Considerations on "Double Doping" and an Enhanced p-Dopant Complex

The double ionization of molecular dopants enables the doping efficiency to rise above 100%. This means increased conductivity can be achieved with fewer dopant molecules, resulting in less disruption to the semiconductor film’s structure. However, current models of doped organic semiconductors based on Fermi-Dirac statistics with two distributions that represent the host and dopant density of states, fail to explain double dopant ionization, and also the analogous situation of bipolaron formation on a host semiconductor polymer. This shortcoming is addressed by considering the energy level renormalization of the frontier molecular orbital levels upon electron transfer between host and <i>p</i>-dopant. Varying the model parameters host-dopant energy level offset, energetic disorder, reorganization energy, and Hubbard <i>U</i> sublevel splitting, robust trends in the fractions of doubly-ionized, singly-ionized, and neutral species are found.<br/>The conventionally used <i>p</i>-dopants based on tetracyanoquinodimethane (TCNQ) and its derivatives feature limited oxidation strength especially for modern polymer semiconductors with high ionization energy (IE). These small molecular dopants also exhibit pronounced diffusion in polymer hosts. A facile approach is presented to increase the oxidation strength of several TCNQ-derivatives by coordination with four tris(pentafluorophenyl)borane (BCF) molecules, using a single-step solution mixing process, resulting in bulky dopant complexes. Using a series of polymer semiconductors with IE up to 5.9 eV, the efficiency of doping using such complexes is found to be significantly higher compared to using either of the dopants alone. Additionally, the bulkier structure of the complexes is shown to result in a higher stability against dopant drift in thin films under an applied electric field. The simple approach of solution-mixing of readily accessible molecules thus offers access to enhanced molecular <i>p</i>-dopants.