Observation of Biexcitons in Nanocrystal Solids in the Presence of Photocharging

Abstract

In nanocrystal quantum dots (NQDs), generating multiexcitons offers an enabling tool for enhancing NQD-based devices. However, the photocharging effect makes understanding multiexciton kinetics in NQD solids fundamentally challenging, which is critically important for solid-state devices. To date, this lack of understanding and the spectralÀtemporal aspects of the multiexciton recombination still remain unresolved in solid NQD ensembles, which is mainly due to the confusion with recombination of carriers in charged NQDs. In this work, we reveal the spectralÀtemporal behavior of biexcitons (BXs) in the presence of photocharging using near-unity quantum yield CdSe/CdS NQDs exhibiting substantial suppression of Auger recombination. Here, recombinations of biexcitons and single excitons (Xs) are successfully resolved in the presence of trions in the ensemble measurements of time-correlated single-photon counting at variable excitation intensities and varying emission wavelengths. The spectral behaviors of BXs and Xs are obtained for three NQD samples with different core sizes, revealing the strength tunability of the XÀX interaction energy in these NQDs. The extraction of spectrally resolved X, BX, and trion kinetics, which are otherwise spectrally unresolved, is enabled by our approach introducing integrated time-resolved fluorescence. The results are further experimentally verified by cross-checking excitation intensity and exposure time dependencies as well as the temporal evolutions of the photoluminescence spectra, all of which prove to be consistent. The BX and X energies are also confirmed by theoretical calculations. These findings fill an important gap in understanding the spectral dynamics of multiexcitons in such NQD solids under the influence of photocharging effects, paving the way to engineering of multiexciton kinetics in nanocrystal optoelectronics, including NQD-based lasing, photovoltaics, and photodetection.

Slow Auger Recombination of Charged Excitons in Nonblinking Perovskite Nanocrystals without Spectral Diffusion

“…2(g), the average area ratio between the central and the side g (2) (τ) peaks was calculated to be ~0.08 (~0.04) for this single NC excited at <N> = ~0.02 (~1.7), implying that the PL efficiency of XXs is at most ~4% relative to that of Xs [21,37,38]. This extremely-low PL efficiency of XXs contradicts the fact that the fast lifetime component contributes ~40% of the total photons in the "on" intensity level from the calculation of [39]. It should be noted that, although we focused on a specific CsPbI 3 NC in Fig.…”

Effect of Spectral Overlap and Separation Distance on Exciton and Biexciton Quantum Yields and Radiative and Nonradiative Recombination Rates in Quantum Dots Near Plasmon Nanoparticles

“…In contrast to molecular fluorophores, in semiconductor quantum dots (QDs), there can exist several intimately tethered excited states (so-called multiexitons). [1,2] Multiexcitons or, more specifically, biexcitons can be generated via two-photon absorption from the QDs ground state [3] or via sequential absorption of two or more photons, [4] but simultaneous formation of two exciton states in QDs, single exciton (EX) and biexciton (BX), can also be caused by a reversed Auger-like process (i.e., impact ionization). [5] Potentially, the latter process allows achieving a photovoltaic quantum efficiency of more than 100%, [6] because in this case two excitons can be formed upon absorption of only one photon.…”

Type-tunable amplified spontaneous emission from core-seeded CdSe/CdS nanorods controlled by exciton–exciton interaction

“…11,[16][17][18] The other advantage of having a lower energy barrier for electrons in CdSe/CdS NRs is that, by simply changing their core and shell sizes, it is possible to tailor the electron and hole wavefunction overlap and tune CdSe/CdS NRs from type-I-like to quasi-type-II-like band alignment, which can be an important design consideration when engineering the gain/loss mechanisms in practical lasing systems. [19][20][21][22][23] Moreover, due to the narrower band gap of CdS with respect to ZnS, the absorption cross-section of CdSe/CdS core/shell NRs is greatly enhanced. 13 With all of these aforementioned promises, in the nanocrystal lasing context, CdSe/CdS NRs have become one of the most heavily studied materials systems.…”