In this work, a hybrid structure of white electroluminescent (EL) devices has been designed. CdSe/ZnS core-shell quantum dots (QDs) are introduced as color convertors. The photometric and colorimetric characteristics of this device are further experimentally investigated with varying frequency and voltage. Compared with that of conventional white EL devices, this novel device achieves a twice luminance and more stable white light. Moreover, the concept of “color conversion index” is proposed to measure the color conversion ability of QDs in EL devices in consideration of human visual effect.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The alternating current electroluminescent (EL) device is ideally suited for the flat light emitting due to its unique properties, compactness, robustness, and flexibility [1–6]. This planar, all-solid state and colored light device can be a potential alternative to popular OLED [7,8] and QLED [9,10] devices. Generally, it is simply composed of a phosphor layer and a dielectric layer, which are sandwiched between two plane electrodes. This structure is usually produced by screen printing technique. In recent years, new materials [11,12] and advanced techniques [13,14] have been introduced into EL devices, resulting in many inspiring advancements.
Quantum dots (QDs) are newly invented optoelectronic semiconductors with tunable spectra, wide absorption bands, narrow emission bands, which have been utilized in photoresistor, solar cell, and light-emitting diode [15–17]. In particular, core-shell and perovskite quantum dots  are prominent with high stability and quantum yield of close to 95%, which are very suitable as color convertors for blue-green EL devices.
The emission colors of EL devices depend on the different luminescent centers incorporated in the phosphor, which can be manipulated by applied power source as well. Due to lack of reliable red phosphor, white light of EL is usually generated by combining the blue-green emission from phosphor with the red emission from color-conversion materials, known as color-by-blue . However, this approach comes with some inherent problems. For example, the degradation of pigments by sunlight can lead to chromaticity drift over time, and fluorescence in ambient light conditions results in a nonneutral background .
In this paper, we introduce CdSe/ZnS core-shell QDs as color convertors for the blue-green EL from phosphor. A type of white EL device is designed on the basis of hybrid structure. The photometric and colorimetric characteristics of this device are experimentally explored. Furthermore, the advantages and luminescence mechanism of this architecture are investigated as well.
2. Experiment sections
All the chemicals are used without further purification. CdSe/ZnS core-shell QDs are obtained from Suzhou Xingshuo Nanotech Co., Ltd. of China. The red fluorescent pigments RTS-4 are supplied by Swada Color in UK. Copper and chlorine doped ZnS phosphor (25 µm in diameter, D502) and BaTiO3 powder are supplied by Obest Electronic Co., Ltd.. Transparent conductive films coated with indium tin oxide (ITO, 100 Ω/sq) and carbon conductive ink are from Wuhan RunYishang P.E. Tech. Co., Ltd..
2.2 Structure and fabrication of devices
Three types of white EL devices are fabricated by spin coating and named as |HQ|, |HP| and |LP| as shown in Figs. 1(a)-(c), respectively. Figure 1(a) illustrates the |LP| device with a conventional structure as Ref. . The copper doped ZnS phosphor paste is deposited on the top of cleaned ITO film as the emitting layer. Then, a dielectric paste containing BaTiO3 and red fluorescent pigments RTS-4 is printed on it. Finally, conductive carbon paste is printed as the back electrode. For |HP| device, the hybrid emitting layer is designed, which is composed both of phosphor and dielectric materials. The color conversion material RTS-4 is deposited on another surface of the ITO film. For |HQ| device, the structure is as same as that of |HP|, which has the hybrid emitting layer as well. However, the CdSe/ZnS core-shell QDs substitute the pigments RTS-4 as color convertors.
2.3 Characterization and analysis
Optic properties of QDs, pigments RTS-4 and phosphor are illustrated in Fig. 2. The photoluminescence (PL) and absorption (UV-vis) spectra of two color convertors in solution are obtained by a spectrophotometer (Hitachi F-4500) using a 480 nm wavelength laser and ultraviolet-visible spectrophotometer (Shimadzu UV-3600), respectively. Alternating current (AC) power is supplied by the Variable-frequency Power (DH-1005) from Jinan Shengxin Electronic Tech Co., Ltd.. Mostly, the emitting properties of these three type devices are measured in dark ambient conditions with a spectrum radiometer (Photo Research SpectraScan PR-705). All the testing of devices is performed in ambient condition without any packaging.
3. Result and discussion
3.1 Static analysis
The ability absorption profile of QDs and RTS-4 is revealed as shown in Fig. 2(a), which serve as effective EL color convertors. Both of them present efficient absorption in the blue-green spectral region, and emit strong PL light in red region (∼ 600 nm). As shown in the inset, the EL spectrum of ZnS: Cu phosphor exhibits a wide emission band from 410 nm to 590 nm by a stable and standard AC power of 400 Hz and 115 Vrms. The absorption spectra of QDs and pigments overlap well with the EL spectrum of phosphor. For this reason, two types of color conversion materials readily absorb the blue-green EL phosphor emission and convert into red PL light. As a result, the excited red light and residual blue-green light are mixed to produce white light.
