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

We present a study on n doping of 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 mol %, accompanied by a decrease in the activation energy from 0.64 to 0.15 eV compared to the undoped 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 C-60 thin films and show that deep donor states are formed in AOB-doped C-60 thin films. A field effect mobility of 0.2 cm(2)/V s is achieved for a doping level of 1.8 mol %. Near Infrared (NIR) and Fourier transform infrared (FTIR) spectra demonstrate electron transfer from the dopant to the matrix: For C-60 doped with AOB, 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 (AOBH(+))] is also observed in the FTIR spectrum of C-60:AOB, where AOBH(+) corresponds to AOB with one additional proton attached. Electrochemical data of AOB and AOBH(+) in acetontrile suggest that the AOB radical cation is not stable, but is rapidly transformed into a compound with similar properties to AOBH(+). Conductivities of C-60 thin films doped with bis(ethylenedithio)-tetrathiafulvalene were also investigated to confirm that the doping effect of AOB in C-60 does not result from a simple electron transfer from AOB to C-60.(c) 2006 American Institute of Physics.