Elsevier

Chemical Physics

Volume 471, 1 June 2016, Pages 54-58
Chemical Physics

Hot electron transfer from PbSe quantum dots molecularly bridged to mesoporous tin and titanium oxide films

https://doi.org/10.1016/j.chemphys.2015.11.005Get rights and content

Highlights

  • Efficient hot electron transfer occurs in 3 nm PbSe quantum dots molecularly linked by 3-mercaptopropionic acid to mesoporous TiO2 anatase sensitized films at room temperature.

  • Inefficient hot electron transfer is resolved for 3 nm PbSe quantum dots molecularly linked by 3-mercaptopropionic acid to mesoporous SnO2 sensitized films at room temperature.

  • We illustrate how, from the overall time-resolved conductivity response, the QD-oxide ET dynamics can be extracted by controlled photo-oxidation of the QDs-oxide system.

Abstract

The selective extraction of hot electrons to suitable energy contacts is a key aspect towards the development of hot carrier solar cells. Here we employ Time Resolved THz Spectroscopy (TRTS) to evaluate the extent to which hot electron transfer (HET) takes place from the 1Pe states of colloidal 3 nm PbSe quantum dots molecularly linked by 3-mercaptopropionic acid to mesoporous SnO2 and TiO2 sensitized films. For PbSe–3MPA–SnO2 samples, we show that the efficiency of hot electron transfer is negligibly small at room temperature, i.e. within the ∼10% detection limit of our measurements. The impact of spurious signals on TRTS data arising from carrier dynamics regarding QDs aggregates – which can be misinterpreted as HET – is discussed in detail. In contrast, in line with previous reports, hot electron transfer is observed to take place from the Pe states of colloidal PbSe QDs in the PbSe–3MPA–TiO2 system, with an efficiency ⩾80%. These results are rationalized in terms of a stronger donor–acceptor coupling between QD and oxide for the TiO2 electrode when compared with a SnO2 electrode, a factor that ultimately defines the kinetic competition between electron transfer rate towards the oxide and intraband cooling within the QDs.

Introduction

Carrier thermalization through electron/hole-phonon interactions following hot charge carrier photo-generation is one of the fundamental energy loss mechanisms in solar energy conversion. Theoretically, the extraction of hot carriers (before thermalization) to selective energy contacts has been proposed as a route for high efficiency solar cell devices [1], [2]. This approach is commonly known as “hot carrier solar cells” (HCSCs) and much research has been focused on trying to identify suitable materials and architectures to simultaneously implement the “hot absorbers” (HAs) and the selective energy contacts (SECs) required for such a device. SECs have been demonstrated to be feasible by engineering double barrier resonant structures made of silicon quantum dots (QDs) [3]. Concerning the “hot absorber” design, most of the work has been focused on identifying bulk materials where carrier cooling is slowed down in wide “phononic band gap” binary bulk semiconductor compounds [4], [5] and potentially in QD superlattices [6]. On excitonic architectures, Tisdale et al. [7], [8] have reported hot electron transfer (HET) in PbSe sensitizing bulk rutile TiO2 (with HET sensitive to temperature and interfacial chemistry). While signatures of HET have been inferred as well from quantum efficiency measurements on PbS quantum dots sensitizing a bulk TiO2 anatase crystals [9].

Here, we evaluate the efficiency of hot carrier extraction from 1Pe states (further denoted as Pe) of 3 nm PbSe QDs molecularly linked (by 3-mercaptopropionic acid, 3-MPA) to mesoporous TiO2 and SnO2 films at room temperature. For these systems, we show, using Time Resolved THz Spectroscopy (TRTS), that while the HET towards TiO2 electrodes is highly efficient, HET towards SnO2 electrodes is negligible at room temperature. These results are consistent with the kinetic competition between hot carrier cooling within the QD and QD-oxide electron transfer rates, the latter being determined by the coupling strength between QD and oxide.

