Sensitization of TiO2 with PbSe Quantum Dots by SILAR: How

Letter pubs.acs.org/JPCL

Sensitization of TiO2 with PbSe Quantum Dots by SILAR: How Mercaptophenol Improves Charge Separation Néstor Guijarro,†,‡ Teresa Lana-Villarreal,† Thierry Lutz,‡ Saif A. Haque,‡ and Roberto Gómez*,† †

Institut Universitari d’Electroquímica i Departament de Química Física, Universitat d’Alacant, Ap. 66, E-03080, Alicante, Spain Department of Chemistry and Centre for Plastic Electronics, Imperial College London, Exhibition Road, South Kensington, London SW7 2AZ, United Kingdom

‡

S Supporting Information *

ABSTRACT: The use of PbSe quantum dots (QDs) as sensitizers for TiO2 samples has been primarily hampered by limitations on charge injection. Herein, a novel successive ionic layer adsorption and reaction (SILAR) method, allowing for an intimate TiO2/PbSe contact and a strong quantum confinement, is described. Photoelectrochemical experiments and transient absorption measurements reveal that charge separation indeed occurs when using either aqueous sulfite or spiro-OMeTAD as a hole conductor and that it can be further enhanced by attaching p-mercaptophenol (MPH) to the QD surface. These results suggest that MPH can promote an efficient funneling of the photogenerated holes from the PbSe to the hole scavenging medium, thereby increasing the yield of electron injection into TiO2. In a more general vein, this work paves the way for the fabrication of PbSe-sensitized solar cells, emphasizing the importance of controlling the QD/hole scavenger interface to further boost their conversion efficiency. SECTION: Energy Conversion and Storage; Energy and Charge Transport

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heterojuntion.18,19 Likewise, but using a rutile single crystal as electron acceptor, Timp et al. emphasized the difficulty for electron transfer,20 whereas Tisdale et al. observed that injection could occur through hot electron transfer.9 However, these results do not rule out altogether the possibility of cold electron transfer from PbSe to TiO2 but suggest that a proper energetic alignment and good TiO2/PbSe coupling are required. Concerning the former, it is worth mentioning that not only the quantum confinement21 but also the adsorption of molecular dipoles on the QD surface22,23 can modify the driving force for electron injection in the oxide by tailoring the energy level position. In this work, a successive ionic layer adsorption and reaction (SILAR) method for the in situ growth of crystalline PbSe QDs on mesoporous TiO2 is presented. A systematic study of the interfacial charge transfer at TiO2/PbSe films in contact with either liquid (aqueous sulfite) or solid (2,2′,7,7′-tetrakis-(N,Ndi-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-OMeTAD)) hole scavengers is addressed by photoelectrochemical and transient absorption measurements, respectively. These results show that charge separation is critically enhanced after modifying the surface of the PbSe QDs with p-mercaptophenol molecules, which are proven to work as efficient mediators in the hole transfer toward the scavenger. These findings could pave the way for the fabrication of PbSe-sensitized solar cells,

ince the advent of photovoltaics, the photogeneration of charge carriers (i.e., electron−hole pairs) in a light-absorber material, followed by their efficient separation and collection, has been established as the essential principle of solar cells, fueling successive generations of devices. Up to now, various designs and a broad variety of materials have been explored in pursuit of an efficient photon-to-current conversion.1−23 Semiconductor nanocrystals (NCs) and specifically metal chalcogenide quantum dots (QDs) have emerged as promising candidates to be employed in photoconverters.4−7 In particular, PbSe QDs surpass other chalcogenide QDs owing to their high extinction coefficient and strong quantum confinement effects.8 Interestingly, the latter triggers the electronic wave function to extend well beyond the QD surface favoring coupling with adjacent NCs, slows down the relaxation (cooling) rate of photogenerated carriers,9 and allows us to tune the band gap over a wide range10 (from 0.4 eV for larger particles to 2 eV for NCs below 1 nm in diameter). As a result of these outstanding properties, processes such as multiple exciton generation (MEG)11,12 and hot electron transfer9 have been profusely studied using this material. Recent breakthroughs on PbSe QDbased solar cells, such as efficiencies of up to 4.5%,15 along with the long awaited confirmation of MEG in devices16 have been obtained in Schottky type architectures. In contrast, rather poor results have been reported for bulk heterojuntion assemblies.17 Surprisingly, reports dealing with the sensitization of metal oxides using PbSe QDs are rather scarce. Terahertz timedomain spectroscopy has shown that electron injection from PbSe QDs to mesoporous TiO2 is negligible, primarily due to unfavorable energy level alignment at the donor−acceptor © 2012 American Chemical Society

