Ligand and Solvent Effects on Hole Transport in Colloidal Quantum

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Ligand and Solvent Effects on Hole Transport in Colloidal Quantum Dot Assemblies for Electronic Devices Liming Liu, Satria Zulkarnaen Bisri, Yasuhiro Ishida, Daisuke Hashizume, Takuzo Aida, and Yoshihiro Iwasa ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01205 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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ACS Applied Nano Materials

Ligand and Solvent Effects on Hole Transport in Colloidal Quantum Dot Assemblies for Electronic Devices Liming Liu †, Satria Zulkarnaen Bisri ‡*, Yasuhiro Ishida ‡*, Daisuke Hashizume ‡, Takuzo Aida †‡, Yoshihiro Iwasa ‡§ † Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. ‡ RIKEN Center for Emergent Matter Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan. § Quantum-Phase Electronics Center and Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. KEYWORDS: colloidal nanocrystals; quantum dots; molecular ligands; field-effect transistors; p-type doping

ABSTRACT

Semiconductor quantum dots (QDs) in colloidal form have attracted growing interest for their potential applications in solution-processable electronic devices. Controlled electronic doping in

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QD assemblies is one of the challenges required for advancing the development of novel solar cells, photodetectors, and transistors based on this system. While several n-type QD films with excellent conductivity have been successfully demonstrated, in general, p-type QD films have shown poor conductivity. Ligand and solvent engineering were found to permit significant enhancements of hole transport in lead sulfide (PbS) QD films. Capping with a carboxylate ligand generally produces p-type doping of PbS QD films; furthermore, among various carboxylate ligands, thiophene-2,5-dicarboxylic acid provides PbS QD films with exceptionally high hole mobility values, and solvents with a high solvency power for the ligand are important for enhancing carrier mobility. With an appropriate combination of ligand molecule and solvent, QDs can be packed more closely into films, resulting in orders-of-magnitude enhancement in the field-effect hole mobility, reaching values of 0.20 ± 0.06 cm2·V–1·s–1. The new guideline presented in this study will be vital for constructing high-performance QD-based p–n junctiontype devices, especially photovoltaics.

Main text

Colloidal quantum dots (QDs) are of remarkable research interest because of their possible wide range of applications as components of energy-generating devices, such as solar cells,1 thermoelectrics,2 or low-power-consumption electronic devices, such as photodetectors3 and light-emitting devices.4 The quantum confinement of carrier wave functions in QDs leads to the formation of discrete electronic energy levels and to the tunability of their electronic bandgaps through alteration of the size of the QD.5 Among the broad variety of QDs, lead sulfide (PbS) is one of the most attractive materials because of the well-developed control of its synthesis,6 leading to high quality, high absorbance, and fine-tuning of band gaps. It is also especially


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appealing for solar cell applications due to its large confinement effect with a large excitonic Bohr radius (20 nm).7 To date, PbS QDs can be treated as a model material for investigations of charge carrier transport properties in colloidal QD assemblies, from which any gained knowledge can be translated to other QDs of different compounds.

As-synthesized PbS QDs are generally covered with long-chain insulating ligands [e.g., oleic acid; (9Z)-octadec-9-enoic acid] that act as native capping agents to stabilize the QDs in dispersions. However, from a device application perspective, these long ligands effectively block charge carrier transport. To impart electronic conductivity in the QD film, these insulating ligands should be replaced by short-chain ligands to enhance the electronic coupling, crucial for device applications.8 Indeed, ligand exchange permits the tuning of various properties of QDs, such as the inter-QD distances, the degree of crosslinking, the energy levels of trap states, and doping levels, that determine the performance of devices incorporating the QDs.9 Colloidal QDs behave as extrinsic semiconductor since their electronic properties are strongly affected by their stoichiometry and their surface conditions. Apart from them, it is the ligands that can shift the energy level and also the Fermi level of the associated QD system. Therefore, to some extent, ligand modification of the QD surface can resemble a doping process.

As a method for

modifying the properties of QDs, ligand exchange is attractive because of ready accessibility to a vast library of ligands and their ease of handling.

