1, University of Cambridge, Cambridge, , United Kingdom
3, University of California, Davis, Davis, California, United States
In recent years, interest in molecular doping of organic semiconductors has grown significantly. Doped organic semiconductors have applications in many device applications, including thermoelectrics, photovoltaics, and LEDs. However, doping also often has significant effects on film morphology or solubility, suggesting that doping processes could also play a role in device fabrication. As an example, doping induced solubility control (DISC) patterning can generate topographic features sizes below 300 nm, better than generally achievable by photolithography. The same process can also be used to generate bilayers of mutually soluble polymers. We recently reviewed prospects for these types of processes in Jacobs and Moule, Adv. Mater. 2017, 1703063.
In order to feasibly use molecular dopants to control film solubility or for patterning, we must develop sequential methods for adding or removing dopants to/from films. Several groups have studied sequential doping processes, however relatively little attention has been given to the reverse process—removing dopants from doped films. These dedoping processes, which are similar to compensation doping in inorganic semiconductors, are also important in device applications. Compensation doping leaves behind immobile ions, while dedoping involves subsequent removal of the compensated charges. In organic semiconductor device applications, dedoping processes are likely preferable to compensation due to the low doping efficiencies typically observed.
Here, we discuss dedoping mechanisms in organic semiconductors, focusing on poly(3-hexylthiophene) : 2,3,5,6-tetrafluoro-7,7’,8,8’-tetracyanoquinodimethane (P3HT:F4TCNQ) as a model system. We identify a series of reactions between F4TCNQ and amines that allow for quantitative dedoping, leaving film fluorescence yield unchanged, or in some cases higher than as-cast films (see Jacobs et al. Chem. Mater. 2017, 29, 832). Along with absorption, conductivity, and fluorescence microscopy measurements, our results suggest that primary amines react with intrinsic doping defects in P3HT, providing a simple method to improve film quality. We also present an analogous photochemical reaction between F4TCNQ and tetrahydrofuran (see Fuzell and Jacobs, et al., J. Phys. Chem. Lett. 2016, 7, 4297). This reaction allows for direct modulation of film doping at sub-micron length scales using focused laser light (Jacobs et al., Adv. Mater. 2017, 29, 1603221), and thus could be used to directly fabricate nanoscale devices (e.g. transistors). Similar reactions are almost certainly widespread in all classes of molecular dopants.