Solution-processed Phosphorus-Tungsten Oxide Film As Hole Injection Layer For Application In Organic Light-Emitting Diode

The research on new functional layer materials and optimization of fabrication in organic light-emitting diode（OLED）has been continuously concerned.In this work,phosphotungstic acid solution used as a precursor is spin-coated onto ITO anode,and then a phosphorus-tungsten oxide film is obtained after annealing treatment（from100°C to 300°C）.Various parameters which include film composition,surface morphology,work function,optical property and conductivity are measured.It suggests that the film has the composition of 24 WO₃·P₂O₅ annealed at 200°C in air,which exhibits work function of 5.55 eV,surface root mean square（RMS）roughness value of1.53 nm,optical transmittance of over 92%and sheet resistance of 7.79Ω/sq.The phosphorus-tungsten oxide film is used as hole injection layer（HIL）,and the OLED with the structure of[ITO/HIL/TPD（25 nm）/Alq₃（60 nm）/LiF（1 nm）/Al（100 nm）]is fabricated.The performance of OLEDs based on phosphorus-tungsten oxide film with different treatments are measured.The result suggests that phosphorus-tungsten oxide film has efficient hole-injection when spin-coated at 2000 rpm and annealed at 200°C.The performance of device shows turn-on voltage of 2.6 V,maximum luminance of 13553 cd/m²,maximum current efficiency of 5.87 cd/A,maximum power efficiency of 3.44 lm/W and maximum external quantum efficiency（EQE）of 1.983%,respectively.Furthermore,the phosphorus-tungsten oxide film annealed in air or vacuum is used as HIL in OLED,which has the structure of[ITO/HIL/NPB（25 nm）/C545T:Alq₃（1 wt%,40 nm）/Alq₃（15 nm）/LiF（1 nm）/Al（100nm）].It indicates that the work fuction of film is controlled,and hole-injection ability of HIL film is improved,which results in the increase of device efficiency.At last,the maximum EQE of device is 3.401%.Tungstic oxide（WO₃）,Poly（3,4-ethylenedioxy-thiophene）:poly（styrenesulfonate）（PEDOT:PSS）,phosphorus-tungsten oxide and phosphorus-molybdenum oxide are applied in OLEDs.Comparing the performance of OLEDs,phosphorus-tungsten oxide posses high hole-injection ability.The maximum current efficiency and the maximum power efficiency of the divice are 5.87 cd/A and 3.44 lm/W,respectively.The results provide a new way for exploring HIL materials particularly the transition metal oxide and their deposition process,as well as the application in high performance OLED.