Mitsuyoshi Saito

Photoelectric Properties of Copper Phthalocyanine Langmuir-Blodgett Film

Photovoltaic and structural characteristics of Langmuir-Blodgett film photocells have been studied using asymmetrically substituted copper phthalocyanine (asy-CuPc). By polarized-light absorbance measurements, each phthalocyanine molecule was found to be arranged in the film with its ring facing towards the dipping direction. The polarity of photoelectric characteristics showed typical features expected for an organic p-type Schottky diode, while the dark characteristics exhibited a reversed polarity. The maximum short-circuit photocurrent was on the order of 10-8 A/cm2 for 100 µW/cm2 monochromatic light, indicating the superiority of asy-CuPc as a film-forming molecule for photovoltaic LB films.

Origin of the In-Plane Anisotropy in Langmuir-Blodgett Films

An extended model of flow orientation has been developed for analyzing the inplane anisotropy in Langmuir-Blodgett (LB) films by taking into account the viscoelastic nature of a condensed monolayer at the air-water interface as a Bingham plasticity. The model is based on a two-dimensional fluid and a line-sink as representing the monolayer and the substrate, respectively. The predictions from the present model with respect to the dichroic behavior have been experimentally verified employing a mixed LB system of a merocyanine dye and a fatty acid. The model is also consistent with the remarks from ESR line-shape analysis reported previously and those from analysis of the optical absorption spectra using the extended dipole theory. These results show that the inner stress due to the velocity gradient in the monolayer during the deposition is the origin of the in-plane anisotropy.

Thermochromic behaviour of merocyanine Langmuir-Blodgett films

Abstract A kinetic study of the thermal decay of the red-shifted bands of dye-containing Langmuir-Blodgett films was carried out for three different merocyanines. The decay process was traced by repeated application of a cycle consisting of spectral measurements, heating at a constant temperature for a given time and then cooling to room temperature. For one of the three dyes the red-shifted band was found to be restored on cooling, while for the other two the spectral changes were quenched. This irreversible decay behaviour is characterized as a first-order rate process with values of the frequency factor τ −1 0 and the activation energy W of 10 11 –10 15 s −1 and 1.1−1.3 eV respectively. The values of W are much smaller than the photon energy hv ∼ 2 eV for the absorption maxima of the bands or the dissociation energy of the chemical bonds, indicating that the present decay process is a steric rearrangement of chromophores.

Effect of dye concentration in Langmuir multilayer photoconductors

Abstract The behaviour of anisotropic photoconductivities is studied in Langmuir films of rhodanine containing merocyanines with three different donor nuclei (benzoxazole (DO), benzothiazole (DS) and benzoselenazole (DSe) diluted with arachidic acid. DS and DSe films, which possess a J-like aggregate, share a common feature in the spectral shape of their photoconductivities; the J-like band is effective only in Δσ‖ and only at higher concentrations. The spectral shape for DO remains invariant for different concentrations. The magnitude of Δσ‖ distinguishes DSe from the others; Δσ‖ for DSe increases monotonically with concentration up to values a decade or more greater than those for DO and DS, which tend to saturate towards higher concentrations. The greater Δσ‖ for DSe disappears after heat treatment, as does the J-like band. These results suggest the important role of molecular coordination in the photoconduction process.

Analytical model of flow dichroism in Langmuir-Blodgett films

Abstract A model of flow orientation has been developed for analyzing the dichroism in Langmuir-Blodgett (LB) films by introducing a two-dimensional fluid and a sink, which represent the monolayer and a substrate, respectively. The angular distribution function derived using the velocity gradient given by the ideal-fluid approximation satisfactorily explains the data of dichroism in the mixed LB films of a dye and a fatty acid prepared with different velocities. The results are consistent also with those of the line-shape analysis of ESR spectrum reported previously.

Modification of optical and photoelectric characteristics by vapour phase treatments in Langmuir-Blodgett films of merocyanine dyes

Abstract The effects of acid and basic vapour phase treatments (AT and BT respectively) have been studied on Langmuir-Blodgett films thicker than 9 monolayers of a merocyanine (DI) and its 6-CH 3 , substituted product (DII). Optical measurements indicate that the red-shifted (J-like) band structure, which is dissociated on AT as well as by heat treatment, can be restored by applying BT. A polymorphism is seen for DI among the as-deposited and the restored J-like structures. The absence of polymorphism for DII indicates a stereospecific role of the 6-CH 3 group in the chromophore aggregation. The lateral photoconductivity Δσ | is controlled in a range of nearly two decades by these treatments and their combinations. The largest Δσ | is attained by a successive application AT → BT.

Stable radical species in Langmuir-Blodgett films of merocyanine dyes studied by electron spin resonance

ESR measurements were performed on Langmuir-Blodgctt (LB) films of merocyanine dyes, diluted with arachidic acid. Stable signal due to two radical species was detected and the spectra showed clear anisotropy both for the in-plane and out-of-plane of the film. Measurements at X- and Q-bands revealed that the in-plane structure of the spectra arises from the hyperfine splitting of a nitrogen nucleus of the dye molecule. Based on this fact, the anisotropic line shape was analyzed by a computer simulation and the angle between the preferred direction of p π orbital axis of the nitrogen atom and the dipping direction of the film was determined to be 60° to 65° in the film plane. A possible correlation between the obtained angle and the structure of the dye aggregates is discussed in relation to the intermolecular charge transfer model for the origin of the radicals.

Photoelectric effects in heterojunction Langmuir-Blodgett film diodes☆

Abstract Photoelectric characteristics of heterogeneous Langmuir-Blodgett (LB) film diodes have been studied using mixed monolayers of surfactant derivatives of semiconducting dyes and arachidic acid (C 20 ). The current-voltage and capacitance-voltage curves measured were qualitatively the same as those of amorphous dye devices. Evaluation of the photoelectron decay range showed that the dye chromophore modifies the LB films to act as electron transport layers. A simple model is proposed, which assumes the formation of energy bands at the chromophore-containing interfaces and shows an analogy between the LB film diodes and amorphous organic devices. With this model, the loss of photoelectrons at an unmodified interface of C 20 is evaluated as 28% and 52% for the hydrophobic and the hydrophilic regions, respectively.

Photoelectric effects in dye-sensitized Langmuir- Blodgett film diodes

Abstract Schottky and p-n junction diodes were fabricated from Langmuir-Blodgett films based on merocyanine and triphenylmethane dye derivatives. The photovoltage was observed and the spectral dependence of the photocurrent was measured. The diodes exhibit essentially the same behaviour as the corresponding evaporated dye film diodes in spite of the presence of inert hydrocarbon chains in the dye-sensitized Langmuir-Blodgett films.

Molecular p-n Junction Photodiodes of Langmuir Multilayer Semiconductors

The electrical and photoelectric characteristics of Langmuir-Blodgett film heterojunctions have been studied using organic p-n diodes consisting of monolayers of four kinds of merocyanine (MX) dyes stacked on monolayers of distearyl paraquat (PQ). A rectifying effect observed in the dark indicates that the PQ and MX monolayers act as n-type and p-type regions, respectively. The photocarriers are identified as electrons generated in the photosensitive MX monolayers. Comparison between different types of MX dyes suggests that laterial motion of carriers occurs in the p-n junctions, and that the acceptor nuclei, i.e., rhodanine in MX, play a dominant role in determining the diffusion potential with respect to the PQ n-type region.