Probing photoinduced dynamics within dye-sensitised solar cells

Abstract

The low efficiencies of p-type nickel oxide (NiO) dye-sensitised solar cells currently hinder the performance of tandem devices, which combine both dye-sensitised photocathodes and photoanodes to create a multi-junction cell. The maximum potential efficiency of such devices is over 40 %, relative to a limit of 33 % for single-junction devices. Fast charge-recombination has been identified as a major limiting factor of dye-sensitised NiO systems, but there is little correlation between solar cell efficiency and the lifetime of the dye- |NiO+ charge-separated state. This suggests that charge-recombination dynamics are complex with multiple intermediates often observed when these systems are illuminated. In this thesis the photoinduced charge transfer dynamics of a selection of light-absorbing compounds are investigated by ultrafast time-resolved spectroscopy, with the aims of further understanding the processes which occur inside a NiO dye-sensitised solar cell and influencing the design of future systems. Chapter 4 investigates a number of broadly-absorbing push-pull dyes which all form a charge-separated state when adsorbed on NiO. Dyes with boron dipyrromethene (bodipy) acceptor groups perform better in dye-sensitised solar cells when they contain a thiophene bridge and when there is stronger electronic communication between the donor and acceptors. A combination of single-point and global analysis of transient absorption data is used to extract the lifetimes associated with the charge-separated state. Similar techniques are used to investigate dyes with cationic acceptor groups, and it is proposed that interactions between the charged acceptor group and the iodide/triiodide redox couple reduce recombination processes. A key result of this chapter is the characterisation of a bodipy-localised triplet excited state in the infrared region, which is observed for a push-pull dye in dichloromethane but not in acetonitrile or when adsorbed on NiO. i In Chapter 5 a series of bodipy chromophore-polyoxometalate hybrid molecules are presented and the photoinduced dynamics are compared for systems where the metal, linker group, counter cation and bridge length are modified in two solvents. A global analysis method is used to identify the presence of charge-transfer intermediates where the spectra of excited state species overlap. It is found that all structural changes have an effect on the dynamics, suggesting that these systems can be effectively tuned to different applications. Most interestingly, when the solvent is changed from acetonitrile to dichloromethane for a bodipy-polyoxomolybdate, recombination of the charge-transfer excited state occurs via a bodipy-localised triplet excited state, which is again characterised in the infrared region. The relationship between solar cell performance and structure is investigated in Chapter 6 for an extensive series of ruthenium sensitisers with cyclometalating ligands. Small variations in the structure have a large effect on the photophysical properties of the dyes, but less impact on the performance of ruthenium-sensitised titanium dioxide solar cells. Finally, Chapter 7 presents a new molecule for the sensitisation of p-type metal oxides. The design of this dye combines properties which were found to be beneficial in the preceding chapters, including a push-pull structure, spectroscopic handles and simple synthesis. In general the properties of the dye are favourable and a promising internal quantum efficiency of 13 % is achieved for initial dye-sensitised solar cell tests