Electronic Structure of PbS Colloidal Quantum Dots on Indium Tin

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Electronic Structure of PbS Colloidal Quantum Dots on Indium Tin Oxide and Titanium Oxide Tae Gun Kim, Hyekyoung Choi, Sohee Jeong, and Jeong Won Kim J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 12 Nov 2014 Downloaded from http://pubs.acs.org on November 12, 2014

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The Journal of Physical Chemistry

Electronic Structure of PbS Colloidal Quantum Dots on Indium Tin Oxide and Titanium Oxide Tae Gun Kim†,§,#, Hyekyoung Choi‡,§,#, Sohee Jeong‡,§* and Jeong Won Kim†,§* †

Korea Research Institute of Standards and Science (KRISS), 267 Gajeong-ro, Daejeon 305-340,

South Korea ‡

Nano-Mechanical Systems Research Division, Korea institute of Machinery and Materials

(KIMM), 156 Gajeongbuk-ro, Daejeon, 305-343, South Korea §

Korea University of Science and Technology (UST), 217 Gajeong-ro, Daejeon 305-350, South

Korea

ACS Paragon Plus Environment

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ABSTRACT

The size of colloidal quantum dot (CQD) materials and their surface modification by chemical ligands can change electronic properties thereby affecting device performances made thereof. In this study, direct measurement of the electronic structure within CQD thin film upon solid-state ligand exchange from oleic acid to 1,2-ethanedithiol has been made by photoelectron spectroscopy. Specifically, we analyzed valence band structures as a function of PbS CQD thickness on two kinds of substrates, indium tin oxide and titanium oxide, respectively which gives the trace of band bending and its saturation. Consequently the energy level alignment of the PbS CQD reveals downward band bending to the substrate but with different magnitude and depletion width depending on substrate. Wide depletion width and barrierless electron injection on TiO2 substrate indicate the importance of junction design and drift length for efficient CQD photovoltaics, which can be addressed discernibly via photoelectron spectroscopy.

KEYWORDS: PbS quantum dot ·

quantum confinement effect ·

1,2-ethanedithiol ·

photovoltaic cells · depletion width · substrate effect · photoelectron spectroscopy


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1.

INTRODUCTION Colloidal quantum dot (CQD) photovoltaic (PV) devices made of lead chalcogenide (PbX, X =

Se, S, Te) have drawn extensive attention for next-generation PV cells due to their potentials as low-cost and highly-efficient energy harvesters.1 Especially, the band gap engineering adjusting quantum confinement effect by control of CQD size can lead to a wide variety of opto-electronic application due to large exciton Bohr radius (20 nm for PbS, 46 nm for PbSe) of PbX CQDs.2-5 However, the surface to volume ratio increases as the CQD size becomes smaller, which leads to the lots of defect sites influencing PV performance at the CQD surface.6 That is why people have paid lots of attention to the ligands on CQD surface to effectively control not only distance between CQDs but also surface defect sites. Among various ligand materials, oleic acid (OA) is well known for effective dispersion of CQDs in non-polar solution.7 However, it has shown a poor charge carrier mobility due to its long alkyl chain acting as transport barrier.8 Recently, there are many researches on the exchange of long ligand molecules by shorter ligand molecules such as organic, inorganic, or hybrid ligand to enhance the carrier mobility and lifetime for better transport properties.6, 8-12 Once an active film of conductive CQDs is created and shined, photogenerated carriers are transferred to contact electrodes via Schottky or depleted-heterojunction (DH) architecture in CQD film based photovoltaics.13-20 Both device structures require an optimized energy level alignment for effective charge separation and transport. Together with keeping the CQD dispersion and controlling the charge mobility as described above, the ligand exchange in CQDbased PVs has a huge influence on the energy level alignment upto 0.9 eV.13 It brings many changes in PV cell parameters and relevant device architecture.12 Thus, accurate measurement of energy level alignment on CQD films is one of key parameters to achieve in PV devices.


