Efficient Hot Electron Transfer in Quantum Dot-Sensitized Mesoporous

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Efficient Hot Electron Transfer in Quantum DotSensitized Mesoporous Oxides at Room Temperature Hai I. Wang, Ivan Infante, Stephanie ten Brinck, Enrique Cánovas, and Mischa Bonn Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01981 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

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Efficient Hot Electron Transfer in Quantum Dot-Sensitized Mesoporous Oxides at Room Temperature

Hai I. Wang1, 2,*, Ivan Infante3, Stephanie ten Brinck3, Enrique Cánovas1,4,* & Mischa Bonn1,* 1

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.

2

Graduate School of Material Science in Mainz, University of Mainz, Staudingerweg 9,

55128 Mainz, Germany. 3

Department of Theoretical Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, De

Boelelaan 1083, 1081 HV Amsterdam, The Netherlands. 4

Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA Nanociencia), Faraday

9, 28049 Madrid, Spain.

*

Correspondence

to:

[email protected],

[email protected]

For TOC only

ACS Paragon Plus Environment

[email protected],

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Abstract Hot carrier cooling processes represent one of the major efficiency losses in solar energy conversion. Losses associated with cooling can in principle be circumvented if hot carrier extraction towards selective contacts is faster than hot carrier cooling in the absorber (in socalled hot carrier solar cells). Previous work has demonstrated the possibility of hot electron extraction in quantum dot (QD)-sensitized systems, in particular at low temperatures. Here we demonstrate room-temperature hot electron transfer (HET) with unity quantum efficiency in strongly coupled PbS quantum dot-sensitized mesoporous SnO2. We show that the HET efficiency is determined by a kinetic competition between HET rate (KHET) and the loss of excess energy, thermalization (KTH), in the dots. KHET can be modulated by changing the excitation photon energy; KTH can be modified through the lattice temperature. DFT calculations demonstrate that the HET rate and efficiency is primarily determined by the density of the state (DoS) of QD and oxide. Our results provide not only a new way to achieve efficient hot electron transfer at room temperature, but also new insights on the mechanism of HET and the means to control it.

Keywords: Hot electron transfer, Strong coupling, PbS quantum dots, quantum dot sensitized solar cells, Terahertz spectroscopy


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In 1961 Shockley and Queisser theoretically established the limiting efficiency for single bandgap solar converters as ~33% under 1 sun illumination.1 The upper limit is dictated ultimately by a trade-off between two major losses in the absorber, which are: (i) the inability to absorb photons with energy below the bandgap and (ii) the cooling down to the conduction band edge of hot carriers generated by the absorption of photons with energies exceeding the bandgap energy.2, 3 The latter loss mechanism accounts for up to ~33% efficiency drop in optimized ~1 eV bandgap single absorber solar cells,4 but could, in principle, be substantially reduced if e.g. hot carriers are extracted towards energy selective contacts before thermalization in the absorber, i.e. hot electron transfer (HET) is achieved.4, 5 Quantum dot (QD)-sensitized oxide represents an appealing system to harvest hot carriers for solar energy conversion.6-8 Strongly coupled quantum dot sensitized oxides systems, in which the mixing of donor and acceptor wavefunctions can be substantially enhanced using short, conjugated bridges between QD and oxide, represent a practical way to extract hot carriers on ultrafast time scales.9-11 Using this strategy, hot electron transfer has been reported in PbSe QD-sensitized TiO2 systems bridged by e.g. 1, 2-ethanedithiol molecules at 77 K.12,

13

However, many questions remain regarding the HET mechanism and to the best of our knowledge, efficient HET at room temperature has not been reported. Additionally, previous HET studies have focused exclusively on TiO2 for harvesting hot carriers. It is unclear, whether efficient HET can also take place when other types of oxide with lower density of electronic states in the conduction band are used as acceptors.14 For instance, SnO2 has been widely used in QD-sensitized system for solar cells and photocatalysis.15, 16 Herein we demonstrate efficient room-temperature HET in a strongly coupled QD-oxide system by directly nucleating PbS QD on SnO2 nanoparticles. By increasing the photon excess energy, we demonstrate that HET rate can be substantially enhanced: with Eex above ~0.5 eV, sub-150 fs HET with unity quantum yield is observed. Employing DFT calculation,


