Numerical study of InAs/GaAs quantum dot solar cells

Solar energy conversion is a promising way to provide future energy demand since it is a clean energy. Unfortunately, the photovoltaic (PV) conversion of the solar energy is expensive, therefore, making attempts to increase the efficiency of PV is essential. A conventional single junction solar cell presents an efficiency limit that is determined by the Shockley-Queisser detailed balance principle (i.e. 40.7% under full sun concentration). The limit comes from the fact that only photons with energy close to the energy bandgap are efficiently converted. Below energy gap, photons are not absorbed since the cell is transparent to them and high energy photons only contribute part of their energy that is equal to the energy bandgap. Many concepts have been developed in order to increase the efficiency limit of solar cells. Among them the intermediate band solar cell (IBSC) has gained considerable attention. In principle, IBSCs have the potential to overcome Shockley-Queisser (SQ) limit of single junction solar cells by providing high current while preserving large voltage. The theoretical limit calculated for an ideal IBSC under full sun concentration is 63.1%. One of the most promising ways to realize the IBSC is to incorporate a QD superlattice in the active region of p-i-n single junction solar cells. The nano-size QDs behave like 3D potential well for the carriers and create discrete energy levels within the forbidden bandgap that allows sub-bandgap photon absorption. Stranski-Krastanov (S-K) growth mode (also called 'layer-plus-island growth') is one of the most common methods to fabricate QDs. This method has been used in many experimental studies for InAs/GaAs heteroepitaxial system which has lattice mismatch of 7.2%. Although InAs/GaAs is not an optimal material system for the IBSC performance, its properties and parameters are well reported in literature compared to other material systems. The drift-diffusion model is the most widely used mathematical approach to describe semiconductor devices. However, in case of quantum dot solar cells, the physics governing the device performance is not sufficiently covered and up to now, modeling of QDSCs has been treated as IBSC modeling through detailed balance principle and semi-analytical or numerical drift diffusion approaches. In this dissertation, QDSCs are investigated in detail by numerical simulation using a QD-aware physics-based model. The influence of selective doping in QDSCs is investigated considering different scenarios in terms of crystal quality. Regarding high-quality crystal, close to radiative limit, large open circuit voltage recovery is predicted in doped cells, due to the suppression of radiative recombination through QD ground state. In case of defective crystal, significant photovoltage recovery is also attained owing to the suppression of both non-radiative and QD ground state radiative recombination. The interplay between non-radiative and QD radiative recombination channels, and their interplay with respect to doping are analyzed in detail. Moreover, a numerical study on the influence of wetting layer states on the photovoltage loss of InAs/GaAs quantum dot solar cells is presented. Almost full open circuit voltage recovery is predicted by combining wetting layer reduction and selective doping. After investigating the inherent limitations of InAs/GaAs QD solar cells regarding realization of the IBSC, a brief description of QDs with type-II staggered band alignment based on GaSb/GaAs material systems (whose interband and intraband dynamics are more promising in view of attaining the IB operating regime) is given and a preliminary study of the competition between thermal and optical escape processes is presented.