Efficient carrier multiplication in InP nanoparticles

Abstract

Efficient carrier multiplication in InP nanoparticles is reported; ultrafast transient absorption measurements at the band edge were used to determine the number of excitons per photoexcited nanoparticle for a range of both excitation fluences and photon energies. At photon energies greater than 2.1Ϯ 0.2 times the band gap, an average of more than 1 exciton per photoexcited nanoparticle was found even in the limit of vanishing fluence. The average number of excitons generated by an absorbed photon was measured to be 1.18Ϯ 0.03 for excitation photons with energies 2.6 times the band gap.

Multiple Exciton Generation and Dynamics in InP/CdS Colloidal Quantum Dots

“…For InP/ZnS/ZnO Type I CQDs with a similar an MEG of 1.18  0.03 for a photon energy equivalent to 2.6 has been previously reported. 15 However, that work used the position of the absorption edge as a measure of , rather than the PL peak as in this study; using instead the PL peak, yields a photon energy equivalent to 2.7 . For its reported efficiency of 0.3, the Type I InP/ZnS/ZnO CQDs would thus be expected to have a MEG of 1.27  0.04 for a pump photon energy equal to 3 .…”

“…10,11,15,25 A Ti:Sapphire regenerative amplifier (Spectra-Physics Spitfire-Pro) seeded by a mode-locked Ti:Sapphire oscillator (Spectra-Physics Tsunami) produces pulses of 100 fs duration and ~1 mJ energy at a wavelength of ~800 nm and 1 kHz repetition rate. Using a beam splitter, 95% of this beam is passed to an optical parametric amplifier (OPA, Light Conversion Ltd. TOPAS-C) with harmonic generating crystals to produce the pump beam, tuneable from the infra-red to the ultra-violet with a minimum wavelength of 232 nm.…”

“…A Type I CQD design can be used to reduce the trapping of hot carriers by the surface states that compete with MEG. A study of dispersions of InP/ZnS/ZnO (core/shell/shell) Type I CQDs reported an efficiency of 0.3, 15 defined as the rate of increase with photon energy (in units of )…”

“…As mentioned above, Type I CQDs based on InP cores exhibit good MEG efficiency. 15 The addition of a CdS shell can redshift the absorption edge significantly, 18 so that it closer to the ideal value for optimal exploitation of the solar spectrum. 5 A CdS shell has also been shown to be an effective means of passivating surface traps, as evidenced by the increase in photoluminescence (PL) and reduction in PL intermittency (also known as 'blinking') resulting from the addition of such a shell.…”

Optimization schemes for efficient multiple exciton generation and extraction in colloidal quantum dots

“…12,[16][17][18] Lead chalcogenide QDs (PbS and PbSe), cadmium chalcogenide QDs, indium based QDs (InAs and InP) and silicon QDs are some of the intensively studied QD systems for exploring the efficiency of MEG. 2,[19][20][21][22][23][24] MEG is commonly measured by ultrafast transient absorption spectroscopy, 25 which allows one to capture the rapid processes of bi-exciton formation and Auger recombinations on the ps time scale. 26 These processes occur at much faster time scale as compared to the lifetimes of single 27 Recently, we have established a model to study this time-dependence, 28 which describes bi-exciton formation in good agreement with time-resolved measurements.…”