Hydroxyl-Terminated CuInS2 Based Quantum Dots: Toward Efficient

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Hydroxyl-Terminated CuInS2 based Quantum Dots: Towards Efficient and Bright Light Emitting Diodes Zelong Bai, Wenyu Ji, Dengbao Han, Liangliang Chen, Bingkun Chen, Huaibin Shen, Bingsuo Zou, and Haizheng Zhong Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04480 • Publication Date (Web): 21 Jan 2016 Downloaded from http://pubs.acs.org on January 22, 2016

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Chemistry of Materials

Hydroxyl-Terminated CuInS2 based Quantum Dots: Towards Efficient and Bright Light Emitting Diodes Zelong Bai†§, Wenyu Ji₸§, Dengbao Han†, Liangliang Chen†, Bingkun Chen†, Huaibin Shen*‡, Bingsuo Zou⊥ and Haizheng Zhong*† †

Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems, School of Materials Science & Engineering, Beijing Institute of Technology, 5 Zhongguancun South Street, Beijing 100081, China ‡

Key Laboratory for Special Functional Materials of Ministry of Education, Henan University, Kaifeng 475004, China

₸

State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China

⊥

Micro/Nano Research Center, School of Physics, Beijing Institute of Technology, 5 Zhongguancun South Street, Beijing 100081, China ABSTRACT: CuInS2 based quantum dots are emerging as low toxic materials for new generation white lighting technology due to their broad and color-tunable emissions as well as the large Stokes shift. Here, we developed a simple and insitu ligand exchange strategy for the fabrication of hydroxyl-terminated CuInS2 based quantum dots capped with 6mercaptohexanol. During the ligand exchange, long-chain methyl-terminated oleylamine on quantum dots’ surface can be effectively and efficiently replaced by the short-chain hydroxyl-terminated 6-mercaptohexanol, enabling their solubility in polar organic solvents such as methanol, ethanol and dimethyl formamide. Moreover, the resulted hydroxylterminated quantum dots exhibit well-preserved photoluminescence properties with quantum yields of ~70%. Using these hydroxyl-terminated CuInS2 based quantum dots as emitting layer, we fabricated efficient and bright light emitting diodes by adopting an inverted device structure. The optimized devices show maximum luminance of 8,735 cd/m2 and external quantum efficiency of 3.22 %. Furthermore, the performance enhancement can be explained by considering the decreased energy barriers between electron transport layer and the emitting layer. The combination of high efficiency and enhanced brightness as well as the potential all-solution processability using green solvents makes hydroxyl-terminated quantum dots capped with 6-mercaptohexanol to be new generation materials for lighting emitting applications.

Introduction Quantum-dot light-emitting diodes (QD-LEDs) are emerging as complement technology to enhance the performance of lighting and display systems due to their broad emission wavelength tunability, wide color gamut, as well as the low-cost solution process that fit for large area fabrication.1,2 Since the first report of QD-LEDs utilizing CdSe quantum dots (QDs), significant progress has been made in improving the brightness and efficiency of QD-LEDs.3-10 Especially, the performance of solution processed CdSe based QD-LEDs are comparable to state-ofthe-art organic light emitting diodes produced by vacuum deposition.11-14 To overcome the toxicity problem, other alternative materials including CuInS2 based QDs15, 16, InP QDs17, carbon dots18, 19, ZnSe QDs20, 21 as well as metal clusters22 have been developed and applied for the fabrication of QD-LEDs. However, the device performance of these materials lag far behind the well-developed cadmium based counterparts.15-22

CuInS2 based QDs are low toxic materials with a large tunable spectral wavelength of 500-800 nm, high quantum yields (QYs) up to 80% and full width at half maximum (FWHM) of 90-120 nm.23-25 All these features make CuInS2 based QDs to be excellent candidates for new generation of white lighting technology.15, 26-29 Nevertheless, many issues still hinder the development of CuInS2 based QDs for electroluminescence (EL) devices, such as (i) the low efficiency of charge injection from charge transport layers to QDs emitting layer,13 (ii) the exciton quenching caused by accumulated space charges near interface between charge transport layer and QDs30-32 and (iii) the layer intermixing of neighboring layers during solution process.7 Many efforts are underway to solve these issues by developing more appropriate charge transport layer materials or new structures for EL devices.3, 7, 11, 14, 33, 34 From the material perspective, we are always motivated to develop suitable, efficient and high quality QDs that are prerequisite to fabricate highly efficient devices. Considering the application of QDs in sandwiched EL devices,

