Interface Engineering in Inorganic-Absorber Nanostructured Solar

Perspective pubs.acs.org/JPCL

Interface Engineering in Inorganic-Absorber Nanostructured Solar Cells Katherine E. Roelofs,†,‡ Thomas P. Brennan,‡ and Stacey F. Bent*,‡ †

Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States

‡

ABSTRACT: Nanostructured solar cells have the potential to provide a low-cost alternative to more traditional thin film solar cell technologies. Of particular interest are nanostructured solar cells with inorganic semiconductor absorbers, due to their favorable absorption properties. Such devices include quantum-dot-sensitized solar cells (QDSSCs), extremely thin absorber solar cells (ETASCs), and colloidal quantum dot solar cells (CQDSCs). However, these device architectures suffer from high rates of internal recombination and other problems associated with their extensive internal surface areas. Interfacial surface treatments have proven to be a highly effective means to improve the electronic properties of these devices, leading to overall gains in efficiencies. In this Perspective, we focus on three types of interfacial modification: band alignment by molecular dipole layers, improved CQD film mobilities by ligand exchange, and reduced recombination by interfacial inorganic layers. Select examples in each of these categories are highlighted to provide a detailed look at the underlying mechanisms. We believe that surface modification studies in these devicesQDSSCs, ETASCs, and CQDSCsare of interest not only to these fields, but also to the broader photovoltaics community.

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absorber; these include quantum-dot-sensitized solar cells (QDSSCs), extremely thin absorber solar cells (ETASCs), and colloidal quantum dot solar cells (CQDSCs). Until the recent advent of perovskite devices (discussed below),4 DSSCs had far out-stripped other nanostructured solar cells in device efficiency. DSSCs have held power conversion efficiencies (PCE’s) of over 10% since 1997,5 with the current record device at 12.3%.6 However, DSSCs face difficulties in achieving strong broadband absorption with a single dye species. Inorganic semiconductor absorbers are an attractive alternative to dye molecules, as semiconductors absorb strongly at all photon energies above the band gap. Inorganic absorbers also offer the potential for improved device stability compared to organic absorbers (i.e., dye molecules and polymers), which are susceptible to degradation when exposed to sunlight. Moreover, inorganic semiconductor absorbers have comparable absorption coefficients to that of the high-performance dyes currently used in DSSCs.7 Indeed, the promise of inorganic absorbers has been demonstrated in just the past year with the introduction of lead-iodide-based perovskite absorbers.4,8−12 Perovskite absorber devices have reached PCEs of over 15% in both nanostructured12 and planar11 architectures. Device Architectures and Eff iciencies. Schematics of QDSSC, ETASC, and CQDSC architectures, shown in Figure 1, highlight their similarities. We note that QDSSCs and ETASCs fall into the broader category of semiconductor-sensitized solar

he introduction of high-efficiency dye-sensitized solar cells (DSSCs) has had a large impact on the field of thin film solar cells, and has motivated further research into nanostructured solar cells. In 1991, O’Regan and Grätzel introduced the nanoporous anode architecture to the DSSC field, resulting in a leap in DSSC efficiencies from 98%); 1,4butanedithiol (BuDT); 1,5-pentanedithiol (PenDT); 1,6-hexanedithiol (HDT). The electron and hole mobilities were measured in ambipolar field-effect transistors. Reprinted with permission from ref 67. Copyright 2010 American Chemical Society.

Taking this inverse relationship between ligand length and film mobility one step further, Tang et al. have provided an indepth study of ligand exchange down to single-atom ligands, producing the first such report of CQDSCs with single-atom ligands.71 The authors investigate the impact ligand exchange has on charge transport in the PbS CQD film, as well as any passivation effects of the ligand exchange on extant electronic defects on the PbS QD surfaces. Electron mobilities of halidetreated CQD films measured by field effect transistor methods were found to increase in the Br¯ capped CQD films, as compared to the ethanedithiol-capped CQD films. This increase in electron mobility with the shift to a single-atom ligand is significant since a high minority carrier mobility is critical to achieving high CQDSC efficiencies. Indeed, the resulting atomic-capped PbS CQDSCs achieved device efficiencies of 5%. The authors took the further step of determining the cause of the higher mobility in the halide treated films, by timeresolved infrared (TRIR) spectroscopy measurements shown in Figure 7. In the TRIR measurements, a 523 nm laser was used to excite the CQD band gap, and an IR light source was used to probe the lower-energy trap-to-band transitions. The results (Figure 7a) indicated that the excited electrons are trapped in energetically shallower states in Br¯ capped CQD films, as compared to the CQD films with organic ligands. To confirm their interpretation of the TRIR peaks, the authors tracked the absorption decay time of the TRIR peaks (Figure 7b), which indicates the rate at which the population of the trap state is 353

