Photosensitization of Single-Crystal Oxide Substrates with Quantum

Invited Feature Article Cite This: Langmuir XXXX, XXX, XXX−XXX

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Photosensitization of Single-Crystal Oxide Substrates with Quantum Confined Semiconductors Lenore Kubie and Bruce A. Parkinson*

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Department of Chemistry and School of Energy Resources, University of Wyoming, Laramie, Wyoming 82071, United States ABSTRACT: Dye-sensitized solar cells have been studied for many years as a potential inexpensive and scalable alternative to silicon solar cells. They have recently expanded their list of photosensitizers to include quantum dots. In recent years, there has been substantial progress in the field of quantum dot solar cells, with certified efficiencies now reaching 13.4%. Fundamental studies on nanomaterial/semiconductor electrode coupling have led to a deeper understanding of photoinduced electron-transfer processes that are important for both of these devices. This Feature Article will highlight the use of a model system, nanomaterials sensitizing single-crystal oxide substrates, that is useful for investigating how changes in nanomaterial shape, dimensionality, size, and local environment affect the photoinduced charge separation efficiency.

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INTRODUCTION Photosensitization has its roots in traditional photographic film, where photoinduced electron transfer (PET) from a dye (photosensitizer) to a silver halide (I−VII semiconductor) grain reduces silver ions, Ag+, to silver metal, Ag0, producing a nucleation site for the developer (a reducing agent) to grow the silver metal grain, producing a negative image.1 Before the advent of dye sensitization, these silver halide grains, with wide band gaps of between 2.7 and 3.2 eV, were sensitive only to blue and ultraviolet light.2 In 1883, the first dye-sensitized silver plates became available commercially.3 These orthochromatic films extended film sensitivity into the blue and green regime of the visible spectrum. The development of new dye sensitizers in the mid-1890s and early 1900s led to the production of panchromatic films that were sensitive to all wavelengths of visible light.3 The use of dye photosensitizers expanded into the field of photovoltaics in the 1960s with the sensitization of ZnO with dye molecules.4,5 In these systems, dyes are used to sensitize a semiconducting electrode, and PET injects an electron into the conduction band of the electrode. Unlike in photography, where the injected charge is used to reduce a silver cation to silver metal, the injected charge in the electrode can be collected to produce a measurable photocurrent, making it possible to study this process in real time and examine how dye/electrode band alignment, electrode band bending, regenerator chemistry, and electrode doping affect PET.6−15 These early studies led to the development of the dyesensitized solar cell (DSSC) or Grätzel cell, wherein a transparent nanocrystalline TiO2 electrode, sensitized with dye molecules, is used as the top plate of a sealed cell containing an iodide redox couple/electrolyte and a bottom plate often having some platinum electrocatalyst.16 This closed-system design allows for the continuous generation and collection of photogenerated charges from the dye molecules to produce electrical power. While mesoporous metal oxide substrates, such as those used in the Grätzel cell, provide high surface areas and thus © XXXX American Chemical Society

provide a means for the production of large photocurrents,17−20 the use of single-crystal metal oxide substrates provides an avenue for fundamental studies of the photosensitizer/electrode interface at the expense of a decreased surface area and thus photocurrent magnitude.11 Several fundamental studies on dye/single-crystal metal oxide interfaces have been performed wherein photocurrents from monolayer coverages of dyes of oxide crystals are explored. These studies have found that the crystal face has a large effect on dye loading (and thus the incident photon conversion efficiency (IPCE)) and that there is an ideal range of doping densities to produce maximum IPCEs. For a recent review of dye-sensitized single-crystal electrodes, see King et al.21 More recently, the field of photocurrent generation with sensitized metal oxide electrodes has expanded to explore nanomaterialsensitized solar cells that include, but are not limited to, quantum-dot-sensitized solar cells (QDSSCs).

