Color-Tunable, Spectra-Stable Flexible White Top-Emitting Organic

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Color-tunable, spectra-stable flexible white top-emitting organic light-emitting devices based on alternating current driven and dual-microcavity technology Xiang Zhang, Teng Pan, Jiaxin Zhang, Letian Zhang, Shihao Liu, and Wenfa Xie ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.9b00900 • Publication Date (Web): 08 Aug 2019 Downloaded from pubs.acs.org on August 8, 2019

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Color-tunable, spectra-stable flexible white top-emitting organic light-emitting devices based on alternating current driven and dual-microcavity technology Xiang Zhang, Teng Pan, Jiaxin Zhang, Letian Zhang, Shihao Liu*, Wenfa Xie* State Key Laboratory of Integrated Optoelectronics, College of Electronics Science and Engineering, Jilin University, Changchun, 130012, People’s Republic of China ABSTRACT White top-emitting organic light-emitting diodes (TOLEDs) are attractive due to their freedom of choice in substrates. However, tunable color and stable spectra for white TOLED remain considerable challenges. Here, we conceive a novel in-planar-electrodes dual-microcavity OLED driven by alternating current signals to build white TOLEDs. Consequently, microcavity effect in the dual-microcavity OLED can be separately optimized for blue and yellow emissive units. The color temperature of the device also can be adjusted by using alternating current signals. We finally achieve color-tunable, spectra-stable flexible white TOLEDs on PET substrate and paper substrate. This work will be beneficial to the further development of white top-emitting devices, even including quantum dots LEDs and perovskite LEDs. KEYWORDS: Dual-microcavity, White top-emitting, Alternating Current (AC), Color-tunable, OLED

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Organic light-emitting devices (OLEDs) are promising candidates for future lighting and display applications due to their excellent characteristics.1-3 Through many efforts in optimizing organic materials and device structures, great progresses have been achieved in the OLEDs area.2-12 In 2018, the radical-based light-emitting diodes with doublet emission not only achieve almost 100% internal quantum efficiency, but also realize an excellent external quantum efficiency (EQE) of 27%, providing an innovative method to overcome the limitation of quantum-mechanical effects.13 Normal OLEDs are fabricated with bottom-emitting structures, in which transparent substrates, like glasses or polyethylene terephthalate (PET), are required.14-16 Compared with bottom-emitting OLEDs (BOLEDs), top-emitting OLEDs (TOLEDs) break through the limitations of substrates. And TOLEDs are more appropriated for display applications because light emission from top side would significantly improve the aperture ratio.17, 18 Moreover, the spectra will be narrower and stronger due to the multiple-beam interference and wide-angle interference. Therefore, color saturation and efficiency can be improved effectively by optimizing the microcavity effects.19-21 However, the existence of microcavity effect makes white emission hard to be achieved by TOLEDs.17, 22 Microcavity effect can strongly alter the photon density of states by introducing the electromagnetic boundary conditions in the optical microcavity. The photonic mode density near the cavity resonance mode is enhanced, and the others are weakened. The emission of device will be redistributed to the electroluminescent wavelengths near the cavity resonance wavelength. However, white light is a combination of lights with different wavelengths in the visible spectrum. Actually, the combination of three primary colors red, green and blue emission or two primary colors yellow and blue emission is needed to form white light. Therefore, even though white emission is easily realized in BOLEDs, TOLEDs with white emission are still hardly found in the reported papers due to the single resonant cavity. As a result, although monochromatic top-emitting OLEDs, quantum-dot light-emitting devices and perovskite light-emitting devices have been widely studied,19-21, 23-25 only several white top-emitting OLEDs can be found in the reported papers.22, 26-31 To solve this problem, the common method is increasing the transmittance of top electrode to suppress the microcavity effect so that the redistribution of spectra caused by microcavity effect would be weakened and the spectra would be more stable.17,

