57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

Organic semiconductor blends based on the bulk heterojunction approach have been studied intensely as materials for photovoltaic devices. These materials are attractive candidates for solar cells because they can be produced inexpensively in large quantities and deposited onto flexible substrates. However, even the best conjugated polymer-based devices still fall short of crystalline silicon solar cells in performance. The long-term stability of organic materials also remains an issue for their use in outdoor applications.

This project was inspired by recent reports that the power conversion efficiencies of solar photovoltaic devices can be improved by incorporating plasmonically active metal nanostructures into the device. Local enhancement of electromagnetic fields by plasmonically active nanostructures manifests itself in a number of other ways including surface enhanced Raman scattering (SERS). Standard electromagnetic SERS theory holds that the Raman scattering enhancement is simply the product of the enhancements of the local electromagnetic field intensities at the incident and Raman scattered wavelengths. Therefore, the magnitude of the SERS enhancement should provide a reasonable measure of the electromagnetic field enhancement, and the SERS experiment can be carried out in an actual device structure or closely related geometries. In addition, any structural changes in the molecules nearest to the metal surface are manifested as changes in the positions and/or relative intensities of the peaks in the SERS spectrum compared to the unenhanced Raman spectrum. This structural sensitivity allows for examination of chemically specific effects of metal nanoparticles on the organic materials which would also be expected to affect the performance of photovoltaic cells.

We performed a SERS study of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), a polymer blend often used as a hole collecting contact in organic solar cells. Many of the device geometries reported to display power conversion efficiency enhancement have the metal nanoparticles in direct contact with PEDOT:PSS. In the oxidized form used, PEDOT:PSS is essentially transparent in the visible and increased light absorption should not be an available enhancement mechanism. We observed significant changes in the spectrum of the PEDOT component in the presence of metals, particularly Ag. These were interpreted to show that at low light intensities, silver nanoparticles promote the re-oxidation of chemically reduced PEDOT and may therefore protect against the formation of nonconducting dedoped regions of PEDOT:PSS coated ITO. At higher light intensities there was evidence for silver-induced photooxidation of PEDOT by addition of oxygen to the thiophene rings, disrupting the pi-conjugation.

We then examined films of poly(3-hexylthiophene)/[6,6]-phenyl C61-butyric acid methyl ester (P3HT/PCBM), a widely studied polymer blend used in bulk heterojunction organic polymer solar cells. The polymer blend was deposited over triangular silver nanoprisms. These nanoprisms have many desirable properties including large electromagnetic field enhancements, broad tunability of their plasmon resonances across the visible spectrum, and self-assembly without aggregation onto silanized glass surfaces. Furthermore, there is direct spectroscopic evidence for enhanced charge-carrier generation in this system. We used the SERS enhancements to measure the extent of electromagnetic field enhancement caused by the nanoparticles, and also to examine any changes in the Raman spectral characteristics that might indicate effects of the metal on the chemistry or morphology of the nearby organic material.

Resonance Raman spectra were obtained at excitation wavelengths ranging from 364 to 633 nm, both in the presence and absence of triangular Ag nanoprisms with varying plasmon resonance frequencies. For ~35 nm polymer films deposited over nanoprisms, the nanoprisms enhance the sample-averaged Raman scattering intensities by factors of 2 to 20, depending on wavelength and nanoprism density. The weak blend fluorescence is enhanced by approximately the same factor as the Raman scattering, implying negligible excited-state quenching by the metal. The Raman peak positions and relative intensities are unaffected by the presence of either the triangular nanoprisms or spherical Au or Ag nanoparticles, indicating negligible morphological or chemical changes to the P3HT in contrast to our previous observations on PEDOT:PSS. The magnitudes of the Raman enhancements are qualitatively consistent with the Ginger group's observed enhancements in charge-carrier (positive polaron) yields for P3HT/PCBM deposited over silver nanoprisms.

PRF-ND funding resulted in two peer-reviewed publications in J. Phys. Chem. C and several conference presentations. One of them led to a collaboration with the group of David Ginger at the University of Washington; he and two of his postdocs are co-authors on one of our publications. My graduate student Marina Stavytska-Barba was supported in part as a research assistant on this grant and is the first author on both publications. She defended her Ph.D. dissertation during summer 2012, becoming UC Merced's fourth chemistry Ph.D. A new graduate student, Joshua Baker, worked during summer 2012 on developing a sample scanning system for our Raman microscope to improve the collection of Raman data from these thin-film samples without photodegradation.