Theoretical Study and Characterisation of Hot Carrier Solar Cell Materials

Abstract

Computational simulations of nitride materials and their corresponding multiple quantum well superlattices (MQW-SL) have been performed in an effort to search for suitable hot carrier absorbers for the Hot Carrier Solar Cell (HCSC). One goal of the study is to examine whether the suppression of the Klemens phonon decay mechanism truly can increase the lifetime of a non-equilibrium optical phonon population created by electron-phonon interactions. Another aspect of this study is to verify whether a MQW-SL could extend the lifetime of hot electrons by means of slow electron-phonon interactions or slow phonon decay processes. Bulk InN, GaN and a thin InN/GaN MQW-SL were simulated within the Density Functional Theory (DFT) framework in order to test those two speculations. Lattice vibrations of selected bulk materials were studied with both Density Functional Perturbation Theory (DFPT) and two semi-classical models, the Adiabatic Bond-Charge Model (ABCM) and the Valence Force Field Model. In this work, the parametrisation of the ABCM has been done for several binary compounds, including but not limited to InAs, GaAs, InN, GaN, GaP and SiC. Phonon dispersions of several MQW-SLs and Si Quantum Dot (QD) systems were predicted by the ABCM using force constants extracted from bulk materials. Two characterisation techniques have been applied, Inelastic X-ray Scattering (IXS) for measuring phonons and Transient Absorption (TA) for probing hot carrier dynamics in materials. Several samples were measured with IXS, including selected III-V arsenides, III-V nitrides, transition metal nitrides, and their MQW-SLs. A HfN thin film sputtered on MgO substrate was measured with ultra fast TA experiment. This work showed that electron-phonon interaction via deformation potential was slightly suppressed in an InN/GaN MQW SL compared to that in the bulk counterparts. The contributions to the optical phonon decay from non-Klemens mechanisms were comparable with the Klemens mechanism. The ABCM was shown to match well with DFT dispersions for the bulk materials and yielded good predictions for a strain balanced InGaAs/GaAsP MQW-SL as was verified by the IXS. The TA experiments performed on the HfN thin film suggested a nanosecond scale hot carrier lifetime. The hot carrier temperature was estimated to be 330 K using a temperature dynamic theory developed in this thesis.