Text Box: Metal nanoparticle interactions with organic polymer solar cell materials

	Organic polymer blends have great potential in photovoltaic devices as low-cost materials that can be deposited readily on large-area flexible substrates.  Typical organic solar cell devices use a semiconducting polymer as the primary light absorbing and hole transport material and  a C60 derivative as the electron transport material.   Power conversion efficiencies of about 5% have been reported for devices based on poly(3-hexylthiophene) (P3HT) and the C60 derivative (6,6)-phenyl-C61–butyric acid methyl ester (PCBM).   These efficiencies are still considerably lower than those achievable with silicon, and a number of strategies for further increasing the efficiencies of such cells are being pursued.  A particularly intriguing one is the incorporation of metallic nanoparticles within the bulk polymer blend or at the electrode interface.   While several mechanisms have been proposed for the enhancement of organic solar cell efficiency by metal nanoparticles, most involve the strong absorption and scattering of light through excitation of surface plasmons, collective oscillations of the conduction electrons.   The plasmon resonance frequencies depend on the size and shape of the particle but typically fall within the visible spectrum for silver and gold.  Absorption of light by metallic nanoparticles can therefore increase the fraction of the solar spectrum absorbed by the cell.   These surface plasmons also produce large local electromagnetic fields near the nanoparticle surface, which have long been exploited in surface-enhanced Raman spectroscopy (SERS).   These large local fields are proposed to increase the absorption of light by the organic materials, increasing overall efficiency.   

	We are using optical and surface enhanced resonance Raman spectroscopies to better understand—and, hopefully, learn how to optimize—the enhancement of organic polymer solar cell efficiency by metal nanoparticles.  

	PEDOT:PSS is a polymer blend widely used to improve the efficiency of charge transfer at the interface between the active polymer layer (e.g. P3HT/PCBM) and the indium tin oxide (ITO) electrode.  Addition of Ag or Au to the PEDOT:PSS layer has been found to enhance solar cell performance.  We find that the addition of the metal nanoparticles enhances the resonance Raman scattering from the organic material and also changes the frequencies and intensities of some of the vibrations.  Addition of silver nanoparticles to chemically reduced PEDOT appears to promote re-oxidation to the useful doped form, suggesting that nanostructured Ag may help protect against de-doping as a damage mechanism.  However, Ag nanoparticles also promote the formation of photoproducts that involve addition of oxygen to the sulfur atoms of the thiophene units, calling into question the long-term stability of silver nanoparticle-containing devices under solar irradiation.  In contrast, the components of the active (light-absorbing and charge-transporting) layer, P3HT/PCBM, do not appear to undergo any chemical or morphological changes in the presence of metal nanoparticles.   The Raman spectra are enhanced by the plasmonically active nanoparticles but are not otherwise changed.  This suggests that the main effect of metal nanoparticles on the active layer is simply to increase the amount of light absorbed through plasmonic enhancement of the electromagnetic field of the light.

Recent publications in this field

Marina Stavytska-Barba and Anne Myers Kelley.  Surface enhanced Raman study of the interaction of PEDOT:PSS with plasmonically active nanoparticles.  J. Phys. Chem. C 114, 6822-6830 (2010).

Marina Stavytska-Barba, Michael Salvador, Abhishek Kulkarni, David S. Ginger, and Anne Myers Kelley.  Plasmonic enhancement of Raman scattering from the organic solar cell material P3HT/PCBM by triangular silver nanoprisms.  J. Phys. Chem. C 115, 20788 (2011).

List of all publications