ZnO Quantum Dots - ACS

Subscriber access provided by UNIV OF TEXAS DALLAS

Article

Light-Emitting Devices Based on Type-II InP/ZnO Quantum Dots Onuralp Karatum, Houman Bahmani Jalali, Sadra Sadeghi, Rustamzhon Melikov, Shashi Srivastava, and Sedat Nizamo#lu ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01618 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 3, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Light-Emitting Devices Based on Type-II InP/ZnO Quantum Dots Authors Onuralp Karatum1, Houman Bahmani Jalali2, Sadra Sadeghi3, Rustamzhon Melikov1, Shashi Bhushan Srivastava1 and Sedat Nizamoglu1, 2, 3, * Affiliation 1. Department of Electrical and Electronics Engineering, Koç University, Istanbul, 34450, Turkey. 2. Department of Biomedical Sciences and Engineering, Koç University, Istanbul, 34450, Turkey. 3. Graduate School of Materials Science and Engineering, Koç University, Istanbul, 34450, Turkey. ABSTRACT: One of the major challenges for present-day quantum dot light-emitting diode (QLED) technology is the transition from toxic heavy metal to “green” material-based devices. This report proposes an alternative cadmium-free material of type-II InP/ZnO core/shell quantum dots (QDs) for QLEDs. In this study, InP/ZnO core/shell QDs are nanoengineered by adjusting the shell coverage for optimum in-film quantum efficiency, and device parameters are investigated to reach a maximum QLED performance. The fully solution processed QLEDs made of biocompatible and environmentally benign QDs presented in this study exhibit low turn on voltage of 2.8 V, external quantum efficiency of 0.53% and a current efficiency of 1 cd/A with a saturated color emission in yellow-orange spectral region. This study paves the way towards non-toxic and efficient LEDs using type-II QDs.

KEYWORDS: Colloidal quantum dots, light-emitting devices, indium phosphide, type-II band alignment, current efficiency, external quantum efficiency

1 ACS Paragon Plus Environment

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Consumer electronics industry is rapidly growing due to massive amount of demand. In parallel with this high demand, the electronic waste (e-waste) problem grows as well. The total e-waste generated worldwide was estimated to be around 41.8 million tones in 2014, and it is expected to reach 50 million tones in 2018.1 Consumer electronic products are typically made of indecomposable, nonbiocompatible and sometimes even toxic materials, which may pose serious threats to human health, variety of species and environment. The regulations regarding the use of such materials in consumer goods is getting stricter day by day. “Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment” (RoHS) and “Waste Electrical and Electronic Equipment Directive” (WEEE) by the European Union are examples of such regulations. Thus, the need for “green material” based devices that are environmentally friendly and biocompatible for electronics industry is rapidly increasing. Lighting and display applications account for a considerable portion of electronics industry. For example, almost one-fifth of the electricity generated is being consumed by lighting applications.2 Semiconductor based light-emitting diodes (LEDs), organic LEDs, and more recently quantum dot lightemitting devices (QLEDs) have been increasingly used in such applications. Ever since the first demonstration of quantum dot light-emitting diodes (QLEDs) 3, incorporating QDs as emissive layer in LED architectures has attracted significant attention.4-7 This interest stems from the excellent optical and electrical properties of QDs such as high fluorescent efficiency, narrow emission spectra, low-cost solution processability and spectral tunability via quantum confinement effect.8-12 QLEDs especially generated interest for lighting and display applications after reaching comparable performances with organic light-emitting diodes (OLED) and semiconductor based LEDs.13-15 Up to now, the most wellunderstood and optimized QDs for LED applications are cadmium-based, because of their outstanding characteristics such as near unity photoluminescence quantum yield (PL QY), narrow full width at half maximum (FWHM) and well-developed size control that can cover full visible spectrum. However, cadmium (Cd) is a highly toxic metal and Cd-containing systems can have significant negative effects on ecosystem.16, 17 As a replacement of Cd-based compounds, many alternative QDs were proposed such as CuInS2 (CIS), AgInS2, ZnSe, and InP is one of the most suitable QDs for light-emitting applications owing to its high quantum yield and tunable band-gap. The previous studies on InP-based QLEDs were based on heterostructures with type-I core/shell QDs.18-21 Application of type-II QDs to LEDs was demonstrated in very few reports, in which only toxic-heavy metal based QDs were used.22, 23 Nontoxic InP-based type-II QDs have not been studied yet for QLEDs.


