The magic world of nanocrystals, from batteries to solar cells

Abstract We shall discuss recent research on junctions that are made up of a network of nanometer-sized oxide particles such as zinc oxide, tungsten trioxide niobium oxide and titanium dioxide, which are interconnected to allow for electronic conduction to take place. The pores between the particles are filled with a semiconducting or a conducting medium, such as a p-type semiconductor, a polymer, a hole transmitter or an electrolyte, constituting a junction of extremely large contact area. In this way interpenetrating bicontinuous network composites are formed which are phase-separated by a heterojunction. Electrons can percolate rapidly across the network of nanoparticles allowing the huge surface area to be addressed electronically. At present these mesoscopic oxide films have already found a number of important applications. Of particular interest are high power lithium insertion batteries and dye-sensitized solar cells (DSC). The latter have emerged as credible alternatives to conventional silicon based devices. The transparent nature of these films and the high conversion efficiency offers new opportunities for applications, e.g., as electric power producing windows. Mesoscopic oxide semiconductor films offer also great promise for a new generation of electroluminescent and electrochromic systems. Examples of these devices will be presented and their mode of operation will be discussed.