dx.doi.org/10.1021/jz4023656 | J. Phys. Chem. Lett. 2014, 5, 348−360

The Journal of Physical Chemistry Letters

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Figure 7. CQDSCs with ligand exchange down to a single-atom ligand, studying ethanedithiol (EDT), 1,6-hexane dithiol (HDT), and mercaptocarboxylic acid (MPA), and the single-atom ligand of Br. (a) Time-resolved infrared (TRIR) spectroscopy measurements of CQD films; peaks in TRIR spectra correspond to trap-to-band transitions. In the Br¯-capped CQD films, the TRIR peak occurs at a lower energy level, indicating that Br¯-capped films have shallower trap states. (b) Decay times extracted from absorption decay curves of the TRIR peak, tracking the rate at which trap states are depopulated. (c) Higher CQD film mobilities were found in CQD films with lower trap-to-band transition energies. Reprinted with permission from Macmillan Publishers Ltd.: Nature Materials (ref 71), copyright 2011.

Figure 8. Interfacial band structure depicting the potential impacts of inorganic surface treatments in QDSSCs. Here, a nanocrystal of the nanoporous anode (i.e., a TiO2 nanocrystal) is depicted in navy, the inorganic coating layer in magenta, and the light-absorbing quantum dot (QD) in yellow. The hole-transport material (HTM) is shown in light blue. (a) The interfacial band structure prior to deposition of the metal oxide layer. Only recombination pathways from the photoanode to the QD are shown, from (α) the anode conduction band and (β) the anode density of states (DOS), although recombination can also occur with the HTM. (b) Recombination barrier layers will reduce both (α) and (β) recombination processes due to the tunneling effect. (c) Recombination barriers may also alter the DOS of the anode, by passivating trap states, leading to further reductions in the (β) recombination path. (d) The recombination barrier layer may also act as a surface dipole, shifting the anode conduction band upward. (e) Due to the difficulties of conformally growing angstrom-thick inorganic layers on a nanoporous anode, it is possible that multiple geometric configurations of the anode/inorganic coating/QD exist within the same device.

QDSSCs for improved interfacial charge transfer. For one, ligand design is necessarily important in QDSSCs in which the QDs are fabricated colloidally and later infiltrated into the nanostructured anode. Moreover, a study by Dibbel et al. has found ligand length to be a critical factor in electron injection from the QDs into the anode material, using spectroscopic measurements of colloidal suspensions of TiO2 nanocrystals and CdS CQDs.72 Finally, in QDSSCs functionalized with molecular dipole layers, the molecular dipole layer can interfere with hole transfer from the QD to the HTM; thus ligand design strategies from CQDSCs can also inform the choice of molecules for molecular dipole layers in QDSSCs. Reduced Recombination by Interfacial Inorganic Layers. In addition to the organic surface treatments discussed above, inorganic materials also find use in interface modification of nanostructured solar cells. Although inorganic surface treatments have been used for a variety of purposes, including use as a protective capping layer on the absorber or for modification

depleted due to trap-to-band transitions. As expected, shallower trap states (lower transition energies) had faster rates of trapto-band transitions. The authors also found higher electron mobilities correlated with shallower trap states (Figure 7c). This result can be explained by the following mechanism: for CQD films with shallower trap states, electrons spend less time in any given trap state, and thus have a faster rate of progress through the film. Overall, this single-atom ligand study demonstrates that ligand exchange can be used both for improving CQD film mobilities (QD-to-QD charge transfer rates), as well as for passivating QD surface defects by modifying the energy levels of the trap states associated with those surface defects. We believe that ligand choice guidelines taken from the CQDSC literature will benefit studies in QDSSCs, such as in molecular design of organic surface functionalization of QDSSCs. For example, while the studies of ligand exchange discussed above were undertaken in the context of increasing film mobility in CQDSCs, the results could also guide studies in 354