Received: September 27, 2012 Accepted: November 2, 2012 Published: November 2, 2012 3367

dx.doi.org/10.1021/jz301528a | J. Phys. Chem. Lett. 2012, 3, 3367−3372

The Journal of Physical Chemistry Letters

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cycles (Figure 1D,E) lead to a higher coverage with larger PbSe particles, showing a broader size dispersion, as evidenced by the corresponding size distribution histograms. (See Figure S2 in the Supporting Information.) In addition, it is observed that smaller NCs seem to present a nanoplate-type shape in contrast with the quasi-spherical shape of larger ones. (See Figure S3 in the Supporting Information.) A similar evolution of the particle morphology with the number of SILAR cycles has recently been reported for SILAR-deposited CdSe QDs.24 Furthermore, it is worth mentioning that the method described here generates PbSe NCs smaller than those obtained through previously described chemical bath deposition methods.25 Finally, an elemental analysis by means of energy-dispersive X-ray (EDX) spectroscopy reveals the presence of Pb and Se in approximately stoichiometric amounts, although an excess of Pb is detected for three SILAR cycles. To assess the yield for electron injection into the TiO2, we performed photocurrent transients of PbSe-sensitized photoanodes in a three-electrode cell at −0.25 V versus Ag/AgCl/ KCl(sat) using a sulfite solution as sacrificial hole scavenger26,27 (Figure 2A,B). The photocurrent value depends, apart from the irradiance on the electrode, on the light-harvesting efficiency, the yield for electron injection, and the charge collection efficiency. However, the experimental conditions employed here (viz. absence of electron scavenger in the electrolyte and sufficiently positive potential) minimize recombination (leaving

stressing the importance of modifying QD interfaces to enhance charge separation. The in situ growth of PbSe QDs has been carried out by means of an aqueous SILAR method similar to that previously reported by some of us.24 In brief, mesoporous TiO2 films were successively dipped in 10 mM Pb(NO3)2 + 0.1 M sodium acetate solution (lead precursor) for 15 s and in a selenosulfate solution (selenium precursor) for 30 s, soaking them after each immersion in a 0.1 M sodium acetate solution for 1 min to remove the excess of precursors not attached to the substrate. One dipping in both precursor solutions is referred to as one “cycle”. (See the Supporting Information for more details.) Figure 1 presents the absorption spectra and transmission electron microscopy (TEM) images for TiO2/PbSe thin films

Figure 1. Absorption spectra for TiO2/PbSe films as a function of the number of SILAR cycles (1−3) (A). TEM images of TiO2 before (B) and after sensitization with 1 (C), 2 (D) or 3 (E) cycles of PbSe SILAR deposition (scale bar 5 nm). The value of the Pb:Se ratio obtained by EDX is shown on each image.

as a function of the number of SILAR cycles. As shown, the sensitization of the film allows extending its absorption throughout the UV−vis-NIR region. By increasing the number of cycles, the absorption spectra are both augmented and redshifted, without showing well-defined peaks or shoulders (Figure 1A). This behavior, commonly observed for SILAR deposits, is accounted for by a progressive loss of quantum confinement and the existence of a broad size distribution.24 From an inspection of the TEM images (Figure 1B,C), it is clear that TiO2 nanoparticles become evenly decorated by small and well-dispersed NCs after only one SILAR cycle. Further

Figure 2. Photocurrent transients for TiO2 electrodes sensitized with PbSe deposited by SILAR before (A) and after modification with MPH (B) using a N2-purged 0.5 M Na2SO3 solution as electrolyte. Photoanodes are termed as TiO2/PbSe(number of SILAR cycles). Photocurrent experiments for a TiO2/PbSe(1) photoanode in N2purged 0.5 M Na2SO4 solution before (black line) and after the addition of MPH (10 mM) to the electrolyte (red line) (C). Irradiance: 45 mW·cm−2 from a Xe arc lamp equipped with filters to allow for illumination only in the 380−1100 nm range. 3368