Page 2 of 16

Page 3 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Here, for the first time, we incorporate toxic-heavy-metal-free, biocompatible and type-II core/shell InP/ZnO QDs into LEDs. Owing to type-II band alignment of InP/ZnO QDs, it is possible to reduce the reabsorption losses by utilizing Stokes shift24-26 and to decrease the energy transfer among neighboring QDs by covering the core with a shell.27-29 To this end, we synthesized InP core, InP/1ZnO (i.e, InP core surrounded by 1 monolayer (ML) of ZnO shell) and InP/2ZnO (i.e, InP core surrounded by 2 monolayers (MLs) of ZnO shell) QDs, and compared their PL QY in solution and in film. We found out that even though InP/2ZnO achieves lower efficiency loss when transferred into film, InP/1ZnO shows higher infilm efficiency because of higher PL QY. Thus, InP/1ZnO is chosen as the emissive layer for QLEDs. The fabrication of the multilayered devices is fully solution processed by spin coating the constituent layers. Afterwards, current density-luminance-voltage, current efficiency, external quantum efficiency and electroluminescence (EL) spectrum of the devices are investigated. Our devices showed low turn-on voltage (2.8V) and saturated emission color with (x, y) tristimulus coordinates of (0.52, 0.47). Maximum luminance (600 cd/m2), current efficiency (1 cd/A) and EQE values (0.53%) of the devices show promise as the first demonstration of type-II Indium-based QLEDs. RESULTS AND DISCUSSION As shown in the band alignment of InP/ZnO QDs in Fig. 1a, holes are confined in the core, while electrons tend to delocalize to the shell because of the similar electron affinities of the core and shell. Due to the quasi-type-II energy alignment of core-shell structure, the emission is dominated by recombination of core excitons, whereas the absorption is predominantly caused by the shell, which is especially the case for QDs with thick shell.25,

26

This can be utilized to decrease the spectral

absorption and emission overlap, which in turn reduces the reabsorption and also decreases PL efficiency drop of QDs in solid state. To this end, we synthesized InP core, InP/1ZnO and InP/2ZnO QDs for investigating the effect of shell thickness on the optical properties of QDs, whose synthesis details, core/shell formation procedure, structural details, and biocompatibility tests were provided in our previous studies24,

30

(See also Methods section for synthesis description). Fig. 1b provides the

absorption and emission spectra of synthesized QDs. With increasing shell thickness, we observe a red shift in the emission peak wavelength expectedly due to the progressive delocalization of electron wavefunction to the shell, which induces a decrease in the degree of spatial confinement for electron.31,

32

As a result, having a higher Stokes shift between PL and absorption, the minimum

reabsorption is achieved for InP/2ZnO QDs. Since a thicker shell also increases the central distances between individual QDs, it reduces the energy transfer among QDs, which is strongly dependent on inter-dot spacing (PL decay dynamics of InP core, InP/1ZnO and InP/2ZnO that show the decrease of energy transfer for thicker shells can be seen in Fig. S5).


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Quantum yields of the QDs must be particularly considered for LED applications. It is noted that the QY can be significantly enhanced by growing multiple shells around the QD core, owing to better passivation of surface trap states.33 However, in case of further increasing the shell thickness after certain coverage, the QY starts to decrease like in CdSe/CdS core/shell nanostructures34, possibly due to the strain caused by lattice mismatch (of %11 in this study30) between core and shell. This is what we observe in Fig. 1c, which presents the absolute quantum yields of InP core, InP/1ZnO and InP/2ZnO QDs in solution and after spin-coating them onto the glass substrate. The QY (in solution) improves from 8% to 30% after 1 ML coverage, but then it diminishes dramatically to 12% after further 2 MLs coverage. Similar behavior can also be observed in both cadmium-based and indium-based counterparts.34, 35 Consequently, even though InP/2ZnO has lower percentage of efficiency loss (20%) in comparison with InP/1ZnO QDs (30%) when transferred into thin film, this drop is acceptable considering one order of magnitude loss reported in the literature, and still InP/1ZnO QDs show higher in-film efficiency of 21%.36 Hence, InP/1ZnO appeals as an appropriate emissive layer for LED application considering its higher QY and also its rather low efficiency loss in solid state.


Page 4 of 16

Page 5 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Figure 1. a) InP/ZnO core-shell QD having type-II energy band alignment.30 b) Absorption (dashed line) and emission (solid line) spectra of InP core, InP/1ZnO and InP/2ZnO QDs (Tauc plots for each QD can be seen in Fig. S4). c) Absolute quantum yield measurements of InP core, InP/1ZnO and InP/2ZnO QDs in solution and in film (N=3).

The device structure of fully solution processed and bottom-emitting QLEDs consists of the following layers

on

a

glass

substrate:

a

transparent

indium

tin

oxide

(ITO)

electrode,

poly(ethylenedioxythiophene)-polystyrene sulfonate (PEDOT-PSS), poly(N,N9-bis-(4-butylphenyl)N,N9-bis(phenyl)-benzidine) (Poly-TPD), InP/1ZnO core/shell QDs, ZnO nanoparticles, and the top Al electrode, which is shown in Fig. 2a (The details of device fabrication, synthesis of QDs, and synthesis of ZnO nanoparticles are provided in Methods section). As expected from the energy band diagram of Fig. 2b, Poly-TPD is chosen as HTL due to the lower highest occupied molecular orbital (HOMO) energy level of -5.4 eV37, 38 and a high hole mobility, which are in favor of the hole injection and transport, and also higher lowest unoccupied molecular orbital (LUMO) energy level of -2.2 eV37, 38, which helps to increase the recombination rate by confining electrons within the QD layer due to the rather large energy offset (∼1.5 eV) at Poly-TPD/QD interface. ZnO is chosen as ETL due to high electron mobility, 5 ACS Paragon Plus Environment

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

large valence band offset at the QD/ZnO interface (∼1.5 eV), and efficient electron injection ability with electron affinity of ∼4.11 eV and ionization potential of ∼7.22 eV (Ultraviolet Photoelectron Spectroscopy measurements of InP/ZnO QDs and ZnO nanoparticles can be seen in Fig. S1 and Fig. S2).

