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Graphene-based buffer layers for light-emitting diodes
Graphene-based buffer layers for light-emitting diodes
6
Quyet Van Le 1 , Soo Young Kim 2 1 Institute of Research and Development, Duy Tan University, Da Nang, Vietnam; 2 Department of Materials Science and Engineering, Korea University, Seongbuk-gu, Seoul, Republic of Korea
6.1
Introduction
Buffer layers play a crucial role in light emitting diodes (LEDs), promoting charge injection by reducing the energy barrier between the electrodes and the active layers. The frequently used electrode, indium tin oxide (ITO), exhibits a work function (WF) of 4.1e4.7 eV; this is much lower than the ionization energy of organic materials, reducing the possible device performance [1]. To overcome this challenge, a thin layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (WF w5.2 eV) is sandwiched between the ITO and emissive layers. However, the use of PEDOT:PSS as a hole injection layer (HIL) severely reduces the stability of LED devices as a result of its highly acidic and hygroscopic nature. Recently, a number of inorganic materials have been effectively employed to replace PEDOT:PSS; examples are MoO3 [2], V2O5 [3], WO3 [4], WS2 [5e8], MoS2 [5,9,10], TaS2 [5,11], and graphene oxide (GO) [12e14]. Among these, GO appears to be the most promising as a result of its tunable WF, high stability, high charge-carrier mobility, low cost, and solution processability. This chapter reports on current progress in the application of GO as an HIL in LEDs. First, the structure, electrical, and optical properties are described. Next, the use of GO as the buffer layer in various LEDs, including organic LEDs (OLEDs), polymer LEDs (PLEDs), and quantum dot LEDs (QLEDs) is reviewed. The effect of the structure and thickness of the GO on the performance of the LEDs as well as the HIL mechanism of GO are clarified. In addition, the combination of GO with other materials to form composite HILs is discussed.
6.2 6.2.1
Graphene oxide buffer layer Structures, properties, and synthesis
GO is generally derived from graphite through an oxidation and exfoliation process in an organic solvent or water [15,16]. GO contains various functional groups such as hydroxyl, epoxy, carboxy, carbonyl, phenol, lactone, and quinine [17]. However, the precise structure of GO is still under debate because of its complexity [15]. Several structural models of GO have been proposed, as shown in Fig. 6.1aee [18]. Hofmann Graphene for Flexible Lighting and Displays. https://doi.org/10.1016/B978-0-08-102482-9.00006-X Copyright © 2020 Elsevier Ltd. All rights reserved.
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Figure 6.1 Previous structure models of GO: (a) Hofmann, (b) Ruess, (c) Scholz-Boehm, (d) Nakajima-Matsuo, (e) Lerf-Klinowski. (f) Aberration-corrected TEM image of a single sheet of suspended GO. The scale bar is 2 nm. Expanded view (A) shows (left to right) a 1 nm2 enlarged oxidized region of the material; a proposed atomic structure for this region (carbon atoms in gray and oxygen atoms in red); the average of simulated TEM images (see Supporting Information) of the proposed structure and of another structure where the position of oxidative functionalities has been changed. Expanded view (B) focuses on the white spot in the graphitic region. This spot moved within the graphitic region but was stationary for three frames (6 s) at a hydroxyl position (left portion of expanded view (B)) and for seven frames (14 s) at a (1,2) epoxy position (right portion of expanded view (B)). The ball-and-stick figures below the microscopy images represent the proposed atomic structure for such functionalities. The simulated TEM image for the suggested structure (see Supporting Information) agrees well with the TEM data. Expanded view (C) shows a 1 nm2 graphitic portion using an exit-plane wave reconstruction of a focal series of GO images and the atomic structure in this region. (e) Reproduced with permission T. Szabo, O. Berkesi, P. Forg o, K. Josepovits, Y. Sanakis, D. Petridis, I. Dékany, Evolution of surface functional groups in a series of progressively oxidized graphite oxides. Chem. Mater. 18 (2006) 2740e2749 Copyright 2006 American Chemical Society. (f) Reproduced with permission K. Erickson, R. Erni, Z. Lee, N. Alem, W. Gannett, A. Zettl, Determination of the local chemical structure of graphene oxide and reduced graphene oxide. Adv. Mater. 22 (2010) 4467e4472, Copyright 2010 Wiley-VCH.
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and Holst’s structure is composed of epoxy functional groups that are widely distributed on a basal plane of graphite having an sp2 hybridized configuration, whereas Ruess’s consists of hydroxyl groups and an sp3 hybrid system. Later, Scholz and Boehm proposed a model with a corrugated backbone containing regular quinoidal species instead of epoxide and ether groups [15]. Nakajima and Matsuo presented a model with a stage 2 graphite intercalation based on poly(carbon monofluoride), (CF2)n [19]. More recently, based on NMR studies, Lerf and Klinowski introduced a widely used model that describes GO as a random distribution of flat aromatic regions (unoxidized benzene rings) and wrinkled regions of alicyclic 6-membered rings [20]. The atomic-scale features of GO were recorded for the first time by Erickson et al., using high-resolution transmission electron microscopy (HRTEM), as shown in Fig. 6.1f [21]. It can be observed from the high contrast HRTEM image that GO contains three different types of region: disordered regions, holes, and graphitic regions. It was proposed by the authors that the holes which appeared on the GO sheets originated from oxidation and exfoliation, releasing CO and CO2, whereas the graphitic region results from the incomplete oxidation of the basal plane. It was also suggested that the disordered regions of the basal plane originate in the high-density oxygen-containing functional groups. GO exhibits superior thermal, mechanical, electrical, and optical properties as a result of its unique 2D structure, which is embedded with various functional groups. The electrical properties of GO significantly depend on its structure and level of oxidization. Specifically, GO is an insulator with a large energy bandgap and high Rs (up to 12 U/sq) arising from a large degree of sp3 hybridization [22e24]. However, the sheet resistance of GO can be decreased through a thermal or chemical reduction treatment [25,26]. As a result, GO can be tuned from an insulator to a semiconductor or a graphene-like semimetal [22,27e32]. A typical demonstration of the tunableresistance behavior of GO is shown in Fig. 6.2aed. The optical properties of GO have also been intensively studied [33e35]. It was discovered that GO is fluorescent over a wide range of wavelengths, from the ultraviolet through the visible and into the near-infrared region, as a result of heterogeneous atomic and electronic structures [36]. Differing from other semiconductors, where the fluorescence results from band-edge transitions, in GO it originates from recombination of electronehole pairs in localized electronic states in which sp2 domains are isolated within the carboneoxygen sp3 matrix [37]. The relation between fluorescence, conductivity, and absorbance and GO reduction, which was reported by Eda et al., is shown in Fig. 6.2eeg [37]; the level of GO reduction was controlled through the exposure time to hydrazine vapor. The IeV characteristics of GO-based devices for various reduction times are shown in Fig. 6.2e. The low bias current observed in 2 mine reduced GO is drastically increased with 5 min reduction, indicating a significant change in conductivity. The insulating properties of 2 minereduced GO are further confirmed in Fig. 6.2f. From Fig. 6.2g, it can be seen that the absorption and PL are
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(c)
(b)
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1011
(d)
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Au/Ti SiO2 Si
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Figure 6.2 Thermal reduction of graphene oxide field effect devices. (a, b, c) Optical images of graphene oxide single sheets prepared with metal leads for resistance measurements. (a) Electrical circuit diagram, showing the Si substrate as a back-gate electrode and including a mini hot-plate with incorporated heater and thermometer. (d) The graphene oxide sample
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markedly increased after around 20 s of exposure as a result of the increased number of localized sp2 sites. It should be noted that the energetic coupling between these sites remains small in the first stage of reduction. However, in the later stages (after 5 min of exposure), the PL of GO is abruptly reduced by the increased interconnectivity between sp2 sites; this facilitates the hopping of excitons to nonradiative combination centers, resulting in PL quenching. Graphite powder is the most common starting material for synthesizing GO. The conversion of graphite powder to GO can be carried out by various techniques, such as Brodie’s [38], Hofmann’s [39], and Hummers’s methods [40]. Briefly, the graphite power is chemically reacted with a strong acid such as HNO3, H2SO4, or HCl, with subsequent intercalation employing an alkali metal compound such as KClO3, KMnO4, or NaNO3. Fig. 6.3 illustrates the oxidation and intercalation processes used to obtain GO from graphite, based on the Hummers’s method [41].
