Highly Flexible Green Light-emitting Diode Based on CsPbBr3 Perovskite Quantum Dots

GUO Jie, LU Min, SUN Si-qi, HU Qiang, ZHANG Jia, BAI Xue

(State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China)

Abstract: Flexible information displays have the promising potential for the future optoelectronic application. However, achieving the highly efficient and stable flexible light-emitting diodes (LEDs) is still a big challenge due to the limited choices of electrode materials and flexible substrates. Herein, the flexible perovskite light-emitting diodes (Pe-LEDs) by combining the flexible substrate based on a photopolymer with the CsPbBr3 quantum dots (QDs) emitting layer were fabricated. In order to improve electrons injection and transport, Ag was used as the cathode. As a result, the green light emitting Pe-LEDs with high luminance of 10 325 cd·m-2 and high color purity with the full-width-at-half-maximum (FWHM) of 19 nm were obtained. In addition, the Pe-LEDs exhibit good flexibility and mechanical ductility, and the as-prepared flexible device still maintains its optoelectronic performance even after repeated bending 100 cycles under a bending angle of about 180°. The research presents a further step toward the future application of flexible displays.

Key words: CsPbBr3 QDs; photopolymer; Ag electrode; flexible light-emitting diode

1 Introduction

Over the past few years, organic-inorganic hybrid perovskite materials have attracted considerable research interest due to their excellent properties, which result in the wide applications, especially for the flexible optoelectronic devices[1-12]. Nevertheless, organic-inorganic hybrid perovskite materials exhibit the inferior stability that restricting their practical applications. Compared to the hybrid organic-inorganic halide counterparts, all inorganic cesium lead halide perovskite(CsPbX3,X=Cl, Br, I or mixed) quantum dots(QDs) exhibit an enhanced stability, and meanwhile they also demonstrate a remarkable optical and optoelectronic performance including the excellent photoluminescence quantum yield(PL QY) levels reaching 90%, high color purity, wide color gamut, and high charge-carrier mobility[13-18]. Therefore, they have the huge potential as an ideal emissive layer materials to fabricate highly efficient perovskite light-emitting diodes (Pe-LEDs)[5,19].

For display applications, flexible, curved displays are key elements in future optoelectronic devices[20]. However, there are still challenges in the fabrication of flexible Pe-LEDs owning to the sophisticated fabricated steps, including the solution-processed spin-coating with various chemicals, high-temperature annealing and vacuum evaporation. These multiple procedures considerably limit the device architecture designs as well as the choices of appropriate flexible substrates and electrodes. The soft plastic substrate is generally a proper alternative for flexible substrate due to its low cost and high flexibility[21-22]. However, they can be easily corroded by some acidic reagent and organic solvents (toluene, chloroform) during the fabrication of LEDs. For the electrodes, although the indium-tin oxide(ITO) has been widely used in the LEDs[23-24], it isn’t suitable for flexible LEDs due to its poor mechanical robustness[25].Therefore, to achieve high-performance flexible Pe-LEDs, we need to choose a suitable electrode and substrate material.

Here, we fabricated a flexible Pe-LED based on all inorganic CsPbBr3perovskite QDs emissive layers, in which the perovskite QDs exhibit an efficient green light emission with the PL QY of 75%. Photopolymer NOA 63 was used as a flexible substrate by spin-coating it on the Si substrate and being stripped from the Si. Simultaneously, Ag film was used as a cathode by simple thermal evaporation[18,26]. Compared with the ITO electrode, the Ag has higher conductivity and better mechanical ductility. In addition, Ag as cathode reduced energy barrier between the cathode and electron transport layer (ETL), which is more conducive to electron injection. Consequently, the as-prepared green light emitting Pe-LEDs exhibit good flexibility, high brightness of 10 325 cd·m-2, high color purity with the full-width-at-half-maximum (FWHM) of 19 nm and low threshold voltage about 2.6 V. The device performance still maintained when the flexible Pe-LEDs were repeatedly bent 100 cycles. These results illustrate that the flexible Pe-LEDs have a great application prospect in the field of the flexible displays.

2 Experiments

2.1 Materials

Cs2CO3and PbBr2were purchased from Sigma-Aldrich. Oleic acid (OA, 90%) and 1-octadecene (ODE, 90%) were purchased from Alfa Aesar. Oleylamine (OLA, 80%-90%) was purchased from Aladdin.

