A self-consistent theory for calculation of built-in voltage (U-bi) of metal-organic semiconductor-metal (MOSM) structures is developed based on Gaussian energy distribution of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). It is shown that the built-in voltage depends not only on the work function difference of the two electrodes, but also on the mean energy level of HOMO and LUMO, as well as the Gaussian width of the energy distribution. The theory predicts that the spreading of HOMO and LUMO levels will results in an increase of U-bi, and that U-bi decreases with increasing temperature. (C) 2009 Elsevier B.V. All rights reserved.