We present here the development of a new solution-processable n-type dopant, N-DMBI. Our experimental results demonstrated that a well-known n-channel semiconductor, [6,6]-phenyl C-61 butyric acid methyl ester (PCBM), can be effectively doped with N-DMBI by solution processing; the film conductivity is significantly increased by n-type doping. We utilized this n-type doping for the first time to improve the air-stability of n-channel organic thin-film transistors, in which the doping can compensate for the electron traps. Our successful demonstration of n-type doping using N-DMBI opens up new opportunities for the development of air-stable n-channel semiconductors. It is also potentially useful for application on solution-processed organic light-emitting diodes and organic photovoltaics.