27-30

But the transmittance of top 2


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electrode is strongly limited by the intrinsic properties of metal materials, and the efficiency of TOLEDs would be lower since it cannot take the advantage of the microcavity effect in improving device performances. For white emission, multiple micro-cavities structure should be a solution to utilize the microcavity effect. Till now, only vertical dual-microcavity is proposed to shape white light from BOLEDs. However, due to the strong absorption of two stacked sliver semitransparent electrodes, the performances of these OLEDs are not ideal.32, 33 As a result, planar dual-microcavity is used to build white TOLEDs here. To drive the TOLEDs with planar dual-microcavity, alternating current (AC) driven signals are very suitable because AC-driven signals will introduce some new properties into the device.8,

34

Although AC-driven

signals have been widely used in BOLEDs, they are very rare in TOLEDs.8,

34-42

Thus, in this

research we demonstrate dual-microcavity top-emitting white organic light-emitting devices (DMT-OLEDs) which are driven by AC signals. Dual microcavity effects based on two planar light-emitting units allow us to optimize the microcavity effects for different colors, respectively, solving the problem in normal multi-color TOLEDs. What’s more, the spectra and brightness of DMT-OLEDs can also be tuned independently by AC-driven signals. The color can be changed from blue through white to yellow easily. In addition, DMT-OLEDs can be made on flexible opaque substrates, like paper, breaking through the limitations of substrates. ■ RESULTS Microcavity effects are universal in TOLEDs. For a better understanding, TOLED can be considered as a Fabry-Pérot resonator, where bottom metal and top metal are regarded as parallel mirrors.43 The cavity resonance wavelength  of TOLED can be calculated by equation (1). 4𝑛𝑖𝑑𝑖

∑𝑖



― 𝑏 ― 𝑡 = 2𝑚𝜋

(1)

Here n𝑖 is the refractive index and 𝑑𝑖 is the thickness of all organic layers, 𝑏 and 𝑡 are the phase shift of bottom metal (Mg:Ag 120 nm) and top metal (Ag 20 nm), respectively, and m is the mode index. Figure 1 (a) shows the calculated results of equation (1) when m=0. The intersection points in Fig. 1 (a) means that the equation (1) can only be satisfied in this situation, and the position of these intersection points denotes the cavity resonance wavelength. Obviously, only one cavity 3


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resonance wavelength can be found in a single resonant cavity when the cavity length is fixed. Different cavity lengths are needed for different colors with different wavelength. It is not possible to optimize the microcavity effects for different colors in a single resonant cavity. The spectral emission intensity I() of forward emission from the microcavity has also been calculated by equation (2).

𝐼() =

[

(4𝜋𝑛𝑧 )  ] 4𝜋𝑛𝑑 𝐼0() 𝑅𝑏𝑅𝑡cos ( ) 

𝑇𝑡 1 + 𝑅𝑏 + 2 𝑅𝑏cos 1 + 𝑅 𝑏𝑅 𝑡 ― 2

(2)

Here 𝑇𝑡 is the transmittance of the top metal, 𝑅𝑏 and 𝑅𝑡 are the reflectivity of bottom metal and top metal, respectively, d is the cavity thickness and z is the distance from emitter to bottom metal. 𝐼0() means the emission of radiating molecules. Based on the equation (2), cavity emission spectra of TOLEDs with different cavity lengths have been calculated and the results are shown in Fig. 1 (b). The parameters used for the calculation are presented in Figure S1. It can be seen that a red-shift of the spectra has been caused by the increase of cavity length. The full width at half maxima (FWHM) of cavity emission spectra is limited to dozens of nanometers. Therefore, the microcavity effect in a resonant cavity can be optimized for only monochromatic light rather than white light. White emission should consist of two or three sub-color. The normalized EL spectra of white bottom-emitting OLEDs with different cavity lengths are presented in Figure S2. Since the micarocavity effect is very weak, white emission consisting of two comparable peaks from FirPic and PO-01 can be achieved by the bottom-emitting OLEDs with different cavity lengths. However, the case is different in bicolor TOLEDs due to the strong microcavity effect. Fig. 1 (c) shows the normalized EL spectra of bicolor TOLEDs with similar structures to the white bottom emitting devices. The structure and performance of these bicolor devices used in Fig. 1 (c) can be found in Figure S2. Fig. 1 (d) shows the Commission Internationale de L’Eclairage (CIE) coordinates of spectra in Fig. 1 (b) & (c).Apparently, it is impossible for TOLEDs to obtain comparable EL intensity for each sub-color based on a single resonant cavity structure. Therefore, to solve this problem, dual-microcavity top-emitting OLEDs with two light-emitting units have been fabricated on flexible PET substrates. Dual-microcavity allows to independently 4