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far,9,20 the well-known exponential distribution of states just below the conduction band of the anatase NCs in the P25 TiO2 sample30,31 would facilitate electron transfer.28 Molecular dipoles have been recently introduced in QDsensitized solar cells based on either liquid electrolytes or solid hole-transporting materials (HTMs), yielding a significant enhancement in performance.23,32 Figure 2B shows the photocurrent transients for photoanodes before and after modification with p-mercaptophenol (MPH). The modification was carried out by immersing the sensitized electrodes in a 10 mM MPH solution in toluene overnight. As observed, the attachment of MPH to TiO2 gives rise to a slight increase in photocurrent, whereas the modification of the photoanode sensitized by one SILAR cycle strikingly improves its response. By taking into account the total irradiance on the electrode (45 mW·cm−2), the transmittance spectra of the filters, and the absorption spectrum of the electrode (Figure 1A, curve 1), the flux of absorbed photons was estimated and thus the maximum attainable photocurrent density, jph,max (7.5 mA·cm−2). The ratios between the observed and the maximum photocurrent densities, jph/jph,max, were equal to approximately 0.06 and 0.19 before and after MPH modification. These values are not particularly high. However, it is worth noting that they were obtained without any particular optimization of the TiO2 film (thickness, particle size, etc.). In fact, these experiments show for the first time that significant photocurrents can be obtained with TiO2 electrodes sensitized with PbSe QDs. Similar treatments with other benzenethiol derivatives have already proven to enhance electron injection from the QD into the oxide.22 It is accepted that such kind of molecular dipoles adsorb specifically on the surface of chalcogenide QDs through the thiol group, inducing a shift in the QD energy levels that modifies the band alignment across the TiO2/QD interface.22,32 As a result, the driving force for electron injection can be increased, boosting the electron transfer and hence the photocurrent, as observed experimentally. In contrast with the results obtained for electrodes sensitized with one SILAR cycle, the treatment with MPH barely enhances the photocurrent for electrodes modified with either two or three SILAR cycles. (See Figure S4 in the Supporting Information.) Presumably, the donor energy level of the corresponding PbSe QDs is well below the conduction band edge of TiO2,20 making the energy level shift induced by MPH insufficient to bring about electron injection. Finally, the photocurrent increase observed for the bare TiO2 electrode upon MPH modification is probably due to the formation of a surface charge-transfer complex that could inject electrons into the metal oxide upon visible-light excitation.33,34 In fact, a change of color from white to pale yellow was noticed upon modification with MPH. From another perspective, it is widely accepted that thiols work as efficient hole scavengers for semiconductor QDs such as CdSe and CdTe,35 among others. Figure 2C shows photocurrent transients performed for TiO2/PbSe(1) electrodes in a sodium sulfate solution before and after the addition of MPH to the electrolyte. In the absence of MPH, the observed monotonous decrease in the photocurrent is primarily attributed to the inability of the sulfate ions to scavenge holes, thus favoring both electron−hole recombination in the QDs and their photooxidation. Interestingly, upon the addition of MPH, stable and higher photocurrents are recorded, exposing the role of the MPH as hole scavenger. Therefore, after QD modification with MPH (Figure 2C), an improve-

