Showing papers by "Brian C. O’Regan published in 2008"

Experimental determination of the rate law for charge carrier decay in a polythiophene: Fullerene solar cell

Abstract: We use transient photovoltage and differential charging experiments, complemented by transient absorption data, to determine charge carrier lifetimes and densities in a poly(3-hexylthiophene): methanofullerene solar cell at Voc as a function of white light-bias intensity. For a typical device, the charge carrier decay dynamics are observed to exhibit an approximately third order dependence on charge density (dn∕dt∝n3).

Bimolecular recombination losses in polythiophene: Fullerene solar cells

Abstract: Transient absorption spectroscopy is employed to monitor charge carrier decay dynamics in an annealed poly(3-hexylthiophene): methanofullerene solar cell. Comparisons of device and film data and data under applied bias demonstrate that these dynamics are dominated by bimolecular recombination. These data allow us to quantify the rate constant for bimolecular recombination, found to be strongly carrier density dependent, and thus determine the bimolecular recombination flux. By comparison with the device short-circuit photocurrent we conclude that the open-circuit voltage is primarily limited by bimolecular recombination.

Catalysis of recombination and its limitation on open circuit voltage for dye sensitized photovoltaic cells using phthalocyanine dyes.

Abstract: In order to increase the energy efficiency of dye-sensitized solar cells beyond 10%, an improved dye needs to be developed with greater light absorption in the red and near-infrared. Many dyes have been tested for this purpose; however, no dye with significant absorption beyond 750 nm has functioned properly. We have examined a series of ruthenium phthalocyanines, a dye class with large and tunable absorption in the red. For these dyes we observe a large reduction in the output voltage of the cells relative to the benchmark dye (N719). By examination of photovoltage transients and charge density measurements, we demonstrate that this reduction in voltage is caused by a 100-fold increase in the rate constant for recombination (iodine reduction) at the TiO2/electrolyte interface. N719, however, does not seem to catalyze this reaction. By examination of the literature, we propose that catalysis of the recombination reaction may be occurring for many other classes of potentially useful dyes including porphyri...

Charge extraction analysis of charge carrier densities in a polythiophene/fullerene solar cell: Analysis of the origin of the device dark current

Abstract: We demonstrate the use of a simple charge extraction measurement to determine the charge carrier densities n in annealed poly(3-hexylthiophene):methanofullerene solar cells under operating conditions. By applying charge extraction to the device under forward bias in the dark (Jdark), we find Jdark∝n2.6. This dependence on charge density is the same as that we find for bimolecular recombination losses observed in such devices under irradiation at open circuit, suggesting that the dark current originates from bimolecular recombination at the polymer/fullerene interface.

Interfacial Charge Recombination Between e−−TiO2 and the I−/I3− Electrolyte in Ruthenium Heteroleptic Complexes: Dye Molecular Structure−Open Circuit Voltage Relationship

Abstract: A series of heteroleptic ruthenium(II) polypyridyl complexes containing phenanthroline ligands have been designed, synthesized, and characterized. The spectroscopic and electrochemical properties of the complexes have been studied in solution and adsorbed onto semiconductor nanocrystalline metal oxide particles. The results show that for two of the ruthenium complexes, bearing electron-donating (−NH2) or electron-withdrawing (−NO2) groups, the presence of the redox-active I−/I3− electrolyte produces important changes in the interfacial charge transfer processes that limit the device performance. For example, those dyes enhanced the electron recombination reaction between the photoinjected electrons at TiO2 and the oxidized redox electrolyte. In an effort to understand the details of such striking observations, we have monitored the charge transfer reactions taking place at the different interfaces of the devices using time-resolved single photon counting, laser transient spectroscopy, and light-induced ph...

A new ruthenium polypyridyl dye, TG6, whose performance in dye-sensitized solar cells is surprisingly close to that of N719, the ‘dye to beat’ for 17 years

Abstract: A new ruthenium polypyridine sensitizer for dye-sensitized solar cells (DSSCs) is proposed containing a hexasulfanyl–styryl-modified bipyridyl group as an ancillary ligand. The advantages of this dye are the much larger absorption coefficient and the small shift of the absorption envelope to the red. We compare this new dye, TG6 (cis-bis(thiocyanato)(2,2′-bipyridyl-4,4′-dicarboxylato){4,4′-bis[2-(4-hexylsulfanylphenyl)vinyl]-2,2′-bipyridine}ruthenium(II) mono(tetrabutylammonium) salt), to the current best-performing dye, N719 (cis-bis(thiocyanato)bis(2,2′-bipyridine-4,4′-dicarboxylato)ruthenium(II) bis(tetrabutylammonium) salt). We have applied a suite of evaluation tools including: varying the electrolyte, varying the TiO2 film thickness, charge density and recombination rate constant measurements, fluorescence lifetime and magnitude, and transient absorption techniques. The combined results indicate that TG6, as presently constructed, can surpass the performance of N719 under some conditions, but is likely to need some modification before surpassing cells designed to give record energy efficiency using N719. The higher absorption coefficient may be relevant to mass-produced DSSCs on metal where thinner TiO2 films are advantageous. The main disadvantage is the slight catalysis of the electron/electrolyte recombination, which is possibly due to the extended π-system. A factor that requires further optimization involves the complex interaction of the slightly lower LUMO position, the composition of the electrolyte, the band edge position of the TiO2, and the electron injection rate. We show why the maximum output from TG6 cells will not occur using the exact electrolytes used to maximize N719 cells.

The role of para-alkyl substituents on meso-phenyl porphyrin sensitised TiO2 solar cells: control of the eTiO2/electrolyte+ recombination reaction

Abstract: We aim to investigate the effect of adding hydrophobic alkyl chains substituents to unsymmetrical free base tetra-phenyl porphyrins used for the preparation of dye sensitised solar cells (DSSC). We have used two different unsymmetrical meso-tetraphenyl substituted free base porphyrins attending to two objectives: (1) to observe how the substitution of three para positions of the meso-phenyl groups with hydrophobic alkyl chains influences the formation of molecular aggregates onto the semiconductor nanoparticles and (b) to deduce the influence that the substitution exerts over the eTiO2/electrolyte+ recombination reaction in operating devices. To achieve these goals we have focussed on the study of the electron transfer processes that take place at the different interfaces of the photovoltaic device using electrochemistry, steady-state and time resolved spectroscopic techniques.

The effect of molecular aggregates over the interfacial charge transfer processes on dye sensitized solar cells

Abstract: The electron transfer reaction between the photoinjected electrons in the nanocrystalline TiO2 mesoporous sensitized films and the oxidized electrolyte in dye sensitized solar cells (DSSC) plays a major role on the device efficiency. In this communication we show that, although the presence of molecular aggregates on the free base porphyrin DSSC limits the device photocurrent response under illumination, they form an effective hydrophobic barrier against the oxidized electrolyte impeding fast back-electron transfer kinetics. Therefore, their drawback can be overcome by designing dyes with peripheral moieties that prevent the formation of the aggregates and are able to achieve efficiencies as high as 3.2% under full sun.