Exciton Dissociation in CdSe Quantum Dots by Hole Transfer to Phenothiazine

Ultrafast charge transfer from quantum dots (QDs) is essential for their application in solar cells. Photoinduced hole transfer from CdSe QDs (with first exciton peak at 462 nm and estimated radius of 1.8 nm) to phenothiazine (PTZ) was investigated by time-resolved fluorescence and transient absorption spectroscopy. Fluorescence lifetime measurements showed that the presence of PTZ reduced the exciton lifetime of CdSe QDs, and the quenching rate increased with increasing PTZ concentration. Transient absorption spectra of CdSe−PTZ complexes directly revealed the formation of PTZ cation radical, suggesting that the excitons in CdSe dissociated by hole transfer to PTZ. A kinetic model based on a Poisson distribution of the number of adsorbates on each QD was found to adequately describe the adsorbate concentration-dependent fluorescence decay kinetics. According to this model, the hole transfer time was ∼2.5 ns in 1:1 CdSe−PTZ complexes and reached ∼300 ps in samples with an average of ∼6 PTZ per QD.