PbSe QDs have several properties that make them attractive as absorbers for photovoltaic applications[10] and, specifically, for hot carrier solar cells: (i) the bulk PbSe bandgap (0.27 eV) is such that the QD absorption edge can be tuned over the IR and visible spectrum by quantum confinement; (ii) PbSe is characterized by a large Bohr radius (aB = 46 nm) [11], so that the electron wavefunction leakage outside the dot is expected to be substantial, implying high carrier delocalization that enhances electronic coupling to its environment, thereby facilitating charge extraction [8]; and (iii) due to the similar effective masses for electrons and holes (me = mh) PbSe QDs possess enhanced discretization for both electron and hole states (though a truly mirror-like energy band diagram has been ruled out [12]). An enhancement in hole confinement will enhance hot electron lifetimes due to the reduced probability of electron–hole Auger interactions [13]. Indeed, relaxation times from the PbSe QD first excited state (Pe) to the exciton ground excite state (Se) of several picoseconds have been reported in the strong quantum confinement regime [11], [14].

We have previously demonstrated the potential of Time Resolved THz Spectroscopy (TRTS) to unambiguously quantify the efficiencies and time scales of ET processes in so-called QD sensitized solar cells [15], [16], [17], [18], [19], an extension of the design employing dyes [20] as sensitizers. TRTS is unique because it allows to directly monitor the QD-oxide ET by determining the photoconductivity within the oxide. Following selective photo-excitation of QDs, the THz probe pulse, covering the frequency range 0.2–2 THz in our setup, is used to perform an ultrafast, contact-free measurement of the complex (real and imaginary) conductivity [21]. Immediately after photo-excitation, when excitons are confined within the QDs of the sensitized samples, the conductivity has zero real part, and is purely imaginary; this has been demonstrated in detail for photo-excited quantum dots in solution, where it was shown that the imaginary conductivity can be related to the polarizability of the electronic and hole wavefunctions within the quantum dot [22], [23], [24], [25]. However, upon ET from the QD to the oxide, the THz field can interact strongly with the free electrons populating the oxide conduction band, and the real part of the conductivity becomes finite. The real part of the conductivity is dissipative – it corresponds to absorption of the terahertz pulse. The absorption is directly proportional to the real conductivity of the samples, which in turn is given by the product of the number of charge carriers N and their mobility μ: Re(σ)  N·μ. Details about the TRTS setup and THz measurements are given elsewhere [17], [18].

Section snippets

3.2 nm PbSe QDs molecularly linked to SnO2 by 3-mercaptopropionic acid

Samples consisting of on 3.2 nm PbSe QDs sensitizing mesoporous SnO2 electrodes were prepared as described in detail elsewhere [17], [18]. Fig. 1 shows the relative band alignment (represented as ΔG) between the PbSe QDs electronic ground and excited states (Se and Pe, respectively in Fig. 1) and the oxide CB obtained from reported QD and oxide band alignments vs vacuum [26], [27], [28]. Solid black and gray lines represent the absorbance of the QDs before and after functionalization of the SnO2

Discussion and conclusions

Our findings need to be rationalized: why HET is so efficient for TiO2 while almost absent in SnO2, independent of the distinct donor–acceptor energetics (ΔG) depicted in Fig. 1, Fig. 4? QD-oxide coupling strength is determined, amongst others, by the electronic density of states (DOS) of SnO2 and TiO2 electrodes at the relevant energies, as the tunneling efficiency through the 3-MPA backbone is proportional to the DOS. A simple tunneling picture also suggests that by increasing the DOS of the

Conflict of interest

No conflict of interest.

Acknowledgements

This work has been financially supported by the Max Planck Society. H. Wang is a recipient of a fellowship of the Graduate School Materials Science in Mainz (MAINZ) funded through the German Research Foundation in the Excellence Initiative (GSC 266). M. Karakus acknowledges the fellowship of the International Max Planck Research School for Polymer Material Science (IMPRS-PRS) in Mainz. E. Cánovas acknowledge funding from the Max Planck Graduate Center (MPGC).

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