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However, the direct measurement of energy levels such as valence band maximum (VBM) and Fermi level in CQD films has rarely been reported due to vulnerable surface oxidation and low signal to noise ratio. Furthermore, a conductive substrate used to avoid charging effect during measurement of ultraviolet photoelectron spectroscopy (UPS) leads to additional band bending between substrate and CQDs depending on the electronic property of the substrate. Here we carefully measure photoelectron spectroscopy to observe the energy level alignment of PbS CQD films and demonstrate the band diagram on the basis of our result by ligand exchange, film thickness, and substrate type. The 1,2-ethanedithiol (EDT) ligand exchange from oleic acid (OA) is successfully carried out on PbS CQD films. The band alignment of PbS CQDs capped with EDT on indium tin oxide (ITO) and TiO2 substrates as representative of Schottky and DH types is compared. Depending on the substrate and CQD film thickness, it shows identical bandbending direction but different saturation thickness. Finally, the depletion width and band bending magnitude are described using the saturation thickness of the CQD films.


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EXPERIMENTS Materials such as Lead(II) oxide (PbO, Aldrich, 99.999%), oleic acid (OA, Alfa, 99%), 1-

octadecene (ODE, Aldrich, 90%), and bis(trimethylsilyl)sulfide (TMS2S, Aldrich, 99.999%) were used as purchased without further purification. PbS CQDs were synthesized by a general method described elsewhere.14 All manipulations were performed using the standard Schlenk line techniques. In a typical synthesis, PbO (0.46 g), OA (1.2 g), and ODE (10 mL) was degassed in three-neck flask for 30 minutes under vacuum. The solution was then heated to 110 oC and reacted for 1.5 hours. Hereafter, the solution was changed under nitrogen (N2) atmosphere and allowed to cool down to 75 oC. TMS2S (180 ㎕) in 4 mL of ODE was loaded into a 12 mL syringe and then rapidly injected into the solution. The flask was then transferred to N2-filled glove box. PbS CQDs were isolated from reaction solution by precipitation using acetone. The resulting precipitate was dispersed in hexane and washed two times with acetone. Lastly, PbS CQDs were dispersed in octane at 10 mg/mL for film fabrication. TiO2/FTO substrate was made using TiO2 nanoparticles from SOLARONIX. TiO2 nanoparticles were deposited on FTO substrate by spin-cast processing at 2500 rpm for 60 seconds. This film was annealed at 450 oC. PbS CQD solids were fabricated using layer-by-layer spin-cast method on ITO and TiO2/FTO substrate in N2 filled glove box. For each layer, the PbS CQDs in octane (10 mg/ml) were dropped while substrate spun at a 2500 rpm. Solid state ligand exchange was processed by previous report.15 1 vol. % of EDT in acetonitrile, acetonitrile, and octane were dropped in the same way as PbS CQD solution. This coating cycle was repeated until thickness of film was targeted (1, 5, 10, and 50 cycles). This film was annealed at 90 oC for 5 minutes.


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Once the PbS CQD films were made, those were mounted on each sample holder and encapsulated in N2 glove box without air exposure. The encapsulated PbS CQD samples are transferred to an entry glove bag filled with dry N2. This transfer scheme minimized any air-bone contaminants and preserves original sample surfaces intact. The base pressure of the analysis chamber was maintained under low 10-10 Torr. The ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS) measurement were performed using a hemispherical electron energy analyzer with a CCD camera (SES-100, VG-Scienta). The UPS measurement used a He I (hω = 21.22 eV) gas discharge lamp as an excitation source with sample bias of -10V for secondary electron cut-off region. The XPS measurement used an Al Kα (hω = 1486.5 eV) without monochromator. The energy resolutions were 0.1 and 1.0 eV, respectively16.


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3.

RESULTS and DICSCUSION

Figure 1. (a) Absorption spectra and (b) transmittance electron microscopy (TEM) image of PbS QDs with particle size of 2.8±0.15 nm.

Figure 1 shows absorption spectrum of PbS CQDs and their TEM image. The absorption peak is at 780 nm (1.59 eV) while the TEM image shows the PbS QD nanocrystalline size of 2.8±0.15


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nm in diameter. Relatively uniform CQDs are synthesized and transferred into a well-dispersed film.

