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 TiO2 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 TiO2 films. The adsorption constant K-ad for submonolayer coverage was (6.7 +/- 2.7) x 10(3) M-1 for direct adsorption and (4.2 +/- 2.0) x 10(4) M-1 for MPA-linked assemblies. Prolonged exposure of a TiO2 film to a CdSe QD suspension resulted in the assembly of aggregated particles regardless of the method of adsorption. A greater coverage of TiO2 was achieved with smaller QDs due to reduced size constraints. Ultrafast transient absorption spectroscopy demonstrated faster electron injection into TiO2 from directly adsorbed QDs (k(ET) = 7.2 x 10(9) s(-1)) compared with MPA-linked QDs (k(ET) = 2.3 x 10(9) s(-1)). The adsorption kinetic details presented in this study are useful for controlling CdSe QD adsorption on TiO2 and designing efficient photoanodes for QDSSCs.