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we further reveal that the enhanced HET rate and efficiency can be primarily assigned to an increased density of states of both donor and acceptor at high excess energies. Furthermore, we show that the HET efficiency is defined by a kinetic competition between interfacial HET and carrier cooling within the QDs. Accordingly, the HET efficiency can be increased either by increasing the coupling strength (e.g. via increasing the DoS of the QDs by increasing their size), and/or by reducing carrier cooling rate (by lowering the temperature). In this study, PbS QDs were nucleated directly onto a mesoporous SnO2 nanoparticle matrix by the established method of successive ionic layer adsorption and reaction (SILAR).17-19 The size of PbS QDs can be controlled through the number of SILAR cycles with size dispersion less than 15%. For increasing number of SILAR cycles (Cn, n = 1, 2, 3, 4), the QD base diameter increases from 1.9 to 2.9 nm (1.9; 2.4; 2.7; 2.9 nm respectively); the aspect ratio (the height vs the radius) for the dots increases from 0.41, to 0.5, to 0.84, and finally to ~1 (nearly half-spherical shape).19 The bandgaps inferred from optical (reflectance and absorption) spectroscopy amount to: 1.35 ± 0.1 (C1), 1.1 ± 0.1 (C2), 1.0 ± 0.1 (C3) and 0.9 ± 0.1 eV (C4). Finally, following our previous report, all QDs were molecularly passivated by 1,4mercaptobenzoic acid, which substantially enhances the ET efficiency from QDs to oxide.17, 18, 20

To unambiguously monitor HET, we have employed optical-pump THz-probe spectroscopy,

which has been used extensively to quantify electron transfer rates and efficiencies for QDoxide systems.10,

17-19, 21, 22

Briefly, following selective optical excitation of QDs with a

femtosecond optical laser pulse, the photoconductivity of the sample is monitored by a second pulse in the THz frequency range (0.2 – 2 THz), with sub-picosecond time resolution. Immediately after excitation, the charge carriers are confined within the QDs, and the real conductivity is zero. Upon ET from the QD to the oxide, the real conductivity becomes finite owing to the presence of free electrons populating the oxide conduction band. The time-


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dependent rise in the conductivity therefore directly reflects the ET process, which can thus be tracked in a non-contact, quantitative manner.

Photon-energy dependent hot electron transfer (HET) To achieve efficient HET at QD-oxide interfaces at room temperature, the electronic coupling strength should be sufficiently strong to ensure that ET is sufficiently fast to effectively compete with hot carrier cooling in QDs taking place on ~1 ps time scales.18,

23-25

To

investigate the HET for direct-contact QD-oxide interfaces, QDs are excited with femtosecond laser pulses with variable wavelengths ranging from 1200 nm to 400 nm. This excitation scheme, with photon energies below the bandgap of the oxide, ensures selective excitation of QDs and the population of QD electronically excited states over a wide and tunable range of excess energies (Eex). Clearly, if hot electron cooling in the QDs outpaces HET, all electrons end up in “cold” states (1Se, the first excited state) before ET occurs. Fast cooling would therefore result in pump wavelength-independent ET dynamics. As shown in figure 1 (A), with increasing pump photon energy we observe that the ET rate increases dramatically: a direct signature of HET occurring in our samples.


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Figure 1. Photon-energy dependent hot electron transfer (HET) (A) Excitation wavelength-dependent electron transfer (ET) dynamics (from 400 nm to 1200 nm) for the sample C3 (PbS QDs with ~2.7 nm diameter). The solid lines are bi-exponential fits based on hot and cold electron transfer (HET, on sub-picosecond timescales; and CET on a ~10 ps timescale, see arrows); (B) The weight of the fast HET component in the dynamics shown in (A) vs. hot electron excess energy in the QDs; (C) HET rates vs. the excess energies of hot electrons. The CET rate was found to be independent of excess energy and fixed to 10.2 ps (grey dotted-dash line). In panels (B-C), the dashed black lines are to guide the eye; the red dotted line represents the time resolution of our setup.

The acceleration of ET with increasing photon energy can be qualitatively understood by noting that the density of electronic states in the oxide dramatically increases with increasing energy, and electrons with more excess energy can therefore transfer more quickly. Next, we quantify HET rates and efficiencies based on the results shown in figure 1(A). By pumping QDs at their band-edge using 1200 nm light, the electrons occupy only the “cold” states (the first excited 1Se state). In this situation, we find that ET taking place from the cold states (termed cold electron transfer, CET, forthwith) can be well described by a single exponential as shown as the solid grey line in figure 1(A), with an ET time constant ~ 10.2 ± 0.2 ps. For


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shorter excitation wavelengths, the ET dynamics can be well described by a linear combination of electron injecting into oxide from the cold state (with a fixed CET time 10.2 ps) and hot states: ∗ (1 −