Figure 2. a) Schematic of multilayered device structure. b) Energy band diagram for constituent layers. The displacement of electrons (filled circles) and holes (empty circles) is shown.

The device performance is highly dependent on the thicknesses of charge transport layers (CTL) and QD layer.39 Thicknesses of charge transport layers have an impact on the efficient charge injection into the emissive layer, thus, optimization of HTL and ETL thicknesses enhances device efficiency by enabling recombination to take place predominantly within the QD layer. Fig. 3a demonstrates the current efficiency and external quantum efficiency (EQE) of the devices as a function of Poly-TPD layer thickness. The maximum efficiencies are achieved at 40 nm thickness with current efficiency of 1 cd/A and EQE of 0.53%. Further increase in the HTL thickness causes device efficiency to decrease due to more parasitic emission from Poly-TPD. As expected, the efficiencies are more sensitive to ETL thickness than HTL thickness due to the interference effect of the reflected light from the cathode.40 As seen in Fig. 3b, increasing the ZnO layer thickness from 20 nm to 40 nm leads to nearly two-fold improvement in EQE. The current efficiency is less affected by ETL thickness, since luminance and current density vary proportionally to each other. After 40 nm, the device efficiency starts to drop upon further increasing the ETL thickness. Optimization of QD layer thickness is also required; and we measured the current efficiency and EQE for 20 nm, 30 nm, 40 nm and 50 nm QD layer thickness. On the one hand, while QD layer gets thinner, it decreases the device efficiency by inducing higher leakage currents through voids and spaces between QD monolayers; on the other hand, while QD layer gets


Page 6 of 16

Page 7 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

thicker, it makes charge transportation to QDs more difficult.

41

Based on the results in Fig. 3c, the

optimal thickness for QD layer is determined to be 30 nm.

Figure 3. Current efficiency and EQE of the QLEDs as a function of a) Poly-TPD layer thickness (ZnO and QD layer thicknesses were kept constant at 40 nm and 30 nm, respectively), b) ZnO layer thickness thickness (Poly-TPD and QD layer thicknesses were kept constant at 30 nm and 30 nm, respectively), c) QD layer thickness (Poly-TPD and ZnO layer thicknesses were kept constant at 30 nm and 40 nm, respectively). Error bars represent the measurement variations for the average of 3 different devices. Different thicknesses were achieved by changing the spin speed (details are provided in Methods section).

Fig. 4a presents current density-luminance-voltage characteristics of the optimized QLED devices. We observe a low 2.8 V turn-on voltage, which is defined as the voltage at which luminance reaches 1 cd/m2, due to efficient band alignment with 0.53 eV offset at HTL-QD interface and 0.38 eV offset at QD-ETL interface (Figure S3), which can be easily overcome by electrons and holes under forward bias. Also, unlike type-I core/shell QDs for which electrons and holes need to overcome the energy barrier of the shell to reach the core, type-II QDs do not constitute an energy barrier for one type of charge (electrons in our case), which is another reason for achieving low turn-on voltage in our QLEDs. In Fig. 4a, maximum luminance reaches up to 600 cd/m2 which, to the best of our knowledge, is the second highest luminance up to date among red InP based QLEDs.20, 21 This is mainly due to efficient charge injection and recombination with optimized layer thicknesses and low PL quenching of type-II QDs 7 ACS Paragon Plus Environment

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

when they are condensed into solid film state. The maximum luminance can further be improved by increasing the in-solution PL QY of InP/ZnO QD structures.

Figure 4. a) Current density-voltage-luminance (JVL) characteristics of the QLEDs (JVL characteristics of QLEDs with InP core, InP/1ZnO and InP/2ZnO can be seen in Fig. S6). b) Current efficiency and EQE of the QLEDs as a function luminance.

