Polymer electroluminescent materials and transport materials

Polymer electroluminescent materials are mainly concentrated in three types of common polymers :(1) polystyrene (PPV); (2) polythiazate (PTh); (3) polyp-phenylene (PPP) and polyalkyl-sue (PAF).
Polymer transport materials mainly include polymer hole transport materials and polymer electron transport materials.

Polystyrene
Polystyrene (PPV) is a typical representative of polymer electroluminescent materials, which is the most studied, the most extensive, and is considered to be the most promising. Polystyrene (PPV) is a bright yellow luminous polymer, its luminescence is yellow-green, and its primary emission spectrum is at 520nm(2.4eV), and the secondary emission is at 551nm (2.25eV). Classical polystyrene (PPV) is an insoluble, non-melting and difficult to process polymer, which can not meet the needs of light-emitting devices, but the prepolymer route can solve the processability problem of PPV.
According to this Wessling route, the water-soluble or alcohol-soluble PPV prepolymers are obtained first
Using the solution processability of prepolymers, thin films for various devices can be prepared. Then, the prepolymer film is heat treated under vacuum at 180~300℃ for 12h to obtain the PPV film. If the negative ions in the water-soluble polyelectrolyte are bromine ions rather than chloride ions, the thermal conversion temperature can be reduced to less than 100℃.
The remarkable characteristics of Wessling route preparation of PPV are simple reaction and mild conditions
Therefore, electrochemical PPV derivatives such as phenolethylene, polyoxylan ethylene and polynaphthalene ethylene were obtained by Wessling route. It is also used to prepare polythiazol ethylene, polyfuran ethylene, polynaphthalene ethylene and so on.
In addition, insoluble and insoluble PPV can be obtained by chemical vapor deposition, electrochemical polymerization and ring-opening shift polymerization of special structural monomers. In 1990, the Friend research group at the University of Cambridge in the UK first used insolubilization
8,y South structure ITO/PPV/Al single-layer light-emitting device. The results show that the device is 520 and 551nm at 14V, with a quantum efficiency of 0.05%." Then, electricity is transferred to the PBD team in ppV/PBD: methyl acid, and the electron transport layer is introduced between the PPV layer and the metal electrode, and the ITOPPV/PBD is designed:
The PMMA/AI double-layer structure device increases the quantum efficiency to 2%. Yang et al. used metallic calcium with low work function as electrode to make ITO/PPV/Ca single-layer device with quantum efficiency of 2% and working life of 1000h at initial brightness of 100 ed/m2.
If the anode uses conductive polyaniline coated on a flexible substrate such as PET composite film, with PPV as the luminous layer, metal calcium as the negative electrode, it can be made of flexible, flexible, folding all-plastic LED devices. Typical device performance: drive voltage 2~3V; Quantum efficiency 1%.
Although the insoluble and insoluble PPV prepared by the prepolymer route has good luminous performance and thermal stability, it is difficult to make a large area of pinhole-free homogeneous film, which is not conducive to the production of large screen flat display devices. Therefore, solution-processable PPV has become the development target of PPV-class luminous materials.
Soluble and processable PPV is mainly a derivative of PPV, especially substituted PPV, that is, introduced in the benzene ring of PPV
Substituents (alkyl, alkoxy, phenyl or aryl). The substituents on the ring mainly play three roles: first, improve the solubility of the polymer, facilitate the direct spin coating film, simplify the device manufacturing process; The second is the regulation of electronic structure and luminous color, especially the introduction of alkoxy group leads to luminous redshift; The third is to increase air resistance or molecular distortion, reduce molecular aggregation, and reduce concentration quenching.
MEH-PPV is an orange-red luminous polymer that is soluble in a range of common organic solvents such as chloroform, tetrahydrofuran, xylene, etc. The single-layer device ITO/PANi/MEH-PPV/Ca, made of 1%MEH-PPV THF solution, is spun into film. Its luminance is orange red, the wavelength is 591nm, the driving voltage is 4V, and the brightness reaches 4000 cd/m2, the highest brightness
Over 10000 cd/m2, the external quantum efficiency is 2%~ 2.3%, and the working life is 200061 when the initial ground height of the t double p square is 10O~200 cd/m2 and the degree is 400~500 cd/m2. It has recently been reported that the initial brightness is 100~200 cd/m2
The working life exceeds 10000h. The biggest feature of MEH-PPV for polymer IED device assembly is that the band position (HOMO and LUMO) has a good match with the two electrodes (ITO and Ca), so it is very suitable for single-layer device assembly.
Another representative light-emitting polymer in the MEH-PPV family is OCC0-PPV, which was jointly developed by Philips/Hoechst. Compared to MEH-PPV,OCCo-PPV's luminescence is slightly redshifted, emitting red light. The single-layer device has an opening voltage of about 2.8V, an external quantum efficiency of 2.1%, and a lumen efficiency of about 3 lm/W. When flexible devices are made, lumen efficiency is slightly reduced. At the drive voltage of 3.4V and the current density of 4.5mA /cm2, the lumen efficiency is about 2 lm/wl
Cn-ppv is A deep red color friend, especially the presence of a strong electron-absorbing group - CN. On the one hand, compared with PPV-type polymers, CN-PPV has a high luminous efficiency of the device. On the other hand, the electron affinity of the polymerized high polymer is reduced, which is conducive to electron injection and improves the luminous efficiency of the device. On the other hand, the band gap of the polymer is reduced. There are P ppV/CN-PPV/AI, whose luminescence is deep red PPV as the luminescence layer and aluminum as the cathode. The assembled double-layer device ITO/PPV/CN-PPV/Al, whose luminescence is deep red, the wavelength is 710 nm, and the internal quantum efficiency is 4%. This value is the highest value of polymer LED devices at present. It is worth emphasizing that the random copolymer XYZ-PPV with phenyl-substituted PPV and alkoxy-substituted PPV is not only easy to regulate the luminous color, but also has very high fluorescence quantum efficiency.


