Systematic Study of the Fundamental Charge Carrier Trapping Sites in Narrow Bandgap Semiconductor Quantum Dots: Characterization and Control via Synthetic Parameters

Description

When a semiconductor quantum dot (QD) absorbs a photon of light a pair of chargedcarriers (a hole and an electron) are created, as an exciton within the nanoparticle.Depending on the particular application, we may want the two geminate carriers torecombine some time later and re-emit a photon of light. Alternatively we may want todissociate the charges and extract them from the material as a photocurrent (e.g. in anoptical detector or a photovoltaic solar cell). In most cases it is unhelpful if one or bothof the photoinduced charges becomes localized and trapped, within the QD or elsewhereoutside the QD in the device structure. Additional energy is required to de-trap carriers,and they may be trapped and unable to recombine for much longer than the normaltimescale for band edge recombination. Where recombination involving de-trappedcarriers does occur, and where the trap state energy lies between the conduction andvalence band energy levels of the QD, the recombination is red shifted. Depending on thetrap energy level and the depth of the trap, carriers may not necessarily re-combineradiatively. In such cases a charge other than the original geminate photochargecombines with the trapped carrier, or the trapped carrier may be involved in a chemicalreaction at the site and that might have implications for the stability of the material. Inphotovoltaic devices especially, traps within the QDs, at their surfaces or within thematerial surrounding the QDs can have a very limiting impact on the efficiency andservice lifetime of the solar cell. In other devices (photoconductive detectors) however, itcan be beneficial to have a limited number of shallow traps, provided the trappedcarriers do not undergo any deleterious chemical reactions whilst localized. There aremany different types of potential trap sites in QDs and in composite devices with QDsembedded in dielectric solids. The energetics and carrier dynamics can span a widerange (from the fs to over seconds timescale). We propose to study and understand theformation of charge traps in low bandgap QD materials and composites by correlating arange of electrical, electro-optic and optical measurement techniques and physicalcharacterization of the QDs, with the synthetic methods used in their preparation. In thisway we aim to improve the performance of our QD materials by engineering traps in acontrolled manner.