6.2.2
GO buffer layer in optoelectronics
The use of GO as a buffer layer in optoelectronic devices was presented by Li et al. in 2010 [42]. Typically, a thin layer of GO is sandwiched between ITO and an active layer consisting of a blend of poly(3-hexylthiophene) and the fullerene derivative, phenyl-C61-butyric acid methyl ester, in an organic solar cell. The use of GO as an interlayer drastically increases the power conversion efficiency of the solar cell from 1.8% to 3.5%, which is then comparable to that of a PEDOT:PSS-based device (3.6%), and indicates a role for GO as a buffer layer. In 2011, Zhong et al. successfully incorporated GO as a HIL in OLEDs [43]. Specifically, the GO-based OLEDs showed a current efficiency of 23 cd A1 and a power efficiency of 14 lm W1, which are significantly higher than those of PEDOT:PSS-based devices (15 cd A1 and 11 lm W1, respectively). The device structure and device characteristic of the GO-based OLED are shown in Fig. 6.4bef. To avoid the formation of dark spots and pixel shrinkage over time caused by absorbed water on the GO, as well as by restacking
=
resistance (red solid curve; left-axis) and temperature profile (dotted blue; right-axis), as functions of time. The resistance at point A (temperature 172 C, point 1 in the first heating cycle) is equal to that at point B (125 C) in the second heating cycle. A similar behavior was observed in each subsequent cycle, with the three sample resistances (for the same temperature, roughly 125 C) forming a decreasing sequence. The resistance sequence is indicated by the three dotted horizontal arrows; temperatures are indicated by boxes on the heating curve. (e) IeV and (f) transfer characteristics of individual GO sheet devices at different stages of reduction (duration of exposure to hydrazine is noted in the legend). (g) Summary plot for a GO thin film, showing the maximum PL intensity (circles; left axis), absorbance at 550 nm (triangles; right axis), and current at 1 V (squares; right axis), as functions of reduction time. (d) Reproduced with permission I. Jung, D.A. Dikin, R.D. Piner, R.S. Ruoff, Tunable electrical conductivity of individual graphene oxide sheets reduced at “low” temperatures, Nano Lett. 8 (2008) 4283e4287, Copyright 2008 American Chemical Society. (g) Reproduced with permission G. Eda, Y.Y. Lin, C. Mattevi, H. Yamaguchi, H.A. Chen, I.S. Chen, C.W. Chen, M. Chhowalla, Blue photoluminescence from chemically derived graphene oxide, Adv. Mater. 22 (2010) 505e509, Copyright 2010 Wiley-VCH.
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(I)
(III)
35ºC /3h
0–5ºC /1.5h 10g graphite
(II) 5g K2FeO4
6g K2FeO4, 4g KMnO4 100mL H2SO4
250mL H2O
95ºC /30min : H2SO4
: KMnO4
: K2FeO4
: O∙
Figure 6.3 Illustration of GO preparation, based on a newly improved Hummers’s method. Reproduced with permission H. Yu, B. Zhang, C. Bulin, R. Li, R. Xing, High-efficient synthesis of graphene oxide based on improved Hummers method, Sci. Rep. 6 (2016) 36143, Copyright 2016 Nature Publishing Group.
of the GO sheets during the preparation process, a surface grafting with 4-(octoxy)benzenediazonium tetrafluoroborate was applied. The phenylated GO (P-GO) was achieved by simply treating GO with 4-(octoxy)benzenediazonium tetrafluoroborate) for 10e60 min. It should be noted that the functionalization of GO depends significantly on the reaction time. The solubility of P-GO in dimethylformamide increases from 0.6 to 1.0 and 1.8 mg mL1 as the reaction time is increased from 10 to 30 and 60 min, respectively. A single layer of P-GO can be easily produced by spin coating (Fig. 6.4a). This research also shows that the two-layer P-GOebased device exhibits better performance than the one-layer device. The mechanism by which a GO HIL improves the device was later discovered by Lee et al. in 2012 [44]. The structure and energy band alignment of a device employing poly(phenylvinylene): super yellow (SY), (Merck Co., Mw ¼ 1,950,000 g mol1) as an emissive layer (thickness 150 nm) is displayed in Fig. 6.5a. It is reported that GO containing epoxy and epoxy-hydroxyl groups, which disturb the sp2 conjugation in the hexagonal plane, is an insulator with a large optical bandgap of 3.6 eV [42]. To clarify the role of GO, reduced GO (rGO) and rGO-based devices were also fabricated. The removal of the functional groups in rGO effectively restores the sp2 conjugation and thereby reduces the optical bandgap to 1.15 eV [45]. The device performance is optimized with a GO thickness of 4.3 nm, showing a maximum luminance (Lmax) and an external quantum efficiency (EQE) of 39,000 cd m2 and 6.7%, respectively, which are superior to those of a PEDOT:PSS-based device (Lmax ¼ 33,800; EQE ¼ 3.5%) (Table 6.1). In contrast, the performance of rGO-based PLEDs is drastically lower, with an Lmax of 8300 cd m2 and EQE of 1.8%. The turn-on voltage is found to be independent of the HIL used. To explain these phenomena, the WFs of PEDOT: PSS, GO, and rGO were measured by ultraviolet photoelectron spectroscopy; the
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(a)
(b) LiF / Al cathode Alq3 (37.5 nm) Alq3:C545T (37.5 nm) NPB (70 nm) Hole-injecting buffer ITO anode Glass
(c) PEDOT (40nm) P-GO (1 nm) P-GO (2 nm)
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Figure 6.4 (a) AFM images of phenylated GO (P-GO) (sample B) films spin coated onto freshly cleaved mica. (b) Structure of the OLED device. (c) The CeVeL characteristics of the OLED with the P-GO layer as a hole-injecting buffer layer. (d) Power efficiency. (e) Current efficiency. (f) Luminance. Reproduced with permission Z. Zhong, Y. Dai, D. Ma, Z.Y. Wang, Facile synthesis of organosoluble surface-grafted all-single-layer graphene oxide as hole-injecting buffer material in organic light-emitting diodes. J. Mater. Chem. 21 (2011) 6040e6045, Copyright 2011 The Royal Society of Chemistry.
WF of GO (4.89 eV) is lower than that of PEDOT:PSS (4.95 eV). This suggests that the electron-blocking behavior of GO is more efficient than that of PEDOT:PSS and rGO; this was confirmed using PLEDs with the structure shown in Fig. 6.5a. It is seen in Fig. 6.5b and c that the PLEDs with the device structure ITO/SY/SPR-001/ LiF/Al (SPR-001 denotes a red-emitting polymer) display red emission and those with ITO/SPR-001/GO/SY/LiF/Al exhibit green emission. In contrast, the device
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Graphene for Flexible Lighting and Displays
(a)
1.29 eV
Al LiF
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Super yellow Graphene oxide
ITO 3.64 eV O
OH
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Figure 6.5 (a) Device schematics of PLEDs with a GO layer; chemical structure of the GO; schematic energy diagrams of the flat band conditions of PLEDs with a hole-transport layer (HTL) (rGO, GO, and PEDOT:PSS). (b,c) Electron-blocking properties of GO. Schematic energy diagrams of the devices used to evaluate the electron-blocking behavior of GO: (b)
Graphene-based buffer layers for light-emitting diodes
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Table 6.1 Device performance of PLEDs with PEDOT:PSS, GO, and rGO as HILs [44]. Maximum luminance (cd mL2) (at voltage)
Maximum luminous efficiency (cd AL1) (at voltage)
Maximum power efficiency (lm WL1) (at voltage
Maximum EQE (%) (at voltage)
Turn on voltage (V)
ITO/SY/LiF/ Al
700
1.4 (8.4 V)
0.6 (6.6 V)
0.6 (8.4 V)
2.8
ITO/PEDOT: PSS/SY/ LiF/Al
33,800 (12.6 V)
8.7 (9.6 V)
3.9 (5.2 V)
3.5 (9.2 V)
1.8
ITO/GO [2.0 nm]/ SY/LiF/Al
31,400 (12.4 V)
8.8 (9.4 V)
4.2 (5.0 V)
3.3 (8.2 V)
1.8
ITO/GO [2.6 nm]/ SY/LiF/Al
35,100 (12.0 V)
14.3 (8.6 V)
6.6 (5.4 V)
5.0 (8.4 V)
1.8
ITO/GO [4.3 nm]/ SY/LiF/Al
39,000 (10.8 V)
19.1 (6.8 V)
11.0 (4.4V)
6.7 (6.8 V)
1.8
ITO/GO [5.2 nm]/ SY/LiF/Al
28,500 (11.2 V)
13.9 (7.4 V)
8.6 (4.0 V)
5.0 (7.4 V)
1.8
ITO/rGO [4.3 nm]/ SY/LiF/Al
8300 (13.0 V)
19.1 (6.8 V)
11.0 (4.4V)
6.7 (6.8 V)
1.8
Device configuration
= ITO/SPR-001/(GO or rGO)/super yellow (SY)/LiF/Al. (c) Electroluminescence (EL) spectra of diverse device configurations. (d) Photoluminescence (PL) spectra of SY films on quartz, PEDOT:PSS/quartz, rGO/quartz, and GO/quartz substrates. (e) Transmittance of HTLs (PEDOT:PSS, GO) on ITO, measured using a UV-Vis spectrometer. (f) Time-resolved PL signal of SY, PEDOT:PSS/SY, rGO/SY, and GO/SY films, measured by time-correlated single photon counting (TCSPC). (g) Exciton lifetime of SY, PEDOT:PSS/SY, rGO/SY, and GO/SY films. Reproduced with permission B.R. Lee, J.-W. Kim, D. Kang, D.W. Lee, S.-J. Ko, H.J. Lee, C.-L. Lee, J.Y. Kim, H.S. Shin, M.H. Song, Highly efficient polymer light-emitting diodes using graphene oxide as a hole transport layer, ACS Nano 6 (2012) 2984e2991, Copyright 2008 American Chemical Society.
