Color Tunable Light Source Research Articles

Red InGaN Micro-LEDs on Silicon Substrates: Potential for Multicolor Display and Wavelength Division Multiplexing Visible Light Communication

Red micro light-emitting diodes (micro-LEDs) on silicon substrates are crucial for the realization of large-scale, high-quality, low-cost micro-LED displays, and are also beneficial for high-speed visible light communication (VLC). In this letter, micro-LEDs with different sizes were fabricated. The maximum light output power density of the 20 μm pixel can reach 3.36 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which is 14 times higher than that of the largest pixel (150 μm). By adjusting the pixel size and injection current, a direct color shift from red to green can be observed. Moreover, multicolor emission with uniform brightness can be realized by adjusting duty cycle, which has broad application prospect in monolithic, multicolor displays. Despite the significant blue shift phenomenon, the peak wavelength of all pixels is still greater than 630 nm at the current density of 100 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which means that red light can be emitted in a large current density range to meet various application scenarios requiring different driving conditions. We have realized red, yellow and green micro-LED transmitters on Si substrate by changing the driving current density and modulation bandwidth up to 533.15 MHz is achieved when emitting green light. This is the report of red-emission micro-LEDs on Si substrates for visible light communication for the first time. And we proposed a proof-of-concept monolithic, multicolor wavelength division multiplexing scheme that achieved a total allowable transmission data rate of 2.35 Gbps. Such micro-LED is expected to be applied in multicolor displays, high-speed multi-channel VLC, color-tunable light sources, etc. In particular, due to its high integration and miniaturization, it is expected to be used in future practical application scenarios such as wearable communication devices and smartwatches.

Purely Organic Fluorescence Afterglow: Visible‐Light‐Excitation, Inherent Mechanism, Tunable Color, and Practical Applications with Very Low Cost

AbstractPurely organic materials with visible light excitable fluorescence afterglow are promising for applications. Herein, fluorescence afterglow with various intensity and duration was observed on fluorescent dyes once being dispersed in polymer matrix, thanks to the slow reverse intersystem crossing rate (kRISC) and long delayed fluorescence lifetime (τDF) derived from the coplanar and rigid chemical structure of the dyes. To verify the mechanism, different polymers were used to tune singlet‐triplet splitting energy based on solvent effect. And commercial acriflavine (Acf) film showed blue shifted fluorescence compared to purified one, with slower kRISC (≈100 s−1) and longer τDF (0.6 s). Via energy transfer from Acf to rhodamine B, the afterglow color was further regulated, with the largest fluorescence quantum yield of 42.4 %. It was demonstrated that the materials worked on color tunable light sources, and low‐cost ($2 for 50 000 labels) anti‐counterfeit labels recognized by white light.

Purely Organic Fluorescence Afterglow: Visible-Light-Excitation, Inherent Mechanism, Tunable Color, and Practical Applications with Very Low Cost.

Purely organic materials with visible light excitable fluorescence afterglow are promising for applications. Herein, fluorescence afterglow with various intensity and duration was observed on fluorescent dyes once being dispersed in polymer matrix, thanks to the slow reverse intersystem crossing rate (kRISC ) and long delayed fluorescence lifetime (τDF ) derived from the coplanar and rigid chemical structure of the dyes. To verify the mechanism, different polymers were used to tune singlet-triplet splitting energy based on solvent effect. And commercial acriflavine (Acf) film showed blue shifted fluorescence compared to purified one, with slower kRISC (≈100 s-1 ) and longer τDF (0.6 s). Via energy transfer from Acf to rhodamine B, the afterglow color was further regulated, with the largest fluorescence quantum yield of 42.4 %. It was demonstrated that the materials worked on color tunable light sources, and low-cost ($2 for 50 000 labels) anti-counterfeit labels recognized by white light.

Spatially variant color light source using amplified spontaneous emission from organic thin films

Color tunable light source has great potential for display, lighting, bio-imaging, and so on. Current broadband light sources including halogen lamp, super continuum laser, inorganic and organic light emission diode have been widely used for these purposes. However, each of them has their own limitations in beam divergence, cost and device size. In this work, we demonstrate a spatially variant light source with tunable color emission property by using two cascaded organic thin films, which emit blue and green light respectively under optical pumping. By spatially selecting the overlapping of the directional amplified spontaneous emission from the cascaded films, we show that the color of light emission can be continuously tuned from blue, white to green. The method we propose here also indicates a potential way to design stripe light source for high resolution bio-imaging.

