We present a study of spectroscopic proprieties of poly(n-vinylcarbazole) (PVK) and PVK doped with iridium complexes tris[2-phenylpyridinato-C2,N]iridium(III) (Ir(ppy)3) films prepared by spin-coating from toluene and chlorobenzene solutions. A different molecular organization of the polymer on the substrate during the spin-coating process can be produced using solvents with different boiling temperatures. The modified molecular rearrangement affects the emission properties of the PVK material and the consequent energy transfer to the doping molecules. Both static and dynamic fluorescence emissions properties have been studied, for pure PVK and PVK doped with different weight percentage of Ir(ppy)3. Different organic light-emitting devices, using a simple architecture, have been prepared with both solvents to test the change in electroluminescence spectral shape and in electrical characteristic, and the final efficiencies of the devices have been evaluated.
In this work we have compared the effect of using solvents with different boiling temperature on the spectroscopic
emission proprieties of poly(n vinylcarbazole) (PVK) and PVK doped with Iridium complexes
tris[2-phenylpyridinato-C2,N]iridium(III) (Ir(ppy)3) films, due to the different organization of the polymer on the substrate during the spin
coating process. The modified molecular rearrangement affects the emission properties of the PVK material and the
consequent energy transfer to the doping molecules. Both Static and dynamic emissions proprieties have been studied,
for the case of pure PVK and PVK doped with different weight percentage of Ir(ppy)3. Different devices have been
prepared to test the change in electroluminescence spectral shape and in electrical characteristic and the final efficiencies
of the devices have been evaluated.
Control of charge interfacial charge transfer is central to the design of photovoltaic devices. A an elegant approach to control those dynamics, is the use of an insulating metal oxide blocking layer at a nanocrystalline inorganic / organic semiconductor interface. We show that the conformal growth of a ~1 nm thick overlayer of MgO on a preformed nanocrystalline SnO2 film results in a ~4-fold retardation in the rate of charge recombination at such an interface This observation shows a good correlation with the current/voltage characteristics of dye sensitised nanocrystalline solar cells fabricated from such films, with the MgO coating resulting in ~ 50% improvement in overall device efficiency.
This study focuses on systems consisting of high hole-mobility MEHPPV based polymers or a fluorene-bithiophene co-polymer in contact with different nanocrystalline TiO2 films. We use photoluminescence quenching, time of flight mobility measurements and optical spectroscopy to characterize the exciton transport, charge transport and light harvesting properties, respectively, of the polymers, and correlate these material properties with photovoltaic device performance. We find that the polymer properties with greatest influence on device efficiency are the polymer exciton diffusion length and absorption range, followed by the hole mobility. We have also studied the photovoltaic performance of these TiO2/polymer devices as a function of active layer thickness. Device performances are significantly improved by introducing a PEDOT layer between the polymer and the top Au electrode and by reducing the thickness of the active layers. The optimized devices have peak external quantum efficiencies ≈ 40 % at the polymer's maximum absorption wavelength and yield short circuit current densities ≥ 2 mA cm-2 for air mass (AM) 1.5 conditions (100 mW cm-2, 1 sun). The AM 1.5 open circuit voltage reaches 0.64 V and the fill factor 0.43, resulting in an overall power conversion efficiency of 0.58 %.
Interfacial electron transfer dynamics of dye sensitised metal oxide films have been widely studied by transient optical techniques. In this paper, we extend such studies to complete dye sensitised solar cells, and show how such transient optical studies can be correlated both with transient photovoltage studies and current / voltage analyses of device photovoltaic performance.
Control of charge interfacial charge transfer is central to the design of photovoltaic devices. We report herein the application of insulating metal oxide blocking layers to control the charge recombination kinetics at a solid-state dye sensitised nanocrystalline inorganic/organic semiconductor interface. We show that the conformal growth of a ~1 nm thick overlayer of Al2O3 on a preformed nanocrystalline TiO2 film results in a ~3-fold retardation in the rate of charge recombination at such an interface. This observation shows a good correlation with the current/voltage characteristics of dye sensitized nanocrystalline solar cells fabricated from such films, with the Al2O3 coating resulting in a 40% improvement in overall device efficiency
We study the charge recombination kinetics and photovoltaic performance of composites of poly (9,9-dioctylfluorene-co-bithiophene) polymer with nanocrystalline TiO2. Transient optical spectroscopy confirms that photoexcitation of the polymer leads to electron transfer to the TiO2 and indicates that charge recombination is slow with a half-time of 100 μs to 10ms. Polymer penetration into thick porous TiO2 layers is improved by melt-processing and treatment of the TiO2 surface. We study the photovoltaic characteristics of devices with different layer thickness and interface morphology. Quantum efficiency (QE) of all devices is increased by reducing the TiO2 and polymer layer thickness. Inserting a thin porous TiO2 layer in to a thin bi-layer device increases the QE by a factor of five. The improved device shows peak QE and monochromatic power conversion efficiencies of over 11% and 1% at 440nm respectively. The device produced a short-circuit current density of 300μAcm-2, a fill factor of 0.24 and an open-circuit voltage of 0.8V under AM1.5 illumination. The fill factor is increased from 0.24 to 0.40 by introducing an additional dip-coating layer and overall power conversion efficiency is increased by 50%. However, the device produced degraded current-voltage characteristics. We investigate this using an alternative polymers and different top contact metals.
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