Determination of the carrier temperature in weakly confined semiconductor nanocrystals using time-resolved optical spectroscopy

Many applications of nanocrystals rely on their use in light detection and emission. In recent years, nanocrystals with more relaxed carrier confinement, including so-called 'bulk' and 2D implementations, have made their stake. In such systems, the charge carriers generated after (photo-)excitation are spread over a semi-continuous density of states, behaviour controlled by the carrier temperature T. Current established methods to measure this dynamically changing temperature include transient absorption and luminescence spectroscopy, yet they very often fail to agree on the exact temperature leading to contradicting reports. Here, we show through a combined side-by-side experimental and theoretical study on state-of-art II-VI and perovskite nanocrystals under weak confinement that only transient PL can yield unambiguously the correct T. In particular, temperatures extracted from TA are heavily affected by the effective masses of the electron and hole bands involved leading to overestimations. Our results pave a way to a more robust extraction of carrier temperature and will help to consolidate ensuing structure-property relations derived from it.