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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 24981−24986
Energy Level Alignment of Molybdenum Oxide on Colloidal Lead Sulfide (PbS) Thin Films for Optoelectronic Devices Diogenes Placencia,†,‡ Paul Lee,§ Joseph G. Tischler,‡ and Erin L. Ratcliff*,†,⊥ ‡
U.S. Naval Research Laboratory, 4555 Overlook Avenue Southwest, Washington, D.C. 20375, United States Department of Chemistry & Biochemistry, University of Arizona, 1306 East University Boulevard, Tucson, Arizona 85721, United States ⊥ Department of Materials Science & Engineering, University of Arizona, 1235 East James E. Rogers Way, Tucson, Arizona 85721, United States Downloaded via TUFTS UNIV on August 2, 2018 at 03:13:50 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
§
S Supporting Information *
ABSTRACT: Interfacial charge transport in optoelectronic devices is dependent on energetic alignment that occurs via a number of physical and chemical mechanisms. Herein, we directly connect device performance with measured thickness-dependent energy-level offsets and interfacial chemistry of 1,2ethanedithiol-treated lead sulfide (PbS) quantum dots and molybdenum oxide. We show that interfacial energetic alignment results from partial charge transfer, quantified via the chemical ratios of Mo5+ relative to Mo6+. The combined effect mitigates leakage current in both the dark and the light, relative to a metal contact, with an overall improvement in open circuit voltage, fill factor, and short circuit current.
KEYWORDS: nanocrystals, interfaces, optoelectronic devices, solar cells, metal oxide, photoemission spectroscopy
L
transport level, whereas the wide-band gap prohibits electron extraction under low fields. Alternatively, wide-band-gap, high-work-function n-type semiconductors (MoO3, V2O5, and WO3) have been used as buffer layers at the electrode/active layer interface. These materials have been proposed to act as charge-inversion layers, injecting an electron into the hole-transport level (quasi Fermi level) of the active layer (electrons instead of holes are transported across the interface).27 For PbS active layers, Brown et al. suggested that incorporation of molybdenum oxide (MoOx) in photovoltaic devices improves device efficiency through the elimination of a reversed-bias Schottky diode at the PbS/hole-collecting electrode interface, relative to a metal contact.25 This conclusion was supported by device performance, with an ideal diode behavior observed for 50 nm of MoO3 although the energetic alignment was not measured. Gao et al. determined that deposition of a 10 nm thick MoOx layer improved device performance, due to two potential mechanisms for improved hole extraction: (i) an interface dipole at the PbS/oxide heterojunction that could enhance band bending at the interface and/or (ii) midgap states of the oxide lent a pathway for hole extraction, despite the n-type nature of the material.26
ead sulfide (PbS) nanocrystal-based optoelectronics have gained significant attention recently because of their performance as detectors for infrared applications,1,2 photovoltaics,3,4 and light-emitting devices.5,6 The combination of a facile hot-injection synthetic procedure,7,8 the large effective mass of PbS that allows for band gap tuning over a broad range of nanocrystal sizes,9,10 as well as relatively high charge mobilities,11 and ease of processing through solution ligandexchange,12−14 makes this material suitable for large-scale production of next-generation optoelectronic devices.15,16 Critical to the operation of these devices are energy level alignments at the interface between the nanocrystal film and electrode contact due to its effect on carrier extraction, injection, recombination, and therefore overall device efficiency.17,18 Multiple strategies have employed nanocrystalmodifying processes to tune energy levels to control charge transport (e.g., ligand-exchange, active layer gradients, etc.).17,19−21 Yet these colloidal materials must still be interfaced with electrodes for complete control over charge transport. A common approach to control transport across the interface is the insertion of a charge-selective interlayer whose properties are more favorable toward electron or hole selectivity. For hole-selective processes, the use of inorganic salts,3,22 organic semiconductors,23,24 or transition metal oxides are typically employed.25,26 These interlayers are often wideband gap, p-type materials that favor hole-extraction from the active layer through energetic alignment with the hole © 2018 American Chemical Society
Received: May 9, 2018 Accepted: July 17, 2018 Published: July 17, 2018 24981
DOI: 10.1021/acsami.8b07651 ACS Appl. Mater. Interfaces 2018, 10, 24981−24986
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ACS Applied Materials & Interfaces
Figure 1. (a) Linear current−voltage (JV) plots in the light and (b) dark semilog JV plots for devices with (red) or without (black) the inclusion of a 15 nm-thick molybdenum oxide hole transport layer between the EDT-treated PbS and Au top contact. Inset in (b) shows a picture of the device layout, where the active area of each device is 0.019 cm2, along with a spectrum of the nanocrystals in tetrachloroethylene (TCE).
