Hot Carrier cells aim to tackle the carrier thermalisation loss in solar cells. The concept is to slow the rate of photoexcited carrier cooling to allow time for the carriers to be collected whilst they are still at elevated energies (“hot”), and thus allowing higher voltages to be achieved from the cell [1,2]. Significant reduction in cooling has been observed at very high illumination intensities via a ‘phonon bottleneck’ mechanism which has been demonstrated to be enhanced in QW nanostructures. In order to reduce the illumination intensities at which this mechanism gives significantly slower cooling towards one sun intensities, it is necessary to block the decay of optical phonons into acoustic phonons.

The current work seeks to emulate the behaviour of some bulk materials which have a large gap between acoustic and optical phonon modes by the use of Bragg reflection of phonon modes in quantum confined nanostructures. Developments in this modeling which indicate the critical importance of the interface are also presented.

In addition to an absorber material that slows the rate of carrier relaxation, a hot carrier cell must allow extraction of carriers from the device through contacts which accept only a very narrow range of energies (energy selective contacts or ESC). Previous work has proved the concept of these selective energy contacts using resonant tunneling in Si QD double barrier structures. This can be characterisaed by demonstration of negative differential resistance. Experimental work on electrical and optical excitation of these structures will be presented.