1. Introduction

Light-emitting diodes (LEDs) have been developed and applied in various systems for a half century. Recently, various materials and structures were used to fabricate LEDs. However, they are still undergoing with an increasing pace due to the requirement of new creative applications and performance improvement. In the dominant market, inorganic materials were used to fabricate LEDs [1–5]. However, in view of the inherent advantages, such as simple fabrication, flexibility, and large area production, organic materials have received considerable attention in the research and market of organic light-emitting diodes (OLEDs) [6–10]. In spite of the quantum confinement for improving performances of OLEDs, various colors can be obtained by blending various dimensional quantum dots into the emissive layer of OLEDs [11–13]. Although single color LEDs are widely used in various applications, white light-emitting diodes (WLEDs) are required for applications in back light of display system, indoor lighting, and optical biosensors [14]. To fabricate white organic light-emitting diodes (WOLEDs), single emissive layer and tandem stacked layers are constructed, recently [15–20]. Among them, to simplify fabrication process, the single emissive layer would be a promising structure. In this work, the films of blended poly(N-vinylcarbazole) (PVK) polymer with various dimensional CdSe/ZnS core-shell quantum dots were utilized as the single emissive layer of WQLEDs. To conquer the disadvantage of low carrier mobility in organic materials and enhance carrier transport surface area, various periodic indium-tin-oxide/zinc oxide (ITO/ZnO) nanorod arrays were formed to work as electron transport/injection layer. The dependence of the resulting performances on the period of the ITO/ZnO nanorod arrays was measured and analyzed. Furthermore, to control the luminous color in white region, various weight ratios of different dimensional CdSe/ZnS quantum dots were blended with PVK polymer in this work.

2. Experiment

Figures 1(a) and 1(b) show the schematic configuration of polymer hybrid WQLEDs without and with ITO/ZnO nanorod arrays, in which 20-nm-thick n-type ZnO, 65-nm-thick quantum dots:poly(N-vinylcarbazole) (QDs:PVK), and 10-nm-thick p-type MoO₃ were used as electron injection layer, emissive layer, and hole injection layer, respectively. Besides, the ITO and Au were employed as cathode electrode and anode electrode. To fabricate ITO nanorod array, after the 250-nm-thick ITO-coated glass substrates were cleaned using acetone, methanol, and deionized water, a 35-nm-thick ITO layer was deposited by using a radio frequency (RF) magnetron sputter. For depositing the ITO layer, a RF power of 75 W was applied to ITO target in the RF magnetron sputter under an Ar gas flow rate of 30 sccm and a working pressure of 5 mtorr. Afterwards, a positive photoresist (AZ 6112) was spread on the deposited 35-nm-thick ITO layer by using a spin coater. An interference grating with various periods was patterned using a two-beam He-Cd laser interference photolithography system [21]. By changing the intersection angle of the two-beam He-Cd lasers, various periodic straight strips were patterned. Afterwards, by rotating an angle of 90°, the perpendicularly crossed grating patterns were defined by exposing the sample using the same process parameters again. Prior to etching the ITO layer about 35 nm in a chemical solution of HCl:deionized water (25:25) for 5 min, the samples were developed by a developer solution for 45 s. To study the dependence of the resulting WQLEDs on the period of the nanorod arrays, the period of 1.0, 1.5, 2.0, and 3.0 µm was formed in this study. Then, the RF magnetron sputter was used to respectively deposit 20-nm-thick ZnO layers on the planar ITO layer and the ITO nanorod array as shown in Figs. 1(a) and 1(b). To deposit the ZnO layer, a RF power of 100 W was applied to ZnO target in the RF magnetron sputter under an Ar gas flow rate of 40 sccm and a working pressure of 5 mtorr. Consequently, the ITO/ZnO nanorod arrays with various periods were formed. The n-type ZnO layer could be utilized as the electron injection layer and passivated the ITO layer for avoiding damage by organic materials. After mixing toluene solvent (1 mL) with blended PVK (5 mg) and various dimensional CdSe/ZnS (25 mg) core-shell quantum dots, the dispersion solution was stirred for one day and further sonicated for 1 h. The 65-nm-thick QDs:PVK emissive layers were formed by respectively spreading the prepared dispersion solution on the planar ITO/ZnO layers and the ITO/ZnO nanorod array using a spin coater with a speed of 2250 rpm for 30 s. To emit red (R), green (G), and blue (B) lights for obtaining white emission, the dimension of 5.2 nm (R), 4.2 nm (G), and 1.6 nm (B) of the CdSe/ZnS core-shell quantum dots was chosen, respectively. After the samples were annealed in a glove box at 90 °C for 30 min, 10-nm-thick MoO₃ layer and 100-nm-thick Au layer were sequentially deposited on the emissive layer using a thermal evaporator.

 

Fig. 1. Schematic configuration of polymer hybrid white quantum dots light-emitting diodes (a) without and (b) with ITO/ZnO nanorod array.