For applications of QDs in electronic devices, there are significant demands for advanced techniques to control carrier doping. Many efforts have been performed for electron doping. For example, n-type PbS QD films have been successfully obtained by capping PbS QDs with


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appropriate ligands such as hydrazine10 or halide ions (I–, Br–, or Cl–).11,12 However, this was not the case for p-type doping. The lack of high-quality p-type PbS QD assemblies has been identified as one of the limiting factors for QD solar cell performance, despite its prospects. It also hampers the development of the other QD electronic devices in which well-controlled ptype and n-type characteristics are necessary. So far, p-type PbS QD films have generally been obtained by intentional or unintentional oxidation of the films under ambient conditions,13,14 but their hole mobilities were very low ( L9 > L4). In fact, the molecule lengths of L10 (6.75 Å) and L9 (6.17 Å) are almost twice that of L4 (3.60 Å). Also, both L9 and L10 have similar molecular structures and lengths, which translate into similar distances between the QDs (Figure 3 and Figure S7, Supporting Information). Furthermore, we also find that the order of the hole mobility values with different ligands cannot be explained by the solubility of the ligands in DMSO, and the observed trends in the hole mobility also cannot simply be attributed to the ligand length or the distance between the QDs. These results suggest that the π conjugation in the ligand molecules is more important rather than the size of ligands in the carrier transport of the QD assemblies. From the abovementioned discussions of the ligand choice and the solvent use combination, several factors determine the hole and electron transport in colloidal QD assemblies. The first is that conjugation within the ligand molecules will have more significant influence rather their size and their solubility in a particular solvent. However, the second thing that should be noted is that the use of a solvent that can dissolve more the particular molecular ligand will enhance the capability of the ligand to provide a shorter inter-QD distance, thus enhancing the carrier mobility values. These two points can become a guideline to choose the ligand molecule and to optimize the use of a solvent for its ligand exchange process.

CONCLUSIONS In summary, a novel short ligand, thiophene-2,5-dicarboxylic acid (L10) imparted p-type doping behavior on PbS QDs, with an excellent hole mobility of 0.2 ± 0.06 cm2·V–1·s–1, among the highest reported for PbS QD FETs. This high hole mobility originated from an optimal


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choice of the ligand among a library of ligands containing two carboxylate groups, which generally produce p-type doping. Furthermore, we also observed that the solvent used for fabricating the FETs has an unexpected crucial effect on the properties of the resultant FETs, irrespective of the type of ligand. To realize high carrier mobility, one should use an appropriate solvent for the selected ligand. These findings, which were clarified through our ligand/solventengineering approach, provide valuable guidelines for further development of various devices based on colloidal QDs that require efficient transport of either holes or electrons, such as photovoltaic devices, transistor circuits, or thermoelectrics.

METHODS PbS QDs synthesis: The QDs were synthesized by using a standard hot-injection method.23 In a three-neck reaction flask, PbAc2·3H2O (2.0 mmol, 0.76 g, 99.99%; Sigma-Aldrich), octadec-1ene (ODE, 25 mL, technical grade; Sigma-Aldrich), and oleic acid (L1, 4.0 mmol, 1.49 mL, >85.0%, TCI) were kept at 120 °C under vacuum for two hours with magnetic stirring until the lead salt dissolved in the solution. The mixture was then cooled and kept at 100 °C with magnetic stirring. A solution of hexamethyldisilathiane (TMS2S, 1.0 mmol, 0.21 mL, >97.0%, TCI) in anhydrous oxygen-free ODE (5 mL), which was prepared in a N2 glovebox, was quickly injected to the first solution at 100 °C. The resultant mixture was magnetically stirred for three minutes then cooled to room temperature by using an ice bath to give a crude dispersion of PbS QDs. The QDs in the crude dispersion was thoroughly washed by repeated redispersion– precipitation cycles as follows. The crude dispersion was mixed with hexane and ethanol and then subjected to centrifugation. The precipitate was redispersed in hexane, precipitated by


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adding ethanol and collected by centrifugation. This redispersion–precipitation cycle was repeated three times. The third cycle was conducted in an N2 glovebox. The resultant precipitate was dispersed in anhydrous chloroform (≥99%, Sigma-Aldrich) and precipitated by adding anhydrous methanol (99.8%, Sigma-Aldrich). This dispersion–precipitation cycle was repeated twice. The resultant precipitate was dried in vacuum at room temperature to afford PbS QDs. For storage, the resulting PbS QDs were dispersed in anhydrous chloroform.