Figure 2. XPS core level spectra of (a) Pb 4f, (b) S 2s, and (c) O 1s for CQD films with oleic acid (OA) and 1,2-ethanedithiol (EDT) termination. The S 2s and O 1s core levels are fitted with Voigt functions. Surface chemical environment and elemental stoichiometry strongly influence on the electronic properties of PbS CQDs.14, 17-18 To tackle these aspects, the XPS measurement of the CQDs before (OA) and after (EDT) ligand exchange was carried out. Figure 2a shows Pb 4f core level spectra for the PbS films. The Pb 4f doublet reveals the spin-orbit splitting of 4.85 eV and almost symmetric features throughout the species. There is no other species such as highly oxidized one (PbOx) or Pb metallic phase.19-22 The Pb 4f7/2 peak position of 138.40 eV for OA-


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PbS CQD film is slightly higher than that of the EDT-PbS CQDs (138.0 eV) due to the higher electronegativity of oxygen than sulfur.22-24 Additional attention is paid to the full width at halfmaximum (fwhm) of Pb 4f7/2 peaks. The OA-PbS CQD film shows the fwhm of 1.35 eV while EDT-PbS 1.20 eV. The broader 4f peak for OA-PbS is reasonable because the OA-PbS CQDs have an extra kind of chemical bonding between Pb and oxygen in carboxylate in addition to the common Pb-S bonding.4, 14, 25-26 In a control experiment, pristine PbS thin film made by thermal evaporation under vacuum shows also symmetric line shape and fwhm of 1.20 eV for the Pb 4f (Supporting Information Figure S1A). Usually in XPS experiment, the sulfur atom is characterized by S 2p core level but the S 2p peak region is overlapped with Pb 4f inelastic scattering background signal such as plasmon loss feature. Instead Figure 2b shows S 2s core level spectra and their fitting results. The S 2s core level spectra are fitted with two components of Pb-S at 225.5 eV and a shoulder peak at 2.5 eV higher binding energy by deconvolution of Voigt line-shape functions. The additional peak accounts for C-S or O-S bonding from each molecule.27-28 This is confirmed by the control experiment for pristine PbS thin film where a single S 2s component is changed to doublet by the addition of EDT (Supporting Information Figure S1B). Figure 2c shows O 1s core level spectra for each PbS CQD film. The OA-PbS CQDs exhibit an asymmetric peak fitted with two components at 531.7 and 533.1 eV. Those are related to carbonyl (C=O) and hydroxyl (C-OH) species, respectively.29 The peak at 531.7 eV probably includes other emission from Pb-OH which is suggested as a stable species on (111) face of PbS CQD crystal.30 On the other hand, EDT-treated PbS films give no meaningful O 1s core level intensity. Thus, the EDT-PbS CQDs is assured that they are oxygen-free at the surface as the complete ligand exchange is committed.


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For UPS measurement of PbS CQD films, the ITO and TiO2 substrates are used. In Schottky devices using the ITO substrate and metal electrode, photoexcited electrons flow into the low work function metal (such as Al, Ca, etc.) and photoexcited holes move into the ITO substrate.31 However, the limiting factor for the PV performance is that minority carriers (electrons) are required to travel long distance to the metal electrode after excitation. Also, carriers are prone to electron-hole recombination loss during the travel. On the other hand, the CQD film acts as a light-absorbing p-type semiconductor, and the n-type transparent metal oxide (TiO2 or ZnO) material with a deep VB serves as electron acceptor and hole-blocking material in DH PV cells.32-34 To fully understand such substrate effects on the PbS QD electronic structure, a careful UPS measurement has been carried out by controlling the EDT-PbS CQD layer thickness on two different types of substrates, ITO and TiO2. Each film thickness was measured and calibrated by cross-sectional SEM (Supporting Information Figure S2).


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Figure 3 UPS spectra as a function of EDT-PbS CQD layer thickness on either ITO (a-c) or TiO2 (d-f) substrate. In Figure 3b and e, the arrows indicate EDT-PbS CQD characteristic peaks. The figure 3c and f shows semi-logarithmic scale of intensities near the Fermi level and each valence band edge position (vertical bars). Figure 3a-c show UPS spectra of EDT-PbS CQD films on the ITO substrate as function of thickness and Figure 3d-f on the TiO2 substrate. In Figure 3a and d, the secondary cut-off (SECO) regions to measure work function (WF) are displayed. The WFs of the two substrates are 4.67 and 4.48 eV, respectively. When EDT-PbS CQD films are added, their values are saturated to 4.36 and 4.40 eV, respectively. Comparing the figure 3b and e, the relative peak intensities from the valence band (VB) are a little different but their characteristic peak positions are well matched each other (arrows). The EDT-PbS CQDs show distinct four peaks at 3.4 eV (S 3p), 6.0 eV (EDT S nonbonding), 7.5 eV (EDT C-S), and 8.8 eV (Pb 6s), respectively.17, 35-38 Such sharp and clear discrimination of the five peaks is only possible on oxygen-free samples within the framework of quantum confinement effect. The OA-PbS CQD films noted by brown curves in Figure 3b exhibit only a broad VB structure near 7 eV and a small peak at 4 eV of O 2p and S 3s-3p states.38 Figure 3c and f shows the semi-logarithmic plots of UPS spectra near the Fermi level (EF = 0) to determine the VB edge (vertical bars) with respect to the EF. The VB edge determination based on linear intensity plot largely depends on nearby peak position, background shape, and arbitrary extrapolation scheme. When determining the VB edge of a low band-gap CQD film using the linear plot, the conduction band edge is often lower than EF, which has no sense at equilibrium. Rather than that, the semi-logarithmic plot reflects a small amount of surface DOS of CQDs

