In the present work, in order to improve electron injection and transport at the interface of the hole blocking layer (HBL) and the electron transport layer (ETL) in the hole-domain solution processed phosphorescent organic light emitting devices (PhOLEDs), the mixed interface layer (MIL) was fabricated by partially co-doping hole blocking material 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) and electron transport material tris(8-quinolinolato) aluminum (Alq3) between HBL/ETL. The MIL thickness was kept at 10nm, while the doping ratio of these two materials varied. Under a given electric field, the devices with the MIL at any mixed ratios all show much higher luminance and current density than those with a typical interface. For example, the luminance power and current density at 10 V for a typical device are 1.03 μW and 5.13 mA·cm-2, while in case of mixed interface are 3.64 μW and 18.1 mA·cm-2, respectively. From data results and theoretical analysis, the possible derivation of these improvements is considered to be the reduced electron accumulation at the interface resulting from the reduced electron injection energy barrier and lowered transport mobility by BCP material, which leads to an increase in electron amount in the emission layer and therefore the higher luminance and current density.