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Electrolyte-Gated Red, Green and Blue Organic Light-Emitting Diodes Jiang Liu, Dustin Yuan Chen, Xinning Luan, Kwing Tong, Fangchao Zhao, Chao Liu, Qibing Pei, and Huaping Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00463 • Publication Date (Web): 23 Mar 2017 Downloaded from http://pubs.acs.org on March 26, 2017

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ACS Applied Materials & Interfaces

Electrolyte-Gated Red, Green and Blue Organic Light-Emitting Diodes Jiang Liu‡1,2, Dustin Chen‡2, Xinning Luan‡1, Kwing Tong2, Fangchao Zhao1,2, Chao Liu2, Qibing Pei2 * and Huaping Li1* 1

Atom Nanoelectronics Inc. 440 Hindry Avenue, Unit E, Inglewood California 90301 USA

2

Department of Materials Science and Engineering, University of California Los Angeles, 420

Westwood Plaza, Los Angeles, California 90095, USA KEYWORDS Gated Phosphorescent Small Molecular OLEDs, Polymer Electrolyte, Nano Porous Cathode, Electrochemical Doping, Electron Injection Layer, Full Color OLED Display

ABSTRACT We report vertical electrolyte-gated red, green and blue phosphorescent small molecule organic light emitting diodes (OLED), in which light emission was modified by tuning the electron injection via electrochemical doping of the electron injection layer 4,4-bis(Ncarbazolyl)-1,1-biphenyl (CBP) under the assistance of a polymer electrolyte. These devices comprise an electrolyte capacitor on top of a conventional OLED, with the interfacial contact between the electrolyte and electron injection layer CBP of OLEDs achieved through a porous cathode. These phosphorescent OLEDs exhibit the tunable luminance between 0.1 to 10 000 cd

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m-2, controlled by an applied bias at the gate electrode. This simple device architecture with gatemodulated luminance provides an innovative way for full color OLED displays.

Introduction Flat Panel Displays (FPDs) are ubiquitous in everyday consumer electronics ranging from laptop, mobile devices, televisions, to automobile displays.1 Amongst existing FPDs, thin film transistor-liquid crystal displays (TFT-LCDs) dominate the marketplace despite their limitations in color, contrast, and response time.2 In recent years, display consumption has been shifting from TFT-LCDs to active matrix organic light-emitting diodes (AMOLEDs), not only due in part to the superior display qualities of OLEDs in aspects of wide color gamut, deep contrast and fast response times, but also in part to cost reduction driven by recent technological advancement.3 Current AMOLED displays are fabricated by depositing OLED pixels onto TFT backplanes. Two methods for achieving full color AMOLED displays have most commonly been applied: (i) direct patterning of RGB OLED subpixels; (ii) filtering white-OLEDs by RGB color filters.4 In both cases, each OLED pixel is driven by control circuits comprising multiple TFTs.5 However, due to technical challenges and fabrication complexity, AMOLED displays to date still have a high fiscal cost. In efforts to mitigate this cost, we proposed a simple and economically efficient OLED driving scheme through electrolyte-gated OLEDs (EG-OLEDs),6 in which the luminance of the OLED is controlled by an internal gate electrode instead of external TFT circuits. With this design, the gate potential redistributes the mobile ions through a nano-porous cathode to modify the doping level of the light emitting polymer, leading to modulation of the electron-hole balance and thus a controllable luminance of the OLED devices. This successful approach illustrates the possibility of driving AMOLEDs without using external TFT control circuits. Furthermore, these