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optimize the microcavity effects for different colors. The architecture of DMT-OLEDs can be seen on Fig. 1 (g). For blue emission unit, the structure is Mg: 6.7wt% Ag (120 nm)/MoO3 (3 nm)/TAPC (30 nm)/26DCzPPy: 20wt% FirPic (30 nm) (EMLB)/Bphen (35 nm)/Liq (2 nm)/Ag (20 nm). And the architecture of yellow light-emitting unit is Mg: 6.7wt% Ag (120 nm)/MoO3 (3 nm)/TAPC (40 nm)/CBP: 10wt% PO-01 (30 nm) (EMLY)/Bphen (40 nm)/Liq (2 nm)/Ag (20 nm). Figure S3 shows the fabrication process of DMT-OLEDs. Although DMT-OLEDs consist of two light-emitting units, the manufacturing process is not complicated because the same layers of the two units can be deposited at the same time. Here, AC signals are selected to drive DMT-OLEDs, as shown in Fig.1 (g). So we not only can optimize the microcavity effects for different colors (Fig. 1 (e) & (f)), but also can tune the spectra and intensity of DMT-OLEDs (Fig. 1 (h) & (i)). The color can be changed from blue through white to yellow easily (Fig. 1 (j)), which will be discussed later. Since the optical and electrical properties of DMT-OLEDs can also be investigated by changing the polarity of direct current (DC) signals, the performances of DMT-OLEDs under forward bias and reverse bias have been measured, respectively. The methods of measurement can be found in the Figure S4.

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Figure 1. (a) Phase shift in TOLEDs. (b) Cavity emission of TOLEDs. (c) Spectra of bicolor TOLEDs consisting of blue and yellow sub-color under different cavity length. (d) CIE coordinates of spectra in Fig. 1 (b) & (c). Spectra of (e) Blue unit and (f) yellow unit under different cavity length. (g) The structure of DMT-OLEDs driven by AC signals. EL spectra of DMT-OLEDs respond to AC voltage, (h) when Vn = -6 V, Vp changes from 0 V to 6.5 V. (i) when Vp = 6 V, Vn changes from 0 V to -6.5 V. (j) Photographs of DMT-OLEDs with scatter instruments change from high color temperature to low color temperature. As DMT-OLEDs realized by side-by-side alignment of blue and yellow two units, microcavity effects of blue and yellow emission in these units can be optimized by changing the cavity length, respectively. Here, to avoid influencing the carrier dynamics, the cavity lengths of units are changed by adjusting the thicknesses of Bphen layers, which would be better for the transportation of inductive electrons. Fig. 1 (e) & (f) show normalized spectra of the two units with different cavity lengths. As shown in Fig. 1 (e), the spectra of blue unit changes with the cavity length obviously. When 6


