Abstract

Molecular p-doping allows for an increase in the conductivity of organic semiconductors, which is regularly exploited in thermoelectric devices. Upon doping, integer and fractional charge transfer have been identified as the two competing mechanisms to occur, where the former is desired to promote the generation of mobile holes in the semiconductor host. In general, high dopant electron affinity is expected to promote integer-charge transfer, while strong coupling between the frontier molecular orbitals of dopant and host promotes fractional charge transfer instead. Here, we investigate the role that the width of the density of states (DOS) plays in the doping process by doping the conjugated polymer poly(3-hexylthiophene) (P3HT) with tetracyanoquinodimethane (TCNQ) derivatives of different electron affinities at a 2% dopant ratio. Cyclic voltammetry confirms that only the electron affinity of F4TCNQ (tetrafluorotetracyanoquinodimethane) exceeds the ionization energy of P3HT, while TCNQ and FTCNQ (2-fluoro-7,7,8,8-tetracyanoquinodimethane) turn out to have significantly lower but essentially identical electron affinities. From infrared spectroscopy, we learn, however, that ca. 88% of FTCNQ is ionized while all of TCNQ is not. This translates into P3HT conductivities that are increased for F4TCNQ and FTCNQ doping, but surprisingly even reduced for TCNQ doping. To understand the remarkable discrepancy between TCNQ and FTCNQ, we calculated the percentage of ionized dopants and the hole densities in the P3HT matrix resulting from varied widths of the P3HT highest occupied molecular orbital (HOMO)-DOS via a semi-classical computational approach. We find that broadening of the DOS can yield the expected ionization percentages only if the dopants have significantly different tendencies to cause energetic disorder in the host matrix. In particular, while for TCNQ the doping behavior is well reproduced if the recently reported width of the P3HT HOMO-DOS is used, it must be broadened by almost one order of magnitude to comply with the ionization ratio determined for FTCNQ. Possible reasons for this discrepancy lie in the presence of a permanent dipole in FTCNQ, which highlights that electron affinities alone are not sufficient to define the strength of molecular dopants and their capability to perform integer-charge transfer with organic semiconductors.