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the long-chain ligands on their surface strongly influence the charge injection between transport and emitting layers, and also affect the carrier transport within the QD layer. Moreover, surface ligands also have the ability to tune the solubility and processability of QDs. However, the surface engineering of colloidal QDs through ligand exchange always induces PL decrease, which hinders the application for efficient EL devices. Here, we report the use of hydroxyl-terminated QDs as the emitting layer for the fabrication of efficient and bright inverted QD-LED, based on 6-mercaptohexanol (MCH) capped CuInS2/ZnS QDs (CIS-MCH). Compared to conventional long alkyl chain capped QDs (such as oleylamine capped CuInS2/ZnS, CIS-OLA), the MCHcapped QDs possess significantly increase of the charge transport within QDs films and improve electron injection from electron transport layer (ETL) into QDs emitting layers as well as the processability in green polar solvent such as ethanol. The combination of these advantages results in significant enhancement in the device performance as follows: brightness up to 8,735 cd/m2, the external quantum efficiency (EQE) up to 3.22 %, current efficiency of 9.43 cd/A and power efficiency of 6.09 lm/W. These parameters have increased 3 folds in luminance and nearly 2 folds in efficiency compared to the other I-III-VI QDs based QD-LEDs that ever reported and are the highest values reported for non-cadmium based QD-LEDs.15-19, 22, 26, 28, 35-39

Experimental Section Synthesis of CuInS2/ZnS QDs: 4 mmol Copper(I) iodide (CuI, 98%), 16 mmol indium(III) acetate (In(OAc)3, 99.99%) , 10 mL 1-dodecanethiol (DDT, 98%), 5 ml oleic acid (OA, 90%) and 100 ml 1-octadecene (ODE, 90%) were mixed in three-necked flask. After being degassed under vacuum for 20 min, the solution was heated to 230 °C and reacted for 15 min under nitrogen flow. Subsequently, 80 mL Zn stock solution (Zinc stock solution was prepared by mixing 30 mmol zinc acetate dehydrate (Zn(OAc)2, 97%), 20 mL oleylamine (OLA, 90%), 60 mL 1octadecene (ODE, 90%), and then heated the mixture to 130 °C under nitrogen flow was added dropwise (~1 mL/min) into the reaction mixture at 230 °C for coating ZnS shell onto the CuInS2 core. Ligand Exchange: Ligand exchange process was accomplished by adapting a modified procedure of previous report.40 At the end of the formation of CuInS2/ZnS coreshell NCs, the reaction mixture was cooled to 180 °C and 200 mmol 6-mercaptohexanol (MCH, 95%) was injected into the reaction mixture immediately. After reacting for 15 min, CuInS2/ZnS NCs capped with MCH ligands precipitated from ODE. After decanting the ODE, CIS-MCH QDs was dispersed into ethanol and then re-precipitated by adding n-hexane. The precipitate was centrifuged and the purification process was repeated for two or three times. Then the purified QDs were dried into powders at 60 °C. Characterization of CIS QDs: Transmission electron microscope (TEM) images of the purified CuInS2/ZnS-MCH QDs were obtained on a JEOL-JEM 2100F microscope op-