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of the interfacial band alignment, the majority of studies have explored inorganic treatments as a means to reduce interfacial recombination. Due to their success in improving device efficiencies, inorganic modifications have found widespread use in QDSSCs, ETASCs, and CQDSCs. Metal oxide layers (such as Al2O3,30 HfO2,73 or ZrO274) have been deposited at the interface by solution deposition methods75−77 and by vapor phase growth techniques.49,59,77,78 Metal chalcogenide layers (such as CdSe, ZnS, or PbSe) have also been used in QDSSCs and CQDSCs to coat the QD surface.79,80 Inorganic layers can reduce recombination through chemical, electronic, or physical mechanisms. For example, inorganic layers can decrease recombination by electronically altering the interfacial band structure, chemically passivating surface defects, or acting as a physical barrier between different components. Decreased interfacial recombination can improve device efficiencies through both increased JSC values and increased VOC values for the following reason. Interfacial recombination represents the loss of excited charge carriers, which in turn reduces the photocurrent (JSC). Hence, reducing interfacial recombination can help improve JSC as well as decrease dark current (J0); both of these effects can improve VOC according to the relationships of general p−n junction theory. Figure 8 depicts schematically a few mechanisms by which the electronic structure at the anode/absorber/HTM interface can be altered by the deposition of inorganic layers: presenting a tunneling barrier to recombination of TiO2 electrons with the absorber or the HTM (Figure 8b), changing the density of states (DOS) of the anode (Figure 8c), or shifting the band levels of the anode (Figure 8d). In Figure 8b, if the energetic barrier is too high for the electron to surpass by thermal or other energetic means, the electron can quantum mechanically tunnel through the barrier. The interpretation of the effects of inorganic layers on device performance (J−V curves) can thus be quite complicated. Any of the changes in Figure 8 could lead to increases in VOC, either through decreases in the recombination rates or by an upward shift in the anode CB. As a final consideration, it is worth noting that a given device can have different geometric arrangements at the photoanode/QD/ HTM interface (Figure 8e), depending on whether the QD nucleates on the barrier layer or on the exposed photoanode surface sites with the barrier deposited afterward.49 Figure 9 shows data from a study by Kim et al. on inorganic layer surface modifications of liquid QDSSCs.81 In a CdS QDSSC (n-TiO2/CdS-QD/polysulfide-electrolyte), the authors investigated TiO2 surface coatings of varying thicknesses deposited on the TiO2 anode. Varying TiO2 thicknesses were achieved by varying lengths of soak time in a TiCl4 aqueous solution for the chemical bath deposition of TiO2. This work by Kim et al. is highlighted here since the results lead the authors to conclude that the TiO2 surface coating influenced device efficiencies via two different mechanisms, explaining the initial increase and subsequent decrease in device efficiencies. Transient photovoltage measurements (Figure 9b) were used to show the decrease in interfacial recombination achieved with increasing thicknesses of the TiO2 layer. However, at longer TiCl4 soak times, the increased thickness of the TiO2 coating was found (by XRD measurements) to lead to lattice strain in the TiO2 nanoparticles. As shown schematically in Figure 9c, the authors conclude that the observed initial increase in device efficiency was caused by a suppression of photoanode-to-electrolyte recombination (pathway 6, Figure 2), while the subsequent decrease in

Figure 9. The role of TiCl4 treatment in QDSSCs (n-TiO2/CdS QDs/electrolyte). (a) J−V curves as a function of TiO2 CBD time. The highest efficiency is achieved at 30 min. (b) Open circuit voltage decay curves under AM 1.5 illumination, show that the highest electron lifetimes (slowest rate of VOC decay) are achieved for the 30 min samples. (c) Schematic of the authors’ proposed mechanism. With increasing TiO2 CBD times, the resulting TiO2 coating is believed to decrease interfacial recombination, an effect that dominates at the 30 min CBD time point. For thicker TiO2 coatings, though, it is believed that the detrimental effects of transport resistance lead ultimately to the lower observed electron lifetimes at 60 min. Reprinted from ref 81, Copyright 2012, with permission from Elsevier.

