Theoretical investigation of materials and performance for the hot carrier solar cell

Abstract

The hot carrier solar cell (HCSC) is a novel concept on solar energy conversion, which has the potential of exceeding the Shockley-Queisser efficiency limit. Such a device extracts the hot photo-generated carriers before their thermalization. It requires an absorber layer that inhibits the energy dissipation of hot carriers, and two energy selective contacts (ESC) that extract electrons and holes respectively, through small energy windows. This is to prevent excessive entropy generation during the extraction processes.

In this thesis, several device models are developed for calculating the conversion efficiency of the hot carrier solar cell. Incorporation of actual data, from literature, for impact ionization and Auger recombination rates, into the model enables self-modulation of the carrier average energies, allowing for stable solutions when carriers are extracted at specific energy levels. The carrier-carrier scattering also takes an important role as it contributes to the renormalisation of carrier energy distributions. The proposed model of energy/particle conservations applies to the case of normalized carrier populations while the relaxation-time model can apply to arbitrary carrier statistics. These device models are then generalized to any solid-state systems, yielding a generalized version of the opto-electronic reciprocal relation. The effect of energy-selective contacts on the device performance has also been quantitatively analyzed. Accounting for real properties of carrier ballistic tunneling through the contacts, the efficiency variation has been calculated for different transmission profiles, yielding optimized contact structures.

To optimize the selection of materials for this device, the electronic and phononic properties of multiple-quantum-well superlattices have been calculated. Tight-binding methods and bond charge models are adopted for the calculation of electronic and phononic structures respectively. The rates of hot electrons emitting phonons are then calculated for various structural parameters, with the carrier screening effect being formulated and incorporated. The anharmonic interactions between phonons are also calculated to evaluate the phonon-bottleneck effect. In correspondence to the proposed device model, the relaxation times of carrier renormalisation are evaluated by calculating the rates of carrier-carrier scattering using real material parameters. The electron tunneling properties through real material systems have also been calculated, with inelastic scattering by phonons incorporated. The work on material modeling provides comprehensive tools for theoretical analysis of real material systems, whose results are to be readily adopted by the device models for accurate performance predictions.