Current efficiency and EQE of the QLEDs as a function of luminance are seen in Fig. 4b. Highest current efficiency is achieved with 1 cd/A at a luminance of 300 cd/m2. This luminance value is at convenient levels for display applications, which typically require 102-103 cd/m2 luminance, but it needs to be increased for lighting applications, which require 103-104 cd/m2 luminance.16 After that point, although luminance keeps increasing up to 600 cd/m2, current efficiency decreases due to parasitic emission from Poly-TPD. EQE is one of the most crucial EL characteristics for QLEDs. It is defined as the ratio of the number of the photons emitted by the device per unit time to the number of electrons that are injected per unit time, and formulized as: 16

𝐸𝑄𝐸 = 𝜂𝑟 𝜒 𝜂𝑃𝐿 𝜂𝑂𝐶 (1) where 𝜂𝑟 is the fraction of injected charges that form excitons in active layer (charge carrier balance factor), 𝜒 is the fraction of total excitons whose states have spin-allowed transitions, 𝜂𝑃𝐿 is the PL QY of the active layer material and 𝜂𝑂𝐶 is the fraction of emitted photons that are coupled out of device (light out-coupling efficiency). Due to the structural similarity between QLEDs and OLEDs, we expect our devices to result in a similar out-coupling efficiency value. It is known that in planar devices approximately 50% of the emitted light is generally lost in waveguided and plasmon modes. Other 25% is trapped in the substrate due to total internal reflection and around 5% is absorbed. The remaining 20% of the emitted light is the out-coupling fraction, 𝜂𝑂𝐶.42-44 PL QY of the active layer is 20% in our QLEDs. The value of 𝜒 is 25% and 75% for fluorescent and phosphorescent materials, respectively, and it can essentially be increased for fluorescent materials by certain mechanisms such as thermally activated delayed fluorescence. For QDs, transitions from dark to bright band-edge excitonic state 8 ACS Paragon Plus Environment

Page 8 of 16

Page 9 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

frequently take place even at room temperature due to the small energetic barrier between dark and bright excitonic states, which is on the order of 8-10 meV for InP.45, 46 Thus, the value of 𝜒 for QDs approaches 1 owing to the efficient crossings of excitons from dark states, in which spin-forbidden transitions to ground state occur, to bright states, where spin-allowed transitions to ground state take place. Based on these rational assumptions (i.e., 𝜒 is nearly 1 and 𝜂𝑂𝐶 is nearly 20%), and considering the maximum 0.53% EQE of our QLEDs, we obtain the value of 𝜂𝑟 around 20%. In principle, it is possible to achieve values close to 1 for charge carrier balance factor, and in fact, there are some studies that get very close to that.47, 48 However, for our devices, even though the layer thicknesses are optimized, we think there are two main factors that prevent the achievement of such high charge carrier balance factor: i) electron delocalization to shells decreases the probability of electron and hole wavefunction overlap, ii) high dielectric constant of InP reduces the strength of attractions between opposite charges.

Figure 5. a) Electroluminescence spectra of the QLEDs at different biases. b) CIE coordinates of the emission at 6V, 9V and 12V.

The electroluminescence spectra of the QLEDs at operation voltages of 4V, 6V, 9V and 12V are shown in Fig. 5a. The emission peak wavelength is 585 nm and full width at half maximum (FWHM) is around 90 nm. We observe a weak emission in blue-wavelength region starting from 9V forward bias, which is originating from Poly-TPD layer. This blue emission can be ascribed to the small number of electrons attaining sufficient energy to overcome the energetic barrier at Poly-TPD-QD interface. As a result, this leaky emission leads to a widening in the lower wavelength region of the EL spectrum. At low voltages up to 8V, our devices exhibit saturated and pure colors with (x, y) tristimulus coordinates of (0.52, 0.47) as demonstrated by the Commission Internationale de l’Eclairage (CIE) chromaticity diagram shown in


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 5b. In the same figure, we also observe that with increasing voltage, appearance of leaky blue emission causes a shift in the color coordinates and affects color saturation adversely. CONCLUSION In summary, we presented the application of environmentally benign, biocompatible, type-II core/shell InP/ZnO QDs to LEDs for the first time. Investigation of PL QY in solution and in film showed that the efficiency loss of QDs when they are transferred into film is exceptionally low owing to the ability of low reabsorption and energy transfer losses because of multiple shells on the core. We determined that the optimum choice for LED application is InP/1ZnO, which is InP core with 1 ML ZnO coverage, due to its high efficiency in solid state. Integrating InP/1ZnO as emissive layer into the fully solution processed LED architecture, our devices displayed low turn-on voltage of 2.8 V and saturated emission color with promising performance levels up to current efficiency of 1 cd/A and EQE of 0.53%. This study presents a new opportunity for the ability of engineering non-toxic QDs for minimizing solid-state efficiency loss by forming shells owing to type-II band alignment, and demonstrates luminance levels, which is suitable for display applications. Moreover, engineering toxic-content-free QDs at nanoscale in new architectures offers a significant room for further progress. Therefore, the results point to a novel direction in using eco-friendly quantum dots for light-emitting devices. METHODS Synthesis of Quantum Dots (QDs): InP cores were synthesized using our previous report.24 For the ZnO shell formation, 1060 μL (For InP/1ZnO) and 2120 μL (For InP/2ZnO) of the prepared zinc oxide stock solution which contains 0.1 mmol Zn(acac)2, 32 μL of oleic acid, 1 mL of oleylamine and 6 ml 1Octadecene was added to the indium phosphide core solution at 60 C. Then, solution was heated up to 280 °C and stirred for 20 min under N2 atmosphere. The final QDs was purified by centrifugation at 6000 rpm for 15 min then dispersed in toluene and kept in 5 C. Biocompatibility of the QDs were investigated in our previous report.30 Synthesis of ZnO Nanoparticles: ZnO nanoparticles were synthesized using a previously reported method.39 In a typical synthesis, 20 ml solution of tetramethylammonium hydroxide (TMAH) dissolved in ethanol (0.55 M) was dropwisely added (1 ml/min) to a 30 ml solution of zinc acetate dihydrate dissolved in dimethyl sulfoxide (DMSO) (0.5 M). The solution was stirred vigorously for 1h at room temperature. Afterwards, it was washed with toluene and re-dispersed in ethanol at a concentration of 30 mg/ml. Device Fabrication: The QLEDs were fabricated on glass substrates covered with patterned indium tin oxide (ITO). The substrates were first cleaned with detergent solution, acetone, DI water and iso10 ACS Paragon Plus Environment

