Formation of Ion Pairs and Charge-Transfer Complexes in the p-Doping of Organic Semiconductors
Authors: Hase Hannes , Liu Jiang Tian , Forgione Pat , Salzmann Ingo 
Published: May 2020

The formation of ion pairs (IPA) through integer charge transfer and ground-state charge-transfer complexes (CPXs) with only fractional charge transfer are the two known mechanisms in the molecular doping of conjugated polymers and molecules, which form the material class of organic semiconductors (OSCs). As it entails no immediate ionization of the OSC, CPX formation is regarded as detrimental to the doping efficiency. For IPA formation to occur, a match between the electron affinity (EA) of the p-dopant and the ionization energy (IE) of the OSC is expected as crucial, while their pronounced frontier molecular orbital overlap should promote CPX formation instead [1].
To explore these complementary scenarios, we first studied the thermal de-doping of the prototypical conjugated polymer poly(3-hexylthiophene) (P3HT), p-doped with the common strong electron acceptor tetrafluoro-tetracyanoquinodimethane (F4TCNQ) [2]. We combined grazing incidence X-ray diffraction (GIXRD) with Fourier-transform infrared spectroscopy (FTIR) to determine the microstructure, and absorbance spectroscopy (UV/vis/NIR) to observe the spectral fingerprint of the two doping scenarios. We found two microstructural environments through their distinguished thermal stabilities, where, the dopants were observed (i) alternatingly stacked with the polymer backbone as well as (ii) dispersed in its sidechain region. From FTIR and UV/vis/NIR we deduce that all dopants are fully ionized, although packing between dopant and polymer backbone would be expected to favor CPX formation instead. Notably, while characteristic shifts of the mid-infrared cyano-stretch modes of TCNQ derivatives are known to well indicate the degree of charge transfer, we show here that they allow further to assess the dopant site in the polymer environment.
We further investigated the influence of EA with respect to IE by contrasting F4TCNQ with its derivatives of lower degree of fluorination, which translates into reduced EA while leaving the spatial aspects of the dopant largely intact. We find that using F2TCNQ and FTCNQ still results in IPA formation in spite of EA < IE, which we understand through the lower degree of spatial, and therefore energetic order in OSCs as compared to inorganic semiconductors, which therefore possess no sharp band edges. Surprisingly, at high doping concentrations (additional) CPX formation occurred for all the differently strong dopants, which highlights the complexity of these systems going beyond the sole impact of the energetics of the individual constituents.
Finally, we juxtaposed the known tendency of small molecular OSCs to form CPXs and that of polymer OSCs to form IPAs. To maximize the comparability with P3HT, alkylated oligothiophenes of different length were synthesized in-house (from 4 to 10 thiophene repeat units) and doped with F4TCNQ. For the longest oligomer, we were able to observe the switch from the CPX regime into that of IPA, i.e., we approach the doping behavior polymer limit. Knowing this threshold is valuable for applications in organic electronics where vacuum processible small molecular OSCs are preferred over polymeric OSCs, which are only processible via solution-based methods.
[1] I. Salzmann, G. Heimel, M. Oehzelt, S. Winkler, N. Koch, Acc. Chem. Res. 2016, 49, 370.
[2] H. Hase, K. O’Neill, J. Frisch, A. Opitz, N. Koch, I. Salzmann, The Journal of Physical Chemistry C 2018, 122, 25893.