Schematic diagram of devices R, G and B based on blue top-emitting OLED(TOLED) is shown in Fig. 1. The blue TOLED is used as a pump source, and a red quantum dot down-conversion film and a green quantum dot down-conversion layer with a micro-cylindrical lens array are covered on the blue TOLED, respectively. Red and green quantum dots absorb part of blue light to produce red and green photoluminescence(PL) emission, respectively. Then, color purity of red, green and blue emission is further improved through filtering action of color filters. Here, RGB emission can be obtained by combining blue TOLED, quantum dot down-conversion layers and color filters, and the whole devices(including color filter and down-conversion layer) used to achieve R, G and B emission are marked as device R, G and B, respectively. The details of structure of the blue TOLED is glass substrate silver (Ag, 100 nm)/molybdenum trioxide(MoO3, 3 nm)/4, 4′-cyclohexyl bis(N, N-bis(4-methylphenyl)aniline (TAPC, 40 nm)/4, 4′, 4′-tris(carbazol-9-yl)triphenylamine(TCTA, 5 nm)/TCTA:bis(4, 6-difluorophenylpyridine-N, C2) Picolinyl hydrazide(TCTA:Firpic, 25 nm)/1, 3, 5-tris((3-pyridyl)-3-phenyl) benzene(TmPyPB, 50 nm)/8-hydroxyquinoline lithium(Liq, 2 nm)/Samarium(Sm, 20 nm)/TAPC(60 nm), wherein the doping ratio of host TCTA and guest Firpic in emitting layer is 8:1.
(Color online)Schematic diagram of devices R, G and B based on blue TOLED
Transmittance of the used color filters is shown in Fig. 2. Each filter only allows light with specific wavelengths to pass and blocks light with other wavelengths, which can greatly improve the color purity of RGB emission. The transmittance of the filters is also very important for the device performances. Higher transmittance leads to that more emission is utilized, and thus the efficiency of the entire device is improved.
Transmittance of the used color filters
Fig. 3 is absorption spectra of red and green quantum dots, and normalized electroluminescence (EL) spectra of devices TOLED, R, G and B. EL spectrum of blue TOLED shows a main peak at 475 nm and a side peak at 500 nm. Device B obtained by combining device TOLED with blue color filter shows pure blue emission with a main peak at 475 nm. The main peak of device G is located at 550 nm, which is mainly derived from the PL of the green light quantum dot. The green emission from device G also has a side peak at 508 nm, and it should be attributed to the EL emission of device TOLED that is not absorbed by the green light quantum dots. The emission peak of device R is at 635 nm, corresponding to PL emission peak of red quantum dots.
Normalized EL spectra of devices R, G, B and TOLED, and absorption(Abs.) spectra of red and green QDs
As shown in Fig. 4, chromaticity coordinates of devices R, G and B are (0.70, 0.30), (0.24, 0.62), and (0.10, 0.20), respectively, and 75.2% NTSC color gamut is achieved by such RGB emission. Apparently, properties of quantum dot materials, such as narrow emission peaks and high color purity, make it easy to achieve wide color gamut. And color filters, which are widely used in liquid crystal display technology, can effectively improve the color quality. Here, full color emission is achieved by a simple method using the combination of blue top-emitting OLED, quantum dots down-conversion layers and color filters. Furthermore, there is still room for improvement in color gamut, such as selecting quantum dot materials with specific wavelength and matched filter. Emission wavelength of quantum dot materials can be easily tuned to cover entire visible wavelength range by changing particle sizes. Therefore, quantum dot materials with specific emission wavelength can be selected according to requirements for achieving wide color gamut.
Chromaticity coordinates of devices R, G and B
As a pump source, blue TOLED is critical to the resulting devices R, G and B, therefore high performance blue TOLED is important for constructing full color display using the method with down-conversion layer and color filter. As shown in Fig. 5, after a series of optimizations on the device structure, the optimized blue TOLED was obtained with a maximum current efficiency of 37 cd/A. After down-conversion and filter actions, devices R, G and B can still obtain a maximum current efficiency of 3.6, 21.9, and 10.6 cd/A, respectively. The inset is photographs of red and green down-conversion films consisting of quantum dots and micro-cylindrical lens array under ultraviolet light. It can be seen that the down-converting films produce very bright PL emission. The current density-voltage-luminance characteristics of these devices are shown in Fig. 6. Since the down-conversion film is placed outside the device and does not change the internal device structure, the current density of each device is nearly same. The feature allows blue TOLED and down-conversion films to be individually optimized, increasing the flexibility of device design.
Current efficiency-luminance characteristics of devices R, G, B and TOLED, the insets are photographs of down-conversion films with red and green QDs under UV light
Current density-voltage-luminance characteristics of devices R, G, B and TOLED
Imprinting technology is a common method used for fabricating microstructures due to its simplicity, short manufacture time and low cost. Using imprinting technology, microstructure with high resolution and consistency can be obtained. As shown in Fig. 7, the down-conversion film obtained by the imprint method has a uniformly distributed cylindrical array on its surface. These micro-cylinders have a regular shape with a clear outline, and its diameter and height are about 20 μm and 5 μm, respectively. In our previous work, we compared the performance of OLEDs based on a down-conversion film without microstructure. Results showed that the microstructure of the down-conversion film is beneficial to improve light extraction and angular characteristics of devices.
Scanning electron micrograph(SEM) of down-conversion film with a micro-cylindrical lens array
We also tested the spectral stability of devices R, G and B. Fig. 8 shows normalized spectra of devices R, G and B at different voltages. It can be seen that as voltage increases from 4 V to 8 V, the spectra of devices R, G and B remain stable. And the chromaticity coordinates of devices R, G and B only change (0.000 9, 0.001 6), (0.000 4, 0.000 6) and (0.000 9, 0.001 2), respectively. The stable spectra of devices R, G and B are obtained because their host device TOLED is a monochromatic device whose spectrum does not change with voltage. Under excitation of blue emission, the emission intensity of quantum dots is also stable so that the resulting devices R, G and B have very high spectral stability.
Normalized EL spectra of devices R, G and B at different voltages
Fig. 9 shows normalized EL spectra of devices R, G and B under different viewing angles. Device R has a very high spectral angular stability since its spectrum hardly changes with viewing angle. The chromaticity coordinates of device R only change from (0.696 2, 0.304 1) at 0° to (0.692 9, 0.305 5) at 60°. When the viewing angle changes from 0° to 60°, the intensity of the side peak(at 508 nm) of the green device is gradually weakened relative to the main peak(at 550 nm), and the chromaticity coordinates also change from (0.244 1, 0.619 8) to (0.265 3, 0.622 1). It is mainly because the emission of the green device consists of two parts, wherein the main peak is derived from PL emission of green light quantum dot and the side peak is derived from EL emission of TOLED. And the PL emission of green quantum dots is not affected by the microcavity effect, while the EL emission of TOLED is. Due to microcavity effect, EL emission of TOLED is mainly concentrated the forward direction. As the angle increases, intensity of the EL emission of TOLED decreases. However, the microstructure of the down-conversion film can scatter emission from device TOLED and down-conversion layer to make emission more dispersed. Eventually, the spectral angular stability of the green device is still very high. Since emission of the blue device is completely derived from the device TOLED so that blue emission is influenced by the microcavity effect. Nevertheless, as the angle increases, the spectrum of device B still keep stable. The color coordinates of device B only change from (0.100 7, 0.195 5) to (0.105 2, 0.174 7) when viewing angle increases from 0° to 60°.
Normalized EL spectra of devices R, G and B under different viewing angles