Organic semiconductors are widely discussed for applications requiring large area and low processing cost. Thin film organics are already used in applications not requiring high speed or efficiency, such as display LEDs. To understand the materials limits of such semiconductors we study fundamental transport processes in devices made from single crystals of small molecules such as pentacene and rubrene. Photoluminescence, FET transconductance, and gate modulated activation energy spectroscopy (GAMEaS) allow the identification of oxygen as the dopant in rubrene. Using these techniques, we demonstrate for the first time, controllable doping in an organic semiconductor. I will discuss the implications of this work for solar energy.