Performance Improvement Of Organic Small Molecule Light-emitting Diodes

In this thesis, organic light-emitting diodes(OLEDs) were prepared by vacuum thermal evaporation, four aspects as follows were studied:1. OLEDs with a structure of ITO/TPD/Alq3/LiF/Al were prepared, the influence of VOx insert layer on devices performance had been studied. Compared the photoelectric characteristics of devices with 20 nm VOx with that of devices without VOX, the results showed that operating voltage decreased 3.5 V at 100 cd/m2 or 1000 cd/m2, maximum luminance and power efficiency increased from 5808 cd/m2 and 0.88 lm/W to 9234 cd/m2 and 2.63 lm/W, respectively.2. OLEDs with a structure of ITO/CuPc/NPB/Alq3/C6o/LiF/Al were prepared, the influence of C60 insert layer on devices performance had been investigated. Inserting 15 nm C6o between Alq3 and LiF scarcely affected current density-voltage curves, this confirmed that band bending of Alq3/C60. C60 insert layer could block exciton quenching caused by cathode, so that high efficient OLEDs with a thin light-emitting layer could be made. After inserting 15 nm C60 and reducing Alq3 thickness to 30 nm, the best devices performance (the maximum power efficiency was 1.03 lm/W) could be obtained.3. Double heteroj unction OLEDs with structures of ITO/NPB/Alq3/BCP/Alq3 or C60/LiF/Al were prepared, the influence of Alq3 and C60 used as electron transport layers(ETL) on devices performance had been compared. Comparing two groups of devices performance with Alq3 and C60 as ETL, respectively, the results illuminated that OLEDs with C60 had better performance (the maximum power efficiency was 0.89 lm/W) than OLEDs with Alq3. In OLEDs with Alq3, when the Alq3 ETL was too thin, it could not effectively protect BCP layer, however, when it's too thick, this would lead to a significant increase in operating voltage. With high electronic conductivity C60 as ETL, the operating voltage did not increase with C60 thickness, and 15 nm C60 could protect BCP layer well.4. OLEDs with a simple structure of ITO/NPB/Alq3/Al/Ag were prepared, the influence of Ag cathode deposition on devices performance had been investigated. The experimental results showed that, when Ag was deposited rapidly, the thicker the Ag layer, the worse the devices performance. However, if 200 nm Ag layer was deposited slowly, the devices performance was also poor. This illuminated that high-temperature during rapid deposition of thicker Ag cathode should give rise to glass transition in organic layers, which resulted in a bad performance. If thicker Ag was deposited slowly, the devices would be exposed to high temperature in a long time, which led to worse glass transition in organic layers. After inserting BCP/C6o/LiF spacer layer between Alq3 and Al cathode, even if there was 280 nm Ag deposited rapidly, maximum current density, maximum brightness, maximum current efficiency of the device were as high as 248.6 mA/cm2,5380.7 cd/m2,3.52 cd/A, respectively.