Preparation Of Hole Injection Layer Based On MoO₃ For Organic Light-emitting Diode

Organic light-emitting diode(OLED),the most promising new-generation display technology,has developed rapidly in recent years due to the advantages of selfluminescence,low power consumption and wide viewing angle.Introducing hole injection layer(HIL)with controllable and efficient property is an effective way to improve the performance and further promote marketization of OLEDs.In this paper,based on transition metal oxides,HILs with controllable electronic properties and efficient hole injection ability were constructed by solution processing.Importantly,OLEDs endowed with high performaces and OLEDs without transport layer were realized.Based on Mo O3 units,ammonium phosphomolybdate(AMP)was selected as precursor.HILs with controllable work function were fabricated through solutionprocessed method by modulated the molar ratio of AMP/H2O2 and annealing atmosphere.The obtained HILs were endowed with high optical transmittance(> 85%),more smooth morphology(RMS ? 3.00 nm)and high WF ranging from 5.5 to 5.9 e V.Accordingly,WF of the HILs was positively proportional to AMP/H2O2 content while gap-state density was inversely correlated with AMP/H2O2.To investigate the effect on OLEDs by films obtained in different conditions,green fluorescent OLEDs with structure of ITO/HIL/NPB/Alq3/Li F/Al were prepared.After the optimization,based on HIL with AMP/H2O2(1:14)in precursor solution and argon annealing atmosphere,OLED had the lowest low turn-on voltage(Von)(2.5 V)and maximum current efficiency(6.82 cd/A),1.2-flod higher than that of PEDOT:PSS based devices.To clearly illustrate the enhancement mechanism of OLEDs by introducing HILs,hole only devices and the interfacial properties of inorganic/organic were investigated.It suggested that the improvement in device performances were attributed to the energy level alignment between the gap states with the HOMO level of NPB.OLEDs with structure of ITO/HIL/TCTA:Ir(mppy)3/Li F/Al were designed.Based on precursor solutions with different ratios of AMP/H2O2,HILs were annealed in an argon atmosphere to investigate the effect of different hole injection materials on the performance of simple devices.The results showed that the balance of carriers and the complex region of carriers play an important role in the performance of OLEDs.With the decrease of AMP/H2O2 ratio,the hole injection property increased and OLEDs were endowed with reduced Von from 4.1 to 3.4 V.By comparison,OLEDs based on HIL,obtained by precursor solution with AMP/H2O2(1:7)and argon annealing atmosphere,were endowed with minimum efficiency roll down.The research results of this paper show that effective control of the injection property of the HILs is an ideal method to identify the balancing of charge carriers to obtain higher current efficiency.