Confirmation of cation-exchange doping
UV-Vis measurements were employed to evaluate the doping efficiency of our method in a widely studied polymeric semiconductor, 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))26. P(NDI2OD-T2) thin films were spin-coated on glass substrates and then thermally annealed. The thin films were chemically doped by immersing them in a doping solution containing cobaltocene (CoCp2) with or without salts of organic cations. All the fabrication processes were conducted in a N2-purged glove box, and absorption measurements were conducted in ambient air without encapsulation. The absorption spectra of a pristine (undoped) P(NDI2OD-T2) thin film is shown in Fig. 1c, where the lowest energy absorption peak is ca. 704 nm. The spectrum of a thin film doped using a solution containing only CoCp2 is nearly identical to that of the pristine film. Although the thin film showed color changes upon chemical doping in the glove box, its color returned to that of the undoped film upon air exposure. This behavior is ascribed to the dedoping of the once-electron-doped thin film upon air exposure owing to the instability of CoCp223,25. When a combination of CoCp2 and tetrabutylammonium (TBA) chloride was employed, the features of electron-doped P(NDI2OD-T2)25 were observed via the measurement in ambient air (Fig. 1d), indicating that unlike the CoCp2-doped film, this thin film does not undergo rapid dedoping. Here, the intensity of the lowest energy excitation of pristine P(NDI2OD-T2) with a peak at 704 nm was decreased. At the same time, new features with peak at 496, 696, and 798 nm emerged. The peak observed in the pristine film at 392 nm moves to slightly shorter wavelength of 370 nm. All of these features agree with the changes in UV-Vis spectra observed during electrochemical doping of this polymer from neutral to radical anion state27. In this study, TBA+ salts with PF6− and bis(trifluoromethanesulfonyl)imide (TFSI−) were also employed (Fig. 1d). The observed ambient stability of the doped state indicates that the anionic charges in the doped polymer thin films are not compensated by CoCp2+ but rather by TBA+ through the mechanism described in Fig. 1a. Note that these spectral changes were not observed when P(NDI2OD-T2) thin films were immersed in salt solutions without CoCp2 (Supplementary Note 2), suggesting both salts and CoCp2 play key roles. Improvements in the ambient stability of all the employed counter-anions imply that this phenomenon is not a anion-specific reaction but likely to be an ion-exchange reaction where anions simply serve as spectator ions. Combinations of CoCp2 and the bulky organic cations of tetraphenylphosphonium (TPP+) and dMesIM+ were also tested; in this case, absorption spectra similar to those obtained with the TBA+ salts were obtained (Fig. 1e). While a pristine P(NDI2OD-T2) thin film showed conductivity on the order of 10−8 S cm−1, all the samples doped with combinations of CoCp2 and salts showed conductivity on the order of 10−5 S cm−1 in air, which supports that the observed changes in UV-Vis spectra are ascribed to introduction of electrons in P(NDI2OD-T2) thin films. Detail of conductivity measurements and IV curves are available in Supplementary Note 3.
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