from initial mono-morphological nanostructures, i.e. nanowires (ZNWs) and nanodisks (ZNDs), were produced from sequential nucleation and growth after a hydrothermal process. Hierarchical nanostructures with 1D nanowire and 2D nanodisk building blocks were realized using zinc nitrate and zinc sulphate as the source of zinc ions, respectively. Compared to their initial mono-morphological counterparts, the grown hierarchical nanostructures demonstrated superior gas sensing properties. ZNLs and ZNFs showed a significant improvement in the sensitivity and fast response to acetone. In addition to the high surface-to-volume ratio, due to the ultrathin sheet building blocks, the enhanced gas sensing properties of the ZNLs and ZNFs are chiefly ascribed to the increased proportion of exposed (0001) polar facets. The current study offers a path for the structure induced development of gas sensing properties by designing a necessary nanostructure, which could be used to fabricate high performance nanostructured gas sensors based on other metal oxides.

Simultaneous optical and electrical modeling of plasmonic light trapping in thin-film amorphous silicon photovoltaic devices

Abstract. Rapid prototyping of photovoltaic (PV) cells requires a method for the simultaneous simulation of the optical and electrical characteristics of the device. The development of nanomaterial-enabled PV cells only increases the complexity of such simulations. Here, we use a commercial technology computer aided design (TCAD) software, Silvaco Atlas, to design and model plasmonic gold nanoparticles integrated in optoelectronic device models of thin-film amorphous silicon (a-Si:H) PV cells. Upon illumination with incident light, we simulate the optical and electrical properties of the cell simultaneously and use the simulation to produce current–voltage (J−V) and external quantum efficiency plots. Light trapping due to light scattering and localized surface plasmon resonance interactions by the nanoparticles has resulted in the enhancement of both the optical and electrical properties due to the reduction in the recombination rates in the photoactive layer. We show that the device performance of the modeled plasmonic a-Si:H PV cells depends significantly on the position and size of the gold nanoparticles, which leads to improvements either in optical properties only, or in both optical and electrical properties. The model provides a route to optimize the device architecture by simultaneously optimizing the optical and electrical characteristics, which leads to a detailed understanding of plasmonic PV cells from a design perspective and offers an advanced tool for rapid device prototyping.