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QUANTUM-DOT-SENSITIZED SOLAR CELLS Quantum dots have several advantages over dyes for use as photosensitizers. First, dyes have a propensity to photobleach, limiting their stability and the potential lifetime of a dyesensitized solar cell (DSSC). While some types of quantum dots will undergo oxidation (e.g., PbS quantum dots), this issue has been addressed using well-established techniques such as core/shell structures and other surface passivation techniques.22−33 Quantum dots also have the advantage of being ligated by exchangeable ligands that have a negligible effect on the optical properties of the quantum dots but can have large effects on the efficiency of a QDSSC produced using these QDs. This is due to the ligands’ effect on the electronic coupling both between the quantum dots in thicker (>1 monolayer) films and at the quantum dot/electrode interReceived: March 5, 2018 Revised: August 16, 2018 Published: August 27, 2018 A

DOI: 10.1021/acs.langmuir.8b00720 Langmuir XXXX, XXX, XXX−XXX

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face.34−43 Simply, the more strongly electronically coupled the QDs are to each other and the electrode, the more efficient the photoinduced electron-transfer process. A key feature of any semiconducting quantum confined system is the tunable band gap that changes with particle size (akin to the quantum mechanics solution to a particle in a box where a smaller quantum dot will have a larger bandgap). This has obvious implications in adjusting the absorbed wavelengths by simply changing the size of the quantum dot. The most intriguing advantage of QDSSCs over DSSCs, or even traditional bulk semiconductor solar cells, is the decreased threshold for multiple exciton generation (MEG).44 In multiple exciton generation, a single high-energy photon, of at least 2 times the band gap energy, can produce multiple excitons, and thus multiple free carriers can be collected from a single photon (Figure 1), resulting in higher

using a model single-crystal system.46 Using this model system, the importance of ligand exchange from long, nonpolar ligands to short, electronically coupled ligands has been demonstrated to be of upmost importance.46 However, it has also been shown that in situ ligand exchange, where ligands are exchanged after film deposition, causes irreproducible IPCE values, likely due to uneven QD sensitization. However, ex situ ligand exchange produces reliable IPCE values and near monolayer QD sensitization.46 Another advantage of using single-crystal substrates is that it allows for using scanning probe microscopies to examine the coverage, thickness, and organization of the QDs on the nearly atomically flat singlecrystal surfaces (Figure 2). Much like dyes, the IPCE of CdSe QDs on single-crystal TiO2 have been shown to be dependent on the doping density of the electrode.47 However, the factors influencing the IPCE are far more complicated in the case of QDs compared to dyes. The cause of this is twofold. First, the increased physical size of a QD compared to a dye allows for a larger physical separation between the injected electron and the hole remaining on the QD. This affects the recombination pathways by lowering the Coulombic attraction between the separated carriers. Second, QDs cannot be thought of as uniform entities and instead contain several regions including core, surface, and ligand. Specifically, we have found that the trapping of a free carrier has significant effects on carrier recombination (Figure 3).47 Without the use of the single-crystal electrode model system, the effects of these pathways could not be easily elucidated from other germinate and nongerminate recombination pathways found in thicker QDSSC systems. Exciton and free-carrier recombination is a major hurdle that must be overcome to make QDSSCs commercially viable. Because exciton and free-carrier recombination is often dominated by surface trap state pathways, shielding of the exciton from these trap states through producing QDs with an outer shell of a larger band gap material has been employed.22−25,48 Core−shell QDs such as these form type I heterostructures, confining the exciton to the core of the QD (Figure 3). While this technique has been shown to increase the lifetime of the exciton, it also creates a barrier to charge extraction. Thus, care must be taken when considering shell thickness.22 The trade-off between the exciton lifetime and charge injection into the electrode manifests itself through an

Figure 1. MEG process whereby a photon with energy >2Eg is absorbed by a QD to produce a high-energy exciton. During the relaxation of the high-energy exciton to the band edge, the excess energy is transferred to produce a second exciton.

efficiencies of photoconversion at these shorter wavelengths. While this phenomena does occur in bulk materials, the threshold for MEG is lower in quantum confined systems (e.g., the MEG is 2 to 3 times more efficient in PbSe QDs than in bulk PbSe).45 The increased MEG yield in quantum confined systems is attributed to several traits of a quantum confined system, including an increase in the probability of Auger energy-transfer processes and less-strict momentum conservation rules.44,45 The Parkinson laboratory demonstrated that fundamental studies of QD sensitization on electrodes can be performed

