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This paper reports on the color-tunable emission from organic semiconductor films composed of Alq3 doped with DCM. The composite system was excited using an ultraviolet laser to enable the emission of light from green to red regions of the visible spectra based on the nonradiative Förster transfer mechanism. A cholesteric liquid crystal film was used as a blue emitter, while simultaneously serving as a temperature-controlled optical filter based on Bragg reflection to tune the emission color of the organic semiconductor films. The corresponding color points on the CIE chromaticity diagram located near the black-body locus exhibited a wide color-tuning range within a narrow temperature range.
© 2017 Optical Society of America
1. Introduction
Smart lighting has become an emerging trend in lighting technology [1,2], seeking to improve energy efficiency by enabling automatic adjustment of operating parameters based on ambient conditions. The atmosphere of an interior space can also be adjusted by introducing lights of different colors. Studies have shown that lighting can affect one’s psychological state and behavior, with effects on the speed and accuracy with which tasks are completed as well as on learning effectiveness [3–5]. Tunable light sources can be used to alter the lighting conditions in the workplace as well as at home. Researchers have devised several methods by which to achieve this goal using light-emitting diodes [6–10]; however, high cost and limitations to the range of chromaticity tuning has hindered their development.
In this study, thin films of the organic semiconductor tris(8-hydroxyquinoline) aluminum (Alq3) doped with the laser dye 4-dicyanmethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) were used as an emissive medium. Alq3 has been used extensively as a green emitter in organic light-emitting diodes. DCM is commonly used in the red spectral region, providing high quantum yield and photostability. The composite system proposed in this paper was excited using an ultraviolet (UV) laser to enable the emission of light from green to red regions of the visible spectra based on the nonradiative Förster transfer mechanism [11]. The blue component required for white light was obtained from UV-excited cholesteric liquid crystals (CLCs) combined with the organic semiconductor film. The CLC also served as a temperature-controlled optical filter based on Bragg reflection for tuning the color of emissions. Compared to the voltage tuning used in the LC-based approach [8–10], our method uses temperature tuning to continuously shift the reflection band of the CLC from the red to the blue region of visible light in order to produce a diversity of emission colors. The proposed light-emitting device is inexpensive, employs a simple configuration, and enables a wide range of chromaticity within a narrow temperature range.
2. Experiment
The organic semiconductor films were fabricated using a solution process. Alq3 and DCM were incorporated within a poly(9-vinylcarbazole) (PVK) polymer matrix. Six solutions of toluene with PVK:Alq3:DCM in six different weight ratios were spin-coated onto glass substrates and then baked in an oven at 100°C for 30 min. The resulting polymer films were labeled as Films A, B, C, D, E, and F. Table 1 lists the weight ratios of the three organic materials in each film.
Table 1. Weight ratios of PVK:Alq3:DCM in six films
The six films were excited using a He-Cd laser at a wavelength of 325 nm. Measurements of the photoluminescence spectra were used to calculate the 1964 Commission Internationale de l'Eclairage (CIE) chromaticity coordinates. To produce white light, Films D and E were combined respectively with CLCs to fabricate light-emitting devices whose emission color can be controlled via changes in temperature. Figure 1 illustrates the structure of the samples and experiment setup. Liquid crystal (LC) host E7 and chiral agent S811 were mixed at a weight ratio of 75.8:24.2 to produce a helical CLC structure. Poly(vinyl alcohol) (PVA) films used to coat the glass surfaces were rubbed to enable the homogeneous alignment of LC molecules, resulting in a planar CLC texture with gap of 23 μm. The samples were placed on a hot stage and excited by a He-Cd laser at 325 nm. An optical fiber was used for the collection of photoluminescence emissions to a spectrometer.
