Photo-charge Generation Efficiency Research Articles

Photosensitization of novel ruthenium-functionalized photoconductive polymers: Effect of ruthenium complex as photosensitizer

Abstract Owing to their efficient photo-charge generation, ruthenium derivatives are emerging as an important class of photosensitizers. Despite their promising characteristics, several deficiencies, such as the relatively low quantum yield and short lifetime of the lowest energy metal-ligand charge transfer, have limited their applications. Here, we present new photoconducting polymers that contain ruthenium complexes as a photosensitizer. Ru(phen) 2 (m-COOH)(PF 6 ) 2 complexes, which covalently bind the poly( N -vinyl carbazole) (PVK) side chain, provided a high quantum yield and sufficient lifetime of the lowest energy metal-ligand charge transfer state for photo-charge generation. A series of PVK–Ru(phen) 2 (m-COOH)(PF 6 ) 2 complexes were thus prepared with various concentrations of Ru. We employed a helium-neon (HeNe) laser (633 nm) and a xenon lamp to measure the photo-charge generation efficiency and photoconductivity of the studied complexes. The wavelength dependence of the photo-charge generation efficiency and photoconductivity was found to be similar to their metal-ligand charge transfer absorption spectra. This suggests that the Ru complexes can extend the photosensitivity of the conductive polymers to longer wavelengths due to the lowest metal-ligand charge transfer energy. The photo-charge generation efficiency and photoconductivity of these polymers was found to be greater than those of PVK–Ru-complex composites.

Enhanced photorefractive performance of polymeric composites through surface plasmon effects of gold nanoparticles.

We investigated the photorefractivity enhancement of polymeric composites by introducing gold nanoparticles (NPs). The gold NPs enhance the photocharge generation rate of sensitizers through plasmon resonance coupling achieved between NPs and sensitizers. Systematic studies show that the presence of gold NPs has increased photocharge generation efficiency, photoconductivity, diffraction efficiency, refractive index modulation, and photorefractive (PR) grating formation rate. The gold-NP-doped PR sample exhibits 2 times larger photocharge generation efficiency and photoconductivity, and 2.5 times faster PR grating formation rate compared to the control sample without the NPs.

Type-II core/shell nanoparticle induced photorefractivity

We demonstrate engineering the photocharge generation efficiency of nanoparticles on the nanometer scale by using a type-II band-gap structure. Compared to bare CdSe cores, the dispersion of type-II core/shell nanoparticles in photorefractive polymer led to an average 100% increase in photocurrents. An improvement to the refractive-index construction time, and a near 100% enhancement to the two beam coupling net-gain coefficients and four-wave mixing internal diffraction efficiencies have been achieved at moderate biases.

Ionization Potential Dependence of Photoconductive Properties of Polysiloxanes Bearing Hole-Conductors Sensitized with C60

Photo-charge generation efficiencies and photoconductivities were measured at 632.8 nm in a series of polysiloxanes bearing hole-conductors doped with C60 as sensitizer. Generation and transport of mobile carriers were strongly dependant on the charge-transfer (CT) complex formed between the hole conducting polymer and the sensitizer. We investigated the dependence of the ionization potential of hole conducting polymer on the photoconductive properties of the CT complex. The composite containing polymer with lower ionization potential had the highest photo-charge generation efficiency and photoconductivity.

Determination of the glass transition temperature of polymeric photorefractive composite from photo-charge generation efficiency measurements

In summary, it has been shown that DSC is not an appropriate technique for determining theTg of polymeric PR composite, since the transitions observed in the DSC curves are weak and relatively broad. Thus, an alternative method has been proposed, consisting on studying the temperature dependence of photo-charge generation efficiency and identifyingTg with the temperature at which tendency of photocharge generation efficiency has reversed.

Temperature Dependence on the Grating Formation in a Low-TgPolymeric Photorefractive Composite

We investigated a dependence of the grating formation on the temperature in polymeric photorefractive (PR) composite, in terms of magnitude and buildup speed of the PR grating. For polymeric PR materials, the temperature is one of the most important factors together with the external electric field because it is closely related on photocharge generation efficiency, mobility of generated carrier, electro-optic coefficient tensor, and so on. Above the glass transition temperature, the diffraction efficiency of degenerate four-wave mixing decreased with increasing the temperature; it can be explained with the magnitude of space-charge field and the electro-optic behavior at different temperatures. The space-charge field decreased linearly with increasing temperature due to a decrease in the photocharge generation efficiency and an increase in the hole detrapping by the high dark conductivity. Also as we expected, the PR grating buildup speed, which is strongly dependent on the photoconductivity, steeply decreased with increasing the temperature, and its tendency was similar to the temperature dependence of the phase shift.

