**3. Low-temperature solution-processed oxide semiconductor thin films and transistors**

#### **3.1. Introduction**

Inorganic semiconductors including silicon and chalcogenides have been solution-processed into high-performance semiconductors that have better mobility (>10 cm<sup>2</sup> V−1 s−1) and stability in comparison with organic semiconductors [37, 38]. These solution-processed inorganic semiconductors, however, generally require high annealing temperature, necessary for generating crystalline phases, impurity-free, and dense structures for device-quality films.

On the contrary, in oxide semiconductors, the amorphous phases are capable of exhibiting electron mobilities comparable to those of their crystalline phase counterparts [39–41], which allow the exclusion of a high-temperature annealing process for obtaining crystalline phases. The conduction band minimum (CBM), which constitutes the electron conduction pathway, is composed of vacant metal cation s-states, and the spatial expanse of these s-states is greater than the inter-cation distances. The s-state spatial overlap is primarily determined by the principal quantum number (n), and therefore, heavy post-transition metal cations with (n–1)d10 ns<sup>o</sup> electronic configurations, where n ≥ 5, are ideal oxide semiconductor candidates [42]. Both In3+ and Sn4+, which have the same [Kr](4d10)5so electronic configuration, meet this requirement, and a highly dispersed CBM is also found in ZnO due to the small interaction distances [43]. To date, most amorphous oxide semiconductors (AOSs) are deposited on rigid substrates, such as glass and metal foil, and are processable at a high temperature (>400°C). Recent efforts to lower the annealing temperature have shown that device-quality AOSs, exhibiting high performance and device stability that are not easily achievable in organic semiconductor-based electronics, have been successfully grown on plastic substrates. In the following part, we summarize the recent advances in the development of solution-processed AOS at a low temperature. In particular, we discuss the chemical pathways (colloidal-based process, sol-gel routes, auto-combustion chemistry, and impurity-free precursor-based approach) and physical approaches by newly developed annealing techniques (photo-assisted, microwave, and high pressure), which effectively enable the fabrication of low-temperature, solution-processable, high-performance AOS-TFTs.
