From Isotropic to Anisotropic Conductivities in P(NDI2OD-T₂) by (Electro-)Chemical Doping Strategies

Both electrochemical and chemical doping of the n-type regioregular polymer poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)) and its regioirregular counterparts are presented. Electrochemical doping clearly shows that a 2-fold reduction with maximum conductivity in the radical-anion state is possible with the polymers. Chemical doping with the state-of-the-art high-performance dopants 4-(2,3-dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzeneamine (N-DMBI) and tetrakis(dimethylamino)ethylene (TDAE) gives conductivity values between 10–4 and 10–3 S cm–1 for as-cast films with clear involvement of the radical-anion species. Anisotropic conductivities of vapor-doped blade-coated films give up to 6 times higher values along the polymer chain direction compared with the perpendicular direction, both in face-on and edge-on oriented chains in the bulk of the films. The polymorph concomitantly changes from a mixed stacking of naphthalenediimide and bithiophene units (form II) to a segregated stacking mode (form I), indicating a strong effect of the dopant on the overall film structure. Maximum achievable conductivity along the chain is 8 × 10–3 S cm–1. Our study underlines that regardless of regioisomerism and film preparation, higher maximum conductivity seems not to be achievable with P(NDI2OD-T2), at least with the herein used chemical dopants. Reasons might include the following: (1) strong localization of charges on the NDI units and (2) too low reduction potentials of the dopants, both evidenced by electrochemistry; (3) another factor is the high volatility of TDAE, which is elucidated by transmission electron microscopy, electron diffraction experiments, and spectroscopy.