ARTICLE pubs.acs.org/JPCC
Energetics of CdSe Quantum Dots Adsorbed on TiO2 Tal Z. Markus,†,|| Stella Itzhakov,‡,§,|| Yafit Itzhaik Alkotzer,§ David Cahen,§ Gary Hodes,§ Dan Oron,*,‡ and Ron Naaman*,† †
Department of Chemical Physics, ‡Department of Physics of Complex Systems, and §Department of Materials and Interfaces, Weizmann Institute, Rehovot 76100, Israel
bS Supporting Information ABSTRACT: Understanding how quantum dot (QD)-sensitized solar cells operate requires accurate determination of the offset between the lowest-unoccupied molecular orbital (LUMO) of the sensitizer quantum dot and the conduction band of the metal oxide electrode. We present detailed optical spectroscopy, low-energy photoelectron spectroscopy, and two-photon photoemission studies of the energetics of size-selected CdSe colloidal QDs deposited on TiO2 electrodes. Our experimental findings show that in contrast to the prediction of simplified models based on bulk band offsets and effective mass considerations, band alignment in this system is strongly modified by the interaction between the QDs and the electrode. In particular, we find relatively small conduction bandLUMO offsets, and near “pinning” of the QD LUMO relative to the conduction band of the TiO2 electrode, which is explained by the strong QD-electrode interaction. That interaction is the origin for the highly efficient QD to electrode charge transfer, and it also bears on the possibility of hot carrier injection in these types of cells.
’ INTRODUCTION In recent years it has been recognized that band offsets in nanocrystalline semiconductor heterojunctions often differ from those extrapolated from bulk properties1 and from simple physical models of semiconductor nanocrystals (quantum dots, QDs).2,3 This has been experimentally observed for core/shell heterostructure nanocrystals, where, for example, lattice strain was shown to significantly affect band alignment,4 as well as for QDs deposited on bulk substrates,5,6 where the electronic interaction with the surface can modify band offsets. An exact description of band offsets in such systems is of particular importance for understanding and optimizing QD-based solar cells and particularly QD-sensitized solar cells (QDSSCs). QDSSCs are a promising low-cost alternative to present photovoltaic cells, such as crystalline silicon and thin inorganic films. They are analogous in structure to dye-sensitized solar cells7 but use inorganic QDs rather than organic or organometallic dyes as sensitizers.8,9 In this type of cell, lower band gap QDs such as CdSe are bound to a wide band gap nanoporous metal oxide semiconductor electrode, commonly made from nanocrystalline TiO2 or nanostructured ZnO. Upon photoexcitation of the QDs, electrons are injected from an excited state of the QDs into the electrode, effecting charge separation. The remaining holes are then usually harvested via an electrochemical reaction with an electrolyte or through a hole conductor, completing the circuit for a photovoltaic cell. Clearly, in such a design, a crucial requirement from the QD sensitizer is that it injects carriers into the porous electrode, i.e., that the lowest-unoccupied molecular orbital (LUMO) of the QD lies above the conduction band of the electrode. Such band offsets have been studied by a variety of techniques. Most notably, transient absorption and luminescence spectroscopies, r 2011 American Chemical Society
usually in the visible or near-infrared spectral range8,1012 but also in the terahertz range,13 which characterize the charge carrier dynamics, have been used to identify if carrier injection occurs. This is, however, only an indirect way to infer the band alignment. A more direct handle on band offsets can be provided by cyclic voltammetry, although care has to be taken to perform it on the full system, since a variety of external factors, such as surface coverage by monolayers or pH, may affect the band offset. The most direct measurement of band offsets is offered by photoelectron spectroscopy, either in the UV or X-ray spectral range, which provides access to the position of the valence band edge and the highest-occupied molecular orbital (HOMO) in the fully assembled system. When complemented by optical absorption spectroscopy, the full band diagram can be obtained from either of the two latter types of measurements, as has recently been done for the TiO2/PbSe system.1416 Here we study the band alignment of the prototypical system of CdSe QDs deposited on TiO2 via a short organic linker. This system was extensively studied via transient absorption and via luminescence spectroscopy,8,10,12 even down to the single nanocrystal level.17 In the transient absorption experiments, the absorption bleach decay time, associated with electron injection to the TiO2, was found to be size-dependent, with smaller QDs exhibiting faster electron transfer. For the TiO2/CdSe system, electron transfer rates varied from 5 1010 s1 for larger QDs to about 2 1011 s1 for smaller ones. The current understanding of carrier dynamics in these experiments relies on estimation of the free energy associated with electron transfer using bulk Received: April 18, 2011 Revised: May 29, 2011 Published: June 02, 2011 13236
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electron affinities and effective mass modeling. Alternatively, these quantities are extrapolated from separate measurements of the constituents (i.e. QDs and the metal oxide electrode) vs known reference electrodes (as measured for CdSe QDs, for example18). Such an approach led to an estimated conduction band-LUMO offset of at least 0.5 V between the TiO2 and CdSe QDs and predicted that, due to the much smaller effective mass of the electron relative to the hole in CdSe, the conduction bandLUMO offset should strongly depend on QD size. In contrast with these predictions, results of a single recent UV photoelectron spectroscopy (UPS) measurement on chemicalbath deposited CdSe QDs on TiO2,19 showed that the TiO2 conduction band and the CdSe LUMO are, within experimental error, aligned with each other. By use of chemical bath deposition it is, however, difficult to systematically study the coupling between the TiO2 and CdSe QDs as a function of size because of the relatively broad inhomogeneous size distribution of the grown nanocrystals. In the following we report on a combined UPS, low-energy photoelectron transmission (LEPET) spectroscopy and two-photon photoemission (TPPE) spectroscopy study of the energetics of size-selected CdSe QDs on TiO2. Our goal is to identify the effects of coupling between the QDs and the TiO2 electrode on band offsets in such a system. We find that the effective-mass based models fail to predict the observed results.
