Investigation of Factors Influencing Multiple Carrier Generation in Nanocrystals of Narrow-bandgap Semiconductors and Their Alloys

Description

Multiple charge carrier pairs (electrons and holes paired as excitons) can be generated in semiconductor nanoparticles by the absorption of individual photons. When this happens the light to electronic charge generation process is much more energy efficient than normal. This has the potential to make solar cells more efficient, and nanocrystal based optoelectronic devices (lasers, optical switches, etc), much more feasible and less hindered by inefficient heat generation and other deleterious processes. Though multiple carrier generation (MCG) has been observed in many types of semiconductor nanoparticles, the precise mechanisms are not yet fully understood. There may be several processes depending on the type of semiconductor, and factors such as material quality and surface states influence the degree to which MCG is manifest. Initial enthusiasm for its application in the solar field has been tempered: some see the theoretical benefits to be less important in practice than initial experimental measurements appeared to indicate. Even so it would be advantageous to: (i) understand the basic nature of the MCG process(es); (ii) understand how material quality factors (degree of crystallinity, presence of surface charge trap sites, nature of the external environment at the particle surface) influence MCG and; (iii) be able to manipulate the electronic energy level structure of nanocrystals (through composition and size control) to favour enhanced MCG (lower photon energy threshold and increased probability).MCG occurs in bulk semiconductors and is a well known phenomenon, occurring via a process known as Impact Ionization (also termed a reverse Auger effect). This is the mechanism behind avalanche photodetectors and it has been used in low bandgap and semi-metal systems such as mercury cadmium telluride alloys to develop highly sensitive IR detectors. In that system MCG is enhanced by choosing alloy compositions that put the split-off band energy at resonance with the material’s bandgap energy. We believe that an analogous approach may be possible in our nanocrystal version of this particular material system, but with the added degree of freedom of tuning the bandgap via the nanocrystal’s size, to allow the two energies to be independently adjusted. A goal will be to obtain efficient MCG over a wider range of bandgap energies than in the bulk material. In addition, these materials can be engineered to have very low electron effective masses and this reduces the energy threshold for multiplication to very close to the ideal of twice the bandgap energy.