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All-inorganic perovskite CsPbBr3 quantum dots (QDs) with an emission peak of around 520 nm were synthesized by a hot-injection method, and were systematically studied as green phosphor for light-emitting diodes (LEDs). Highly pure green light with an emission peak of 534 nm and a full-width at half-maximum (FWHM) about 20 nm was achieved using CsPbBr3 QDs and GaN LEDs. Commission Internationale Ed I’Eclairage coordinate of the fabricated green LEDs was (0.203, 0.757). Compared to GaN LEDs, the current-voltage characteristic of the green LED did not show any degradation. Moreover, the green LEDs displayed a luminous efficiency of 31.92 lm/W under an injection current of 10 mA.
© 2016 Optical Society of America
1. Introduction
Light-emitting diodes (LEDs) have attracted significant interest due to their advantages of small size, low energy consumption, high efficiency and long life time. In general, LEDs were recognized as one kind of the economical and efficient solid-state lighting sources [1, 2]. In order to achieve high color rendition (Ra > 80) of white LEDs, the component ratio of green light and red light should be larger. Furthermore, high-purity green light plays an important role in backlight of liquid crystal display [3]. Unfortunately, green GaN-based LEDs suffer from a rapid degradation in efficiency when the emission wavelength is in the range of 500 nm to 600 nm, which is called “green gap” phenomenon [4, 5]. In general, the external quantum efficiency of green GaN-based LEDs chip is about 30% which is much lower than that of blue GaN-based LEDs. One of the alternative routes to obtain green light effectively is by combining blue or near-ultraviolet LEDs chip with green phosphor. For example, Osram reported a green LEDs prototype emitting at 530 nm with a spectral width of 35 nm [6]. Beside commercial report, a series of academic studies about green inorganic phosphors doped with rare-earth elements have been reported recently. However, the green light excited from these phosphors raised concerns about poor monochromaticity [7, 8] or wide bandwidth (> 50 nm) [9], which cannot satisfy the demand for high-purity green light. Therefore, it is necessary to develop green phosphor with good monochromaticity and narrow bandwidth.
Recently, a number of quantum dots, such as ZnAgInS, CuInS, CdSe/ZnS and PbSe, have been applied as color converter for white LEDs and near-infrared LEDs, since they have advantages of tunable emission wavelength, narrow bandwidth and high photoluminescence (PL) efficiency [10–15]. Besides, QDs have negligible scattering due to their ultra small sizes (<10 nm). Over the past two years, inorganic cesium lead halide perovskite (CsPbX3, X = Cl, Br and I) QDs attracted intense attention for their combined superior optical performance of both QDs and halide perovskites, which exhibit great potential in the field of LEDs, lasers and solar cells [16–18]. In particular, CsPbBr3 QDs have a pure green PL emission of around 520 nm with a bandwidth of about 20 nm and PL quantum yields of about 90%, indicating that CsPbBr3 QDs have huge potential to be a candidate material of green phosphor. Despite of these superior properties that CsPbX3 QDs possess, current research are mainly focused on synthetic techniques to control their morphology [19]. To our best knowledge, there are few reports on unitizing CsPbBr3 QDs as green phosphor for the fabrication of green LEDs [20].
In this paper, all-inorganic perovskite CsPbBr3 QDs were synthesized by a facile hot-injection method and their luminescent properties were investigated. Then the green LEDs were successfully fabricated by combining CsPbBr3 QDs and blue GaN LEDs chips with emission wavelength of 435 nm. Moreover, the optoelectronic characteristics of the green LEDs under different driving currents were investigated in detail.
2. Experimental
2.1 Synthetic procedures
The inorganic perovskite QDs were prepared by a hot-injecting method [17]. A mixture of 100 mg Cs2CO3, 0.6 ml of oleic acid (OA), and 5 ml of octadecene (ODE) was degassed under nitrogen flow in a three-neck flask at 150 °C for 1 hr until all the Cs2CO3 reacted with OA, which is used as Cs-precursor solution. Meanwhile, a 100 ml three-necked flask with 5 ml ODE and 0.36 mmol of PbBr2 was placed in a heating jacket with flowing nitrogen, and the flask was heated to 150 °C for 1 hour. Then, 0.5 ml oleylamine (OLA) and 0.5 ml OA were injected into the PbBr2 crude solution quickly, and remain the temperature 150 °C for several minutes until the PbBr2 is solubilized completely. 0.4 ml Cs-precursor solution was swiftly injected and the reaction kept for about 10 seconds. After that, the flask was cooled by the ice bath to guarantee the crystallization of CsPbBr3 QDs. For the purification of CsPbBr3 QDs, the crude solution was centrifuged by dissolving them in toluene.
