Probing Defect States and Surface Restructuring of Prototypical Solar Materials

This thesis examines two important solar material surfaces: rutile TiO2 (011)-2×1 and the hybrid organic-inorganic perovskite CH3NH3 PbBr3. Supported by ultraviolet photoemission spectroscopy, resonant two-photon photoexcitation at the TiO2 (011)-formate interface asserts that formate adsorption stabilises the small polarons residing within TiO2 (011) by ∼0.2 eV, possibly pointing to a reversion of the substrate from a 2×1 to a 1×1 reconstruction as has been observed previously upon water adsorption. In contrast to rutile TiO2 (110), no additional photoexcitation channel is observed. Using X-ray photoemission spectroscopy on in situ cleaved single crystal CH3NH3 PbBr3, water adsorption is found to occur with no threshold. Scanning tunnelling microscopy reveals that the associated restructuring is consistent with the Volmer-Weber model, while ultraviolet photoemission spectroscopy shows an increased band gap state density following water vapour exposure, which is attributed to surface defect formation due to lattice swelling. Finally, resonant photoemission is applied to in situ cleaved CH3NH3 PbBr3 to reveal an alternative photoexcitation pathway equal to the band gap energy. Constant final state analysis using coherent two photon photoemission is proposed as a complementary technique for determining the valence band maxima of semiconductor systems.