Jane Falgenhauer

Stable Sensitization of ZnO by Improved Anchoring of Indoline Dyes

To establish photovoltaics on a large scale of terawatts (TW), following scenarios of a sustainable energy supply based on renewable sources while avoiding large contributions to CO2 emission by burning of fossil fuels, technologies are needed that are based on abundant elements and that have a small energy payback time. Dyesensitized solar cells (DSCs) have the potential to fulfil these conditions since they consist of either TiO2 or ZnO, which are abundant in the earth crust. Energy payback times of 0.6–0.8 years were calculated for cells based on TiO2, [5] well comparable to the 0.8 years of presentday record CdTe cells. DSCs based on ZnO as semiconductor instead of TiO2 provide further potential to reduce this time because they can be electrodeposited from aqueous electrolytes without additional annealing steps and therefore allow cell preparation based on light-weight, low-energy polymer substrates. For ZnO, however, the classical established dyes, such as substituted Ru-trisbypyridine complexes, are chemically too aggressive and new sensitizer molecules have to be established. 7] Among the dyes tested as sensitizers for ZnO, an indoline dye (D149) has proven to reach the highest efficiencies in such cells (5.6 %). However, D149 is not stably fixed at the ZnO surface but is dissolved from the electrode when in contact to standard electrolytes. Herein, we study differently substituted indoline dyes with additional carboxylate anchoring groups. We show that these dyes (Scheme 1) strongly bind to the ZnO surface and that the stability of the cells can be increased while maintaining their efficiency. For our experiments, we used fluorine-doped tin oxide (FTO) films on glass onto which we first deposited a compact ZnO blocking layer (~1 mm) by polarizing the electrode at 1.06 V (vs. Ag/AgCl) in an O2-saturated aqueous solution of ZnCl2 (5 mm) with KCl (0.1 m) as supporting electrolyte for 10 min. Following this, eosin Y (50 mm) was added as a structure-directing agent and a porous ZnO layer was deposited at 0.96 V (vs. Ag/AgCl) for 20 min; eosin was then removed in a pH 10.5 KOH solution. The dyes D149, DN216 or DN285 were then adsorbed from 0.5 mm solutions in acetonitrile/tert. butanol (1:1 by vol.) at room temperature for 10 min. To obtain a measure of the energetic position of the electron in the excited state of the sensitizer, we measured the redox potentials of the molecules dissolved in dimethylformamide (DMF). The potential of the first reduction was used as a measure of the LUMO (lowest unoccupied molecular orbital) energy which further is assumed to be characteristic for the energy of an electron in the first excited singlet state. Figure 1 a shows the results of cyclic voltammetry (CV) measured at a Pt sheet in contact with the dye solutions. Beyond an inert potential window ( 0.3 V to + 0.7 V), clear waves were observed for the reduction and oxidation of the molecules. The redox characteristics of DN216 and DN285 to a large extent resembled those of D149. To obtain a safe assignment of the observed waves to changes in the HOMO (highest occupied molecular orbital) or LUMO population of the chromophore system (Table 1), which is decisive for the injection characteristics of a sensitizer, spectroelectrochemcial experiments were performed (Figures 2 and 3). The spectra were measured at constant potentials subsequent to a peak in the CV to obtain a sufficiently large signal (Figure 1 a). The absorption of the sensitizers measured in DMF (Figure 1 b) showed a peak maximum at 392 nm, assigned to a p!p* transition, and the main peak of internal charge transfer in the visible range at lmax = 534 nm, which is observed for all dyes independently of substitution. The extinction coefficient was determined to be e= 46700, 77600 and

Peripheral ligands as electron storage reservoirs and their role in enhancement of photocatalytic hydrogen generation

The contrasting early-time photodynamics of two related Ru/Pt photocatalysts with very different photocatalytic H2 generation capabilities are reported. Ultrafast equilibration (535 ± 17 fs) creates an electron reservoir on the peripheral ligands of the ester substituted complex, allowing a dramatic increase in photocatalytic performance. This insight opens the way towards a novel design strategy for H2 generating molecular photocatalysts.

Charge transfer at organic-inorganic interfaces—Indoline layers on semiconductor substrates

We studied the electron transfer from excitons in adsorbed indoline dye layers across the organic-inorganic interface. The hybrids consist of indoline derivatives on the one hand and different inorganic substrates (TiO2, ZnO, SiO2(0001), fused silica) on the other. We reveal the electron transfer times from excitons in dye layers to the organic-inorganic interface by analyzing the photoluminescence transients of the dye layers after femtosecond excitation and applying kinetic model calculations. A correlation between the transfer times and four parameters have been found: (i) the number of anchoring groups, (ii) the distance between the dye and the organic-inorganic interface, which was varied by the alkyl-chain lengths between the carboxylate anchoring group and the dye, (iii) the thickness of the adsorbed dye layer, and (iv) the level alignment between the excited dye ( π*-level) and the conduction band minimum of the inorganic semiconductor.

Characterization of porphyrin nanorods on fluorine doped tin oxide glass sheet

Porphyrin nanorods (PNR) have been fabricated by electrostatic self-assembly of two oppositely charged porphyrin molecules. The free base meso-tetra-(4-phenylsulphonate) porphyrin (TPPS4) served as negatively charged counterpart for the positively charged metallo meso-tetra(4-N-methylpyridyl) porphyrins (MTM’PyP) with either Sn, Co, Mn or In as central metal M. Films of PNR were prepared on fluorine doped tin oxide glass sheets (FTO) by using a drop-dry method. The electronic spectra revealed J-aggregation of the charged molecules for the colloid PNR as well as for the films. Transmission electron microscopy confirmed the formation of porphyrin nanorods. The laser microscope and scanning electron microscope (SEM) images of the PNR/FTO films showed the formation of three kinds of structures in the films which consist of differently branched or linear needles with their main axis grown in the direction of the solvent flow during preparation. During cyclic voltammetry either applying negative potentials from 0.0 V to -1.0 V or positive potentials from 0.0 V to +2.2 V irreversible reduction or oxidation reactions were detected for the films. Consistently, SEM images taken following cyclic voltammetry showed the disintegration of the PNR on the films into smaller subunits. Spectroelectrochemical measurements showed the formation of porphyrin anionic radicals during oxidation by a decrease in the absorption intensities and broadening of spectra with an additional band appearing around 900 nm. A similar trend was observed when negative potentials were applied but in this case the cationic radical was produced. In both cases the decrease of the intensity of the J-aggregate confirms a loss of intermolecular coupling, again consistent with the smaller subunits observed in SEM analysis.