Development and prospect of electroluminescent materials

In  1963, PoPe[1] et al. observed blue EL of anthracene when 400V DC voltage was applied to both sides of anthracene single crystal using electrolyte solution as electrode. Subsequently, Helfrich et al. [2] made a further study on EL of anthracene single crystal. Dresener[3] et al. introduced solid electrodes into organic EL devices in 1969 due to the complexity of the manufacturing process of electrolyte solution electrodes. These early organic EL devices were difficult to grow single crystals and had very high driving voltages (400~2000V) and had little practical use, but these early studies established an understanding of the entire organic electroluminescence process.

In 1973, Vityuk et al. [4] replaced single crystal with vacuum deposition of anthracene film; In 1982, Vincett[5] et al. fabricated organic EL devices using aluminum and gold as cathode and anode and 0.6μm anthracene film as luminescent layer. The driving voltage was greatly reduced (about 30V), but the device life was still very short and the luminescent efficiency was still very low.

Organic EL really took off in the 1980s. In 1987, Tang et al. [6] of Eastman Kodak Produced organic light-emitting diode (OLED) by taking aromatic diamine with good hole transmission effect as hole transport layer, 8-hydroxyquinoline aluminum as luminescent layer, transparent ITO conductive film and magnesium silver alloy as anode and cathode respectively. The device is double-layer film sandwich structure, green light, its driving voltage is less than 10V, luminous efficiency is 1.5lm /W, luminous brightness is up to 1000cd/m2. This kind of ultra-thin flat panel device has attracted great attention due to its high brightness, high efficiency and low driving voltage. Subsequently, Adachi[7] et al. from Kyushu University in Japan introduced an electron transport layer into the device to make a three-layer sandwich structure, which further reduced the driving voltage and improved the luminous efficiency of the device.

In 1990, Bradley et al. [8] from The University of Cambridge used the polymer material POLY (p-phenylacetylene PPV) thin film as the luminescent layer for the first time to produce a single-layer thin film sandwich polymer electroluminescent device. The device's opening voltage was 14V and bright yellow and green light was obtained, and the internal quantum efficiency was about 0.05%.

In 1993, Greenhma[9] et al. inserted another layer of polymer between the two layers to achieve carrier matching injection, which improved the internal quantum efficiency of luminescence by 20 times, which not only broadened the understanding of OLED device mechanism, but also indicated the industrialization of OLED.

In 1998, Baldo et al. [10] studied and developed organic light-emitting devices prepared with general organic materials or doped with fluorescent dyes. Due to the constraints of quantum mechanical transition law of spin conservation, its maximum internal luminescence quantum efficiency is 25%. Organic light-emitting diodes (OLeds) were doped with PtOEP, and the luminescence efficiency was 4% and the internal quantum efficiency was 23%, and the luminescence efficiency increased with the increase of doping concentration.

In 1999, O 'Brien et al. [11] proposed to use BCP (an organic matter that transmits electrons) as a hole blocking layer and phosphotic dye PtOEP doping to prepare OLED with luminescence efficiency of 5.6% and internal quantum efficiency of 32% after studying the exciton transmission law. In August 2000, the research group also doped diphenylpyridine Iridium (Ir (PPY)3) into TAZ or CBP (electron transport material) to prepare organic light-emitting devices with luminescence efficiency up to (15.4±0.2) % and internal quantum efficiency close to 100% under low brightness conditions [12].

In recent years, Zhilin Zhang, Xueyin Jiang et al. [13-14] from Shanghai University have made some achievements in polychromatic organic thin film electroluminescent devices and white electroluminescent devices. A lot of useful work has been done in organic/polymer electroluminescent devices [15] and rare earth doped organic electroluminescent devices [16]. Tsinghua University [17], South China University of Technology, Zhejiang University [18] and other famous universities have also joined the research on organic electroluminescent devices.

With the continuous development of research, productized organic light-emitting display devices are emerging. In 1997, Idemitsu Kosan[19], A Japanese company, successfully developed monochrome video display with gray level of 256, resolution of 240×960, 60 frames /s, and 3cm, as well as red, green and blue (RGB) multi-color organic electroluminescence display. In the same year, Pioneer Electronics[20] of Japan produced the first commercial OLED product, namely automobile communication information system instrument. The company has since introduced passive matrix-driven color OLED displays that can display video images, with high-resolution displays that are almost as good as traditional cathode-ray displays. Eastman Kodak, an American firm, has teamed up with Sanyo, a Japanese firm, to make OLED displays powered by low-temperature polysilicon thin-film transistors that are as thick as a penny. Philips, Uniax and Germany's Covin have also developed organic OLED displays with high efficiency, high brightness and long life.

Electroluminescent EL display is characterized by active luminescent cold light source, surface luminescence and uniform brightness without light spot, low power consumption, long life (more than 5000h), wide operating temperature range (-40~+70℃), ultra-thin, can be tailored according to requirements of any shape and size, its impact resistance, shock resistance is good. EL electroluminescent screen is widely used in active display or backlight display of LCD module, mobile phone, IC phone, magnetic card phone, battery powered display screen, BP machine, watch, car dashboard, audio and TV remote control, handheld GPS receiver, portable computer, etc. With the development of technology, the emergence of dot-matrix module, EL large screen display display will develop rapidly, in the advertising industry, transportation hub, meeting display, etc.