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CeNS Colloquium

Place: Kleiner Physik-Hörsaal, Geschwister-Scholl-Platz
Date: 07.05.10, Time: 15:30 h

The influence of nanostructure and electronic interfaces in solid-state dye-sensitized solar cells

Prof. Henry J. Snaith
Photovoltaic and Optoelectronic Device Group, University of Oxford

The conventional liquid electrolyte based dye-sensitized solar cell (DSC) can generate between 11 to 12% solar to electrical power conversion efficiency. This level of power conversion efficiency is high enough for commercialisation, however successful commercialization is still challenging due to difficulties of scaling up thin films devices containing corrosive liquids. Our research is focussed on hybrid concepts for dye-sensitized solar cells which employs an organic hole-transporter in place of the iodide based electrolyte. For this solid-state concept power conversion efficiencies of around 5% are achievable, however significant further improvements are foreseeable by enhancing our understanding and control of the photovoltaic conversion process. In this seminar I will give an introduction to dye-sensitized solar cells, and then present a few different areas of our recent research.

To extend the light absorption in the solar cells we sensitize the system with different dyes with complementary absorption spectra. We observe a significant increase in the device efficiency for the cosensitized device, but the increase in photocurrent is even greater than that expected from the enhanced light harvesting alone. Through a spectroscopic study, we find that a rapid energy transfer (~200ps) from wide band gap to low band gap co-adsorbed dyes facilitates significantly improved efficiency in this system. This mechanism opens a second parallel channel for charge generation, which significantly increases to likelihood of electron transfer to the metal oxide and opens new possibilities for dye design.

TiO2 is usually employed as the electron transporter in solid-state DSCs. Here we investigate replacing TiO2 with SnO2. For SnO2 we observe exceptionally high photocurrents and good power conversion efficiencies are observed. We find the performance very sensitive to the nature of the surface treatment of the SnO2 electrode, but with optimized treatments open-circuit voltages of up to 0.8V and power conversion efficiencies of 3% have been achieved.

Traditionally, small molecules have shown superior performance in solid-state DSCs as compared to hole-conducting polymers, such as PEDOT. Here we present a mechanistic investigation into the operation of solid-state DSCs employing poly(3-hexylthophene) as the hole-transporter. We find that infiltration of the polymer into mesoporous films of up to 2 microns thickness is surprisingly easy. Interestingly, the pore walls are wet by the polymer, but the pore filling fraction is as low as 15 to 25%. Despite the thin surface layer within which charges can percolate, we estimate up to 98% charge collection efficiency through micron thick films. The external quantum efficiency is only in the order of 50% however, suggesting major losses in the charge generation step exist. Overall power conversion efficiencies of around 3% for this polymer based SDSC make this competitive with the molecular hole-transporter based concept.