James R. Durrant

The origin of slow electron recombination processes in dye-sensitized solar cells with alumina barrier coatings

We investigate the effect of a thin alumina coating of nanocrystalline TiO2 films on recombination dynamics of dye-sensitized solar cells. Both coated and uncoated cells were measured by a combination of techniques: transient absorption spectroscopy, electrochemical impedance spectroscopy, and open-circuit voltage decay. It is found that the alumina barrier reduces the recombination of photoinjected electrons to both dye cations and the oxidized redox couple. It is proposed that this observed retardation can be attributed primarily to two effects: almost complete passivation of surface trap states in TiO2 that are able to inject electrons to acceptor species, and slowing down by a factor of 3–4 the rate of interfacial charge transfer from conduction-band states.

Hybrid nanocrystalline TiO2 solar cells with a fluorene–thiophene copolymer as a sensitizer and hole conductor

We report the effects of layer thickness, interface morphology, top contact, and polymer–metal combination on the performance of photovoltaic devices consisting of a fluorene–bithiophene copolymer and nanocrystalline TiO2. Efficient photoinduced charge transfer is observed in this system, while charge recombination is relatively slow (∼100 μs–10 ms). External quantum efficiencies of 13% and monochromatic power conversion efficiencies of 1.4% at a wavelength of 440 nm are achieved in the best device reported here. The device produced an open-circuit voltage of 0.92 V, short-circuit current density of about 400 μA cm−2, and a fill factor of 0.44 under simulated air mass 1.5 illumination. We find that the short-circuit current density and the fill factor increase with decreasing polymer thickness. We propose that the performance of the indium tin oxide/TiO2/polymer/metal devices is limited by the energy step at the polymer/metal interface and we investigate this situation using an alternative fluorene-based ...

State selective electron injection in non-aggregated titanium phthalocyanine sensitised nanocrystalline TiO2 films

We describe a novel titanium phthalocyanine that shows no aggregation when anchored to nanocrystalline TiO2 films through its axial carboxylated ligand without the use of co-adsorbents; state selective electron injection into the TiO2 is demonstrated, resulting in efficient photocurrent generation in dye sensitised photoelectrochemical solar cells.

Efficient charge collection in hybrid polymer/TiO2 solar cells using poly(ethylenedioxythiophene)/polystyrene sulphonate as hole collector

We report a study of the optimization of power conversion efficiency in hybrid solar cells based on nanostructured titanium dioxide and a poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV) based conjugated polymer. Charge collection efficiency is enhanced by introducing a poly(ethylenedioxythiophene)/polystyrene sulphonate (PEDOT) layer (under the gold electrode) as the hole collector. Device performance is maximized for a device with a net active layer thickness of 100 nm. The optimized device has peak external quantum efficiencies ≈40% at the polymer’s maximum absorption wavelength and yield short circuit current density ⩾2mAcm−2 for air mass (AM) 1.5 conditions (100mWcm−2, 1 sun). The AM 1.5 open circuit voltage for this device is 0.64 V and the fill factor is 0.43, resulting in an overall power conversion efficiency of 0.58%.

Slow charge recombination in dye-sensitised solar cells (DSSC) using Al2O3 coated nanoporous TiO2 films

The conformal growth of an overlayer of Al2O3 on a nanocrystalline TiO2 film is shown to result in a 4-fold retardation of interfacial charge recombination, and a 30% improvement in photovoltaic device efficiency.

Flexible dye sensitised nanocrystalline semiconductor solar cells

We report here flexible solid-state solar cells based upon dye-sensitised nanocrystalline Al2O3 coated TiO2 films and an I2/NaI doped solid-state polymer electrolyte. Such devices show remarkably high solar-light to electrical energy conversion efficiencies of approximately 5.3% under 10 mW cm-2 AM1.5 illumination.

Solid state solar cell made from nanocrystalline TiO2 with a fluorene-thiophene copolymer as a hole conductor

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.

Slow charge recombination at a dye-sensitized nanocrystalline TiO2/organic semiconductor heterojunction employing Al2O3 coatings

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