After precise adjustment of the concentration of color convertors, all three devices emit light close to standard white (0.33, 0.33) as shown in Fig. 2(b).
It can be seen that the device |HP| and |LP| exhibit similar emitting characteristics, sharing the same emission peaks and spectra shapes. However, the emission intensity of |HP| with hybrid structure is nearly twice that of conventional structure |LP| in all optical wavelength, indicating a better optic performance of hybrid structure. The enhanced emission intensity can be due to the improved microstructure of the hybrid emitting layer. The microstructures of the |LP| and |HP| are shown in Fig. 3. It can be seen that there is a large number of pores between the phosphor in |LP| as shown in Fig. 3(a). In contrast, the BaTiO3 dielectric particles are filled there in instead of air in |HP|, as shown in Fig. 3(b). Since an EL device is a lossy capacitor-like device in substance, the dielectric property of emitting layer plays an important role for its performance . For hybrid structures, the electric filed on phosphor particles surrounded by dielectric materials is enhanced. Moreover, the hybrid one-layer structure between two electrodes reduces the number of interfaces and energy losses in insulated resin, leading to a higher luminance and emitting efficiency of |HP|.
On the other hand, there is an obvious shape difference between the emission spectra of devices |HP| and |HQ| with the same structure, as shown in Fig. 2(b). With less blue EL component (400 nm - 460 nm) and more green component (500 nm - 580 nm), the emission spectrum of |HQ| is more concentrated in the middle wavelength range with higher spectral luminous efficiency. Besides, the PL intensity of color convertor is much stronger with a narrow half-peak width. These phenomena are due primarily to the diverse absorbance of QDs and RTS-4. As shown in Fig. 2(a), the absorbance of QDs is increased with the wavelength decreasing, which is unequal for blue and green light. However, the pigment RTS-4 absorbs the whole visible light spectrum almost fully and equally. Moreover, the RTS-4 apparently absorbs red light (500 nm - 700 nm) which overlap its emission bands and, thereby, serious self-reabsorption. The absorbance of two color convertors for phosphor emission are different in two devices, especially for green component. As a result, the QDs do not quench green emission as much as RTS-4. This is important for QDs to become potential application as red color convertors in white-light-emitting color-conversion devices that rely on a combination of a blue-green EL source and red emitters.
Apart from a more concentrated spectrum, the device |HQ| exhibits a 5.4% higher luminance than that of |HP|. The luminous efficiency value for |HQ| and |HP| is 160.92 lm/W and 145.38 lm/W, respectively, which is comparable to commercially available EL devices . Since the QDs and RTS-4 are excited by the same phosphor, the luminous efficiency of devices is associated with their luminance. The extent to which red PL replaces blue EL is usually assessed in terms of “down conversion efficiency”, which is defined as the ratio of the watts of QDs emission collected to the watts of phosphor emission created . However, the down conversion efficiency of |HQ| is 34.3%, which is lower than that of |HP| in 47.2%. This comparison result is exact opposite of their luminous efficiency, and is caused by the different spectral luminous efficiency for human eyes in different wavelength . To eliminate this disturbing factor and measure the color conversion process effectively, we propose a new concept of “color conversion index” (CCI) to measure the color conversion efficiency for the color convertors according to the formula:
Where PPL-col and Pabs-col are the power of created PL and absorbed phosphor EL by convertors in visual spectral, respectively. Eemi(λ) and EEL(λ) are the emission intensity of the devices and phosphor EL at different wavelength, respectively. V(λ) is the visual spectral luminous efficiency function. λ0 represents the wavelength where the absorbance and PL of color convertor is balanced. Eemi-col(λ) and EEL-col(λ) are the visual emission intensity of the devices and phosphor EL, respectively, in the visual emission spectra as shown in Fig. 4.
Calculated by this new formula, the CCI for RTS-4 and QDs is 68.62% and 68.86%, respectively. The gap between them is very small, which is consistent to human visual effect as shown in the insets in Fig. 2(b). It is worth mentioning that the device |HQ| provides a whiter surface at non-luminous area than that of |HP|, which can be due to the high transparency of QDs and the hiding power of pigments.