efficiency was a result of increased transport resistance due to increased TiO2 lattice strain and pore-filling issues. In addition to metal oxides, wide band gap metal chalcogenide layers have also been employed successfully in QDSSCs to decrease interfacial recombination. ZnS layers were first introduced as a means to prevent photocorrosion of QDs,82 with gains in efficiency also observed. Recently, Guijarro et al. have studied in depth the means by which efficiency improvements are achieved with ZnS layers.83 In this work, CQDSC devices (n-TiO2/CdSe CQD/polysulfide electrolyte) were treated with SILAR-grown ZnS layers. The authors found evidence of both QD surface defect passivation, determined by UV−vis absorbance and photoluminescence, and increased charge transfer resistance (i.e., decreased recombination), measured by electrochemical impedance spectroscopy. The increased charge transfer resistance was attributed to increased separation between the photoanode and the electrolyte, and led to increases in JSC. This study shows that ZnS 355



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Figure 10. (a) Inorganic layers of Al2O3, BaTiO3, and MgO deposited at the interface in ETASCs. (b) All three metal oxides served to improve the VOC, a good indication of decreased interfacial recombination, though only in the MgO case did JSC values improve as well. (c) A double layer treatment of BaTiO3 followed by MgO provided even better improvements in performance than either treatment alone. The device parameters of the J−V curves are included, and have the following units: JSC (mA/cm2), VOC (V), FF (unitless), and η (%). Reprinted from reference 75. Copyright 2012 American Chemical Society.

to act both as a barrier to recombination and as a passivant of QD surface defects.83 Considering that surface treatments inevitably have multiple effects, a better strategy when considering interfacial modifications may be to purposefully choose surface modifications that will have multiple beneficial effects, or to employ a combined approach, using multiple compatible surface treatments at a given interface. Indeed, a combined approach has been successfully pursued in all three device architectures. For instance, in QDSSCs, there have been studies employing such combinations as F− chemical treatments and ZnS inorganic layers,79 molecular dipole layers and ZnS inorganic layers,62 and Al2O3 passivation layers with the supersensitization of a light-absorbing dye molecule.84 In addition to considering the possibility that a given surface treatment may engender multiple effects, it is also important to identify which interface is being modified in any given treatment. For surface modifications in QDSSCs, ETASCs, and CQDSCs, there are two surfaces in the active layer that can be modified: the anode surface and the absorber surface. In ETASCs, the distinction between the anode/absorber interface and the absorber/HTM interface is clear, since these two interfaces are physically separated. In QDSSCs, the story is more complicated, as surface treatments on the QDs will invariably affect the metal oxide substrate as well, given the sparse coverage of the metal oxide by the QDs.37 Thus, in the case of QDSSCs, molecular layers are an appealing tool for interfacial modification, since the headgroup of the molecule can be chosen to attach specifically to either the QD surface or the anode surface, giving a higher degree of control over interfacial engineering. While the use of inorganic absorbers offers more opportunities to control interfacial properties, it necessitates a careful and comprehensive sample set in order to isolate changes in device performance. In CQDSCs, while much effort has been devoted to modifications of the QD surfaces in the CQD film, charge transfer at the anode/CQD-film interface is also of importance,16,35 and CQDSCs would benefit from further research on interfacial modifications for the optimization of the anode/CQD-film interface. In our own work with QDSSCs, we have taken advantage of the two interfaces available for modification, by depositing inorganic barrier layers both prior to and after QD deposition in CdS QDSSCs49 and in PbS QDSSCs.78 Depositing the barrier layer before QD deposition modifies the anode/absorber