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the cavity length is 95 nm or 100 nm (Bphen: 35 nm), the spectrum of blue unit only has one main peak at 470 nm. And if the cavity length is added to 105 nm, the side peak at about 495 nm increases clearly. Once the cavity length is further increased, the intensity of side peak @ 495 nm is stronger than that of the peak at 470 nm and it becomes the new main peak. The red-shift of spectra in this process corresponds to the calculated results of the cavity resonance wavelength  in Fig 1 (a) & (b). Therefore, 95 nm or 100 nm is suitable for the cavity length of blue unit. Similarly, the spectra of yellow unit can also be adjusted by the cavity length, as shown in Fig. 1 (f). When the length is added from 105 nm to 125 nm, only one main peak can be observed. If the cavity length is further increased, the full width at half maximum will be wider due to the red-shift of the cavity resonance wavelength . So, the cavity length of yellow unit should be no more than 125 nm to ensure one main peak. Besides optical properties, electrical properties of blue and yellow units are also optimized, as shown in Fig. 2. The current densities of two units decrease gradually with the increase of cavity lengths. For blue unit, the spectrum only has one main peak at 470 nm when cavity length is 95 nm or 100 nm. Compared with the 95 nm cavity-length unit, the 100 nm cavity-length unit shows improved current efficiencies, as shown in Fig. 2 (d). Therefore, 100 nm is suitable for the cavity length of blue unit because blue unit shows high color purity and comparable electrical performance at this time. For yellow unit, optical properties indicate that the cavity length of yellow unit should be no more than 125 nm. Fig. 2 (e) shows that 125 nm cavity-length unit shows highest current efficiency. However, the current of 125 nm cavity-length unit decreases obviously compared with the 105 nm and 115nm cavity-length unit. Not only injected electrons, inductive electrons will also be transported in yellow unit of DMT-OLEDs under the positive half of the AC cycle because the two units are connected in series, which has been discussed in our previous work.39 The current of DMT-OLEDs would be lower if we add the cavity lengths, which would result in the lower brightness and efficiency. Consequently, the cavity length of yellow unit should be 115 nm because yellow unit will show comparable electrical performance with high color purity at this time.

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Figure 2. (a) j-V-L and (d) current efficiency of blue unit under different cavity lengths. (b) j-V-L and (e) current efficiency of yellow unit under different cavity lengths. Normalized EL spectra of (c) blue unit and (d) yellow unit at different viewing angles. The insert is angular emission patterns of two units and the comparison with that of the Lambertian emitter. To improve the viewing characteristics, TAPC is used as out-coupling layer and different thicknesses of out-coupling layers are discussed, as shown in Figure S5. Fig. 2 (c) & (f) shows the normalized EL spectra of blue and yellow units with 70 nm TAPC out-coupling layer under different viewing angles, respectively. It is obviously that there is only a little change in the spectra with the increase of viewing angles. Due to the different electromagnetic boundary conditions, the photon density of states in optical microcavity is different from that in noncavity. The spontaneous emission properties of emitting material placed in such a cavity will be then changed by the altered photon density of states, resulting in the a directional enhancement of emission, as shown in the inset of Fig. 2 (f). White color which combined of these two units also shows excellent angular independent characteristics, as shown in Figure S6. The CIE coordinates only changes (-0.0063, 0.0045) when viewing angel changes from 0° to 60°. Based on above discussions, the optical and electrical properties of blue and yellow emission have been optimized, respectively. Then dual-microcavity flexible top-emitting organic light-emitting devices have been demonstrated. Fig. 3 (a) and (c) exhibit the photographs of DMT-OLEDs under forward bias and reverse bias, respectively. It shows that only blue emission can 8


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be seen under forward bias, without any yellow emission. For reverse bias, we can also get the contrary result. It means that, when we drive DMT-OLEDs by AC signals, only one unit of the two units would be under forward bias at the same time and only the unit under forward bias would emit its specific light. Thus, excitons which formed by holes and electrons only realize radiative recombination in the unit which is under forward bias. It will be further verified later. In the positive half of the AC cycle, similar to our previous work, the injected electrons from Ag electrode of blue unit form excitons in EMLB with inductive holes from Mg:Ag metal layer.39 Then blue emission can be observed. Meanwhile, inductive electrons from Mg:Ag metal layer drift across the yellow unit toward Ag electrode. Then inductive electrons can be neutralized with injected holes. This will restore the DMT-OLEDs to its original neutral state, as shown in Fig. 3 (f). Based on this process, free charges can be generated in Mg:Ag metal layer again and then are used in the subsequent negative half of the AC cycle. Thus, yellow color emission can be seen in yellow unit and charge neutralization can be realized in blue unit. DMT-OLEDs will come back to its original neutral state again and will be ready for the next cycle.

