Investigation on the influence of interfacial and morphological effects in organic field effects transistors

Abstract

Organic eld e ect transistors (OFETs) are important building blocks of organic electronic circuits and systems. OFETs nd applications in large-area based electronic displays because of their inherent advantages like low temperature and largearea processability on exible substrates. Their applications include exible displays, large area based sensing and active matrix imaging. However, the commercialization of OFETs faces several technical obstacles. The low mobility of organic semiconductors limits their current-carrying e ciency and switching speed, the high operation voltage restrict their use as low power devices, easy degradation in air causing shorter device lifetime and contact resistance limiting the charge injection/extraction into/from the semiconductor. Many of the aforementioned issues relate to interfacial properties in OFETs. Among two important interfaces in OFETs namely the interface between organic semiconductor and the dielectric layer and between metal and organic semiconductor near the source/drain electrodes, the interface area present in the latter is critical for carrier injection and extraction e ciency of the device. Apart from interface area, the thin lm morphology of organic semiconductor near the interface and in the bulk, too becomes crucial and both these factors contribute to the current carrying e ciency of the device. Moreover, these factors also a ect the optical response of these devices and will be extremely signi cant from the perspective of applications such as organic phototransistors (OPTs).In this regard the optoelectronic performance of top and bottom contact OFET con- gurations were compared through systematic studies by numerical simulations and fabrication of OFETs. Moreover, some non-contact mode techniques were adopted to quantitatively reveal the morphological changes happening at the metal-organic interface in the two con gurations. The analysis of obtained results showed that despite of the fact that organic semiconductor morphology was severely disrupted in top contact structures due to metal penetration e ect, these structures showed better performance over the bottom contact devices which is attributed to large interface area for carrier injection and extraction. While bottom contact structures were observed to produce inferior performance due to the cumulative e ects of limited carrier injection/extraction area and poor semiconductor morphology near thecontacts. Moreover, the OFETs fabricated using di erent polymers and polymer deposition techniques showed that its performance under dark and illumination were critically dependent on semiconductor morphology.