A. Yasuda

Tandem dye-sensitized solar cell for improved power conversion efficiencies

The power conversion efficiency of dye-sensitized solar cells crucially depends on the ability of the sensitizer to absorb light in a broad range of the solar spectrum. In order to increase this range of absorption, a tandem structure with two different sensitizer dyes in two different compartments of the cell, respectively, was realized. Overall power conversion efficiencies as high as η=10.5% and short circuit current densities of JSC=21.1 mA/cm2 were achieved under air mass 1.5 illumination with red dye and black dye in the upper and lower compartment of the cell, respectively.

Upconversion with ultrabroad excitation band: Simultaneous use of two sensitizers

The authors demonstrate the ability to combine sensitizers effectively working with single emitter in order to increase the excitation window for noncoherent upconversion. They show effective upconversion of the red part of the sun spectrum realized by ultralow excitation intensity (as low as 1Wcm−2) and ultrabroad excitation spectrum (Δλ∼80nm).

On the origin of increased open circuit voltage of dye-sensitized solar cells using 4-tert-butyl pyridine as additive to the electrolyte

A combination of electrical measurements and surface analysis techniques was used to investigate the electronic structure of the TiO2/electrolyte interface and its change upon addition of 4-tert-butyl pyridine to the electrolyte. A reduced interface defect density and a resulting reduction of charge carrier recombination were found to be the main mechanism for an increased open circuit voltage of dye-sensitized solar cells with 4-tert-butyl pyridine in the electrolyte. The reduction of interface defect states was traced back to specific binding of 4-tert-butyl pyridine at defect sites on the TiO2 surface.A combination of electrical measurements and surface analysis techniques was used to investigate the electronic structure of the TiO2/electrolyte interface and its change upon addition of 4-tert-butyl pyridine to the electrolyte. A reduced interface defect density and a resulting reduction of charge carrier recombination were found to be the main mechanism for an increased open circuit voltage of dye-sensitized solar cells with 4-tert-butyl pyridine in the electrolyte. The reduction of interface defect states was traced back to specific binding of 4-tert-butyl pyridine at defect sites on the TiO2 surface.

Two pathways for photon upconversion in model organic compound systems

We have studied the phenomenon of photon upconversion in systems of two model compounds as highly efficient blue emitters sensitized with metallated macrocycle molecules in thin films. The bimolecular upconversion process in these systems is based on the presence of a metastable triplet excited state of the macrocycles giving rise to dramatically different photophysical characteristics relative to the other known methods for photon upconversion such as two-photon absorption, parametric processes, second harmonic generation, and sequential multiphoton absorption. The chosen blue emitter molecules have suitably positioned triplet levels: in the case of an oligofluorine—essentially higher and in the case of diphenylanthracene—lower than the sensitizer porphyrin platinum triplet level and thus two excitation pathways for photon upconversion were observed and investigated.

Upconversion photoluminescence in poly(ladder-type-pentaphenylene) doped with metal (II)-octaethyl porphyrins

We report on the optimization of the upconversion photoluminescence in films of conjugated polymers doped with platinum porphyrin. The upconversion emission was observed to take place at pump intensities as low as 0.5kW∕cm2. Comparison between the photoluminescence integral intensity of polyfluorene and ladder-type pentaphenylene polymer shows a five-fold increase of the efficiency of the upconversion process for the latter. The higher upconversion efficiency can be partially attributed to the reduced reabsorption of the photoluminescence in the case of the ladder-type pentaphenylene matrix.

Direct measurement of increased light intensity in optical waveguides coupled to a surface plasmon spectroscopy setup

The local increase of light intensity in waveguide structures was monitored by a combination of conventional surface plasmon spectroscopy and a dye-sensitized solar cell (DSSC). The sensitized nanoporous TiO2 layer of the DSSC served both as waveguide structure and light absorber in the DSSC. If the conditions for the excitation of guided modes in the TiO2 waveguide structure were fulfilled, increased light intensity in the porous layer was monitored by an increase of short circuit current in the DSSC. Comparison with direct illumination yields an increase of intensity by a factor of 19±6, in good agreement with transfer matrix calculations.

Response to "Comment on 'Two pathways for photon upconversion in model organic compound systems' [J. Appl. Phys. 101, 023101 (2007)]"

We have studied the phenomenon of photon upconversion in systems of two model compounds as highly efficient blue emitters sensitized with metallated macrocycle molecules in thin films. Aggregation between the active molecules (metallated porphyrins and anthracene derivatives) indeed plays a very important role in the triplet-triplet annihilation supported photon upconversion process. Contrary to the suggestions of Steer [J. Appl. Phys. 102, 076102 (2007)] aggregation (followed from phase separation) is a highly undesired effect: external quantum yields of the level of 1%–3% are observed only in systems with a low degree of aggregation (solutions), whereas the solid state films exhibit at least order of magnitude lower upconversion quantum yield than solutions. Additionally, the “only excitons in aggregates model” cannot explain the increase of the linear absorption of the sensitizer at the presence of the emitter. Furthermore, this hypothesis cannot explain why the upconversion quantum yield increases whe...

V2O5 nanofiber‐based chemiresistors for ammonia detection

Vanadium pentoxide (V2O5) nanofibers with a cross‐section of 15 nm2 were obtained by self‐assembly in aqueous solution. These fibers — the length of which can reach 10–15 μm — have been deposited as a thin network on interdigitated electrode structures with 10 μm electrode gap. Exposure to ammonia at room‐temperature changes the conductance of the networks, which makes them useful as chemiresistors for gas‐sensing applications. By evaluating the concentration dependence of the sensor response, we found that the gas adsorption follows the Langmuir adsorption isotherm.