only that associated with the TiO2/PbSe interface). Consequently, the photocurrent becomes mainly determined, in this particular case, by the yield of electron injection and the light-harvesting efficiency.27 As shown in Figure 2A, after one SILAR cycle a photocurrent higher than that obtained for bare TiO2 is recorded. Importantly, increasing the number of SILAR cycles leads to a dramatic decrease in the photocurrent, giving values even lower than those of naked TiO2, even though the light-harvesting efficiency increases. We note that the photocurrent obtained for bare TiO2 electrodes is likely due to excitation by the UV radiation not removed by the UV filter used in the experiment (cutoff λ < 380 nm, see Figure S1 in the Supporting Information). Our results indicate that only sufficiently small PbSe QDs (1 SILAR cycle) are able to transfer photoinduced electrons into the TiO2. Most likely, the strong quantum confinement of these NCs and the direct contact with the TiO2 achieved by the SILAR method promote an efficient coupling between electron donor and acceptor, along with a favorable energy alignment across the TiO2/PbSe interface. These results are in agreement with those reported by Strel’tsov et al., where the photocurrent response of nanoporous TiO2 electrodes was extended toward the visible region upon electrodepositing small PbSe NCs.28 In contrast, the decrease in photocurrent observed as the number of cycles increases is evidence that large PbSe QDs are inefficient for electron injection. In addition, the fact that photocurrent values lower than those obtained for bare TiO2 are recorded upon sensitization by three SILAR cycles suggests that large PbSe QDs efficiently absorb most of the incident UV photons, limiting TiO2 excitation. Acharya et al. have argued that only PbSe QDs smaller than 5 nm in diameter are able to inject electrons into TiO2, whereas a type I heterojuntion at the TiO2/PbSe interface is formed for larger QDs, impeding electron transfer.29 In this sense, TEM analysis reveals that most of the NCs are well below the threshold of 5 nm (i.e., the average diameter and height are around 2.0 and 1.5 nm, respectively) after one SILAR cycle (Figure S2 in the Supporting Information). In contrast, a significant part of the NCs are over 5 nm in diameter after two and three SILAR cycles. (See Figure S2 in the Supporting Information.) It is worth noting that the presence of such large PbSe NCs, with a low band gap, limits light harvest by the coexisting smaller NCs, which actually are the active ones for electron injection, thereby reducing the photocurrent. These findings indicate that a strong quantum confinement in PbSe QDs is crucial to inject photogenerated electrons into the TiO2 film; otherwise, photoinduced carriers recombine in the QDs because of the poor driving force for charge transfer. Hot electron injection could promote charge transfer despite the loss of quantum confinement, but this process has proven to be inefficient when a high density of defects is present at the TiO2/PbSe interface.18 Given that the PbSe SILAR method presented here resembles that previously reported for CdSe QDs,24 and having shown that in the latter case significant recombination occurs at the TiO2/QD interface, we do not expect that the mechanism of hot electron transfer could operate in the present case. It should be mentioned that up until now the poor charge separation reported for TiO2/PbSe assemblies could be attributed to the large size of QDs used in previous studies (5.518 and 6.7 nm9) or the weaker coupling between the QD and the metal oxide because QDs were attached by molecular wires18 or directly precipitated9 on the oxide surface. Moreover, in contrast with the rutile single crystals used as substrate so 3369



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effective masses of electrons (me = 0.04) and holes (mh = 0.03) in PbSe QDs are fairly similar,28 both the HOMO and the lowest unoccupied molecular orbital (LUMO) levels will significantly shift when the QD size changes. Consequently, it is not unexpected that the HOMO level for PbSe QDs deposited after two or three SILAR cycles could be located above the MPH HOMO level, precluding the advantageous cascade effect for hole extraction. Finally, it is worth mentioning that for bare TiO2 films the addition of MPH in the sulfate electrolyte drastically reduces the photocurrent (see Figure S5 in the Supporting Information), which indicates that MPH adsorption blocks the surface sites involved in the water photooxidation process. The MPH molecule adsorbed via the phenol group would mainly act as a recombination center. Recent efforts are focused on the preparation of solid-state QD-sensitized solar cells, which entails replacing the liquid electrolyte by a solid material that functions as a hole transporting medium. So as to gain insights into the behavior of PbSe QDs in such a system, sensitized TiO2 films were combined with spiro-OMeTAD, a widely extended organic hole conductor. Transient absorption spectroscopy (TAS) experiments were performed to study the charge recombination dynamics at the TiO2/PbSe/spiro-OMeTAD heterojunction before and after PbSe surface modification by MPH. In this case, photogenerated electrons are injected into the TiO2 matrix, whereas holes oxidize spiro-OMeTAD molecules, until both carriers recombine over time. Transient decays presented in Figure 3 were carried out on the microsecond-to-second time scale, probing the spiro-OMeTAD+ cation at 1600 nm.39,40 In fact, the amplitude of the transient band at 1600 nm can be directly correlated with the quantity of spiro-OMeTAD+ photogenerated by means of its extinction coefficient (22 800 M−1 cm−1). Hence, we can determine the number of spiroOMeTAD+ species generated per photon absorbed (termed as spiro-OMeTAD+ yield henceforth).38 As observed, upon modifying the PbSe surface with MPH, the spiro-OMeTAD+ yield is increased from 4 to 21% (measured at 1 μs), whereas the lifetime remains barely unaltered (>10 ms). The similarity of the lifetime values suggests that a small portion of the oxide surface is in contact with spiro-OMeTAD molecules and hence, in both cases, the lifetime is determined by the electron back flow from TiO2 to the hole-transporting material. The increase in the spiro-OMeTAD+ yield upon MPH modification obviously corresponds to an increase in the quantity of holes extracted. As discussed when using sulfite electrolyte, two different mechanisms might account for the improvement of charge separation in the presence of MPH. On the one hand, the MPH adsorption on the NC surface would induce an upward shift of the PbSe energy levels, increasing thereby the driving force for electron injection and the population of holes in the NCs available to be scavenged by the spiro-OMeTAD. On the other hand, MPH molecules could work as efficient mediators in hole extraction. In this respect, it is expected that these molecules, specifically anchored to the QD surface, quickly capture the hole and funnel it toward the spiroOMeTAD, taking advantage of favorable interactions among organic moieties. In contrast, the lack of specific interactions between the QD and the spiro-OMeTAD would likely hamper the hole withdrawal. In this sense, Im et al. recently pointed out that specific interactions between semiconductor sensitizers and hole conductors are crucial to ensure electronic coupling and efficient charge separation.41 Moreover, the location of the energy levels suggests a cascade alignment that would account

