Soluble small red molecule No. 2 was introduced as the emitter for light-emitting diodes （PLED） with different cathodes. N-arylbenzinmidzoles （TPBI） was used as hole-blocking layer （HBL） to lower cathode barrier and facilitate electron injection.With TPBI/Ag cathode， the quantum efficiency was increased by 57%.The reason for the efficiency increase is that interfacial dipole layer was formed at the interface between TPBI and Ag, and possibly coordinated reaction between Ag and N atom in TPBI leads to a superior contact between TPBI/Ag.