1536-125X (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TNANO.2018.2846230, IEEE Transactions on Nanotechnology 1 Abstract—The electrical and optical characteristics of bottom- gate top-contact poly (3, 3’’’- dialkylquaterthiophene) (PQT-12) polymer based organic thin film transistors (OTFTs) fabricated by floating-film transfer method (FTM) have been investigated in this paper. The atomic force microscopy (AFM), UV-Vis spectroscopy, and photoluminance (PL) characteristics of the FTM based PQT-12 films have been compared with the PQT-12 films deposited by the conventional spin-coating method. The improved electrical characteristics of FTM based OTFT have been found as compared to those of the spin-coated OTFTs. Due to better properties of the FTM based PQT-12 films over the spin coated films, the electrical and optical characteristics of the FTM based OTFT have been compared under dark and illuminated conditions. The FTM film based OTFT shows the respective values of field effect mobility and threshold voltage of 7.8×10 -2 cm 2 /Vs and -8.1 V under dark and 8.9×10 -2 cm 2 /Vs and -5.3 V under illumination of 200 μW/cm 2 at 540 nm. The maximum responsivity of 11.3 A/W is found at the light intensity of 5 μW/cm 2 at 540 nm. Index Terms—PQT-12, OTFT, Floating-film transfer method, Photoresponse, Responsivity. I. INTRODUCTION RGANIC thin film devices have drawn considerable attention for photodetection, gas sensing, memory cell and solar cell applications [1]–[5]. The conducting polymers with conjugate bonds have been widely used in the organic thin film transistors (OTFTs) due to low-temperature and low- cost solution processed fabrication method [1], [6]–[8]. The OTFTs are sensitive to the various intensity and wavelength of light due to generation of extra charge excitons pairs when exposed to particular intensity and/or wavelength of the light [6], [9]–[11]. The generated photoexcitons enhanced the charge carrier density in the OTFT channel [6], [12]. These photoexcitons are responsible for shifting the Fermi level close to the HOMO level in the p-type polymers and close to the LUMO level in n-type polymers [12]. This results in enhancing the field effect mobility which, in turn, increases Chandan Kumar, Gopal Rawat, Hemant Kumar, Yogesh Kumar, and Satyabrata Jit are with the Department of Electronics Engineering, IIT (BHU), Varanasi, India. (e-mail/ chandank.rs.ece14@iitbhu.ac.in, sjit.ece@iitbhu.ac.in). Rajiv Prakash is with the School of Material Science and Technology, IIT (BHU), Varanasi, India the on-current of the OTFTs. The photodetection properties of the conducting polymers have been largely investigated by the researchers. Deen et al. [6] have reported poly(3-octylthiophene) (P3OT) based phototransistors with field effect mobility of 4×10 -3 cm 2 /Vs and maximum responsivity of 0.16 A/W. Narayan and Kumar [13] have reported a P3OT polymer based phototransistor with a responsivity of 1 A/W at 1 mW/cm 2 intensity of 532 nm wavelength. A poly(9 9-dioctylfluorene-co-bithiophene) (F8B2) polymer based phototransistor has been reported by Hamilton and Kanicki [14] with the field effect mobility of 5×10 -4 cm 2 /Vs and maximum responsivity of ~10 A/W. The various hybrid composites of conducting polymers are also employed for the selective and enhanced photoresponse in the conducting polymers based phototransistors [15], [16]. Han et al. [17] have reported poly(3-hexylthiophene): hexylthiophene-end capped poly(3-hexylthiophene-co- benzothiadiazole) (P3HT:THBT-ht) blend based phototransistor with a maximum responsivity of 4 A/W at a wavelength of 470 nm. Few conducting polymers based phototransistors are observed to have higher responsivity than the single-crystal silicon phototransistors [18]. The poly (3, 3’’’- dialkylquaterthiophene) (PQT-12) polymer has been explored for optoelectronic applications due to its high photo-sensitive characteristics and high open-air stability [4], [19], [20]. The PQT-12 polymer was first used in the OTFTs as the modified form of P3HT obtained by alkyl chain engineering [7], [21]. Later, it has been explored for various devices such as diode, transistor, gas sensor, photodetector, solar cells etc. [3], [4], [12], [22]–[27]. Wasapinyokul et al. [20] have fabricated a top gate and bottom source/drain structure based phototransistor using spin-coating of PQT-12 polymer on a glass substrate. They have obtained a very low field effect mobility of 1.1×10 -3 cm 2 /Vs with a maximum responsivity of 6.6 A/W at 525 nm and the light intensity of 3 μW/cm 2 . In general, the solution processed spin-coating method is used for the thin film deposition of the polymers on the substrates [2], [4], [6], [20]. However, a new deposition technique called floating-film transfer method (FTM), originally proposed by Morita et al. [28] and later optimized by others [23], [29]–[31], have been explored as a superior technique by many researchers for synthesizing well aligned Electrical and Optical Characteristics of PQT-12 Based Organic TFTs Fabricated by Floating- Film Transfer Method Chandan Kumar, Gopal Rawat, Hemant Kumar, Yogesh Kumar, Rajiv Prakash, and Satyabrata Jit, Senior Member, IEEE O