Effect of UV irradiation on solution processed low voltage flexible organic field-effect transistors
Graphical abstract
Introduction
An extensive research advancement in the field of organic field-effect transistors (OFETs) has already lead to their successful integration in many applications such as flexible displays [1], [2] radio frequency identification (RFID) tags [3], wearable devices [4], and various types of sensors including chemical sensors [5], bio-sensors [6], gas sensors [7], and pressure sensors [8]. Use of OFETs in various optoelectronic applications like photo-sensing [9], [10], [11], optical memory elements and photo-switches [12], [13], [14] has also been very widely explored due to photo-sensitive nature of organic semiconductors. However, majority of these reports along with a recent study by our group [15] deal with photo-sensitive properties of organic semiconductors in the visible range of electromagnetic spectrum. Effect of ultra-violet (UV) irradiation on organic semiconductors largely remains under-explored with very less reports available. Nonetheless, a detailed study of the effects of UV irradiation on organic semiconductors and corresponding devices is highly imperative for development of several low cost, low power and high performance civil and military applications including smoke and fire detection, missile warning, combustion monitoring and ozone sensing [16], [17]. To the best knowledge of authors, for most of the organic semiconductors, which have been explored as UV detectors, corresponding devices are in the diode architecture and have been fabricated on rigid substrates [18], [19], [20], [21], [22], [23]. However, a photo-OFET is always a better choice over a photo-diode due to simultaneous photo-detection and amplification along with higher sensitivity. Despite this fact, reports on UV sensitive photo-OFETs are unfortunately obscure and effect of UV irradiation on the performance of OFETs remain under-addressed. For this reason, it is highly essential to study the effect of UV irradiation on the electrical characteristics of OFETs.
In this study, we report the effect of UV irradiation on the electrical performance of low voltage, flexible, solution processed OFETs. Organic semiconductor TIPS-pentacene was selected for this study due to its high performance and air stability [24], [25], [26]. Pristine OFETs function at moderate operating voltage of −10 V and show a mobility of 0.11(±0.08) cm2 V−1 s−1 with current on-off ratio of ∼104. UV irradiation was found to increase the off currents and threshold voltage (VTH) in positive VGS direction due to photo-generation of excitons. Photo-OFETs yielded a maximum drain current modulation (ratio of irradiated and dark currents) of ∼500, a maximum photo-responsivity of ∼43 mA/W for UV irradiation with an intensity of 1.8 mW/cm2. A Higher exposure time under UV irradiation led to a positive shift in VTH and reduction in the saturation current and mobility. This reduction was attributed to decrease in the degree of crystallinity of organic semiconductor due to prolonged UV exposure. On increasing the gate bias during irradiation, similar positive shift in VTH and reduction in mobility was observed. In addition, these OFETs showed a repeatable switching response to periodic illumination pulses, signifying their capability to be used as UV switches.
Section snippets
Experiments
Flexible polyethylene terephthalate (PET) substrates (thickness: 127 μm) having a 130 nm thick layer of indium tin oxide (ITO) with surface resistivity of 60 Ω/sq, were used to fabricate bottom-gate top-contact OFETs. Procedures of substrate cleaning and deposition of bi-layer gate dielectric consisting of high-k HfO2 and poly(4-vinylphenol) (PVP) on cleaned substrates were similar as described in our previous reports [15], [27]. A 0.5 wt % solution of TIPS-pentacene solution was prepared in
Results and discussion
A uniform PVP layer and terraced TIPS-pentacene crystals as obtained in our previous reports [28], were verified in this study also. Fig. 1(c) shows the absorption spectrum of TIPS-pentacene, which indicates higher absorbance in the ultra-violet region of the spectrum. Transfer characteristics of a representative TIPS-pentacene device is shown in Fig. 1(d). An average field effect mobility of 0.11(±0.08) cm2 V−1 s−1 was obtained with on-off current ratio of ∼104, as also recently reported by
Conclusion
Effect of UV irradiation (λpeak = 365 nm) on solution processed, TIPS-pentacene flexible OFETs has been analyzed. Due to trapping of photo-generated electrons, a positive shift in VTH was observed, whereas a quick collection of photo-generated holes resulted in an increase in off current. Maximum current modulation of ∼500 and maximum photo-responsivity of ∼43 mA/W were obtained from these OFETs for UV illumination intensity of 1.8 mW/cm2 while operating at low voltage of −5 V. Increasing UV
Acknowledgements
This work was supported in part by the research project funded by Science and Engineering Research Board, Department of Science and Technology, Government of India (SR/FTP/ETA-47/2012).
References (42)
- et al.
Organic RFID transponder chip with data rate compatible with electronic product coding
Org. Electron.
(2010) - et al.
Fabrication of low-cost electronic biosensors
Mater. Today
(2009) - et al.
