Effect of trap states on the electrical doping of organic semiconductors

• Direct charge transfer from trap states of host to p-dopant raises the doping effect.
• p-Dopant having higher LUMO level can dope host molecules having lower HOMO level.
• Mobility of host molecules having deep trap states was recovered with p-doping.

We demonstrate that direct charge transfer (CT) from trap states of host molecules to the p-dopant molecules raises the doping effect of organic semiconductors (OS). Electrons of the trap states in 4,4′-N,N′-dicarbazolyl-biphenyl (CBP) (EHOMO = 6.1 eV) are directly transferred to the p-dopant, 2,2′-(perfluoronaphthalene-2,6-diylidene) dimalononitrile (F6-TCNNQ) (ELUMO = 5.4 eV). This doping process enhances the conductivity of doped OS by different ways from the ordinary doping mechanism of generating free hole carriers and filling trap states of doped OS. Trap density and trap energy are analysed by impedance spectroscopy and it is shown that the direct charge transfer from deep trap states of host to dopants enhances the hole mobility of doped OS and the I–V characteristics of hole only devices.

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