Electronic Structure of PbS Colloidal Quantum Dots on Indium Tin ...

Article pubs.acs.org/JPCC

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 ‡

S Supporting Information *

ABSTRACT: The size of colloidal quantum dot (CQD) materials and their surface modification by chemical ligands can change electronic properties thereby affecting device performances. In this study, direct measurement of the electronic structure within CQD thin film upon solid-state ligand exchange from oleic acid to 1,2ethanedithiol 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, which give 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.

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 because of 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 optoelectronic application because of 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 a lot of defect sites influencing PV performance at the CQD surface.6 This 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 nonpolar solution.7 However, it has shown a poor charge carrier mobility because of its long alkyl chain acting as transport barrier.8 Recently, there has been much research 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 © 2014 American Chemical Society

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 up to 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 the key parameters to achieve in PV devices. However, the direct measurement of energy levels such as valence band maximum (VBM) and Fermi level in CQD films has rarely been reported because of 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 we 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 band-bending direction but different saturation Received: August 29, 2014 Revised: November 9, 2014 Published: November 12, 2014 27884

dx.doi.org/10.1021/jp508737r | J. Phys. Chem. C 2014, 118, 27884−27889

The Journal of Physical Chemistry C

Article

thickness. Finally, the depletion width and band-bending magnitude are described using the saturation thickness of the CQD films.

2. 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) were degassed in a three-neck flask for 30 min under vacuum. The solution was then heated to 110 °C, and it reacted for 1.5 h. Hereafter, the solution was changed under nitrogen (N2) atmosphere and was allowed to cool to 75 °C. TMS2S (180 μL) in 4 mL of ODE was loaded into a 12 mL syringe and then was rapidly injected into the solution. The flask was then transferred to a N2-filled glovebox. PbS CQDs were isolated from the reaction solution by precipitation using acetone. The resulting precipitate was dispersed in hexane and was washed two times with acetone. Finally, 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 s. This film was annealed at 450 °C. PbS CQD solids were fabricated using layer-by-layer spin-cast method on ITO and TiO2/FTO substrate in a N2 filled glovebox. For each layer, the PbS CQDs in octane (10 mg/mL) were dropped while substrate spun at 2500 rpm. Solid-state ligand exchange was processed by a previous report.15 One volume percent of EDT in acetonitrile, acetonitrile, and octane was dropped in the same way as PbS CQD solution. This coating cycle was repeated until a thickness of film was targeted (1, 5, 10, and 50 cycles). This film was annealed at 90 °C for 5 min. Once the PbS CQD films were made, they were mounted on each sample holder and were encapsulated in a N2 glovebox without air exposure. The encapsulated PbS CQD samples were transferred to an entry glovebag filled with dry N2. This transfer scheme minimized any air-bone contaminants and preserved 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) measurements were performed using a hemispherical electron energy analyzer with a CCD camera (SES-100, VGScienta). The UPS measurement used a He I (ℏω = 21.22 eV) gas discharge lamp as an excitation source with sample bias of −10 V for secondary electron cutoff region. The XPS measurement used an Al Kα (ℏω = 1486.5 eV) without monochromator. The energy resolutions were 0.1 and 1.0 eV, respectively.16

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

CQDs.14,17,18 To tackle these aspects, the XPS measurements of the CQDs before (OA) and after (EDT) ligand exchange were 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 a highly oxidized one (PbOx) or Pb metallic phase.19−22 The Pb 4f7/2 peak

3. RESULTS AND DISCUSSION Figure 1 shows the absorption spectra of PbS CQDs and their transmittance electron microscopy (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 nm in diameter. Relatively uniform CQDs are synthesized and transferred into a well-dispersed film. Surface chemical environment and elemental stoichiometry strongly influence the electronic properties of PbS

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. 27885



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Figure 3. UPS spectra as a function of EDT-PbS CQD layer thickness on either (a−c) ITO or (d−f) TiO2 substrate. In b and e, the arrows indicate EDT-PbS CQD characteristic peaks. Panels c and f show the semilogarithmic scale of intensities near the Fermi level and each valence band edge position (vertical bars).