Photoconducting polymers containing covalently attached ruthenium complexes as a photosensitizer

We have prepared photoconducting polymers that have ruthenium (Ru) complexes as photosensitizers. Ru complexes were introduced to utilize their high quantum yield and the long lifetime of the lowest energy metal–ligand charge transfer (MLCT) state for photo-charge generation. Ru(bpy)2(m-COOH-4-methylbpy)(PF6)2 was covalently attached to poly (N-vinylcarbazole) (PVK) using Friedel–Crafts acylation. Ru(bpy)2[m-(oxo-PVK)bpy](PF6)2 was prepared to have various weight percentages of Ru complexes. We have measured photo-charge generation efficiency and photoconductivity of PVK-Ru complexes with 632.8nm He–Ne laser and xenon lamp of the visible region. The wavelength dependence of photo-charge generation efficiency and photoconductivity was similar to their 3MLCT absorption spectra. It was expected that Ru complexes would extend the photosensitivity of polymers to the region of longer wavelengths through the presence of their lowest MLCT energy. The photo-charge generation efficiency and photoconductivity of these polymers was found to be greater than that of PVK: Ru-complex composites.

The influence of photosensitizers on the photorefractivity in poly[methyl-3-(9-carbazolyl)propylsiloxane]-based composites

We investigated the photosensitizers effect on the photorefractive (PR) properties in five poly[methyl-3-(9-carbazolyl)propylsiloxane] (PSX-Cz)-based PR composites which were doped with various photosensitizers having each different electron affinity, such as 2,4,5,7-tetranitro-9H-fluorine-9yilden malonitrile (TeNFM), 2,4,7-trinitro-9-fluorenone (TNF), 9-dicyanomethylene-2,4,7-trinitro-fluorenone (TNFM), tetracyanoethylene (TCNE), and 7,7,8,8-tetracyanoquinnodimethane (TCNQ). At 632.8 nm, photo-charge generation efficiencies, photoconductivities, space charge field, four wave mixing diffraction efficiencies, and PR grating buildup times were measured as a function of external electric field. The photo-charge generation, which is dependent on the light absorption, was achieved through the charge transfer (CT) complexes between the PSX-Cz and each of the photosensitizers. The photon energy of the CT transition decreased with increasing electron affinity of the photosensitizer. In composites doped with TeNFM, TNF, and TNFM, the space charge field ( E sc) increased as the photo-charge generation efficiency increased; the grating buildup in these composites is rate-limited by the photo-charge generation speed. In sample doped with TCNE, and TCNQ, the hole mobility was reduced due to the larger amount of photosensitizer anion traps produced by photoreduction of the photosensitizer. Then, the grating buildup speed became hole mobility limited, and smaller buildup rates were observed. The magnitude of space charge field was sustained as the charge and trap density increased. In all composites, the refractive index modulation is increased with the magnitude of space charge field.

Temperature dependence of photo-charge generation of polymeric photorefractive composite in the glass transition temperature region

For photo-charge generation of polymeric photorefractive (PR) composites, temperature is important factor with electric field, but no study for temperature dependence of photo-charge generation efficiency ( φ) at the glass transition temperature ( T g) region has been done yet. In this work, we investigate the temperature dependency of φ of a low T g PR composite. With increasing temperature the photo-charge generation efficiencies increased below T g but decreased above T g. This behavior could be attributed to temperature dependence of heat capacity and the dielectric constant of PR composite. The space charge field was strongly dependent on temperature and increased with increasing φ.

Dependency of the Photo-Charge Generation Efficiency on the Photoconductivity and on the Space-Charge Field in Photorefractive Polymeric Composites

Space-charge field is considered as an important factor determining photorefractive efficiency of polymer composites. Recently, we proposed a simple method which can measure the space-charge field built in photorefractive materials. Six polymeric photorefractive composites with different electron acceptor as a photosensitizer were prepared to vary the photo-charge generation efficiency and consequently to vary the space-charge field only. We investigated the dependency between photo-charge generation efficiency, photoconductivity, and space-charge field.

Photogeneration, charge transport, and photoconductivity of a novel PVK/CdS-nanocrystal polymer composite

We report on the photoconductive characteristics of an inorganic:organic hybrid composite in which PVK serves as a polymeric charge-transporting matrix and quantum dots composed of surface passivated cadmium sulfide serve as a charge-generating sensitizer. The PVK:CdS nanocomposites prepared are directly compared with a similar composite composed of PVK and C 60, an extensively studied system due to its promising characteristics. We demonstrate the possibility of tuning the band-gap of these sensitizing nanocrystals through careful discretion in the method of synthesis such that their spectral response can be adjusted to suit a particular wave-length of operation. In this way materials can be fabricated which posses photoconductive characteristics exceeding those observed in the PVK:C 60 system. In this paper results concerning photosensitivity will be presented and the Onsager formalism developed for organic systems will be employed in order to extract the parameters r 0 and Φ 0 from the photocharge generation efficiency data.