Influence of Source/Drain Catalytic Metal and Fabrication Method on OTFT-Based Hydrogen Sensor

The effects of source/drain (S/D) catalytic metal (Pd, Pt, and Ni) and fabrication method (sputtering and electron-beam evaporation) on the sensing characteristics of organic thin-film transistor (OTFT)-based hydrogen sensor are investigated. The highest solubility of H₂ in Pd contributes to the highest sensitivity of the sensor with Pd as the S/D electrodes. Although Ni has higher H₂ solubility than Pt, the device with Ni S/D electrodes has the lowest sensitivity among the three devices with different catalytic metals. This should be resulted from more chemisorbed oxygen, higher hydroxyl formation rate, and lower heat of chemisorption of hydrogen on Ni than on Pd and Pt. All these lead to lower surface coverage of hydrogen and thus less catalytic dissociation of hydrogen for the Ni electrode. In addition, a native oxide layer is formed on the Ni surface in moist air at room temperature, which inhibits the hydrogen adsorption on Ni. On the other hand, it is demonstrated that the three devices with catalytic metals fabricated by sputtering have higher sensitivity than their counterparts made by E-beam evaporation because larger thermal load generated during E-beam deposition results in higher thermal stress and thus poorer metal contact to reduce the sensitivity. Last, both the higher thermal stress in the S/D electrode and the poorer S/D contact are demonstrated to increase the response time of the sensor.