Organic light-emitting diodes (OLEDs) consisted of a novel fluorene derivative of 5,6-bis(9,9-dihexyl-9H-fluoren-2-yl)-
2,3-diisocyano-2,3-dihydropyrazine (BDHFLCNPy) and a hole transporting material of N,N'-Di-[(1-naphthalenyl)-
N,N'-diphenyl](1,1'-biphenyl)-4,4'-diamine (NPB) were fabricated, and electroluminescence (EL) spectrum of devices
were investigated. It was found that light emission around 650 nm observed in devices came from exciplex generated at
heterojunction interface by NPB molecules worked as electron donor and BDHFLCNPy molecules worked as electron
acceptor. Moreover, a shoulder peak around 500 nm ascribed to BDHFLCNPy exciton was observed. To systemically
study the effect of heterojunction structure in exciplex formation, OLEDs with different thickness of acceptor were
fabricated. The results illustrated that a shoulder peak around 600 nm occurred in EL when acceptor thickness increases,
and BDHFLCNPy exciton emitting strength is relatively altered. The emission band around 600 nm is due to electroplex.
The L-V-J properties of OLEDs show that device with the thinnest acceptor layer has the highest luminance and current
density. On the contrary, OLEDs with thicker acceptor layer have higher luminance efficiency. The different
recombination mechanism of exciton, exciplex and electroplex in heterojunction were studied. Furthermore, the acceptor
thickness effect of exciplex and electroplex generating mechanism and energy transferring mechanism between them was
also discussed.
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