Figure 2. Ultraviolet photoelectron spectroscopy (UPS) data obtained for the sequential deposition of molybdenum oxide upon EDT-treated PbS nanocrystals. (a) The secondary kinetic energy edges and (b) high kinetic energy edges have been plotted with respect to the surface work function and system Fermi level, respectively. The intersecting points between the dashed lines represent where the surface work functions and valence band maximums were obtained.
reaction and is consistent with prior reports of changes in device efficiency with MoOx thickness.25,26 PbS nanocrystal synthesis (air-free workup) and films were fabricated in accordance to previously established procedures.17 Films were deposited upon gold-on-glass substrates, in an argon-filled glovebox. The samples were introduced into the vacuum system from the glovebox with no exposure to atmosphere, followed by sequential XPS/UPS characterization and deposition of molybdenum trioxide powder, and finally a thin film of gold. Characterization was carried out in a Kratos Axis-Ultra using an Al Kα source (1486.6 eV) and He(I) excitation source (21.2 eV) for XPS and UPS, respectively. A −10.0 V bias was applied to the sample to further enhance collection of the lowest kinetic energy electrons during UPS experiments.28 Charging (or lack thereof) was verified by acquiring C 1s spectra with and without the charge neutralizer
In this Letter, we studied the complex processes at the PbS/ molybdenum oxide interface (herein termed MoOx, due to the substoichiometric nature of the observed interface) via UV and X-ray photoemission spectroscopy (UPS, XPS), with direct connection to device behaviors in the presence and absence of the MoOx layer. We show, for the first time, that the interface rectification arises from reduction−oxidation (redox) reactions between the MoOx and the PbS quantum dots, respectively. The presence of a chemical redox reaction is unique from band bending, where band bending is descriptive of the reorganization of free carriers. The observed midgap states coincide with the presence of Mo5+ at the interface and reorganization in electron density about the bonds in the quantum dot, as evident in changes in both the Pb and S core levels. Moreover, the measured interface dipole is shown to vary with MoOx thickness, consistent with shielding of the localized chemical 24982
DOI: 10.1021/acsami.8b07651 ACS Appl. Mater. Interfaces 2018, 10, 24981−24986
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ACS Applied Materials & Interfaces to determine if any shifts in the core level peaks were noticeable. Devices were fabricated and tested via previously established protocols and tested in accordance to standard testing procedures.3,17 Figure 1 shows the linear photocurrent and semilog dark current density−voltage (JV) plots for an average of 15 devices obtained from four different sample test chips, with inserts showing the device layout (Figure 1b, left) and attenuance of the nanocrystals used (Figure 1b, right). Additional data and device parameters are given in the Supporting Information section (Figure S1 and Table S1). In the absence of the MoOx layer, devices show a wide distribution in photoactive performance (Figure 1a), as evident by the large standard deviation. Inclusion of the MoOx layer (15 nm) resulted in an increase in the open-circuit voltage (VOC), the short-circuit photocurrent (JSC), and fill factor (FF), consistent with prior reports.25,26 We note that our devices differ slightly from those of Brown et and Gao et al. in that we have used a PbS-I photoactive layer, capped by PbS-EDT for the necessary comparison to prior work.25,26 Likewise, in Figure 1b, dark behavior is improved with the inclusion of the optimized MoOx layer, with a 10-fold reduction in the dark saturation current and a favorable increase in the shunt resistance. Prior work has demonstrated a decrease in dark current is consistent with the suppression of injected carriers through leakage pathways.29 However, it is impossible to definitively ascertain interface behavior from device data alone. To improve upon device understanding, we conducted photoelectron spectroscopy