Organic field-effect transistor and its photoresponse using a benzo[1,2-b:4,5-b′]difuran-based donor–acceptor conjugated polymer
Org. Electron.
(2014) - et al.
Highly light-responsive ink-jet printed 6,13-bis(triisopropylsilylethynyl) pentacene phototransistors with suspended top-contact structure
Org. Electron.
(2010) - et al.
Organic ultraviolet photovoltaic diodes based on copper phthalocyanine as an electron acceptor
Sol. Energy Mater. Sol. Cells
(7/24/2006) - et al.
OPV devices based on functionalized lanthanide complexes for application in UV–light detection
Sol. Energy Mater. Sol. Cells
(8/15/2007) - et al.
Spectral response tuning and realization of quasi-solar-blind detection in organic ultraviolet photodetectors
Org. Electron.
(2011) - et al.
Organic field-effect transistors based on single-crystalline active layer and top-gate insulator consistently fabricated by electrostatic spray deposition
Org. Electron.
(2015) - et al.
Performance enhancement in mechanically stable flexible organic-field effect transistors with TIPS-pentacene:polymer blend
Org. Electron.
(2016) - et al.
Flexible organic field-effect transistors with TIPS-Pentacene crystals exhibiting high electrical stability upon bending
Org. Electron.
(2016)
Effect of intrinsic polymer properties on the photo sensitive organic field-effect transistors (Photo-OFETs)
Microelectron. Eng.
Highly-sensitive solution-processed 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TESADT) phototransistors for optical sensing applications
Org. Electron.
Pentacene TFT driven AM OLED displays
IEEE Electron Device Lett.
Flexible AM OLED panel driven by bottom-contact OTFTs
IEEE Electron Device Lett.
Active devices based on organic semiconductors for wearable applications
IEEE Trans. Inf. Tech. Biomed.
Unencapsulated air-stable organic field effect transistor by all solution processes for low power vapor sensing
Sci. Rep.
Organic field-effect transistor-based gas sensors
Chem. Soc. Rev.
Ultralow voltage pressure sensors based on organic FETs and compressible capacitors
IEEE Electron Device Lett.
Flexible organic phototransistor array with enhanced responsivity via metal–ligand charge transfer
ACS Appl. Mater. Interfaces
High-performance top-gated organic field-effect transistor memory using electrets for monolithic printed flexible NAND flash memory
Adv. Funct. Mater.
Stable switching characteristics of organic nonvolatile memory on a bent flexible substrate
Adv. Mater.
Cited by (13)
An intensive study on organic thin film transistors (OTFTs) for future flexible/wearable electronics applications
2024, Micro and NanostructuresIn situ memory characteristics of thermal disturbance in low-voltage organic field-effect transistors
2022, Journal of Physics and Chemistry of SolidsUV assisted non-volatile memory behaviour using Copper (II) phthalocyanine based organic field-effect transistors
2021, Organic ElectronicsCitation Excerpt :Under external biasing condition, some of the photo-generated electrons are trapped at the semiconductor/dielectric interface causes lowered the trapped densities available, which eventually resulting in lower the VTH of the device. Whereas, photo-generated holes are drifted towards the drain terminal, resulting in enhancement in drain-to-source (IDS) of the device [11,24]. Similar to our previous programming condition, the transfer characteristics of the representative CuPc OFET remained in low-conductance state (“OFF state” or logic 0) in both condition at VGS = 0V i.e. in the “initial” state in dark condition as well as in the UV-light photo-illumination condition; representing no “writing” process were performed using this programming conditions.
Influence of molecular weight of polymer dielectric on the photo-response of solution-processed OFETs
2021, PolymerCitation Excerpt :These photo-generated electrons are trapped at various locations in the semiconductor causing reduction in potential barrier at the semiconductor/dielectric interface, resulting in positive shift in the transfer characteristics i.e. reduction of VTH of the representative device [27,32]. On the other side, photo-generated holes are drifted towards the drain terminal resulting in enhancement in drain-to-source current (IDS) of the device [27,32]. From the transfer characteristics, figure-of-merit of photo-transistor such as photo-current modulation (P) and photo-responsivity (R) were extracted, and are shown in Fig. 6 for each representative TIPS-pentacene OFET.
Influence of intensity on copper phthalocyanine based organic phototransistors
2021, Materials Today: ProceedingsUV-enhanced room-temperature ultrasensitive NO gas sensor with vertical channel nano-porous organic diodes
2020, Sensors and Actuators, B: ChemicalCitation Excerpt :However, for organic semiconductor-based material, the conductivity is not determined by the oxygen deficiencies. In prior reports, the effect of UV irradiation on solution processed organic semiconductor-based devices (OFETs, photodetectors, and optical memory elements) has been investigated [63–65]. In those reports, UV exposure caused a remarkable change in the electrical characteristics of the devices.