Pawel Wagner

Porphyrins for dye-sensitised solar cells: new insights into efficiency-determining electron transfer steps

Porphyrin molecules offer immense potential as the light harvesting component of dye-sensitised nanocrystalline TiO(2) solar cells. Synthetic porphyrin dyes were amongst the first dyes trialled for sensitisation of inorganic semiconducting oxides. Today, they exhibit the best performance reported for dye-sensitised solar cells. Accompanying the significant performance improvement over the last two decades is a much improved understanding of efficiency-determining fundamental electron transfer steps, from charge photogeneration to recombination. In this feature article we highlight our recent discoveries of the influence of porphyrin molecule structure on efficiency determining electron transfer kinetics and device performance by systematically changing the molecular structure and observing electron injection and recombination kinetics using time-resolved optical and electrical probes. Despite our observation of ultrafast charge injection for all porphyrin dyes studied by transient absorption spectroscopy, the injection yield estimated using an internal standard remains below 100% and depends strongly on the molecular structure. The observed discrepancy between kinetic competition and the injection yield is attributed to non-injecting dyes, probably arising due to inhomogeneity. A very interesting sub-ns (0.5 ns to 100 ns) charge recombination channel between photo-injected electrons and porphyrin cations is observed, which is found to be more prominent in free-base porphyrin dyes with a conjugated linker. Charge recombination between the acceptor species in the redox containing electrolyte and injected electrons is shown to be an important limitation of most porphyrin-sensitised solar cells, accelerated by the presence of porphyrin molecules at the TiO(2)-electrolyte interface. This recombination reaction is strongly dependent on the porphyrin molecular structure. Bulky substituents, using a porphyrin dimer instead of a porphyrin monomer, a light soaking treatment of freshly prepared films and co-sensitization of TiO(2) with multiple dyes are shown to be successful strategies to improve electron lifetime. Finally, new developments unique to porphyrin dye-sensitised solar cells, including performance enhancements from a light exposure treatment of a zinc porphyrin dye, a significant performance improvement observed after co-sensitisation of TiO(2) with free-base and zinc porphyrin dyes and the use of porphyrin dimers with increased light harvesting in thin-film TiO(2) solar cells are described.

Zn−Zn Porphyrin Dimer-Sensitized Solar Cells: Toward 3-D Light Harvesting

Zn-Zn porphyrin dimers have been incorporated into thin dye-sensitized solar cells (DSSCs) to boost their light harvesting efficiency. The photoexcited dimers show efficient and fast electron injection into TiO(2) indicating that both photoexcited chromophores contribute to current generation. The improved light harvesting ability coupled to enhanced DSSC performance demonstrates the potential of 3-D light harvesting arrays as next generation light harvesters for artificial solar energy conversion systems.

Direct exfoliation of graphite with a porphyrin – creating functionalizable nanographene hybrids

Exfoliation of graphite was achieved using a free-base porphyrin 1 resulting in an efficient fabrication of single-layer nanographene (NG)- hybrid platelets that can be further functionalized with other nanomaterials. The novel nanographene-porphyrin hybrids reveal efficient charge transfer in the excited state.

Determining the orientation and molecular packing of organic dyes on a TiO2 surface using X-ray reflectometry.

The determination of the orientation and molecular density for several porphyrin dyes adsorbed on planar TiO(2) surfaces using X-ray reflectometry (XRR) is reported. Adsorption of nanoscale water layers occurred rapidly upon exposure of freshly prepared TiO(2) surfaces to ambient conditions; however, this was successfully eliminated, resulting in clearly discernible adsorbed dye layers for sensitized surfaces. Adsorbed dye orientations, determined from computations constrained by the measured dye layer thickness, were calculated to have a binding tilt angle of 35°-40°. Combining the XXR data with the orientation models indicates that the porphyrins form densely packed surfaces with an intermolecular spacing of 3-4 Å, consistent with π-π stacking interactions. Changes in the molecular size of probe dyes were reflected in corresponding changes in the measured dye layer thickness, confirming the ability of this technique to resolve small variations in dye layer thickness and consequently adsorption orientation. Application of these results to understanding the behavior of dye-sensitized devices is discussed.

Improved performance of porphyrin-based dye sensitised solar cells by phosphinic acid surface treatment

Chemical surface treatment of porphyrin-sensitised titania films using bis-(4-methoxyphenyl)phosphinic acid after dye adsorption, results in large improvements in DSSC efficiencies which originate primarily from higher short circuit currents. The result was attributed to a positive shift in the TiO2 quasi-Fermi level with simultaneous retardation of charge recombination. High device performances have been achieved even using simplified electrolyte matrices devoid of the common additives, LiI and t-butylpyridine.

