Multiple Exciton Generation in Films of Electronically Coupled PbSe Quantum Dots

Abstract

We study multiple exciton generation (MEG) in electronically coupled films of PbSe quantum dots (QDs) employing ultrafast time-resolved transient absorption spectroscopy. We demonstrate that the MEG efficiency in PbSe does not decrease when the QDs are treated with hydrazine, which has been shown to greatly enhance carrier transport in PbSe QD films by decreasing the interdot distance. The quantum yield is measured and compared to previously reported values for electronically isolated QDs suspended in organic solvents at approximately 4 and 4.5 times the effective band gap. A slightly modified analysis is applied to extract the MEG efficiency and the absorption cross section of each sample at the pump wavelength. We compare the absorption cross sections of our samples to that of bulk PbSe. We find that both the biexciton lifetime and the absorption cross section increase in films relative to isolated QDs in solution.

Carrier multiplication in germanium nanocrystals

“…The time scale of this conversion process is estimated to be 0.1-1 ps. [38][39][40] Consequently, the amplitude of the TA signal is increased by the higher exciton multiplicity (see step 3 in Figure 4b). This multiple-exciton state, however, is relatively short lived and decays through Auger recombination.…”

“…The traces can now be described by fitting a double-exponential to the data, as illustrated by the dashed lines with the lifetimes t 1 53 ns and t 2 5170 ps. The additional fast component t 2 is typical for Auger recombination in lead selenide NCs 38,41 and in silicon NCs in a silicon-dioxide matrix. 42 The occurrence of CM is confirmed by the higher A/B ratio as well as the steeper rise of the initial amplitude A upon increasing pump fluence for this short wavelength excitation.…”

Increasing impact ionization rates in Si nanoparticles through surface engineering: A density functional study

“…Motivated by this proposal, the presence of MEG was indeed demonstrated experimentally in several different semiconductor nanocrystals 6-9 including colloidal silicon NPs 10 and Si NPs embedded in a SiO 2 matrix 11,12 . While the efficiency of MEG in nanoparticles became the subject of debate 3, [13][14][15][16] , recently 17,18 a consensus emerged that MEG is indeed capable of generating up to 2.5 electrons per incoming photon in colloidal NPs 17,[19][20][21] , albeit at higher energies than initially reported. Finally, a remarkable paper reported very recently the first successful extraction of the multiple excitons from PbSe NPs to the charge transport layers of a functioning solar cell with an external quantum efficiency of 120% 22 .…”

Schottky Solar Cells Based on Colloidal Nanocrystal Films

“…Demonstrating an EQE above 100% at hυ ) 3E g to 4E g in a PbSe NC PV cell would provide unequivocal proof that multiple exciton generation 11 (MEG), observed in many colloidal semiconductor NCs [12][13][14][15][16][17][18][19] and in NC films, 20 can be observed and exploited in devices. However, because our cells show a maximum EQE of only ∼65%, we must rely on a determination of the IQE or a comparison of the shape of the EQE and optical absorption spectra to identify any anomalously large photocurrents that may be attributable to MEG.…”