Prof Henry Snaith FRS

Abstract:

Upon the addition of lithium salts to the hole-transporter matrix, 2, 2′, 7, 7′ -tetrakis (N,N -di- p -methoxypheny-amine)- 9, 9′ -spirobifluorene (spiro-MeOTAD), the authors observe a 100-fold increase in conductivity through spiro-MeOTAD within a Ti O2 mesoporous network. The authors demonstrate this to be a bulk effect and not due to improved injection at the electrodes. By testing "hole-only" diodes of pure spiro-MeOTAD and those doped with lithium salts, the authors calculate that the hole mobility increases from 1.6× 10-4 to 1.6× 10-3 cm2 V s. The authors discuss the possible mechanisms for this significant enhancement in charge mobility and its implication to the dye-sensitized solar cell operation. © 2006 American Institute of Physics.

Abstract:

We present the first application of an amorphous "liquid" organic semiconductor in an optoelectronic device, demonstrating that it is highly suited for use as a hole-transporting material in nanostructured dye-sensitized solar cells. For such devices, we obtain power conversion efficiencies of up to 2.4% under simulated air mass 1.5 solar spectrum at 100 mWcm(-2), and incident photon-to-electron quantum efficiencies in excess of 50%.

Abstract:

We present an analytical and experimental investigation into the origin of the open-circuit voltage in the solid-state dye-sensitized solar cell. Through Kelvin probe microscopy, we demonstrate that a macroscopically uniform electric field exists throughout the nanocomposite between the electrodes. Considering a balance between drift and diffusion currents, and between charge generation and recombination, we develop an analytical expression for the open-circuit voltage which accurately follows experimental data. We find the open-circuit voltage increases with light intensity as 1.7 kTq, where T is absolute temperature, however it decreases with increasing temperature and device thickness. The intensity dependence arises from the charge generation rate increasing more strongly with intensity than the recombination rate constant, resulting in increased chemical potential within the device. We find that the temperature dependence arises from a reduction in the charge lifetime and not from increased charge diffusion and mobility. The thickness dependence is found to derive from two factors; first, charge recombination sites are distributed throughout the film, enabling more charges to recombine in thicker films before influencing the potential at the electrodes, and second, the average optical power density within the film reduces with increasing film thickness. © 2006 The American Physical Society.

Abstract:

The role of Schottky barrier at an electron-collection electrode in solid state dye-sensitized solar cells was investigated. A compact TiO2 layer was inserted between the fluorine-doped tin-oxide (FTO) anode and the nanoporous TiO2 in order to reduce loss through recombination between holes in the hole transporter and electrons in the FTO. An apparent positive shift in the conduction band was observed under UV illumination, which results in improving electron injection from the lowest unoccupied molecular orbital (LUMO) energy level of the dye molecules into the TiO2 conduction band, or into newly created states. It was demonstrates that a Schottky barrier exists at the FTO/TiO2 interface when a TiO2 compact layers is used between the FTO and the nanoporous film. Incorporation of an oxygen-rich compact layer in a solid state dye-sensitized solar cell is found to be improving the device performance.

Abstract:

A Li+ coordinating sensitizer, NaRu(4-carboxylic acid-4'-carboxylate)(4,4'-bis[(triethylene glycol methyl ether) methyl ether]-2,2'-bipyridine)(NCS)2 (coded as K51), has been synthesized, and the effect of Li+ coordination on its performance in mesoscopic titanium dioxide dye-sensitized solar cells has been investigated. Fourier transform infrared spectra suggest that Li+ coordinates to the triethylene oxide methyl ether side chains on the dye molecules. With the addition of Li+ to a nonvolatile liquid electrolyte, we observe a significant increase in the photocurrent density, with only a small decrease in the open-circuit voltage, contrary to a non ion coordinating dye which displays a large drop in potential with the addition of Li+. For a solar cell incorporating an organic hole-transporter, we find the potential rises with increasing the Li+ concentration in the hole-transporter matrix. For the liquid electrolyte and solid-state cells, we obtain power conversion efficiencies of 7.8% and 3.8%, respectively, under simulated sunlight.