Charge transfer in and conductivity of molecularly doped thiophene-based copolymers

ABSTRACT The electrical conductivity of organic semiconductors can be enhanced by orders of magnitude via doping with strong molecular electron acceptors or donors. Ground‐state integer charge transfer and charge‐transfer complex formation between organic semiconductors and molecular dopants have been suggested as the microscopic mechanisms causing these profound changes in electrical materials properties. Here, we study charge‐transfer interactions between the common molecular p‐dopant 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane and a systematic series of thiophene‐based copolymers by a combination of spectroscopic techniques and electrical measurements. Subtle variations in chemical structure are seen to significantly impact the nature of the charge‐transfer species and the efficiency of the doping process, underlining the need for a more detailed understanding of the microscopic doping mechanism in organic semiconductors to reliably guide targeted chemical design. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 58–63 The electrical conductivity of thiophene‐based copolymer semiconductors is enhanced by orders of magnitude via doping with the strong molecular electron acceptor 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane. Two mechanisms are found to contribute to doping: integer charge transfer dominates with thiophene units and charge transfer complex formation with quinoxaline units. The achieved conductivity appears to be enhanced when integer charge transfer dominates in the material.