is the deposition of oxide semiconductors that already demonstrated to be suitable for low power electronics [8] and for the development of logic circuits on paper substrates. [9] Recent research pointed to easier, faster, and less expensive approach for film deposition by using commercially available rollerball pens for the transfer of functional inks onto a wide range of substrates. This trend paves the way for the introduction of the PoP technique, where simple applications, as protein inks deposition, [10] conductive tracks, [11] piezoelectric devices, [12] or 3D antennas have already been demonstrated. [13] Nevertheless, in order to produce active devices, it is crucial to have as well a highly reliable and consistent method to deposit semiconductor materials using the same approach. By doing so, we pull printing and particularly PoP to a revolutionary technology stage, permitting the creation of low-cost and eco-friendly paper electronics "on-the-fly" by simply using a pen, proper functional inks, and a sheet of paper.To the best of the authors' knowledge, the successful deposition of inorganic semiconducting materials by the PoP technique has not yet been explored. Here, we report for the first time a method for a reliable deposition of ZnO nanoparticles (NPs) based dual-phase layer on paper substrates at room temperature (RT) by the PoP approach. These layers were used to fabricate hybrid fully printed/hand-drawn UV sensors and field effect transistors, where paper is simultaneously used as the physical support and as dielectric. Although the use of conventional rollerball pens is very appealing as a portable patterning instrument for paper-based printed/written electronics, only narrow lines (typically between 250 µm and 1 mm) can be drawn and the ink throughput is not always continuous. [13] For more complex and wider patterns, various parallel lines need to be drawn, where each line has to be in contact with the neighboring one to achieve a bidimensional functional film. This approach turns rollerball pen deposition process unpractical and difficult to control. In order to overcome this bottleneck we used a parallel metal plate pen, well-known from calligraphy applications, capable of dispensing ink over a large area (see Figure 1). Figure 1a shows the complete head of the used parallel pen and the two insets in Figure 1c illustrate the parallel plate structure, seen as side-and top-view in SEM, respectively (nib size of 6.0 mm). Here, the concept of direct ink passage becomes visually clear: whereas a rollerball pen disperses the ink by means of a rotating sphere, the parallel pen permits a direct ink flow between the two parallel metal plates. Thus, the ink flow depends on the capillary forces between the two plates and the The present work reports on the handwriting of electronic circuits on paper based on the deposition of an inorganic oxide semiconductor, exploiting the pen-on-paper (PoP) approach. The method relies on the use of a parallel metal plate pen, well known from calligraphy applications...

Fully printed and flexible inorganic electrolyte gated transistors (EGTs) on paper with a channel layer based on an interconnected zinc oxide (ZnO) nanoparticle matrix are reported in this work. The required rheological properties and good layer formation after printing are obtained using an eco-friendly binder such as ethyl cellulose (EC) to disperse the ZnO nanoparticles. Fully printed devices on glass substrates using a composite solid polymer electrolyte as gate dielectric exhibit saturation mobility above 5 cm2 V−1 s−1 after annealing at 350 °C. Proper optimization of the nanoparticle content in the ink allows for the formation of a ZnO channel layer at a maximum annealing temperature of 150 °C, compatible with paper substrates. These devices show low operation voltages, with a subthreshold slope of 0.21 V dec−1, a turn on voltage of 1.90 V, a saturation mobility of 0.07 cm2 V−1 s−1 and an Ion/Ioff ratio of more than three orders of magnitude.

Low-cost and large-scale production techniques for flexible electronics have evolved greatly in recent years, having great impact in applications such as wearable technology and the internet of things. In this work, we demonstrate fully screen-printed UV photodetectors, successfully fabricated at a low temperature on a cork substrate, using as the active layer a mixture of zinc oxide nanoparticles and ethylcellulose. The photoresponse under irradiation with a UV lamp with peak emission at 302 nm exhibited a quasi-quadratic behavior directly proportional to the applied voltage, with a photocurrent of about 5.5 and 20 μA when applying 1.5 V and 5 V, respectively. The dark current stayed below 150 nA, while the rise and falling times were, respectively, below 5 and 2 s for both applied voltages. The performance was stable over continuous operation and showed a degradation of only 9% after 100 bending cycles in a 45 mm radius test cylinder. These are promising results regarding the use of this type of sensor in wearable applications such as cork hats, bracelets, or bags.

Printed electronics answers to the emerging trend of using truly inexpensive and easily accessible techniques to design and fabricate low‐cost and recyclable flexible electronic components. Nevertheless, printing of inorganic semiconductor materials arises some barriers for flexible electronics, as they usually may require high annealing temperatures to enhance their electronic performances, which are not compatible with paper. Here, the formulation of a water‐based, screen‐printable ink loaded with zinc oxide nanoparticles that does not require any sintering process is reported. The ink is used to create the channel in fully printed electrolyte‐gated transistors on paper, gated by a cellulose‐based ionic conductive sticker. The high conformability of the electrolyte‐sticker mitigates the effect of the surface roughness of the channel, yielding transistors that operate under low voltage (<2.5 V) with a current modulation above 104 and μSat ≈ 22 cm2 V−1 s−1. These devices operate even under moderate outward bending conditions. The screen‐printed transistors are readily integrated in “universal” logic gates (NOR and NAND) by using ubiquitous calligraphy accessories for patterning of conductive paths and graphitic load resistances. This demonstrates the manufacturing of reliable and recyclable cellulose‐based iontronic circuits with low power consumption, paving the way to a new era of sustainable “green” electronics.

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