2.2 Green-LEDs fabrication
Blue GaN LEDs chip (1 mm × 1 mm) was welded in a groove with a radius of about 2 mm. To realize pure green emission from the blue LEDs, the quantity of CsPbBr3 QDs coating GaN chip is key parameter. Thus, 5 mg CsPbBr3 QDs taken out from the extracted powder were re-dispersed in 10 mL toluene, and then ultrasonic treatment was executed to disperse the QDs sufficiently. Following, with assist of a pipettor, 0.03 mL QDs solution was directly dropped into the LEDs groove. Then, the LEDs was putted onto a heater with a temperature of 50 °C to volatilize toluene and to form QDs film. The thickness of the film created by 0.03 mL QDs solution was meaured to be about 120 nm, and also the film thickness has linear relation with the droped solution.
2.3 Characterization
For the characterization of CsPbBr3 QDs, the crystal phases were characterized by X-ray diffraction (XRD) with Cu Ka radiation (XRD-6100, Shimadzu, Japan). The transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) spectrum were recorded by a electron microscope (Libra 200 FE, Zeiss, Germany). The absorption spectrum was recorded at room temperature ranging from 300 to 800 nm by a UV-vis spectrophotometer (UV-2100, Shimadzu, Japan). The PL spectrum was measured by a fluorescence spectrophotometer (Cary Eclipse, Agilent, America) including a Xe lamp as excitation source with optical filters.
3. Results and discussion
3.1 Characterization of CsPbBr3 QDs
The structure and composition of the synthesized CsPbBr3 QDs were characterized by combined techniques. The XRD spectrum suggested the CsPbBr3 QDs had a cubic atomic structure, as shown in Fig. 1(a). According to Bragg’s law nλ = 2dsinθ, the lattice distance in (200) direction was 0.58 nm, which had been further confirmed by high-resolution transmission electron microscopy (HRTEM) image shown in Fig. 1(c). The HRTEM image also demonstrated that the CsPbBr3 QDs had a high crystallinity. TEM image in Fig. 1(b) indicated the QDs exhibited good monodispersity with average size of about 15 nm. The EDX spectrum presented in Fig. 1(d) confirmed that the QDs were composed of mainly Cs, Pb and Br. No impurity element had been identified. Following the identification of structure and composition, optical properties of CsPbBr3 QDs were investigated. A picture of toluene containing CsPbBr3 QDs under 365 nm light excitation was presented as the inset of Fig. 2.
Fig. 2 Absorption and PL intensity spectra of the CsPbBr3 QDs, and insert is the fluorescence photograph of the CsPbBr3 QDs under 365 nm light excitation.
The solution gave out evenly-distributed and intensive green light. The quantum efficiency of the perovskite CsPbBr3 QDs solution was about 85%, which was measured by the Quantaurus-QY from HAMAMATSU. The absorption and PL spectra of the QDs in toluene were given in Fig. 2. The absorption peak was at 515 nm while the PL peak located at 520 nm. The PL spectrum was a good Gaussian curve and its full-width at half-maximum (FWHM) was about only 20 nm, indicating high-purity green emission. It is worth noting that there was a large overlap region between the absorption and PL spectra.
3.2 Optical properties of the as-fabricated green LEDs during coating procedure
Since the CsPbBr3 QDs are capable of emitting narrow-band green light, they may be potentially used as green phosphor. Blue LEDs with emitting wavelength of 435 nm were used as excitation source, whose emission spectrum was given as the black curve in Fig. 3(a). To analyze the suitable quantity that realized near-pure green light emission, the emission spectrum of the as-coated LEDs was recorded by a HR-2000 + spectrograph as soon as the LEDs was lighted by the driving current at room temperature.
Fig. 3 (a) Emission spetrum change of LEDs with the addition of QDs solution, the insert is the light component changes of the emission light; (b) Chromaticity coordinates change of the as-coated LEDs with the addition of QDs solution, the insert is the green light purity analyses.
Along with the addition of QDs solution directly deposited into the groove of LEDs, the spectrum variation under a constant driving current of 10 mA was presented in Fig. 3(a). Two emission bands peaked at 435 nm and 530 nm could be observed, and they could be assigned to the emission of blue GaN chip and CsPbBr3 QDs, respectively. Along with the QDs solution quantity (layer thickness) increasing, the blue emission intensity of 435 nm reduced gradually, and it almost disappeared while the QDs layer was thick enough to absorb the emission of blue GaN chip completely. The intensity of green emission presented an obvious increase, but it reached a maximum at a deposition volume of 0.15 mL, and then decreased with further addition of QDs solutions. Thus, it can be concluded that the thickness of the QDs layer strongly affected both the absorption of blue light and the emission of green light.
A directly spectral integration of the blue and green lights of the measured spectrum was carried out to analyze components of the emission light. As shown in the insert of Fig. 3(a), the percentage of emitted green light increased with the addition of QDs. The energy transfer efficiency from blue to green light might be estimated by the ratio of the light integration, and it was calculated to be 42.1% at 0.15 mL. When the deposited volume of QDs solution reached 0.48 mL, the emission of the coated LEDs was close to pure green light that exceeded 99% of the whole spectrum, and the thickness of the film on GaN LEDs chip was nearly 2 μm.
The change of emission spectrum allowed realization of color rendering. It could also be seen from Fig. 3(b) that the Commission Internationale Ed I'eclairage (CIE) chromaticity coordinates of the spectrum changed from pure blue to nearly pure green followed by the increased deposition volume of QDs solution. And finally, the chromaticity coordinates of the near-pure green LEDs reached (0.203, 0.757) when the QDs solution quantity was 0.48 mL. Light purity analyses were carried out to quantitate the emitted green light of the as-coated LEDs [21]. According to the variational chromaticity coordinates in Fig. 3(b), it could be seen that the coordinate reach green region while the quantity of QDs solution achieved 0.21 mL. Thus, this coordinate corresponding to 0.21 mL was taken as the first point. For the calculation of light purity, determination of dominant wavelength of the as-coated LEDs is key step, and it can be speculated by the chromaticity coordinates of the as-coated LEDs and pure white light as shown in Fig. 3(b). Then, the green light purity can be calculated by a/(a + b). As given in the insert of Fig. 3(b), the green light purity of the as-coated LEDs increased gradually along with the QDs solution increasing, and it finally achieved 94.3%.
Meanwhile, it is worth noting that along with the addition of CsPbBr3 QDs, the green emission (around 530 nm) had a red shift, as evidenced by the enlarged and normalized plot in Fig. 4(a). And also compared with the PL emission in solution shown in Fig. 2, the emission peak had a red shift. It was speculated that both of these red shifts might be induced by the same reason, i.e., QDs particles aggregation in solid state. According to spectral overlap between absorption and emission spectra shown in Fig. 4(b), the absorption intensity of the wavelength that overlapped the short wavelength of emission was larger than that overlapped the long one, which resulted in stronger self-absorption in the short wavelength. This kind of spectral overlap usually can be described in terms of Förster distance (R0), and red-shift induced by the re-absorption always occurs in small interparticle distance less than R0 [22, 23]. In our case, the QDs particles would aggregate together after toluene volatilized. Therefore, the short wavelength (490~520 nm) of the emission was weakened, and as a consequence, the long wavelength of the emission was comparatively strengthened. Along with the addition of QDs, the number of aggregated QDs increased, resulting in a continual red shift shown in Fig. 4(a). The red shift of green light peak as a function of solution quantity was given in Fig. 4(c), where linear fitting of the red shift was also presented. The emission peak of green light increased along with the solution quantity increasing and the linearity achieved 0.997. According to the fitting result, it can be speculated that the red shift is 24.437 nm per mL.
Fig. 4 (a) Enlarged and normalized emission peak of green light in Fig. 3(a); (b) Normalized intensity of absorption and PL spectra of CsPbBr3 QDs; (c) Red shift of green light peak as a function of solution quantity.
3.3 Optoelectronic properties of the fabricated green LEDs
Following the achievement of near-pure green LEDs, the green LEDs was injected with different currents from 1 mA to 200 mA. Spectrum was also recorded by the HR-2000 + spectrograph and measured results were given in Figs. 5 and 6.
