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Title: A spectroscopic study of strain-balanced InGaAs/GaAsP quantum well structures as absorber materials for hot carrier solar cells
Author: Hirst, Louise
ISNI:       0000 0004 2732 4208
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2012
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In this thesis, five intrinsic loss mechanisms which fundamentally limit solar energy conversion efficiency are identified. The three dominant mechanisms are thermalisation loss, below Eg loss and Boltzmann loss. Targeting these three losses through alternative device design is the only way substantial efficiency enhancement might be achieved. The hot carrier solar cell targets these dominant mechanisms and hence has a theoretical limiting efficiency, under maximum solar concentration, in excess of 80%. Despite clear efficiency advantages, a hot carrier solar cell has never been experimentally demonstrated because two key development challenges remain: energy selective contacts and absorber materials which maintain a hot carrier distribution under realistic levels of incident solar irradiation. In this study, strain-balanced InGaAs/GaAsP QW structures with a range of QW parameters were characterised spectroscopically in order to determine the suitability of this material system as a hot carrier absorber. In a deep, wide well sample, a temperature gradient between the carrier distribution and the surrounding lattice of 150 K was demonstrated using continuous wave photoluminescence spectroscopy. This technique was also used to calculate a thermalisation coefficient for each sample, allowing for comparison with other hot carrier studies. Time resolved photoluminescence measurements were used to identify cooling pathways occurring in this material system. Bi-exponential cooling behaviour was observed, indicating that two different mechanisms with different characteristic cooling lifetimes were dominating carrier cooling. In the deep, wide well sample it was determined that peak LO phonon distribution temperatures of at least 500 K above that of the surrounding lattice would be required to produce the observed carrier cooling.
Supervisor: Ekins-Daukes, Ned Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral