Abstract
An important property of a ‘Hot Carrier cell’ is to slow the rate of carrier cooling to allow hot carriers
to be extracted and do useful work in the external circuit. Photo-generated hot carriers cool by emitting optical phonons which further decay into acoustic phonons by the Klemens mechanism. Inhibition of this Klemens’ decay can slow carrier cooling rates and this can be achieved if the energy of the acoustic phonons emitted is blocked in the phonon dispersion of an absorber material.
III-V bulk materials with a large anion:cation mass ratio have a large gap between acoustic and optical modes sufficient to block this Klemens’ decay. InN, GaN, InP, BBi and BSb should have such large “phononic band gaps”, but only InN, BBi and InP have electronic band gaps narrow enough to absorb a reasonable fraction of the solar spectrum. Time resolved photoluminescence data are presented supporting the suppression of Klemens’ decay in InP as compared to GaAs, which has no phonon band gap but a similar electronic band gap.
In principle similar gaps in the phonon dispersion open up in quantum dot nanostructures through Bragg reflection. 1D modelling data on Group IV alloys are presented which indicate such mini-gaps large enough to block Klemens’ decay, at least in very small QD superlattices. However these mini-gaps are significantly reduced in 3D modelled nanostructures. It is not yet possible to model the larger mass difference Group IV alloys, but ‘model’ QD superlattice structures, modelled in 3D, with large mass differences do indicate large mini-gaps across all of reciprocal space sufficient to block Klemens’ decay. The potential of some of these structures to also block the next most likely mechanism, Ridely decay, is also discussed and some suggestions are made for the potential to fabricate some of these structures.
Keywords: Hot Carrier – 1; Quantum dot – 2; Phonon decay – 3; Carrier cooling – 4; Quantum well – 5; Optical phonon – 6; Acoustic phonon - 7
to be extracted and do useful work in the external circuit. Photo-generated hot carriers cool by emitting optical phonons which further decay into acoustic phonons by the Klemens mechanism. Inhibition of this Klemens’ decay can slow carrier cooling rates and this can be achieved if the energy of the acoustic phonons emitted is blocked in the phonon dispersion of an absorber material.
III-V bulk materials with a large anion:cation mass ratio have a large gap between acoustic and optical modes sufficient to block this Klemens’ decay. InN, GaN, InP, BBi and BSb should have such large “phononic band gaps”, but only InN, BBi and InP have electronic band gaps narrow enough to absorb a reasonable fraction of the solar spectrum. Time resolved photoluminescence data are presented supporting the suppression of Klemens’ decay in InP as compared to GaAs, which has no phonon band gap but a similar electronic band gap.
In principle similar gaps in the phonon dispersion open up in quantum dot nanostructures through Bragg reflection. 1D modelling data on Group IV alloys are presented which indicate such mini-gaps large enough to block Klemens’ decay, at least in very small QD superlattices. However these mini-gaps are significantly reduced in 3D modelled nanostructures. It is not yet possible to model the larger mass difference Group IV alloys, but ‘model’ QD superlattice structures, modelled in 3D, with large mass differences do indicate large mini-gaps across all of reciprocal space sufficient to block Klemens’ decay. The potential of some of these structures to also block the next most likely mechanism, Ridely decay, is also discussed and some suggestions are made for the potential to fabricate some of these structures.
Keywords: Hot Carrier – 1; Quantum dot – 2; Phonon decay – 3; Carrier cooling – 4; Quantum well – 5; Optical phonon – 6; Acoustic phonon - 7
| Original language | English |
|---|---|
| Pages | 178-186 |
| Number of pages | 9 |
| Publication status | Published - 25 Sept 2009 |
| Externally published | Yes |
| Event | 24th European Photovoltaic Solar Energy Conference 2009 - Hamburg, Germany Duration: 21 Sept 2009 → 25 Sept 2009 Conference number: 24 |
Conference
| Conference | 24th European Photovoltaic Solar Energy Conference 2009 |
|---|---|
| Abbreviated title | EUPVSEC 2009 |
| Country/Territory | Germany |
| City | Hamburg |
| Period | 21/09/09 → 25/09/09 |
| Other | September 21-24 2009 |