Figure 2. AFM images of CdSe QD-sensitized TiO2 crystals. (a) Monolayer coverage of QDs on TiO2. Lattice planes of the atomically flat TiO2 can be seen underneath. (b) The QDs have been “scraped” off by switching to contact mode AFM to reveal the clean TiO2 crystal beneath. Reprinted with permission from ref 47. Copyright 2018 American Chemical Society. B


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Figure 4. Absorbed photon conversion efficiency (APCE) versus (A) photon energy and (B) photon energy relative to the QD band gap. MEC is observed at ∼2.5 Eg. Reprinted with permission from ref 50. Copyright 2010 AAAS.

into the TiO2 by measuring photocurrents from these higherenergy excitonic states in PbS QDs where the lowest excitonic state (1s) was below the conduction band of TiO2. Expanding on this, in 2011, the NREL group produced the first solar cell to demonstrate MEG that pushed the APCE above 100% in the blue region of the spectrum.51 Since these initial findings proving that MEG is possible in QDSSCs, there has been a focus on developing QD structures that would exhibit lower MEG thresholds. The two key concepts in such QD designs are efficient exciton separation into free carriers and the subsequent fast extraction of free carriers. One possibility for decreasing MEG thresholds is the design of type II heterojunction core−shell QD structures, which allow for the dissociation of an exciton into an electron and hole at the interface between the two materials (Figure 5).

Figure 3. Energy-level band diagram of the sensitizing CdSe quantum dots on the TiO2 surface. A photoexcited electron is depicted in the QD conduction band, with a hole left behind in the valence band. The hole can then be quickly trapped by states at the QD surface (ss), leaving the electron free to be injected into the TiO2 conduction band. Possible recombination pathways are shown as blue dashed lines. Reprinted with permission from ref 47. Copyright 2018 American Chemical Society.

increase in the open-circuit voltage (VOC) and a decrease in the short circuit current (JSC), respectively.23 Another method by which the surface of lead chalcogenide QDs can be passivated is the ligand exchange to halide ligands such as iodide and chloride.26−33 Both core/shell methods and halide capping also have the added benefit of protecting lead chalcogenide QDs from oxidation. Using monolayer coverages of QDs on single-crystal electrodes, the effect of oxidizing agents on quantum dots can be monitored in a uniform manner. A 2016 article showed that even under highly photocorrosive conditions (0.25 M aqueous KI electrolyte), after an initial rapid decay a residual sensitized photocurrent from ZnS/CdSe shell/core QDs persists for >18 h, indicating in this case that a thin shell is probably only completely protecting about 25% of the QDs.49 Core-only cadmium selenide quantum dots covalently attached to and photosensitizing single-crystal TiO2 surfaces corrode under illumination in aqueous electrolyte containing iodide as a regenerator. A comparison of photocurrent spectra before and after long-term monochromatic illumination indicated that the CdSe QD sensitizers photocorroded and decreased in size until their band gap energy exceeded the excitation energy. This wavelength-dependent photoelectrochemical etching mechanism can be used to tune the size distribution of surface-adsorbed QDs.32 Monolayer coverages of quantum dots on single-crystal electrodes allows for a fundamental study of MEG in QDSSCs. In 2010, the Parkinson laboratory was the first to definitively measure not only MEG but also multiple exciton dissociation and multiple exciton collection (MEC) in a QDSSC system. In this system, PbS QDs were used to sensitize anatase TiO2 single crystals. Detailed measurements of sample absorbance were then used to calculate absorbed photon conversion efficiencies (APCEs), which exceeded 100% at shorter wavelengths.50 In this report, the threshold for MEG was found to be (2.5 ± 0.25) Eg (Figure 4). This study also identified electron injection from higher-energy excitonic states

Figure 5. Core, type I, and type II core−shell QD structures are shown (top) with corresponding band alignments and wave functions (bottom). In a type I structure, both the electron and hole wave functions are mostly confined to the core of the QD, whereas in a type II structure either the hole (shown) or the electron is confined to the shell while the other carrier is confined to the core.