FET fabrication: All device fabrication was performed inside an N2 glovebox (Tr.O2 < 1 ppm, Tr.H2O < 1 ppm). All ligands were purchased and used as received: ethane-1,2-dithiol (EDT, L2; Sigma-Aldrich), 3-mercaptopropionic acid (MPA, L3; Sigma-Aldrich), oxalic acid (L4; Wako Chemicals), succinic acid (L5; Wako Chemicals), L-malic acid [(2S)-2hydroxysuccinic acid, L6; Wako Chemicals], fumaric acid [(2E)-but-2-enedioic acid, L7; Wako Chemicals], terephthalic acid (benzene-1,4-dicarboxylic acid, L8; TCI), furan-2,5-dicarboxylic acid (L9; TCI), thiophene-2,5-dicarboxylic acid (L10; TCI), and 3,4-propylenedioxythiophene2,5-dicarboxylic acid (3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine-6,8-dicarboxylic acid, L11; Sigma-Aldrich). On a 230-nm SiO2/Si wafer with lithographically patterned interdigitated electrodes (30 nm Au), the PbS QDs film was deposited by a layer-by-layer sequential spincoating technique under conditions optimized as shown in Figure S8 (Supporting Information). Each layer was formed as follows. A dispersion of PbS QDs in chloroform (5 mg mL–1, 40 µL) was spin-coated on the wafer at 1400 rpm and dried at 4000 rpm. The PbS QDs film on the wafer was covered with a droplet of a solution of the appropriate ligand (50 mM, 100 µL) in anhydrous DMSO (≥99%, Sigma-Aldrich), NMF (99%, Wako Chemicals), or MeOH (99.8%, SigmaAldrich) for 60 s, and then spin-dried at 4000 rpm. The morphology of the films was investigated


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by scanning electron microscopy (SEM, Hitachi SU8010) and atomic force microscopy (AFM; Asylum, Cypher).

FET measurements: All FET measurements were performed in an N2 glovebox by using a probe station equipped with low-noise probes that were connected to an Agilent B1500A semiconductor parameter analyzer equipped with high-resolution source measurement units. All mobility values reported in this paper is the field effect mobility obtained using a capacitance of C = 15 nF/cm2 for the 230 nm thick SiO2 gate dielectric. Also, the mobility was calculated from the linear regime transfer curves from the forward scan (|VD| increases in time) to avoid the screening effect of trapped carriers.33 The mobility values reported in the paper are the averages of those obtained in more than ten devices, all of which showed similar transistor behavior. This statistical approach guarantees the reproducibility and reliability of the measurements. We fabricated EDLT on the FET devices described above. A drop of ionic liquid [EMIM][TFSI] was put on top of the channel, and then a platinum foil was placed on top of the droplet as a gate electrode. A tungsten probe was inserted into the ionic liquid to measure the reference potential. The transfer curves were obtained at low (0.1 V) source-drain bias, and the applied gate voltage (VG) was scanned at a rate of 50 mV/s within the electrochemical window of the electrolyte. The calculation of mobility in EDLTs is described in Supporting Information.

FTIR measurements: FTIR absorption measurements were performed on a Shimadzu Tracer100. The PbS QD films were deposited on double-polished silicon wafers by following the protocol used for FET fabrication. The measurements were made in the ATR mode with an average of 60 scans.


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TEM measurements and analysis: TEM images were recorded on a JEOL JEM-1230 (normal resolution) or JEOL JEM-2100 (high resolution) electron microscope. The PbS QDs film was deposited onto a carbon grid by drop-casting a diluted chloroform dispersion of the native PbS QDs (capped with the native ligand L1), followed by immersion of the grid in a solution (MeOH or DMSO) of the appropriate ligand and drying at 70 °C. To estimate the diameters of the QDs and the distances between them, we analyzed the TEM images using ImageJ software Ver. 1.50i (National Institute of Health, USA). Diameters and distances for >100 particles were measured and averaged (Figure S12, Supporting Information).

