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Comparing Halide Ligands in PbS Colloidal Quantum Dots for Field-Effect Transistors and Solar Cells Dmytro Bederak, Daniel M. Balazs, Nataliia V. Sukharevska, Artem G. Shulga, Mustapha Abdu-Aguye, Dmitry N Dirin, Maksym V. Kovalenko, and Maria Antonietta Loi ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01696 • Publication Date (Web): 09 Nov 2018 Downloaded from http://pubs.acs.org on November 12, 2018
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Comparing Halide Ligands in PbS Colloidal Quantum Dots for Field-Effect Transistors and
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Solar Cells
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Dmytro Bederak, † Daniel M. Balazs, † Nataliia V. Sukharevska, † Artem G. Shulga, † Mustapha Abdu-
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Aguye, † Dmitry N. Dirin, ‡,§ Maksym V. Kovalenko, ‡,§ Maria A. Loi*†
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Affiliations:
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†
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Groningen, The Netherlands,
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‡
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8093, Switzerland,
Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747AG
Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1, Zürich
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§
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Dübendorf 8600, Switzerland
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*Corresponding author email:
[email protected] 13
Keywords: colloidal quantum dots, lead sulfide, halides ligands, charge transport, solar cells
Empa-Swiss Federal Laboratories for Materials Science and Technology, Uberlandstrasse 129,
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Abstract
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Capping colloidal quantum dots (CQDs) with atomic ligands is a powerful approach to tune their
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properties and improve the charge carrier transport in CQD solids. Efficient passivation of the CQD
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surface, which can be achieved with halide ligands, is crucial for application in optoelectronic devices.
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Heavier halides, i.e. I- and Br-, have been thoroughly studied as capping ligands in the last years, but
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passivation with fluoride ions had not received sufficient consideration. In this work, effective coating
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of PbS CQDs with fluoride ligands is demonstrated and compared to the results obtained with other
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halides. The electron mobility in field-effect transistors of PbS CQDs treated with different halides
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shows an increase with the size of the atomic ligand (from 3.9×10-4 cm2/Vs for fluoride-treated to
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2.1×10-2 cm2/Vs for iodide-treated), while the hole mobility remains unchanged at around 10-5 cm2/Vs.
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This leads to a relatively more pronounced p-type behaviour of the fluoride- and chloride-treated films
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compared to the iodide-treated ones. Cl-- and F--capped PbS CQDs solids were then implemented as p-
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type layer in solar cells; these devices showed similar performance to those prepared with 1,2-
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ethanedithiol in the same function. The relatively stronger p-type character of the fluoride- and
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chloride-treated PbS CQD films broadens the utility of such materials in optoelectronic devices.
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Introduction
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Colloidal quantum dots (CQDs) are solution-processable semiconductor nanocrystals, which have
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been used to fabricate various optoelectronic devices, such as solar cells, photodetectors and field
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effect transistors.1 As-synthesized CQDs are capped with long-chain insulating oleic acid ligands to
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stabilize their colloidal state and passivate their surface. Only replacing these bulky native ligands it is
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possible to enable electronic coupling between the individual quantum dots.
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Ideally, the ligand exchange must ensure an efficient passivation of the quantum dot surfaces, preserve
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quantum confinement, in particular when the CQD solid is used as active layer for solar cells and
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photodetectors,2 and allow for rational adjustment of the electronic characteristics (doping levels,
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carrier mobilities etc.). Towards these goals, capping CQDs with inorganic ligands received much
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attention with the introduction of metal chalcogenide complexes.3 Soon thereafter various small
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inorganic ligands, such as chalcogenides, halides, pseudohalides and halometallate complexes
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showing chemical versatility and providing better processability were introduced for colloidal
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quantum dot solids.4–10
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The research on halide ligands is mainly fueled by their prospects in solar cells. Iodide, bromide,
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chloride have been used intensively as ligands for CQD solids.6,7,11–14 PbS quantum dot films with Br-
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and I- ligands show n-type transport behaviour and exhibit a high degree of surface passivation.6,11
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Furthermore, iodide capping results in exceptionally air-stable n-type CQD solids, which are widely
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employed as main light absorber layer in CQD solar cells.15
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The treatment of PbS CQDs with halides results in a short interdot distance favoring good charge
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transport.16,17 Fluoride is the smallest ion in the halide series and one of the smallest possible ligands,
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potentially giving the shortest interdot spacing compared to any other ligand. Despite the prospects,
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limited attention has been dedicated to the fluoride-treated lead chalcogenide CQDs. Previous reports
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have mainly focused on the stability of the films towards oxidation based on the study of their optical
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properties.18,19 While, the transport properties of fluoride-treated lead chalcogenide CQD films have
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not been investigated up until now.
