Impact of Energy Level Shift of Hole Injection Layer on the Recombination Rate in OLEDs

This paper reports the numerical study on the impact of energy level variation of the hole injection layer (HIL) in the organic light emitting diodes (OLEDs). We used a thin CuPc layer for the hole injection and S-TAD (2,2',7,7'-tetrakis-(N,N- diphenylamino)-9,9-spirobifluoren) for the hole transfer layer, S-DPVBi (4,4'-bis (2,2'-diphenylvinyl) -1,1'-spirobiphenyl) for the emission layer, and Alq3 (Tris (8-hyroxyquinolinato) aluminium) for the electron transfer layer. Device B has tri-layer structure except HIL. We used three models which are four layer structures for our simulation. HIL of Device A has 5.3 eV of highest occupied molecular orbital (HOMO) level and 3.8 eV of lowest unoccupied molecular orbital (LUMO) level. Also, we varied the LUMO level and HOMO level of the thin CuPc layer, which are named as Device C and Device D, respectively. Our numerical study represented that Device D has the most amount of recombination rate due to Device D has the most carrier density in the emission layer. Device A has 4.381 x 10(25) cm(-3)s(-1) of recombination, Device B has 6.439 x 10(25) cm(-3)s(-1) of recombination and Device D has 7.499 x 10(25) cm(-3)s(-1) of recombination. Consequently, nevertheless the insertion of HIL does not always improve recombination rate and we observed the recombination rate can improve about 16% according to HOMO level of HIL.