Abstract In the present work, current–voltage ( I– V) characteristics of fullerene devices (ITO⧹C 60⧹Al) are reexamined by in situ electrical measurement in high vacuum and by infrared imaging analysis. Two kinds of I– V curves are detected: ‘ohmic’ and nonohmic. Degradation processes of the two different devices are measured, and ‘ohmic’ degradation processes are ascribed to short-circuiting. ITO⧹C 60⧹Al devices in high vacuum are confirmed to be intrinsically nonohmic. Surface temperature distribution of the two different devices is measured and localized heat is detected further confirming the existence of short-circuiting in ‘ohmic’ devices. To avoid short-circuit, organic buffers are inserted between fullerene layer and cathode and this is found to be effective.