position of 138.40 eV for OA-PbS CQD film is slightly higher than that of the EDT-PbS CQDs (138.0 eV) because of the higher electronegativity of oxygen than sulfur.22−24 Additional attention is paid to the full width at half-maximum (fwhm) of Pb 4f7/2 peaks. The OA-PbS CQD film shows the fwhm of 1.35 eV, while EDT-PbS shows the fwhm of 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 (see Figure S1A of the Supporting Information). 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 a doublet by the addition of EDT (see Figure S1B of the Supporting Information). 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. These 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 the (111) face of PbS CQD crystal.30 On the other hand, EDTtreated PbS films give no meaningful O 1s core level intensity. Thus, the EDT-PbS CQDs are assured that they are oxygenfree at the surface as the complete ligand exchange is achieved. 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, and so forth), and photoexcited holes move into the ITO substrate.31 However, 27886



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Figure 4. Schematic band diagrams of EDT-PbS CQDs on (a) ITO and (b) TiO2 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 the device, respectively. (a) ITO represents Schottky device model, and (b) TiO2 represents depleted heterojunction device.

emission near the VB edge (not shown here).39 On the basis of this method, the VB edge of EDT-PbS CQD film is determined by the point of intersection of two tangent lines along the 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 CQD layer thickness between 45 and 90 nm, but on the TiO2 substrate, the VB edge is saturated at 0.75 eV between 180 and 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. The schematic band diagrams of PbS CQD films on ITO and TiO2 on the basis of 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 has widely been measured with absorption spectroscopy and can be 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 because of the large dielectric constant of PbS CQDs.40 The value is generally dependent on particle size and is expected to be about 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 the 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 the intrinsic material properties, remains 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 the band gap primarily owing to the ligand dipole moment.13 The doping polarity of PbS QD solid (p-type, ntype, 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 p-type characteristics over the transfer curve analysis in field-effect transistor (FET) 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

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 has been measured and calibrated by cross-sectional scanning electron microscopy (SEM; see Figure S2 of the Supporting Information). Figure 3a−c shows UPS spectra of EDT-PbS CQD films on the ITO substrate as a function of thickness, and Figure 3d−f shows it on the TiO2 substrate. In Figure 3a and d, the secondary cutoff (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 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 with each other (arrows). The EDT-PbS CQDs show four distinct 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).17,35−38 Such sharp and clear discrimination of the four 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 semilogarithmic 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 does not make sense at equilibrium. Rather, the semilogarithmic plot reflects a small amount of surface density of states of CQDs38,39 and produces reliable results. The control experiment indicates that PbS pristine film and adsorption of EDT do not show any extra 27887



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magnitude of band bending, and the barrier height for hole injection on ITO substrate and hole blocking on TiO2 substrate through band diagram on the basis of UPS results. The hole injection barrier of 0.25 eV observed on the 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 their way to accurately design photovoltaic junction for highly efficient carrier extraction.

work function of TiO2 and ITO in Figure 4, both interfaces show the same 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 at 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 the 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 must tunnel through it to be collected toward the electrode on the 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.32,44 Consequently, the same direction of VB bending in two different types of devices acts as opposite roles. The interface dipole induces a charge depletion region in the CQD layers. The depletion tends to expand toward the lightly doped side up to the 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 3 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. Because ITO has a high free electron density (∼1020 cm−3) like metal, the depletion region of ITO can be negligible in PbS-EDT CQDs on the ITO substrate.45 On the contrary, TiO2 layers have a partially depleted region because of their lower n-type carrier density of about 1016 cm−3. Thus, the mobile charges are widely distributed in PbS CQD film on TiO2 substrate with a 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, because the observed Wd is shorter than the usual carrier drift length (0.2−1 μm), the Wd is yet to be improved for overall device performance.46−48 Additionally, our photoelectron spectroscopy (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 has much success in many material research fields.

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ASSOCIATED CONTENT

S 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.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions #

Both authors contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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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4. CONCLUSION The electronic structure of PbS CQD films prepared by solidstate 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 (10−420 nm) to examine the junction between the substrate and the PbS CQD film. We observed the depletion width from each film thickness of saturation, the 27888



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