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with the structure ITO/SPR-001/rGO/SY/LiF/Al shows red emission as a result of insufficient electron blocking. These results suggest that electron transport from SPR-001 to SY, or SY to SPR-001, is efficiently blocked by GO. The enhancement of emission from GO-based PLEDs is further verified by PL measurement, shown in Fig. 6.5deg. Briefly, the PL intensity and exciton lifetime for an SY layer are higher on GO than on PEDOT:PSS or on rGO. Thus, the use of GO improves the hole injection and electron blocking, increasing the electronehole recombination rate in the emissive layer and improving the device performance. Since the performance of GO as a buffer layer in OLEDs was initially demonstrated, many efforts have been made toward further improvement. For example, Yang et al. found that GO used as a HIL in OLEDs showed better performance after light reduction under water vapor at 200 C, compared to pristine GO [46]. Yang et al. also compared the performance of rGO fabricated from a LangmuireBlodgett (rGO LB) film and by spin coating. The device performance for various HILs is summarized in Table 6.2. The rGO-based OLED exhibits a maximum luminance around 2 times higher than the GO-based version and is increased a further 1.5 times by using an rGO LB film. The LangmuireBlodgett film has a better arrangement of the nanosheets, which increases the surface coverage on the ITO. These data from Yang et al. suggest the important roles of the oxidation level and nanosheet arrangement of the GO film for high device performance. Lee et al. functionalized GO with (3-glycidyl oxypropyl)-trimethoxysilane (GPTMS) and triethoxymethylsilane (MTES), increasing the WF from 4.8 eV to 4.9 and 5.0 eV, respectively. The maximum luminance efficiencies of PGTMS-GO and MTES-GO were measured to be 13.91 and 12.77 cd A1, respectively, which are higher than that for the PEDOT: PSS-based device (12.34 cd A1) [13]. The role of GO in QLEDs has also been demonstrated. For instance, Park et al. used PVK/GO/V2O5-x as an efficient HIL in a QLED with the structure PVK/GO/V2O5-x/ Table 6.2 Performance of OLEDs with the structure ITO/HIL/TPD/Alq3/Al, using various HILs [46]. Driving voltage (V) for 100 mA cmL2
Maximum luminance (cd mL2, 12 V)
Luminance efficiency (cd AL1, 12 V)
PEDOT:PSS spin-coating film
11.7
4435
2.9
GO spin-coating film
12.1
2253
1.9
rGO spincoating film
11.4
4107
3.0
rGO LB film
9.6
6232
3.8
HIL
Graphene-based buffer layers for light-emitting diodes
(a)
109
Type A: w/ GO layer
Type B: w/ GO layer Al cathode
(–)
(–)
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(+)
(+)
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anode
Glass
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–0.34 eV
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EFermi Sub-gao Δh1 states =0.49 eV E =2.8 eV
–1.23 eV
4.63 eV
4.97 eV
φ=4.71 eV 5.12 eV
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Evac
=0.16 ev
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Δ h0 =0.16 eV
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:Lying-down orientation (π–π stacking)
Cross-linking (V-O-VC)
Al
OH
O O O
V O
Hydrogen bonding
OH
Hydrogen bonding
O
O OO O
OH
Oxygen Vanadium
O O
V OO O
V OO O
VO
VO V 3d and PVK C 2p wave function hybridization
Figure 6.6 QD-LEDs with and without a GO interlayer. (a) Schematic illustrations depicting structures of QD-LEDs with and without the GO interlayer. EL spectra of QD-LEDs (b) with GO interlayer and (c) without GO interlayer at the same applied bias voltage of 6.0 V. Insets in (b) and (c) are photographs of red emission from types A and B QD-LEDs, respectively, also taken under the same dark room and camera conditions. Electronic energy level alignments of (d) type A and (e) type B QD-LEDs derived from UPS spectra. The insertion of the GO layer
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Graphene for Flexible Lighting and Displays
(CdSe/CdZnS core/shell QDs)/(ZnO nanoparticles)/Al, as shown in Fig. 6.6a [47]. (PVK designates poly(vinylcarbazole), used as a hole-transport layer (HTL)). The QLEDs with a GO-based HTL yielded maximum luminance, luminous efficiency, and EQE several times (up to 10 times) higher than those without GO. The EL spectra of QLEDs with and without GO is shown in Fig. 6.6b and c. The improvement in devices employing GO results from the reduction of the hole barrier between the QDs and the HIL from 1.74 to 0.75 eV, as seen in the downward shift of the PVK energy levels (Fig. 6.6dee).
6.3
Graphene-based composite buffer layer
Apart from the stand-alone use of GO as a HIL, the combination of GO with other materials such as 2-dimensional transition metal dichalcogenides (2D-TMDs) and PEDOT:PSS has also been actively investigated. The use of a MoS2 nanosheet (NS)/GO composite as a HIL was demonstrated for the first time by Park et al. [12]. Typically, MoS2 NSs are synthesized using the lithium intercalation method; the resulting NSs were mixed with GO in the ratios 10:0, 8:2, 6:4, 4:6, and 2:8. The fabrication scheme, device structure, and device performance are shown in Fig. 6.7aee. The highest power efficiency among (MoS2 NS)/GO-based OLEDs, 3.77 lm W1, is found with the (MoS2 NS):GO ratio of 6:4. However, stand-alone GO as a HIL showed the best device performance, with a power efficiency of 4.94 lm W1, as a result of the high surface coverage and the GO WF. To improve the device performance, the uniformity and WF of (MoS2 NS)/GO will need to be improved. Other 2D-TMDs such as WS2, TaS2, and TiS2 may be investigated as 2D-TMDs/GO HILs. For comparison, the use of PEDOT:PSS composite as a HIL significantly improves the device performance by increasing the maximum luminance from 647 to 725 cd m2 while decreasing the turn-on voltage from 5.35 to 3.65 V [48] [reference]. The improvements in PEDOT:PSS-based devices originate in the increased conductivity of the PEDOT:PSS layer on GO incorporation. However, excess GO results in decreased conductivity, transmittance, and WF leading to poor device performance (Fig. 6.8aec). Therefore, the mixing ratio between PEDOT:PSS and GO is an important factor in obtaining high-efficiency OLEDs; the performance dependence is shown in Fig. 6.8def and Table 6.3. Similarly, Diker et al. improved the luminous
=
downshifts the electronic energy levels of poly(vinylcarbazole) (PVK), resulting in a reduced hole injection barrier at the PVK/QD interface. The boxed panels are schematics depicting the heterointerfacial chemical structures of (f) PVK/GO/V2O5ex and (g) PVK/V2O5ex for types A and B QD-LEDs, respectively. Reproduced with permission M. Park, T.P. Nguyen, K.S. Choi, J. Park, A. Ozturk, S.Y. Kim, MoS2-Nanosheet/Graphene-Oxide composite hole injection layer in organic light-emitting diodes, Electron. Mater. Lett. 13 (2017) 344e350, Copyright 2010 Wiley-VCH.
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Figure 6.7 (a) Current densityevoltage, (b) luminanceevoltage, (c) luminance efficiencyecurrent density, and (d) power efficiencyecurrent density characteristics of organic lighteemitting diode devices with MoS2-graphene oxide composite as hole injection layer. (e) Synthesis scheme of materials and structures for organic lighteemitting diode devices. Reproduced with permission M. Park, T.P. Nguyen, K.S. Choi, J. Park, A. Ozturk, S.Y. Kim, MoS2-Nanosheet/Graphene-Oxide composite hole injection layer in organic light-emitting diodes, Electron. Mater. Lett. 13 (2017) 344e350, Copyright 2017 Springer.
