A key breakthrough in modern electronics was the introduction of band structure engineering, the design of almost arbitrary electronic potential structures by alloying different semiconductors to continuously tune the band gap and band-edge energies. Implementation of this approach in organic semiconductors has been hindered by strong localization of the electronic states in these materials. We show that the influence of so far largely ignored long-range Coulomb interactions provides a workaround. Photoelectron spectroscopy confirms that the ionization energies of crystalline organic semiconductors can be continuously tuned over a wide range by blending them with their halogenated derivatives. Correspondingly, the photovoltaic gap and open-circuit voltage of organic solar cells can be continuously tuned by the blending ratio of these donors.

The functionality of organic semiconductor devices crucially depends on molecular energies, namely the ionisation energy and the electron affinity. Ionisation energy and electron affinity values of thin films are, however, sensitive to film morphology and composition, making their prediction challenging. In a combined experimental and simulation study on zinc-phthalocyanine and its fluorinated derivatives, we show that changes in ionisation energy as a function of molecular orientation in neat films or mixing ratio in blends are proportional to the molecular quadrupole component along the π-π-stacking direction. We apply these findings to organic solar cells and demonstrate how the electrostatic interactions can be tuned to optimise the energy of the charge-transfer state at the donor−acceptor interface and the dissociation barrier for free charge carrier generation. The confirmation of the correlation between interfacial energies and quadrupole moments for other materials indicates its relevance for small molecules and polymers.

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.

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