2.2 Synthesis of CsPbBr3 QDs

The CsPbBr3perovskite QDs[13]were synthesized on the base of the previously reported method. Cesium oleate (Cs-OA) was prepared by mixing Cs2CO3(0.814 g), OA(2.5 mL), and ODE(30.0 mL) in a 100 mL three-necked flask, which was degassed and dried under vacuum for 1 h at 120 ℃, then heated to 150 ℃ under N2until a clear solution was obtained. For the synthesis of CsPbBr3QDs, 10.0 mL ODE, 0.138 g PbBr2, 1.0 mL OA and 1.0 mL OLA were loaded into a 50 mL three-necked flask, degassed and dried by applying vacuum for 1 h at 120 ℃; after the solution became clear, the temperature was raised to 180 ℃ and 1 mL of cesium oleate solution was quickly injected. 5 seconds later, the reaction mixture was cooled down to room temperature in an ice-water bath. The reaction mixture needs to be centrifuged for 10 min at 5 000 r/min, and the obtained precipitate was redispersed in 5.0 mL of hexane, centrifuged down again for 10 min at 10 000 r/min, and precipitate was redispersed in 1.0 mL of hexane.

2.3 Device Fabrication

Firstly, photopolymer (NOA 63, Norland) was spin-coated onto a cleaned silicon wafer at a speed of 1 500 r/min for 40 s and exposed to an ultraviolet lamp for 3 min. Then, the Ag electrode was grown on the photopolymer filmviathermal evaporation. Subsequently, ZnO nanocrystals (NCs) solution was spin-coated onto the Ag electrode at a speed of 1 000 r/min for 40 s, and annealed on the hot plate at 150 ℃ for 10 min, after down to room temperature. Then, the substrate was transferred into a glove box. A solution of polyethylenimine (PEI) (dissolve in 2-methoxyethanol in a mass fraction of 0.2%) was spin-coated onto the ZnO film at 3 000 r/min for 40 s and annealed at 125 ℃ for 10 min. CsPbBr3QDs were spin-coated at a speed of 1 000 r/min for 40 s. Next, 4, 4′- Bis (carbazole-9-yl) biphenyl (CBP), 4,4′,4″-tris(carbazol-9-yl)-triphenylamine(TCTA), MoO3, and Au were sequentially deposited using a thermal evaporation under a base pressure of 7×10-4Pa. At last, the flexible LEDs was stripped from the silicon template.

2.4 Characterizations

Absorption spectra were measured using a Shimadzu UV-2550 spectrophotometer. The PL spectra were measured using an Ocean Optics. X-ray diffraction(XRD) patterns were acquired using a Bruker D8 Advance X diffractometer (Cu Kα,λ=0.154 06 nm). The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained on a FEI Tecnai F20 microscope. The current density-voltage-luminance of LED was measured by A Keithley 2612B source measure unit. The LED brightness was determined using a Photo Research Spectra Scan spectrometer PR650. The electroluminescence(EL) spectra were recorded with a Maya spectrometer(Ocean Optics) coupled to an optical fiber.

3 Results and Discussion

The UV-visible absorption and PL emission spectra of CsPbBr3perovskite QDs are shown in Fig.1(a). The first exciton absorption peak and PL emission peak are located at 500 nm and 513 nm, respectively. The CsPbBr3perovskite QDs exhibit a small Stokes displacement and a narrow FWHM about 18 nm. The inset in Fig.1(a) shows the photographs of the CsPbBr3QD solution under a fluorescent lamp and UV lamp (excitation wavelength of 365 nm), respectively. A bright green emission of the CsPbBr3QD solution especially under the UV irradiation was observed. These results indicated that CsPbBr3QDs are benefitting for fabricating the high color purity monochromatic LEDs. Fig.1(b) presents the XRD pattern of CsPbBr3QD powder (JCPDS No. PDF#75-412). The 15.2°, 21.5°, 30.6°, 34.4°, 37.7°, 43.8°, 46.7° correspond to (100), (110), (200), (210), (211), (220) and (300) crystal plane, well consisting with the cubic crystal structure in the literature[13]. The TEM and HRTEM images of CsPbBr3QDs are shown in Fig.1(c) and 1(d), respectively, from which the CsPbBr3QDs are monodisperse and have a cube structure with an average size of 9 nm. The lattice spacing of CsPbBr3QDs is 0.58 nm, corresponding to (100) crystal plane of cubic crystal structure, thus further illustrating the CsPbBr3QDs were successfully synthesized.

The fabricated processes of flexible photopolymer substrate are shown in Fig.2(a). The photopolymer NOA 63 was spin-coated on Si wafer and cured under ultraviolet ozone to form a film, and then the film was peeled off from Si wafer. In order to achieve the balanced charge injection and efficient transport in LEDs, the inverted device architecture of Ag/ZnO/PEI/CsPbBr3QDs/CBP/TCTA/MoO3/Au was designed, as shown in Fig.2(b). Corresponding energy-level diagram of each layer within the flexible device is shown in Fig.2(c), and all the energy levels of these materials used in Pe-LEDs were obtained from the published studies[7]. The Ag and ZnO were used as cathode and ETL, respectively.

Fig.1 (a)UV-Vis absorption and PL spectra of CsPbBr3QDs under excitation wavelength of 365 nm, with insets showing the photograph of QD solutions under ambient light (left) and UV light (right). (b)XRD patterns of CsPbBr3QDs powder. (c)TEM image of CsPbBr3QDs. (d)HRTEM image of CsPbBr3QDs.

