Background
Dye sensitised solar cells (DSSC) are one of the promising avenues that are being explored to harvest the sun's energy. They have been the subject of much research since these photovoltaic devices were first demonstrated by Grätzel & O'Regan [1] in the early 1990's. A cartoon showing how DSSC's work is shown in Fig. 1. Future improvements will come from new pairings of substrate/dye/redox couples, with TiO2 nanoparticles being the currently favoured substrate material [2].
Fig 1. DSSC--Dye-sensitized solar cells: Light transmitted by the transparent electrode is absorbed by a dye (red), which coats TiO2 nanoparticles (grey). The process forms electron-hole pairs (e-/h+). Electrons travel through the TiO2 layer to one electrode as holes travel through an electrolyte (blue) to the other electrode, generating electric current.
Fundamental studies aimed at achieving an atomic level understanding of the overall DSSC system have constructed models to explain transient absorption spectra for ZnO and TiO2 substrates [3]. A comparison of these two substrates is commonly made because although TiO2 has a higher efficiency, ZnO has many advantages over TiO2 in terms of the variety of nanostructures that can be synthesised. The bulk material also has higher electron mobility. Two 2016 papers describe experimental and computational studies of the commonly employed N3 dye on ZnO and TiO2 [4,5]. These papers differ in their interpretation of the superiority of TiO2 substrates over that of ZnO. In the experimental study [4], a pump probe measurement on thin films was interpreted to indicate that an interfacial electron-cation complex is formed on ZnO that slows electron injection. In contrast, the computational study predicted that the larger density of states at the conduction band edge of TiO2 is the reason for its superior performance [5]. This discrepancy points to the challenges faced in studies of these complex systems. Nevertheless, it is clear that measurements of the photodynamics hold the key to understanding charge transfer and energy dissipation in DSSC. Here we propose to investigate N3 in the gas phase and compare it to the dye dip-coated onto atomically characterised single crystal substrates of ZnO and TiO2. We will study the photodynamics using two-photon photoemission (2PPE) with variable photon energy, and with time resolved photoemission using a visible light pump and XUV probe. Both these measurements will be carried out on the fs timescale. This is a challenging and ambitious project that will provide results that will transform our understanding of DSSC functionality.
Objectives
1. Use 2PPE to explore the photodynamics of N3 in the gas phase and on ZnO and TiO2 and explore the role of Ti 3d band gap states in the photoexcitation.

2. Use time resolved photoemission (TRPES) to follow the population and decay of the LUMO and other states in the dye as well as band gap and conduction band edge states in the substrate.
Methodology
Figure 2 shows the principles of 2PPE and HHG TRPES along with sample spectra recorded in connection with our previous work related to TiO2/H2O photocatalysis. The 2PPE work allowed us to identify a photoexcitation process in addition to band gap excitation that could be important in photocatalysis [6]. This involves hydroxyl-localised excitation from band gap states to states in the conduction band region. TRPES spectra point to the rapid recombination (<50 fs) of hot electrons created by pumping the band gap states with 1 eV light. A longer component (ca. 1 ps) is also observed if the pump is band gap light (3.1 eV). This is interpreted as VB-CB exciton pair recombination. In this earlier work, 2PPE measurements were carried out at UCL, with TRPES measurements at the Artemis facility at Harwell. In the current project this would be augmented with an existing, but as yet uncommissioned HHG apparatus at UCL.