polythiophene
The outstanding feature of polythiophene (PTh) as a polymer luminescent material is red light, but the disadvantage is that the fluorescence quantum efficiency is low, resulting in the electroluminescence efficiency and brightness of its device are low, that is, the comprehensive performance of polythiophene is not as good as PPV. Therefore, the research work is far less extensive, systematic, and in-depth than that of PPV-like polymers.
The luminescence properties of polythiophene are mainly studied in solution processable poly (3-alkylthiophene). Ohmori et al. first made an orange-red LED device using poly (3-alkylthiophene)(PAT) as the luminescence layer, and studied the effect of alkyl chain length on the electroluminescence characteristics. It was found that with the increase of alkyl chain segment, its luminescence intensity increased, but the luminance of the device was low.
Through the steric hindrance effect of 3-substituents, it is beneficial to regulate the coplanarity of polythiophene, so as to adjust the effective co-frequency degree of the luminous chain segment, and implement the effective regulation of the luminous wavelength (color). For example, using different polythiophene derivatives PCHMT, PCHT, PTOPT and POPT as luminescence layers, LED devices with blue (440nm), green (520nm), orange (590nm) and red (660nm) can be produced respectively, and their external quantum efficiency can reach 0.1%~1%[67.68]. With PTOPT as the luminescence layer and PBD as the electron transport layer, the luminescence color is voltage dependent. Therefore, the luminous color can be controlled by voltage to achieve white luminous.
Recently, it has been found that sulfonated polystyrene (PSS) doped polythiophene derivative (PEDOT) is a transparent conductive polymer, because of its higher work function and suitable electrical conductivity than ITO, has been recognized as a very good performance of hole transport material, widely used to modify conductive glass ITO.


