Investigation of the hole transport characterization and mechanisms in co-evaporated organic semiconductor mixtures

The hole transport characteristics in small molecule semiconductor mixtures of HAT-CN : NPB, HATCN : TAPC and HAT-CN : CBP in the ratio of 2 : 1 have been investigated by admittance spectroscopy measurements. It is found that the hole mobility (10(-5)-10(-4) cm(2) V-1 s(-1)) variation with electric field in HAT-CN : NPB is the same as that in HAT-CN : TAPC. The hole transport in HAT-CN : CBP is spacecharge-limited current (SCLC) with exponential distribution traps, different from SCLC with free trap distributions of HAT-CN : NPB and HAT-CN : TAPC mixtures. The hole mobility in the HAT-CN : CBP mixture is obtained as 10(-7)-10(-5) cm(2) V-1 s(-1), which is one or two orders of magnitude lower than those of the other two mixtures. It can be seen that the mobility exhibits strong field-dependence in low electric field region. The mobility slightly increases as the electric field increases and almost saturates in high electric field region. It is shown that the trapping effect leads to the much lower hole mobility. The hole transport mechanisms in the three mixtures have been further studied through energy level analysis and atomic force microscopy.