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Effects of size and coverage density of silver nanoparticles (AgPs) on the fluorescence emission and fluorescence lifetime of poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films were investigated. AgPs of 64 nm diameter showed greater effects on the fluorescence decay process of P3HT films as compared with 7 nm AgPs. The fluorescence lifetime (FL) of P3HT decreased from 0.61 to 0.22 ns in the presence of 64 nm AgPs, while no appreciable change (0.60 ns) was seen in the case of 7 nm AgPs. The results suggest that the 64 nm AgPs showed a greater effect on the enhancement of the decay rate of excited P3HT. The photoelectric conversion of thin films consisting of P3HT and phenyl-C61-butyric acid methyl ester (PCBM) was also investigated. AgPs of 7 or 64 nm diameters were first deposited on indium-tin-oxide substrates with controlled surface coverage densities from ~1 to 40%. When the coverage densities of deposited AgPs were ~20% for both 7 and 64 nm, the enhancement of photoelectric conversion efficiency reached maximum. The degree of enhancement in the case of 64 nm AgPs was larger than in the case of 7 nm AgPs.

In the present work silver nanoparticles (cubes and spheres) with sizes of 40 ± 5 nm have been synthesized by a similar experimental method in order to optimize both the photoluminescence and photoelectric conversion enhancement factor of poly(3-hexylthiophene) photoelectrodes with thickness comparable to those used in organic solar cells. The silver nanoparticles were assembled into monolayers with various coverage densities using the Langmuir−Blodgett technique. The dependence of surface plasmon evanescent field and energy transfer has been investigated with the largest enhancement in photoluminescence observed at approximately 14% coverage density of silver nanocubes. We attribute the enhancement to a strong local electric field as well as scattering properties associated with particles of this geometry that translate to significant photocurrent enhancements. The mean photoluminescence lifetimes of the photoelectrode decreased with decreasing silver nanocube spacing and represent an increase in the total decay rate by a factor of 1.24 at 14% coverage of the nanoarrays. The origin of this increase is due to enhancement in either the radiative and/or nonradiative decay rates. ■ INTRODUCTIONRecently, the interaction of noble metal nanoparticles (NPs) with organic materials such as conjugated polymers and dyes has attracted significant attention due to their diverse applications in material, chemical, and biological science. 1−6 The great usefulness of these NPs stems from the ability to fine-tune their physical and optical properties. 4,5,7 For example, metal NPs can adopt a wide range of sizes and shapes (cubes, prisms, rods, cages, etc.) which give rise to a wider range of spectral properties. When excited by light of appropriate wavelength, they exhibit localized surface plasmon resonance (LSPR) due to collective oscillation of their conduction electrons, which in turn induces strong electromagnetic fields on and around the NP surface. 3−7 This generally leads to an enhancement in the radiative and nonradiative electronic properties of the NPs.During the past decade, numerous experimental and theoretical results have revealed large enhancement in light absorption efficiency, 8−12 fluorescence, 12−17 surface-enhanced Raman scattering 3,18−20 (SERS), and photocurrents 1,3,6,21−34 due to the proximal effects of NPs on conjugated polymers and dye systems. The observed enhancements are generally attributed to resonant coupling between radiation and particle plasmons in NPs and the possibility of tuning this resonance to coincide with the electronic transition levels of nearby molecules. The degree of enhancement depends on the oscillating electric field intensity in the vicinity of the metal NPs, whereas the electric field intensity depends on the metal type, size, shape, proximity of neighboring particles, and dielectric environment. 4 Our group recently developed photoelectrodes using gold NPs with monolayers of organic dye molecules and demonstrated that photocurrents could be about 10−50 times enhanced as compa...

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We have investigated the effect of silver nanoparticles (AgPs) on the photocurrent generation of a polyphiophene-fullerene photovoltaic film. Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methylester (PCBM) were used for the electron donor and acceptor, respectively. First, AgPs were electrostatically deposited upon the surface of an indium tin oxide (ITO) electrode via a polycation. Then, a film of P3HT or a mixture of P3HT and PCBM was prepared by spin coating. The thickness of the film was evaluated by atomic force microscopy. Absorption and fluorescence spectral measurements were carried out to investigate the effects of AgPs. Photocurrent spectra were also measured, and the effects of AgPs on photocurrent enhancement were verified.