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3. Experimental results and discussion

Figures 2(a), 2(b), 2(c), and 2(d) show the scanning electron microscopy images of the ITO/ZnO nanorod arrays with a period of 1.0, 1.5, 2.0, and 3.0 µm, respectively. Figure 2(e) shows the nanorod profile in the 3.0-µm-periodic ITO nanorod array measured using an atomic force microscopy (AFM). The ITO nanorod revealed a hill morphology with a height of 35 nm. Figure 3 illustrates the photoluminescence spectra of the CdSe/ZnS core-shell quantum dots with a dimension of 5.2 nm, 4.2 nm, and 1.6 nm that were obtained using the He-Cd laser as an excited source. The associated emission peaks located at 611 nm, 523 nm, and 451 nm, which corresponded with the emission of red, green, and blue colors, respectively. To obtain white emission with Commission Internationale del’ Eclairage (CIE) coordinate of (0.33, 0.33), various weight ratios of CdSe/ZnS core-shell quantum dots were blended with PVK polymer as emissive layers of WQLEDs. By keeping the fixed amount of the total CdSe/ZnS core-shell quantum dots (25 mg) and the PVK polymer (5 mg) in the WQLEDs with 1.5-µm-periodic ITO/ZnO nanorod array, Fig. 4 shows the CIE coordinates of the WQLEDs operating at an applied voltage of 10 V. The associated CIE coordinates were (0.311, 0.530), (0.308, 0.395), (0.301, 0.294), (0.305, 0.330), and (0.329, 0.331) corresponded to the weight ratio of RGB quantum dots of (1.3:2.8:0.9), (1.3:1.9:1.8), (1.3:1.0:2.7), (1.3:1.3:2.4), and (1.5:1.3:2.2), respectively. It was found that the white emission corresponded with near (0.33, 0.33) could be obtained by using the weight ratio of 1.5:1.3:2.2 of RGB quantum dots. Besides, its color rendering index (CRI) and color temperature were 73 and 5538 K, respectively. To obtain white emission, the weight ratio of 1.5:1.3:2.2 of RGB quantum dots was chosen in the following studies, hereafter.

 

Fig. 2. Scanning electron microscope images of ITO/ZnO nanorod array with a period of (a) 1.0 µm, (b) 1.5 µm, (c) 2.0 µm, and (d) 3.0 µm, and (e) AFM nanorod profile in 3.0-µm-periodic ITO nanorod array.

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Fig. 3. Photoluminescence spectra of CdSe/ZnS core-shell quantum dots with dimension of 5.2, 4.2, and 1.6 nm.

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Fig. 4. CIE coordinates of polymer hybrid white quantum dots light-emitting diodes using various weight ratios of RGB quantum dots and 1.5-µm-periodic ITO/ZnO nanorod array.

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When the weight ratio of the RGB quantum dots was kept at 1.5:1.3:2.2, the CIE coordinate was maintained at (0.329, 0.331) for the WQLEDs without and with various periodic ITO/ZnO nanorod arrays. It was found that the CIE coordinate did not influenced by the period of the various-periodic ITO/ZnO nanorod arrays. Besides, when various voltages were applied to the WQLEDs, the CIE coordinates were slightly changed. The change of CIE coordinates was attributed to the different emission efficiency of quantum dots at different injection current. The stability of CIE coordinate was determined by the quantum dots used in WQLEDs. Figure 5 shows the current density as a function of apply voltages of the various WQLEDs. At the same apply voltage, the current density increased with a decrease period of the ITO/ZnO nanorod array. Since the contact surface area between the QDs:PVK emissive layer and the ITO/ZnO nanorod array of the WQLEDs increased with a decreased period of ITO/ZnO nanorod arrays, it was deduced that one factor of the increased current density was attributed to the increased contact injection surface area. Comparing Fig. 1(a) with Fig. 1(b), the separation between anode electrode and cathode electrode was shortened in the bulge region of the ITO/ZnO nanorod array. Consequently, at the same apply voltage, the resulting electric field in the bulge region of WQLEDs with nanostructure was larger than that in the WQLEDs with planar structure. The increased electric field was one factor for increasing current density. Furthermore, the total bulge area increased with a decreased period of ITO/ZnO nanorod array. Consequently, the current density increased in a smaller periodic ITO/ZnO nanorod array. Using a PR-655 spectroradiometer with a Keithley 2400 source meter to measure luminance, the inset of Fig. 5 presents the dependence of luminance on apply voltage of the various WQLEDs. By applying the same voltage, one factor of the increased luminance by reducing the period of the ITO/ZnO nanorod array was attributed the resulting increased current density. Besides, because of the low carrier mobility in the QDs:PVK emissive layer, its low carrier transport velocity was resulted. Therefore, to shorten the carrier transport distance and increase the carrier transport velocity (due to the increased electric field) by using the ITO/ZnO nanorod array to reduce the separation between anode electrode and cathode electrode, the carrier recombination opportunity would be increased to increase luminance. At the apply voltage of 10 V, compared with the luminance of 7191 cd/m² of the WQLEDs without ITO/ZnO nanorod array, it was increased to 17816 cd/m² of the WQLEDs by using 1.0-µm-periodic ITO/ZnO nanorod array. Moreover, the luminance of 9828, 13244, and 16333 cd/m² was obtained for the WQLEDs with a period of 3.0, 2.0, and 1.5 µm, respectively. The luminance increased with a smaller period of ITO/ZnO nanorod array was attributed to the larger carrier recombination opportunity and the more injection current density caused from the larger injection surface area of ITO/ZnO electron transport/injection layer. Due to the constant total amount of QDs existing in the QDs:PVK emissive layer, to obtain effective hole-electron recombination through QDs, the injection current density was restricted. Although the injection current density of the WQLEDs using the 1.0-µm-periodic ITO/ZnO nanorod array was larger than that using the period of 1.5 µm, the slight increase of luminance was attributed to the restricted effective injection current density for recombination via limited amount of QDs.