Polyp-phenyl and polyalkyl-su
Polyp-phenylene (PPP) and polyalkyl w(PAF) are the typical representatives of blue polymers due to their large band gap. The outstanding feature is very good light and thermal stability. The luminescence of polystyrene is at 420 nm; Poly (alkyl Sue due to good works, a total of, the light wavelength redshift near to 470 nm.
Like PPV, PPP is also insoluble and difficult to process. Therefore,PPP research has focused on soluble polystyrene derivatives, especially substituted PPP and trapezoidal PPP(L-PPP).
Yang et al. synthesized three soluble alkoxy-substituted PPP derivatives DO-PPP,EHO-PPP and CN-PPP. Using single-layer structure device ITO/ DO-PPP/Ca, the luminous wavelength is about 420nm, the external quantum efficiency is 1.8%, and made of double-layer structure device ITO/PVK/PPP/Ca, the external quantum efficiency of DO-PPP is 3%,EHO-PPP is 2%, and CN-PPP is 1.4%. If air stable metal AI, Ag, etc. is used as the cathode, the quantum efficiency is also between 0.3% and 0.8%. Remmers et al. reported that with P3V/P5V as the luminous layer and calcium metal as the cathode, the assembled device emits blue light (460 nm), and the internal quantum efficiency is as high as 4%.
As mentioned earlier, the introduction of substituents can easily lead to molecular distortion and weaken the interaction between molecules. However, the distorted molecules have poor coincident properties and unexpected blue shifts in the spectrum. However, trapezoidal PPP does show better planarity, and its spectrum is even red shifted to yellow light.
Polyalkyl Fang is the first blue-light polymer (with a luminous wavelength of 470 nm), first reported by Yoshino's research group in Japan. Due to the difficulty of synthesis and poor structural regularity, it has not been studied much.
Recently, it has been found that polyalkylum with high molecular weight and regular structure obtained by improved synthesis methods such as Suzuki reaction route has high luminous efficiency. In addition, due to the introduction of alkyl side chains on the bridge carbon, it has good solubility and processability, and is considered to be a promising class of luminous polymers.
In recent years, the creative contribution of DOW Company in the United States has made polyalkylphenoid polymers not only emit blue light, but also emit green light and red light, which has pushed the research of polyalkylphenoid luminous polymers to a climax and attracted much attention. For example: based on polyalkyl w polymer assembly of green light devices, driving low voltage (2V), high brightness (3.1V drive, brightness 1000 cd/m2; At 6V, brightness up to 10000 cd/m2) high efficiency (lumen efficiency up to 22 lm/w)[12]. 15.4.4 Polyvinyl carbazole (PVK) is a classical polymer hole transport material. It is the transfer of holes by jumping electrons between carbazole groups. The hole mobility is only 10-7cm2/V·s, and only under high electric field can the holes be transferred effectively.
Most common polymer materials can be doped with P-type, that is, P-type common polymer has hole transport characteristics [10]. Therefore, P-type polymers can be used not only as luminescent materials, but also as hole transport materials. For example, PPV is not only an excellent luminescent polymer, but also a good hole transport material. As mentioned earlier, the University of Cambridge in the United Kingdom used CN-PPV to produce a polymer LED with the highest internal quantum efficiency, and its hole transport layer is PPV.
Polysilane is another representative polymer hole transport material. Compared with T-T copolymer, the molecular structure of polysilane is S-copolymer. The delocalization of charge on Si-Si bond reduces ionization energy and increases hole mobility. The hole mobility at room temperature is 10-4~10-5cm2/V·s, which is slightly lower than that of triaromatic amine derivatives (10-3cm/V·s) and 2~3 orders of magnitude higher than that of PVK.
It is expected that polymer hole transport materials can solve the problems of thermal stability, easy crystallization and solution processing of organic small molecule hole transport materials. Usually, organic hole transport materials with excellent properties such as TPD are introduced into the polymer side chain and main chain to construct new polymer hole transport materials. All double-layer devices using polymer hole transport materials show high luminous efficiency and brightness.

Polymer electron transport material
As mentioned earlier, polymer luminescent materials generally have good hole transport properties, while electron transport properties are poor.
In order to achieve the balance of electron/hole injection and transmission, the introduction of electron transport layer between the cathode and the luminescent layer is undoubtedly beneficial to improve the performance of the device.