Solution-based sol-gel technology has attracted interest in various fields due to the accessibility of advanced materials with ‘tailor-made’ functionalities through inexpensive and environmentally viable processing routes. One of the reasons for the continued advancement of sol-gel technology is the ease of control of the nano-architecture of the resultant materials and the plethora of different material constructs that can be developed. The method’s versatility lies in the comfort of integrating sol-gel technologies with other forms of processing, allowing multidisciplinary approaches to occur with minimal effort. 

And metal oxide semiconductors have emerged as alternative channel materials for thin-film transistors (TFTs) in the field of displays and transparent electronics due to their attractive electrical properties, strong oxidizing power, good chemical inertness, low cost, non-toxicity, large surface area, and unique optical properties. Interest in flexible oxide TFTs has increased notably due to advances with a-IGZO in the active matrix display industry. Recent research focuses on new metal oxides that exclude rare elements like In and Ga while achieving higher TFT mobility. Sn-based oxides (e.g. ITZO, IGTO, ZATO, and SnO₂) have shown excellent electrical performance and high mobility, making them potential alternatives; Sn⁴⁺ can replace In due to its similar electronic structure, abundance, and cost-effectiveness, and it forms stronger bonds with oxygen than Zn²⁺. Among the metal oxide semiconductors, SnO₂ with relatively high mobility (250 cm²/Vs at 300 K), wide bandgap (3.6 eV), and low crystallization temperature have shown great potential for the application. On top of that, ZrO₂ with its high dielectric constants (>20), sufficient bandgap (~ 5.8 eV), good compatibility with SnO₂, and excellent electrical/chemical reliability, has the potential to serve as the switching layer in low-power operation, which is required for next-generation switching devices.

Hence, many efforts have been made to apply SnO₂ and/or ZrO₂ by sol-gel processing to maximize each other’s advantages. However, there is a critical issue that needs to be addressed to practically utilize these solution-based technologies. A considerable amount of energy is required to convert the precursor into an oxide film and sufficiently remove internal organic residues, and for this, high-temperature annealing of > 400 °C is usually accompanied incompatible with most flexible plastic substrates after precursor deposition. However, there are still challenges in implementing solution-based low-temperature processes so far.

In our study, (1) by adding the combustion effect to the conventional sol-gel process, we demonstrated the possibility of fabricating ZrO₂/SnO₂ TFTs on a flexible substrate even at an annealing temperature below 250 °C (please see below figure). The internal energy generated by the exothermic reaction of the precursor removes organic impurities. It converts the precursor into a metal-oxygen-metal system, resulting in the formation of high-quality metal oxide films at low temperatures.

(2) Also, due to the synergistic chemical stability between ZrO₂ and SnO₂, high-performance electrical properties have been achieved (please refer to our Supplementary Table 4, which compares the characteristics with previous studies): a field-effect mobility of 26.16 cm²/Vs, a subthreshold swing of 0.125 V/dec, and an on/off current ratio of 1.13 × 10⁶ at a low operating voltage of 3 V. Zr⁴⁺ (760 kJ/mol) demonstrate stronger bonding with oxygen compared to others (e.g. Al³⁺ (511 kJ/mol)), which allows for the formation of more complete metal-oxygen bonds and enables a reduction in interface trap sites between the insulator and the semiconductor.

 (3) Moreover, we demonstrated flexible ZrO₂/SnO₂ TFTs with strong mechanical stability, enduring 5,000 bending cycles at a <2.5 mm radius by reducing device dimensions.

 (4) In addition to electrical and mechanical properties, we investigated the chemical and material characteristics more clearly and in-depth through various analyses. we determined the temperature at which the precursors are converted to oxides by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Also, the structural properties of SnO₂ and ZrO₂ films were investigated by grazing incidence X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS).

The aforementioned results suggest that the combustion-assisted solution process enables the fabrication of high-performance flexible ZrO₂/SnO₂ TFTs with relatively low process temperatures and that it is a suitable process for the full implementation of solution-processed metal oxide electronics and flexible electronics where high-temperature processes should be avoided as well.