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and produces reliable results. The control experiment indicates that PbS pristine


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film and adsorption of EDT do not show any extra emission near the VB edge (not shown here).39 Based on this method, the VB edge of EDT-PbS CQD film is determined by the point of intersection of two tangent lines along peak and background in Figure 3c and f. When the thickness of EDT-PbS CQD film is very thin, the initial VB edge position of both substrates is about 0.9 eV. However, each VB edge shows a little different behavior as the EDT-PbS CQD layer thickness increases. Thus, we demonstrate that one should consider the type of substrate and film thickness when measuring UPS. On the ITO substrate, the VB edge position is saturated to the value of 0.65 eV at the CQDs layer thickness between 45–90 nm but on the TiO2 substrate, the VB edge is saturated at 0.75 eV between 180–420 nm. This is because the junction between TiO2 and EDT-PbS CQD films generates a wider depletion than between ITO and EDT-PbS CQD films.

Figure 4. Schematic band diagrams of EDT-PbS CQDs on ITO (a) and TiO2 (b) substrates. Evac, eD, Ec, Ev, EF, Eg, and Wd represent vacuum level, interface dipole, conduction band edge, valence band edge, Fermi level, energy band gap, and depletion width respectively. Green and red arrows show the direction of movement of photoexcited electrons and holes in device,


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respectively. ITO (a) represents Schottky device model and TiO2 (b) represents depleted heterojunction device. The schematic band diagrams of PbS CQD films on ITO and TiO2 based on the UPS measurements in Figure 3 are drawn in Figure 4a and b, respectively. The conduction band edge (Ec) of PbS CQDs is estimated by adding the band gap measured by absorption spectra in Figure 1a. The optical gap of CQDs and CQD-solids can be easily measured with absorption spectroscopy and widely used in most reports on CQD solar cells. The optical gap is smaller than the transport gap by the amount of the exciton binding energy, which is negligible due to large dielectric constant of PbS CQDs.40 The value is generally dependent on particle size and is expected to be on the order of 100 meV in our QD-size regime.8 The ITO is normally regarded as a degenerated semiconductor because the Ec is located below the Ef and TiO2 substrate is a well-known n-type semiconductor in which its Ec is close to Ef and its electronic affinity is greater than 4.2 eV.41 The band bending at TiO2 surface is ignored on the diagram. Ionization potential of PbS CQDs, one of intrinsic material properties, keeps constant at 5.1±0.2 eV throughout the whole film thicknesses. The EF of EDT-PbS CQD films is aligned almost at the middle of band gap primarily owing to the ligand dipole moment.13 The doping polarity of PbS QD solid (p-type, n-type, or ambipolar) can vary easily by the coordinating ligands, level of oxidation, and stoichiometry.42 Also, the stoichiometry and doping polarity of PbS solids depend very much on the size of PbS QDs.17 EDT-PbS solids above 6 nm in diameter mostly show ptype characteristics over the transfer curve analysis in field-effect transistor architecture.42 The EDT-PbS CQD solids behave as ambipolar in our size regime below 4 nm in diameter, which has recently been characterized by a way of FET measurement.43 Moreover, Because of relatively high work function of TiO2 and ITO in Figure 4, both interfaces show the same


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direction of interface dipoles (eD = 0.56 and 0.18 eV, respectively). Therefore the electron depletion at the EDT-PbS CQD film side near ITO or TiO2 makes the same direction (downward) of band bending. The magnitudes of the VB bending for each PbS CQD film are estimated 0.25 and 0.10 eV, respectively in the same direction. The green and red arrows show the flow direction of photoexcited electrons and holes in device, respectively. The directions of photoexcited electrons and holes are opposite in the Schottky and DH type cells. Thus the same direction of VB bending means a hole injection barrier in Figure 4a and a hole blocking barrier in Figure 4b in Schottky and DH cells respectively. Thus the photoexcited holes must have enough energy to overcome the VB bending (0.25 eV) or tunnel through it to be collected toward electrode on ITO substrate in Figure 4a. Otherwise, severe electron-hole recombination occurs crossing this injection barrier. However the slight VB bending in DH cells in Figure 4b will aid the separation of the charge carriers because of the absence of any electron injection barrier at the cathode.32,