 

Fig. 5. Current density as a function of apply voltage of polymer hybrid white quantum dots light-emitting diodes without and with various periodic ITO/ZnO nanorod arrays. The inset shows dependence of luminance on apply voltage of various polymer hybrid white quantum dots light-emitting diodes.

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Figure 6 shows the luminous efficiency as a function of apply voltage of the various WQLEDs. It could be seen that the luminous efficiency increased with an increase of apply voltage until the maximal luminous efficiency and then decreased by further increasing apply voltage. Since the injected current density increased with an increase of apply voltage, the resulting hole-electron recombination rate was increased to enhance luminance and associated luminous efficiency by increasing apply voltage. However, the total number of quantum dots in the QDs:PVK emissive layer was kept constant. Consequently, to obtain effective hole-electron recombination occurred at active emissive quantum dots through inactive PVK polymer matrix, the injected current density was restricted. It was deduced that the resulting luminance was saturated at a higher current density by applying a larger voltage as shown in the inset of Fig. 5. Therefore, the luminous efficiency was reduced by further applying voltage beyond the voltage corresponded to the maximum luminous efficiency due to the saturation effect of the effective hole-electron recombination.

 

Fig. 6. Luminous efficiency as a function of apply voltage of various polymer hybrid white quantum dots light-emitting diodes.

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As shown in Fig. 6, it was found that the maximum luminous efficiency of 2.30 cd/A was obtained for the WQLEDs without ITO/ZnO nanorod array. For the WQLEDs using the periodic ITO/ZnO nanorod arrays with a period of 3.0 µm, 2.0 µm, 1.5 µm, and 1.0 µm, their maximum luminous efficiency was 2.50 cd/A, 2.85 cd/A, 3.13 cd/A, and 2.95 cd/A, respectively. It was worth noting that the maximum luminous efficiency increased with a decrease period of the ITO/ZnO nanorod array until 1.5 µm and then decreased by further reducing period to 1.0 µm. As mentioned above, more carriers injected from the larger contact surface area and faster carrier transport velocity caused by increasing electric field in the smaller periodic ITO/ZnO nanorod array under the same apply voltage. Consequently, the electron-hole recombination opportunity could be enhanced owing to the shorter carrier transport distance and the faster carrier transport velocity using the smaller periodic ITO/ZnO nanorod array. Therefore, at the same apply voltage, the luminous efficiency increased in the WQLEDs using a smaller periodic ITO/ZnO nanorod array until a period of 1.5 µm due to more injected carrier to make more hole-electron recombination rate. However, the luminance was slightly larger and the injection current density was larger of the WQLEDs using the 1.0-µm-periodic ITO/ZnO nanorod array. Consequently, the luminous efficiency of the WQLEDs using 1.0-µm-periodic ITO/ZnO nanorod array was smaller than that of using 1.5-µm-periodic ITO/ZnO nanorod array.

4. Conclusions

In this work, QDs:PVK single emissive layer and nanostructured ITO/ZnO electron injection layer were proposed to fabricate WQLEDs. By blending the PVK polymer with the weight ratio of 1.5:1.3:2.2 for RGB emissive quantum dots, CIE of (0.329, 0.331), CRI of 73, and color temperature of 5538 K were respectively obtained for the resulting WQLEDs. To shorten carrier transport distance, increase electric field, accelerate carrier transport velocity, and increase injection current density, various periodic ITO/ZnO nanorod arrays were fabricated. By using 1.5-µm-periodic ITO/ZnO nanorod array, the luminance of 16333 cd/m² and the luminous efficiency of 3.13 cd/A were obtained in the WQLEDs. The proposed nanostructured nanorod array and blended single emissive layer of inactive PVK polymer matrix and active emissive quantum dots would be a promising construction for fabricating white light-emitting diodes. To further improve performances of the proposed WQLEDs, the optimal thickness of QDs:PVK single emissive layer and the optimal height of ITO/ZnO nanorod array would be worthily important issues.