Figure 3. Photographs of DMT-OLEDs under (a) forward bias, (b) 50Hz AC signal, (c) reverse bias, 9


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respectively. (d) J-V-L characteristics and (e) CE-L-PE characteristics of DMT-OLEDs under forward and reverse bias, respectively. (f) Energy level schematic of DMT-OLEDs under forward bias. Fig. 3 (d) shows the J-V-L of DMT-OLEDs under forward and reverse bias. The turn on voltage (Von) of DMT-OLEDs is ~4.5 V under forward bias and ~3.2 V under reverse bias. The turn on voltage under forward bias is higher than that under reverse bias. One reason is that the triplet energy level of blue emitter with higher triple energy level is much higher than that of yellow emitter. The other reason is that the cavity length of yellow unit is longer than that of blue unit, making inductive electrons more difficult to drift across yellow unit. Nevertheless, the turn on voltage under forward bias can be reduced by adopting better blue emitter or increase the mobility of transport materials. The maximum luminance of DMT-OLEDs is 11153 cd/m2 under forward bias and 19563 cd/m2 under reverse bias, respectively. From Fig. 3 (e), it is found that the maximum current efficiency (Power Efficiency) of DMT-OLEDs reaches 34 cd/A (18 lm/W) under forward bias and 82 cd/A (67 lm/W) under reverse bias, respectively. These data suggests that DMT-OLEDs show high brightness and efficiency with low turn voltage. Compared with our previous work, the performance of DMT-OLEDs has been improved significantly.39 Besides the influence of optimized microcavity effects, another reason for this improvement is that more inductive electrons can be transported and neutralized in the unit which is under reverse bias. To prove this, two kinds of devices have been fabricated, as shown in Figure S7. The ITO electrode in device A is anode and the ITO electrode in device B is cathode. Here, LiF layer near the ITO electrode is used to prohibit the charge injection from ITO electrode. Thus, the current in device A and device B is determined by electrons and holes injected from Mg:Ag electrode, respectively. From Figure S7, the improvement in DMT-OLEDs can be attributed to the fact that the current in device A is larger than that in device B obviously. Hence, more inductive electrons can be transported and neutralized in the unit. And efficient DMT-OLEDs with low turn on voltage are achieved. From the results shown above, we know that DMT-OLEDs can be operated under both forward bias and reverse bias, which means DMT-OLEDs allow us to selectively operate the individual units by using AC signals. When a square-wave AC signal with low frequency is adopted to drive DMT-OLEDs, the two units in DMT-OLEDs will alternatively turn on and emit pulse of blue and 10


