We demonstrate a novel doping mechanism of silicon, namely n type transfer doping by adsorbedorganic cobaltocene CoCp2 molecules. The amount of transferred charge as a function of coverage is monitored by following the ensuing band bending via surface sensitive core level photoelectron spectroscopy. The concomitant loss of electrons in the CoCp2 adlayer is quantified by the relative intensities of chemically shifted Co2p components in core level photoelectron spectroscopy which correspond to charged and neutral molecules. Using a previously developed model for transfer doping, the evolution in relative intensities of the two components as a function of coverage has been reproduced successfully. A single, molecule specific parameter, the negative donor energy of amp; 8722; 0.50 0.15 eV suffices to describe the self limiting doping process with a maximum areal density of transferred electrons of 2 1013 cm amp; 8722;2 in agreement with the measured downward band bending. The advantage of this doping mechanism over conventional doping for nanostructures is addressed