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erating at 200 kV. X-ray diffraction (XRD) measurements were carried out on a Rigaku D/max-2500 X-ray diffractometer with graphite monochromated Cu Kα radiation (λ= 1.5418 Å) with a scanning rate of 1 degree min−1. Ultraviolet–visible spectroscopy (UV-Vis) absorption spectra of QDs solutions were measured on a UV-6100 UV-Vis spectrophotometer and photoluminescence (PL) spectra were taken using an F-380 fluorescence spectrometer. The absolute photoluminescence quantum yields (PLQYs) of diluted QDs solutions were determined using a fluorescence spectrometer with integrated sphere (C9920-02, Hamamatsu Photonics, Japan) excited at a wavelength of 450 nm using blue LED light source. The PLQYs of CIS QDs were obtained with absorbance of QDs solution nearly 0.1. Fourier Transform Infrared (FTIR) Spectra were recorded on an IRPrestige-21 spectrophotometer. The infrared reflection spectra of CIS QDs powders were measured at room temperature. The infrared reflection spectra of ligand molecules were measured by depositing OLA or MCH onto potassium bromide (KBr) plate. Hydrogen Nuclear Magnetic Resonance (1H-NMR) spectra were recorded on a Varian mercury-plus 400 spectrometer and the solvents for CIS-OLA QDs and CIS-MCH QDs were deuterated chloroform (CDCl3) and deuterated methanol (CD3OD), respectively. The X-ray photoelectron spectroscopy (XPS) was collected on an ESCA Lab220I-XL X-ray photoelectron spectrometer using an Al Kα X-rays as the excitation source. Cyclic voltammetry (CV) measurements were recorded on a 660D electrochemical workstation, using glassy carbon as the working electrode and platinum as the counter electrode. Reference electrode was Ag/Ag+ (Ag wires with 0.01 M AgNO3 in acetonitrile). CIS-OLA QDs dissolved in n-hexane or CISMCH QDs dissolved in ethanol were dropped on the working electrode to form a uniform film. CV curve was obtained under a scanning rate of 10 mV/s. Time-resolved photoluminescence (TRPL) measurement was collected using fluorescence lifetime measurement system (Edinburgh FL920) at an excitation wavelength of 405 nm. Device Fabrication: All the devices were fabricated on ultrasonically cleaned indium tin oxide (ITO) coated glass (15 Ω/sq). ZnO QDs (obtained from Nanophotonica, Inc. and filtered with 0.45 µm PVDF filter) was spin-coated onto the substrate and then dried at 150 °C on a heater for 15 min. Then, CIS-MCH QDs dispersed in ethanol were spin coated at 2000 rpm for 60 s on top of the ZnO QDs and dried for 15 min at 80 °C. Subsequently, layers of 30 nm 4,4’-N,N’-dicarbazole-biphenyl (CBP), 8 nm 4,4’,4’’Tri(9-carbazoyl) triphenylamine (TCTA), 8 nm MoO3 and 100 nm thick Al cathode were thermally deposited layer by layer in a vacuum chamber. Characterization of Devices: Film thicknesses were measured using a calibrated quartz crystal microbalance. The EL spectra of CuInS2/ZnS based QD-LEDs were obtained using an Ocean Optics Spectrometer (USB 2000) and a Keithley 2400 power source. Current density-voltage (J-V) characteristics of QD-LEDs were measured using an Agilent 4155C semiconductor parameter analyzer. The luminance was calculated from the photocurrent of a calibrat-


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ed silicon detector (Newport 818 UV) that was placed close to QD-LEDs assuming Lambertian emission. Results and Discussion Scheme 1. Schematic diagram of synthesis and in-situ ligand exchange process, molecular formula of ligands and surface structure of CIS QDs before and after ligand exchange.

The MCH capped CuInS2/ZnS QDs can be welldispersed in alcoholic solvents such as methanol, ethanol and dimethyl formamide (DMF) (Figure 1a). More significantly, this ligand exchange strategy is very effective and efficient. One batch experiment can produce ~10.3 g of CIS-MCH QDs (Figure 1b). As shown in Figure 1c, TEM image shows that the CIS-MCH QDs are also monodispersed. The statistical result of particle mean diameter is nearly 3.1 nm (Figure S2). Meanwhile, the XRD data (Figure S3) also demonstrate the size and lattice structure of QDs are maintained after ligand exchange. Figure 1d shows UV-Vis absorption spectra and PL spectra of both CIS-OLA QDs and CIS-MCH QDs, and the main fluorescent properties of CIS-MCH QDs, such as emission wavelength, FWHM and PLQYs, are similar with that taken from the sample before ligand exchange. The PLQYs measurement using integrated sphere shows the absolute PLQYs of CIS QDs are 72.3% and 70.5% for the samples before and after ligand exchange, respectively. We also explored the versatility of this ligand exchange of MCH on other preformed QDs, such as CdSe/ZnS QDs by injection of MCH into the mixture of as-prepared QDs and ODE at 180 °C. The results are presented in the supporting information (Figure S1).

