Inkjet-printed Organic Thin-film Transistors Research Articles

Monolithically printed all-organic flexible photosensor active matrix

Upcoming technologies in the fields of flexible electronics require the cost-efficient fabrication of complex circuitry in a streamlined process. Digital printing techniques such as inkjet printing can enable such applications thanks to their inherent freedom of design permitting the mask-free deposition of multilayer optoelectronic devices without the need for subtracting techniques. Here we present an active matrix sensor array comprised of 100 inkjet-printed organic thin film transistors (OTFTs) and organic photodiodes (OPDs) monolithically integrated onto the same ultrathin substrate. Both the OTFTs and OPDs exhibited high-fabrication yield and state-of-the-art performance after the integration process. By scaling of the OPDs, we achieved integrated pixels with power consumptions down to 50 nW at one of the highest sensitivities reported to date for an all-organic integrated sensor. Finally, we demonstrated the application potential of the active matrix by static and dynamic spatial sensing of optical signals.

Improved Circuit Model Fitting of Inkjet-Printed OTFTs and a Proposal for Standardized Parameter Reporting

Within the field of organic thin-film transistors (OTFTs), a large variety of materials are available. As a consequence, it may not be possible for one circuit model to accurately replicate the behavior of all OTFT devices. We propose modifications to two popular circuit models in order to better match the characteristics observed in the devices manufactured using our materials system. Using measured data for complementary n- and p-type organic devices, modeling parameters are extracted through optimization. Due to the ubiquitous use of the square-law model to characterize organic devices, modeling parameters are also extracted using this traditional approach. Extracted parameters are then compared and discussed. As there is a clear variation in device parameters based on the model used, a new standardization scheme is proposed which attempts to provide a standardized quality assurance metric, which simplifies the comparison of the reported device parameters. This scheme provides an indication of the goodness of fit between the model being used to describe the device and the extracted modeling parameters.

Flexible Organic Light-Emitting Diode Displays Driven by Inkjet-Printed High-Mobility Organic Thin-Film Transistors

A 3.2-in flexible color display, with a resolution of 50 ppi and composed of bottom-emission multiphoton organic light-emitting diodes (OLEDs) and inkjet-printed organic thin-film transistors (OTFTs) with a bottom-gate/bottom-contact structure on a color filter, was developed. The device could successfully display color videos while being bent, and achieved a maximum luminance of 125 cd/m2 with white light emission. The gate dielectrics of the OTFTs used on the backplane were bilayers of cardo polymer and Parylene, and the material used for the organic semiconductors was dithieno [2,3-d;2’,3’-d’]benzo[1,2-b;4,5-b’]dithiophene blended with polystyrene in tetralin solvent, which was coated using inkjet printing to sufficiently fill the banks composed of a fluorine-based polymer. OTFTs with a channel length of 5 $\mu \text{m}$ were created using the above process, and the structure achieved a high mobility of 1.2 $\mathrm {cm}^{{{2}}}/({\mathrm{ V}}\cdot \text{s}$ ), making it suitable for flexible color OLED displays. The mobility was about three times as high as that obtained using solution shearing methods.

Developement of Printed OTFTs and Logic Circuits

We report the development of inkjet-printed organic thin-film transistors (OTFTs), inverters and logic circuits on PET substrate. The mobility of 1 cm2/Vs and 0.1 cm2/Vs achieved on p-type and n-type printed OTFTs with an operation voltage of 15 V. The printed uniform and thin dielectric layer achieved by introducing coffee ring effect on printing process. A gain value of 24 obtained from the printed CMOS inverter and the device exhibits an excellent air-stability without encapsulation. Our results demonstrated the potential of the developed printed transistors for flexible electronics in the near future.

Fully inkjet-printed short-channel organic thin-film transistors and inverter arrays on flexible substrates

We report fully inkjet-printed organic thin-film transistors (OTFTs) and inverters with an average channel length of 9 μm and minimized overlap capacitance between gate and source/drain (S/D) electrodes on a flexible plastic substrate. Metal-organic precursor-type silver ink, poly(4-vinylphenol), and 6, 13-bis (triisopropylsilylethynyl)pentacene were used to form the gate and S/D electrodes, the gate dielectric, and the active semiconductor layer, respectively. Well-defined S/D electrodes with narrow width and spacing can be realized by introducing a separate printing process and interdigitated structure with a cartridge that can eject a 1 picoliter volume of ink droplets without any assistant surface treatments or structures. Fully inkjet-printed short channel OTFT arrays presenting an average mobility of 0.065 cm2 V−1 s−1 and on/off ratio of 4 × 103 and inverter arrays exhibiting a voltage gain of 5.5 at a VDD of −30 V and good frequency response were demonstrated.