coatings can decrease recombination to the HTM through bare TiO2 surface regions, as well as recombination mediated by the CdSe QDs surface defects. Inorganic layers are also used frequently in ETASCs, with the intent of decreasing interfacial recombination. Tsujimoto et al. have studied interfacial layers of Al2O3, BaTiO3, and MgO as recombination-blocking layers in ETASCs;75 device performances are shown in Figure 10. The authors chose a device architecture of n-TiO2/Sb2S3/CuSCN; current record-efficiency ETASCs employ Sb2S3 as the absorber (Table 1)22 and the use of an inorganic, solid-state HTM (CuSCN) makes this an all-solid-state, all-inorganic device. All three metal oxide layers increased the device efficiency, primarily through increases in the VOC or the FF. As discussed above, such a result could be expected if the barriers were acting to decrease J0, and thus increase the VOC. Then, by combining two treatments, with a BaTiO3 layer followed by an MgO layer, the authors were able to achieve even higher efficiencies than had been observed in either individual treatment. This study shows the benefits of a combinatorial approach to interfacial modifications: while the authors reported that a single material grown too thick always led to losses in efficiency, the deposition of two thin layers of different materials provided an additive benefit in which each outweighed any negative impact of either (that is, the decreased JSC observed when BaTiO3 was employed on its own). We have also observed in our work with QDSSCs (see Outlook below) that the deposition of inorganic layers at the interface leads to losses in efficiency when the inorganic layer is grown too thick.49 Outlook. As with DSSCs, power generation in QDSSCs, ETASCs, and CQDSCs hinges on interfacial charge transfer processes. By examining several examples of interfacial modifications in this Perspectivenamely, molecular dipole layers, capping ligands, and inorganic layerswe have aimed to demonstrate the versatility of surface modifications and their ability to improve the performance of QDSSCs, ETASCs, and CQDSCs. It is tempting to approach the idea of surface modifications by employing a single modification to achieve a single goal (e.g., adjusting band alignment, passivating surface defects, or providing a recombination barrier). We have seen, however, that any given surface modification has the ability to act in multiple beneficial ways. For instance, as discussed in the previous section, interfacial inorganic layers in QDSSCs have the potential 356



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For the CdS QDSSCs, while the J−V curve behavior was qualitatively similar in comparing the two Al2O3 layer configurations (prior to QD deposition versus after QD deposition), we found differences in the interfacial recombination processes, as tracked by measurements of the lifetimes of excited electrons in the TiO2. Figure 11 illustrates the electron lifetime results from the CdS QDSSC study, showing that devices with the Al2O3 layers deposited prior to the QDs had higher electron lifetimes than those with Al2O3 layers deposited after the QDs. Based on these results and others, we concluded that a likely explanation was as follows: Al2O3 layers deposited prior to the QDs could block both recombination to the oxidized QD and the HTM, whereas Al2O3 layers deposited after the QD could only block recombination to the HTM (Figure 11b). In our work with PbS QDSSCs we also found that Al2O3 deposited prior to the QDs gave higher electron lifetime values than Al2O3 deposited after the QDs.78 In this series of studies, by experimentally separating out the different recombination losses that can occur, we showed that while passivation of either interface can improve QDSSC performance, the carrier losses are more severe at the TiO2/QD interfaces in these cells.

Great benefits can be reaped in nanostructured solar cells through surface treatments, due to the extensive interfacial areas In conclusion, great benefits can be reaped in nanostructured solar cells through surface treatments, due to the extensive interfacial areas in these devices. In QDSSC, ETASCs, and CQDSCs, the use of inorganic absorber materials provides additional opportunities for interfacial modification, as compared to DSSCs, due to the ability to modify both the anode and the absorber surface. We believe that further research exploring the exact mechanisms at work behind surface treatments will lead not only to a deeper understanding of device operation, but also to further gains in efficiency. There is great opportunity for crossover in these devices; that is, that interfacial modifications and strategies, when proved successful in one device architecture, stand a good chance of being beneficial in another. As fundamental understanding of interfacial surface treatments in these devices advances, there is a great potential for employing a combined approach, of several distinct surface treatments in a given device. Overall, we believe that in nanostructured solar cells, interfacial modifications will play a key role in moving power conversion efficiencies into the sphere in which they are competitive with other thin film technologies (e.g., α-Si, CdTe, and CIGS).