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In this work, the optical and electronic properties of thin films of PbS CQDs treated with the four
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halide atomic ligands are compared. Not only the fluoride ligand is introduced for the fabrication of
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PbS CQD devices but its performance are also compared with the other three halide ligands in a
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homogeneous set of experiments. The properties of the halide-treated films are found to obey a trend-
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wise dependence on the ligand size. The fluoride-treated sample displays the lowest electron mobility,
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which increases two orders of magnitude moving down in the periodic table (F-Cl-Br-I). Interestingly,
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the hole mobility for all the halides remains roughly the same. Moreover, in the fluoride-treated PbS
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quantum dot transistors we observe a time-dependent decreasing of the electron current, suggesting a
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high sensitivity of this active layer to the traces of contaminants in the glovebox. Quasi-pn junction
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solar cells were fabricated by replacing the conventional 1,2-ethanedithiol (EDT)-capped CQDs by
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chloride- or fluoride-treated PbS CQDs, making use of their relative p-type behavior, while keeping
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the iodide-treated CQDs as main absorber and n-type layer. Halides are less toxic and more
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environmental friendly than thiols, therefore are more compatible with an eventual industrialization of
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the process. Solar cells, fabricated with fully halide-treated films show power conversion efficiency
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similar to that of devices using an EDT-treated layer, therefore showing their prospects as solar cell
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material.
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Results and Discussion
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In a few reports, treatment of lead chalcogenide quantum dots with fluoride was performed by using
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tetrabutylammonium fluoride (TBAF).18,20 Tetrabutylammonium halide salts are typically available in
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high purity, setting them favorable for photovoltaic research. Unlike other tetrabutylammonium
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halides, dried TBAF is reported to decompose at room temperature by Hofmann elimination;21 hence
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TBAF is only available in hydrated form or in a solution of water or tetrahydrofuran.22 The exposure
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of CQDs to humidity is known to have a significant influence on the material properties,23–25 making
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hydrated fluoride source rather undesirable. Anhydrous TBAF can be generated in situ from the
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reaction of hexafluorobenzene with tetrabutylammonium cyanide and used for synthetic purposes, but
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the experimental conditions are rather incompatible with the use in the CQD solids.26 Consequently,
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an anhydrous fluoride compound that can contend tetrabutylammonium halide series is required for a
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good comparison between the different halide-treated CQD films. Herein, a tetramethylammonium
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fluoride is chosen as the source for the fluoride atomic passivation due to its simplicity and facile
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preparation in anhydrous form.21 Thus, four anhydrous tetraalkylammonium halide salts, namely
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tetramethylammonium fluoride (TMAF), tetrabutylammonium chloride (TBACl),
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tetrabutylammonium bromide (TBABr) and tetrabutylammonium iodide (TBAI), were used for the
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solid-state ligand exchange. The ligand exchange and the chemicals used in this work are
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schematically illustrated in Figure 1A.
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CQDs with the size between 2.7 and 3.5 nm diameter, as calculated from the first excitonic peak
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energy of the absorbance spectrum, were used for all the experiments.27 Thin films of PbS quantum
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dots treated with the four halides were prepared by spin-coating with subsequent ligand exchange and
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washing, using the layer-by-layer approach reported earlier.28 Samples for electron microscopy were
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prepared by drop-casting on carbon-coated copper grids, the ligand exchange and washing were
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performed as for the device fabrication. More details on the sample preparation are reported in the
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Experimental methods section.
