Ambipolar charge transport in a solution-processed small molecule 4,7-bis{2-[2,5-bis(2-ethylhexyl)-3-(5-hexyl-2,2':5',2 ''-terthiophene-5 ''-yl)-pyrrolo[3,4-c]pyrrolo-1,4-dione-6-yl]-thiophene-5-yl}-2,1,3-benzothiadiazole (BTDPP2) transistor has been investigated and shows a balanced field-effect mobility of electrons and holes of up to similar to 10(-2) cm(2) V-1 s(-1). Using low-work-function top electrodes such as Ba, the electron injection barrier is largely reduced. The observed ambipolar transport can be enhanced over one order of magnitude compared to devices using Al or Au electrodes. The field-effect mobility increases upon thermal annealing at 150 degrees C due to the formation of large crystalline domains, as shown by atomic force microscopy and X-ray diffraction. Organic inverter circuits based on BTDPP2 ambipolar transistors display a gain of over 25.