) + ∗ (1 −

) , in which and are

the relative weights of electron transfer from the hot and cold states ( + = 1); and are the ET time constants for HET and CET, respectively. This two-state model (hot and cold injection) is clearly a simplification. In reality, a continuum of hot states exists, in which non-thermalized carriers will have (time-dependent) excess energies. The observed, effective HET rate is a function of the time-dependent occupation of high-energy electron states, and the ‘true’ HET rate from each of those states. Consequently, as multiple hot states are involved in the HET process, the “HET rates” in this work should be treated as effective HET rates. The description of HET with one effective rate provides a very reasonable description of the data, as evident from the lines in figure 1 (A). The model provides wavelength-dependent HET rates = 1⁄ and its relative weight , vs. the hot electron excess energy (Eex) as shown in figures 1(B-C). Given the symmetric band structure and the similar effective masses for both electrons and holes in PbS26, 27, the excess energy of hot carriers is expected to be distributed evenly between hot electrons and hot holes.26 As such, the excess energy of hot electrons can be approximated by: Eex=(hv-Eg)/2. As shown in figure 1(B), with increasing Eex, the HET weight rises quickly and reaches nearly 100% for Eex exceeding 0.5 eV. The HET rate (figure 1(C)) correlates well with the HET weight: it increases with Eex and dives below our time resolution of ~ 150 fs for Eex > 0.6 eV. The observed correlation between HET rate and efficiency is not surprising, as an increasing HET rate will lead to an increasingly favorable competition for HET to effectively outpace carrier cooling. Furthermore, one can estimate the coupling strength which can be parameterized as electronic coupling energies ∆ of QD hot electronic states based on the


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uncertainty principle: ∆~

ħ

= ħ ∗ . Based on this discussion and data in figure 1(C),

we conclude that the coupling strength of hot electrons in PbS QDs to SnO2 increases with hot electron excess energy Eex. For Eex > 0.6 eV, the faster-than-150 fs injection times correspond to electronic coupling energies exceeding 7 meV, in line with strong interfacial coupling characterizing the directly contacted QD-oxide heterojunction.9, 12, 13 Enhanced HET with increasing Eex has been ascribed to a larger wavefunction leakage for hot states12 with higher Eex and/or enhanced DoS both in QD and the oxide. To investigate the relative contributions to the HET process of these two mechanisms, we performed density functional theory (DFT) calculations of a PbS half-sphere QD model (~ 1.0 nm radius) adsorbed on anatase tin-oxide surface (see figure 2). The caculation details can be found in the S1 section of supporting information (SI). The electronic structure of the PbS-SnO2 system is illustrated in figure 2. Here, each horizontal line corresponds to a molecular orbital (MO) of the combined PbS: SnO2 system at the gamma point. The colors indicate the moiety contributing to the orbital: PbS (blue), Sn (black) and Oxygen (red). It is apparent that for the most energy, and in particular the CB region, the PbS states are strongly mixed with those of SnO2. This implies that the contribution to the coupling from wavefunction function overlap is largely independent of Eex. This is further corroborated by the wavefunctions delocalized over both PbS and SnO2 for different excess energies (see figure 2). We therefore conclude, that the increased HET with increasing excess energy can be primarily assigned to an increased in density of states of donor and acceptor at high excess energies.


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Figure 2. The electronic structure of the PbS-SnO2 system calculated by density functional theory (DFT). Each horizontal line corresponds to a molecular orbital (MO) of the combined PbS: SnO2 system at the gamma point. The colors indicate the moiety contributing to the orbital: PbS (blue), Sn (black) and Oxygen (red). (A) and (B) represent the corresponding molecular orbitals under 400 and 1200 nm excitation.

After discussing the rate constant and underlying mechanism for HET, now we turn to estimate its efficiency. The ultrafast sub-150 fs injection time observed here for HET, ensures that HET can effectively compete not only with carrier cooling, but also with trapping processes in PbS QDs, especially for the ligand-passivated QDs employed here.17,

18

The

absence of trapping is illustrated by experiments in which the incident photon flux was kept constant, but the excitation wavelength was varied between 800 and 400 nm (see figure S1 in SI). The injection rates differ by approximately an order of magnitude between 800 and 400 nm excitation, yet the observed long-time photoconductivities are indistinguishable. This indicates that for a given absorbed photon density, the number of the injected electrons into oxide is independent of the pump energy, i.e. ET rate. This observation is consistent with


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carrier trapping in QDs occurring on 1-1000 ps timescales28-32, being much slower than HET in this study (< 150 fs). Thus, we conclude that hot electron transfer occurs very efficiently, up to unity quantum efficiency in this directly connected PbS: SnO2 system, owing to HET rates being substantially larger than all other competing rates.