Figure 1. The photoemission schemes: In the case of LEPET (a) electrons are ejected from below the Fermi level by photons with energy that exceeds the ionization potential of the nanoparticles. In the TPPE experiments (b), the electrons are ejected by a two-photon process in which the first photon excites electrons in the nanoparticles to unoccupied states located below the vacuum level. The excited electrons relax to originally unoccupied states from which they are ejected by a second photon.
’ RESULTS AND DISCUSSION Figure 1 presents a scheme describing the photoemission processes. In LEPET, electrons are ejected from the HOMO, while the TPPE studies provide information on the LUMO of the system. The photoelectron energy distribution is shown in Figure 2, for photoelectrons ejected by the LEPET process, from bare ITO, from ITO covered by five layers of TiO2 coated with the organic linker 3-mercaptopropionic acid (3-MPA), and from the same substrate after the adsorption of CdSe QDs of different sizes. The photon energy was 6.42 eV. As was explained before5 the work function (WF) is given by WF ¼ hυ ðEK max LECOÞ
ð1Þ
where the LECO is the low-energy cut-off in the photoemission spectrum, EKmax is the maximum kinetic energy as obtained for electrons ejected from just below the Fermi energy, and hυ is the photon energy. The LECO is related to the vacuum level of the sample since electrons ejected with kinetic energy just above the vacuum level appear with the lowest possible energy at the detector. Hence, the LECO reflects the work function of the sample, as electrons at the LECO are those with the highest binding energy. The work function of the samples containing QDs was found to depend on the density of the QDs. No attempt was made in this study to quantify the WF since the alignment of the energy states of the QDs is not affected by the QD density, as shown in the Supporting Information (Figure S4 of Supporting Information) The most pronounced effect in the spectra in Figure 2 is the shift in the HECO (high-energy cut-off) for the different samples studied. The HECO results from electrons ejected from occupied electronic states lying closest to the Fermi level; hence the electrons at the HECO are those with the lowest binding energy. When the HECO is shifted to lower kinetic energy, it means that the weakest bound occupied electronic state is positioned at
Figure 2. The LEPET spectra of ITO (gray), ITO coated with 3-MPA on 5 layers of TiO2 (black), and with 3 different sizes of CdSe QDs with diameters of 3.7 nm (red), 4.7 nm (blue), and 6.4 nm (dark yellow). The LECO for the samples with CdSe QDs is shown. The insert is a zoom on the QDs’ HECO region which shifts according to the QD size.
a lower energy relative to the Fermi level, namely, it is more strongly bound. Since the spectra of the samples with the QDs were obtained with a much lower laser intensity (∼50 pJ) than that used for obtaining the signal from TiO2 samples that do not contain QDs (∼600 pJ), these spectra are entirely due to the presence of the QDs and therefore the HECO is related to the HOMO (or valence band edge) of the QDs. This conclusion is supported by the shift of the HECO as a function of the size of the QDs. If the HECO had a significant contribution from electrons originating from the TiO2 or the organic molecules, no dependence on the QD size would be expected. Thus the shift between the HECOs indicates a shift between the HOMOs of ∼60 meV for each size of QDs studied (insert Figure 2), with the smallest QDs having the lowest-lying HOMO and the largest QDs the highest lying HOMO. We are able to place the HOMOs of the QDs relative to the Fermi level by comparing the spectra to that obtained from a 13237
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Figure 3. (a) TPPE spectra of ITO coated by TiO2/MPA and with QDs (6.4 nm) using photon energy of 4.43 eV. When the laser intensity is increased, it is possible to observe a peak from TiO2 surface states at energies below the Fermi level (dotted black line). (b) Spectra of ITO, coated with TiO2, MPA, and QDs (size 6.4 nm) using three different wavelengths, showing the HECOs. Notice that the shifts between the HECO energies are ΔEk= Δhν, indicating a relaxation into long-lived states, prior to ejection by the “probe” photon.