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Consequently, the same direction of VB bending in two different types of devices acts opposite roles. The interface dipole induces a charge depletion region in the CQD layers. The depletion tends to expand toward the lightly doped side upto flat band region. The photogenerated carriers in the depletion region are moved and separated via drift along the electric field generated in the depletion region. The PV cells with narrow depletion width suffer from low charge separation field. The charge depletion width (Wd) of the PbS CQD film on TiO2 substrate is three times as wide as on ITO substrate. According to the Poisson equation, the Wd is related to the mobile charge carrier density difference in each junction region. Since ITO has a high free electron density (~1020 cm-3) like metal, depletion region of ITO can be negligible in PbS-EDT CQDs on


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the ITO substrate.45 On the contrary, TiO2 layers have partially depleted region due to their lower n-type carrier density of an order of 1016 cm-3. So, the mobile charges are widely distributed in PbS CQD film on TiO2 substrate with longer charge drift length than on ITO substrate. The direction of space charge field on TiO2 is not a limiting factor in the aspect of electron collection efficiency in DH type PVs. However, since the observed Wd is shorter than usual carrier drift length (0.2 ~ 1 ㎛), the Wd is yet to be improved for overall device performance.46-48 Additionally, our PES analysis suggests that, in the Schottky type cell, the slight hole injection barrier should be avoided to improve collection efficiency in Figure 4a. Thin layers of transition metal oxide with high work function might be a good candidate as a hole extraction layer to switch the interface dipole and space charge field on the ITO.49 The interface engineering with such insertion layers is of much success in many material research fields.


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4.

CONCLUSION The electronic structure of PbS CQD films prepared by solid-state ligand exchange has been

investigated by photoelectron spectroscopy under air-free environment. The energy levels such as EF and VB edge for EDT-PbS CQD film were obtained depending on the type of substrate (ITO and TiO2) and the film thickness (10 to 420 nm) to examine the junction between the substrate and PbS CQD film. We observed the depletion width from each film thickness of saturation, the magnitude of band bending, and the barrier height for hole injection on ITO substrate and hole blocking on TiO2 substrate through band diagram based on UPS results. The hole injection barrier of 0.25 eV observed on ITO substrate suffers from the recombination of carrier pairs, which might lead to low fill factor in PV cells. Our study further shows a wide depletion width and barrierless electron injection on TiO2, which indicates a strong advantage of DH type PV cells over Schottky type. PES studies of PbS CQDs electronic structure on various substrates including metals are on its way to accurately design photovoltaic junction for highly efficient carrier extraction.


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ASSOCIATED CONTENT

Supporting Information XPS spectra of control samples. Cross-sectional SEM images of CQD films. This material is available free of charge via the Internet at http://pubs.acs.org. ■ #

AUTHOR INFORMATION

Both authors contributed equally to this work

Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

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Acknowledgement

We acknowledge the support from National Research Foundation (NRF) grant No. 2014-007296, Nano Material Technology Development Program (2014M3A7B6020163), and the Global Frontier R&D Program (2011-0031566) by the Center for Multiscale Energy Systems funded by the NRF under the Ministry of Science, ICT and Future Planning.