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yellow colors, as shown in supplementary movie. If the frequency of AC signal increases to be sufficiently high (50 Hz), the flicker of two units will unable to be noticed by human eyes. Then, stable and static light of blue and yellow color is observed. As shown in Fig. 3 (b), we can see blue and yellow color at the same time in DMT-OLEDs. In addition, by changing the voltages of positive and negative half cycle of the AC signal, the intensity of two lights can be adjustable. Thus, color-tunable DMT-OLEDs can be realized by mixing two colors based on AC driven signals. The methods for mixing two colors have been discussed in our previous work.39 Scatter instruments are used to obtain mixed color here. To demonstrate the characteristics of color-tunability, square wave AC signals at a frequency of 50 Hz and with a consistent duty cycle of 50% are used to drive DMT-OLEDs. Fig. 1 (h) and (i) present the normalized EL spectra of DMT-OLEDs operated by AC signals with different positive voltages (Vp) and negative voltages (Vn). As shown in Fig. 1 (h), with a combination of Vp of 0 V and Vn of -6 V, DMT-OLEDs only emit yellow light, which is the emission from PO-01. When Vp increases to 5 V, blue light emerges and the intensity grows gradually with the further increase of Vp. However, the intensity of yellow emission has hardly changed when Vp increases. In Fig. 1 (i), as expected, only blue emission from FirPic can be observed when Vp is 6 V and Vn is 0 V. As Vn decreases from -5 V to -6.5 V gradually, the intensity of yellow emission increases correspondingly and the intensity of blue emission keeps unchanged. It is worth to notice that, under proper combinations of Vp and Vn, the same emission from DMT-OLEDs can be adjusted to different luminance. Fig. 1 (j) shows some pictures of DMT-OLEDs with different color temperature. Therefore, DMT-OLEDs show brightness and color tunability from blue through white toward yellow at different Vp and Vn combinations, which is not easy to be achieved in conventional direct current driven OLED. Then, it is beneficial to compensate the color shift caused by different lifetime of emitters and achieve stable white emission.39 And the operation lifetime of the monochromatic devices could also be improved because the backswing voltage can suppress the accumulation of charge carriers in the device.44 We believe that this property is benefit to increase the advantages of OLED lighting source. As discussed in our previous work, besides scatter instruments, reducing the gap between units can be used to obtain mixed color.39 What’s more, brightness and color tunability also can be turned by the duty cycle of AC signal. Therefore, DMT-OLEDs with variety methods to mix and tune the color emission exhibit a 11


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huge potential application prospect.

Figure 4. Transient EL intensities of DMT-OLEDs respond to AC driven signals with different frequency, (a) 50 Hz, (b) 1K Hz, (c) 10K Hz. (d) Pictures of DMT-OLEDs based on paper substrates. To demonstrate DMT-OLEDs indeed emit blue and yellow color alternatively under AC driven signals, the transient EL properties of DMT-OLEDs have been further measured. Sinusoidal AC driven signals with different frequency are used here. The voltage (9 V) is turned to make DMT-OLEDs emitting enough light intensity and the EL responses have been shown in Fig. 4 (a) ~ (c). It can be found that there are indeed EL response at the peaks and valleys of AC driven signal, proving the inference above. And once the frequency is too high (10K Hz), the EL intensities of blue and yellow units will unable to reduce to zero and consequently rise again when the voltage is higher than Von again. In addition, DMT-OLEDs based on paper substrates have been fabricated, as shown in Fig. 4 (d). These demonstrate the universality of this device concept and show the advantages that breaking through the limitations of substrates. ■ MATERIALS AND METHODS All the OLEDs discussed in this paper are fabricated on clean PET substrates or paper substrates. 12


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The surface of paper substrates needs to be modified before they are used. A cross-linked poly(4-vinylphenol) layer is deposited by spin coating and subsequently cross-linked at 160°C for 2 h. Then electrodes and organic functional layers are deposited by thermal evaporation under high vacuum without breaking the vacuum (~6×10-4 Pa). The thicknesses and evaporation rates of these layers are monitored by quartz oscillators. Organic

small

molecular

materials

in

this

study:

hole

transport

layer

adopts

Di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane (TAPC). 4,4'-Bis(carbazol-9-yl)biphenyl (CBP) and 2,6-Bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) are used as hosts in emitting layer (EML).

