The effect of bilayer hole transporting layers using thermal crosslinking technology on the characteristics of organic light-emitting diodes

Abstract

A persistent problem with solution processes is dissolution of the first deposited layer by the solution of the second deposited layer. Because the energy barrier between the highest occupied molecular orbital (HOMO) levels of Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4-(N-(4-s-butylphenyl)diphenylamine)] (TFB) used as a first hole transporting layer (HTL) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) used as a light-emitting layer (EML) is so large, poly(N-vinylcarbazole) (PVK) was inserted as a second HTL between TFB and F8BT to facilitate hole injection from the TFB layer into the F8BT layer. In this paper, we demonstrate the effect of bilayer HTLs in solution-processed organic light-emitting diodes (OLEDs) using thermally-annealed TFB. The characteristics of OLEDs with HTLs, with and without thermally annealed TFB, are compared. The maximum external quantum efficiency (EQE) of an OLED, with thermally-annealed TFB and spin-coated PVK on the TFB used as bilayer HTLs, increases significantly to 0.617%, whereas the EQE of the OLED, without thermally annealed TFB and PVK spin-coated directly on the as-coated TFB, is greatly reduced to 0.195%. This result indicates that insertion of PVK between thermally annealed TFB and F8BT layers can facilitate easy hole injection from the TFB layer into the F8BT layer and enhance OLED performance.