Ultrafast Supercontinuum Spectroscopy of Carrier Multiplication and Biexcitonic Effects in Excited States of PbS Quantum Dots

Abstract

We examine the multiple exciton population dynamics in PbS quantum dots by ultrafast spectrally-resolved supercontinuum transient absorption (SC-TA). We simultaneously probe the first three excitonic transitions over a broad spectral range. Transient spectra show the presence of first order bleach of absorption for the 1S h -1S e transition and second order bleach along with photoinduced absorption band for 1P h -1P e transition. We also report evidence of the one-photon forbidden 1S h,e -1P h,e transition. We examine signatures of carrier multiplication (multiexcitons for the single absorbed photon) from analysis of the first and second order bleaches, in the limit of low absorbed photon numbers ( abs N ~ 10 -2 ), at pump energies from two to four times the semiconductor band gap. The multiexciton generation efficiency is discussed both in terms of a broadband global fit and the ratio between early-to long-time transient absorption signals..Analysis of population dynamics shows that the bleach peak due to the biexciton population is red-shifted respect the single exciton one, indicating a positive binding energy.

Molecular‐Level Switching of Polymer/Nanocrystal Non‐Covalent Interactions and Application in Hybrid Solar Cells

“…These observations account a photo‐induced electron transfer from P3HT to PbS NCs which is kinetically controlled by exciton diffusion at the hybrid interface, however indicating partial recombination on comparable time scale. At longer delays (>100 ps), the dynamics at the first excitonic peak of PbS‐ArS NCs in the composite and neat solid are similar (black and dark grey lines in Figure b, respectively), pointing to an analogous recombination mechanism in both samples after the observed initial P3HT‐to‐NC electron transfer; moreover, P3HT lower excited state bleaching shows an increasing signal (at 610 nm, circles in Figure b) which follows the dynamics of the photo‐induced absorption feature at 650 nm (triangles in Figure b) attributable prevalently to PbS‐ArS NCs (see Supporting Information Figures S31–33) . In order to determine the presence of long‐lived excited species, we employed quasi steady‐state photo‐induced absorption (PIA) spectroscopy: besides P3HT and PbS NC bleaching (at around 610 and 920 nm, respectively), PIA spectrum of the hybrid composite displays a strong broad feature reminiscent of P3HT delocalized polarons, which gives a positive contribution to the differential absorbance signal centered around 700 nm displaying TA dynamics on the millisecond time scale (black spectrum and decay in Figure c,d, respectively).…”

Enhanced carrier multiplication in engineered quasi-type-II quantum dots

“…Indeed, as was indicated by the first measurements of zero-dimensional PbSe quantum dots (QDs)15 in 2004 and follow-up QD studies1617181920212223, nanomaterials can show CM efficiencies that surpass those of bulk solids. A key development in reaching this important conclusion was the elucidation of the role of uncontrolled photocharging of QDs2425, which was a major source of discrepancies in earlier CM measurements172627282930 if its effects were not ameliorated by using stirred2425262731 or flowed32 QD solution samples.…”

Stable and high-power few cycle supercontinuum for 2D ultrabroadband electronic spectroscopy

“…SC generation in hollow core, photonic crystal fibers, and filamentation in gaseous or liquid media has been extensively explored [5][6][7][8][9][10][11][12]. These new SC sources offer opportunities to investigate new spectral and temporal regimes and have potential in applications, including optical imaging, microscopy [13][14][15], spectroscopy [16][17][18], optical metrology [19,20], and telecommunications [21,22]. However, SC remains problematic for multidimensional coherent spectroscopies because of the inherent instability of the nonlinear SC generation process, greatly limiting its practical implementation.…”