Acridine orange base as a dopant for n doping of C60 thin films

Abstract

We present a study on $n$ doping of ${\mathrm{C}}_{60}$ thin films by acridine orange base [3,6-bis(dimethylamino)acridine(AOB)] combining conductivity, field effect, and Seebeck measurements. An increase of more than six orders of magnitude in conductivity is observed for a doping ratio of $6\mathrm{mol}\%$ , accompanied by a decrease in the activation energy from $0.64\text{to}0.15\mathrm{eV}$ compared to the undoped ${\mathrm{C}}_{60}$ . We observe a clear doping effect immediately after sample preparation, but also a further activation by annealing or illumination. The field effect and Seebeck measurements confirm $n$ -type conduction of ${\mathrm{C}}_{60}$ thin films and show that deep donor states are formed in AOB-doped ${\mathrm{C}}_{60}$ thin films. A field effect mobility of $0.2{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s}$ is achieved for a doping level of $1.8\mathrm{mol}\%$ . Near Infrared (NIR) and Fourier transform infrared (FTIR) spectra demonstrate electron transfer from the dopant to the matrix: For ${\mathrm{C}}_{60}$ doped with AOB, $\mathrm{C}_{60}{}^{-}$ is present in NIR absorption and FTIR spectra. On the other hand, a peak corresponding to acridine orange [3,6-bis(dimethylamino)acridinium chloride $\left(\mathrm{AOB}{\mathrm{H}}^{+}\right)$ ] is also observed in the FTIR spectrum of ${\mathrm{C}}_{60}:\mathrm{AOB}$ , where $\mathrm{AOB}{\mathrm{H}}^{+}$ corresponds to AOB with one additional proton attached. Electrochemical data of AOB and $\mathrm{AOB}{\mathrm{H}}^{+}$ in acetontrile suggest that the AOB radical cation is not stable, but is rapidly transformed into a compound with similar properties to $\mathrm{AOB}{\mathrm{H}}^{+}$ . Conductivities of ${\mathrm{C}}_{60}$ thin films doped with bis(ethylenedithio)-tetrathiafulvalene were also investigated to confirm that the doping effect of AOB in ${\mathrm{C}}_{60}$ does not result from a simple electron transfer from AOB to ${\mathrm{C}}_{60}$ .