Takahiro Osasa

Solid-State Dye-Sensitized Solar Cells Using Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] as a Hole-Transporting Material

Solid-state dye-sensitized solar cells (DSSCs), consisting of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (MEHPPV) as a hole-transporting material, an indium tin oxide (ITO) electrode, and a titanium dioxide (TiO2) electrode, were studied with respect to the factors affecting their properties. The morphology of TiO2 electrodes, which was controlled by changing preparation conditions, affected the performance of solid-state DSSCs, and cells with a TiO2 electrode having a smooth surface showed better properties. The carrier transportation between the TiO2 layer and the MEHPPV layer was one of the most important factors that determined the overall efficiency of the solar cells. This carrier transportation process was improved by the addition of an interlayer consisting of potassium iodide (KI) and iodine (I2). In addition to improving the carrier transportation, this KI/I2 blend interlayer also improved the pore filling of the solid-state DSSC. By controlling these parameters, a solid-state DSSC was obtained, with a short-circuit current density of about 1.51 mA/cm2, an open circuit voltage of about 0.65 V, a fill factor of about 0.5, and an energy conversion efficiency of about 0.51%.

Photocurrents generated under forward biases in organic thin-film solar cells with organic heterojunction structure

The mechanism of photocurrent generation for organic solar cells with a heterojunction formed between metal phthalocyanine and C60 (or a perylene derivative) was studied. Photocurrent was observed under both reverse and forward biases. Under reverse biases, photocurrent action spectra showed that excitons generated in both organic layers contribute to photocurrent, whereas under forward biases, the excitons generated in only one of them contributed to photocurrent. Forward photocurrent was attributed to the electron transfer reaction between excitons and charge carriers accumulated at the organic/organic junction, the charge carriers depending on the rate of carrier injection from the electrodes. Forward photocurrent showed quantum efficiencies higher than 100% under very weak irradiation, and was attributed to current multiplication at the organic/electrode interface. Since this phenomenon is closely associated with the interfacial structure, it can be used as a measure of interface quality.

CuOx Films as Anodes for Organic Light-Emitting Devices

CuOx films were prepared on glass plates by the vacuum sublimation of Cu films followed by an oxygen-plasma treatment. These CuOx films showed a strong hole-injection ability and found to be applicable to organic light-emitting devices as anodes, although their optical transmittance and conductivity were slightly lower than those of indium–tin-oxide layers. The most important merit of employing the films as the anodes lies in that they are easily shaped in patterns by vacuum sublimation using shadow masks in device fabrication.

Organic Electroluminescence Devices Based on Mixture of Aluminum–Hydroxyquinoline Complex and Compound with Low Dielectric Constant

The quantum efficiency and lifetime of fluorescence of an aluminum–hydroxyquinoline (Alq3) complex in solution increased with decreasing polarity of the solvent. Similarly, the fluorescence quantum efficiency and lifetime of Alq3 films were increased by mixing the complex with a compound with a low dielectric constant. These results suggested that the nonradiative decay process of Alq3 is retarded in less polar media. The quantum efficiency and lifetime of fluorescence of Alq3 films were 20±2% and 17 ns, respectively. When Alq3 was mixed with a less polar substance in the film state, the quantum efficiency and lifetime were increased to 25±2% and 20 ns, respectively. These results indicate that the nonradiative decay process in the film state is also retarded as the polarity decreases. Using a film consisting of Alq3 and a compound with a low dielectric constant as the light emitting layer of organic electroluminescence devices, the luminance efficiency was increased by about 20%.

Formation of new bulk-heterojunction structure in organic thin film solar cells

Organic solar cells were fabricated by stacking aromatic amine and C60 layers. The energy conversion efficiency of these solar cells was low because of poor photoabsorption by these layers and short diffusion length of excitons. However, the photocurrent density was increased by about 3 times by the application of heat treatment to the stacked organic layers at about 140 °C, and the maximum energy conversion efficiency reached 1.1% under AM 1.5, 100 mW/cm2 simulated solar light. The effect of the heat treatment was attributed to the infiltration of the amorphous aromatic amine compound into grain boundaries of the microcrystalline C60 layer. From observation by electron microscopy, the mixed form of these two compounds near the interface was found to be suited to solar cells because the C60 and aromatic amine phases wedge each other in a direction normal to two electrodes.