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results demonstrate the potential for OLED displays to be fabricated by facile, low cost, manufacturing processes. In our previous works, we employed solution-processed fluorescent polymers (a yellow emission polyphenylene vinylene derivative) as emitters.6 However, because fluorescent molecules can only utilize up to 25% of the generated excitons, phosphorescent dopants are commonly employed in OLEDs for a theoretical 100% internal quantum efficiency.7 In this contribution, we adopted EG-OLED technology for phosphorescent emitters, and report on the fabrication and characterization of the red, green and blue EG-OLEDs with phosphorescent small molecules emitters, demonstrating a full color display based on this technology. The nano-porous cathode was also comprehensively studied. Results and Discussion Device Structure and Fabrication Figure 1a illustrates a schematic of an EG-OLED. This device can be segregated into two integrating portions: (1) an OLED stack with a porous cathode, and (2) a gating capacitor comprising the porous Al cathode (also the cathode of the OLED stack) / polymer electrolyte / gate Al. From bottom to top, the OLED structure consists of an ITO anode, a hole injection layer (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), a hole transport layer (4,4-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC)) of 40 nm,

a light-

emission layer (4,4-bis(N-carbazolyl)-1,1-biphenyl (CBP)) of 40 nm, and a porous aluminum (Al) cathode of 25 nm. The portion of the CBP acting as the EML (10 nm) is doped with three different phosphorescent guests, the red guest bis(1-phenylisoquinoline)-(acetylacetonate) iridium (III), the green guest (tris[2-(p -tolyl)pyridine]iridium(III) ), and the blue guest bis[2-


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(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III), while the remaining 30 nm of undoped CBP acts as the electron transport layers. All materials’ structures are shown in Figure 1b. Detailed description of the device fabrication is provided in the experimental section. The morphology of the 25 nm, porous Al cathode was imaged using atomic force microscopy (AFM) and is depicted in Figure 1c, with an in-depth analysis of the porous electrode provided in the latter section. The electrolyte layer is an amalgamation of a salt (CF3SO3Li), an ion conductor (poly(ethylene oxide) (PEO)) and a solidifier poly(methyl methacrylate) (PMMA)) in a weight ratio of 1:5:10, while the top gate electrode is a 100 nm Al layer. The resulting capacitance value of the gating capacitor of porous Al cathode / polymer electrolyte / gate Al is measured to be 0.4 µF/cm2 at 1000 Hz (Figure 1d). More details for the fabrication steps can be found in the Supporting Information.


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Figure 1. (a) The 3D structural sketch of an EG-OLED device. (b) Chemical structures of molecules employed in EG-OLED devices. (c) AFM height image of porous Al source showing discontinuous islands network (25 nm in deposition thickness). (d) Capacitance of the gating capacitor of porous Al / electrolyte / Al was plotted in the frequency range from 1 kHz to 1 MHz.

Device Characterization All device characterization was carried out in a nitrogen filled glovebox using a Keithley 4200 semiconductor characterization system. The current density (J) and luminance (L)transfer


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characteristics were measured by grounding the source electrode (S), applying a voltage of 8 V at drain electrode (VDS) in reference to S, and sweeping a voltage (-2 V 8 V -2 V) at the gate electrode (VGS) in reference to S. As can be seen from Figure 2(a)-2(c), at VGS = 0 V, the luminance was below 10 cd/m2 ; at VGS= 8 V the luminance reaches 2400 cd/m2 (red), 9500 cd/m2 (green) and 3900 cd/m2 (blue), respectively. The devices turn off (~0.1 cd/m2) at a VGS of 2 V, indicating an optical contract ratio over 104. The low luminance at VGS=0 V is due to an unbalanced charge injection, with the hole population significantly higher than their electron counterpart. In contrast, the high luminance intensity at VGS= 8 V is a result of improved electron injection and a resultant carrier balance. Optical images of the working devices can be seen in the inset of Figure 2(a)-2(c), exhibiting almost 100% aperture ratio in the active device area. The gate leaking current density is approximately 1% of the S-D current density as illustrated in Figure S1. It is of note that although the luminance varies by approximately four orders of magnitude, J between drain and source is less than 30%. The J varies from 22 mA/cm2 to 41 mA/cm2, from 26 mA/cm2 to 39 mA/cm2 and from 16 mA/cm2 to 21 mA/cm2, for red, green and blue devices respectively. This disparity can be explained by the change of gate potential modulating only electron injection, while the hole carriers are the dominant carrier in these OLEDs. As a result, the increase of electron injection with positive gate potential results in improved electron-hole balance, and thus brighter and more efficient electroluminescence.6 The current efficiency (CE), an indicator of charge injection balance, is calculated and plotted at different VGS (Figure 2d-2f). At zero gate bias, the CE is lower than 1 cd/A. This low efficiency indicates that due to insufficient electron injection, hole current is the predominant contributor to the overall current density, and most holes are flowing through the channel without recombining


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with electrons. The CE gradually increases with increasing VGS, and peaks at around 6 cd/A (red), 26 cd/A (green) and 15cd/A (blue), as more electrons are injected into the channel for more balanced electron-hole recombination. In contrast to the efficiency roll-off phenomena in OLEDs,8 the efficiency is observed to increase in the EG-OLEDs with increasing luminance (Figure 2g-2i) due to more balanced charge carrier injection. The hysteresis in the transfer curve indicates a delay in luminance in response to the current density, which is primarily attributed to the nature of slow ion motion and is also observed in other ion-containing device, such as electrolyte-gated transistors,9 ion-containing perovskite LEDs10 and OLED devices incorporating electrolyte materials.11 The slow ion motion, and thus device response, can be accelerated with elevated temperature. In addition, an ion conductor with a glass transition temperature (Tg) lower than ambient temperature may also be used12 To improve the temporal response of EG-OLED devices. The stability of EG-OLED was measured at VDS = 5V and VGS = 8 V for a period of 11 hours (supporting information Figure S2). The device’s luminance and current density decreased about 20% and 11% of their initial values, respectively.


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Figure 2. Transfer characteristics: (a-c) Current density and Luminance transfer characteristics, (d-f) Current efficiency verse different gate voltage, (g-i) Current efficiency verse luminance of the red (a, d and g), green (b, e and h) and blue (c, f, and i) EG-OLED devices. The device photos are shown in the inset of (a-c). All curves were recorded with VGS sweeping from -2 V – 8 V and VDS of 8 V. Output characteristics were measured by sweeping VDS from 0 V to 8 V with different VGS (-2 V, 0 V, 2V, 4V, 6V, and 8V). The current density output curves are shown in Figure 3a-3c, with the luminance illustrated in Figure 3d-3f. From the current density curves rectifying diode behavior is clearly showed. As can be seen from Figure 3d, at VGS = -2 V, the maximum luminance of the red device is below 1 cd/m2; at VGS=0 V, the turn-on voltage (VTO, the D-S voltage when the


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luminance=1 cd/m2) is around 8 V; when VGS=2,4,6,8 V, VTO decreased to 4.5 V, 3.5 V, 3.2V, 3.2V. Similar trends were observed for green and blue devices, in addition to their VTO dropped significantly when VGS was increased (Figure 3e, 3f). These decreases in VTO indicate, again, that the gate potential modulates the injection barrier of electrons. Like the observed current density profiles in transfer curves, the output current densities also show slight changes with various gate potentials. The optical spectrum of each R, G and B device was also measured as shown in Figure 3g-3i, and are seen to remain the same at different gate potentials.

Figure 3. The output characteristics with VDS swept from 0 V to 8 V at different VGS (-2 V, 0 V, 2V, 4V, 6V, and 8V ). (a)-(c) the current density curves; (d)-(f) luminance curves. (g)-(i) the


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emission spectra verse different gate voltage of the red (a, d and g), green (b, e and h) and blue (c, f, and i) EG-OLED devices.

Electroluminescence properties of red, green and blue EG-OLEDs are summarized in Table 1. Table 1. The electroluminescence properties of red, green and blue EG-OLED devices Dopant

VTO*

CIE

(V)

Max L

Optical

Max C.E.