to monitor both the hole transport levels and the corresponding chemical states of the interface with MoOx thickness (i.e., as the interface is formed). Figure 2 shows a subset of the UPS spectra (for clarity) for a series of MoOx depositions upon a PbS-EDT surface, plotted with respect to surface work function (2a) and energy with respect to the Fermi level (2b). The complete data set is provided in the SI (Figure S2). Semiquantitative band-edge offsets based on data obtained from Figure 2a, b and S2 are shown in Figure 3 for the PbS-EDT/MoOx interface, as a function of oxide thickness, in addition to the oxide/gold offset (UPS spectra, Figure S3). The optical gap for the PbS nanocrystals in tetrachloroethylene was used, whereas the band gap for MoOx was obtained from previous work.30 The corresponding data and band-edge offset diagram for Au deposition of PbS-EDT surface is given in the SI (Figure S4). In Figure 2, the initial PbS-EDT film (0 nm) is representative of what has been previously described, with the multicomponent peak closest to the Fermi level attributed to S 3p levels, as indicated by arrows.31 The valence band maximum (VBM) for the PbS-EDT film was determined via the intersection of the spectrum baseline and the rise in intensity from the peak closest to the Fermi level, followed by the application of a correction factor (here only ∼0.1 eV) due to the low density of states at the VBM, as detailed by Miller and co-workers.31 The addition of 0.3 nm of MoOx results in only a small change in surface work function (0.2 eV), in addition to the growth of the VBM of the oxide under the background of the still visible PbS-EDT onset feature (ca. −4.3 eV), as indicated by the arrow in Figure 2b. Moreover, the interface is clearly not undergoing band bending, as the vacuum level shifts in the opposite direction relative to the onset in occupied states (Figure 3). At thicknesses above 1.5 nm, large changes in the surface work function (5.5 to 6.7 eV) and the distinct VBM
Figure 3. Band-edge offsets of the EDT-treated PbS/molybdenum oxide interface, showing the positions of the EVBM and work function, as a function of molybdenum oxide layer thickness.
feature of vacuum-deposited MoOx are visible, which has been associated with the growth of a continuous film.30 At a total thickness of 3.0 nm, a small feature near the Fermi edge (−0.87 eV below the Fermi) is noticeable, disappearing with additional oxide deposition. This midgap feature has been attributed to interfacial states of the MoOx when interfaced with organic materials.30,32 The formation of midgap states is a strong indication of a chemical reaction at the PbS-EDT/ MoOx interface. Evidence of a chemical reaction between the PbS-EDT to the MoOx is shown in Figure 4a. At low MoOx coverages, the Mo 3d3/2 and 3d5/2 shows a multicomponent nature, where by the lower binding energy feature is reduced with increasing MoOx coverage. A multicomponent MoOx species was also demonstrated by Meyer et al. for MoOx on graphene-on-Si30 and by Shallcross et al. for MoOx on poly(3-hexylthiophene).33 In both systems and herein, the low binding energy components at low coverages is attributed to the Mo5+ oxidation state (Mo 3d5/2 ≈ 231.6 eV; Mo 3d3/2 ≈ 234.7 eV), while higher coverages (>1.5 nm) is dominated by the Mo6+ oxidation state (Mo 3d5/2 ≈ 232.4 eV; Mo 3d3/2 ≈ 235.5 eV). Peak fitting of the spectra after removing the high-frequency noise through FFT filtering (Figure 4b−f) resulted in the quantitative determination of Mo(VI) to Mo(V) for each deposition, in accordance to previously published work.34 Specifically, at the low 0.4 nm coverage, the ratio of Mo(VI) to Mo(V) was assessed at 52 to 48%, respectively, indicating a high fraction of Mo5+ species confined to the interface. This coincides with the low fraction of MoOx valence states observed in the UPS spectra of Figure 2b. At moderate thicknesses, (i.e., 1.5 nm), the MoOx film comprised ∼75% Mo6+ species, which further increase with increasing MoOx thickness (∼93.% Mo6+ character at 9.0 nm). The predominantly Mo(VI) species correlates well with the observed large dipole shifts and the distinct metal oxide VBM observed in the 24983