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Figure 6.8 (a) Transmission spectra and (b) transmittance at 550 nm of PEDOT:PSS films, for different doping concentrations of GO (0e0.08 wt%), (c) the conductivity of PEDOT:PSS mixed with 6 wt% DMSO and 0.00e0.08 wt% GO. The effect of GO concentration on the (d) JeV curves, (e) luminanceeV curves, and (f) EL spectra of PLEDs, using PEDOT:PSS/GO composite as the HTL. Reproduced with permission H.S. Dehsari, E.K. Shalamzari, J.N. Gavgani, F.A. Taromi, S. Ghanbary, Efficient preparation of ultralarge graphene oxide using a PEDOT:PSS/GO composite layer as hole transport layer in polymer-based optoelectronic devices, RSC Adv. 4 (2014) 55067e55076, Copyright 2014 The Royal Society of Chemistry. Table 6.3 Summary of properties for PLEDs with PEDOT:PSS and PEDOT:PSS/GO composite HTLs [48]. GO (wt%)
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efficiency from 0.071 to 0.156 mA cm2, and the power efficiency from 0.043 to 0.08 lm W1, by optimizing the ratio between GO and PEDOT:PSS [49]. These results confirm the importance of GO composites as HILs in light-emitting devices.
6.4
Conclusion
This chapter provides an overview of the current applications of GO as the HIL in light-emitting devices. GO possesses a 2D structure with various functional groups such as hydroxyl, epoxy, carboxy, carbonyl, phenol, lactone, and quinine, resulting in unique optical and electrical properties that can be modified by controlling those groups. The use of GO and GO-composites as HILs significantly improves the performance of LED devices. However, the uniformity of GO films and the reliability of GO-based devices remain a big challenge for commercialization; these issues will require more attention and investigation in future.
References [1] D.J. Milliron, I.G. Hill, C. Shen, A. Kahn, J. Schwartz, Surface oxidation activates indium tin oxide for hole injection, J. Appl. Phys. 87 (2000) 572e576. [2] J. Li, Q. Guo, H. Jin, K. Wang, D. Xu, G. Xu, X. Xu, Low-temperature solution-processed moox as hole injection layer for efficient quantum dot light-emitting diodes, RSC Adv. 7 (2017) 27464e27472. [3] H. Zhang, S. Wang, X. Sun, S. Chen, Solution-processed vanadium oxide as an efficient hole injection layer for quantum-dot light-emitting diodes, J. Mater. Chem. C 5 (2017) 817e823. [4] Y.H. Kim, S. Kwon, J.H. Lee, S.M. Park, Y.M. Lee, J.W. Kim, Hole injection enhancement by a Wo3 interlayer in inverted organic light-emitting diodes and their interfacial electronic structures, J. Phys. Chem. C 115 (2011) 6599e6604. [5] C. Kim, P. Nguyen Thang, V. Le Quyet, J.M. Jeon, W. Jang Ho, Y. Kim Soo, Performances of liquid-exfoliated transition metal dichalcogenides as hole injection layers in organic light-emitting diodes, Adv. Funct. Mater. 25 (2015) 4512e4519. [6] T.P. Nguyen, Q. Van Le, K.S. Choi, J.H. Oh, Y.G. Kim, S.M. Lee, S.T. Chang, Y.-H. Cho, S. Choi, T.-Y. Kim, S.Y. Kim, MoS2 nanosheets exfoliated by sonication and their application in organic photovoltaic cells, Sci. Adv. Mater. 7 (2015) 700e705. [7] Q.V. Le, C.M. Kim, T.P. Nguyen, M. Park, T.-Y. Kim, S.M. Han, S.Y. Kim, (NH4)2WS4 precursor as a hole-injection layer in organic optoelectronic devices, Chem. Eng. J. 284 (2016) 285e293. [8] V. Le Quyet, P. Nguyen Thang, Y. Kim Soo, Uv/ozone-treated WS2 hole-extraction layer in organic photovoltaic cells, Phys. Status Solidi (RRL) e Rapid Res. Lett. 8 (2014) 390e394. [9] Q.V. Le, T.P. Nguyen, H.W. Jang, S.Y. Kim, The use of uv/ozone-treated MoS2 nanosheets for extended air stability in organic photovoltaic cells, Phys. Chem. Chem. Phys. 16 (2014) 13123e13128.
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[10] G.J. Choi, Q.V. Le, K.S. Choi, K.C. Kwon, H.W. Jang, J.S. Gwag, S.Y. Kim, Polarized light-emitting diodes based on patterned Mos2 nanosheet hole transport layer, Adv. Mater. 29 (2017) 1702598. [11] Q.V. Le, T.P. Nguyen, K.S. Choi, Y.-H. Cho, Y.J. Hong, S.Y. Kim, Dual use of tantalum disulfides as hole and electron extraction layers in organic photovoltaic cells, Phys. Chem. Chem. Phys. 16 (2014) 25468e25472. [12] M. Park, T.P. Nguyen, K.S. Choi, J. Park, A. Ozturk, S.Y. Kim, MoS2-Nanosheet/ Graphene-Oxide composite hole injection layer in organic light-emitting diodes, Electron. Mater. Lett. 13 (2017) 344e350. [13] C.Y. Lee, Q.V. Le, C. Kim, S.Y. Kim, Use of silane-functionalized graphene oxide in organic photovoltaic cells and organic light-emitting diodes, Phys. Chem. Chem. Phys. 17 (2015) 9369e9374. [14] K. Soon Choi, Y. Park, K.-C. Kwon, J. Kim, C. Keun Kim, S.Y. Kim, K. Hong, J.-L. Lee, Reduced graphite oxide-indium tin oxide hybrid materials for use as a transparent electrode, J. Electrochem. Soc. 158 (2011) J231eJ235. [15] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, The chemistry of graphene oxide, Chem. Soc. Rev. 39 (2010) 228e240. [16] R. Ruoff, Calling all chemists, Nat. Nanotechnol. 3 (2008) 10. [17] D. Chen, H. Feng, J. Li, Graphene oxide: preparation, functionalization, and electrochemical applications, Chem. Rev. 112 (2012) 6027e6053. [18] T. Szabo, O. Berkesi, P. Forgo, K. Josepovits, Y. Sanakis, D. Petridis, I. Dékany, Evolution of surface functional groups in a series of progressively oxidized graphite oxides, Chem. Mater. 18 (2006) 2740e2749. [19] T. Nakajima, A. Mabuchi, R. Hagiwara, A new structure model of graphite oxide, Carbon 26 (1988) 357e361. [20] A. Lerf, H. He, M. Forster, J. Klinowski, Structure of graphite oxide revisited, J. Phys. Chem. B 102 (1998) 4477e4482. [21] K. Erickson, R. Erni, Z. Lee, N. Alem, W. Gannett, A. Zettl, Determination of the local chemical structure of graphene oxide and reduced graphene oxide, Adv. Mater. 22 (2010) 4467e4472. [22] C. Mattevi, G. Eda, S. Agnoli, S. Miller, K.A. Mkhoyan, O. Celik, D. Mastrogiovanni, G. Granozzi, E. Garfunkel, M. Chhowalla, Evolution of electrical, chemical, and structural properties of transparent and conducting chemically derived graphene thin films, Adv. Funct. Mater. 19 (2009) 2577e2583. [23] D.W. Boukhvalov, M.I. Katsnelson, Modeling of graphite oxide, J. Am. Chem. Soc. 130 (2008) 10697e10701. [24] H.A. Becerril, J. Mao, Z. Liu, R.M. Stoltenberg, Z. Bao, Y. Chen, Evaluation of solutionprocessed reduced graphene oxide films as transparent conductors, ACS Nano 2 (2008) 463e470. [25] G. Eda, G. Fanchini, M. Chhowalla, Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material, Nat. Nanotechnol. 3 (2008) 270. [26] S. Wang, P.J. Chia, L.L. Chua, L.H. Zhao, R.Q. Png, S. Sivaramakrishnan, M. Zhou, G.S. Goh Roland, H. Friend Richard, T.S. Wee Andrew, K.H. Ho Peter, Band-like transport in surface-functionalized highly solution-processable graphene nanosheets, Adv. Mater. 20 (2008) 3440e3446. [27] G. Eda, C. Mattevi, H. Yamaguchi, H. Kim, M. Chhowalla, Insulator to semimetal transition in graphene oxide, J. Phys. Chem. C 113 (2009) 15768e15771.
