Theoretical and experimental investigation of quantum well intermediate band solar cells

Abstract

In order for photovoltaic energy conversion to compete with conventional energy sources and become a realistic alternative source of low-carbon renewable energy, significant cost-per-watt reduction is required. One obvious way to achieve this is to increase the conversion efficiency of solar cells, which is currently limited to around 30% even with modern technology. The Intermediate Band Solar Cell (IBSC), the focus of this project, is a concept that promises photovoltaic power conversion efficiencies of up to 63.2% through the introduction of an extra energy band in the bandgap of a semiconductor. However, many attempts to achieve IBSCs using quantum dots superlattice show poor conversion efficiency due to their small absorption cross-section and short-lived intermediate state. In this project, we attempt to overcome this issue by proposing an innovative Photon Ratchet (PR) quantum well cascade structure designed to improve the efficiency, through increasing the absorption cross-section and the lifetime of electrons in the intermediate state. The goal of this project is to prove the benefits of this concept, both theoretically and experimentally. In this thesis, theoretical and experimental work on quantum well solar cells is presented. The basic concept of solar cells, IBSC and PR-IBSC as well as their advantages and disadvantages are discussed in chapter 1, along with theory of quantum mechanics and optical transitions in quantum wells. Chapter 2 focuses on theoretical work, which includes limiting efficiency calculation and fundamental loss calculation in solar cells, in order to determine the fundamental benefit of the PR-IBSC when compared with conventional IBSCs. The result of this work was published in Applied Physics Letter in 2012 to propose the concept of the PR-IBSC for the first time. Experimental work on existing quantum well solar cells is presented in chapter 3, along with basic characterisation techniques. The InGaAs quantum well with GaAs barrier in a p-i-n diode is optically and electrically characterised and we describe how we have observed an increase in photocurrent due to sequential absorption of photons via the intermediate band (IB), which arises from the one-dimensional confinement in the quantum well, for the first time. This is an important result and has been published in the Journal of Photovoltaics in 2014. In order to study the interband and inter-subband transitions in quantum wells individually, we also designed a new set of samples, along with their reference samples, which consist of n-i-n and p-i-n diodes with identical single quantum wells in the i-region. The details of the samples, along with a model which simulates the transitions in quantum wells and achieves basic characterisation of the samples, are presented in chapter 4. Finally, in chapter 5, we draw up our conclusions and future work on the new samples is discussed.