3.2 Dynamic analysis
In order to explore the inherent regulations of color conversion process, the emitting properties for three devices are investigated with frequency range of 400-2000Hz and voltage range of 35-145 V. Figure 5 demonstrates the frequency dependent emission spectra of three devices operated at 115 Vrms in detail. It can be found that the EL intensity from phosphor is enhanced with the increasing frequency, and accordingly the excited PL intensity from color convertors. Moreover, the blue-Cu emission band from ZnS:Cu,Cl phosphor is accentuated in intensity relative to the green-Cu band when the frequency increases . As a result, there emerges a new peak in the blue light region about 445 nm in devices |HP| and |LP|. Since the QDs have higher absorbance in the short wavelength direction, the new peak in device |HQ| is not so obvious as in |HP|. At the same time, the emission peaks in red region are never changed, certifying the energy levels of QDs and RTS-4 remain unchanged with the increasing frequency.
Figure 6 illustrates the white photometric and colorimetric characteristics of three EL devices operated at 115 Vrms and the frequency increasing from 400 to 2000 Hz. It can be seen that the luminance (Fig. 6(a)) and CCI (Fig. 6(b)) of all three devices keep rising continuously, which is opposite to color rendering index (CRI, Ra) as shown in Fig. 6(c). Moreover, the device |HQ| exhibits the maximal luminance, CCI and CRI than other devices at all frequencies. Under standard power driving, the device |HQ| exhibits the maximal luminance of 61.67 cd/m2, which is more than twice that of the conventional structure |LP| of 30.59 cd/m2. On the other hand, device |HP| with the hybrid structure shows an increased luminance by 5.4%, compared to that of |LP|.
The luminance of hybrid devices |HP| and |HQ| rises much faster than that of |LP| as the frequency increases, as shown in Fig. 6(a), indicating that the EL intensity for hybrid structure is more sensitive to frequency. On the other hand, it can be observed that the luminance gap between |HP| and |HQ| is enlarged with increasing frequency, which is consistent with the increased gap between their CCI in Fig. 6(b). It proves that the CCI is a reliable valuation parameter for the performance of color convertors.
The three devices present frequency dependence of Commission Internationale de ĺEclairage (CIE) coordinate, and their coordinates are switching to blue-area in color with the increasing frequency, as shown in Fig. 6(d). The device |HQ| exhibits the most stable chromaticity when the frequency changes from 400 Hz to 2000 Hz. It can be found that the CIE coordinate of |HQ| shifts from (0.3343, 0.3574) to (0.3268, 0.2961), which of |HP| moves from (0.3345, 0.3093) to (0.3131, 0.2524). There is not much difference in the y-ordinate variation between them, yet the x-ordinate of |HQ| changes much smaller than that of |HP| and |LP|.
Then, the three white EL devices are operated at 400 Hz and varying voltage as shown in Fig. 7. The luminance and CCI of them increases with the increasing applied voltage, as shown in Fig. 7(a) and Fig. 7(b). It can be seen that the device |HQ| exhibits the maximal luminance and CCI at all frequencies. Besides, the CCI of |HQ| rises much more slowly than that of |HP| and |LP|. It means that, at varied voltages, the color conversion efficiency by QDs in |HQ| is much steady. Since the increasing voltage enhances the EL intensity at all wavelengths almost equally , the color conversion processes in |HP| and |LP| by RTS-4 are more affected by the EL intensity. On the other hand, the luminance of hybrid devices |HP| and |HQ| rises much faster than that of |LP| as the voltage increases, as shown in Fig. 7(a), indicating that the EL intensity of hybrid structure is more sensitive to voltage as well.
The varying voltage induces imperceptible changes on CIE coordinates, as shown in Fig. 7(c). Similarly to the cases with varying frequency, the device |HQ| exhibits best performance with the smallest chromaticity drift. Meanwhile, the green-light component of all three devices basically unchanged. With the increase of voltage, interestingly, the CIE coordinates for devices |HP| and |HQ| with hybrid structures are moving in opposite directions to that for |LP|. The CIE coordinates of |LP| shows a small red-shift, which is same with that of a pure blue-green EL device . In devices |HP| and |HQ|, however, the phosphor EL intensity is enhanced larger than the increase of convertors PL intensity. As a result, the CIE coordinates of devices with hybrid structure exhibit blue-shifts. Since their CCI is increased with voltage, this reversal is getting smaller and may vanish when the voltage is increased over a threshold value.
To summarize, we have demonstrated a novel white EL device |HQ| based on hybrid structure with QDs as color convertors. The dense sturcture of hybrid emitting layer increases the dielectric property and emission intensity of the device. On the other hand, the QDs have an advantage as red color convertors with high CCI in blue-green EL devices. The photometric and colorimetric characteristics of this device are experimentally investigated as functions of the excitation AC voltage and frequency. Except for higher luminance and Ra, the device |HQ| realizes higher chromaticity stability. In addition, we propose the formula of “color conversion index” to measure the extent to which red PL replaces blue EL. The results show that CCI is a reliable valuation parameter for the efficiency of color convertors.
Lab of Green Platemaking and Standardization for Flexographic Printing (ZBKT201903, LGPSFP-01).
The authors declare no conflicts of interest.
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