However, these structures leave either the electron or hole trapped in the core of the QD and thus limit the rate of charge extraction. To overcome this barrier, it is possible to design Janus type II QD heterostructures that would leave both the hole and electron exposed.52,53 A final way in which the energy onset of MEG can be realized is by decreasing the band gap of the material. InSb has very favorable properties with a large 1s Bohr exciton radius (56 nm), a small band gap, and favorable carrier effective masses for MEG (me = 0.014 mo and mh = 0.25 mo, where mo is the mass of a free electron).54 Ongoing work in the Parkinson laboratory is exploring the possibility of three or more electrons being collected per photon using InSb QDs as sensitizers. C


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QUASI-ONE-DIMENSIONAL NANOSTRUCTURES IN SOLAR CELLS While the majority of work on the incorporation of nanomaterials into solar cells has focused on quantum dots, it is important to discuss the other common shapes of nanomaterials: quasi-one-dimensional and quasi-two-dimensional (vide infra). Because these systems are also dominated by surface states, many of the lessons learned from QD systems apply to these systems. Both quasi-one- and quasi-twodimensional systems have the benefit of an increased density of states (DOS) at the band edge compared to their quasizero-dimensional counterparts (Figure 6).55 The increased DOS in these systems translates to an increased optical crosssection and thus increased light harvesting in a nanomaterial sensitized solar cell (NMSSC).

relationship between the driving force of the free-carrier transfer and the carrier collection efficiency (Figure 7). These results support previous work by Ihly et al. where a Marcus-like relationship is observed in photoinduced electron transfer (PET) between s-SWCNTs and C60 derivatives.77

Figure 7. Relationship between the relative photon conversion efficiency and driving force on (a) SnO2 and (b) TiO2. CoMoCAT SWCNT chiralities are shown in black, and HiPCO chiralities are shown in red. Both plots are normalized to (7, 5) SWCNTs. Reprinted with permission from ref 76. Copyright 2018 American Chemical Society. Figure 6. Density of states [D(E)] of 2D, 1D, and 0D systems as a function of energy (E).55

Importantly, though SWCNTs do not have ligands but instead are often wrapped by surfactant, we found that the surfactant and concentration of excess surfactant played a large role in the photoconversion efficiencies. For example, no measurable photocurrent was obtained for sodium dodecyl sulfate (SDS)- or sodium dodecyl benzenesulfonate (SDBS)wrapped SWCNTs; however, both sodium cholate (SC)- and poly[9,9-dioctylfluorenyl-2,7-diyl] (PFO)-wrapped SWCNTs produced photocurrent. In both the SC and PFO cases, excess surfactant in solution caused a large decrease in photocurrent.76 We attribute this to the binding of excess free surfactant to the metal oxide electrode, forming an insulating barrier between the SWCNTs and the electrode. Additionally, the carboxylate group on the cholate is capable of binding directly to the TiO2 surface, presumably resulting in stronger electronic coupling. One hurdle that remains when studying these 1D structures is obtaining higher/near-monolayer coverages of these highaspect-ratio materials. In order to limit tube−tube interactions, low submonolayer coverages must be used, which drastically decrease IPCEs. SWCNT alignment methods may allow for higher nanotube loading and thus higher IPCEs. Additionally, improvements in thin-film absorbance measurements on doped (colored) crystals would allow us to convert IPCEs to IQEs and thus paint a better picture of SWCNT carrier collection efficiencies.