ment in the rate of hole extraction, concomitant to the energy level shift mentioned above, should be considered. The MPH molecule directly tethered to the QD would be able to withdraw the hole faster than sulfite ions, thus working as a mediator in QD regeneration. Consequently, the reduction of recombination inside the QD, induced by a faster hole scavenging, would result in higher electron injection yields and photocurrents. In summary, two complementary mechanisms may explain the remarkable enhancement of the interfacial electron transfer upon MPH adsorption. This molecule could act as a mediator in the transfer of holes to solution or as a molecular dipole shifting the PbSe QD energy levels upward with respect to those of TiO2. Although both processes would operate in the same direction, the rather small change in the ionization energy (∼60 meV)32 reported by Chi et al., upon QD modification with adsorbates having dipole moments similar to that of MPH,36,37 points to the fact that channeling the hole transfer through the MPH molecules could also be crucial in improving the photoanode behavior. In addition, the energy level diagram shown in Figure 3B reveals a

Figure 3. Transient kinetics for TiO2/PbSe/spiro-OMeTAD films before and after modification with MPH (A). All measurements were performed under N2, exciting samples at 450 nm (fluency 40 μJ·cm−2) and probing them at 1600 nm. Values of mΔOD were scaled to the number of photons absorbed at the pump wavelength. Estimated energy level positions and main carrier transfer processes (B). See the Supporting Information for details.

cascade alignment of the PbSe valence band edge (after 1 SILAR cycle), and MPH and sulfite energy levels, which would funnel photogenerated holes toward the electrolyte. It is also conceivable that a fast hole extraction would leave a negatively charged QD, shifting upward the donor energy level and hence further boosting the electron transfer yield.38 According to this model, the MPH highest occupied molecular orbital (HOMO) should be located between those of the QD and hole conductor to enhance hole extraction. In this respect, it is pertinent to bear in mind that repeating SILAR cycles leads to a loss of quantum confinement. Taking into account the fact that the 3370



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Council for financial support via the U.K.-India and Excitonic Supergen programmes. S.A.H. thanks the Royal Society for a University Research Fellowship.

for the fast hole funneling toward the hole conductor, reducing its internal recombination with photogenerated electrons and thereby increasing the yield of electron injection (Figure 3B). As previously discussed in the case of a sulfite electrolyte, we note that the rapid hole capture by MPH and its subsequent transfer to spiro-OMeTAD molecules may account for the remarkable enhancement in charge separation, although its role as a modifier of band alignment cannot be discarded. In summary, in this letter we report on the sensitization of TiO2 films with PbSe QDs proving with photoelectrochemical experiments that photogenerated electrons in the PbSe QDs can be injected into the TiO2. The PbSe QDs were prepared according to a novel SILAR method and characterized by TEM and EDX analysis. Charge separation in TiO2/PbSe films has been studied using either liquid (aqueous sulfite) or solid (spiro-OMeTAD) hole capturing phases by means of photocurrent transients and TAS measurements, respectively. It has been shown that only strongly quantum-confined NCs are able to inject electrons into TiO2. Interestingly, the modification of the QD/hole scavenger interface with MPH significantly enhances charge separation in both media, leading to higher photocurrents and spiro-OMeTAD+ yields. More importantly, the fact that similar results are obtained independently of both the liquid or solid nature of the hole scavenger medium and the techniques used (electrochemical or spectroscopic) provides strong evidence of the beneficial effect of the MPH molecule on charge separation. As far as we know, in all previous works published on this topic, the enhancement of the interfacial electron transfer was solely attributed to the increase in the driving force induced by the molecular dipole together with a possible passivation of QD surface states. Herein we propose another mechanism whereby the MPH works as an efficient hole-transfer mediator, funneling the hole from the QD toward the hole conductor. The faster hole withdrawal with MPH would efficiently reduce the recombination in the QD, improving the electron injection yield as a result. These findings outline the potential of PbSe as a sensitizer for TiO2based solar cells and emphasize the importance of the manipulation of the QD/electrolyte (or QD/HTM) interface to further boost the yield of charge separation.