XRD measurements: XRD measurements of the PbS QDs were performed on a Rigaku SmartLab with a Cu Kα source, operated at 40 kV and 30 mA, using a θ-2θ scanning method. L10-capped PbS QDs were obtained from the native PbS QDs (capped with the native ligand L1) through a phase-transfer ligand-exchange method.34

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: ************. Figure S1−S15 (PDF)


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ORCID Satria Zulkarnaen Bisri: 0000-0002-3922-6248 Author Contributions L.L., S.Z.B., and Y.Is. conceived and designed the project. L.L. performed and analyzed experiments; D.H. assisted XRD measurements. L.L., S.Z.B., Y.Is., T.A., and Y.Iw. co-wrote the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was partly supported by Grant-in-Aid for Specially Promoted Research (No. 25000003) on “Emergent Iontronics”, Grant-in-Aid for Young Scientist (A) (Grant no: JP17H04802) from The Japan Society for the Promotion of Science (JSPS), and 2016 RIKEN Incentive Research Grant. L.L. thanks JSPS for a Young Scientist Fellowship. We thank Prof. Maksym Kovalenko (ETH Zurich, Switzerland) for suggestions and discussions.

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Figure 1. (a) Chemical structures of ligands for capping lead sulfide (PbS) quantum dots (QDs). Native ligand L1 (oleic acid), reference ligands L2–L4 (L2: EDT, L3: MPA, L4: oxalic acid),15,25 and library ligands L5–L11 (L5: succinic acid, L6: L-malic acid, L7: fumaric acid, L8: terephthalic acid, L9: furan-2-5-dicarboxylic acid, L10: thiophene-2,5-dicarboxylic acid, L11: 3,4-propylenedioxythiophene-2,5-dicarboxylic acid) employed in this work. (b) A schematic illustration of a FET device of a PbS QD film. (c) Effect of the solvent used for fabrication of the FETs on the packing of PbS QDs.


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Figure 2. (a) AFM image of the L10 (thiophene-2,5-dicarboxylic acid)-capped PbS QD film. The PbS QD film was fabricated into a FET device by using MeOH as the ligand-exchange solvent. (b) FTIR spectra of the L10 (thiophene-2,5-dicarboxylic acid)-capped PbS QD films fabricated with DMSO (red), NMF (green), or MeOH (blue). As a reference, the spectrum of the film of the native L1 (oleic acid)-capped PbS QDs is also shown (black). (c) ID–VG transfer characteristics at VD = –10 V of FETs made from L10 (thiophene-2,5-dicarboxylic acid)-capped PbS QD films fabricated with DMSO (red), NMF (green), or MeOH (blue).


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Figure 3. (a–f) Transmission electron micrographs of L10 (thiophene-2,5-dicarboxylic acid)capped PbS QD films fabricated with MeOH (a,c,e) or DMSO (b,d,f). TEM images of the films deposited on carbon grids by dip-coating (a,b). Histograms of the distances between the centers of QDs (c,d). Schematic illustrations of the neighboring QDs (e,f). In the inset images of (a) and (b), the distances between the QDs are shown by white arrows.


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Figure 4. ID–VG transfer characteristics of the FETs made of the films of PbS QDs capped with (a) L2 (EDT), (b) L3 (MPA), (c) L4 (oxalic acid), and (d) L9 (furan-2,5-dicarboxylic acid), which were fabricated with DMSO (red) and MeOH (blue).


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Figure 5. The comparison between solid gated and ionic liquid gated transistors. (a) A schematic of ionic-liquid-gated field effect transistor and the measurement circuit. ID-VG transfer characteristics of devices prepared with various ligands (L4: oxalic acid, L9: furan-2,5dicarboxylic acid, L10: thiophene-2,5-dicarboxylic acid) in DMSO with (b) ionic liquid gate and (d) SiO2 solid gate. (c) Hole mobility values of the FETs made of PbS QDs films capped with various ligands (L4: oxalic acid, L9: furan-2,5-dicarboxylic acid and L10: thiophene-2,5dicarboxylic acid) gated with either SiO2 solid gate (blue) or ionic liquid gate (red). The error bars reflect the statistical standard deviation.


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ToC figure


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Ligand and Solvent Effects on Hole Transport in Colloidal Quantum

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