Abstract The emissive mechanism of conventional fluorescent material coumarin C545T was studied at the 3DTAPBP/TPBi-based exciplex interface and CBP/TPBi-based non-exciplex interface. 3DTAPBP is short for 2,2’-Bis(3-(N, N-di-p-tolylamino) phenyl) biphenyl, CBP for 4,4-bis(N-carbazolyl)-2,2-biphenyl and TPBi for 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl) benzene. The device structures were ITO/MoO3/3DTAPBP/C545T/TPBi/LiF/Al and ITO/MoO3/CBP/C545T/TPBi/LiF/Al, respectively. The photo-luminescence (PL) spectra of 3DTAPBP, CBP and TPBi films are peaked at 415, 411 and 380 nm, respectively. The PL of 3DTAPBP/TPBi heterojunction is centered at 412 and 490 nm. The peak of 412 nm is originated from 3DTAPBP. Nevertheless, the peak at 490 nm neither belongs to the emission of 3DTAPBP nor TPBi, which should be from the emission of exciplex at the 3DTAPBP/TPBi interface. There is no additional peak for PL of the CBP/TPBi heterojunction film, which is the superposition of PL of CBP and TPBi films. That is, exciplex cannot be formed at the CBP/TPBi interface. The electroluminescent (EL) emission of C545T is dominant for the ITO/MoO3/3DTAPBP/C545T/TPBi/LiF/Al and ITO/MoO3/CBP/C545T/TPBi/LiF/Al devices. In both types of devices, C545T can be diffused into 3DTAPBP and TPBi or CBP and TPBi layers. For a doping system, the emissive mechanism of guest material could be divided into two possibilities: one is energy transfer between the host and the guest, and the other is direct charge trapping of the guest. In the exciplex system of 3DTAPBP/TPBi, the Förster energy transfer from the host (3DTAPBP/TPBi) to guest (C545T) is predominant since there is a large overlap between the emission spectra of 3DTAPBP/TPBi and excitation spectrum of C545T. Consequently, the curves of current density vs. voltage (J-V) have been little or no influence by C545T thickness (concentration). In the non-exciplex system of CBP/TPBi, direct charge trapping is the main emission mechanism due to the small overlap between photoluminescence of CBP/TPBi and C545T excitation. The J-V characteristics show a reduction trend with the increase of C545T thickness for operating voltage could be increased with the thickness of C545T induced by the recombination of C545T via direct charge carrier capture.
Key words：Photoluminescence; C545T; Energy transfer; Carrier capture; Exciplex
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