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Abstract
The hot carrier solar cell (HCSC) concept requires its absorber material to be capable of significantly slowing down hot carrier thermalisation, which can possibly be realised by the "phonon-bottleneck" effect. A suitable electronic bandgap, low carrier effective mass, high optical absorption and high environmental stability are also required.
Density functional theory (DFT)-based ab-initio simulations were used to investigate possible absorber material candidates. Calculations were mainly performed to obtain the structural, electronic and phononic properties of the materials.
Based on phononic calculation results, bulk compounds with low symmetries and complex structures are generally not suitable as the absorber material. Thus we concentrated on high-symmetry binary compounds, especially III-V binaries, which are mostly semiconductors.
Among the bulk materials we investigated, BSb is a promising candidate [Sol. Energ. Mat. Sol. C. 111 (2013): 123-126] which is capable of completely blocking one major phonon decay mechanism, possibly producing a "phonon bottleneck". The experimental electronic bandgap, optical absorption [J. Crys. Growth 305.1 (2007): 149-155] and electron effective mass of BSb also fulfil the requirements of absorbers very well. We also investigated the feasibility of BSb fabrication with chemical vapour deposition (CVD), and the theoretical analysis of our proposed CVD reaction gave
very positive results.
A method of estimating hot phonon decay losses was developed to compare the thermalisation suppression performance of absorber candidates. Compared to conventional methods, which employs the lifetime of certain hot carriers or phonon modes as the indicators, this method considers the existence of a hot optical phonon population and is thus more comprehensive with regards to HCSCs.
Phononic calculation results of superstructures showed that in the confined direction, some phononic bands become nearly discrete. Thus phonon decay channels fulfilling energy and momentum conservation are rare. This can help reduce optical phonon decay rate. Additionally, we found some optical phonons are confined in different layers in superstructures, which reduces the coupling between these phonon states and supresses phonon decay. A low-cost statistical method of aligning the electronic bands of on-stoichiometric compounds is developed to assist superstructure design.