Publication date: 3rd July 2020
We report on time-resolved differential transmission measurements on colloidal InP/ZnS and InP/ZnSe core/shell QDs. We can distinguish between electron and hole relaxation processes by optically exciting and probing individual transitions. This, in turn, allows us to determine how the initial excess energy of the charge carriers affects the relaxation processes. An efficient Auger-like electron−hole scattering mechanism circumvents the expected phonon bottleneck for the electron. Accordingly, electrons relax faster than holes. Moreover, the hole relaxation is slowed down significantly for two reasons. First, a small wave function overlap between core and shell states reduces the scattering probability. Second, holes can be trapped at the core/shell interface. This leads to either slow detrapping or nonradiative recombination, thus, reducing the radiative efficiency.
Overall, these results demonstrate that it is crucial to consider charge carrier relaxation when constructing devices. An energetic alignment of the relevant states (in charge transport materials and QDs) should help to maximize radiative efficiency.
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