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[47] R. Park Young, S. Choi Kyoung, C. Kim Jong, S. Seo Young, Y. Kim Soo, Y.J. Kim, K. Choi Won, Y. Jeong Hu, S. Yang Woo, J. Hong Young, Graphene oxide inserted poly(N-Vinylcarbazole)/Vanadium oxide hole transport heterojunctions for high-efficiency quantum-dot light-emitting diodes, Adv. Mater. Interfaces 4 (2017) 1700476. [48] H.S. Dehsari, E.K. Shalamzari, J.N. Gavgani, F.A. Taromi, S. Ghanbary, Efficient preparation of ultralarge graphene oxide using a PEDOT:PSS/GO composite layer as hole transport layer in polymer-based optoelectronic devices, RSC Adv. 4 (2014) 55067e55076. [49] H. Diker, G.B. Durmaz, H. Bozkurt, F. Yes¸il, C. Varlikli, Controlling the distribution of oxygen functionalities on go and utilization of PEDOT:PSS-GO composite as hole injection layer of a solution processed blue oled, Curr. Appl. Phys. 17 (2017) 565e572.
6
Quyet Van Le 1 , Soo Young Kim 2 1 Institute of Research and Development, Duy Tan University, Da Nang, Vietnam; 2 Department of Materials Science and Engineering, Korea University, Seongbuk-gu, Seoul, Republic of Korea
6.1
Introduction
Buffer layers play a crucial role in light emitting diodes (LEDs), promoting charge injection by reducing the energy barrier between the electrodes and the active layers. The frequently used electrode, indium tin oxide (ITO), exhibits a work function (WF) of 4.1e4.7 eV; this is much lower than the ionization energy of organic materials, reducing the possible device performance [1]. To overcome this challenge, a thin layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (WF w5.2 eV) is sandwiched between the ITO and emissive layers. However, the use of PEDOT:PSS as a hole injection layer (HIL) severely reduces the stability of LED devices as a result of its highly acidic and hygroscopic nature. Recently, a number of inorganic materials have been effectively employed to replace PEDOT:PSS; examples are MoO3 [2], V2O5 [3], WO3 [4], WS2 [5e8], MoS2 [5,9,10], TaS2 [5,11], and graphene oxide (GO) [12e14]. Among these, GO appears to be the most promising as a result of its tunable WF, high stability, high charge-carrier mobility, low cost, and solution processability. This chapter reports on current progress in the application of GO as an HIL in LEDs. First, the structure, electrical, and optical properties are described. Next, the use of GO as the buffer layer in various LEDs, including organic LEDs (OLEDs), polymer LEDs (PLEDs), and quantum dot LEDs (QLEDs) is reviewed. The effect of the structure and thickness of the GO on the performance of the LEDs as well as the HIL mechanism of GO are clarified. In addition, the combination of GO with other materials to form composite HILs is discussed.
6.2 6.2.1
Graphene oxide buffer layer Structures, properties, and synthesis
GO is generally derived from graphite through an oxidation and exfoliation process in an organic solvent or water [15,16]. GO contains various functional groups such as hydroxyl, epoxy, carboxy, carbonyl, phenol, lactone, and quinine [17]. However, the precise structure of GO is still under debate because of its complexity [15]. Several structural models of GO have been proposed, as shown in Fig. 6.1aee [18]. Hofmann Graphene for Flexible Lighting and Displays. https://doi.org/10.1016/B978-0-08-102482-9.00006-X Copyright © 2020 Elsevier Ltd. All rights reserved.
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Figure 6.1 Previous structure models of GO: (a) Hofmann, (b) Ruess, (c) Scholz-Boehm, (d) Nakajima-Matsuo, (e) Lerf-Klinowski. (f) Aberration-corrected TEM image of a single sheet of suspended GO. The scale bar is 2 nm. Expanded view (A) shows (left to right) a 1 nm2 enlarged oxidized region of the material; a proposed atomic structure for this region (carbon atoms in gray and oxygen atoms in red); the average of simulated TEM images (see Supporting Information) of the proposed structure and of another structure where the position of oxidative functionalities has been changed. Expanded view (B) focuses on the white spot in the graphitic region. This spot moved within the graphitic region but was stationary for three frames (6 s) at a hydroxyl position (left portion of expanded view (B)) and for seven frames (14 s) at a (1,2) epoxy position (right portion of expanded view (B)). The ball-and-stick figures below the microscopy images represent the proposed atomic structure for such functionalities. The simulated TEM image for the suggested structure (see Supporting Information) agrees well with the TEM data. Expanded view (C) shows a 1 nm2 graphitic portion using an exit-plane wave reconstruction of a focal series of GO images and the atomic structure in this region. (e) Reproduced with permission T. Szabo, O. Berkesi, P. Forg o, K. Josepovits, Y. Sanakis, D. Petridis, I. Dékany, Evolution of surface functional groups in a series of progressively oxidized graphite oxides. Chem. Mater. 18 (2006) 2740e2749 Copyright 2006 American Chemical Society. (f) Reproduced with permission K. Erickson, R. Erni, Z. Lee, N. Alem, W. Gannett, A. Zettl, Determination of the local chemical structure of graphene oxide and reduced graphene oxide. Adv. Mater. 22 (2010) 4467e4472, Copyright 2010 Wiley-VCH.
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and Holst’s structure is composed of epoxy functional groups that are widely distributed on a basal plane of graphite having an sp2 hybridized configuration, whereas Ruess’s consists of hydroxyl groups and an sp3 hybrid system. Later, Scholz and Boehm proposed a model with a corrugated backbone containing regular quinoidal species instead of epoxide and ether groups [15]. Nakajima and Matsuo presented a model with a stage 2 graphite intercalation based on poly(carbon monofluoride), (CF2)n [19]. More recently, based on NMR studies, Lerf and Klinowski introduced a widely used model that describes GO as a random distribution of flat aromatic regions (unoxidized benzene rings) and wrinkled regions of alicyclic 6-membered rings [20]. The atomic-scale features of GO were recorded for the first time by Erickson et al., using high-resolution transmission electron microscopy (HRTEM), as shown in Fig. 6.1f [21]. It can be observed from the high contrast HRTEM image that GO contains three different types of region: disordered regions, holes, and graphitic regions. It was proposed by the authors that the holes which appeared on the GO sheets originated from oxidation and exfoliation, releasing CO and CO2, whereas the graphitic region results from the incomplete oxidation of the basal plane. It was also suggested that the disordered regions of the basal plane originate in the high-density oxygen-containing functional groups. GO exhibits superior thermal, mechanical, electrical, and optical properties as a result of its unique 2D structure, which is embedded with various functional groups. The electrical properties of GO significantly depend on its structure and level of oxidization. Specifically, GO is an insulator with a large energy bandgap and high Rs (up to 12 U/sq) arising from a large degree of sp3 hybridization [22e24]. However, the sheet resistance of GO can be decreased through a thermal or chemical reduction treatment [25,26]. As a result, GO can be tuned from an insulator to a semiconductor or a graphene-like semimetal [22,27e32]. A typical demonstration of the tunableresistance behavior of GO is shown in Fig. 6.2aed. The optical properties of GO have also been intensively studied [33e35]. It was discovered that GO is fluorescent over a wide range of wavelengths, from the ultraviolet through the visible and into the near-infrared region, as a result of heterogeneous atomic and electronic structures [36]. Differing from other semiconductors, where the fluorescence results from band-edge transitions, in GO it originates from recombination of electronehole pairs in localized electronic states in which sp2 domains are isolated within the carboneoxygen sp3 matrix [37]. The relation between fluorescence, conductivity, and absorbance and GO reduction, which was reported by Eda et al., is shown in Fig. 6.2eeg [37]; the level of GO reduction was controlled through the exposure time to hydrazine vapor. The IeV characteristics of GO-based devices for various reduction times are shown in Fig. 6.2e. The low bias current observed in 2 mine reduced GO is drastically increased with 5 min reduction, indicating a significant change in conductivity. The insulating properties of 2 minereduced GO are further confirmed in Fig. 6.2f. From Fig. 6.2g, it can be seen that the absorption and PL are
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Figure 6.2 Thermal reduction of graphene oxide field effect devices. (a, b, c) Optical images of graphene oxide single sheets prepared with metal leads for resistance measurements. (a) Electrical circuit diagram, showing the Si substrate as a back-gate electrode and including a mini hot-plate with incorporated heater and thermometer. (d) The graphene oxide sample
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markedly increased after around 20 s of exposure as a result of the increased number of localized sp2 sites. It should be noted that the energetic coupling between these sites remains small in the first stage of reduction. However, in the later stages (after 5 min of exposure), the PL of GO is abruptly reduced by the increased interconnectivity between sp2 sites; this facilitates the hopping of excitons to nonradiative combination centers, resulting in PL quenching. Graphite powder is the most common starting material for synthesizing GO. The conversion of graphite powder to GO can be carried out by various techniques, such as Brodie’s [38], Hofmann’s [39], and Hummers’s methods [40]. Briefly, the graphite power is chemically reacted with a strong acid such as HNO3, H2SO4, or HCl, with subsequent intercalation employing an alkali metal compound such as KClO3, KMnO4, or NaNO3. Fig. 6.3 illustrates the oxidation and intercalation processes used to obtain GO from graphite, based on the Hummers’s method [41].
