Encapsulating the active Layer of organic Thin-Film Transistors

Organic thin-film transistors (OTFTs) with W = 1000 μm and L = 1 μm were produced with a high batch reproducibility of the on-current of -63.3 μA +/- 17 μA (-40 V
DS
, - 40V
GS
) and the threshold voltage of 1.3 V +/- 1.44V. Unprotected organic devices suffer from degradation due to water damp and oxygen incorporation. To validate the function of an OTFT capsulation, interdigital transistor structures (W = 46.8cm, L = 20 μm) were prepared on p-type silicon wafers to drive a high current (initially -6.8 mA at -40 V
DS
, - 40V
GS
) in order to detect an explicit reaction to degradation. Subsequently, the OTFT's active layer was encapsulated with 1.5 μm of sputtered polytetrafluoroethylene (PTFE) driving a current of -6.2 mA. A degradation experiment over 4 months in dark laboratory conditions revealed a reduced degradation compared to earlier experiments. The threshold voltage shifted in positive direction suggesting degradation only from oxygen. Obviously, the degradation from humidity was blocked. Otherwise, it would have caused a negative threshold voltage shift.