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REFERNCES 1. Kamat, P. V., Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters. J. Phys. Chem. C 2008, 112, 18737-18753. 2. Tian, J.; Cao, G., Semiconductor Quantum Dot-Sensitized Solar Cells. Nano rev. 2013, 4, 22578. 3. Fu, X.; Pan, Y.; Wang, X.; Lombardi, J. R., Quantum Confinement Effects on ChargeTransfer between PbS Quantum Dots and 4-Mercaptopyridine. J. Chem. Phys. 2011, 134, 024707. 4. Nam, M.; Park, J.; Kim, S.-W.; Lee, K., Broadband-Absorbing Hybrid Solar Cells with Efficiency Greater Than 3% Based on a Bulk Heterojunction of PbS Quantum Dots and a LowBandgap Polymer. J. Mater. Chem. A 2014, 2, 3978-3985. 5. Jasieniak, J.; Califano, M.; Watkins, S. E., Size-Dependent Valence and Conduction Band-Edge Energies of Semiconductor Nanocrystals. ACS nano 2011, 5, 5888-5902. 6. Tang, J., et al., Colloidal-Quantum-Dot Photovoltaics Using Atomic-Ligand Passivation. Nat. Mater. 2011, 10, 765-771. 7. Patel, J., Simple Non-Aqueous Fabrication Route for Oleic Acid Capped Luminescent Cadmium Sulphide Quantum Dots at Relatively Low Temperature. Soft Nanoscience Letters 2011, 1, 61-65. 8. Choi, J. J.; Luria, J.; Hyun, B. R.; Bartnik, A. C.; Sun, L.; Lim, Y. F.; Marohn, J. A.; Wise, F. W.; Hanrath, T., Photogenerated Exciton Dissociation in Highly Coupled Lead Salt Nanocrystal Assemblies. Nano Lett. 2010, 10, 1805-1811. 9. Ip, A. H., et al., Hybrid Passivated Colloidal Quantum Dot Solids. Nat Nano 2012, 7, 577-582. 10. Anderson, N. C.; Hendricks, M. P.; Choi, J. J.; Owen, J. S., Ligand Exchange and the Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile MetalCarboxylate Displacement and Binding. J. Am. Chem. Soc. 2013, 135, 18536-18548. 11. 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, 16914- 16919. 12. Chuang, C.-H. M.; Brown, P. R.; Bulović, V.; Bawendi, M. G., Improved Performance and Stability in Quantum dot Solar Cells through Band Alignment engineering. Nat Mater 2014, 13, 796-801. 13. Brown, P. R.; Kim, D.; Lunt, R. R.; Zhao, N.; Bawendi, M. G.; Grossman, J. C.; Bulović, V., Energy Level Modification in Lead Sulfide Quantum Dot Thin Films through Ligand Exchange. ACS nano 2014, 8, 5863-5872. 14. Choi, H.; Ko, J. H.; Kim, Y. H.; Jeong, S., Steric-Hindrance-Driven Shape Transition in PbS Quantum Dots: Understanding Size-Dependent Stability. J. Am. Chem. Soc. 2013, 135, 5278-5281. 15. Choi, H.; Kwan Kim, J.; Hoon Song, J.; Kim, Y.; Jeong, S., Increased Open-Circuit Voltage in a Schottky Device Using PbS Quantum Dots with Extreme Confinement. Appl. Phys. Lett. 2013, 102, 193902. 16. Kim, Y. H.; Kwon, S.; Lee, J. H.; Park, S. M.; Lee, Y. M.; Kim, J. W., Hole Injection Enhancement by a WO3 Interlayer in Inverted Organic Light-Emitting Diodes and Their Interfacial Electronic Structures. J. Phys. Chem. C 2011, 115, 6599-6604.


