Solution Processed Semiconductor Nanostructures and Nanocomposites for Dye Sensitized Solar Cells

Dye sensitized solar cells (DSSC) are low cost alternatives to silicon solar cells. The conventional DSSC consists of two sandwiched pieces of conducting glass, one of them coated with mesoporous layer of nanoparticulate TiO2 with a self-assembled monolayer of chemisorbed dye molecules, filled with an electrolyte for dye regeneration. Conventional dye sensitized solar cells use TiO2 nanoparticles as electron transport material. Electron transport is critical for the performance of the dye sensitized solar cells, as they recombine with the electrolyte if they are not collected fast. Using nanoparticles introduce high surface area, but slows down the electron transport and prevent electron collection. We synthesized novel hybrid nanostructures with fast electron transport and high surface. Tested the new hybrid structures with different electrolytes and compared with conventional photoanodes. The band alignment of the nanocomposites is studied. The band edge engineering of the nanocomposites is studied for improved electron transport. The recombination in the DSSC is studied and blocking layers synthesized using ALD and TiCl4 treatment are compared and the properties resulting to improved efficiencies in DSSC are studied. The photocurrent of the large ZnO nanorods based DSSC are improved from 0.4 mA/cm2to 4.4 mA/cm2 using the above techniques

Blue and white light emission from zinc oxide nanoforests

Blue and white light emission is observed when high voltage stress is applied using micrometer-separated tungsten probes across a nanoforest formed of ZnO nanorods. The optical spectrum of the emitted light consistently shows three fine peaks with very high amplitude in the 465–485 nm (blue) range, corresponding to atomic transitions of zinc. Additional peaks with smaller amplitudes in the 330–650 nm range and broad spectrum white light is observed depending on the excitation conditions. The spatial and spectral distribution of the emitted light, with pink–orange regions identifying percolation paths in some cases and high intensity blue and white light with center to edge variations in others, indicate that multiple mechanisms lead to light emission. Under certain conditions, the tungsten probe tips used to make electrical contact with the ZnO structures melt during the excitation, indicating that the local temperature can exceed 3422 °C, which is the melting temperature of tungsten. The distinct and narrow peaks in the optical spectra and the abrupt increase in current at high electric fields suggest that a plasma is formed by application of the electrical bias, giving rise to light emission via atomic transitions in gaseous zinc and oxygen. The broad spectrum, white light emission is possibly due to the free electron transitions in the plasma and blackbody radiation from molten silicon. The white light may also arise from the recombination through multiple defect levels in ZnO or due to the optical excitation from solid ZnO. The electrical measurements performed at different ambient pressures result in light emission with distinguishable differences in the emission properties and I–V curves, which also indicate that the dielectric breakdown of ZnO, sublimation, and plasma formation processes are the underlying mechanisms

Atmospheric pressure microplasmas in ZnO nanoforests under high voltage stress

Atmospheric pressure ZnO microplasmas have been generated by high amplitude single pulses and DC voltages applied using micrometer-separated probes on ZnO nanoforests. The high voltage stress triggers plasma breakdown and breakdown in the surrounding air followed by sublimation of ZnO resulting in strong blue and white light emission with sharp spectral lines and non-linear current-voltage characteristics. The nanoforests are made of ZnO nanorods (NRs) grown on fluorine doped tin oxide (FTO) glass, poly-crystalline silicon and bulk p-type silicon substrates. The characteristics of the microplasmas depend strongly on the substrate and voltage parameters. Plasmas can be obtained with pulse durations as short as similar to 1 mu s for FTO glass substrate and similar to 100 ms for the silicon substrates. Besides enabling plasma generation with shorter pulses, NRs on FTO glass substrate also lead to better tunability of the operating gas temperature. Hot and cold ZnO microplasmas have been observed with these NRs on FTO glass substrate. Sputtering of nanomaterials during plasma generation in the regions surrounding the test area has also been noticed and result in interesting ZnO nanostructures ('nano-flowers' and 'nano-cauliflowers'). A practical way of generating atmospheric pressure ZnO microplasmas may lead to various lighting, biomedical and material processing applications. (C) 2015 Author(s)