Page 10 of 16

Page 11 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

propanol, consecutively, for 15 min each, and then treated with UV generated ozone for 15 min to increase the work function of ITO and further eliminate the residues on the ITO surface. PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) solution (filtered through a 0.45 um filter) was spin coated onto the ITO substrates at 4000 rpm for 45 s, and baked at 150°C for 15 min under ambient conditions. The PEDOT:PSS-coated substrates were transferred into a nitrogen-filled glove box for spin-coating of the poly(N,N9-bis-(4-butylphenyl)-N,N9-bis(phenyl)-benzidine) (poly-TPD) layer. Poly-TPD, used as hole transport layer (HTL) (8 mg/ml in chlorobenzene), was spin coated for 40 s and the spin speed was varied through 1000 rpm, 2000 rpm, 3000 rpm, 4000 rpm to obtain thicknesses of 60 nm, 50 nm, 40 nm and 30 nm respectively; followed by baking at 130°C for 30 min. After that InP/ZnO QD solution (10 mg/ml in toluene) was spin coated for 45 s and the spin speed was varied through 500 rpm, 1000 rpm, 2000 rpm and 4000 rpm to obtain thicknesses of 50 nm, 40 nm, 30 nm and 20 nm, respectively; and baked at 70°C for 20 min. For the deposition of electron transport layer (ETL), ZnO nanoparticles solution in ethanol with a concentration of 30 mg/ml was spin coated for 60 s, and the spin speed was varied through 1000 rpm, 2500 rpm, 4500 rpm and 6000 rpm to obtain thicknesses of 50 nm, 40 nm, 30 nm, and 20 nm, respectively; and baked at 90°C for 30 min. Finally, the top Al cathode layer (100 nm) was thermally deposited under a based pressure of 1 x 10-6 Pa. Devices were encapsulated before taking the measurements. Instrumentation and characterization. UV/Visible absorption and photoluminescence spectra of QDs were performed with Edinburgh Instruments Spectrofluorometer FS5 which includes a 150 W Xenon lamp combined with an excitation monochromator. The excitation wavelength was 375 nm with a band pass filter that has 2 nm full width at half maximum (FWHM). Emission detector was a single photon counting photomultiplier tube (R928P) with 2 nm spectral width. Absolute fluorescence quantum yield values were measured by placing the measurement module, which contains an integrating sphere with inner diameter of 150 mm, into FS5 system. Layer thicknesses are characterized using Dimension Icon Bruker AFM device. Current density-voltage-luminance (J-V-L) characteristics of QLEDs were carried out using a Keithley 2400 voltage-current source unit together with JETI Specbos 1211 Spectro-radiometer. Current efficiency was then calculated by multiplying the luminance values with the device area; and dividing it with the corresponding current values obtained from current-voltage (IV) data. External quantum efficiency (EQE) values were calculated based on a recommended method in literature49, using a calibrated silicon photodetector (Newport 818-UV) with a known responsivity. QLEDs were placed close to photodetector and the edges of the devices were covered with black tape to mask emerging photons from the edges of the substrates. EQE values were then inferred from the photocurrent of the detector. Electroluminescence (EL) spectrum and color coordinates of the devices were obtained using Labsphere integrating sphere. PL decays were taken 11 ACS Paragon Plus Environment

