Novel inorganic materials for solution-processed electronics

While conventional crystalline inorganic semiconductors offer superior charge carrier mobilities, they are generally difficult to form by low cost processes. Crystallization of inorganic semiconductors requires high-temperature treatments that force trade-offs between device performance, cost and compatibility with plastic substrates. The development of applications ranging from displays, photovoltaic cells and light-emitting devices to "smart cards", radio frequency tags and sensors could be accelerated by introducing lower cost alternatives to conventional silicon technology. Solution-based processes such as spin coating, dip coating or inkjet printing offer substantial cost reductions for fabrication of electronic and optoelectronic devices. We provide an overview of several new approaches to solution-processed inorganic semiconductors. Colloidal semiconductor nanocrystals enable room temperature solution-based fabrication of field-effect devices [1]. We fabricated thin-film transistor channels formed by self-assembly of 9 nm PbSe nanocrystals (Figure 1). Cross-linking of the nanocrystals with hydrazine increased exchange coupling, raising film conductance by about 10 orders of magnitude, yielding n-type device with charge-carrier mobility of 1 cm2/Vs. Reversible switching between nand p-type transport in nanocrystal arrays is possible upon adsorption of nor p-type doping molecules on nanocrystal surface (Figure 1d). Annealing of doped nanocrystal arrays at 200°C increased carrier mobility by about an order of magnitude. Electron mobility of 11 cm2/Vs was observed for arrays of 8 nm PbTe nanocrystals [2]. Self-assembly of multifunctional nanoparticle building blocks provides a powerful modular approach to the design of composite materials that combine properties of semiconducting, metallic and magnetic constituents (Figure 2) [3]. We demonstrate that these materials can be employed for solution-processed electronic and optoelectronic devices. Liquid-phase colloidal synthesis allows engineering size, shape and composition of nanomaterials. Various semiconductors can be prepared in form of nanoscale spheres, rods, discs, tetrapods, nanowires and nanorings. Some of these structures are interesting for ultra-small electronic devices. For example, Figure 3 shows a single electron transistor based on a CdTe tetrapod with arms 8 nm in diameter and 150 nm in length [5]. Another promising approach to solution-processed semiconductors is based on using molecular precursors that transform into crystalline inorganic semiconductors upon heating at elevated temperatures. A novel class of inexpensive soluble precursors for high-mobility inorganic chalcogenides has been developed [4]. For example, spin-coated films of In2Se3 exhibited electron mobilities as high as 16 cm2/Vs (Figure 4) [6]. Soluble hydrazine-based precursors can be synthesized for a range of materials with promising electronic, thermoelectric, and photovoltaic properties.