Broadening spectral response in solid-state dye-sensitized solar cells via Forster Resonance Energy Transfer

Dye-sensitized solar cells (DSSCs) are a promising renewable energy technology. However, in the 19 years since the dye solar cell was first popularized by O'Regan and Gratzel there has been little improvement in efficiency. A major hurdle to increasing the efficiency of both solid and liquid dye-sensitized solar cells is the fact that these cells can only convert light in a relatively narrow spectral region. For example, the most popular dye used in DSSCs, N-719 dye only responds to wavelengths of light between 350--650 nm. Its competitor, crystalline silicon will convert light all the way to 1100 nm. While DSSCs can have conversion efficiencies near unity for the light which is absorbed, the fact that they are unable to process a large fraction of the solar spectrum creates a fundamental limit on their overall efficiency.;Without a broader spectral response, DSSCs cannot achieve efficiencies high enough to displace other more expensive technologies. While many different solar dyes have been synthesized in the past two decades, the most efficient cell realized to date, possessing an 11.5% conversion efficiency, still uses a dye with a structure very similar to that used in the original Gratzel cell published in 1991 and has the same spectral response in the range from 350--650 nm. Thus far, efforts to increase efficiency by extending the response of dyes have been unsuccessful. It seems that there is a tradeoff between having a high absorption coefficient and having a broad absorption range. Many of the strongly absorbing red and NIR dyes have narrow response windows which are insensitive to much of the blue and green light. The problem becomes more significant when we consider the case of solid state dye sensitized solar cells (SDSCs). The active layer of an SDSC is required to be very thin due to transport limitations of the low-mobility hole transport materials used. In order to completely absorb light using such a thin layer, we are restricted to using only the aforementioned high absorbing dyes with narrow spectral response windows.;The question then becomes "what can we do given the materials available to us?" The obvious solution is to employ two or more sensitizers with complementary absorption spectra which can together create a solar cell with a broader spectral response. Historically, the most successful examples of this concept have been dye cocktails, in which two or more dyes are absorbed onto titania and work in parallel. To date, however none of these dye cocktails has demonstrated a performance better than if the best constituent dye was used alone.;The focus of this dissertation is a newly developed approach which relies on Forster Resonance Energy Transfer (FRET). In this scheme two dyes work in series instead of in parallel. A short wavelength absorbing donor material dispersed in the hole transport material is coupled with a long wavelength absorbing acceptor material anchored to the surface of titania. The donor material absorbs the short wavelength light, becomes excited, and transfers this excitation to the acceptor dye. In addition to the energy transferred from the donor, the acceptor also absorbs the long wavelength light matching with its own absorption spectra, which would be its only energy source in the absence of the donor material. Combining the materials in this way overcomes several of the losses inherent in the dye cocktail system, and because the donor and acceptor are not in physical contact they are insensitive to many of each other's properties.;FRET has the potential to overcome current efficiency limitations for SDSCs yet, at the inception of this work, FRET had not yet been demonstrated in SDSCs. The purpose of this dissertation is to develop and demonstrate efficient FRET systems, and indentify appropriate donor and acceptor materials. Herein, efficient working systems are demonstrated using off-the-shelf laser dyes as donors and commercially available solar cell dyes as acceptors.