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

by PicoQuant MicroTime 100 time-resolved confocal fluorescence microscope. The samples were excited by an 8 mW picosecond diode laser (λexc = 375 nm) pulsed at 60 MHz repetition rate to 40X objective lens. PL decays were fit by a two-exponential decay and the average life-time (τavg) was calculated from an amplitude weighted mean28 (Details can be seen in Table S1). Ultraviolet photoelectron spectroscopy measurements were performed using Specs FlexMod system. Supporting Information Ultraviolet Photoelectron Spectroscopy measurements of InP/ZnO QDs and ZnO nanoparticles, Tauc plots of InP core, InP/1ZnO, InP/2ZnO QDs, PL decay dynamics of InP, InP/1ZnO, InP/2ZnO thin films, JVL characteristics of QLEDs with emissive layer as InP core, InP/1ZnO, InP/2ZnO QDs. Corresponding author *Email: [email protected] Notes The authors declare no competing financial interest. Acknowledgements This project has received funding from Technological Research Council of Turkey (TUBITAK) with Project No. 115E242 and 115E115. SN also acknowledges the support by Turkish Academy of Science and Science Academy. We thank KUYTAM (Koç University Surface Science and Technology Center) for AFM infrastructure and Ataturk University DAYTAM for UPS infrastructure. The authors gratefully acknowledge Dr. Amir Motallebzadeh for AFM measurements and Ahmet Emre Kasapoglu for UPS measurements. References (1) Balde, C. P.; Forti, V.; Gray, V.; Kuehr, R.; Stegmann, P., The global e-waste monitor 2017: Quantities, flows and resources. United Nations University, International Telecommunication Union, and International Solid Waste Association: 2017. (2) Light's labour's lost : policies for energy-efficient lighting. Waide, P.; Tanishima, S., Eds. OECD :: Paris :, 2006. (3) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P., Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer. Nature 1994, 370, 354. (4) Coe, S.; Woo, W.-K.; Bawendi, M.; Bulović, V., Electroluminescence from single monolayers of nanocrystals in molecular organic devices. Nature 2002, 420, 800. (5) Shen, H.; Cao, W.; Shewmon, N. T.; Yang, C.; Li, L. S.; Xue, J., High-efficiency, low turn-on voltage blue-violet quantum-dot-based light-emitting diodes. Nano Lett. 2015, 15, 1211-6. (6) Caruge, J. M.; Halpert, J. E.; Wood, V.; Bulović, V.; Bawendi, M. G., Colloidal quantum-dot lightemitting diodes with metal-oxide charge transport layers. Nat. Photonics 2008, 2, 247.


Page 12 of 16

Page 13 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

(7) Zhang, Y.; Xie, C.; Su, H.; Liu, J.; Pickering, S.; Wang, Y.; Yu, W. W.; Wang, J.; Wang, Y.; Hahm, J. I.; Dellas, N.; Mohney, S. E.; Xu, J., Employing heavy metal-free colloidal quantum dots in solutionprocessed white light-emitting diodes. Nano Lett. 2011, 11, 329-32. (8) Nizamoglu, S.; Mutlugun, E.; Özel, T.; Demir, H. V.; Sapra, S.; Gaponik, N.; Eychmüller, A., Dualcolor emitting quantum-dot-quantum-well CdSe-ZnS heteronanocrystals hybridized on InGaN∕GaN light emitting diodes for high-quality white light generation. Appl. Phys. Lett. 2008, 92. (9) Alivisatos, A. P., Semiconductor Clusters, Nanocrystals, and Quantum Dots. Science 1996, 271, 933-937. (10) Kim, S.; Kim, T.; Kang, M.; Kwak, S. K.; Yoo, T. W.; Park, L. S.; Yang, I.; Hwang, S.; Lee, J. E.; Kim, S. K.; Kim, S. W., Highly luminescent InP/GaP/ZnS nanocrystals and their application to white lightemitting diodes. J. Am. Chem. Soc. 2012, 134, 3804-9. (11) Qian, G.; Lin, Y.; Wantz, G.; Davis, A. R.; Carter, K. R.; Watkins, J. J., Saturated and Multi-Colored Electroluminescence from Quantum Dots Based Light Emitting Electrochemical Cells. Adv. Funct. Mater. 