Effect of Oxygen, Moisture and Illumination on the Stability and Reliability of Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) OTFTs during Operation and Storage

Abstract

We report a systemic study of the stability of organic thin film transistors (OTFTs) both in storage and under operation. Apart from a thin polystyrene buffer layer spin-coated onto the gate dielectric, the constituent parts of the OTFTs were all prepared by vacuum evaporation. The OTFTs are based on the semiconducting small molecule dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) deposited onto the surface of a polystyrene-buffered in situ polymerized diacrylate gate insulator. Over a period of 9 months, no degradation of the hole mobility occurred in devices stored either in the dark in dry air or in uncontrolled air and normal laboratory fluorescent lighting conditions. In the latter case, rather than decreasing, the mobility actually increased almost 2-fold to 1.5 cm(2)/(V · s). The devices also showed good stability during repeat on/off cycles in the dark in dry air. Exposure to oxygen and light during the on/off cycles led to a positive shift of the transfer curves due to electron trapping when the DNTT was biased into depletion by the application of positive gate voltage. When operated in accumulation, negative gate voltage under the same conditions, the transfer curves were stable. When voltage cycling in moist air in the dark, the transfer curves shifted to negative voltages, thought to be due to the generation of hole traps either in the semiconductor or its interface with the dielectric layer. When subjected to gate bias stress in dry air in the dark for at least 144 h, the device characteristics remained stable.

Vacuum production of OTFTs by vapour jet deposition of dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) on a lauryl acrylate functionalised dielectric surface

“…Since registration is difficult for the small nozzle, a slight off-set of 0.5 mm was applied to each substrate in the drum width direction, in order to ensure that at least one row of transistors was well registered with the jet. as hole traps reported previously [25].…”

“…The lauryl acrylate buffered sample showed slightly more negative turn-on and threshold voltages than for the PS buffer, likely due to its slightly higher surface polarity and hence affinity for water. It is known that absorbed atmospheric moisture can be a source of hole traps which is reflected in transfer curves as a more negative turn-on voltage and eventually lower mobility [25]. The un-buffered transistor in this work showed carrier mobility of ~0.05 cm 2 /Vs, on/off ratio of 10 4 and turn-on voltage close to 0 V. The low mobility was likely due to the undesirable surface energy which is prone to yielding poor crystallinity at the very near surface in deposited films of small molecule semiconductors as discussed in our recent paper [23].…”

Molecular Semiconductors for Logic Operations: Dead‐End or Bright Future?

“…No drift of µ or V T were observed after 20 000 cycles of the transfer characteristics or after 24 h under constant direct‐current bias stress. OFETs were also air stable >200 d. Thienoacenes, such as DNTT 9 ( R = H or C 10 H 21 ) show excellent device stability over periods of several months . Thermal stability of OFETs was also demonstrated for DNTT 9 ( R = phenyl) .…”

Effect of relative humidity and temperature on the stability of DNTT transistors: A density of states investigation

“…However, this is not the case for DNTT-based TFTs. A recent study showed that moisture led to negative shifts of threshold and turn-on voltages, and an increase in off-current [16]. The measured mobility appeared to be unaffected.…”