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17. Kim, D.; Kim, D. H.; Lee, J. H.; Grossman, J. C., Impact of Stoichiometry on the Electronic Structure of PbS Quantum Dots. Phys. Rev. Lett. 2013, 110, 196802. 18. Luther, J. M.; Pietryga, J. M., Stoichiometry Control in Quantum Dots: A Viable Analog to Impurity Doping of Bulk Materials. ACS nano 2013, 7, 1845-1849. 19. Kim, K. S.; O'Leary, T. J.; Winograd, N., X-Ray Photoelectron Spectra of Lead Oxides. Anal. Chem. 1973, 45, 2214-2218. 20. Akhtar, J., et al., A Greener Route to Photoelectrochemically Active PbS Nanoparticles. J. Mater. Chem. 2010, 20, 2336-2344. 21. Zhang, T.; Zhao, H.; Riabinina, D.; Chaker, M.; Ma, D., Concentration-Dependent Photoinduced Photoluminescence Enhancement in Colloidal PbS Quantum Dot Solution. J. Phys. Chem. C 2010, 114, 10153-10159. 22. Hardman, S. J. O., et al., Electronic and Surface Properties of PbS Nanoparticles Exhibiting Efficient Multiple Exciton Generation. PCCP 2011, 13, 20275-20283. 23. Lobo, A.; Moller, T.; Nagel, M.; Borchert, H.; Hickey, S. G.; Weller, H., Photoelectron Spectroscopic Investigations of Chemical Bonding in Organically Stabilized PbS Nanocrystals. J. Phys. Chem. B 2005, 109, 17422-17428. 24. Dissanayake, D. M. N. M.; Hatton, R. A.; Lutz, T.; Giusca, C. E.; Curry, R. J.; Silva, S. R. P., A PbS Nanocrystal-C60 Photovoltaic Device for Infrared Light Harvesting. Appl. Phys. Lett. 2007, 91, 133506. 25. Todosiciuc, A.; Nicorici, A.; Gutsul, T.; Gramm, F.; Braginsky, L.; Shklover, V., Study of Temperature-Based Synthesis of PbTe Nanoparticles and Their Interaction with Surfactant. Rom. J. Info. Sci. Tech. 2010, 13, 84-97. 26. Cass, L. C.; Malicki, M.; Weiss, E. A., The Chemical Environments of Oleate Species within Samples of Oleate-Coated PbS Quantum Dots. Anal. Chem. 2013, 85, 6974-6979. 27. Aguilera-Sigalat, J.; Rocton, S.; Sánchez-Royo, J. F.; Galian, R. E.; Pérez-Prieto, J., Highly Fluorescent and Photostable Organic- and Water-Soluble CdSe/ZnS Core-Shell Quantum Dots Capped with Thiols. RSC Advances 2012, 2, 1632. 28. Stavrinadis, A.; Rath, A. K.; de Arquer, F. P.; Diedenhofen, S. L.; Magen, C.; Martinez, L.; So, D.; Konstantatos, G., Heterovalent Cation Substitutional Doping for Quantum Dot Homojunction Solar Cells. Nat. Commun. 2013, 4, 2981. 29. Ketteler, G.; Ashby, P.; Mun, B. S.; Ratera, I.; Bluhm, H.; Kasemo, B.; Salmeron, M., In Situ Photoelectron Spectroscopy Study of Water Adsorption on Model Biomaterial Surfaces. J. Phys.: Condens. Matter 2008, 20, 184024. 30. Zherebetskyy, D.; Scheele, M.; Zhang, Y.; Bronstein, N.; Thompson, C.; Britt, D.; Salmeron, M.; Alivisatos, P.; Wang, L.-W., Hydroxylation of the Surface of PbS Nanocrystals Passivated with Oleic Acid. Science 2014, 344, 1380-1384. 31. Clifford, J. P.; Johnston, K. W.; Levina, L.; Sargent, E. H., Schottky Barriers to Colloidal Quantum Dot Films. Appl. Phys. Lett. 2007, 91, 253117. 32. Pattantyus-Abraham, A. G., et al., Depleted-Heterojunction Colloidal Quantum Dot Solar Cells. ACS nano 2010, 4, 3374-3380. 33. Gao, J.; Luther, J. M.; Semonin, O. E.; Ellingson, R. J.; Nozik, A. J.; Beard, M. C., Quantum Dot Size Dependent J-V Characteristics in Heterojunction ZnO/PbS Quantum Dot Solar Cells. Nano Lett. 2011, 11, 1002-1008. 34. Etgar, L., Semiconductor Nanocrystals as Light Harvesters in Solar Cells. Materials 2013, 6, 445-459.


