Project : “ Hot Carrier solar cell : Implementation of the Ultimate PV Converter ” Annual

The UNSW node of the Hot Carrier Solar Cell project is investigating absorbers which slow the rate of carrier cooling an essential element of a successful cell. It has also further developed the work on energy selective contacts for Hot Carrier cells. This work includes modelling the limiting efficiencies of cells using real material properties and modelling the phonon dispersions of ordered arrays of quantum dots. The object of the latter is to determine the potential to modify the allowed phonon energies such as to block the decay of optical phonons, this being the principle decay mechanism for hot photo-generated carriers in an absorber material. This will then offer the possibility of producing a material which substantially decreases the rate of carrier cooling in an absorber and hence allows carriers to be extracted whilst still hot and hence produce a higher external voltage. Thus also boosting the efficiency of such a ‘Hot Carrier solar cell’ very significantly. The investigation has three main themes: 1. Modelling of the phonon dispersions in three dimensions of these quantum dot nanostructures. This will allow the phonon properties of various material combinations to be modelled and assessed. 2. Calculating the limiting efficiencies of cells with specific electronic, and later with specific phononic properties. 3. Fabrication and characterisation of ordered quantum dot arrays. Colloidal dispersion of capped nanocrystals to form 2D supercrystal arrays. These are being characterised for their modulation of phonon dispersions and for the effects of this modulation on the rate of carrier cooling. 4. Fabrication of double barrier resonant structures and characterising with illuminated I-V techniques. In the past year modelling has progressed on the Hot Carrier efficiency with indium nitride as ab absorber. Measurement of double barrier resonant quantum dot structures grown in high quality by collaborators and improved 2/3D modelling has been carried out. For absorbers, modelling of nanocrystals superlattice arrays has been to III-V QD superlattices. Such superlattices have been grown by collaborators and are being measured and colloidal Langmuir-Blodgett growth of silicon Si nanocrsystals arrays has been carried out. Meausurement of carrier cooling rates has been carried out on indium nitride with its large phononic gap, demonstrating the importance of material quality. Finally design of complete hot carrier cell structures has been initiated with the important properties defined more precisely, with plans for fabricating such structures now being implemented.