Linker conjugation effects in rhenium(I) bifunctional hole transport/emitter molecules

Spectroscopic, electrochemical and density functional theory (DFT) methods have been employed to investigate a group of [Re(CO)(3)(HT)(phen)](+) complexes (phen = 1,10-phenanthroline), and in particular the level of electronic communication between various hole-transporting (HT) ligands and the rhenium centre. Here, the HT ligand consists of a coordinating pyridine connected to dimethylaniline group through a single-, double- or triple-bond-connecting system. Electronic absorption, resonance Raman, and steady-state emission spectroscopy combined with lifetime studies and DFT calculations suggest that multiple dpi(Re)-->pi*(phen) metal-to-ligand charge transfers (MLCTs) exist for each complex, two of which significantly absorb at about 340 and 385 nm, and one that emits at approximately 540 nm. In the complexes containing more-conjugated HT ligands, non-emissive intraligand transitions (IL(HT)) exist with energies between the ground and MLCT excited states. The overlap of these IL(HT) transitions and the absorbing MLCT of lowest energy deactivates emission resulting from about 385 nm excitation, and lowers the quantum yield and excited-state lifetimes of these complexes. Cyclic voltammetry experiments indicate that throughout the series investigated, the highest occupied molecular orbital (HOMO) of each complex is centred on the HT ligand, while the occupied molecular orbitals localised on the rhenium are lower in energy.

Remarkable synergistic effects in a mixed porphyrin dye-sensitized TiO2 film

A remarkable 300% efficiency enhancement driven by a matching increase in the short circuit current was observed in a mixed porphyrin dye-sensitized solar cell constructed from two dyes in a 3:1 ratio. Absorbed photon-to-current conversion efficiency measurements indicate an improved charge injection yield for both dyes in the mixture. Several possible origins for the observed performance enhancement are discussed.

Enhanced performance of dye-sensitized solar cells using carbazole-substituted di-chromophoric porphyrin dyes

The purpose of this work is to investigate the origin of improved photovoltaic performance of a series of di-chromophoric carbazole-substituted porphyrin dyes employed as sensitizers in dye-sensitized solar cells. Five di-chromophoric zinc porphyrin dyes with the same porphyrin core, a carbazole unit attached in the meso-position through a phenylethenyl linkage, and substituents spanning a range of electron affinities, in an attempt to tune the electronic level of the carbazole unit, have been synthesized (CZPs). Density functional theory (DFT) calculations predicted the nature of the electronic transitions observed in the CZP systems, showing a large degree of orbital mixing. In contrast, UV-vis absorption, resonance Raman spectroscopy and differential pulse voltammetry investigations suggested negligible interaction between the porphyrin and carbazole chromophores. Carbazole substitution led to a moderate increase in photon absorption intensity within the ∼300 nm to 400 nm wavelength region, a smaller but broader Soret band absorption and slightly increased photon absorption intensity in the 550 nm to 650 nm Q band region. Despite the rather small changes in light harvesting and negligible changes in the HOMO/LUMO electronic levels, the photovoltaic performance of the new dyes is increased by as much as 30% compared to the single chromophore Zn porphyrin dye 5-(4-(2-cyano-2-carboxyethenyl)phenyl-15-phenyl-10,20-bis(2,4,6-trimethylphenyl)porphyrinato zinc(II) (ZP1), leading to over 6% power conversion efficiencies (PCEs). Both open circuit voltage (VOC) and short circuit current (JSC) have increased. The increased VOC is attributed to increased electron lifetimes due to a steric blocking effect. Analysis of the increased short circuit current (ΔJSC) showed that only less than 10% of ΔJSC originates from increased light absorption under simulated air mass 1.5 illumination, while the rest of the improvements are attributed to a steric effect enhancing the electron injection efficiency. These results suggest that developing non-conjugated multichromophoric dyes can lead to simultaneous increases in both the photocurrent and the photovoltage of dye-sensitized solar cells.

Towards Hydrogen Energy: Progress on Catalysts for Water Splitting

This article reviews some of the recent work by fellows and associates of the Australian Research Council Centre of Excellence for Electromaterials Science (ACES) at Monash University and the University of Wollongong, as well as their collaborators, in the field of water oxidation and reduction catalysts. This work is focussed on the production of hydrogen for a hydrogen-based energy technology. Topics include: (1) the role and apparent relevance of the cubane-like structure of the Photosystem II Water Oxidation Complex (PSII-WOC) in non-biological homogeneous and heterogeneous water oxidation catalysts, (2) light-activated conducting polymer catalysts for both water oxidation and reduction, and (3) porphyrin-based light harvesters and catalysts.

Tuning the optical properties of ZnTPP using carbonyl ring fusion.

Zinc meso-tetraphenylporphyrin (ZnTPP) was modified in such a way to allow the effect of an asymmetric structural distortion on its optical properties to be investigated. This involved the fusion of a phenyl group to an adjacent pyrrole ring via a carbonyl bridge. With the aid of Density Functional Theory (DFT) and time-dependent DFT (TD-DFT) calculations it was found that the asymmetric distortion away from planarity induced by the carbonyl fusion resulted in a loss of degeneracy in the two lowest unoccupied molecular orbitals (LUMOs). The effect was a red shift of the electronic absorbance bands, an increased Q:B ratio from 0.046 in ZnTPP to 0.096 in the fused derivative, and the appearance of additional UV-vis peaks. This study therefore suggests that structural distortions, as well as electronic substituents may be used to alter absorbance spectra, a technique which is of interest in the design of light-harvesting dyes.