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35. Walsh, A., Defect Processes in a PbS Metal Organic Framework: A Quantum-Confined Hybrid Semiconductor. J. Phys. Chem. Lett. 2010, 1, 1284-1287. 36. Kimura, K., Handbook of HeI Photoelectron Spectra of Fundamental Organic Molecules: Ionization Energies, Ab Initio Assignments, and Valence Electronic Structure for 200 Molecules; Japan Scientific Societies Press, 1981. 37. Grandke, T.; Ley, L.; Cardona, M., Angle-Resolved UV Photoemission and Electronic Band Structures of the Lead Chalcogenides. Phys. Rev. B 1978, 18, 3847-3871. 38. Timp, B. A.; Zhu, X. Y., Electronic Energy Alignment at the PbSe Quantum Dots/ZnO(101̅0) Interface. Surf. Sci. 2010, 604, 1335-1341. 39. Carlson, B.; Leschkies, K.; Aydil, E. S.; Zhu, X. Y., Valence Band Alignment at Cadmium Selenide Quantum Dot and Zinc Oxide (101̅0) Interfaces. J. Phys. Chem. C 2008, 112, 8419-8423. 40. Timp, B. A.; Zhu, X.-Y., Electronic Energy Alignment at the PbSe Quantum Dots/ZnO (1010) Interface. Surf. Sci. 2010, 604, 1335-1341. 41. Fuke, N.; Fukui, A.; Islam, A.; Komiya, R.; Yamanaka, R.; Harima, H.; Han, L., Influence of TiO2/Electrode Interface on Electron Transport Properties in Back Contact DyeSensitized Solar Cells. Sol. Energy Mater. Sol. Cells 2009, 93, 720-724. 42. Zarghami, M. H.; Liu, Y.; Gibbs, M.; Gebremichael, E.; Webster, C.; Law, M., P-Type PbSe and PbS Quantum Dot Solids Prepared with Short-Chain Acids and Diacids. ACS nano 2010, 4, 2475-2485. 43. Osedach, T. P.; Zhao, N.; Andrew, T. L.; Brown, P. R.; Wanger, D. D.; Strasfeld, D. B.; Chang, L.-Y.; Bawendi, M. G.; Bulovic, V., Bias-Stress Effect in 1, 2-Ethanedithiol-Treated PbS Quantum Dot Field-Effect Transistors. ACS nano 2012, 6, 3121-3127. 44. Liu, H., et al., Electron Acceptor Materials Engineering in Colloidal Quantum Dot Solar Cells. Adv. Mater. 2011, 23, 3832-3837. 45. Horng, R.-H.; Wuu, D.-S.; Lien, Y.-C.; Lan, W.-H., Low-Resistance and HighTransparency Ni/Indium Tin Oxide Ohmic Contacts to P-Type GaN. Appl. Phys. Lett. 2001, 79, 2925-2927. 46. Jeong, K. S., et al., Enhanced Mobility-Lifetime Products in PbS Colloidal Quantum Dot Photovoltaics. ACS nano 2011, 6, 89-99. 47. Zhitomirsky, D.; Voznyy, O.; Levina, L.; Hoogland, S.; Kemp, K. W.; Ip, A. H.; Thon, S. M.; Sargent, E. H., Engineering Colloidal Quantum Dot Solids within and Beyond the MobilityInvariant Regime. Nat. Commun. 2014, 5, 3803. 48. Johnston, K. W.; Pattantyus-Abraham, A. G.; Clifford, J. P.; Myrskog, S. H.; Hoogland, S.; Shukla, H.; Klem, E. J. D.; Levina, L.; Sargent, E. H., Efficient Schottky-Quantum-Dot Photovoltaics: The Roles of Depletion, Drift, and Diffusion. Appl. Phys. Lett. 2008, 92, 122111. 49. Gao, J.; Perkins, C. L.; Luther, J. M.; Hanna, M. C.; Chen, H. Y.; Semonin, O. E.; Nozik, A. J.; Ellingson, R. J.; Beard, M. C., N-Type Transition Metal Oxide as a Hole Extraction Layer in PbS Quantum Dot Solar Cells. Nano Lett. 2011, 11, 3263-3266.


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Table of Contents Only


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(a) Absorption spectra of PbS QDs used and (b) TEM image of PbS QDs with particle size of 2.8±0.15 nm. 172x285mm (72 x 72 DPI)



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XPS core level spectra of (a) Pb 4f, (b) S 2s, and (c) O 1s for CQD films with oleic acid (OA) and 1,2ethanedithiol (EDT) termination. The S 2s and O 1s core levels are fitted with Voigt functions. 165x105mm (192 x 192 DPI)


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UPS spectra as a function of EDT-PbS CQD layer thickness on either ITO (a-c) or TiO2 (d-f) substrate. In Figures 3(b) and (e), the arrows indicate EDT-PbS CQD characteristic peaks. The figures 3(c) and 3(f) show semi-logarithmic scale of intensity near the Fermi level and valence band offset positions (vertical bars). 174x223mm (192 x 192 DPI)



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Schematic band diagrams of EDT-PbS CQDs on ITO (a) and TiO2 (b) substrates. Evac, eD, Ec, Ev, EF, Eg, and Wd represent vacuum level, interface dipole conduction band edge, valence band edge, Fermi level, energy band gap and depletion width respectively. Green and red arrows show the direction of movement of photoexcited electrons and holes in device, respectively. ITO (a) represents Schottky device model and TiO2 (b) represents depleted heterojunction device. 445x185mm (72 x 72 DPI)


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Electronic Structure of PbS Colloidal Quantum Dots on Indium Tin

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