And

Bis(4-phenyl-thieno[3,2-c]pyridinato-C2’)(acetylacetonato)

iridium(III)

Bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III) (FirPic) are

(PO-01),

used as guests in

EML. 4,7-Diphenyl-1,10-phenanthroline (Bphen) and 8-Hydroxyquinolinolato-lithium (Liq) are used as electron transport layer and electron injection layer, respectively. The structures of the materials used in the device have been presented in Figure S8. The current density-voltage-luminance (J-V-L), current efficiency-luminance-power efficiency (CE-L-PE) and electroluminescence (EL) spectra of unpackaged OLEDs are measured simultaneously by Goniophotometric Measurement System based on spectrometer (GP500, Otsuka Electronics Co. Osaka, Japan) at room temperature in air. 50 Hz pulse waveform with suitable voltage is generated by Agilent B2902A. The scatter instruments consist of a convex lens and a scatter plate. The convex lens is sandwiched between DMT-OLEDs and the scatter plate. Sinusoidal waves with different frequency are provided by function signal generator (DG5102, RIGOL, China). Transient EL intensity is measured by fiber input Si biased detector (DET025AFC, Thorlabs, USA) and oscilloscope (DS4054, RIGOL, China). ■ CONCLUSIONS In summary, high efficient dual-microcavity white top-emitting OLEDs based on AC driven signals have been proposed. DMT-OLEDs can optimize the microcavity effects for different colors without complicating the manufacturing process, also providing a new way for the realization of white top-emitting quantum dot or perovskite light-emitting devices. The color temperature and 13


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intensity of DMT-OLEDs can be independently adjusted because two units of DMT-OLEDs only respond to positive or negative half cycle of AC signals, respectively. Then DMT-OLEDs can be tuned from blue through white to yellow easily. Besides, various flexible opaque substrates are suitable for DMT-OLEDs, like paper, breaking through the limitations of substrates. ■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The parameters used in the equation (2); The structures and performances of bicolor top-emitting and bottom-emitting devices in single cavity; The fabrication process of DMT-OLEDs; The methods of measurement; The spectra of blue and yellow units with different thicknesses out-coupling layer at different viewing angles; The spectra of white color with 70 nm out-coupling layer at different viewing angles; The comparison of current in two devices; The structures of the materials used in the device; A short movie demonstrates device operation principle. ■ AUTHOR INFORMATION Corresponding Author *E-mail: S. H. Liu, [email protected]. *E-mail: W. F. Xie, [email protected]. ORCID Shihao Liu: 0000-0002-0645-5319 Wenfa Xie: 0000-0003-0912-8046 Author Contributions X. Z and S. H. L designed and conducted most of the experiments and prepared the manuscript. T. P 14


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assisted in the preparation of paper-based devices. J. X. Z assisted in the preparation of PET-based devices. L. T. Z mainly analyzed the micorcavity effect in TOLEDs. W. F. X supervised the project, analyzed the data and edited the paper. All authors discussed the results and commented on the manuscript. Notes The authors declare no competing financial interest. ■ ACKOWMLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Nos. 61774074, 61474054, 61475060), and Science and Technology Development Planning of Jilin Province (No. 20190101024JH). ■ REFERENCES (1) Tang, C. W.; Vanslyke, S. A. Organic Electroluminescent Diodes. Appl. Phys Lett. 1987, 51, 913-915. (2) Sasabe, H.; Kido, J. Development of high performance OLEDs for general lighting. J. Mater. Chem. C 2013, 1, 1699-1707. (3) Yin, Y.; Ali, M. U.; Xie, W.; Yang, H.; Meng, H. Evolution of white organic light-emitting devices: from academic research to lighting and display applications. Mater. Chem. Front. 2019, 3, 970-1031. (4) Wu, Z.; Ma, D. Recent advances in white organic light-emitting diodes. Mater. Sci. Eng., R 2016, 107, 1-42. (5) Liu, S.; Yu, H.; Zhang, Q.; Qin, F.; Zhang, X.; Zhang, L.; Xie, W. Efficient ITO-free organic light-emitting devices with dual-functional PSS-rich PEDOT: PSS electrode by enhancing carrier balance. J. Mater. Chem. C 2019, 7, 5426-5432. 15


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Color-Tunable, Spectra-Stable Flexible White Top-Emitting Organic

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