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Preparation process for titanium dioxide layer during the construction of solar cell
Preparation of the titanium dioxide layer
The deposition of the TiO2-layer will be done by screen printing. The modification of the titanium dioxide will be anatase.
A printing cycle will comprise the following steps:
(i)With the screen in the upright position, the mesh openings will be filled with the TiO2-paste. Then the screen will be placed on top of the FTO-glass substrate and a squeegee will be used to press the paste through the mesh openings.
(ii)The TiO2 will be kept at room temperature for 5 minutes to decrease the mechanical tension within the layer.
(iii)The layer will be dried at 120°C on a hot plate for 10 minutes to evaporate volatile components of the paste. Then the dried TiO2-layers will be fired at high temperature to burn the organic components of the TiO2-paste and to improve the mechanical contact of the nanoporous TiO2 particles. The following observations are important to be noted:
(i)The maximum temperature should not exceed 550°C because the phase transition from anatase to the thermodynamically more stable rutile starts at this temperature.
(ii)The heating rate should be very slow. Initially the layer contains a lot of organic material, which decomposes in the temperature interval 200 °C < T < 350 °C. The decomposition process induces mechanical tension into the TiO2-layer.
(iii)The cooling rate of the sintered TiO2-electrode also needs to be slow in order to minimize the tension within the glass substrate and the TiO2-layer.Prior to the staining of the electrodes, the TiO2-films will be immersed into a 0.05M solution of TiCl4 in water for 30 minutes at 70 °C and rinsed with water and ethanol afterwards. After the dip-coating, the TiO2-electrodes will be fired for 30 minutes at 450 °C again.After the second heat treatment, the TiO2 electrodes will either be immediately sensitized or stored in a box. If the electrodes are stored, they will again be heated to 450 °C for 30 minutes prior to sensitization to avoid the adsorption of pollutants (CO2, dust, etc). Before optical characterization, the TiO2 film will be characterized for the film thickness.