This thesis involves studies of dye sensitization of dye-sensitized solar cell (DSC)
devices using ultra-fast dyeing and ultra-fast desorption. The work includes studies of adsorption isotherms, uptake kinetics and the development of a new method for dye adsorption, desorption and re-dyeing.
The development of a new method of adsorption, desorption and re-dyeing of devices using different desorption solutions with the aim of developing precise dye loading control for different sensitizers. Initially the adsorption of the Ru(II) complex (N719) and the organic dye (SQl) have been studied on Ti0₂ films and the data modelled using Langmuir and Freundlich isotherms. The adsorption data were best represented by the Langmuir model for N719 and by the Freundlich model for SQ 1. A new method of ultra-fast dyeing, desorption and re-dyeing has been developed which enables much greater control over dye loading than is possible by passive dyeing. It has been shown that one device can be dyed, desorbed and re-dyed use more than ten times giving the similar performance to the first dyeing. Different alkaline solutions have been tested for
desorbing N719 from Ti0₂ such as LiOH, NaOH, KOH, Bu₄NOH and tris-(hydoxymethy)ethylamine prior to device re-dyeing. Ultra-fast desorption by Bu₄NOH and re-dyeing with the same dyes and also with different dyes has been successfully achieved. The method has been developed to control dye loading by partial dye removal using varying volumes or concentration of Bu₄NOH. A new procedure for the selective removal of dyes has also been developed; N719 by LiOH, SQl by dilute Bu₄NOH and D149 by concentrated Bu₄NOH followed by acetone and ethanol. This method has been used to change dyes on the surface and also to quantify dye adsorption in DSC devices.
Ultra-fast co-sensitization of Ti0₂ with more than two dyes (N719 plus various organic dyes) using sequential or multiple dyeing on P25 and commercial DSL18NR-T Ti0₂ films have been also studied. Sequential ultra-fast co-sensitization with new yellow triphenylamine dye (YD) and N719 yielded η 7.5% which is higher than the individual dyes. Multiple dyeing from a mixed 6% SQ1:N719 solutions gave η 7.1% which is also higher than for the corresponding individual dyes. The best ratio for mixing N719:D149 is 1:3 v/v which yielded
η= 8.2%. Co-sensitization with three dyes and four dyes has been also studied and the devices show improved efficiencies compared to the respective individual dyes.
The rates of adsorption of dyes from mixed dye solutions were studied on TiO₂ using the selective desorption methods developed in chapter three. The kinetics of the adsorption of yellow triphenylamine dye (YD) were studied using ultra-fast and passive dyeing. The results show that adsorption of YD by passive dyeing follows a pseudo second order kinetic model with a rate constant of 9.8 x 10^-5 cm2.μg^{- 1}.min^-1 with the TiO₂ surface reaching saturation in ca. 350 min. However, ultra-fast dyed YD followed a pseudo-first order model with a rate constant of 11.16 x 10^-2 min^-1 and the surface was reached saturation in 10 min. The experimental data for the ultra-fast co-adsorption of mixed YD: N719 solutions shows YD data can be modelled by both pseudo first order and pseudo second order models but that N719 best fitted with a pseudo first order model. The kinetics adsorption of mixed 5% SQ1:N719 solutions also were studied and
the results show both dyes followed both pseudo-first order and pseudo second order models. The effect of dyeing time on the IV device data have also been studied showing that efficiency and photo-current increased with increasing dye loading presumably reflecting higher electron injection into the TiO₂ conduction band.