To achieve the MCH capped CuInS2/ZnS QDs, oleylamine capped CuInS2/ZnS core-shell QDs (CIS-OLA) were synthesized at first. Scheme 1 shows the illustration of the synthesis procedure and ligand exchange process for MCH capped QDs. The synthesis process for CuInS2/ZnS core-shell QDs is similar with our previous reports.16 The ligand exchange was performed after the completion of ZnS shell formation of QDs by injecting MCH ligands into the reaction system. Through ligand exchange, MCHcapped CuInS2/ZnS QDs agglomerated and precipitated from the reaction mixtures (See details in experimental section). The aggregation of MCH-capped QDs in ODE can be explained by the formation of hydrogen bonds due to the changing of solubility of QDs, which is similar with the previous report.40

Figure 2. (a) FTIR spectra and (b) H-NMR spectra of CISOLA QDs and CIS-MCH QDs (the solvent residual peak is labeled with asterisk).

Figure 1. (a) The photographs of samples that dispersed in different solvents under day light and UV 365 nm illumination. (b) Photograph of 10.3 gram scaled CIS-MCH QDs. (c) TEM image of CIS-MCH QDs. (d) UV-Vis absorption and PL emission (λex = 400 nm) spectra of OLA and MCH capped CIS QDs.

In order to clarify the type and quantity of surface ligands on the surface of resulting QDs, we performed FTIR and 1H-NMR measurements. Figure 2a shows the FTIR spectra of CIS QDs before and after ligand exchange. In the FTIR spectrum of CIS-MCH QDs, the broad band appearing at 3272 cm-1 (νOH) is the characteristic peak of hydroxyl group on MCH ligands. The peaks at 1049 cm-1 (νCO) and 717 cm-1 (γOH) also indicate the existence of MCH ligands. In the supporting information, Figure S4 compares the FTIR spectra of CIS QDs and their ligand molecules. It is obvious that the fingerprint spectrum of CIS-MCH QDs (~ 1500-500 cm-1) is extremely similar to MCH molecules. This is another strong evidence of MCH ligands covering the surface of CIS QDs. As shown in 1HNMR spectrum (Figure 2b) of CIS-MCH QDs, the 1-5 signals represent the hydrogen on corresponding methylene of MCH ligands. It is revealed from Figure 2b and Figure S5 that hydrogen atom signals of MCH ligands on CIS QDs are slightly shifting and broadened due to the effect of QDs.

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Figure 3. (a) Schematic illustration of the inverted QD-LEDs with using CIS-MCH QDs as the emitting layer. (b) Photographic image of a large-area CIS-MCH QDs based QD-LEDs at a driving voltage of 10V. (c) EL spectra of CIS-MCH QDs based QD-LED. (d) Current density and luminance of the devices as a function of applied bias. (e, f) Current efficiency, power efficiency and EQE of the devices as a function of current density.

The surface composition of QDs was further investigated by applying XPS analysis. The S 2p core levels of CIS QDs before and after ligand exchange are shown in Figure S6. As the S atom in different states have different binding energies (161.2 eV for S2p ZnS and 162.9 eV for S2p RSH), the spectrum of S 2p can be fitted into two peaks. The increasing of area ratio of peak for S2p R-SH suggests MCH ligands are bonded on the surface of CIS QDs. All these results confirm the successful ligand exchange of CIS QDs using MCH as substitutional capping ligands. Once we have ascertained the capping of MCH on the surface of CIS QDs, we fabricated an inverted structure QD-LED with CIS-MCH QDs as the emitting layer.33 As shown in Figure 3a, our device consists of multiple layers stacked in the structure of indium tin oxide (ITO)/zinc oxide nanoparticles (ZnO, 50-60 nm)/CIS-MCH QDs (25 nm) /4,4’-N,N’-dicarbazole-biphenyl (CBP, 30 nm)/4,4’,4’’-tri(9-carbazoyl) triphenylamine (TCTA, 8 nm)/MoO3 (8 nm)/Al (100 nm). The photograph (emitting at 10V) and EL spectra of this QD-LED are demonstrated in Figure 3b and 3c. As can be seen from Figure 3c, orange emission completely originating from QDs is observed for the QD-LED at whole range of applied voltages