White OLED based on a temperature sensitive Eu3+/Tb3+ β-diketonate complex

A mixed lanthanide β-diketonate complex of molecular formula [Eu0.45Tb0.55(btfa)3(4,4′-bpy)(EtOH)] (btfa–=4,4,4–trifluoro–1–phenyl–1,3–butanedionate; 4,4′-bpy=4,4′-dipyridyl; EtOH=ethanol) was synthesized and its structure was elucidated by single crystal X-ray diffraction. The temperature dependence of the complex emission intensity between 11 and 298K is illustrated by the Commission Internacionale l’Éclairage (CIE) (x,y) color coordinates change within the orange-red region, from (0.521, 0.443) to (0.658, 0.335). The existence of Tb3+-to-Eu3+ energy transfer was observed at room temperature and as the complex presents a relatively high emission quantum yield (0.34±0.03) it was doped in a 4,4′-bis(carbazol-9-yl)biphenyl (CBP) organic matrix to be used as emitting layer to fabricate a white organic light-emitting diode (WOLED). Continuous electroluminescence emission was obtained varying the applied bias voltage showing a wide emission band from 400 to 700nm. The white emission results from a combined action between the Eu3+ and Tb3+ peaks from the mixed Eu3+/Tb3+ complex and the other organic layers forming the device. The intensity ratio of the peaks is determined by the layer thickness and by the bias voltage applied to the OLED, allowing us to obtain a color tunable light source.

39.1: Invited Paper: Efficient Color Tunable Light Sources by The Combination of a Transparent and a Non‐Transparent Organic Light Emitting Diodes

AbstractWe have fabricated color tunable light sources by combining a transparent and a non‐transparent organic light emitting diodes (OLEDs). This type of color tunable OLED has efficiency losses due to the absorption of a transparent OLED. To minimize the efficiency losses, we have come up with an innovative transparent OLED, which has high transmittance for the light from a non‐transparent OLED and also high bottom to top emission intensity ratio for the light from a transparent OLED. Therefore, the lights from a non‐transparent and a transparent OLEDs could be maximized simultaneously, which resulted in efficient color tunable light sources.

Color tunable light-emitting diodes with modified pulse-width modulation

In this study, a color tunable light source, operated by a modified pulse width modulation method, is investigated. By utilizing this method along with anti-parallel connected discrete light-emitting diodes (LEDs) and two electrical terminals, a wide range of the chromaticity coordinates is attained and varied by electrical control. Using the combination of a blue LED and a phosphor-converted yellow LED (blue LED plus yellow phosphor), the chromaticity range is varied by electrical control from pure blue to pure yellow. In addition, using the modified pulse-width modulation method and a combination of white and red LEDs, white light with correlated color temperatures ranging from 5000 K to 2000 K is demonstrated. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

White OLED using β-diketones rare earth binuclear complex as emitting layer

In this work, the fabrication and the characterization of a white triple-layer OLED using a β-diketones binuclear complex [Eu(btfa) 3phenterpyTb(acac) 3] as the emitting layer is reported. The devices were assembled using a heterojunction between three organic molecular materials: the N, N′-bis(naphthalen-1-yl)- N, N′-bis(phenyl)benzidine (NPB) as hole-transporting layer, the β-diketones binuclear complex and the tris(8-hydroxyquinoline aluminum) (Alq 3) as the electron transporting layer. All the organic layers were sequentially deposited under high vacuum environment by thermal evaporation onto ITO substrates and without breaking vacuum. Continuous electroluminescence emission was obtained varying the applied bias voltage from 10 to 22 V showing a wide emission band from 400 to 700 nm with about 100 cd/m 2 of luminance. The white emission results from a combined action between the binuclear complex, acting as hole blocking and emitting layer, blue from NPB and the typical Alq 3 green emission. The intensity ratio of the peaks is determined by the layer thickness and by the bias voltage applied to the OLED, allowing us to obtain a color tunable light source.