Tracking the Adsorption and Electron Injection Rates of CdSe Quantum Dots on TiO₂: Linked versus Direct Attachment

Understanding CdSe quantum dot (QD) adsorption phenomena on mesoscopic TiO 2 films is important for improving the performance of quantum dot sensitized solar cells (QDSSCs). A kinetic adsorption model has been developed to elucidate both Langmuir-like submonolayer adsorption and QD aggregation processes. Removal of surface-bound trioctylphosphine oxide as well as the use of 3-mercaptopropionic acid (MPA) as a molecular linker improved the adsorption of toluene-suspended QDs onto TiO 2 films. The adsorption constant K ad for submonolayer coverage was (6.7 ± 2.7) × 10 3 M –1 for direct adsorption and (4.2 ± 2.0) × 10 4 M –1 for MPA-linked assemblies. Prolonged exposure of a TiO 2 film to a CdSe QD suspension resulted in the assembly of aggregated particles regardless of the method of adsorption. A greater coverage of TiO 2 was achieved with smaller QDs due to reduced size constraints. Ultrafast transient absorption spectroscopy demonstrated faster electron injection into TiO 2 from directly adsorbed QDs ( k ET = 7.2 × 10 9 s –1 ) compared with MPA-linked QDs ( k ET = 2.3 × 10 9 s –1 ). The adsorption kinetic details presented in this study are useful for controlling CdSe QD adsorption on TiO 2 and designing efficient photoanodes for QDSSCs.