Elsevier

Journal of Energy Chemistry

Volume 24, Issue 6, November 2015, Pages 712-716
Journal of Energy Chemistry

Characterization of hot carrier cooling and multiple exciton generation dynamics in PbS QDs using an improved transient grating technique

https://doi.org/10.1016/j.jechem.2015.11.002Get rights and content

Abstract

Multiple exciton generation (MEG) dynamics in colloidal PbS quantum dots (QDs) characterized with an improved transient grating (TG) technique will be reported. Only one peak soon after optical absorption and a fast decay within 1 ps can be observed in the TG kinetics when the photon energy of the pump light is smaller than 2.7Eg (Eg: band gap between LUMO and HOMO in the QDs), which corresponds to hot carrier cooling. When is greater than 2.7Eg, however, after the initial peak, the TG signal decreases first and soon increases, and then a new peak appears at about 2 to 3 ps. The initial peak and the new peak correspond to hot carriers at the higher excited state and MEG at the lowest excited state, respectively. By proposing a theoretical model, we can calculate the hot carrier cooling time constant and MEG occurrence time constant quantitatively. When MEG does not happen for smaller than 2.7Eg, hot carrier cools with a time constant of 400 fs. When MEG occurs for larger than 2.7Eg, hot carrier cools with a time constant as small as 200 fs, while MEG occurs with a time constant of 600 fs. The detailed hot carrier cooling and MEG occurrence dynamics characterized in this work would shed light on the further understanding of MEG mechanism of various type of semiconductor QDs.

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Multiple exciton generation (MEG) occurs in PbS QDs when photon energy for is larger than 2.7 Eg. Hot carriers cool within 200 fs, while MEG occurs within 600 fs.

Introduction

Semiconductor quantum dots (QDs) have attracted increasing interest as potential light-harvesting materials in the application to cheap and highly efficient next generation solar cells because of the following unique properties [1], [2], [3], [4]. First, the energy gaps of the QDs can be tuned by controlling their size, and thus the absorption spectra can be tuned to match the spectral distribution of sunlight. Secondly, semiconductor QDs have large extinction coefficients due to the quantum confinement effect. Thirdly, semiconductor QDs have the potential to generate multiple electron–hole pairs with one single photon absorption (multiple exciton generation: MEG) [1], [4], which would lead to incident-photon-to-current (IPCE) efficiencies of over 100%. Therefore, Nozik and Hanna have theoretically predicted that the maximum thermodynamic efficiency for photovoltaic devices with a single sensitizer could be improved to as much as ∼44% by employing semiconductor QDs [5]. Since QD-based solar cells could be made very cheaply using simple chemical methods, they are expected to be a candidate of the promising next-generation cost-effective high efficiency solar cells [1], [2], [3], [4].

Since the initial experimental evidence of efficient MEG in colloidal PbSe QDs was obtained by Schaller and Klimov [6], enhanced MEG has been observed in several types of semiconductor QDs such as PbS, CdSe, PbTe, InAs and Si at threshold photon energies of 2 to 3 times the HOMO–LUMO transition energy (Eg) of the QDs, based on transient absorption (TA) spectroscopy, time-resolved photoluminescence (TRPL) technique and time-resolved terahertz spectroscopy (TRTS) [6], [7], [8], [9], [10], [11], [12]. In addition, more direct signatures of MEG were observed in PbS QD photodetectors, PbS QD-sensitized solar cells and PbSe QD-based heterojunction solar cells as the excitation photon energy exceeds 2.7 times Eg [13], [14], [15]. However, some reports questioned the reported higher MEG quantum yield (QY) in semiconductor QDs (e.g., PbSe QDs) and the existence of MEG in some cases (e.g., in CdSe QDs) [16], [17]. To understand the phenomenon of MEG in QDs, further experimental and theoretical studies are necessary, including dynamics of MEG occurrence and the mechanism of MEG. The most common and useful methods used to characterize MEG in QDs are TA and TRPL techniques [1], [7], [11]. TRTS, which measures the intraband photoinduced absorption, has also been employed to study MEG and provides complementary information [12]. By measuring the transient absorption or transient luminescence kinetics between LUMO and HOMO in the QDs, the recombination dynamics of excitons in the lowest excited state can be monitored. The condition for these measurements is that the pump light intensity should be low enough such that less than one exciton is excited in each QD. Then, if MEG does not happen, the lifetime of a single exciton would be as long as nanoseconds and thus only a slow decay process due to single exciton relaxation with a lifetime as long as nanoseconds can be observed. However, if MEG happens after light absorption, then more than one exciton will be produced in one QD. Then an additional fast decay resulting from Auger recombination of the generated multiexcitons, accompanying with the slow decay due to single exciton relaxation, will be observed in the TA or TRPL kinetics [1], [9], [11]. Thus the signature of MEG is the fast decay dynamics with lifetimes in the range between 10 ps and 100 ps appearing in the TA or TRPL kinetics. However, in these TA and TRPL kinetics, only carrier dynamics for times longer than 3–5 ps after optical absorption were usually utilized [1], [9], [11], by which time hot carrier cooling and MEG are complete [1], [9], [11]. But the dynamics of MEG occurrence (such as at what time MEG starts), which happens as fast as less than a few picoseconds, were not be observed clearly by using the TA, TRPL and TRTS kinetics. Fig. 1 shows the schematic illustration of MEG generation and characterization using the TA method.

