1, University of Cambridge, Cambridge, , United Kingdom
3, Stanford University, Stanford, California, United States
4, University of California, Davis, Davis, California, United States
The molecular doping of organic semiconductors (OSCs) has recently gained significant interest due to applications in thermoelectrics, transistors, and for selective contacts in photovoltaics or LEDs (see review by Jacobs and Moule, Adv. Mater. 2017, 1703063). Taking the p-type case as an example, molecular doping is usually assumed to occur by single electron transfer from the OSC to the dopant molecule, generating an ion pair (IP). However, over the past 5 years it has become apparent that some materials—primarily small molecule OSCs—dope by a qualitatively different process. In these systems, significant orbital hybridization occurs between the dopant and organic semiconductor, generating a charge transfer complex (CTC) which accepts both electrons from the OSC highest occupied molecular orbital (HOMO). These CTCs must thermally dissociate to generate free charge carriers, and therefore are generally seen as detrimental to efficient doping. Unfortunately, at present we do not have a clear picture of what factors control the formation of CTCs. This lack of understanding is due in large part to the fact that all currently known OSC:dopant systems appear to exclusively dope by either CTC or IP formation.
Here, we present the first system which selectively exhibits both IP or CTC formation. By varying the film casting conditions in the well-studied poly(3-hexylthiophene) : 2,3,5,6-tetrafluoro-7,7’,8,8’-tetracyanoquinodimethane (P3HT:F4TCNQ) system, we observe films with qualitatively different UV-vis-NIR absorption spectra than previously reported. These spectra are consistent with CTC formation, as are FT-IR spectra, which indicate fractional charge transfer. Solvent exposure converts these films back to the normally observed IP phase. Grazing incidence XRD studies indicate the CTC phase is crystalline, and remarkably displays an even higher degree of order than undoped P3HT processed under similar conditions. Therefore, disorder-induced charge localization cannot explain the formation of CTCs. DFT studies further support our conclusion that polymorphism directly controls the degree of charge transfer, and therefore the doping mechanism.