Improved efficiency of organic solar cells using Au NPs incorporated into PEDOT:PSS buffer layer

Au based plasmonic phenomenon inside the hole transport layer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) of an organic solar cell based on blend of poly(3-hexylthiophene) (P3HT) and [6:6]-phenyl-C61-butyric acid (PCBM) is investigated. The concentration of the Au nanoparticles synthesized by wet chemical reduction is one of the key factors to strong light trapping when the spherical gold nanoparticles are blended into the PEDOT:PSS solution. Studies of the influence of the concentration of nanoparticles distribution in the PEDOT:PSS were carried out using UV–Vis spectroscopy and atomic force microscopy. Electrical characteristics of the pristine device and of device with metallic nanostructures were analyzed from J –V characteristics to observe the plasmonic effects on the performance in the P3HT:PCBM organic solar cells. The origin of the photocurrent enhancements with varying Au nanoparticles concentrations on PEDOT:PSS are discussed.The University of theWitwatersrand, Material Physics Research Institute, School of Physics & Chemistry; and MMU facilities at Wits, NRF and Material Energy Research Group (MERG).http://scitation.aip.orgcontent/aip/journal/advaam2017Physic

Synthesis and characterisation of metal selenide nanocrystals for use in electronic devices

A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy in the School of Chemistry Faculty of Science, University of Witwatersrand, 2017Advancements in nanotechnology and nanosystems promise to extend limits of sustainable development and environment remediation in an attempt to address some of the world most challenging problems. Specifically, nanotechnology has played an important role in the design, synthesis, and characterization of various new and novel functional nanomaterials possessing extremely unique properties. For example, low dimensional nanostructures such as semiconductor nanocrystals with well controlled sizes, shapes, porosities, crystalline phases, and structures have been prepared via various synthetic methods. In addition these semiconductor nanocrystals have attracted research attention because of their fundamental role in the comprehension of the quantum size effect and great potential applications to save resources and improve the environment. Tremendous studies have established that morphological, optical, catalytic and electronic properties of semiconductor nanocrystals can be manipulated during synthesis by simply varying the growth parameters. Herein we establish the effect of different synthetic methods and several growth parameters on the properties of the as-synthesized semiconducting metal selenides nanocrystals (NixSey and InxSey) including structural, optical, electronic and catalytic properties. For example, reducing coordinating solvent oleylamine was seen to favour a particular morphologies and stoichiometries despite the duration of synthesis. In the case of InxSey nanocrystals, oleylamine favoured indium monoselenide (InSe) nanosheet formation while addition of 1-DDT as a co-surfactant to oleylamine produces In2Se3 nanowires. For NixSey nanocrystals, TOP as a co-surfactant to different ligands favoured the formation Ni3Se2 with different shapes including dots, plates, rods and wires in different solvents. Other parameters studied included the reaction time and temperature. Besides the properties, we probe the potential applications of these materials in dye sensitized solar cells as counter electrodes as well in chemical sensor as the sensing material. NixSey nanocrystals were employed as CE in DSSCs in an attempt to replace the noble expensive platinum conventionally used as CE in most DSSCs. It was established that different stoichiometry played a significant role in the catalytic reduction of I3-. Thus, different photovoltaic performance parameters were obtained with NiSe2 giving a higher PCE of 1.5 % followed Ni3Se4 then Ni3Se2. These values were however very low compared to the ones reported in literature, something that was attributed to low electron mobility, enhanced recombination and reduced catalytic performance as a result of poor device assembly and the organic ligand layer encapsulating the nanocrystal. In another scenerio, indium monoselenide nanocrystals were employed in chemiresistive sensors to detect the presence of a number of VOCs including formaldehyde, methanol, chloroform and acetone in the ambient. Indeed despite the well-known electrical, optical and structural properties previously reported in literature, metal selenides such as CdSe, PbSe and ZnSe among others present lack of investigation for gas sensing. The experimental results showed that different morphologies of InSe nanostructures interacted differently to the analyte gas suggesting difference in the electronic properties of different morphologies. The InSe nanoparticle based sensors gave a good response to HCHO and MeOH fumes and were more selective to HCHO fumes than chloroform and acetone. While those fabricated using the InSe nanosheets though responding well to HCHO recovered half way when exposed back in air and resulted in relatively high noise to signal ratio when exposed to MeOH. The operating temperature range for the InSe sensor devices were determined to be near room temperature. The sensors response was observed to decrease with increasing temperature from 30 °C to 90 °C. Evident from the results, the surface capping molecule (oleylamine) employed to stabilize the nanostructures during synthesis was responsible for the poor sensing abilities of the nanostructures.XL201

Effect of implantation of Sm+ ions into RF sputtered ZnO thin film

The effects of implantation of Samarium ions (Sm+), a rare earth ion (RE) on the properties of ZnO films grown on Si (001) substrate by RF sputtering system are presented. The structural properties of the virgin and Sm–implanted ZnO thin films were investigated by Atomic force microscopy, Rutherford backscattering spectroscopy and Raman spectroscopy. Local lattice softening caused by the incorporation of highly mismatched Sm+ (ionic radii 0.096 nm and 0.113 nm for Sm3+ and Sm2+ respectively) into Zn antisites was detected as a red shift in E2 (high) mode likely caused by reduction in the crystallinity of the ZnO film. Photoluminescence on the pristine ZnO film showed a strong near band gap (NBE) emission and an intrinsic defect related blue, green-orange emission. The NBE is suppressed after implantation of Sm+ while the blue, green – orange emission intensities are enhanced as a result of increased structural defects with mismatched charge states. Moreover the effect of varying the concentration of Sm+ ions is presented and compared with predictions made from Stopping and Range of Ions in Matter (SRIM) calculation.The University of the Witwatersrand, Material Physics Research Institute, School of Physics; the XRD and MMU facilities at Wits, NRF and Material Energy Research Group (MERG).https://aip.scitation.org/journal/advam2020Physic

Hierarchical Nanoflowers of Colloidal WS2 and Their Potential Gas Sensing Properties for Room Temperature Detection of Ammonia

A one-step colloidal synthesis of hierarchical nanoflowers of WS2 is reported. The nanoflowers were used to fabricate a chemical sensor for the detection of ammonia vapors at room temperature. The gas sensing performance of the WS2 nanoflowers was measured using an in-house custom-made gas chamber. SEM analysis revealed that the nanoflowers were made up of petals and that the nanoflowers self-assembled to form hierarchical structures. Meanwhile, TEM showed the exposed edges of the petals that make up the nanoflower. A band gap of 1.98 eV confirmed a transition from indirect-to-direct band gap as well as a reduction in the number of layers of the WS2 nanoflowers. The formation of WS2 was confirmed by XPS and XRD with traces of the oxide phase, WO3. XPS analysis also confirmed the successful capping of the nanoflowers. The WS2 nanoflowers exhibited a good response and selectivity for ammonia