This paper deals with two-dimensional optoelectronic simulation based study of organic phototransistors. The top and bottom contact device structures were simulated to compare their characteristics under dark and under light illumination conditions. It was observed that the performance of these phototransistors can be tuned by shining a monochromatic light of given intensity. Further, the photosensitivity of the top and bottom contact phototransistors was compared and it was concluded that the difference in their photocurrent can be attributed to their different carrier extraction areas, which also leads to the difference in their contact resistances. Our results clearly indicate that the top contact structure has higher photosensitivity compared to the bottom contact structure. Furthermore, we simulated the dependence of photosensitivity on gate voltage, light intensity and channel length of the two structures and found the presence of both photoconductive and photovoltaic effects governing performance of organic phototransistors. Finally by simply scaling the device channel length contact resistance in the bottom and top contact organic photo transistors were estimated under different light illumination conditions.