6.2.2
GO buffer layer in optoelectronics
The use of GO as a buffer layer in optoelectronic devices was presented by Li et al. in 2010 [42]. Typically, a thin layer of GO is sandwiched between ITO and an active layer consisting of a blend of poly(3-hexylthiophene) and the fullerene derivative, phenyl-C61-butyric acid methyl ester, in an organic solar cell. The use of GO as an interlayer drastically increases the power conversion efficiency of the solar cell from 1.8% to 3.5%, which is then comparable to that of a PEDOT:PSS-based device (3.6%), and indicates a role for GO as a buffer layer. In 2011, Zhong et al. successfully incorporated GO as a HIL in OLEDs [43]. Specifically, the GO-based OLEDs showed a current efficiency of 23 cd A1 and a power efficiency of 14 lm W1, which are significantly higher than those of PEDOT:PSS-based devices (15 cd A1 and 11 lm W1, respectively). The device structure and device characteristic of the GO-based OLED are shown in Fig. 6.4bef. To avoid the formation of dark spots and pixel shrinkage over time caused by absorbed water on the GO, as well as by restacking
=
resistance (red solid curve; left-axis) and temperature profile (dotted blue; right-axis), as functions of time. The resistance at point A (temperature 172 C, point 1 in the first heating cycle) is equal to that at point B (125 C) in the second heating cycle. A similar behavior was observed in each subsequent cycle, with the three sample resistances (for the same temperature, roughly 125 C) forming a decreasing sequence. The resistance sequence is indicated by the three dotted horizontal arrows; temperatures are indicated by boxes on the heating curve. (e) IeV and (f) transfer characteristics of individual GO sheet devices at different stages of reduction (duration of exposure to hydrazine is noted in the legend). (g) Summary plot for a GO thin film, showing the maximum PL intensity (circles; left axis), absorbance at 550 nm (triangles; right axis), and current at 1 V (squares; right axis), as functions of reduction time. (d) Reproduced with permission I. Jung, D.A. Dikin, R.D. Piner, R.S. Ruoff, Tunable electrical conductivity of individual graphene oxide sheets reduced at “low” temperatures, Nano Lett. 8 (2008) 4283e4287, Copyright 2008 American Chemical Society. (g) Reproduced with permission G. Eda, Y.Y. Lin, C. Mattevi, H. Yamaguchi, H.A. Chen, I.S. Chen, C.W. Chen, M. Chhowalla, Blue photoluminescence from chemically derived graphene oxide, Adv. Mater. 22 (2010) 505e509, Copyright 2010 Wiley-VCH.
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(I)
(III)
35ºC /3h
0–5ºC /1.5h 10g graphite
(II) 5g K2FeO4
6g K2FeO4, 4g KMnO4 100mL H2SO4
250mL H2O
95ºC /30min : H2SO4
: KMnO4
: K2FeO4
: O∙
Figure 6.3 Illustration of GO preparation, based on a newly improved Hummers’s method. Reproduced with permission H. Yu, B. Zhang, C. Bulin, R. Li, R. Xing, High-efficient synthesis of graphene oxide based on improved Hummers method, Sci. Rep. 6 (2016) 36143, Copyright 2016 Nature Publishing Group.
of the GO sheets during the preparation process, a surface grafting with 4-(octoxy)benzenediazonium tetrafluoroborate was applied. The phenylated GO (P-GO) was achieved by simply treating GO with 4-(octoxy)benzenediazonium tetrafluoroborate) for 10e60 min. It should be noted that the functionalization of GO depends significantly on the reaction time. The solubility of P-GO in dimethylformamide increases from 0.6 to 1.0 and 1.8 mg mL1 as the reaction time is increased from 10 to 30 and 60 min, respectively. A single layer of P-GO can be easily produced by spin coating (Fig. 6.4a). This research also shows that the two-layer P-GOebased device exhibits better performance than the one-layer device. The mechanism by which a GO HIL improves the device was later discovered by Lee et al. in 2012 [44]. The structure and energy band alignment of a device employing poly(phenylvinylene): super yellow (SY), (Merck Co., Mw ¼ 1,950,000 g mol1) as an emissive layer (thickness 150 nm) is displayed in Fig. 6.5a. It is reported that GO containing epoxy and epoxy-hydroxyl groups, which disturb the sp2 conjugation in the hexagonal plane, is an insulator with a large optical bandgap of 3.6 eV [42]. To clarify the role of GO, reduced GO (rGO) and rGO-based devices were also fabricated. The removal of the functional groups in rGO effectively restores the sp2 conjugation and thereby reduces the optical bandgap to 1.15 eV [45]. The device performance is optimized with a GO thickness of 4.3 nm, showing a maximum luminance (Lmax) and an external quantum efficiency (EQE) of 39,000 cd m2 and 6.7%, respectively, which are superior to those of a PEDOT:PSS-based device (Lmax ¼ 33,800; EQE ¼ 3.5%) (Table 6.1). In contrast, the performance of rGO-based PLEDs is drastically lower, with an Lmax of 8300 cd m2 and EQE of 1.8%. The turn-on voltage is found to be independent of the HIL used. To explain these phenomena, the WFs of PEDOT: PSS, GO, and rGO were measured by ultraviolet photoelectron spectroscopy; the
Graphene-based buffer layers for light-emitting diodes
105
(a)
(b) LiF / Al cathode Alq3 (37.5 nm) Alq3:C545T (37.5 nm) NPB (70 nm) Hole-injecting buffer ITO anode Glass
(c) PEDOT (40nm) P-GO (1 nm) P-GO (2 nm)
10000
3 2
1000
1
200
0 2.0
2.5 3.0 Voltage (V)
(d)20
100
3.5
10
100
Power efficiency (lm/W)
300
100000
Current density (cd/m )
Current density (mA/Cm )
400
1
PEDOT (40nm) P-GO (1 nm) P-GO (2 nm)
15
10
5
0 0
5
(e)
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25
0.1 20
0.01
0.1
(f) PEDOT (40nm) P-GO (1 nm) P-GO (2 nm)
1
10
100
Current density (mA/cm )
50000
40000 Current efficiency (cd/A)
Current efficiency (cd/A)
15
Voltage (V)
20
15
PEDOT (40nm) P-GO (1 nm) P-GO (2 nm)
30000
20000
10000 0
10
10
100 Current density (mA/cm )
0
100
200
300
400
500
Current density (mA/cm )
Figure 6.4 (a) AFM images of phenylated GO (P-GO) (sample B) films spin coated onto freshly cleaved mica. (b) Structure of the OLED device. (c) The CeVeL characteristics of the OLED with the P-GO layer as a hole-injecting buffer layer. (d) Power efficiency. (e) Current efficiency. (f) Luminance. Reproduced with permission Z. Zhong, Y. Dai, D. Ma, Z.Y. Wang, Facile synthesis of organosoluble surface-grafted all-single-layer graphene oxide as hole-injecting buffer material in organic light-emitting diodes. J. Mater. Chem. 21 (2011) 6040e6045, Copyright 2011 The Royal Society of Chemistry.
WF of GO (4.89 eV) is lower than that of PEDOT:PSS (4.95 eV). This suggests that the electron-blocking behavior of GO is more efficient than that of PEDOT:PSS and rGO; this was confirmed using PLEDs with the structure shown in Fig. 6.5a. It is seen in Fig. 6.5b and c that the PLEDs with the device structure ITO/SY/SPR-001/ LiF/Al (SPR-001 denotes a red-emitting polymer) display red emission and those with ITO/SPR-001/GO/SY/LiF/Al exhibit green emission. In contrast, the device
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Graphene for Flexible Lighting and Displays
(a)
1.29 eV
Al LiF
2.7 eV
Super yellow Graphene oxide
ITO 3.64 eV O
OH
rGO
O
Al
SY
4.3 eV
ITO 4.7 eV 4.79 eV 4.89 eV 4.95 eV
OH O O
O
PEDOT:SS
O
OH
O
O
5.2 eV OH
O
O
(b)
(c) EL intensity (a.u.)
1.0
Al LiF Super yellow GO
0.8 0.6 0.4 0.2 0.0
SPR-001
(d)
(e)
100 Transmittance (%)
PL intensity (a.u.)