~ 4 - 13 V, and no parasitic EL emission from the adjacent organic layers in the EL spectra is observed, implying the efficient charge injection and radiative recombination of electrons and holes in CIS QDs. Moreover, the optimized device exhibits low turn-on voltage (~3.75 V) (the lowest voltage when luminance reaches to 1 cd/m2), high luminance (~ 8,735 cd/m2), high EQE (3.22 %), high current efficiency (9.43 cd/A) and high power efficiency (6.09 lm/W), as shown in Figure 3d, 3e and 3f. All the results indicate an efficient formation of excitons in CIS QDs, where they decay radiatively. As a control, we also fabricated an EL device with same structure using original CISOLA QDs as the emitting layers. The performance parameters of two QD-LEDs with different emitting layers are compared in supporting information (Figure S7). Compared with the methyl-terminated CIS QDs based QDLED, the brightness of hydroxyl-terminated CIS QDs based QD-LED increased by >20 times and the efficiency increased by nearly 4 times. Considering that MCH capped QDs have similar QYs with OLA capped sample, the dramatically performance enhancement in device is attributed to the ligand engineering with MCH on the surface.16, 36, 37


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Chemistry of Materials EHOMO = - (Eox + 4.71) eV (1) ELUMO = - (Ered + 4.71) eV (2) where the unit of the potentials is V vs. Ag/Ag+ (0.01 M) reference electrode.

Figure 4. (a) Cyclic voltammetry (CV) curves of OLA and MCH capped CIS QDs. (b) Proposed energy levels of the multilayered device.

The use of shorter chain length ligands in QD-LEDs has been an effective strategy to increase the electron mobility within the QD film as well as improve the carrier injection efficiency from transport layer.4, 13 Here, we highlight that the MCH ligands capping on CIS QDs optimize the electron injection efficiency from ZnO ETL to CIS QDs emitting layer in the device. Compared with OLA ligand, MCH ligand has a shorter chain length and a polar group (the hydroxyl group) on tail of alkyl chain, which decreases the charge injection barrier and energy transfer distance between charge transport layers and CIS QDs emitting layer. According to Schottky-Mott rule,41 the injection efficiency of the diodes is mainly determined by the potential barrier height, that is, the difference of the lowest unoccupied molecular orbital (LUMO) (for electron injection) or the highest occupied molecular orbital (HOMO) (for hole injection) between the adjacent semiconductor layers. In our case, the potential barrier height between LUMO energy levels of ZnO ETL and CIS QDs emitting layer severely reduces the electron injection efficiency from ZnO ETL to CIS QDs. The LUMO energy level of ZnO layer is -4.0 eV.33 The energy level of CIS QDs with different ligands were measured by CV measurement and the results are shown in Figure 4a.42, 43 The onset of oxidation and reduction peaks were identified as the point of intersection of tangents and labeled with vertical lines. The corresponding energy levels of each sample were calculated, according to Equations (1) and (2).

Figure 5. Emission decay of (a) CIS-OLA QDs and (b) CISMCH QDs deposited on glass substrate and ZnO film, respectively. Solid red lines represent the kinetic fit using triexponential decay analysis.

In the calculation results, LUMO energy level of CISOLA QDs and CIS-MCH QDs are -3.48 eV and -3.56 eV, respectively. Figure 4b shows a flat energy level diagram of each layer in our device. We found that the potential barrier height between ZnO layer and CIS QDs decreases from 0.52 eV to 0.44 eV after ligand exchange. Furthermore, the potential barrier height can be further reduced as a result of an interface dipole formed due to the hydrogen bonding between hydroxyl group of MCH and ZnO layer, which have been observed in hydroxyl-terminated polymer based EL devices.44 Therefore, it is not difficult to infer that the MCH ligands can dramatically increase the electron injection efficiency from ZnO nanoparticle ETL to CIS QDs emitting layer by decreasing the potential barrier height. Meanwhile, according to our previous reports,32 the decrease of potential barrier height may reduce the radical anion accumulation near the interface of ZnO layer and CIS QDs, which subsequently prevents the associated exciton quenching process. This is also a potential benefit of hydroxyl-terminated QDs for the improvement of the device performance.