Max P.E.**

(cd m-2)

Contrast

(cd A-1)

(lm W-1)

Red

3.1

(0.68,0.32)

2560

~2×104

7.1

2.7

Green

3.2

(0.36,0.59)

9600

~1×105

25.7

9.8

Blue

3.3

(0.16,0.28)

4050

~4×104

14.8

6.0

*VTO was determined at VGS = 8 V. **Max Power Efficiency (P.E.) was estimated at VDS = 8 V. Max: maximum. OLED Displays As a proof of concept, we fabricated an EG-OLED device with patterned R, G and B dopants and letter-shaped gate electrodes, with the device structure shown in Figure 4a. An image of the device under operation at VGS=5 V and VDS=5 V is shown in the inset of Figure 4b. The displayed signage (red “r”, green “g” and blue “b”) is identical to the pattern of the gate electrodes, further confirming that light emissions is controlled by the potential applied to the


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gate electrode of EG-OLEDs. The color coordinates of red, green and blue EG-OLEDs were measured by a spectrometer PR655 and were plotted in CIE 1931 color space as shown in Figure 4b. The CIE coordinates of the red, green and blue EG-OLEDs are determined by the small molecular dopants shown in Figure 1. A seven-segment display module13 in a monochromic green device was also fabricated. In this system, a common source and drain are stacked as shown in Figure 4c, and seven gate electrodes are patterned and controlled individually using a prototyping circuit board (see Figure S2). The gate electrodes can switch on or off at different “pixels” by applying different potentials at different gate electrodes. Figure 4d illustrates this ability by spelling out “ATOM” and “UCLA” with a single device (see Figure 4d). These results indicate the feasibility of using EG-OLEDs in large-area display devices.


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Figure 4. (a) The 3d structural sketch of the RGB display module. (b) R, G and B emission of the full color display module in CIE color space diagram. The inset shows the photo image of turned-on RGB display module. (c) The 3d structural sketch of the 7-segment display module with a monochromic green color; (d) The photo images of the turned-on 7-segment display module displaying different letters of “ATOM” (top) and “UCLA” (bottom).

Working Mechanism To better understand how these devices work, we propose the following working mechanism. When a voltage (VGS) is applied between the gate and source, the electrolyte becomes polarized. As a result, two Helmholtz electrical double layers (EDL)14 with nanoscale thickness are formed at the interfaces of electrolyte/Al and electrolyte/CBP. The latter EDL interacts with the CBP through the porous Al cathode in 3 different ways according to the polarity of VGS. When VGS=0 V (Figure 5a, Figure 5b) The EG-OLED works in a similar manner as a conventional OLED with holes injected from the anode and electrons from the cathode. The hole injection barrier is negligible, as the work function of ITO/PEDOT:PSS anode (5.1 eV) is well aligned with the highest occupied molecular orbital (HOMO) level of TAPC (5.3 eV); while the electron injection is hindered because there is a 1.9 eV gap between the Al cathode (4.3 eV) and the lowest unoccupied molecular orbital (LUMO) of CBP (2.4 eV). This unbalanced hole and electron injection would result in an extremely low luminance at VGS=0 V.


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When VGS>0 V (Figure 5c, Figure 5d) Anions within the electrolyte move towards the gate electrode while cations migrate to the electrolyte/source electrode interface, forming an EDL at gate/electrolyte interface. Since the Al source electrode is porous, cations traverse through the pores and make contact with the CBP. The accumulated ions can stabilize electrochemically n-doped CBP15 that forms an Ohmic contact with the Al source electrode,16-17 improving the electron injection and thus leading to more effective electroluminescence. It is worth mentioning that, CBP as an intrinsically p-type semiconductor, can be effectively converted to n-type upon n-doping (positive gate voltage), and can be reversibly converted to ptype materials through p-doping (negative gate voltage).18 These properties of being interchangeable as an electron or hole transporter are essential for the functionalities of EGOLED devices. In other trials, 2,2’,2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBI) were utilized as ETL materials in EG-OLED structures, but only limited modulation (optical contrast=3, the luminance was modified from around 1000 to 3000 cd/m2) was observed. We speculate that the low modulation is attributed to poor interaction between TPBI with anions. When VGS0 (c,d) and VGS

Electrolyte-Gated Red, Green, and Blue Organic ... - ACS Publications

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