3×10 2×10 1×10
80 60 ITO ITO/PEDOT:PSS ITO/GO(i)(2.0nm) ITO/GO(ii)(2.6nm) ITO/GO(iii)(4.3nm) ITO/GO(iv)(5.2nm)
40 20 0
0 450 500 550 600 650 700 750 800
300
400
10
Quartz/GO/SY(10nm) Quartz/GO/SY(10nm)
–2
10–3 –1 0 1
2 3 4 5 6 7 8 Time (ns)
(g)
600
700
0.5 Exciton lifetime (ns)
PL intensity (a.u.)
10–1
IRF Quartz/SY/(10nm)
500
Wavelength (nm)
Wavelength (nm)
100
700
Wavalength (nm)
4×10
(f)
600
500
400
ITO
0.4 0.3 0.2 0.1 0.0
Quartz
GO
PEDOT:PSS
rGO
Type of substrate
Figure 6.5 (a) Device schematics of PLEDs with a GO layer; chemical structure of the GO; schematic energy diagrams of the flat band conditions of PLEDs with a hole-transport layer (HTL) (rGO, GO, and PEDOT:PSS). (b,c) Electron-blocking properties of GO. Schematic energy diagrams of the devices used to evaluate the electron-blocking behavior of GO: (b)
Graphene-based buffer layers for light-emitting diodes
107
Table 6.1 Device performance of PLEDs with PEDOT:PSS, GO, and rGO as HILs [44]. Maximum luminance (cd mL2) (at voltage)
Maximum luminous efficiency (cd AL1) (at voltage)
Maximum power efficiency (lm WL1) (at voltage
Maximum EQE (%) (at voltage)
Turn on voltage (V)
ITO/SY/LiF/ Al
700
1.4 (8.4 V)
0.6 (6.6 V)
0.6 (8.4 V)
2.8
ITO/PEDOT: PSS/SY/ LiF/Al
33,800 (12.6 V)
8.7 (9.6 V)
3.9 (5.2 V)
3.5 (9.2 V)
1.8
ITO/GO [2.0 nm]/ SY/LiF/Al
31,400 (12.4 V)
8.8 (9.4 V)
4.2 (5.0 V)
3.3 (8.2 V)
1.8
ITO/GO [2.6 nm]/ SY/LiF/Al
35,100 (12.0 V)
14.3 (8.6 V)
6.6 (5.4 V)
5.0 (8.4 V)
1.8
ITO/GO [4.3 nm]/ SY/LiF/Al
39,000 (10.8 V)
19.1 (6.8 V)
11.0 (4.4V)
6.7 (6.8 V)
1.8
ITO/GO [5.2 nm]/ SY/LiF/Al
28,500 (11.2 V)
13.9 (7.4 V)
8.6 (4.0 V)
5.0 (7.4 V)
1.8
ITO/rGO [4.3 nm]/ SY/LiF/Al
8300 (13.0 V)
19.1 (6.8 V)
11.0 (4.4V)
6.7 (6.8 V)
1.8
Device configuration
= ITO/SPR-001/(GO or rGO)/super yellow (SY)/LiF/Al. (c) Electroluminescence (EL) spectra of diverse device configurations. (d) Photoluminescence (PL) spectra of SY films on quartz, PEDOT:PSS/quartz, rGO/quartz, and GO/quartz substrates. (e) Transmittance of HTLs (PEDOT:PSS, GO) on ITO, measured using a UV-Vis spectrometer. (f) Time-resolved PL signal of SY, PEDOT:PSS/SY, rGO/SY, and GO/SY films, measured by time-correlated single photon counting (TCSPC). (g) Exciton lifetime of SY, PEDOT:PSS/SY, rGO/SY, and GO/SY films. Reproduced with permission B.R. Lee, J.-W. Kim, D. Kang, D.W. Lee, S.-J. Ko, H.J. Lee, C.-L. Lee, J.Y. Kim, H.S. Shin, M.H. Song, Highly efficient polymer light-emitting diodes using graphene oxide as a hole transport layer, ACS Nano 6 (2012) 2984e2991, Copyright 2008 American Chemical Society.
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with the structure ITO/SPR-001/rGO/SY/LiF/Al shows red emission as a result of insufficient electron blocking. These results suggest that electron transport from SPR-001 to SY, or SY to SPR-001, is efficiently blocked by GO. The enhancement of emission from GO-based PLEDs is further verified by PL measurement, shown in Fig. 6.5deg. Briefly, the PL intensity and exciton lifetime for an SY layer are higher on GO than on PEDOT:PSS or on rGO. Thus, the use of GO improves the hole injection and electron blocking, increasing the electronehole recombination rate in the emissive layer and improving the device performance. Since the performance of GO as a buffer layer in OLEDs was initially demonstrated, many efforts have been made toward further improvement. For example, Yang et al. found that GO used as a HIL in OLEDs showed better performance after light reduction under water vapor at 200 C, compared to pristine GO [46]. Yang et al. also compared the performance of rGO fabricated from a LangmuireBlodgett (rGO LB) film and by spin coating. The device performance for various HILs is summarized in Table 6.2. The rGO-based OLED exhibits a maximum luminance around 2 times higher than the GO-based version and is increased a further 1.5 times by using an rGO LB film. The LangmuireBlodgett film has a better arrangement of the nanosheets, which increases the surface coverage on the ITO. These data from Yang et al. suggest the important roles of the oxidation level and nanosheet arrangement of the GO film for high device performance. Lee et al. functionalized GO with (3-glycidyl oxypropyl)-trimethoxysilane (GPTMS) and triethoxymethylsilane (MTES), increasing the WF from 4.8 eV to 4.9 and 5.0 eV, respectively. The maximum luminance efficiencies of PGTMS-GO and MTES-GO were measured to be 13.91 and 12.77 cd A1, respectively, which are higher than that for the PEDOT: PSS-based device (12.34 cd A1) [13]. The role of GO in QLEDs has also been demonstrated. For instance, Park et al. used PVK/GO/V2O5-x as an efficient HIL in a QLED with the structure PVK/GO/V2O5-x/ Table 6.2 Performance of OLEDs with the structure ITO/HIL/TPD/Alq3/Al, using various HILs [46]. Driving voltage (V) for 100 mA cmL2
Maximum luminance (cd mL2, 12 V)
Luminance efficiency (cd AL1, 12 V)
PEDOT:PSS spin-coating film
11.7
4435
2.9
GO spin-coating film
12.1
2253
1.9
rGO spincoating film
11.4
4107
3.0
rGO LB film
9.6
6232
3.8
HIL
Graphene-based buffer layers for light-emitting diodes
(a)
109
Type A: w/ GO layer
Type B: w/ GO layer Al cathode
(–)
(–)
ZnO NP QD PVK GO V2O5
(+)
(+)
ITO
anode
Glass
Glass
0.025
1.2
(b)
(c) 0.020 Relative EL intnsity
Relative EL intnsity
1.0 0.8 0.6 0.4
0.015
0.010
0.005
0.2
500
550
600
650
700
0.000 500
750
550
(d) Type A
(i)
(iii)
Δ =0.41 eV
600
650
(e)
(iv)
Type B
(v)
(i)
–0.34 eV
0.65 ev
0.2 ev
3.89 eV
e–
1.15 ev
EFermi Sub-gao Δh1 states =0.49 eV E =2.8 eV
–1.23 eV
4.63 eV
4.97 eV
φ=4.71 eV 5.12 eV
Δh
750
–0.15 eV
Evac
=0.16 ev
700
Wavelength (nm)
Wavelength (nm)
0.16 eV
Δ h0 =0.16 eV
h+ Δh2 =0.50 eV
,
Δ h3 =1.74 eV
Δ h3=0.75 eV 1.36 eV
(f)
ITO
GO
VO
PVK
QDs ZnO (core/shell)NPs
PVK O
GO
PVK O
O
OH
OH O
Monomeric (and polymeric)
(g)
ITO
VO
PVK
QDs ZnO (core/shell) NPs
Al
PVK: Tilted or stand-up orientated
:Lying-down orientation (π–π stacking)
Cross-linking (V-O-VC)
Al
OH
O O O
V O
Hydrogen bonding
OH
Hydrogen bonding
O
O OO O
OH
Oxygen Vanadium
O O
V OO O
V OO O
VO
VO V 3d and PVK C 2p wave function hybridization
Figure 6.6 QD-LEDs with and without a GO interlayer. (a) Schematic illustrations depicting structures of QD-LEDs with and without the GO interlayer. EL spectra of QD-LEDs (b) with GO interlayer and (c) without GO interlayer at the same applied bias voltage of 6.0 V. Insets in (b) and (c) are photographs of red emission from types A and B QD-LEDs, respectively, also taken under the same dark room and camera conditions. Electronic energy level alignments of (d) type A and (e) type B QD-LEDs derived from UPS spectra. The insertion of the GO layer
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Graphene for Flexible Lighting and Displays
(CdSe/CdZnS core/shell QDs)/(ZnO nanoparticles)/Al, as shown in Fig. 6.6a [47]. (PVK designates poly(vinylcarbazole), used as a hole-transport layer (HTL)). The QLEDs with a GO-based HTL yielded maximum luminance, luminous efficiency, and EQE several times (up to 10 times) higher than those without GO. The EL spectra of QLEDs with and without GO is shown in Fig. 6.6b and c. The improvement in devices employing GO results from the reduction of the hole barrier between the QDs and the HIL from 1.74 to 0.75 eV, as seen in the downward shift of the PVK energy levels (Fig. 6.6dee).