Figure 11. Inorganic surface modification of CdS QDSSCs by the deposition of ultrathin Al2O3 layers by atomic layer deposition (ALD). The Al2O3 layer thickness was varied, with 0, 1, and 3 ALD cycles of Al2O3. The Al2O3 layers were deposited both after QD deposition and before QD deposition, resulting in two different configurations: nTiO2/QD/Al2O3 (filled markers) and n-TiO2/Al2O3/QD (open markers). (a) Electron lifetimes were extracted from transient photovoltage measurements, showing longer carrier lifetimes in the case of the n-TiO2/Al2O3/QD devices. Standard deviations represent the spread of lifetimes measured across three different batches of devices (each data point represents ∼6 devices in total). (b) One possible explanation for the results is that the n-TiO2/Al2O3/QD devices could block two interfacial charge recombination processes, whereas the n-TiO2/QD/Al2O3 devices could block only one. Reprinted with permission from ref 49. Copyright American 2013 Chemical Society.

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interface, whereas depositing the barrier layer after QD deposition primarily modifies the absorber/HTM interface. In both the CdS and the PbS studies, we employed ALD for the deposition of Al2O3 layers at the interface in solid-state QDSSCs (n-TiO2/QDs/spiro-OMeTAD). ALD is a self-limiting vapor deposition technique; at most a monolayer of material is deposited in each ALD cycle, due to the separate introduction of precursor material in an ALD run. ALD is a valuable technique for surface modifications in nanostructured solar cells, due to its ability to conformally coat high aspect ratio substrates, to penetrate highly porous films of micrometer-scale thicknesses, and to provide angstrom-scale control of film thicknesses.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 1 650 723-0385. Fax: 1 650 723-9780. Notes

The authors declare no competing financial interest. Biographies Katherine E. Roelofs is a Ph.D. candidate in Materials Science and Engineering at Stanford University. Her research focuses on the surface chemistry and electronic properties of interfaces in quantum dot-sensitized solar cells. 357