Fig. 5 (a) Spectrum intensity and (b) chromaticity coordinates of the green LEDs under different injection current; (c) Schemtical structure of the green LEDs; (d) and (e) photographs of the green LEDs without and with driving current.
Fig. 6 (a) Voltage-current curve of the green LEDs and bare blue LEDs; (b) Emission peak and FWHM variation of the green LEDs along with different driving currents.
The emission spectra of the coated LEDs with the driving currents of 5 mA, 50 mA, 100 mA, 150 mA, 200 mA were given in Fig. 5(a), where the intensity increased gradually along with the current increasing, and no obvious shift of PL spectrum occurred under different driving currents. Figure 5(b) showed CIE chromaticity coordinates of these PL spectra. The chromaticity coordinates were overlapped when the drive current reached above 50 mA, further confirming no spectrum shift occurred. Figures 5(c)-5(e) presented the structure of the coated LEDs and photographs of the LEDs without/with driving currents.
As presented in Fig. 6(a), it could be seen that the voltage-current curve of the coated LEDs was almost the same with the one of bare LEDs, indicating the coating QDs had nearly no influence on the circuit of bare LEDs. Figure 6(b) showed the emission peak position and FWHM variation of the coated LEDs under different currents. In the wide current range from 1 to 200 mA, the emission peaks fluctuated only in between 533.3 nm and 534.2 nm, verifying no shift of PL spectrum occurred. The FWHM basically remained a wideband of 20.39 nm while the driving current was under 160 mA. After the driving current exceeded 160 mA, the FWHM presented an increasing tendency. It had been reported that the FWHM increased consistently while increasing the temperature of QDs [24, 25], which could be attributed to the increasing temperature induced by the increasing current.
3.4 Luminous properties of the fabricated green LEDs
Finally, luminous performance of the coated LEDs was evaluated and luminous parameters were measured by a HAAS-2000 spectroradiometer with increasing driving currents from 10 mA to 200 mA. As the comparision, luminous performance of the used bare LEDs was also tested. As shown in Fig. 7 (a) and (b), results of both bare LEDs and coated LEDs had the same tendency, i.e., luminous flux increased along with the current increasing, but the luminous efficiency decreased.
Fig. 7 (a) Luminous flux and luminous efficiency of the bare LEDs, and (b) Luminous flux and luminous efficiency of the green LED under different driving currents.
The luminous efficiency of the coated LEDs (green light) was 31.92 lm/W under 10 mA driving current, and it decreased to be 11.62 lm/W when the driving current was 200 mA, which is much lower than the commercial reports [6]. Luminous efficiency of the bare LEDs (blue light) also had an ouvious decrease from 7.95 lm/W at 10 mA to 5.9 lm/W at 160 mA, which was one of these factors resulting in the decrease of luminous efficiency of coated LEDs. However, the attenuation rate of bare LEDs was much smaller than that of the coated LEDs. Thus, the major factor might be the QDs fluorescence quenching induced by increased temperature while the LEDs was lighted with bigger current. Also the luminous performance of the coated green LEDs was related to several other factors, including the used CsPbBr3 QDs and coating structure. In further work, all these factors will be considered to improve the luminous performance of coated green LEDs.
4. Conclusions
In summary, all-inorganic perovskite CsPbBr3 QDs utilized as the green phosphor for green-light emission were successfully demonstrated in this paper. High-quality CsPbBr3 QDs with cubic shape and narrowband green emission were synthesized by a hot-injecting method. By coating the synthesized QDs onto blue GaN LEDs, highly pure-green emission with FWHM about 20 nm was obtained. The coated green LEDs showed a red shift with the CsPbBr3 QDs increasing, which could be contributed to the QDs aggregation and overlaps between absorption and emission spectra. Under different driving currents from 1 mA to 200 mA, the chromaticity coordinate of the coated green LEDs was (0.203, 0.757) with almost no variation in the CIE coordinates. Moreover, the luminous efficiency of the coated green LEDs achieved 31.92 lm/W at a 10 mA driving current. These experimental results demonstrated that CsPbBr3 QDs are good candidate for green phosphor and can be applied in green LEDs.
Acknowledgments
This work was financially supported by grants from National Natural Science Foundation of China (NSFC) (61404017, 61520106012), Natural Science Foundation of Chongqing (cstc2015jcyjA1055, cstc2015jcyjA90007), Fundamental Research Funds for the Central Universities (106112015CDJZR125511, 106112015CDJXY120001).
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