Investigation of electron transport properties in Li2CO3-doped Bepp2 thin films

Abstract

The admittance spectroscopy investigations showed that doping lithium carbonate (Li2CO3) into bis[2-(2-hydroxyphenyl)-pyridine] beryllium (Bepp2) greatly improved the electron mobility compared with the pure Bepp2 film. The electron mobility reaches the orders of ∼10−4cm2V−1s−1, almost independent of the electric field. The trap states at low frequencies were clearly observed by capacitance–frequency measurement. The current–voltage and current–thickness characteristics indicated the electron conduction of space-charge-limited current (SCLC) with discrete trap distributions in the intermediate voltage and the SCLC with exponential trap distribution at the higher voltage in the Li2CO3-doped Bepp2 film. We further estimated the density of trap states to be about 4.54×1017cm−3 by the temperature dependent current density characteristics. The investigation of ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS) found that there occurs complicated chemical reaction between Bepp2 and Li2CO3, and the Bepp2 traps more electrons after Li2CO3 doping. This is an effective charge transfer between Bepp2 and Li2CO3, which greatly reduces the electron injection barrier and significantly enhances the electron mobility.