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(13) Lee, H.; Leventis, H. C.; Moon, S.-J.; Chen, P.; Ito, S.; Haque, S. A.; Torres, T.; Nüesch, F.; Geiger, T.; Zakeeruddin, S. M.; et al. PbS and CdS Quantum Dot-Sensitized Solid-State Solar Cells: “Old Concepts, New Results”. Adv. Funct. Mater. 2009, 19, 2735−2742. (14) Briscoe, J.; Dunn, S. Extremely Thin Absorber Solar Cells Based on Nanostructured Semiconductors. Mater. Sci. Technol. 2011, 27, 1741−1756. (15) Pattantyus-Abraham, A. G.; Kramer, I. J.; Barkhouse, A. R.; Wang, X.; Konstantatos, G.; Debnath, R.; Levina, L.; Raabe, I.; Nazeeruddin, M. K.; Grätzel, M.; et al. Depleted-Heterojunction Colloidal Quantum Dot Solar Cells. ACS Nano 2010, 4, 3374−3380. (16) Maraghechi, P.; Labelle, A. J.; Kirmani, A. R.; Lan, X.; Adachi, M. M.; Thon, S. M.; Hoogland, S.; Lee, A.; Ning, Z.; Fischer, A.; et al. The Donor−Supply Electrode Enhances Performance in Colloidal Quantum Dot Solar Cells. ACS Nano 2013, 7, 6111−6116. (17) Hillhouse, H. W.; Beard, M. C. Solar Cells from Colloidal Nanocrystals: Fundamentals, Materials, Devices, and Economics. Curr. Opin. Colloid Interface Sci. 2009, 14, 245−259. (18) Kamat, P. V. Quantum Dot Solar Cells: Semiconductor Nanocrystals as Light Harvesters. J. Phys. Chem. C 2008, 112, 18737−18753. (19) Kamat, P. V. Quantum Dot Solar Cells: The Next Big Thing in Photovoltaics. J. Phys. Chem. Lett. 2013, 4, 908−918. (20) Dittrich, T.; Belaidi, A.; Ennaoui, A. Concepts of Inorganic Solid-State Nanostructured Solar Cells. Sol. Energy Mater. Sol. Cells 2011, 95, 1527−1536. (21) Wang, J.; Mora-Seró, I.; Pan, Z.; Zhao, K.; Zhang, H.; Feng, Y.; Yang, G.; Zhong, X.; Bisquert, J. Core/Shell Colloidal Quantum Dot Exciplex States for the Development of Highly Efficient QuantumDot-Sensitized Solar Cells. J. Am. Chem. Soc. 2013, 135, 15913−15922. (22) Chang, J. A.; Im, S. H.; Lee, Y. H.; Kim, H.-J.; Lim, C.-S.; Heo, J. H.; Seok, S. I. Panchromatic Photon-Harvesting by Hole-Conducting Materials in Inorganic−Organic Heterojunction Sensitized-Solar Cell through the Formation of Nanostructured Electron Channels. Nano Lett. 2012, 12, 1863−1867. (23) Buhbut, S.; Itzhakov, S.; Hod, I.; Oron, D.; Zaban, A. PhotoInduced Dipoles: A New Method to Convert Photons into Photovoltage in Quantum Dot Sensitized Solar Cells. Nano Lett. 2013, 13, 4456−4461. (24) Albero, J.; Clifford, J. N.; Palomares, E. Quantum Dot Based Molecular Solar Cells. Coord. Chem. Rev. 2013, DOI: 10.1016/ j.ccr.2013.07.005. (25) Lee, H. J.; Yum, J.-H.; Leventis, H. C.; Zakeeruddin, S. M.; Haque, S. A.; Chen, P.; Seok, S. I.; Grätzel, M.; Nazeeruddin, M. K. CdSe Quantum Dot-Sensitized Solar Cells Exceeding Efficiency 1% at Full-Sun Intensity. J. Phys. Chem. C 2008, 112, 11600−11608. (26) Im, J.-H.; Lee, C.-R.; Lee, J.-W.; Park, S.-W.; Park, N.-G. 6.5% Efficient Perovskite Quantum-Dot-Sensitized Solar Cell. Nanoscale 2011, 3, 4088−4093. (27) Lee, H.; Leventis, H. C.; Moon, S. J.; Chen, P.; Ito, S.; Haque, S. A.; Torres, T.; Nuesch, F.; Geiger, T.; Zakeeruddin, S. M.; et al. PbS and CdS Quantum Dot-Sensitized Solid-State Solar Cells: “Old Concepts, New Results”. Adv. Funct. Mater. 2009, 19, 2735−2742. (28) Chung, I.; Lee, B.; He, J.; Chang, R. P. H.; Kanatzidis, M. G. AllSolid-State Dye-Sensitized Solar Cells with High Efficiency. Nature 2012, 485, 486−489. (29) de la Fuente, M. S.; Sánchez, R. S.; González-Pedro, V.; Boix, P. P.; Mhaisalkar, S. G.; Rincón, M. E.; Bisquert, J.; Mora-Seró, I. Effect of Organic and Inorganic Passivation in Quantum-Dot-Sensitized Solar Cells. J. Phys. Chem. Lett. 2013, 4, 1519−1525. (30) Brennan, T. P.; Bakke, J. R.; Ding, I. K.; Hardin, B. E.; Nguyen, W. H.; Mondal, R.; Bailie, C. D.; Margulis, G. Y.; Hoke, E. T.; Sellinger, A.; et al. The Importance of Dye Chemistry and TiCl4 Surface Treatment in the Behavior of Al2O3 Recombination Barrier Layers Deposited by Atomic Layer Deposition in Solid-State DyeSensitized Solar Cells. Phys. Chem. Chem. Phys. 2012, 14, 12130− 12140. (31) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. Control of Charge Recombination Dynamics in Dye Sensitized

Thomas P. Brennan recently received his Ph.D. in Chemical Engineering at Stanford University. His dissertation focuses on recombination barriers in dye- and quantum dot-sensitized solar cells and understanding the orientation of dye molecules adsorbed on metal oxide substrates. Stacey F. Bent is the Jagdeep and Roshni Singh Professor of Engineering at Stanford University. She earned her B.S. degree from U.C. Berkeley and her Ph.D. from Stanford University. Following postdoctoral work at AT&T Bell Laboratories, she joined the faculty at New York University before moving to Stanford University in 1998. Her research interests are in understanding surface and interfacial chemistry and materials processing, and applying this knowledge to problems in sustainable energy and nanoelectronics. Further information can be found at http://bentgroup.stanford.edu.

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ACKNOWLEDGMENTS This publication was supported as part of the Center on Nanostructuring for Efficient Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0001060.

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