Lead chalcogenide nanorods (NRs) have been successfully implemented as the active material in a solar cell, with internal quantum efficiencies exceeding 120%.56 Enhanced MEG in NRs over QDs has been observed spectroscopically by several other groups as well.57,58 The increased carrier generation and subsequent collection from NRs compared to QDs is likely due to both an increase in Coulombic exciton attraction57,58 and a decrease in Auger recombination rates.58 Another class of quasi-one-dimensional nanostructures is single-walled carbon nanotubes (SWCNTs). Because of their abundant all-carbon composition and high stability, these materials are extremely attractive for use in photovoltaic devices. Unfortunately, these materials have often been dismissed as inappropriate active-layer materials as a result of their heterogeneous compositions and large exciton binding energies (EBEs).59 Specifically, as-synthesized SWCNTs contain both semiconducting SWCNTs (s-SWCNTs) and metallic SWCNTs (m-SWCNTs). Therefore, before being incorporated as an absorber in a photovoltaic device, the mSWCNTs must be removed. In recent years, several methods of accomplishing the separation of SWCNTs have been developed.60−65 The large EBEs remain problematic but have been shown to be highly dependent on the SWCNT chirality, local dielectric constant, and level of tube−tube interaction.66−71 Therefore, changing the local environment is one possible method of overcoming this limitation. Despite these large binding energies, there has been some success in generating free carriers in SWCNTs; however, until recently direct free-carrier collection by an electrode had been accomplished only by applying a large bias across the nanotube.72−76 Recently, we were able to implement our model singlecrystal system to study photoinduced charge collection from various s-SWCNT chiralities on both TiO2 and SnO2 singlecrystal substrates.76 In this study, we demonstrated that freecarrier collection is possible from all studied SWCNT chiralities and also determined that there is a logarithmic

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QUASI-TWO-DIMENSIONAL NANOSTRUCTURES IN SOLAR CELLS The density of states of an ideal two-dimensional structure is also depicted in Figure 6. Similar to one-dimensional structures, two-dimensional materials have a larger density of states at the band edge compared to their zero-dimensional counterparts. To date, there has been limited work on measuring the photoresponse of single layers of two-dimensional materials.78−81 Recently, we reported the synthesis and characterization (including photoresponse) from Ag2S nanoplatelets (NPLs).82 The formation of nanoplatelets was an unexpected result because of the fact that Ag2S does not D


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charge transfer across these interfaces that may play a role in future energy conversion technologies. We look forward to future studies, from both our laboratory and others in the field, to expand the investigation of the interesting and potentially useful phenomena that are unique to quantum confined semiconductor systems.

inherently form a 2D structure. However, the bulk structure of Ag2S does have planes of silver and sulfur along the (101) plane. We surmise that growth along the other planes is hindered via poisoning of the (101) faces, confining the platelets in one direction. Unlike other 2D materials (e.g., graphene, MoS2), these NPLs are synthesized colloidally; therefore, producing variants of these NPLs should be possible through changes in synthesis conditions and reactants. The sensitization of Ag2S NPLs on single-crystal SnO2 results in photoaction spectra that closely match the absorbance spectra of the sample. However, over time the excitonic features disappear in the photoaction spectra (Figure 8a). Importantly,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lenore Kubie: 0000-0002-0976-8548 Bruce A. Parkinson: 0000-0002-8950-1922 Notes

The authors declare no competing financial interest. Biographies

Figure 8. Ag2S nanoplatelets. (a) IPCE spectra (red to blue) and UV−vis spectrum (black) of single-layer Ag2S. Time proceeds from red to blue. Over time, all excitonic features are lost. (b) UV−vis spectra of Ag2S NPLs during synthesis. Time proceeds from red to purple. First, there is the appearance of an excitonic peak at 1.55 eV (single-layer Ag2S). Over time, this peak disappears and a lowerenergy (1.25 eV) peak appears (multilayer Ag2S). Reprinted with permission from ref 82. Copyright 2017 American Chemical Society.

