Electron migration limits the rate of photobrightening in thin films of cdse quantum dots in a dry N₂ (g) atmosphere

This paper describes a study of the mechanism of photoinduced photoluminescence enhancement, termed "photobrightening" (PB), of ∼150-nm-thick films of CdSe quantum dots (QDs) in a dry N₂ (g) atmosphere. Steady-state photoluminescence (PL) and ultrafast transient absorption measurements of films photoexcited continuously and shot-wise revealed that: (i) PL enhancement occurred in all of the close-packed films during periods of continuous photoexcitation and continued after the excitation source was turned off; (ii) the time-dependence of PB (both during excitation and in the dark) was initially linear and became exponential as the PB reached saturation; (iii) the rate of PB and the maximum PB achieved by the film depended on the degree of surface passivation of the QDs; (iv) the PL peak shifted to lower energy and broadened during PB; and (v) rates of nonradiative trapping of excitonic electrons decreased during PB. These data were utilized to construct a model for PB based on migration of photoexcited electrons within the film. The basis for this model is that PB is limited by the rate of migration of electrons among surface-localized energetically shallow traps in the film, and not by the rate of creation of surface-trapped charge carriers. The migration mechanism provides a rationalization for the seemingly contradictory reports that charging of QD surfaces causes PB in ensembles of QDs but causes photodarkening and blinking in single QDs.