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(1) Grätzel, M. Conversion of Sunlight to Electric Power by Nanocrystalline Dye-sensitized Solar Cells. J. Photochem. Photobiol., A. 2004, 164, 3−14. (2) Kamat, P. V. Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters. J. Phys. Chem. C 2008, 112, 18737−18753. (3) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated PolymerBased Organic Solar Cells. Chem. Rev. 2007, 107, 1324−1338. (4) Rühle, S.; Shalom, M.; Zaban, A. Quantum-Dot-Sensitized Solar Cells. ChemPhysChem 2010, 11, 2290−2304. (5) Yang, Y.; Chen, C.-Y.; Roy, P.; Chang, H.-T. Quantum DotSensitized Solar Cells Incorporating Nanomaterials. Chem. Commun. 2011, 47, 9561−9571. (6) Emin, S.; Singh, S. P.; Han, L.; Satoh, N.; Islam, A. Colloidal Quantum Dot Solar Cells. Solar Energy 2011, 85, 1264−1282. (7) Hetsch, F.; Xu, X.; Wang, H.; Kershaw, S. V.; Rogach, A. L. Semiconductor Nanocrystal Quantum Dots as Solar Cell Components and Photosensitizers: Material, Charge Transfer, and Separation Aspects of Some Device Topologies. J. Phys. Chem. Lett. 2011, 2, 1879−1887. (8) Du, H.; Chen, C.; Krishnan, R.; Krauss, T.; Harbold, J. M.; Wise, F. W.; Thomas, M. G.; Silcox, J. Optical Properties of Colloidal PbSe Nanocrystals. Nano Lett. 2002, 2, 1321−1324. (9) Tisdale, W. A.; Williams, K. J.; Timp, B. A.; Norris, D. J.; Aydil, E. S.; Zhu, X.-Y. Hot-Electron Transfer from Semiconductor Nanocrystals. Science 2010, 328, 1543−1547. (10) Moreels, I.; Lambert, K.; De Muynck, D.; Vanhaecke, F.; Poelman, D.; Martins, J. C.; Allan, G.; Hens, Z. Composition and SizeDependent Extinction Coefficient of Colloidal PbSe Quantum Dots. Chem. Mater. 2007, 19, 6101−6106. (11) Nozik, A. J. Quantum Dot Solar Cells. Physica E 2002, 14, 115− 120. (12) Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P.; Micic, O. I.; Nozik, A. J.; Shabaev, A.; Efros, A. L. Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots. Nano Lett. 2005, 5, 865−871. (13) Schaller, R. D.; Klimov, V. I. High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion. Phys. Rev. Lett. 2004, 92, 186601−4. (14) Trinh, M. T.; Houtepen, A. J.; Schins, J. M.; Hanrath, T.; Piris, J.; Knulst, W.; Goossens, A. P. L. M.; Siebbeles, L. D. A. In Spite of Recent Doubts Carrier Multiplication Does Occur in PbSe Nanocrystals. Nano Lett. 2008, 8, 1713−1718. (15) Ma, W.; Swisher, S. L.; Ewers, T.; Engel, J.; Ferry, V. E.; Atwater, H. A.; Alivisatos, A. P. Photovoltaic Performance of Ultrasmall PbSe Quantum Dots. ACS Nano 2011, 5, 8140−8147. (16) Semonin, O. E.; Luther, J. M.; Choi, S.; Chen, H.-Y.; Gao, J.; Nozik, A. J.; Beard, M. C. Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG in a Quantum Dot Solar Cell. Science 2011, 334, 1530−1533. (17) Noone, K. M.; Anderson, N. C.; Horwitz, N. E.; Munro, A. M.; Kulkarni, A. P.; Ginger, D. S. Absence of Photoinduced Charge Transfer in Blends of PbSe Quantum Dots and Conjugated Polymers. ACS Nano 2009, 3, 1345−1352. (18) Pijpers, J. J. H.; Koole, R.; Evers, W. H.; Houtepen, A. J.; Boehme, S.; Donegá, C. M.; Vanmaekelbergh, D.; Bonn, M. Spectroscopic Studies of Electron Injection in Quantum Dot Sensitized Mesoporous Oxide Films. J. Phys. Chem. C 2010, 114, 18866−18873. (19) Cánovas, E.; Moll, P.; Jensen, S. A.; Gao, Y.; Houtepen, A. J.; Siebbeles, L. D. A.; Kinge, S.; Bonn, M. Size-Dependent Electron Transfer from PbSe Quantum Dots to SnO2 Monitored by Picosecond Terahertz Spectroscopy. Nano Lett. 2011, 11, 5234−5239.

ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental section. Morphological characterization of sensitized films. Photocurrent transients for bare TiO2 electrodes and for electrodes sensitized with two and three SILAR cycles modified with MPH. Estimation of the energy levels. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.G. is grateful to the Spanish MEC for the award of an FPU grant. The Alicante group acknowledges support of the Spanish MICINN through projects HOPE CSD2007-00007 (Consolider Ingenio 2010) and MAT2009-14004 (Fondos FEDER). S.A.H. thanks the Engineering and Physical Sciences Research 3371



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(20) Timp, B. A.; Zhu, X.-Y. Electronic Energy Alignment at the PbSe Quantum Dots/ZnO (1010) Interface. Surf. Sci. 2010, 604, 1335−1341. (21) Robel, I.; Kuno, M.; Kamat, P. V. Size-Dependent Electron Injection from Excited CdSe Quantum Dots into TiO2 Nanoparticles. J. Am. Chem. Soc. 2007, 129, 4136−4137. (22) Shalom, M.; Rühle, S.; Hod, I.; Yahav, S.; Zaban, A. Energy Level Alignment in CdS Quantum Dot Sensitized Solar Cells Using Molecular Dipoles. J. Am. Chem. Soc. 2009, 131, 9876−9877. (23) Barea, E. M.; Shalom, M.; Giménez, S.; Hod, I.; Mora-Seró, I.; Zaban, A.; Bisquert, J. Design of Injection and Recombination in Quantum Dot Sensitized Solar Cells. J. Am. Chem. Soc. 2010, 132, 6834−6839. (24) Guijarro, N.; Lana-Villarreal, T.; Shen, Q.; Toyoda, T.; Gómez, R. Sensitization of Titanium Dioxide Photoanodes with Cadmium Selenide Quantum Dots Prepared by SILAR: Photoelectrochemical and Carrier Dynamics Studies. J. Phys. Chem. C 2010, 114, 21928− 21937. (25) Gorer, S.; Albu-Yaron, A.; Hodes, G. Chemical Solution Deposition of Lead Selenide Films: A Mechanistic and Structural Study. Chem. Mater. 1995, 7, 1243−1256. (26) Guijarro, N.; Lana-Villarreal, T.; Mora-Seró, I.; Bisquert, J.; Gómez, R. CdSe Quantum Dot-Sensitized TiO2 electrodes: Effect of Quantum Dot Coverage and Mode of Attachment. J. Phys. Chem. C 2009, 113, 4208−4214. (27) Guijarro, N.; Shen, Q.; Giménez, S.; Mora-Seró, I.; Bisquert, J.; Lana-Villarreal, T.; Toyoda, T.; Gómez, R. Direct Correlation between Ultrafast Injection and Photoanode Performance in Quantum Dot Sensitized Solar Cells. J. Phys. Chem. C 2010, 114, 22352−22360. (28) Strel’tsov, E. A.; Ivanov, D. K.; Ivanova, Y. A.; Poznyak, S. K.; Kulak, A. I. Photoelectrochemical Processes on TiO2 Electrodes Sensitized by Lead Selenide Nanoparticles. Theor. Exp. Chem. 2012, 48, 33−37. (29) Acharya, K. P.; Alabi, T. R.; Schmall, N.; Hewa-Kasakarage, N. N.; Kirsanova, M.; Nemchinov, A.; Khon, E.; Zamkov, M. Linker-Free Modification of TiO2 Nanorods with PbSe Nanocrystals. J. Phys. Chem. C 2009, 113, 19531−19535. (30) Jankulovska, M.; Berger, T.; Lana-Villarreal, T.; Gómez, R. A Comparison of Quantum-Sized Anatase and Rutile Nanowire Thin Films: Devising Differences in the Electronic Structure from Photoelectrochemical Measurements. Electrochim. Acta 2012, 62, 172−180. (31) Berger, T.; Monllor-Satoca, D.; Jankulovska, M.; Lana-Villarreal, T.; Gómez, R. The Electrochemistry of Nanostructured TiO2 Electrodes. ChemPhysChem 2012, 13, 2824−2875. (32) Chi, C.