Photoexcited carriers in organic light emitting materials and blended films observed by surface photovoltage spectroscopy

Abstract

The electronic structure of the widely-used light emitting materials, 2,5-bis(5-tert-butyl-2-benzoxazolyl) thiophene (BBOT), poly($N$-vinylcarbazole) (PVK) thin films have been characterized using surface photovoltage spectroscopy. The photo-induced charge separation and transfer processes in both blend films of PVK:BBOT and PVK:TPD:BBOT, where TPD is $N,{N}^{\ensuremath{'}}$-diphenyl-$N,{N}^{\ensuremath{'}}$-bis(3-methylphenyl)-[$1,{1}^{\ensuremath{'}}$-biphenyl]-$4,{4}^{\ensuremath{'}}$-diamine have also been investigated. The results of the photo-induced contact potential difference (CPD) change show that BBOT film is an electron-transporting material while PVK film is a hole-transporting one. The photoluminescence and electroluminescence results of the blend films suggest an exciplex interaction between BBOT and PVK or TPD. A positive CPD change due to photo-excitation of the BBOT in PVK:BBOT blend film is attributed to electron trapping at the localized state induced by dispersed BBOT species. In the PVK:TPD:BBOT blend films, a positive CPD change, which starts at the same transition energy as in the former blend film but is significantly enhanced, is observed and explained in terms of charge transfer between the involved energy structures of the blend components. The dependence of the observed effects on the blend composition and ensuing electronic structure is discussed.