Lenore Kubie received her B.A. in chemistry at Western Connecticut State University in 2009 and her Ph.D. in chemistry from the University of Rochester in 2016. At the University of Rochester, Lenore was awarded the IGERT fellowship and worked under the advisement of Drs. Kara L. Bren and Todd D. Krauss. Lenore then became a postdoctoral research scientist at the University of Wyoming under the advisement of Dr. Bruce A. Parkinson. Lenore’s previous research has included fields ranging from inorganic biochemistry to the photophysics of nanomaterials. Her current research focuses on the design of 0D, 1D, and 2D nanocrystals for use in light-harvesting applications. Recently, Lenore began a second postdoctoral position at the National Renewable Energy Laboratory (NREL) under the advisement of Dr. Matthew C. Beard.

we have shown that the 3-mercaptopropionic acid ligands are located only around the edges of these platelets, and we hypothesize that the degradation of the sample over time is due to the lack of ligand on the top and bottom faces of the platelets, allowing for nanoplatelet growth over time. The growth of these platelets perpendicular to the (101) plane does eventually occur during synthesis. In fact, two Ag2S species can be isolated: single layer and multilayer. Unlike QD growth, where the exciton peak slowly shifts to lower energies throughout the course of the reaction (as the QD grows larger), these Ag2S nanoplatelets form two distinct species with the first exciton peaks separated by 0.3 eV (Figure 8b). Currently, we are investigating methods of stabilizing these NPLs to prevent multilayer growth. In summary, it is apparent that single-crystal substrates are quite useful for studying the fundamentals of photoinduced electron transfer across semiconductor/electrolyte interfaces. The detailed structural information, obtainable using scanning probe techniques, is indispensable in interpreting both the spectral and current voltage data that can be measured even at very low coverages of the quantum confined semiconducting species. This combined information is then useful for evaluating the applicability of various physical models of

Bruce A. Parkinson received his B.S. in chemistry from Iowa State University in 1972 and his Ph.D. in chemistry from Caltech in 1977 and was a postdoctoral scientist at Bell Laboratories in 1978. He then E