DOI: 10.1021/acsami.8b07651 ACS Appl. Mater. Interfaces 2018, 10, 24981−24986
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Figure 4. XPS core level spectra of (a) molybdenum (Mo 3d), where the Mo6+ and Mo5+ core level positions are referenced. (b−f) Peak fitting results of the spectra shown in panel a, where the 3d5/2 and 3d3/2 peaks were fitted with the corresponding Mo5+ and Mo6+ for thicknesses of (b) 0.4, (c) 0.8, (d) 1.5, (e) 3.0, and (f) 9.0 nm of MoOx on the PbS film. Colored lines correspond to the following peak fits: dark blue, Mo6+ 3d5/2 ≈ 232.4 eV; red, Mo6+ 3d3/2 ≈ 235.5 eV; light blue, Mo5+ 3d5/2 ≈ 231.6 eV; green, Mo5+ 3d3/2 ≈ 234.7 eV.
about the chemical bonds of the colloidal system, although given that the S 2p signal results from a combination of both the chalcogenide and the ligand, the exact nature of the new chemical state is beyond the scope of this work. We further note that we cannot definitively determine whether the chemical reaction is taking place solely at surface atoms of the nanocrystal, or if changes in electron density are occurring spatially through the crystal due to the limitation of the XPS technique. However, a low binding energy shoulder in the lead and a high binding energy shoulder in the sulfur is indicative of a decrease in polarization and coincides directly with the quantified changes in molybdenum oxidation state shown in Figure 4. In conclusion, we have demonstrated for the first time the presence of an oxidation−reduction reaction at the PbS-EDT/ MoOx interface, enabled by a combined analysis of the chemical and electronic features as the interface is formed. The critical feature of the redox interface states suggests a new path forward to interface engineering, whereby abrupt transitions from p-type dot to n-type oxide can be achieved through
UPS. We thus conclude that the local vacuum level shift that arises from the deposition of MoOx on PbS-EDT reported by prior groups is attributed to an interface dipole as a result of an oxidation−reduction reaction. To probe further into this local vacuum level shift across the heterojunction, we monitored the Pb 4f5/2 and 4f7/2 core levels for position shifts with increasing oxide thickness. A shift in these core level binding energies would be indicative of band bending toward or away from the Fermi level while changes in peak width could suggest chemical reactions.31 Figure S5 shows the Pb 4f core level spectra as a function of MoOx thickness, showing no change in position. Figure S6 provides the results of fitting the Pb 4f regions with three peaks that change in relative percent composition with MoOx coverage (Table S2), indicative of a chemical reaction at the interface. A complementary high binding energy shoulder is observed on the S 2p at low coverages, as indicated by the arrow in Figure S7. Collectively, the changes in peak widths of Pb 4f and S 2p core levels suggest a clear reorganization of electron density 24984
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chemistry at subnanometer length scales, instead of relying on longer-range band bending. Such results are critical to photovoltaic and photodetector applications, where nanometer-scale interfaces often dominate key functional properties.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b07651. Materials and methods section; additional current− voltage curves for devices; table of relevant device parameters; UPS spectra for PbS-MoOx depositions, UPS spectra of the MoOx-gold interface; UPS and band diagram of gold on PbS-EDT surfaces; Pb 4f core level spectra for PbS-EDT/MoOx interface and corresponding core level fits; and S 2p core level spectra for the PbS-EDT/MoOx interface (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ratcliff@email.arizona.edu. ORCID
Erin L. Ratcliff: 0000-0002-2360-8436 Author Contributions †
D.P. and E.L.R. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Office of Naval Research (ONR). E.L.R. acknowledges the 2016 ONR Summer Faculty Research Program.
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REFERENCES
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