Organic Dye Based Opto-Electronic Devices and Sensors
Organic semiconductors have been the subject of intense scientific investigation for the past 50 years. Applications of these materials include chemical, physical and biological sensors, electronic, and photonic devices. Low cost production of these devices is possible by using thermal evaporation, centrifugation, spin coating and drop casting technologies. This thesis describes fabrication and characterization of organic semiconductor sensors for temperature, humidity and light measurements using novel organic materials. Different types of commercially available organic dyes have been used as active materials to fabricate sensors.
In the first part of the thesis, humidity sensors have been fabricated using CuPc/NiPc thin films deposited by drop casting and centrifugation. Another type of humidity sensor based on VOPcPhO, Alq3 and their composites have also been fabricated by spin coating technique, wherein organic films have been grown over a substrate having pre-fabricated aluminum electrodes. Optimum ratio of VOPcPhO and Alq3 has been identified to achieve best capacitive and resistive response of the sensor.
In the second part, optical sensors from composite films of NiPc:CoPc (1:1) have been fabricated by drop casting and centrifugation on a ceramic alumina substrate having surface-type interdigitated Ag electrodes. Variation in impedance and capacitance w.r.t. illumination have been measured. Another type of optical sensors based on organic photodiodes (OPDs), using composite ZnPc:PC71BM dye, have been fabricated and characterized. The optimum ratio of ZnPc:PC71BM (1:0.8) has been determined by detecting photocurrent density with respect to varied illumination levels of impinging light at different reverse biased conditions of the optical sensors. Moreover, the light to dark current density ratio (Jph/Jd) of the devices has been evaluated along with the transient photo-current density response to assess their sensitivity
In the third part, organic field effect transistors (OFETs) functioning as temperature/humidity senors (referred to as multi-functional sensors) have been fabricated using orange dye (OD) and its composite OD:sugar at positive (+1 g) and negative (-1 g) gravities by drop casting. It has been observed that in drop casting method, properties of OFET sensors are dependent upon gravity as well as solution composition employed for OFETs channel definition.
Additionally, OFETs have been fabricated by depositing NiPc of varying thickness (100-300 nm) and semi-transparent thin films of aluminum in sequence by vacuum evaporation on a glass substrate having silver source and drain electrodes. It has been observed that the drain current of OFETs increases with increase in radiation intensity, and the OFETs having 200 nm thick NiPc film, exhibited better performance compared to 100 nm and 300 nm thick films. For temperature evaluation, it has been noticed that the reduction in the impedance is higher when thickness of the film is decreased. It has also been demonstrated that for improved performance, as a temperature sensor, the OFET channel film should be ∼100 nm.