Novel Zinc Oxide Inks with Zinc Oxide Nanoparticles for Low- Temperature, Solution-Processed Thin-Film Transistors Song Yun Cho,* ,† Young Hun Kang, † Jun-Young Jung, ‡ So Youn Nam, † Jongsun Lim, † Sung Cheol Yoon, † Dong Hoon Choi, ‡ and Changjin Lee* ,† † Division of Advanced Materials, Korea Research Institute of Chemical Technology, Daejeon 305-600, Republic of Korea ‡ Department of Chemistry, Korea university, Seoul 136-701, Republic of Korea * S Supporting Information ABSTRACT: Novel zinc oxide (ZnO) inks via mixing a soluble aqueous ZnO precursor with ZnO nanoparticles suitable for low temperature processing of the thin film transistors (TFTs) were prepared. ZnO TFTs produced from the proposed ZnO mixture ink exhibited significantly enhanced field effect mobility of 1.75 cm 2 V -1 s -1 and an on/off ratio of 5.89 × 10 8 even at low processing temperature of 250 °C. Various structural analyses were performed to investigate the influence of ZnO nanoparticles inclusion into the thin film nanostructure on the structural, chemical, and electrical characteristics of the ZnO TFTs. KEYWORDS: thin-film transistor, zinc oxide, nanoparticle/precursor ink, low-temperature process ■ INTRODUCTION Great efforts have been undertaken recently to develop an efficient solution deposition method for ZnO thin film transistors since it is an easy and cost-effective process. In solution-processed ZnO TFTs, the prerequisites for good TFT performance are the processability of materials with a high ZnO content and the processing temperature for highly crystalline thin films. In the absence of high temperature annealing, the TFT process generally produces porous nanoparticle agglom- erates with a high portion of interface states. Agglomerates tend to limit carrier mobility and adversely affect the TFT subthreshold slope as well as the off-current and switching voltages. Many researchers have endeavored to produce the optimum ZnO ink in order to achieve respectable device performance demonstrating significant ZnO solubility. Never- theless, the goal of achieving a high level of device performance and stability at a low processing temperature of 200 to 250 °C remains a significant challenge, which is compatible with processing on plastic or glass substrates in liquid crystal displays or organic light emitting displays. Recently, new approaches have been investigated to achieve the low temperature solution- processed metal oxide TFTs using indium oxide, IZO (InZnO), and IZTO (InZnSnO). Marks group achieved the mobility of 3.5 cm 2 V -1 s -1 in the indium oxide TFTs at a low temperature of 250 °C using self-energy generating combustion chemistry. Chang group realized the high mobility of 11.8 cm 2 V -1 s -1 in the indium oxide TFTs at 250 °C by an O 2 /O 3 atmospheric process. Sirringhaus groups also reported the mobility of 1.2 cm 2 V -1 s -1 in the ZnO TFTs at 270 °C by filling in the internanorod space by coating a thin zinc precursor layer on the first deposited ZnO nanorod film. 1-3 Although many groups have achieved high-performing ZnO TFTs using various ZnO precursors and ZnO nanomaterials based on zinc acetate, zinc nitrate and zinc alkoxide, there are some limitations in terms of materials and fabrication process at lower temperatures. For example, the deposition and sintering procedures of nano- particle suspensions, such as colloidal nanospheres and nanorods, are commonly used in the solution processing of ZnO TFTs. 4-7 However, temperatures higher than 600 °C or an additional annealing process should be required after solution deposition to ensure the thin film for the TFTs is uniform. When various ZnO colloidal nanocrystals are processed at low temperatures (approximately 270 °C), the prepared ZnO TFTs show unsatisfactory electrical perform- ance, with a mobility level of less than 1.2 cm 2 V -1 s -1 and an on/off ratio of about 1 × 10 5 . 3-9 A soluble ZnO precursor is another good candidate for solution processed ZnO inks. However, such precursor-type materials generally require high- temperature processing (above 300 °C) to produce the crystalline structure that confers a high level of carrier mobility on TFTs. To avoid this limitation of the annealing process, a few research groups have introduced high-k dielectric insulator materials, such as aluminum oxide, or reported alkoxide metal oxide precursor suitable for high performance TFTs and low Received: December 5, 2011 Revised: June 18, 2012 Published: August 24, 2012 Article pubs.acs.org/cm © 2012 American Chemical Society 3517 dx.doi.org/10.1021/cm2036234 | Chem. Mater. 2012, 24, 3517-3524