(181k) High Electron Affinity Molecular Dopants for Solution Processing of Organic Semiconductors

Molecular p-type dopants are widely researched to improve the conductivity of organic semiconductors. This field remains active because recently introduced high hole mobility copolymers like PDPP-2T, PDPP-3T, PDPP-4T, and PDPP-T-TT-T have high ionization energies, thus making them harder to dope. The limited selection of dopants available with high enough LUMO levels to dope the DPP copolymer group have low solubilities in common solvents and low electron affinities (EA). For example, the highest electron affinity molecular dopant recently reported by Liu et. al. to be able to dope these polymers is called CN6-CP (EA=-5.87 eV), but it is not solution processable. In this work, we synthesize and characterize the performance of a series of related p-type dopants that were chemically tailored to increase solubility by replacing nitrile groups with ester groups. We present (tri-, di-, and mono-) substituted CN6-CP (TMCN3-CP, DMCN4-CP, and MCN5-CP) that show greatly increased solubility while only slightly decreasing electron affinity (0.1 eV per ester group). We demonstrate that this series of dopants can reach high conductivities nearing 100 S/cm in DPP copolymers using sequential solution processing. Furthermore, we examine the trade-off between increasing electron affinity and dopant stability.