Abstract
We investigate the temperature dependence of excitonic energy migration in CdSe/ZnCdS core/shell colloidal quantum dot (QD) solids using spectrally resolved transient photoluminescence spectroscopy. In line with previous studies, we observe that as excitons hop among different energy sites within the inhomogeneously broadened QD ensemble, the photoluminescence spectrum transiently shifts toward the red. Using shape-engineered nanocrystals having a temperature-independent radiative lifetime, the magnitude of this transient red-shift is found to vary nonmonotonically with temperature. Near room temperature, the magnitude of this transient red-shift is determined by thermal equilibrium within the site energy distribution. As the sample temperature is reduced, the site-to-site hopping rate slows down and excitons become kinetically trapped at local minima in the global energy landscape. For a more homogeneous QD ensemble, reduction in site energy disorder causes the transition from thermodynamic to kinetic-limited behavior to shift to lower temperatures. These results have implications for the design of colloidal QD optoelectronic devices and advance our understanding of exciton dynamics and energy transport in disordered systems.
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