Daniel P. Hagberg

How the Nature of Triphenylamine-Polyene Dyes in Dye-Sensitized Solar Cells Affects the Open-Circuit Voltage and Electron Lifetimes

Three donor-linker-acceptor triphenylamine-based cyanoacrylic acid organic dyes used for dye-sensitized solar cells (DSCs) have been examined with respect to their effect on the open-circuit voltage (V(oc)). Our previous study showed a decrease in V(oc) for DSCs based on dyes with increased molecular size (increased linker conjugation). In the present study, we investigate the origin of V(oc) with respect to (i) conduction band (E(CB)) positions of TiO(2) and (ii) degree of recombination between electrons in TiO(2) and electrolyte acceptor species at the interface. These parameters were studied as a function of dye structure, dye load, and I(2) concentration. Two types of behavior were identified: the smaller polyene dyes show a surface-protecting effect preventing recombination upon increased dye loading, whereas the larger dyes enhance the recombination. How the different dye structures affect the recombination is discussed in terms of dye surface blocking and intermolecular interactions between dyes and electrolyte acceptor species.

A light-resistant organic sensitizer for solar-cell applications.

Finely tuned: A stable dye-sensitized solar cell that contains a molecularly engineered organic dye has been prepared. The efficiency of the cell remains at 90% after 1000 h of light soaking at 60 degrees C. The remarkable stability of the cell is also reflected in the open-circuit voltage value (V(oc)), short-circuit photocurrent-density value (J(sc)), and the fill factor, which also show barely no decline (see picture).

Hydration of lanthanide chloride salts: A quantum chemical and classical molecular dynamics simulation study

We present the results of a quantum chemical and classical molecular dynamics simulation study of some solutions containing chloride salts of La(3+), Gd(3+), and Er(3+) at various concentrations (from 0.05 to 5 M), with the purpose of understanding their structure and dynamics and analyzing how the coordination varies along the lanthanide series. In the La-Cl case, nine water molecules surround the central La(3+) cation in the first solvation shell, and chloride is present only in the second shell for all solutions but the most concentrated one (5 M). In the Gd(3+) case, the coordination number is ∼8.6 for the two lowest concentrations (0.05 and 0.1 M), and then it decreases rapidly. In the Er(3+) case, the coordination number is 7.4 for the two lowest concentrations (0.05 and 0.1 M), and then it decreases. The counterion Cl(-) is not present in the first solvation shell in the La(3+) case for most of the solutions, but it becomes progressively closer to the central cation in the Gd(3+) and Er(3+) cases, even at low concentrations.

Electronic and molecular structures of organic dye/TiO2 interfaces for solar cell applications: a core level photoelectron spectroscopy study

The electronic and molecular properties of three organic dye molecules with the general structure donor-linker-anchor have been investigated using core level photoelectron spectroscopy (PES). The molecules contain a diphenylaniline donor unit, a thiophene linker unit, and cyanoacrylic acid or rhodanine-3-acetic acid anchor units. They have been investigated both in the form of a multilayer and adsorbed onto nanoporous TiO(2) and the experimental results were also compared with DFT calculations. The changes at the dye-sensitized TiO(2) surface due to the modification of either the donor unit or the anchor unit was investigated and the results showed important differences in coverage as well as in electronic and molecular surface properties. By measuring the core level binding energies, the sub-molecular properties were characterized and the result showed that the adsorption to the TiO(2) influences the energy levels of the sub-molecular units differently.

Distance and driving force dependencies of electron injection and recombination dynamics in organic dye-sensitized solar cells.

A series of dyes based on a triphenylamine donor and a rhodanine acetic acid anchor/acceptor for solar cell application has been studied with regards to electron injection and recombination kinetics using femtosecond transient absorption. The series contains three dyes, with estimated electron transfer distances ranging from 17.2 to 11.0 Å, and which have shown significant differences in energy conversion efficiencies. The injection and recombination kinetics were studied in the NIR region where electrons in the conduction band of the TiO(2) are suggested to absorb. For all dyes, the injection rate is larger than (200 fs)(-1) which implicates a quantitative injection efficiency. Surprisingly, the subsequent recombination reaction has a rate that increases with increasing linker length. On the other hand, this behavior is consistent with the concomitant decrease in driving force for this series of dyes. Moreover, the lifetimes show exponential distance dependence when corrected for driving force and reorganization energy, which indicates a superexchange interaction between the electrons in TiO(2) and the radical cations of the dyes. A dependence on probe wavelength of the attenuation factor was found, giving a β value of 0.38 Å(-1) at 940 nm and 0.49 Å(-1) at 1040 nm. The difference is suggested to be due to the difference in electronic coupling between fully separated dye cations and injected electrons versus geminate electron-hole pairs. Addition of tert-butylpyridine, which from previous work is known to give a substantial drop in the IPCE values for the studied dyes, was found to decrease the amount of long-lived electrons in the TiO(2) without affecting the injection rate.

Mapping the frontier electronic structures of triphenylamine based organic dyes at TiO2 interfaces

The frontier electronic structures of a series of organic dye molecules containing a triphenylamine moiety, a thiophene moiety and a cyanoacrylic acid moiety have been investigated by photoelectron spectroscopy (PES), X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES) and resonant photoelectron spectroscopy (RPES). The experimental results were compared to electronic structure calculations on the molecules, which are used to confirm and enrich the assignment of the spectra. The approach allows us to experimentally measure and interpret the basic valence energy level structure in the dye, including the highest occupied energy level and how it depends on the interaction between the different units. Based on N 1s X-ray absorption and emission spectra we also obtain insight into the structure of the excited states, the molecular orbital composition and dynamics. Together the results provide an experimentally determined energy level map useful in the design of these types of materials. Included are also results indicating femtosecond charge redistribution at the dye/TiO(2) interface.