Influences of pH and ligand type on the performance of inorganic aqueous precursor-derived ZnO thin film transistors

The aqueous precursor-derived ZnO semiconductor is a promising alternative to organic semiconductors and amorphous silicon materials in applications requiring transparent thin-film transistors at low temperatures. The pH in the aqueous solution is an important factor in determining the device performance of ZnO-TFTs. Using a basic aqueous solution, the ZnO transistor annealed at 150 °C exhibited a high field-effect mobility (0.42 cm ² V ^-1 s ^-1) and an excellent on/off ratio (10 ⁶). In contrast, the ZnO layer annealed at 150 °C prepared from an acidic solution was inactive. Chemical and structural analyses confirmed that the variation of the device characteristics originates from the existing state difference of Zn in solution. The hydroxyo ligand is stable in basic conditions, which involves a lower energy pathway for the solution-to-solid conversion, whereas the hydrated zinc cation undergoes more complex reactions that occur at a higher temperature. Our results suggest that the pH and ligand type play critical roles in the preparation of aqueous precursor-based ZnO-TFTs which demonstrate high performance at low temperatures.