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(16) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353 (6346), 737−740. (17) Liska, P.; Vlachopoulos, N.; Nazeeruddin, M. K.; Comte, P.; Graetzel, M. Cis-Diaquabis(2,2“-Bipyridyl-4,4-”Dicarboxylate)Ruthenium(II) Sensitizes Wide Band Gap Oxide Semiconductors Very Efficiently Over a Broad Spectral Range in the Visible. J. Am. Chem. Soc. 1988, 110 (11), 3686−3687. (18) Vlachopoulos, N.; Liska, P.; Augustynski, J.; Graetzel, M. Very Efficient Visible Light Energy Harvesting and Conversion by Spectral Sensitization of High Surface Area Polycrystalline Titanium Dioxide Films. J. Am. Chem. Soc. 1988, 110 (4), 1216−1220. (19) O’Regan, B.; Moser, J.; Anderson, M.; Graetzel, M. Vectorial Electron Injection Into Transparent Semiconductor Membranes and Electric Field Effects on the Dynamics of Light-Induced Charge Separation. J. Phys. Chem. 1990, 94 (24), 8720−8726. (20) Enea, O.; Moser, J.; Grätzel, M. Achievement of Incident Photon to Electric Current Conversion Yields Exceeding 80% in the Spectral Sensitization of Titanium Dioxide by Coumarin. J. Electroanal. Chem. Interfacial Electrochem. 1989, 259 (1−2), 59−65. (21) Kilin, D., Ed.; Photoinduced Processes at Surfaces and in Nanomaterials; ACS Symposium Series; American Chemical Society: Washington, D.C., 2015. (22) Lai, L.-H.; Protesescu, L.; Kovalenko, M. V.; Loi, M. A. Sensitized Solar Cells with Colloidal PbS−CdS Core−Shell Quantum Dots. Phys. Chem. Chem. Phys. 2014, 16 (2), 736−742. (23) Neo, D. C. J.; Cheng, C.; Stranks, S. D.; Fairclough, S. M.; Kim, J. S.; Kirkland, A. I.; Smith, J. M.; Snaith, H. J.; Assender, H. E.; Watt, A. A. R. Influence of Shell Thickness and Surface Passivation on PbS/ CdS Core/Shell Colloidal Quantum Dot Solar Cells. Chem. Mater. 2014, 26 (13), 4004−4013. (24) Yang, J.; Wang, J.; Zhao, K.; Izuishi, T.; Li, Y.; Shen, Q.; Zhong, X. CdSeTe/CdS Type-I Core/Shell Quantum Dot Sensitized Solar Cells with Efficiency Over 9%. J. Phys. Chem. C 2015, 119 (52), 28800−28808. (25) Speirs, M. J.; Balazs, D. M.; Fang, H. H.; Lai, L. H.; Protesescu, L.; Kovalenko, M. V.; Loi, M. A. Origin of the Increased Open Circuit Voltage in PbS−CdS Core−Shell Quantum Dot Solar Cells. J. Mater. Chem. A 2015, 3 (4), 1450−1457. (26) Zhang, Z.; Yang, J.; Wen, X.; Yuan, L.; Shrestha, S.; Stride, J. A.; Conibeer, G. J.; Patterson, R. J.; Huang, S. Effect of Halide Treatments on PbSe Quantum Dot Thin Films: Stability, Hot Carrier Lifetime, and Application to Photovoltaics. J. Phys. Chem. C 2015, 119 (42), 24149−24155. (27) Sayevich, V.; Gaponik, N.; Plötner, M.; Kruszynska, M.; Gemming, T.; Dzhagan, V. M.; Akhavan, S.; Zahn, D. R. T.; Demir, H. V.; Eychmüller, A. Stable Dispersion of Iodide-Capped PbSe Quantum Dots for High-Performance Low-Temperature Processed Electronics and Optoelectronics. Chem. Mater. 2015, 27 (12), 4328− 4337. (28) Stadler, P.; Mohamed, S. A.; Gasiorowski, J.; Sytnyk, M.; Yakunin, S.; Scharber, M. C.; Enengl, C.; Enengl, S.; Egbe, D. A. M.; El Mansy, M. K.; et al. Iodide-Capped PbS Quantum Dots: Full Optical Characterization of a Versatile Absorber. Adv. Mater. 2015, 27 (9), 1533−1539. (29) Ning, Z.; Voznyy, O.; Pan, J.; Hoogland, S.; Adinolfi, V.; Xu, J.; Li, M.; Kirmani, A. R.; Sun, J.-P.; Minor, J.; et al. Air-Stable N-Type Colloidal Quantum Dot Solids. Nat. Mater. 2014, 13 (8), 822−828. (30) Niu, G.; Wang, L.; Gao, R.; Ma, B.; Dong, H.; Qiu, Y. Inorganic Iodide Ligands in Ex Situ PbS Quantum Dot Sensitized Solar Cells with I−/I3− Electrolytes. J. Mater. Chem. 2012, 22 (33), 16914− 16919. (31) Shepherd, D. P.; Sambur, J. B.; Liang, Y.-Q.; Parkinson, B. A.; Van Orden, A. In Situ Studies of Photoluminescence Quenching and Photocurrent Yield in Quantum Dot Sensitized Single Crystal TiO2 And ZnO Electrodes. J. Phys. Chem. C 2012, 116 (39), 21069−21076. (32) Sambur, J. B.; Parkinson, B. A. Size Selective Photoetching of CdSe Quantum Dot Sensitizers on Single-Crystal TiO2. ACS Appl. Mater. Interfaces 2014, 6 (24), 21916−21920.

spent time at the Ames Laboratory and the Solar Energy Research Institute (now known as the National Renewable Energy Laboratory) in Golden, Colorado. He moved to the Central Research and Development Department of the DuPont Company in 1985, and in 1991, he became a professor of chemistry at Colorado State University until his departure in 2008 to join the Department of Chemistry and the School of Energy Resources at the University of Wyoming. His current research covers a wide range of areas including materials chemistry, surface chemistry, and photoelectrochemical energy conversion on both Earth and Mars. He is a Fellow of the American Association for the Advancement of Science and the Electrochemical Society and has more than 235 peer-reviewed publications plus 5 U.S. patents. He recently was awarded a Humboldt Research Prize from the German Humboldt Society.

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ACKNOWLEDGMENTS The authors thank the Department of Energy, Office of Science, Basic Energy Sciences, Division of Chemical Sciences for financial support through grant DE-SC0007115.

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REFERENCES

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