2014, 24, 4484-4490. (12) Sadeghi, S.; Ganesh Kumar, B.; Melikov, R.; Mohammadi Aria, M.; Bahmani Jalali, H.; Nizamoglu, S., Quantum dot white LEDs with high luminous efficiency. Optica 2018, 5. (13) Mashford, B. S.; Stevenson, M.; Popovic, Z.; Hamilton, C.; Zhou, Z.; Breen, C.; Steckel, J.; Bulovic, V.; Bawendi, M.; Coe-Sullivan, S.; Kazlas, P. T., High-efficiency quantum-dot light-emitting devices with enhanced charge injection. Nat. Photonics 2013, 7, 407-412. (14) Yang, Y.; Zheng, Y.; Cao, W.; Titov, A.; Hyvonen, J.; Manders, J. R.; Xue, J.; Holloway, P. H.; Qian, L., High-efficiency light-emitting devices based on quantum dots with tailored nanostructures. Nat. Photonics 2015, 9, 259-266. (15) Dai, X.; Zhang, Z.; Jin, Y.; Niu, Y.; Cao, H.; Liang, X.; Chen, L.; Wang, J.; Peng, X., Solutionprocessed, high-performance light-emitting diodes based on quantum dots. Nature 2014, 515, 96. (16) Shirasaki, Y.; Supran, G. J.; Bawendi, M. G.; Bulović, V., Emergence of colloidal quantum-dot light-emitting technologies. Nat. Photonics 2013, 7, 13-23. (17) Panfil, Y. E.; Oded, M.; Banin, U., Colloidal Quantum Nanostructures: Emerging Materials for Display Applications. Angew. Chem. Int. Ed. Engl. 2018, 57, 4274-4295. (18) Wang, H. C.; Zhang, H.; Chen, H. Y.; Yeh, H. C.; Tseng, M. R.; Chung, R. J.; Chen, S.; Liu, R. S., Cadmium-Free InP/ZnSeS/ZnS Heterostructure-Based Quantum Dot Light-Emitting Diodes with a ZnMgO Electron Transport Layer and a Brightness of Over 10 000 cd m(-2). Small 2017, 13. (19) Lim, J.; Park, M.; Bae, W. K.; Lee, D.; Lee, S.; Lee, C.; Char, K., Highly Efficient Cadmium-Free Quantum Dot Light-Emitting Diodes Enabled by the Direct Formation of Excitons within InP@ZnSeS Quantum Dots. ACS Nano 2013, 7, 9019-9026. (20) Jo, J. H.; Kim, J. H.; Lee, K. H.; Han, C. Y.; Jang, E. P.; Do, Y. R.; Yang, H., High-efficiency red electroluminescent device based on multishelled InP quantum dots. Opt. Lett. 2016, 41, 3984-7. (21) Kim, H. Y.; Park, Y. J.; Kim, J.; Han, C. J.; Lee, J.; Kim, Y.; Greco, T.; Ippen, C.; Wedel, A.; Ju, B.-K.; Oh, M. S., Transparent InP Quantum Dot Light-Emitting Diodes with ZrO2Electron Transport Layer and Indium Zinc Oxide Top Electrode. Adv. Funct. Mater. 2016, 26, 3454-3461. (22) Lin, Q.; Song, B.; Wang, H.; Zhang, F.; Chen, F.; Wang, L.; Li, L. S.; Guo, F.; Shen, H., Highefficiency deep-red quantum-dot light-emitting diodes with type-II CdSe/CdTe core/shell quantum dots as emissive layers. J. Mater. Chem. C 2016, 4, 7223-7229. (23) Jin, X.; Li, H.; Huang, S.; Gu, X.; Shen, H.; Li, D.; Zhang, X.; Zhang, Q.; Li, F.; Li, Q., Bright alloy typeII quantum dots and their application to light-emitting diodes. J. Colloid Interface Sci. 2018, 510, 376383. (24) Sadeghi, S.; Bahmani Jalali, H.; Melikov, R.; Ganesh Kumar, B.; Mohammadi Aria, M.; Ow-Yang, C. W.; Nizamoglu, S., Stokes-Shift-Engineered Indium Phosphide Quantum Dots for Efficient Luminescent Solar Concentrators. ACS Appl. Mater. Interfaces 2018, 10, 12975-12982. (25) Coropceanu, I.; Bawendi, M. G., Core/Shell Quantum Dot Based Luminescent Solar Concentrators with Reduced Reabsorption and Enhanced Efficiency. Nano Lett. 2014, 14, 4097-4101. (26) Meinardi, F.; Colombo, A.; Velizhanin, K. A.; Simonutti, R.; Lorenzon, M.; Beverina, L.; Viswanatha, R.; Klimov, V. I.; Brovelli, S., Large-area luminescent solar concentrators based on 13 ACS Paragon Plus Environment