All-inkjet-printed low-pass filters with adjustable cutoff frequency consisting of resistors, inductors and transistors for sensor applications

Low-cost and flexible first and second order low-pass filters with adjustable cutoff frequency were designed and printed by inkjet printing technology. The all-inkjet-printed low-pass filters were characterized and an adjustable cutoff frequency feature in form of an inkjet-printed organic thin-film transistors (OTFTs) was added to the filters for application-oriented fine-tuning. The applicability of these small circuits was evaluated by signal filtering for sensor applications. As a result, low-pass filters with an adjustable cutoff frequency ranging from 82 Hz to 740 Hz were obtained, demonstrating their suitability in signal filtering and their promising applicability for tactile sensing characterized by low frequency signals.

Flexible nano-hybrid inverter based on inkjet-printed organic and 2D multilayer MoS2 thin film transistor

We report a novel platform on which we design a flexible high-performance complementary metal–oxide–semiconductor (CMOS) inverter based on an inkjet-printed polymer PMOS and a two-dimensional (2D) multilayer molybdenum disulfide (MoS2) NMOS on a flexible substrate. The initial implementation of a hybrid complementary inverter, comprised of 2D MoS2 NMOS and polymer PMOS on a flexible substrate, demonstrates a compelling new pathway to practical logic gates for digital circuits, achieving extremely low power consumption with low sub-1nA leakage currents, high performance with a voltage gain of 35 at 12V supply voltage, and high noise margin (larger than 3V at 12V supply voltage) with low processing costs. These results suggest that inkjet-printed organic thin film transistors and 2D multilayer semiconducting transistors may form the basis for potential future high performance and large area flexible integrated circuitry applications.

Ionic self-assembled monolayer for low contact resistance in inkjet-printed coplanar structure organic thin-film transistors

To reduce the contact resistance in inkjet-printed organic thin-film transistors (OTFTs), the use of a newly synthesized ionic self-assembled monolayer (SAM) consisting of an anchoring group, a linker group, and an ionic functional group, is investigated. According to the gated transmission line method (TLM) measurements of a series of OTFT devices, where one type has no charge injection layer, another type having a pentafluorobenzenethiol (PFBT) injection layer, and a third type containing a (6-mercaptohexyl)trimethylammonium bromide (MTAB) ionic SAM, the latter exhibits the lowest contact resistance value of ∼3.1KΩcm. The OTFTs without charge injection layer and with the PFBT SAM have relatively higher contact resistance values of ∼6.4KΩcm and ∼5.0KΩcm, respectively. The reduced contact resistance in the OTFTs with ionic SAMs is attributed to the large charge carrier density induced by the ionic SAM, which allows sufficient tunneling-assisted injection of the carriers from the metal electrode to the polymer semiconductor. These results suggest that the use of appropriate ionic SAM injection layer is an effective way to reduce the contact resistance, hence improving the charge transport characteristics of inkjet-printed OTFTs.

Inkjet-printed organic thin film transistors based on TIPS pentacene with insulating polymers

The blending of the crystalline organic semiconductor, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene), with amorphous polymers exhibits not only excellent solution processability, but also superior performance characteristics in organic thin film transistors (OTFTs). To understand the inkjet-printing behavior of TIPS pentacene/polymer blends, we use amorphous polycarbonate (APC), which is structurally beneficial to the facile phase separation of TIPS pentacene crystals due to the strong segregation strength estimated by the Flory–Huggins interaction parameter. The various inkjet-printing behaviors of TIPS pentacene/APC inks, which depend on the TIPS pentacene/APC compositions, ink viscosities, and different solvent mixtures, are investigated. These behaviors can ultimately determine the phase separation, morphology, shape, and orientation of the TIPS pentacene crystals in OTFT films. Flory–Huggins phase separation theory is applied, and various analytical methods, such as polarized optical microscopy, 3D surface profile, and time-of-flight secondary ion mass spectroscopy (TOF-SIMS), are utilized to explain these relationships. By controlling these inkjet-printing conditions, it is possible to easily regulate the optimal inkjet-printing process for TIPS-pentacene/polymer systems, which can derive the desirable stripe-shaped and vertically phase-separated TIPS pentacene crystals with the proper orientation and enhanced surface morphology. The resultant inkjet-printed films from the TIPS pentacene with APC show excellent device stability and an average mobility of 0.53 cm2 V−1 s−1. Furthermore, the inkjet-printed flexible OTFT array with an average mobility of 0.27 cm2 V−1 s−1 sustains the application of TIPS pentacene/APC in the field of flexible printed electronics.