Coulomb shifts upon exciton addition to photoexcited PbS colloidal quantum Dots

  • Geiregat P
  • Houtepen A
  • Justo Y
 et al. 
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Using ultrafast hyperspectral transient absorption (TA) spectroscopy, we determine the biexciton addition energies in PbS quantum dots (QDs) with different sizes when either a cold or a hot electron−hole pair is added to a QD already containing a cooled exciton. The observed dependence of this so-called biexciton addition energy on the QD diameter and the exciton energy can be rationalized by interpreting the addition energies as the result of an imbalance in the Coulomb interactions between the newly created carriers and the carriers already present in a QD. The obtained results are therefore relevant from both a fundamental and practical point of view. They provide experimental data on Coulomb interaction between charge carriers in confined semiconductors that can be compared with theoretical estimates. Moreover, understanding the way hot−cold biexciton addition energies influence the transient absorption spectrum adds a new element to the transient absorption toolbox for the optoelectronic properties of colloidal QDs. ■ INTRODUCTION Colloidal semiconductor nanocrystals or quantum dots (QDs) are an increasingly prominent class of low-dimensional nanomaterials that combine size-tunable electronic and optical properties with a suitability for solution-based processing. Starting from fundamental research and theoretical modeling on their unique physical properties, QDs are now applied in a variety of domains such as solar energy harvesting, photo-detection, and light-emitting diodes or displays. 1−5 These applications typically rely on the linear optical properties of QDs, i.e., light absorption by unexcited QDs and light emission by radiative recombination in excited QDs. On the other hand, various studies have shown that the spectral and time-dependent properties of excited QDs can strongly enhance the performance of QD-based devices in the above mention applications or enable QDs to be used in completely different applications. Quantum dots excited with photons having energies exceeding twice that of the QD bandgap transition can, for example, dissipate their excess energy by forming biexcitons in a process called multiple exciton generation (MEG) that can considerably enhance the short circuit current of single junction, QD-based solar cells. 2,6−8 Controlling the recombination rate of biexcitons by nonradiative Auger processes allowed for the formation of blinking-free QDs and facilitated the formation of QD-based lasers. 9,10 Moreover, it was proven that excited QDs exhibit a broadband and ultrafast photoinduced absorption related to intraband transitions of either the excited electron or hole, which could be used for optical modulation. 11,12

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