Effect of magnetron sputtered semiconducting TiO2 layers on the performance of dye sensitized solar cells

Semiconducting titanium dioxide (TiO2) is a key component of dye sensitized solar cells (DSC). Apart from devices based on organic hole conductors the current embodiment of the DSC employs nanocrystalline TiO2 films that are applied directly onto the transparent conducting glass support, i.e. a fluorine doped tin dioxide (FTO) film. The mesoporous structure of the TiO2 layer exposes the FTO glass to the redox electrolyte increasing the dark current and reducing cell performance in particular at low light levels. Introducing a compact layer between the FTO glass and the TiO2 nanocrystals should block this undesirable electron flow from the FTO to the tri-iodide ions in the electrolyte. It also allows the use of redox systems that show reversible electrochemical behavior on the FTO glass. The authors report on the deposition of dense TiO2 layers by reactive Pulse Magnetron Sputtering, a high rate PVD technique. The crystal structure and composition of the compact underlayer was found to exhibit a decisive effect on the photovoltaic response of the films. The cells were tested in an AM 1.5 spectral mismatch corrected solar simulator over a wide range of different light intensities. Importantly, these tests revealed that the diode ideality factor of the DSC improved significantly in the presence of the TiO2 underlayer. The loss in the open circuit photovoltage was 60-70 mV upon reducing the light intensity by a factor of ten, indicating the ideality factor of the diode to be between 1 and 1.2 as compared to about 1.2 - 1.4 for the unprotected FTO electrode. As a consequence the DSC maintains its photovoltaic performance well at low light levels. A fill factor of 76 % and an efficiency of about 7 % were obtained even at the low light intensity of 10 W/m(exp 2). Thus the advantage of using the compact underlayer emerges especially at low irradiation levels due to the higher VOC and to the better ideality factor of the photo-diode. The results indicate that the additional dense TiO2 layer improves the performance of the solar cell significantly in particular under indoor light conditions