| dc.description.abstract | Nitrogen doped Ti02 photoelectrodes have been found to be more sensitive to visible
light spectrum with a narrower band gap [Asahi et. al. 2001], This increases the
absorbance of the nitrogen doped films suitable for use in a solar cell [Lindgren et. at.
2003]. Nitration therefore improves light harvesting in the solar cell [Asahi et. al. 2001,
Mwabora et at. 2004]. The effect of Ti02 properties like surface area of the film,
porosity, crystal sizes, transmittance and reflectance, optical and electrical properties on
the efficiency of a cell which are vital to quantify the performance of a solar cell have
been studied.
Ti02 films were made by the pressing method and subsequently doped with nitrogen by
heat-treatment in nitrogen, oxygen and argon gas mixture. Nitrogen gas ratio (if» in the
doping procedure was varied from 0 to 0.024. X-ray diffraction analysis of the films
showed that Ti02 films prepared with if> < 0.008 had a mixture of anatase and rutile
phase, beyond if> = 0.008 anatase phase became prominent, therefore nitrogen induced a
rutile to anatase phase transitions. From scanning electron microscopy it was observed
that the particle sizes of nitrogen doped films reduced with increase of if> from an average
of 36nm to 18nm therefore the film became more compact and reduced in porosity.
Optical measurements revealed that film transmittance improved and reflectance reduced
in the visible light spectrum with nitrogen doping.
Photoeectrochemical measurements showed a large anodi2 shift of the onset potential for
the cathodic scans of the order of 0.24V, electron-hole recombination via nitrogen
induced states in the band gap of Ti02 was suggested to be the-cause of such shifts in the
anodic photocurrent. The peak currents for the nitrogen doped Ti02 photoelectrodes
r • •
increased with increase in if> and decreased with increaseIn heat treatment temperature,
which was attributed to the decreased effective surface area in the films consistent with
porosity and SEM analysis.
Overall energy conversion efficiency, short circuit current and open circuit voltages were
found to depend on nitrogen ratio. The cell fabricated with nitrogen doped Ti02
photo electrode had an overall efficiency of 0.70 % while the undoped had efficiency of
0.96 %. Higher <J> resulted in decrease in film porosity and solar cell efficiency. Nitrogen
induced states in the band gap are also thought to increase with increase in <J> that may act
as a recombination centre for the electron-hole pairs. Ohmic losses in the conducting
glass support and the cell contacts could also contribute to the low efficiency, short
circuit currents and photovoltages.
Fabrication of Ti02 films by pressing method utilizes simple tools and is a promising
technology for mechanization where a high throughput is required. Nitrogen doping
narrows Ti02 band gap by introducing conduction states within the band gap of Ti02. The
influence of such states to the electron loss mechanisms should be investigated. Thicker
Ti02 films are produced by pressing method which improves light harvesting by the film
[Mikhelashvili and Einstein, 2001] however, electron-hole recombination losses may be
introduced with use of thicker films. Therefore film thickness need to be optimized in the
pressing technique. | en |