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Smaller PbS CQDs, ca. 3 nm in diameter, have a truncated octahedron shape, mostly dominated by
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{111} facets, while larger PbS particles have cuboctahedron shape with {111} and {100} facets.29 The
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variation in faceting creates a significant difference in chemical reactivity and assembly of the CQDs.
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Larger lead chalcogenide CQDs, of size from 3.8 to 6 nm typically form square superlattices after
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halide treatment,16,30 while, in general, smaller CQDs give rise to arrays with a much higher level of
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disorder. However such “small” particles attract keen interest due to their application in CQD solar
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cells. In this work, a relatively large CQD size (3.5 nm) was chosen in order to show the effect of
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halide treatment on their assembly, while smaller particles were used for device fabrication.
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Figure 1. (A) Schematic illustration of PbS colloidal quantum dots with native oleic acid ligands and,
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after the ligand exchange, with halide ligands. Fourier-transformed TEM images of single CQD
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domains treated with (B) TBAI and (C) TMAF. The scale bars are 500 µm-1.
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Figure 1 illustrates the difference in the CQDs ordering after the ligand exchange with iodide (panel
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(B)) and fluoride (panel (C)) ligands. The typical square superlattice is observed after the iodide
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treatment of CQDs with cuboctahedron shape, but domains with hexagonal packing are also present.16
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Ligand exchange with F results in a predominantly hexagonal ordering (Figure 1C) with pronounced
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disorder. A similar behavior, with a different degree of disorder is found in bromide- and chloride-
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treated samples (Figure S1). The distance between the single CQDs clearly indicates that the ligand
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exchange took place also when fluoride ligands are used. Moreover, the presence of fluorine in the
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ligand-exchanged films is observed using energy-dispersive X-ray spectroscopy (EDX) as reported in
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Figure S4. However, EDX has low sensitivity to light elements, such as fluorine. Therefore, the
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detection of fluorine in the sample containing a high amount of heavy atoms suggests a significant
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fluorine concentration in the thin film.
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Electron diffraction patterns of the PbS CQDs treated with TBAI and TMAF are shown in Figure S2.
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The lack of some of the reflections in the TBAI-treated sample indicates a dominant out-of-plane
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orientation of the individual CQDs with their {100} facets normal to the surface. Moreover, a
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preferential in-plane orientation with the neighboring CQDs aligning with their {100} facets facing
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each other is also observed as four dominant peaks in the innermost ring. Such organization is not
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observed in the F--treated sample, in which all expected reflections are observable as rings, confirming
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the qualitatively observed disorder of these films.
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The average interdot distance is extracted from the Fourier-transformed TEM images of single
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domains. No clear difference is observed between the fluoride- and iodide-treated samples due to the
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high level of disorder. The average center-to-center interdot spacing in all halide-capped samples is
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4.5 nm, while the oleate-capped CQDs show a 4.9 nm periodicity. The reduction of the interdot
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distance after the halide treatment is a further confirmation of the removal of the long oleate ligands.
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Figure 2. (A) Absorbance spectra of thin films of 3.5 nm PbS CQDs capped with native oleic acid
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(OA) and with the four halide ligands; the curves are vertically shifted for clarity. (B) Normalized PL
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spectra of the same films reported in (A).
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Figure 2 shows the optical properties of thin films treated with the four halide ligands, and of an
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oleate-capped CQD film as reference. The absorbance spectra (Figure 2A) show a trend in the degree
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of confinement. In general, the first excitonic peak shifts towards higher wavelengths, and becomes
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broader upon ligand exchange with halides. A clear peak is observable only in PbS-I and PbS-Br
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samples, and not in the PbS-Cl and PbS-F ones. The different energies of the peak (OA
