Article pubs.acs.org/JPCC
Transition from Thermodynamic to Kinetic-Limited Excitonic Energy Migration in Colloidal Quantum Dot Solids Lisa V. Poulikakos,† Ferry Prins, and William A. Tisdale* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States 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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INTRODUCTION Due to size-tunable optical and electronic properties, quantumconfined semiconductor nanocrystals, also known as colloidal quantum dots (QDs), show great potential for use in emerging optoelectronic technologies such as light-emitting diodes (LEDs),1,2 lasers,3−5 solar cells,6,7 and photodetectors.8,9 For these applications it is essential to understand the dynamics of electron−hole pairs, also known as excitons, within solid QD assemblies.10,11 Kagan et al. were the first to demonstrate excitonic energy transfer in close-packed CdSe quantum dot solids.12,13 They observed that the photoluminescence spectrum of a QD solid is red-shifted relative to the solution photoluminescence and that the magnitude of this red-shift was greater for more inhomogeneously broadened ensembles− indicating that excitons in QD assemblies migrate energetically downhill during their lifetime.13 Based on transient measurements, they attributed their observations to dipole−dipole interactions between neighboring nanocrystals. Later, Crooker et al. investigated spectrally and temporally resolved photoluminescence in CdSe QD assemblies and found an energydependent exciton transfer rate from smaller to larger QDs.14 More recently, Miyazaki and Kinoshita observed energy relaxation by excitonic energy transfer from higher to lower energy sites following site-selective excitation.15,16 Here, we investigate the temperature dependence of exciton hopping in CdSe/ZnCdS quantum dot solids using spectrally resolved transient photoluminescence spectroscopy. We observe a nonmonotonic dependence of the magnitude of the transient red-shift on temperature, while simultaneously observing a monotonic decrease in the hopping rate with decreasing temperature. Taken together, these observations indicate a transition from a fully thermalized exciton distribution near room temperature to a kinetically trapped © 2014 American Chemical Society
exciton distribution at low temperatures. Moreover, we show that the transition from thermodynamic to kinetic-limited hopping behavior occurs at higher temperatures in QD ensembles that are more inhomogeneously broadened. Our ability to isolate the contributions of homogeneous and inhomogeneous broadening to ensemble exciton dynamics is enabled by the use of shape-engineered QDs.5 Many previous investigations of temperature-dependent energy transfer were dominated by temperature-induced changes to the radiative lifetime (usually due to population redistribution among dark and bright exciton states),17−22 whereas the pyramid-shaped QDs used in this work exhibit a temperature-independent exciton lifetime. This study offers insight into the physical mechanisms driving the temperature-dependent behavior of exciton diffusion in QD films and, more generally, advances our understanding of exciton dynamics in disordered systems.
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EXPERIMENTAL METHODS All QD solutions used in this work were supplied by QD Vision, Inc. and used as received. CdSe/Zn0.5Cd0.5S core/shell semiconductor nanocrystals having a pyramid-like shape were used and their sizes determined by TEM (see Figure 1). The smaller “inhomogeneous” CdSe cores have a nominal diameter of 3.2 nm and a median emission energy of 2.24 eV (total emission line width standard deviation of σtot = 55 meV), while the larger “homogeneous” cores have a nominal diameter of 4.2 nm and a median emission energy of 2.06 eV (total emission line width standard deviation of σtot = 38 meV). Both size nanocrystals are coated with a ∼1 nm thick Zn0.5Cd0.5S shell Received: March 25, 2014 Published: April 1, 2014 7894
dx.doi.org/10.1021/jp502961v | J. Phys. Chem. C 2014, 118, 7894−7900
The Journal of Physical Chemistry C
Article
Figure 1. TEM images of the two QD batches used in this study: (a) the inhomogeneous batch (photoluminescence emission spectral width σtot = 55 meV) and (b) the homogeneous batch (photoluminescence emission spectral width σtot = 38 meV). The QDs have a pyramid-like shape.
and are capped with benzylphosphonic acid. Solid films were prepared by spin-casting from a 40 mg/mL solution in a N2 glovebox (
