Surface-plasmon-enhanced photoconversion in organic photovoltaics (2009)

In this thesis, the benefits of including surface-plasmon-active materials into organic photovoltaics are investigated. First, the effect of discontinuous silver thin-films formed by physical vapor deposition at the transparent front electrode of the device is explored. A reproducible near doubling in efficiency is seen in these devices which arises from a near doubling of the short-circuit current. Analysis of the wavelength-dependence of the increase in current shows that the increase in current is due to surface-plasmon-enhanced optical absorption in the active layer of the devices. Additionally, these results are shown to be reproducible over several trials when using a fabrication routine that employs a low-temperature annealing step that retains the surface-plasmon activity of the substrate and prevents delamination of the active layers. The relative dielectric function of the active-layer material was determined at optical frequencies using variable-angle spectroscopic ellipsometry. A Huang-Rhys vibronic progression is used to model the peak energies of excitonic transitions in the film and the resulting parameters are found to be in excellent agreement with previously reported values. Theoretical calculations of the surface-plasmon enhancement are performed using the aforementioned dielectric function. The theoretical calculation of the skin depth of the surface plasmon is shown to be consistent with the observed wavelength dependence of the plasmonically enhanced current in organic photodiodes. In order to better understand the enhancement process and the fate of photogenerated holes and electrons, additional work was done to explore the electronic structure of the organic films using impedance spectroscopy. The results of this work indicate the presence of a Schottky diode at the metal/organic interface in standard device geometries. This result has several implications on charge extraction for standard devices and those including silver thin-films. It is also observed that including silver particles in the active layer of the device leads to a strong increase of the static dielectric constant of the active layer, leading to strong recombination. Successfully using colloidal silver particles to enhance the photocurrent in organic photovoltaic devices will likely require better control of their surface chemistry in order to limit recombination on their surfaces. These results are explored and discussed in detail. Transient photoconductivity measurements were used to further probe the effect of a Schottky diode on charge extraction and charge recombination. The resulting data is unprecedented in the literature and indicates several issues with the approach of using silver thin films at the front transparent electrode ultimately limiting the maximum efficiency increase that can be obtained using this approach with this material system. In a different organic photovoltaic system that does not exhibit space charges, higher increases in the conversion efficiency can probably be obtained. The work presented in this thesis represents the first attempts at enhancing photoconversion in organic bulk heterojunction photovoltaics. The results are put into an experimental framework that explores not only the optical properties of the films, but the electronic properties as well. From the work presented in this thesis, it is clear that the future for surface-plasmon enhanced photoconversion in organic photovoltaics is favorable.