Research On N-, P-Type Doping Of Two Class Of Organic Semiconductor Materials

In recent years, the technology and products of OLEDs have received extensive attention and recognition in the circles of the society for their unique advantages, great potential development and application value. But the performance and wide application of OLEDs are limited for the intrinsical deficiency of organic semiconductor, such as low carrier concentration, low conductivity, poor ability of carrier injection and so on. At present, n-and p-type doping of organic semiconductor materials have been become an important technology to overcome above limitation in OLED field. In this paper, CBP, TPBi and BAlq, three popular host materials in OLEDs, were chosen. The influence of doping on their electrical properties had been researched in detail.Firstly, the p-type doping of CBP had been researched. The p-type doped CBP with FeCl3at various doping concentration, possesses elevated properties of hole injection and transport. At the optimum concentration (12wt%), the nature of transport changes into a trap-free space-charge-limited currents (SCLC) regime (J-V2), indicating that traps has been filled and the injecting barrier is insignificant from ITO/CuPc/NPB to CBP:FeCl3layer. The estimated conductivity is about3.1×10-7S/cm. The estimated hole mobility is about1.26×10-4cm2/V s, nearly six times higher than that of the typical hole transport material NPB (1.63×10-5cm2/V s). In the same way, the p-type doped CBP with MoO3at various doping concentration, possesses elevated properties of hole injection and transport. At the optimum concentration (16wt%), the nature of transport changes into a trap-free space-charge-limited currents (SCLC) regime (J-V2), indicating that traps has been filled and the injecting barrier is insignificant from ITO/CuPc/NPB to CBP:MoO3layer. The estimated conductivity and’hole mobility is2.4×10-7S/cm and6.0×10-5cm2/V s, respectively. Similarly, p-type doped CBP with WO3at the various doping concentration, can improve the hole injection and transport. The estimated conductivity is9.4×10-10S/cm at the optimum concentration (6wt%). Compared with the former two doping materials, effect is poorer. The hole mobility of WO3:CBP is much lower than that of the typical hole transport material NPB.Secondly, the n-type doping of TPBi and BAlq had been researched. The n-type doped TPBi with LiF at the various doping concentration, possesses elevated properties of electron injection and transport. The estimated conductivity is3.6×10-9S/cm at the optimum concentration(4wt%), which is three times of the pure TPBi. Similarly, the n-type doped BAlq with CsF, LiF and Liq at the various doping concentration were investigated, respectively. These dopant can not improve the hole injection and transport of BAlq. The conductivity of pure BAlq is9.7×10-7S/cm, but for the BAlq doped with CsF (6wt%), LiF(2wt%)and Liq(4wt%), the conductivity are8.9×10-9,7.0×10-10and8.4×10-9S/cm, respectively. With doping concentration increases, the nature of electron transport is changed from Ohmic conduction to trap charge limited current. The alkali metal compound doped BAlq can not form alkali metal cation and BAlq anion radical, indicating that BAlq has good chemical stability and cannot produce chemical reaction with alkali metal. Doped alkali metal compounds can form the trap state in the band gap of BAlq, which become the trap of capturing electrons.