Solution-processed electrical doping of organic semiconductors and their application for organic devices

Abstract

Different electronic devices based on organic semiconductors (OSCs) are being developed and promoted every year due to unique and outstanding properties of the organic materials including flexibility, transparency, light weight, and solution-process ability. However, these materials are not fully commercialized due to their intrinsically low electrical performance and poor stability. And in order to overcome these issues several approaches have been developed and one of them is doping. Doping is the most straightforward method to increase electrical conductivity of the materials in the first place. The organic light-emitting diode (OLED) industry already uses this method to finally commercialize these organic devices successfully.1 It should be noted that doping of OSCs is completely different from that of inorganic ones in which conductivity is enhanced via the increase of charge carriers provided by the impurity atoms that replace the atoms in the host lattice. In organic electronics, no replacement of host lattice atoms by impurity atoms occurs. Rather, it is just simple and direct introduction of extra charge carriers to the whole host molecule via a charge transfer process.2 In this case, the effectiveness of the doping process depends on the energy level differences between the materials’ highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). This requirement seriously limits the doping possibilities of the organic materials as dopant and host molecules with matching energy levels are rare. One of the suggested alternatives for doping of OSCs in which the energy levels of the two materials can be ignored, is application of Lewis acids and bases for pand n-type doping, respectively.3‒7 In this work, two cationic organic dyes, Pyronin B (PyB) and Acridine Orange (AO), are investigated as p-type dopants for a conjugated ambipolar polymer diketopyrrolopyrrole-thieno [3,2-b]thiophene (DPPT-TT). The dopants are conjugated molecules with Lewis acid nature.