Figure 4. TPPE spectra from three different sizes of QDs obtained when using a wavelength of 300 nm (4.13 eV). All the spectra are identical, indicating that the photoelectrons in the TPPE process are ejected from the same states in the TiO2.
conductive substrate (bare Au or ITO), where the HECO results from electrons ejected from the Fermi level. We find that the HOMO states are located at 1.62, 1.56, and 1.5 eV below the Fermi level for the 3.7, 4.7, and 6.4 nm diameter QDs, respectively. Absorption experiments performed on the largest QDs deposited on a mesoporous TiO2 indicated that there is no apparent significant shift of the first exciton peak as compared to the solution spectrum (see Figure S5 of Supporting Information). The difference in the energy band gaps obtained by photoluminescence (PL) measurements (optical band gap between the smallest (2.11 eV) and largest QDs (1.94 eV) is 170 meV. Since the energy shift between the HOMOs, as obtained here, is ∼120 meV, the energy difference between the LUMOs of the QDs is expected to be in the order of ∼50 meV. The spectrum of the ITO/TiO2/MPA was obtained at higher laser intensity and results from surface states present in the TiO2. These TiO2 surface states were observed in previous works by UPS, where it was also shown that the valence band of the TiO2 lies ∼3 eV below the Fermi level, so that the conduction band of the TiO2 is only approximately 200 meV above the Fermi level.20,21 This result is in good agreement with results obtained
Figure 5. Energy diagram, as deduced from the PL, LEPET, and TPPE spectroscopy, for ITO/TiO2, ITO/TiO2/MPA, and when CdSe QDs are adsorbed ITO/TiO2/MPA/CdSe-QD, for three different QD sizes. Both the occupied electronic states (blue) and the unoccupied electronic states (red) are shown. As the size of the QDs increases, the HOMOLUMO energy gap decreases and the shift of the HOMO is ∼2.4 times larger than that observed for the LUMO.
by UPS on the samples studied in this work, where the TiO2 valence band was found to be at 3.1 ( 0.1 eV below the Fermi level (Figure S6 of Supporting Information) placing the conduction band less than 200 meV above the Fermi level. Figure 3a presents the TPPE spectra of ITO coated by TiO2/ MPA without QDs and TiO2/MPA with QDs (6.4 nm) using a photon energy of 4.43 eV. The spectrum of the sample with the QDs (red curve) shows one broad band composed from two peaks (deconvolution not shown). The low-energy peak is also present in samples of TiO2 that do not contain QDs (black curve) and is a result of electrons ejected by a single photon process from TiO2 surface states at energies just below the Fermi level (Figure 3a dotted black line). The second peak, at higher 13238
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Figure 6. (a) Normalized emission spectra of 3.7 nm (blue), 4.7 nm (green), and 6.4 nm (red) CdSe QDs in toluene (a) and after being adsorbed onto the ITO coated TiO2 surface via 3-MPA linker (b).
kinetic energies, is present only in samples coated by QDs and is the result of photoelectrons ejected from states above the Fermi level in a two photon process. In TPPE spectroscopy it is important to verify that the spectra obtained are not due to a coherent two-photon absorption process, but that the spectra result from relaxation of the electron excited by the first photon, into a long-lived intermediate state, from which it is excited by the second photon to above the vacuum level. If the TPPE process is indeed sequential, the final kinetic energy, ΔEk, of the electron will be proportional to the change in the photon energy, Δhν. In this case, when varying the photon energy, one expects to obtain ΔEk = Δhν. However, if the process is a coherent two-photon-process then ΔEk = 2Δhν. Figure 3b presents the photoemission spectra obtained from a sample containing the large QDs with three different photon energies. The HECO in these spectra are separated from each other by ΔEk = Δhν, indicating that the TPPE process involves relaxation of the photoexcited electron into long-lived states, before being ejected by the second photon (as shown schematically in Figure 1b). Figure 4 presents the spectra for samples covered with three different sizes of QDs, using a photon energy of 4.13 eV. No QD size dependence is observed in the spectra, indicating that all the electrons are ejected from a common electronic state and not from the LUMO of the QDs. In addition the kinetic energies of the photoemitted electrons are lower than expected based on the calculated energy of the LUMO taken from the optical band gap and the HOMO energy, as obtained from the LEPET experiments. These observations can be explained by assuming that the electrons, once excited in the QDs, are transferred very rapidly to longer-lived, unoccupied states in the TiO2 conduction band. Indeed, no saturation is observed in the signal with increasing light intensity.
