Charge Transport Dilemma of Solution-Processed Nanomaterials

A large variety of nanoparticles were synthesized during the last 25 years and are used now as building blocks for a variety of materials. Bottom-up solution processing of devices emerged as a promising direction of their technological applications because this method can (a) utilize intrinsic ability of nanocolloids to self-organize, (b) reduce high energy and equipment cost of device manufacturing, and (c) impart new functionalities to electronic devices. However the technological impact of solution processable semiconductor materials although potentially considerable-has been so far limited because of the long-standing dilemma between the need for effective colloidal stabilization of nanoparticles and effective charge transport. Surfactants and other organic materials being used to synthesize and/or disperse nanocolloids introduce a barrier for charge transport between the particles. Although these barriers do make it impossible to use them in electronic devices, they certainly make it more difficult. In this review, we look into the latest progress in the solution processable devices and methods to produce electrically conductive thin films from nanoscale dispersions. We are specifically interested in the understanding of the prospects of self-assembly to facilitate charge transport and nanoscale connectivity during solution processing. The updated theoretical description of charge transport in nanoparticle solids and similar nanomaterials is also given. It includes consideration of the key mechanisms such as tunneling and cotunneling, as well as key electrical parameters characterizing transport of electrons through the surfactant-related barriers, such as coupling energy and Coulombic charging energy. Manifestations of these mechanisms in different electronic materials made from nanoparticles, nanowires, nanotubes, and nanosheets and their relative advantages and disadvantages are also discussed. We conclude the topic with a brief description of new opportunities and approaches to improve charge transport in solution processed materials from nanoscale dispersions.