Investigation of the Organic Semiconductor Sensors for the Telemetry System Applications

Abstract

In this work, organic semiconductor sensors for humidity, light, temperature and displacement measurements were fabricated and characterized for the telemetry system applications. Main advantages of the organic devices are its low cost, ease of device fabrication, and the use of flexible substrate for the device fabrication. Organic semiconductor devices have attracted considerable interest in the area of electronic and photonic devices due to their wide range of applications. The techniques which are used in the fabrication of the organic devices are comparatively simple one, resulting in the reduced cost of the device. The organic materials can be easily synthesized. The choice of material selection makes it more attractive for the future technology. The organic materials also have very good compatibility with the silicon technology. The fabricated organic sensors were integrated with standard inorganic semiconductor circuits for the telemetry system applications. Organic humidity sensors were fabricated using Copper Phthalocyanine (CuPc) and Poly Epoxy Propyle Carbazol (PEPC) with the blend of Cellulose (which creates the porosity for the device). Silver (Ag) and gold (Au) were used as electrodes. These humidity sensors were fabricated by the drop casting method. Methanol was used to prepare the 5wt% blend of cellulose with the organic material. Fabricated humidity sensors were characterized for the resistive-humidity, capacitive-humidity, and for the impedance- humidity response. Impedance response can be used to obtain both the effects of resistance and capacitance which makes it more usable parameter in the characterization of the devices. Using the impedance-humidity relationship increases the range of the sensor and improves the sensitivity of the sensor in the humidity interval of 30%-98% RH. It was observed that the impedance is largely effected by the resistance-humidity parameter from 30%-80% RH humidity level and the capacitance-humidity parameter plays its essential role in the interval of 80%-98% RH. To fabricate light and temperature sensors, Copper Phthalocyanine (CuPc) and Gallium Arsenide (GaAs) were deposited on silver electrode. A thin film of CuPc was deposited on GaAs substrate to fabricate Ag/n-GaAs/p-CuPc/Ag structure in which the front electrode has a gradient of thickness which allows the 6% and 10% of the incident light to pass through the silver film at the edge and in the center of the sample, respectively. CuPc was deposited by vacuum thermal evaporation technique and its thickness was observed by a quartz crystal monitor. Light-Resistance and Temperature-Resistance parameters of the multi-functional sensor were then measured. The Light Experiment was performed by opening a small lid from the top of the chamber, and the Temperature Experiment was performed in an air tight closed chamber. After that the sensor was connected with the oscillator circuit, and it was observed that how it affects the oscillating frequency of the voltage controlled oscillator. When illuminated, the cell acts as a photo resistor, and the frequency of VCO varies from 12.2 kHz to 17 kHz depending on the illumination of the cell. When the cell is connected with the VCO as a temperature sensor, the frequency of VCO varies from 2.1 kHz to 11 kHz. The temperature and illumination dependence of the frequency of oscillator can be used for the short range and long range telemetry system applications. Organic displacement sensors were fabricated using carbon nanotubes (CNTs). Multi- walled carbon nanotubes (MWNTs) were deposited on an adhesive plastic substrate connected to two aluminum electrodes. Displacement sensors were fabricated using a technique known as “Press Tablet”. To fabricate this kind of displacement sensor, bulk of the CNT material was put on the Al electrodes with an inter electrode distance of 4-6 mm. The nominal thickness of the MWNTs layers in the samples were ~300 and ~430 μm. The fabricated displacement sensors were put into a plastic casing so that the external environmental effects can be minimized. It was found that by changing the displacement from 0 to 800 μm changes occurred in the sensor resistance by 2.5-3 times which ultimately alter the oscillator’s frequency (from 22-156 kHz for 430 μm thick samples and 42-122 kHz for 300 μm thick samples). This effect can be used for the short range and long range telemetry system applications. A second displacement sensor was fabricated using organic field effect transistor (OFET) in which Silver (Ag) was used as the source and drain electrode, Aluminum (Al) as the gate electrode and the Copper Phthalocyanine (CuPc) was used as the organic semiconducting material. CuPc and Al were deposited by the vacuum thermal evaporation technique. The sensor was characterized for the effect of displacement, and was observed that the OFET changes its resistance in response to displacement effect on the surface of the sensor. The organic field effect transistor changes its resistance by 8 times when the displacement was changed from 0 to 550 μm. Afterwards, the displacement sensitive OFET was connected with the voltage controlled oscillator circuit, altering the frequency of the oscillator from 3.3-11 kHz, which can also be utilized for the telemetry system applications.