3. Results and discussion
Figure 2 presents the absorption spectra of PVK, Alq3, and DCM, as well as the emission spectra of Films A to F, PVK, and CLC. The photoluminescence of Alq3 overlapped the absorption of DCM in the wavelength range of 450 to 550 nm, thereby enabling the efficient non-radiative Förster energy transfer. The rapid long-range transfer of energy via resonant dipole-dipole coupling occurred without photon emissions. An increase in the doping concentration of guest molecules was shown to shift the photoluminescence band of the Alq3:DCM films from the green region (with a peak at 500 nm) to the red region (with a peak at 590 nm). The PVK polymer matrix produced weak blue emissions at 400 nm. The UV-excited CLC exhibited stronger fluorescence at the same wavelength, and therefore provided the main source of blue light to produce white light in these experiments. Figure 3 presents images and corresponding coordinates in the CIE chromaticity diagram for each of the light-emitting films. The coordinates of the additive mixture of two light sources were located along a straight line joining the two corresponding points in the chromaticity diagram of the two lights. Films D and E were combined with CLC to fabricate devices for the emission of white light, labelled Devices D and E, respectively.
Fig. 2 (a) Absorption spectra of fluorescent materials. (b) Emission spectra of Films A to F excited at 325 nm at an intensity of 0.25 W/cm2. The inset presents the emission spectra of PVK and CLC under the same excitation conditions.
Fig. 3 (a) Images of each light-emitting film and (b) corresponding color points on CIE chromaticity diagram. The coordinates of the white point are (1/3,1/3).
Figure 4 presents the reflection spectra of the CLC at various temperatures. CLCs are self-assembled periodic helical structures with a pitch within the wavelength range of visible light. The helical structure enables selective reflection in wavelength and circular polarization. At normal incidence, the center wavelength of the reflection band is the product of the mean refractive index of the LC and the pitch. Circularly polarized light with the same sense as the helix twist is totally reflected, a property that is often referred to as Bragg reflection. The helical pitch of the CLC employed in this study was highly sensitive to temperature, which resulted in a considerable blue shift in the reflection band within a narrow temperature range.
Figure 5 presents the photoluminescence spectra of Devices D and E at various temperatures. As the temperature was increased, the spectral regions corresponding to the reflection bands were partially filtered out by the CLC, which resulted in emissions of various colors, as shown in Fig. 6. Device D produced a variety of colors, whereas Device E produced mostly reddish light or white light at a specific temperature. The reflected light in the absorption bands of Alq3 and DCM was absorbed by the organic semiconductor films, whereas the remaining light was transmitted through the films. The emission spectra shown in Fig. 5 cover most of the visible spectrum, resulting in the high color rendering index (CRI) [12] desired in many illumination applications. CRI is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully. CRI measurements are presented as a comparison with a reference source, such as daylight and blackbody radiation, which contains a wide variety of wavelengths ideal for color rendering. Figure 7 shows the chromaticity coordinates as they vary with temperature. All color points lay close to the black-body locus, which is well-suited to illumination applications. The reflection band of CLC depends on the angle of incidence of the light, which means that the color of the devices changed with the viewing angle. We are currently developing a device capable of providing consistent emission colors when observed from any angle.
Fig. 6 Photoluminescence images of Devices (a) D and (b) E at various temperatures: (from left to right) 29, 30, 32, 34, 36, and 37°C.
Fig. 7 (a) CIE chromaticity coordinates of Devices D (green dots) and E (red dots) at various temperatures. The cyan line indicates the black-body locus; (b) Magnified view of data points, in which gray lines indicate various correlated color temperatures: (from left to right) 10000, 7500, 6500, 5000, 4000, and 3000K.
4. Conclusion
This paper reports on organic semiconductor films that enable the control of emission color via temperature. The guest-host system Alq3:DCM was excited using a UV laser to induce the emission of light from green to red regions, based on the nonradiative Förster transfer mechanism. UV-excited CLC provided the blue component of white light, while simultaneously serving as a temperature-controlled optical filter to block a portion of the visible light based on Bragg reflection. The resulting devices enable the tuning of emission color over a wide range of chromaticity within a narrow temperature range. The corresponding color points on the CIE chromaticity diagram lay close to the black-body locus. This study opens up new avenues for the further development of smart lighting.
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