A series of 9,9-bis(diarylaminophenyl)fluorene derivatives (X-APFs) were synthesized and used as hole-transporting materials in single hetero-junction light-emitting device fabrication. In devices with tris(8-hydroxyquinoline) aluminum (Alq) as the electron-transporting material, these materials served the role of hole-transporters only. The device performance (turn-on voltage, luminance efficiency and etc) shows a dependence on the substituent on the phenyl ring. In devices with 2,2'2'-(1,3,5-phenylene)tris-[1- phenyl-1H-benzimidazole](TPBI) as the electron-transporting material, a complex electroluminescence behavior was observed, depending on the substituent as well as the applied voltage. The results are discussed in terms of energy level matching and the effect of electric field on the molecular geometry.