To study the dynamics of MEG occurrence, we have proposed an improved transient grating (TG) technique [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28] as a new technique to characterize MEG dynamics and applied it to PbS QDs which were used as a model material. The TG technique is one kind of photothermal methods and the transient refractive index change of samples due to pulsed light absorption is measured. Compared to the conventional TG technique, the improved TG technique features a simple optical setup and highly sensitive detection [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. The principle of the improved TG technique has been explained in detail in the previous papers [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28] and is only described briefly here. A pump beam is incident on the transmission grating and the spatial intensity profile of the pump beam has an interference pattern in the vicinity of the other side of the transmission grating. When a sample is brought near the transmission-grating surface, it can be excited by the optical interference pattern. The refractive index of the sample changes according to the intensity profile of the pump light and the induced refractive index profile functions as a different type of transiently generated grating. When the probe beam is incident in a manner similar to that of the pump beam, it is diffracted both by the transmission grating (called a reference light) and the transiently generated grating in the sample (called a signal light). In principle, the two diffractions progress along the same direction; therefore, these two diffractions interfere, which is detected by a detector positioned at a visible diffraction spot of the reference beam. We have succeeded in observation on the dynamics of MEG, including the start and complete of MEG, together with a hot exciton relaxation dynamics in PbS QDs [24]. In addition, the relaxation dynamics of the generated multiple excitons to a single exciton in the QDs through Auger recombination could be clarified clearly. Recently, we have applied the TG technique to study hot carrier cooling and MEG in alloy CdxHg1-xTe colloidal QDs [26]. The TG measurements revealed a composition x dependent multiple exciton generation process which competes with phonon mediated carrier cooling to deplete the initial hot carrier population. The interplay between these two mechanisms is strongly dependent on the electron effective mass which in these alloys has a marked composition dependence and may be considerably lower than the hole effective mass. For a composition x = 0.52, we found a maximum carrier multiplication quantum yield of 199% with pump photon energy 3 times the bandgap energy, Eg, whilst the threshold energy is calculated to be just 2.15Eg. These results indicate that alloy CdxHg1-xTe QDs are promising materials for solar cell application or LED [26].

In this paper, we will quantitatively calculate the time constants of hot carrier cooling time and MEG occurrence in PbS QDs from the ultrafast TG kinetics within a few ps by proposing a theoretical model for the first time. We find that the photoexcited carrier relaxation processes including hot carrier cooling changed clearly when increases from less than 2.7Eg to larger than 2.7Eg, and MEG occurs for is larger than 2.7Eg.

Section snippets

Experimental

In the improved TG technique used for studying the ultrafast carrier dynamics of semiconductor QDs, the laser source was a titanium/sapphire laser (CPA-2010, Clark-MXR Inc.) with a wavelength of 775 nm, a repetition rate of 1 kHz, and a pulse width of 150 fs [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. The light was separated into two parts. One part was used as a probe pulse. The other was used to pump an optical parametric amplifier (OPA) (a TOAPS from Quantronix) to

Results and discussion

For studying MEG in QDs using the TG measurements, the pump light intensity of each wavelength used should be low enough so that less than one exciton would be excited in each QD. Thus, we first measured the pump light intensity dependence of the TG kinetics over a time scale as long as 400 ps and confirmed that the waveforms of the TG kinetics overlap each other very well when they are normalized for each pump wavelength from 290 nm (hν/Eg: 3.42) to 520 nm (hν/Eg: 1.9) for different light

Conclusions

We have characterized hot carrier cooling and MEG occurrence dynamics in PbS QDs using an improved TG technique. By proposing a theoretical model, we could quantitatively analyze the time constant of hot carrier cooling from higher excited states and the time constant of carrier creation at the lowest excited states. For the first time, our results clearly revealed that there is a significant change in the physical processes of photoexcited carrier relaxation in the QDs when the photon energy

Acknowledgments

This work was supported by MEXT KAKENHI Grant no. 26286013 and the PRESTO program Photoenergy conversion systems and materials for the next generation solar cells, Japan Science and Technology Agency (JST).

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