Dye sensitisation of sol-gel derived titanium dioxide films.

This thesis describes the development and characterisation of dye sensitised Inorganic:Organic (10) heterojunction photovoltaic (PV) cells of the form Au/MPc/TiO2/InSnO2 or F-SnO2 (where MPc= copper phthalocyanine, chloroaluminium phthalocyanine or lead phthalocyanine). The transparent TiO2 films were prepared by Sol-Gel techniques and characterised optically, structurally and electrically. The effects of and interactions between Sol-Gel process parameters have shown that the parameters undergo significant interaction with particular effects on the TiO2 film thickness and thickness related properties obtained during dip coating. The film refractive index n was in the range 2.73-1.81 and wavelength dependent. The conductivity of the films derived from Au/TiO2/InSnO2 structure was 4.26x10[-6]S/cm. E[0] varied from 3.4-3.35eV where delta=2 indicting an indirect allowed transition in 1 layer thick films to E[0] ~3.2eV where delta=3 indicating indirect forbidden transition as the number of coating layers increased.The spectral response, dark and illuminated J(V) and dark C(V) characteristics of the dye sensitised 10 PV cells have been determined. Spectral response indicates that all of the organic dyes studied can be used to sensitise Sol-Gel derived TiO2 into the visible region Corresponding photovoltaic and junction parameters were derived. Photovoltaic effects were observed in all devices studied, however cell efficiencies were poor, in the range (eta~0.0001-0.046%). The low quantum efficiencies were anticipated to be a consequence of the presence of recombination centres at the TiO2/MPc heterointerfaces and the high observed series resistance due to the low conductivity of the MPc films.The MPc/TiO2 junction formed a rectifying contact. C(V) analysis indicated that the junction was electrically abrupt. The dark J(V) characteristics were divided into three regimes, (i) A reverse bias regime in which the device acts as a p-n heterojunction, (ii) an intermediate forward voltage regime, where the derived values of m>2 indicating the presence of a high density of interface states (iii) High forward regime. All devices deviate from the standard diode equation as a consequence of space charge effects in the organic layer, the ideality factors m"2. Temperature dependence measurements of PbPc/TiO2 heterojunctions show that the junction currents are a composite of tunnelling and recombination.PV cell parameters were influenced by changes in ambient conditions, the thickness of organic layers and variations in incident intensity. The J[sc] and V[oc] were proportionally and logarithmically dependent on the incident intensity respectively. The high R[s] is believed to be responsible for the poor cell efficiencies reducing FF, J[sc] and eta. A 30 fold increase in eta was observed when the organic film thickness was reduced from 500nm to 100nm. Exposure to atmosphere reduced eta and is likely to be a consequence of O2 trapping centres.The work has demonstrated the feasibility of a low cost solid state 10 heterojunction photovolatic cell. However improvements in efficiency are required to produce a commercially viable device.