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

‘Stokes-shift-engineered’ nanocrystals in a mass-polymerized PMMA matrix. Nat. Photonics 2014, 8, 392. (27) Pal, B. N.; Ghosh, Y.; Brovelli, S.; Laocharoensuk, R.; Klimov, V. I.; Hollingsworth, J. A.; Htoon, H., 'Giant' CdSe/CdS core/shell nanocrystal quantum dots as efficient electroluminescent materials: strong influence of shell thickness on light-emitting diode performance. Nano. Lett. 2012, 12, 331-6. (28) Bahmani Jalali, H.; Melikov, R.; Sadeghi, S.; Nizamoğlu, S., Excitonic Energy Transfer within InP/ZnS Quantum Dot Langmuir− Blodgett Assemblies. J. Phys. Chem. C 2018. (29) Kagan, C. R.; Lifshitz, E.; Sargent, E. H.; Talapin, D. V., Building devices from colloidal quantum dots. Science 2016, 353. (30) Bahmani Jalali, H.; Mohammadi Aria, M.; Dikbas, U. M.; Sadeghi, S.; Ganesh Kumar, B.; Sahin, M.; Kavakli, I. H.; Ow-Yang, C. W.; Nizamoglu, S., Effective Neural Photostimulation Using Indium-Based Type-II Quantum Dots. ACS Nano 2018, 12, 8104-8114. (31) Brovelli, S.; Schaller, R. D.; Crooker, S. A.; Garcia-Santamaria, F.; Chen, Y.; Viswanatha, R.; Hollingsworth, J. A.; Htoon, H.; Klimov, V. I., Nano-engineered electron-hole exchange interaction controls exciton dynamics in core-shell semiconductor nanocrystals. Nat. Commun. 2011, 2, 280. (32) Garcia-Santamaria, F.; Brovelli, S.; Viswanatha, R.; Hollingsworth, J. A.; Htoon, H.; Crooker, S. A.; Klimov, V. I., Breakdown of volume scaling in Auger recombination in CdSe/CdS heteronanocrystals: the role of the core-shell interface. Nano Lett. 2011, 11, 687-93. (33) Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H., Highly Luminescent Monodisperse CdSe and CdSe/ZnS Nanocrystals Synthesized in a Hexadecylamine−Trioctylphosphine Oxide−Trioctylphospine Mixture. Nano Lett. 2001, 1, 207-211. (34) Peng, X.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P., Epitaxial Growth of Highly Luminescent CdSe/CdS Core/Shell Nanocrystals with Photostability and Electronic Accessibility. J. Am. Chem. Soc. 1997, 119, 7019-7029. (35) Kumar, B. G.; Sadeghi, S.; Melikov, R.; Aria, M. M.; Jalali, H. B.; Ow-Yang, C. W.; Nizamoglu, S., Structural control of InP/ZnS core/shell quantum dots enables high-quality white LEDs. Nanotechnology 2018, 29, 345605. (36) Wood, V.; Bulović, V., Colloidal quantum dot light-emitting devices. Nano Rev. 2010, 1, 10.3402/nano.v1i0.5202. (37) Park, N.-G.; Grätzel, M.; Miyasaka, T., Organic-inorganic halide perovskite photovoltaics : from fundamentals to device architectures. 2016. (38) Kim, H. H.; Park, S.; Yi, Y.; Son, D. I.; Park, C.; Hwang, D. K.; Choi, W. K., Inverted Quantum Dot Light Emitting Diodes using Polyethylenimine ethoxylated modified ZnO. Sci. Rep. 2015, 5, 8968. (39) Qian, L.; Zheng, Y.; Xue, J.; Holloway, P. H., Stable and efficient quantum-dot light-emitting diodes based on solution-processed multilayer structures. Nat. Photonics 2011, 5, 543-548. (40) Sun, J. W.; Lee, J. H.; Moon, C. K.; Kim, K. H.; Shin, H.; Kim, J. J., A fluorescent organic lightemitting diode with 30% external quantum efficiency. Adv. Mater. 2014, 26, 5684-8. (41) Sun, Q.; Wang, Y. A.; Li, L. S.; Wang, D.; Zhu, T.; Xu, J.; Yang, C.; Li, Y., Bright, multicoloured lightemitting diodes based on quantum dots. Nat. Photonics 2007, 1, 717-722. (42) Brütting, W.; Frischeisen, J.; Schmidt, T. D.; Scholz, B. J.; Mayr, C., Device efficiency of organic light-emitting diodes: Progress by improved light outcoupling. Phys. Status Solidi A 2013, 210, 44-65. (43) Greenham, N. C.; Friend, R. H.; Bradley, D. D. C., Angular Dependence of the Emission from a Conjugated Polymer Light-Emitting Diode: Implications for efficiency calculations. Adv. Mater. 1994, 6, 491-494. (44) Lu, M. H.; Sturm, J. C., Optimization of external coupling and light emission in organic lightemitting devices: modeling and experiment. J. Appl. Phys. 2002, 91, 595-604. (45) Fu, H.; Zunger, A., InP quantum dots: Electronic structure, surface effects, and the redshifted emission. Phys. Rev. B 1997, 56, 1496-1508. (46) Efros, A. L.; Rosen, M.; Kuno, M.; Nirmal, M.; Norris, D. J.; Bawendi, M., Band-edge exciton in quantum dots of semiconductors with a degenerate valence band: Dark and bright exciton states. Phys. Rev. B 1996, 54, 4843-4856. 14 ACS Paragon Plus Environment

Page 14 of 16

Page 15 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

(47) Konstantatos, G.; Huang, C.; Levina, L.; Lu, Z.; Sargent, E. H., Efficient Infrared Electroluminescent Devices Using Solution-Processed Colloidal Quantum Dots. Adv. Funct. Mater. 2005, 15, 1865-1869. (48) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R., Nearly 100% internal phosphorescence efficiency in an organic light-emitting device. J. Appl. Phys. 2001, 90, 5048-5051. (49) Forrest, S. R.; Bradley, D. D. C.; Thompson, M. E., Measuring the Efficiency of Organic LightEmitting Devices. Adv. Mater. 2003, 15, 1043-1048.


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

For Table of Contents Use Only

Light-Emitting Devices Based on Type-II InP/ZnO Quantum Dots Authors Onuralp Karatum, Houman Bahmani Jalali, Sadra Sadeghi, Rustamzhon Melikov, Shashi Bhushan Srivastava and Sedat Nizamoglu* Light-emitting devices fabricated with biocompatible and non-toxic InP/ZnO quantum dots as emissive layer results in bright and saturated color devices, which might be used for display applications. The structure presented in here might be an alternative to toxic-heavy-metal containing QLEDs in the recent industrial products.


Page 16 of 16

ZnO Quantum Dots - ACS

Recommend Documents