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In order to verify above discussion, TRPL was measured to investigate the influence of surface ligands on electron transfer (ET) between ZnO layer and QDs. Briefly, CISOLA QDs and CIS-MCH QDs were spin coated on glass substrate and ZnO layer, respectively. Afterward, the PL lifetime of each sample was recorded by applying TRPL measurement, as shown in Figure 5a and 5b. Theoretically, as glass is insulated, the ET process can only occur between ZnO layer and CIS QDs. Therefore, the average PL lifetime of the QDs on glass can be calculated as reference,45-47 that is, τQDs-glass = 1/(kR + kNR), where kR and kNR represented radiative and nonradiative decay rates for QDs, respectively. And the PL lifetime of the QDs on ZnO film can be described as τQDs-ZnO = 1/(kR + kNR + kET). Thus the ET rate (kET) and efficiency (ηET) can be calculated as: kET = 1/τQDs-ZnO - 1/τQDs-glass and ηET = 1 - τQDs-ZnO/τQDs-glass, respectively. The calculation results are shown in Figure 5a and 5b, both the ET rates of CIS-OLA QDs and CISMCH QDs are nearly 107 s-1, which is consistent with the results in other relevant studies.47 Here, we found that the rate and efficiency of ET process from QDs to ZnO layer slightly decreases after ligand exchanging. As the ET process from CIS QDs to ZnO layer is exactly the reverse process of electron injection from ZnO layer to CIS QDs, this result supports the decrease of potential barrier height between LUMO energy levels of ZnO ETL and CIS QDs. Thus, to a certain extent, both the charge injection rate and efficiency from ZnO layer to QDs are increased, which improves the performance of QD-LEDs. To further demonstrate the increase in charge injection rate, we fabricated two devices with the structure of ITO/ZnO/CIS-OLA QDs/Al and ITO/ZnO/CISMCH QDs/Al in same thickness and measured the J-V characteristics, respectively. As we can see in Figure S8, the current density of the device using CIS-MCH QDs is higher than that using CIS-OLA QDs. This is also an evidence of the increased charge injection rate from ZnO layer to QDs. Conclusion In summary, hydroxyl-terminated CIS-MCH QDs capped MCH were obtained through a ligand exchange strategy and exhibited an excellent performance in the EL devices. During the ligand exchange process, the solubility of CIS QDs changed from oil-soluble to alcohol-soluble without obvious PL decrease. The influence of ligands on the electron injection between emitting layer and electron transport layer was thoroughly studied by applying CV and TRPL measurements. The results suggest that MCH ligands can tune the potential barrier height between ZnO layer and CIS QDs, which enhances the electron injection efficiency from ZnO layer to QDs. As the ligand exchange strategy possesses a good versatility, this study thus offers a new strategy for high efficiency QD-LEDs. Especially, hydroxyl-terminated QDs capped with MCH have enormous advantages in fabricating multiple emitting layers QD-LEDs for white lighting applications.7, 28, 34

ASSOCIATED CONTENT

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Supporting Information. Photoluminescence spectra and 1 H-NMR spectra of CdSe/ZnS-MCH QDs, statistical result of particle mean diameter, XRD spectra of CIS QDs, FTIR and 1 H-NMR spectra of CIS QDs and ligand molecules, XPS spectra of CIS QDs, Current density and luminance of CIS-OLA and CIS-MCH QD-LEDs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: (Haizheng Zhong) [email protected] *E-mail: (Huaibin Shen) [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. §These authors contributed equally.

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT Zelong Bai and Wenyu Ji contributed equally to this work. Haizheng Zhong would like to thanks Prof. Jialong Zhao, Prof. P.K. Nathan and Dr. Hongbo Li for helpful discussions. This study was supported by The National Basic Research Program of China (No. 2013CB328804), National Natural Science Foundation of China Grant (Nos. 21573018, 61474037), Beijing Nova program (No. xx2014B040) and Beijing Higher Education Young Elite Teacher Project (No. YETP1231).

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