High-Performance Inorganically Connected CuInSe₂ Nanocrystal Thin-Film Transistors and Integrated Circuits Based on the Solution Process of Colloidal Synthesis, Ligand Exchange, and Surface Treatment

Nanocrystal (NC) CuInSe2 thin-film transistors (TFTs), consisting of nontoxic and relatively abundant elements, have great potential in environment-friendly and low-cost electronic devices. However, the high-performance CuInSe2 NC TFTs reported so far utilize toxic compounds, such as CdSe and hydrazine, which often require tedious and complex procedures. TFTs using directly synthesized CuInSe2 NCs as channel layers exhibit promising device performances but are still not comparable to the counterparts of cadmium and lead chalcogenide-based NCs. In this work, an efficient solution-based colloidal synthesis and ligand-exchange process have been developed to effectively remove bulky surfactant ligands from CuInSe2 NCs and produce unique inorganically connected NCs through metal-sulfide bonding using simple metal-free chalcogenide compounds. Such inorganically connected CuInSe2 NC thin films, combined with surface treatment, substantially affect charge transport through trap states and tunneling transport mechanism. The carriers tunneling through the barrier between neighboring NCs with much shorter interparticle distances significantly enhance electronic coupling and improve the electrical transport properties. CuInSe2 NC TFT exhibits the electrical performance with a mobility of 9.6 cm2/(V s), on/off current ratio over 104, and negligible hysteresis at low operating voltages, comparable to those for state-of-the-art II–VI- and IV–VI-type NC TFTs. As a proof of concept, the CuInSe2 NC TFTs are used as building blocks of integrated inverters to demonstrate their promise for low process temperature-fabricated NC circuits.