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C4.02 - Direct Observation of Doping Sites in Temperature-Controlled, p-Doped P3HT Thin Films 
April 22, 2014   11:00am - 11:15am

Over recent years, solution-processed molecular dopants have received much attention due to their potential for the realization of stable and controllable doped transport layers for both p- and n-type materials. Such doped layers are indispensable for a variety of applications including organic light-emitting diodes (OLED), organic photovoltaics (OPV) and transparent conducting electrodes. Yet despite the increasing availability of p- and n-type materials from organic synthesis, the solution doping process and overall distribution of dopants within thin films are poorly understood. To this end, we investigate the relationship between solution-state doping and the corresponding phase separation between dopant and host molecules in thin film. Here we focus specifically on the p-type doping of a prototypical, high performance semiconducting polymer poly(3-hexylthiophene) (P3HT) using F4TCNQ (7,7,8,8-Tetracyano-2,3,5,6-tetrafluoroquinodimethane). Using conducting-AFM (c-AFM), we are able to directly map dopant sites in blended films and observe the dopant-polymer phase separation as a function of doping concentration and solution temperature. Our data, firstly, confirm the existence of the “weak” and “strong” doping regimes and the resultant phase separation previously reported. In the “weak doping” regime, F4TCNQ remains as a neutral species in solution and becomes homogeneously dispersed among amorphous domains in thin films. In the “strong doping” regime, F4TCNQ molecules bind to the polymer backbone in solution, which leads to the formation of new crystalline domains in the solid state. Furthermore, we find that it is possible to tune the doping strength by controlling the solution of the spin casting solutions. A 2% doped film spin casted at 30oC, for instance, exhibits a doping efficiency of nearly one order of magnitude higher than a film spin casted at 80oC. The observed change in doping efficiency also resulted in a drastic change in phase separation in thin film. To our knowledge, this is the first report to successfully image doped sites in F4TCNQ:P3HT thin films and substantiates c-AFM as a powerful tool for characterizing doped polymer systems.

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