’ CONCLUSIONS On the basis of the results presented it is possible to map the energy levels of the system. Figure 5 shows the energy level diagram schematically presenting the fact that the QD HOMO position shifts significantly as a function of QD size, while the LUMO position barely changes. This type of near “pinning” of the LUMO is in sharp contrast to CdSe QDs adsorbed on either Au or ZnO surfaces, where the HOMO level was nearly pinned and the LUMO levels shifted as a function of the QD size.5,22 In both of those experiments, HOMO pinning was explained as a result of the strong interactions between the energy levels of the substrate and the QD HOMO. Here, the interaction between the energy levels of the substrate and the LUMO of QD is dominant and therefore the LUMO is nearly pinned.
In the simplest model of quantum confinement,23 the HOMO and LUMO shifts are inversely proportional to the effective mass of the holes and electrons respectively. Since the effective mass of a hole of CdSe is known to be three times more than that of the electron, this approximation predicts a shift of the HOMO, as a function of the QD size, which is three times smaller than the shift of the LUMO. However, one should appreciate that the electronic coupling, when the QDs are attached chemically to a substrate, may modify substantially the energetics of the system. The effect of the substrate on the adsorbed QD is related to the ratio between the effective mass of the charge carriers in the two phases. Whereas the electron effective mass in ZnO me = 0.23 m024 is only twice as large as that of CdSe, the situation is dramatically different in TiO2, where the electron effective mass is at least of the order of me ≈ 1 m025 (with reported experimentally inferred values of up to 10 m026,27). It is thus one to 2 orders of magnitude larger than in CdSe. Hence, unlike in the case of ZnO, coupling between the QD and the TiO2 surfaces can dramatically modify the energetics of the system, leading to a HOMO shift nearly three times larger than the LUMO shift. This observation suggests that excited electronic states in CdSe adsorbed on TiO2 are dramatically different than those of the bare QD, and contain a significant “TiO2 character”, facilitating electron injection to the electrode and reducing recombination losses in the QD. Indeed, the high electron effective mass in TiO2 was already recognized as an important factor in the relatively high conversion efficiencies observed in DSSCs.28 When conducting TPPE spectroscopy, the electrons were ejected through states of the TiO2, and no ejection of electrons from the LUMO of the QDs could be detected. This observation further validates the very efficient transfer of the photoexcited electrons in the QDs to the TiO2 conduction band. It also leads to the significant conclusion that the question whether hot electrons, having energies above the QD LUMO, are transferred into the TiO2 hot or after cooling to the QD LUMO is ill-posed in this context, because the excited state in the QD state already contains a significant component of TiO2 conduction band states. Our data also support the conclusion that this mixing is stronger in small QDs, in correspondence with the more rapid injection rates obtained in luminescence and transient absorption measurements.
’ METHODS Sample Preparation. On a 25 mm 25 mm glass substrate, coated with ITO, a dense TiO2 layer was formed29,30 with 120 nm average thickness from a sol of 0.125 M titanium 13239
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’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental procedures, UVvis spectra, SEM images, and UPS spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Author Contributions
)
isopropoxide. The CdSe QD deposition onto the dense TiO2 surface was done via an organic linker (MPA). The TiO2/MPA samples were then immersed into the QDs dispersion in toluene for 20 min, washed with toluene, and dried with N2. The detailed synthesis of CdSe QDs, sample preparation and characterization techniques can be found in the Supporting Information section. PL Characterization. The emission spectra from the three different sizes of QD are shown in Figure 6. In general, only a small shift was observed between the emission from solution (Figure 6a) and from the monolayer assemblies (QDs attached to the ITO/TiO2 via 3-MPA) as shown in Figure 6b. The PL spectra were collected in a Fluorolog-3 Spectrofluorometer (Horiba JobinYvon, France) at room temperature using an excitation wavelength of 350 nm. The samples were placed at an angle of 30° to the incident light and the emitted PL was collected (front face PL measurement) through a monochromator. Photoemission Setup. The photoemission experiments are based on ejection of photoelectrons from the QDs that are deposited via 3-MPA on ITO coated with dense TiO2 layers in an ultrahigh vacuum chamber (