QD size dependent HET dynamics As presented in the last section, the increased density of states of donor and acceptor at high excess energies can greatly enhance the rate and efficiency of HET. Following this insight, a convenient way to control the HET process can be to tune the DoS of QDs, simply via engineering the QD size. Bigger QDs with lower bandgaps and higher DoS (for a given photon excitation energy) should therefore enable faster thus more efficient HET compared to the smaller QDs. Additionally, it has been previously reported that the hot carrier cooling slows down with increasing QD sizes.25, 33 Taking both factors into account, by increasing the QD size both HET rate and cooling rate are expected to be modulated (enhanced and reduced, respectively) in such a way to facilitate fast and efficient HET. This expectation is indeed borne out by experiment. As shown in figure 3(A-B), the weight of hot ET increases quickly from ~ 70% for sample C1 (~1.9 nm base length), to 100% for sample C4 (~ 2.9 nm base length), accompanied by a increase of HET rate (see figure S2 in SI). While a typical example of the size dependent carrier dynamics is given in the figure 3(A), the average values of HET rate and their errors are shown in the figure 3(B). The errors are estimated by measuring a series of samples for each SILAR cycle.


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Figure 3. (A) QD-size-dependent ET dynamics at QD-oxide interfaces, following excitation by a 400 nm pulse. Lines are fits based on the model discussed in the main text. For the fitting, the CET rate is fixed, while the HET rate is allowed to vary, following the discussion in the main text; (B) Summary of the size-dependent weight of the fast component in the dynamics shown in (A), vs. excess energies of hot electrons. The error bars are estimated based on measurements of 5-8 samples for sample C1-C3 and 2 samples for C4.

T dependent HET dynamics

Controlling the lattice temperature has been shown to be an effective means to tune the cooling rates thus the hot carrier distributions QDs. In the QD-oxide system in our study where hot electron transfer prevails, this can lead to the HET temperature dependent by involving various hot states at different temperature. For small C1-QDs we find that, at room temperature, carrier cooling apparently competes efficiently with HET, and ~30% of the carriers are not injected from the hot states. If HET indeed competes with carrier cooling, that 70/30 branching ratio should change when carrier cooling is affected by changing the QD lattice temperature (T). At low T, the phonon population is reduced, and carrier thermalization is slowed down. For PbS QDs in the strong confinement regime, T has been reported to have little effect on their electronic structures (e.g., bandgaps).34 As such, we do not expect changes in ET induced by this effect.


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Figure 4. (A) Temperature-dependent ET dynamics in QD-oxide interfaces following 400 nm excitation. The measurements are done based a sample of C2. Solid lines are fitting based on the model discussed in the paper; (B) T dependent the rates and efficiency of the HET vs. hot electron excess energies; the dash lines are guides to the eye.

In figure 4(A), we demonstrate the T dependent HET dynamics for the C2 sample (QD with base diameter 2.4 nm). The inferred HET rates and efficiencies are summarized in figure 4(B). Upon lowering temperature from 293 to 135 K, the HET efficiency goes up from 70% to 90%; the temperature dependent HET rate is shown to be well correlated to the HET efficiency. With lowering the temperature, electrons with higher excess energy are involved in the HET processes. This leads to an increase of apparent HET rates thus efficiency. To summarize, efficient hot electron extraction can be achieved in strongly coupled QD-oxide systems at room temperature. We show that the competition between HET and hot carrier cooling determines the interfacial HET efficiency. By increasing the photon excess energy, we demonstrate that HET rate can be substantially enhanced: with Eex above ~0.5 eV, sub-150 fs HET process with unity quantum yield is observed. Employing DFT calculation, we further show the increased HET rate can be primarily assigned to an increased density of states of donor and acceptor at high excess energies. Furthermore, in line with the rate competition scenario, we show that HET can be enhanced by either increasing the QD size and thus the ACS Paragon Plus Environment

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donor DoS, or lowering T to reduce the cooling rate and efficiency. Our results provide new insights into circumventing thermal losses in sensitized systems, with potential relevance for low-cost solar energy conversion schemes.

Supporting Information Additional details on density functional theory (DFT) calculations; futher discussion and figures on fluence dependence THz conductivity for 400 and 800 nm pump and the sizedependent HET rate

Acknowledgments This work has been financially supported by the Max Planck Society. E. C. acknowledges financial support from the Max Planck Graduate Center and the regional government of Comunidad de Madrid under project (2017-T1/AMB-5207). H. I. W. acknowledges financial support from the Graduate School Materials Science in Mainz through the German Research Foundation in the Excellence Initiative (GSC 266).

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Efficient Hot Electron Transfer in Quantum Dot-Sensitized Mesoporous

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