6.3
Graphene-based composite buffer layer
Apart from the stand-alone use of GO as a HIL, the combination of GO with other materials such as 2-dimensional transition metal dichalcogenides (2D-TMDs) and PEDOT:PSS has also been actively investigated. The use of a MoS2 nanosheet (NS)/GO composite as a HIL was demonstrated for the first time by Park et al. [12]. Typically, MoS2 NSs are synthesized using the lithium intercalation method; the resulting NSs were mixed with GO in the ratios 10:0, 8:2, 6:4, 4:6, and 2:8. The fabrication scheme, device structure, and device performance are shown in Fig. 6.7aee. The highest power efficiency among (MoS2 NS)/GO-based OLEDs, 3.77 lm W1, is found with the (MoS2 NS):GO ratio of 6:4. However, stand-alone GO as a HIL showed the best device performance, with a power efficiency of 4.94 lm W1, as a result of the high surface coverage and the GO WF. To improve the device performance, the uniformity and WF of (MoS2 NS)/GO will need to be improved. Other 2D-TMDs such as WS2, TaS2, and TiS2 may be investigated as 2D-TMDs/GO HILs. For comparison, the use of PEDOT:PSS composite as a HIL significantly improves the device performance by increasing the maximum luminance from 647 to 725 cd m2 while decreasing the turn-on voltage from 5.35 to 3.65 V [48] [reference]. The improvements in PEDOT:PSS-based devices originate in the increased conductivity of the PEDOT:PSS layer on GO incorporation. However, excess GO results in decreased conductivity, transmittance, and WF leading to poor device performance (Fig. 6.8aec). Therefore, the mixing ratio between PEDOT:PSS and GO is an important factor in obtaining high-efficiency OLEDs; the performance dependence is shown in Fig. 6.8def and Table 6.3. Similarly, Diker et al. improved the luminous
=
downshifts the electronic energy levels of poly(vinylcarbazole) (PVK), resulting in a reduced hole injection barrier at the PVK/QD interface. The boxed panels are schematics depicting the heterointerfacial chemical structures of (f) PVK/GO/V2O5ex and (g) PVK/V2O5ex for types A and B QD-LEDs, respectively. Reproduced with permission M. Park, T.P. Nguyen, K.S. Choi, J. Park, A. Ozturk, S.Y. Kim, MoS2-Nanosheet/Graphene-Oxide composite hole injection layer in organic light-emitting diodes, Electron. Mater. Lett. 13 (2017) 344e350, Copyright 2010 Wiley-VCH.
Graphene-based buffer layers for light-emitting diodes
(b)
250
25000
MoS2-GO 10–0 MoS2-GO 8–2 MoS2-GO 6–4 MoS2-GO 4–6 MoS2-GO 2–8 MoS2-GO 0–10
200 150 100 50 0 0
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4
6
15000 10000 5000 0
8 10 12 14 16
4
Power efficiency (lm/W)
10 8 6 4 2 1
10
8
10
12
14
16
5.0 4.5 4.0 3.5 3.0 2.5 2.0 0.1
100
6
Voltage (V)
(d)
12
0.1
MoS2-GO 10–0 MoS2-GO 8–2 MoS2-GO 4–4 MoS2-GO 4–6 MoS2-GO 2–8 MoS2-GO 0–10
20000
Voltage (V)
(c) Luminance efficiency (cd/A)
Luminance (cd/m2)
Current density (mA/cm2)
(a)
111
Current density (mA/cm2)
1
10
100
Current density (mA/cm2)
(e) LiF/Al
BuLi
Exfoilation
Alq Intercalation
BCP
MoS bulk
MoS nanosheet
Li MoS
Alq : C545T NPB
MoS -GO
Hummers method
composit
e
ITO glas s
Graphite
GO Mo
S
Li
C
O
H
Figure 6.7 (a) Current densityevoltage, (b) luminanceevoltage, (c) luminance efficiencyecurrent density, and (d) power efficiencyecurrent density characteristics of organic lighteemitting diode devices with MoS2-graphene oxide composite as hole injection layer. (e) Synthesis scheme of materials and structures for organic lighteemitting diode devices. Reproduced with permission M. Park, T.P. Nguyen, K.S. Choi, J. Park, A. Ozturk, S.Y. Kim, MoS2-Nanosheet/Graphene-Oxide composite hole injection layer in organic light-emitting diodes, Electron. Mater. Lett. 13 (2017) 344e350, Copyright 2017 Springer.
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Graphene for Flexible Lighting and Displays
(a)
(d)
92
700
Current density (mA/cm )
Transmittance (%)
88 86
0.00 wt% 0.02 wt% 0.04 wt% 0.06 wt% 0.08 wt%
84 82 80 350
500 400 300 200 100 0 –100
400
450
500 550 Wavelength (nm)
600
650
0
700
(e)
91
(b)
0.00 wt% 0.02 wt% 0.04 wt% 0.06 wt%
600
90
600 Luminance (cd/m )
Transmittance (%)
3
4 Voltage (V)
5
6
7
0.00 wt% 0.02 wt% 0.04 wt% 0.06 wt%
700
88
2
800
90
89
1
500 400 300 200 100
87
0 –100
86 0
(c)
0.01
0.02
0.03 0.04 0.05 Concentration (wt%)
0.06
0.07
800
2
4
6 Voltage (V)
8
10
0.00 wt% 0.02 wt% 0.04 wt% 0.06 wt%
0.7 0.6 EL intensity (a.u.)
700 650 600 550
12
0.8
(f)
750 Conductivity (S/cm)
0
0.0
0.5 0.4 0.3 0.2 0.1
450 0
0.02
0.04 Concentration (wt%)
0.06
0.08
0 500
550
600 Wavelength (nm)
650
700
Figure 6.8 (a) Transmission spectra and (b) transmittance at 550 nm of PEDOT:PSS films, for different doping concentrations of GO (0e0.08 wt%), (c) the conductivity of PEDOT:PSS mixed with 6 wt% DMSO and 0.00e0.08 wt% GO. The effect of GO concentration on the (d) JeV curves, (e) luminanceeV curves, and (f) EL spectra of PLEDs, using PEDOT:PSS/GO composite as the HTL. Reproduced with permission H.S. Dehsari, E.K. Shalamzari, J.N. Gavgani, F.A. Taromi, S. Ghanbary, Efficient preparation of ultralarge graphene oxide using a PEDOT:PSS/GO composite layer as hole transport layer in polymer-based optoelectronic devices, RSC Adv. 4 (2014) 55067e55076, Copyright 2014 The Royal Society of Chemistry. Table 6.3 Summary of properties for PLEDs with PEDOT:PSS and PEDOT:PSS/GO composite HTLs [48]. GO (wt%)
Threshold voltage (V)
Turn-on voltage (V)
Maximum luminance (cd mL2)
lEL (nm)
EL intensity (a.u.)
0
5.45
5.35
647.1
552.9
0.736
0.02
4.10
3.85
676.8
555.3
0.734
0.04
3.70
3.65
725.6
558.9
0.735
0.06
3.80
3.75
691.1
559.5
0.738
Graphene-based buffer layers for light-emitting diodes
113
efficiency from 0.071 to 0.156 mA cm2, and the power efficiency from 0.043 to 0.08 lm W1, by optimizing the ratio between GO and PEDOT:PSS [49]. These results confirm the importance of GO composites as HILs in light-emitting devices.
6.4
Conclusion
This chapter provides an overview of the current applications of GO as the HIL in light-emitting devices. GO possesses a 2D structure with various functional groups such as hydroxyl, epoxy, carboxy, carbonyl, phenol, lactone, and quinine, resulting in unique optical and electrical properties that can be modified by controlling those groups. The use of GO and GO-composites as HILs significantly improves the performance of LED devices. However, the uniformity of GO films and the reliability of GO-based devices remain a big challenge for commercialization; these issues will require more attention and investigation in future.
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