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Integrating a Triplet-triplet Annihilation Up-conversion System to Enhance Dye-sensitized Solar Cell Response to Sub-bandgap Light

The poor response of dye-sensitized solar cells (DSCs) to red and infrared light is a significant impediment to the realization of higher photocurrents ...

The University of Sydney

Photochemical upconversion is performed, whereby emitter triplet states are produced through triplet energy transfer from sensitizer molecules excited with low energy photons. The triplet emitter molecules undergo triplet-triplet annihilation to yield excited singlet states which emit upconverted fluorescence. Experiments comparing the 560 nm prompt fluorescence when rubrene emitter molecules are excited directly, using 525 nm laser pulses, to the delayed, upconverted fluorescence when the porphyrin sensitizer molecules are excited with 670 nm laser pulses reveal annihilation efficiencies to produce excited singlet emitters in excess of 20%. Conservative measurements reveal a 25% annihilation efficiency, while a direct comparison between the prompt and delayed fluorescence yield suggests a value as high as 33%. Due to fluorescence quenching, the photon upconversion efficiencies are lower, at 16%.

On the efficiency limit of triplet-triplet annihilation for photochemical upconversion.

Luminescent solar concentrators promise to reduce the cost of solar energy, but are hindered by a number of losses. Escape of luminescence through the large waveguide-air interfaces can be attenuated through alignment of the optical transition dipole of the luminophore along the waveguide surface normal, directing the maximum possible proportion of luminescence into waveguide modes. We demonstrate such alignment using a guest-host dye-doped liquid crystal sandwiched between conductive glass slides. Application of a potential while illuminating through a narrow edge caused a drop in the intensity of luminescence escaping the large surfaces, and an increase in the intensity of light escaping the narrow edges of the system. This is explained in terms of alignment of the transition dipoles of the dye. We discuss implementation in a luminescent solar concentrator.

Towards an aligned luminophore solar concentrator.

Four new hetero- and homo-leptic iridium(III) bisterpyridine complexes have been prepared which incorporate aniline (tpy-φ-NH(2)), benzoic acid (tpy-φ-COOH), and benzyl alcohol (tpy-φ-CH(2)OH) substituents at the 4' positions of the tpy ligands (tpy = 2,2':6',2''-terpyridine, φ = phenylene). The electrochemical behaviour and ground and excited state spectroscopic properties of the complexes are reported, and the X-ray crystal structures of a homoleptic benzyl alcohol [Ir(tpy-φ-CH(2)OH)(2)](PF(6))(3), homoleptic aniline [Ir(tpy-φ-NH(2))(2)](PF(6))(3), and heteroleptic benzyl alcohol/aniline substituted complex [Ir(tpy-φ-CH(2)OH)(tpy-φ-NH(2))](PF(6))(3) have been solved. Complexes with aniline substituents were found to display absorption bands at around 430 nm corresponding to intraligand charge transfer (ILCT) that are sensitive to changes in solvent and pH. Strong emission in the visible region involving the ILCT state is observed in two of the complexes (Φ(e) = 0.7% and 2.6%) in acetonitrile. In the heteroleptic aniline/benzyl alcohol complex the Stokes shift is shown to be linearly related to solvent polarisability according to the Lippert equation, but only for solvents with weak hydrogen bonding interactions. Additionally, in water, emission from the ILCT state is quenched and only weak ligand centred (LC) emission is observed. The long lifetimes and quantum yields of these complexes make them interesting candidates for probes in sensing applications, especially [Ir(tpy-φ-CH(2)OH)(tpy-φ-NH(2))(2)](PF(6))(3) due to its unusual sensitivity to the solvent environment.

Photophysical properties of a new series of water soluble iridium bisterpyridine complexes functionalised at the 4' position.

Conventional photochemical upconversion (UC) through homo-geneous triplet-triplet annihilation (TTA) is subject to several enthalpic losses that limit the UC margin. Here, we address one of these losses: the triplet energy transfer (TET) from the sensitizer to the emitter molecules. Usually, the triplet energy level of the emitter is set below that of the sensitizer. In our system, the triplet energy level of the emitter exceeds that of the sensitizer by ∼600 cm(-1). Choosing suitable concentrations for the sensitizer and emitter molecules, we can exploit entropy as a driving force for the migration of triplet excitation from the sensitizer to the emitter manifolds. Thereby we obtain a new record for the peak-to-peak TTA-UC energy margin of 0.94 eV. A modified Stern-Volmer analysis yields a TET rate constant of 2.0 × 10(7) M(-1) s(-1). Despite being relatively inefficient, the upconverted fluorescence is easily visible to the naked eye with irradiation intensities as low as 2 W cm(-2).

Entropically driven photochemical upconversion.

We report the synthesis and ultrafast excited-state dynamics of two new meso-meso, β-β, β-β triply fused diporphyrins, Zn-3DP and Pd-3DP. Both compounds were found to have short excited-state lifetimes: Zn-3DP possessed an average S1 lifetime of 14 ps before nonradiative deactivation to the ground state, whereas Pd-3DP displayed a longer average S1 lifetime of 18 ps before crossing to the T1 state, which itself possessed a very short triplet lifetime of 1.7 ns. The excited-state dynamics of Zn-3DP, compared to similar zinc(II) diporphyrins reported in the literature, suggests that a conical intersection of the S1 and S0 potential energy surfaces plays a major role as a deactivation pathway of these molecules. Furthermore, the short triplet lifetime of Pd-3DP, compared to other diporphyrins that also exploit the intramolecular heavy atom effect, reveals that the position of the heavy atom within the diporphyrin framework influences the strength of spin-orbit coupling. The implications for employing triply fused diporphyrins as NIR-absorbing triplet sensitizers are discussed.

Synthesis and ultrafast excited-state dynamics of zinc and palladium triply fused diporphyrins.

We describe here the synthesis of azide-functionalised iridium(III) bisterpyridines using the "chemistry on the complex" strategy. The resulting azide-complexes are then used in the copper(I)-catalysed azide-alkyne Huisgen 1,3-dipolar cycloaddition "click chemistry" reaction to from the corresponding triazole-functionalised iridium(III) bisterpyridines. The photophysical characteristics, including lifetimes, of these compounds were also investigated. Interestingly, oxygen appears to have very little effect on the lifetime of these complexes in aqueous solutions. Unexpectedly, sodium ascorbate acid appears to quench the luminescence of triazole-functionalised iridium(III) bisterpyridines, but this effect can be reversed by the addition of copper(II) sulfate, which is known to oxidize ascorbate under aerobic conditions. The results demonstrate that iridium(III) bisterpyridines can be functionalized for use in "click chemistry" facilitating the use of these photophysically interesting complexes in the modification of polymers or surfaces, to highlight just two possible applications.

Yuen Yap Cheng

School of Chemistry

The University of Sydney

Integrating a Triplet-triplet Annihilation Up-conversion System to Enhance Dye-sensitized Solar Cell Response to Sub-bandgap Light