Fabrication and Characterization of Photosensitive Organic Field - Effect Transistors

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Mahato, Ajay Kumar
Tiwari, Shree Prakash
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Indian Institute of Technology Jodhpur
Over the past few decades, organic electronics has been extensively explored for various applications such as backplane circuitry for flexible displays, radio-frequency identification (RFID) tags, wearable, and photo-sensing devices due to several advantages such as ease of fabrication, mechanical flexibility, large-area deposition,andsolutionprocessability. For photo-sensitive device applications, organic semiconductors are preferred because of advantages such as higher photo-responsivity towards photo-illumination, and wide photo-sensing spectrum in UV-Vis spectrum. Various types of high-performance photo-sensing devices such as photo-diode, photo-transistors, solar photovoltaic cell, and non-volatile optical memory have been developed over the past two decades. Several groups have demonstrated photo-sensitivity of the order of 1A/W using numerous organic semiconductors, which is comparable to the conventional amorphous silicon (a-Si). It was found that the photo-responsivity and photo-sensing spectrum of organic semiconductor materials could be easily tuned by changing the molecular/chemical structure of organic semiconductor materialsIn this work, the photo-sensitive behavior of organic semiconductor material is focused upon, in organic field-effect transistors (OFETs) fabricated bottom-gate top-contact (BGTC) configuration. TIPS-pentacene, a soluble acene-based organic semiconductor material, is primarily chosen as organic semiconductor, and the corresponding electrical and photo-response properties were studied. It was observed that, though TIPS-pentacene OFETs offered several advantages such as high charge-carrier mobility and air-stability, there are some shortcomings such as sensitivity towards red-light illumination and dependency of device performance on deposition schemes. We systematically worked on these aspects and found that the inferior photo-sensitivity of TIPS-pentacene OFETs towards red-light illumination could be mitigated by using a bilayer combination of TIPS-pentacene/CuPc as active layer. We found that, neat TIPS-pentacene OFETs can sense only blue and green colours, and CuPc OFET can sense only red and green colours; whereas TIPS-pentacene/CuPc bilayer OFET can sense all red, green and blue colours representing enhancement in photo-sensing spectrum.Further, it was also observed that the device performance of solution-processed OFETs were modulated by deposition schemes. In our study, we have fabricated solution processed OFETs using drop-casting, spray-coating and spin-coating deposition schemes using TIPS-pentacene and TES-ADT as active semiconductor materials; and the corresponding thin-film, electrical and the associated photo-response performance were studied. It was found that, the spray-coating of solution-processable organic semiconductor offered superior electrical as well as photo-response performance compared to the conventional drop-casting and spin-coating deposition schemes for both TIPS-pentacene and TES-ADT devices due to thinner active semiconductor layer.Influence of molecular weight of polymer dielectric on the electrical and photo-response performance for TIPS-pentacene OFETs was also investigated. We observed that the overall device performance and photo-response behavior of TIPS-pentacene OFETs were deteriorating with increase in molecular weight of the PVA molecule. The deterioration in the device performance and the photo-response behavior was attributed to the decrease in orientation of hydroxyl (-OH) group concentration perpendicular to the PVA-semiconductor interface with increase in molecular weight of the PVA molecule. Moreover, the quality of the active layer also deteriorates due to increase in surface roughness of PVA film with increase in molecular weight of the PVA molecule, resulting in higher trap density in the active semiconductor film.Moreover, non-volatile memory characteristics using CuPc OFETs were also demonstrated. The memory operation was demonstrated with different programming conditions, and the corresponding change in device transfer characteristics was examined accordingly. We found that, programming of CuPc OFET with an electric pulse under UV-lightiiillumination prominently shifts the device transfer characteristics from “Low-conduction state” to “High-conduction state”, showing non-volatile memory behaviour.
Mahato, Ajay Kumar. (2020). Fabrication and Characterization of Photosensitive Organic Field - Effect Transistors (Doctor's thesis). Indian Institute of Technology Jodhpur, Jodhpur.