Fermi level control by doping is established since decades in inorganic semiconductors and has been successfully introduced in organic semiconductors. Despite its commercial success in the multi-billion OLED display business, molecular doping is little understood, with its elementary steps controversially discussed and mostly-empirical-materials design. Particularly puzzling is the efficient carrier release, despite a presumably large Coulomb barrier. Here we quantitatively investigate doping as a two-step process, involving single-electron transfer from donor to acceptor molecules and subsequent dissociation of the ground-state integer-charge transfer complex (ICTC). We show that carrier release by ICTC dissociation has an activation energy of only a few tens of meV, despite a Coulomb binding of several 100 meV. We resolve this discrepancy by taking energetic disorder into account. The overall doping process is explained by an extended semiconductor model in which occupation of ICTCs causes the classically known reserve regime at device-relevant doping concentrations.

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Increasing the amount of charge carriers by molecular doping is important to improve the function of several organic electronic devices. In this work, we use highly fluorinated fullerene (C 60 F 48) to p-type dope common amorphous molecular host materials. We observe a general relation between the material's electrical conductivity and Seebeck coefficient, both strongly depending on the energy level offset between amorphous host and dopant. This suggests that the doping efficiency at similar doping levels is mainly determined by the electron transfer yield from host to dopant. Indeed, the dopant anion and host cation absorption strength correlate with the ionization energy (IE) of the host material. Host materials with an IE significantly below the electron affinity of the dopant yield the highest doping efficiency. Surprisingly, the doping efficiency reduces only by about one order of magnitude when the IE of the host material is increased by 0.55 eV, which we attribute to the disordered nature of the host materials

p-Type molecular doping of organic materials with high ionization energies (IEs) of above 5.50 eV is still a challenge, limiting the use of doping in high-performance organic light-emitting diodes (OLEDs). Here, we investigate the molecular dopant hexacyano-trimethylene-cyclopropane (CN6-CP) with a high electron affinity of 5.87 eV as p-dopant in OLEDs. We show that CN6-CP can be used not only as a dopant in the traditional hole transport material 4,4′cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC, IE = 5.50 eV) but also effectively dopes the host material tris(4-carbazoyl-9-ylphenyl)amine (TCTA, IE = 5.85 eV), reaching a conductivity of 1.86 × 10 −4 S/cm at a molar ratio of 0.25. Using CN6-CP-doped TAPC as hole injection and transport layer, we achieve a low driving voltage of 2.92 V at the practical brightness of 1000 cd/m 2 and 3.18 V at a current density of 10 mA/cm 2 for a green phosphorescent OLED based on bis[2-(2-pyridinyl-N)phenyl-C](acetylacetonato)iridium-(III) (Ir(ppy) 2 (acac)), together with a maximum external quantum efficiency of 18% and a luminous efficacy of 78 lm/W. The device also exhibits a very low efficiency roll-off at high luminance. Further, by directly adopting CN6-CP-doped TCTA as the injection/transport layer, the driving voltage drops to 2.78 V at 1000 cd/m 2 and 2.93 V at 10 mA/cm 2 . Moreover, conductivity and absorption measurements suggest that CN6-CP could also dope CBP with an IE as high as 6.05 eV. The results show that CN6-CP is an excellent p-type dopant for efficient OLEDs and possesses great potential for future application in organic optoelectronic devices.

The original version of this Article contained an error in Equation 1. A factor of ‘c’ was included in the right-hand term. This has been corrected in the PDF and HTML versions of the Article.