Abhishek P. Kulkarni

Plasmon-enhanced charge carrier generation in organic photovoltaic films using silver nanoprisms.

We use photoinduced absorption spectroscopy to measure long-lived photogenerated charge carriers in optically thin donor/acceptor conjugated polymer blend films near plasmon-resonant silver nanoprisms. We measure up to 3 times more charge generation, as judged by the magnitude of the polaron absorption signal, in 35 nm thin blend films of poly(3-hexylthiophene)/phenyl-C(61)-butyric acid methyl ester on top of films of silver nanoprisms (approximately 40-100 nm edge length). We find that the polaron yields increase linearly with the total sample extinction. These excitation enhancements could in principle be used to increase photocurrents in thin organic solar cells.

Electron accumulation on metal nanoparticles in plasmon-enhanced organic solar cells.

Plasmonic metal nanoparticles have been used to enhance the performance of thin-film devices such as organic photovoltaics based on polymer/fullerene blends. We show that silver nanoprisms accumulate long-lived negative charges when they are in contact with a photoexcited bulk heterojunction blend composed of poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM). We report both the charge modulation and electroabsorption spectra of silver nanoprisms in solid-state devices and compare these spectra with the photoinduced absorption spectra of P3HT/PCBM blends containing silver nanoprisms. We assign a previously unidentified peak in the photoinduced absorption spectra to the presence of photoinduced electrons on the silver nanoprisms. We show that coating the nanoprisms with a 2.5 nm thick insulating layer can completely inhibit this charging. These results may inform methods for limiting metal-mediated losses in plasmonic solar cells.

Efficient blue organic light-emitting diodes based on an oligoquinoline

An oligoquinoline 6,6′-bis(2,4-diphenylquinoline), with excellent thermal stability and a high glass transition temperature (Tg=133°C), was used as a blue-emitting and electron-transport material to achieve efficient blue electroluminescence. The diodes based on poly(N-vinylcarbazole) hole-transport layer and LiF∕Al cathode had a stable blue emission with a peak at 453nm, chromaticity coordinates at (0.15, 0.12), and a maximum brightness of 925cd∕m2 in air. The external quantum efficiency of 4.1% and luminous efficiency of 3.33cd∕A at 100cd∕m2 are comparable to the best blue organic light-emitting diodes (OLEDs) in the literature. These results demonstrate that oligoquinolines are promising as both blue emitters and electron-transport materials for developing high-efficiency OLEDs with a simpler architecture.

Quantum dot/plasmonic nanoparticle metachromophores with quantum yields that vary with excitation wavelength.

Coupled plasmonic/chromophore systems are of interest in applications ranging from fluorescent biosensors to solar photovoltaics and photoelectrochemical cells because near-field coupling to metal nanostructures can dramatically alter the optical performance of nearby materials. We show that CdSe quantum dots (QDs) near single silver nanoprisms can exhibit photoluminescence lifetimes and quantum yields that depend on the excitation wavelength, in apparent violation of the Kasha-Vavilov rule. We attribute the variation in QD lifetime with excitation wavelength to the wavelength-dependent coupling of higher-order plasmon modes to different spatial subpopulations of nearby QDs. At the QD emission wavelength, these subpopulations are coupled to far-field radiation with varying efficiency by the nanoprism dipolar resonance. These results offer an easily accessible new route to design metachromophores with tailored optical properties.

Phase Transfer of Large Anisotropic Plasmon Resonant Silver Nanoparticles from Aqueous to Organic Solution

We describe the phase transfer of large, anisotropic, silver nanoparticles (approximately 50-100 nm edge length) from water to polar organics such as alcohols, acetone, dimethylformamide and to nonpolar hexanes. We transferred the silver nanoparticles to the polar organic solvents via their precipitation in water by centrifugation and redispersion in organics. Using scanning electron microscopy (SEM) imaging and UV-vis extinction spectra, we confirmed that there was little to no shape change in the nanoparticles upon transfer to the polar solvents. The nanoparticles were stable for months in the polar organics. We also transferred the nanoparticles to hexanes with up to 75% phase transfer efficiency by using sodium oleate as a surfactant. We found the extinction spectra and transmission electron microscopy (TEM) images of the nanoparticles were similar in water and hexanes, indicating that exchange into hexanes resulted in an only slight change in shape. The nanoparticles were stable for at least 10 days in hexanes under appropriate conditions. The phase transfer efficiency decreased with an increase in the size of the nanoparticles. These results open the possibility for the conjugation of large, anisotropic plasmon resonant silver nanoparticles with organic dyes or their blends with conjugated polyelectrolytes for fundamental optical studies and applications.

Plasmonic Enhancement of Raman Scattering in P3HT/PCBM by Silver Nanoprisms

Triangular silver nanoprisms enhance both resonance Raman scattering and fluorescence from the solar cell material P3HT/PCBM. The Raman spectra and enhancement profiles are qualitatively consistent with an electromagnetic enhancement mechanism.