Fig.2 (a)Process of fabricating a flexible photopolymer substrate by template-stripping technique. (b)Device structure of the CsPbBr3Pe-LED. (c)Corresponding energy band diagram of the Pe-LED structure.

Herein, we utilized the Ag as cathode mainly because that it is beneficial for electron injection due to the small energy barrier between Ag cathode and ZnO, which is different from the conventional reports in the LED with inverted device structure that ITO normally is used as cathode. The PEI as interface layer not only can modify the energy level of ZnO but also can passivate CsPbBr3films. Simultaneously, the ZnO/PEI layer can also be used as hole block layer (HBL), because the large energy barrier between the valence band maximum (VBM) of CsPbBr3and the highest occupied molecular orbital (HOMO) of ZnO/PEI layer can block the transport of holes from the opposite direction. The CBP and TCTA as hole transport layer (HTL) can form a gradient level due to proper HOMO level between CBP and TCTA, which can facilitate holes transport from anode to CBP layer and inject into emission layer. In addition, they also were used as electron block layer (EBL) to confine electrons in the CsPbBr3emissive layer due to the high lowest unoccupied molecular orbital (LUMO) level. When electrons from cathode and holes from anode recombine in the perovskite emission layer, resulting in EL emission of the flexible Pe-LED.

In order to analysis the performances of flexible LEDs, current density-voltage-luminance(J-V-L) characteristics were performed and shown in Fig.3(a). The device turn-on voltage (generally defined as the driving voltage at which the luminescence of 1 cd·m-2is obtained) is around 2.6 V near the bandgap of the CsPbBr3QDs, which illustrates that an efficient and barrier-free charge injection into the emissive layers is accomplished. Peak luminance of 10 325 cd·m-2is achieved under the applied voltage of 11 V. Fig.3(b) shows the current efficiency-current density-external quantum efficiency(CE-J-EQE) characteristics of the as-prepared flexible Pe-LEDs, which demonstrate a maximum CE of 3.4 cd·A-1and peak EQE of 1.25% at a current density of 158 mA·cm-2. Moreover, the CE and EQE values display little roll-off, indicating a balanced and efficient charge injection in the as-prepared device. The PL spectrum of CsPbBr3QD films and EL spectrum of the corresponding LED device are given in Fig.3(c). Both emissive peaks are located at 513 nm, and without any additional peak from the charge transport materials in the EL spectrum, suggesting that CsPbBr3QDs as the primary exciton recombination centers resulting from the balanced charge recombination during the device operation[27]. However, the EL spectrum has a slight broadening compared with the PL spectrum, which is attributed to the energy transfer from smaller to large QDs in the emissive layers or the dielectric function of the surrounding medium[27-28]. The symmetric emission from the flexible device, corresponding to Commission International de l’Eclairage(CIE) color coordinates of (0.08, 0.73) is shown in Fig.3(d), which reveals the color-saturated green emission and hopeful application prospect in the display.

Fig.3 (a)Current density and luminancevs. driving voltage of flexible Pe-LEDs. (b)External quantum efficiency and current efficiencyvs. current density of flexible Pe-LEDs. (c)PL spectrum of CsPbBr3QD films and EL spectrum of the Pe-LEDs. (d)Corresponding CIE coordinates for the EL spectrum.

The photographs of the curved device before and after lighting are shown in Fig.4(a) and (b), respectively. It is observed that there are no cracks or dark spots while the device was operating. In order to further investigate the mechanical robustness of the flexible Pe-LEDs, we measured theJ-Vand EL characteristics of this device after repeated stretch-release 100 cycles with a bending angle of about 180 degrees. As shown in Fig.4(c) and (d), theJ-Vcurves and EL spectra have no obvious deterioration after repeated bending up to 100 cycles. These results illustrate that the flexible Pe-LEDs based on photopolymer substrate and Ag cathode own high flexibility.

Fig.4 Photographs of the flexible Pe-LEDs before lighting(a), and operated at 5 V(b). (c)Comparison ofJ-Vcharacteristics. (d)EL spectra before and after repeated bending.

4 Conclusion

In this work, we reported a Pe-LED based on the green light emitting CsPbBr3QDs that act as the emissive layers. The Pe-LED manifest a highly flexible performances benefiting from the utilization of a flexible photopolymer substrate, which can avoid the corrosion from an organic solvent in the subsequent Pe-LED fabrication process. Furthermore, we used Ag as a cathode to ensure the conductivity and mechanical ductility of flexible electrode, and moreover the Ag cathode can enhance electrons injection efficiency. Therefore, the flexible Pe-LEDs exhibit a saturated color purity with green emission, low turn on voltage of 2.6 V and high brightness of 10 325 cd·m-2. In addition, the blending test illustrates that flexible Pe-LEDs have excellent flexibility and high mechanical robustness.