-F.; Chen, P.; Lee, Y.-L.; Liu, I.-P.; Chou, S.-C.; Zang, X.L.; Bach, U. Surface Modifications of CdS/CdSe Co-sensitized TiO2 Photoelectrodes for Solid-State Quantum-Dot-Sensitized Solar Cells. J. Mater. Chem. 2011, 21, 17534−17540. (33) Kim, S.; Choi, W. Visible-Light-Induced Photocatalytic Degradation of 4-Chlorophenol and Phenolic Compounds in Aqueous Suspension of Pure Titania: Demonstrating the Existence of a SurfaceComplex-Mediated Path. J. Phys. Chem. B 2005, 109, 5143−5149. (34) Lana-Villarreal, T.; Rodes, A.; Pérez, J. M.; Gómez, R. A Spectroscopic and Electrochemical Approach to the Study of the Interactions and Photoinduced Electron Transfer between Catechol and Anatase Nanoparticles in Aqueous Solution. J. Am. Chem. Soc. 2005, 127, 12601−12611. (35) Wuister, S. F.; Donegá, C. M.; Meijerink, A. Influence of Thiol Capping on the Exciton Luminescence and Decay Kinetics of CdTe and CdSe Quantum Dots. J. Phys. Chem. B 2004, 108, 17393−17397. (36) Hehre, W. J.; Radom, L.; Pople, J. A. Molecular Orbital Theory of the Electronic Structure of Organic Compounds. XII. Conformations, Stabilities and Charge Distributions in Monosubstituted Benzenes. J. Am. Chem. Soc. 1972, 94, 1496−1504. (37) In ref 32, p-methoxybenzenethiol is employed as molecular dipole, instead of MPH used here. Although dipole moments for these molecules have not been reported as far as we know, the rather similar values for phenol (1.45 D) and anisole (1.38 D) reported in ref 36

suggest that their corresponding thiol derivatives would present similar dipole moments. (38) Guijarro, N.; Lutz, T.; Lana-Villarreal, T.; O’Mahony, F.; Gómez, R.; Haque, S. A. Toward Antimony Selenide Sensitized Solar Cells: Efficient Charge Photogeneration at spiro-OMeTAD/Sb2Se3/ Metal Oxide Heterojunctions. J. Phys. Chem. Lett. 2012, 3, 1351−1356. (39) Leventis, H. C.; O’Mahony, F.; Akhtar, J.; Afzaal, M.; O’Brien, P.; Haque, S. A. Transient Optical Studies of Interfacial Charge Transfer at Nanostructures Metal Oxide/PbS Quantum Dot/Organic Hole Conductor Heterojuntions. J. Am. Chem. Soc. 2010, 132, 2743− 2750. (40) Plass, R.; Pelet, S.; Krueger, J.; Bach, U.; Grätzel, M. Quantum Dot Sensitization of Organic-Inorganic Hybrid Solar Cells. J. Phys. Chem. B 2002, 106, 7578−7580. (41) Im, S. H.; Lim, C.-S.; Chang, J. A.; Lee, Y. H.; Maiti, N.; Kim, H.-J.; Nazeeruddin, Md. K.; Grätzel, M.; Seok, S. I. Toward Interaction of Sensitizer and Functional Moieties in Hole-Transporting Materials for Efficient Semiconductor-Sensitized Solar cells. Nano Lett. 2011, 11, 4789−4793.

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Sensitization of TiO2 with PbSe Quantum Dots by SILAR: How

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