Following the ever-expanding technological demands, printed electronics has shown palpable potential to create new and commercially viable technologies that will benefit from its unique characteristics, such as, large-area and wide range of substrate compatibility, conformability and low-cost. Through the last few decades, printed/solution-processed field-effect transistors (FETs) and circuits have witnessed immense research efforts, technological growth and increased commercial interests. Although printing of functional inks comprising organic semiconductors has already been initiated in early 1990s, gradually the attention, at least partially, has been shifted to various forms of inorganic semiconductors, starting from metal chalcogenides, oxides, carbon nanotubes and very recently to graphene and other 2D semiconductors. In this review, the entire domain of printable inorganic semiconductors is considered. In fact, thanks to the continuous development of materials/functional inks and novel design/printing strategies, the inorganic printed semiconductor-based circuits today have reached an operation frequency up to several hundreds of kilohertz with only a few nanosecond time delays at the individual FET/inverter levels; in this regard, often circuits based on hybrid material systems have been found to be advantageous. At the end, a comparison of relative successes of various printable inorganic semiconductor materials, the remaining challenges and the available future opportunities are summarized.

kinds of solid electrolytes [29-40] have been used as the gate insulators. Within this solution processed/printable semiconductor technologies, inorganic oxides hold paramount positions owing to their unparalleled transport properties, abundance and low cost. [41-43] In fact, the performance of printed oxide TFTs have steadily improved over the recent times and have certainly become substantially superior to their organic counterparts. In this regard, among the oxide semiconductors of choice, indium oxide have received specific attention due to their chemical and environmental stability resulting in high device lifetime and particularly high carrier mobility which can be easily reproduced even when they are solution processed. [44-47] In the present study, the indium oxide precursor ink has long been adjusted for its easy printability, homogeneous film formation and improved electronic transport/carrier mobility values. With this thoroughly optimized semiconductor material, different solid electrolytic insulators in the form of CSPEs and ion gels have been examined for their device performance and relative environmental stability. Recently, solution processed high-k oxide dielectrics have also shown good promise with low process temperature and device performance. However, high performance solid electrolytes may always offer crucial advantage in terms of semiconductor/insulator interface quality, high gating efficiency, easy printability, room temperature processability and high capacitance values enabling extremely low operating voltages of the TFTs (<2 V). [38,48-51] The first notable instance of employing electrolytes in fieldeffect transistors (FETs) can be traced back to the work carried out by Bergveld in 1970, which has later been used as ion-sensitive FET sensors. [52] Tardella and Chazalviel in 1985 studied nonaqueous electrolytes. [53] One of the earliest examples of solid electrolyte has been poly(ethylene oxide) (PEO) doped with lithium salt. [54] In this regard, Frisbie and co-workers had reported solid polymer electrolyte based on mixture of lithium perchlorate (LiClO 4) salt in a PEO polymer matrix to be used as gate insulator in electronic devices. [55,56] As a result of the increased cross-linking points due to the interaction between the Li + and the PEO chains, the ion transport in these electrolytes is primarily dependent on the temperature and the Printed oxide thin film transistors (TFTs) have outperformed their organic counterparts in the recent past; printable solid electrolytic insulators have also emerged as a suitable alternative to oxide dielectrics. In the present study, multiple composite solid polymer electrolytes (CSPEs) and an easyto-formulate ion gel are fabricated and characterized for their double-layer capacitance (C DL) values and the quality of electrostatic coupling. In the next step, alongside printed In 2 O 3 as the semiconductor channel, the performance of the solid electrolytes is evaluated using printed top-gate TFTs. The semiconductor ink formulation and the device ar...

The major limitations of solution‐processed oxide electronics include high process temperatures and the absence of necessary strain tolerance that would be essential for flexible electronic applications. Here, a combination of low temperature (<100 °C) curable indium oxide nanoparticle ink and a conductive silver nanoink, which are used to fabricate fully‐printed narrow‐channel thin film transistors (TFTs) on polyethylene terephthalate (PET) substrates, is proposed. The metal ink is printed onto the In2O3 nanoparticulate channel to narrow the effective channel lengths down to the thickness of the In2O3 layer and thereby obtain near‐vertical transport across the semiconductor layer. The TFTs thus prepared show On/Off ratio ≈106 and simultaneous maximum current density of 172 µA µm−1. Next, the depletion‐load inverters fabricated on PET substrates demonstrate signal gain >200 and operation frequency >300 kHz at low operation voltage of VDD = 2 V. In addition, the near‐vertical transport across the semiconductor layer is found to be largely strain tolerant with insignificant change in the TFT and inverter performance observed under bending fatigue tests performed down to a bending radius of 1.5 mm, which translates to a strain value of 5%. The devices are also found to be robust against atmospheric exposure when remeasured after a month.

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