GaAs/AlGaAs quantum dot intermediate band solar cells

This thesis presents my Ph.D. work about quantum dot GaAs/AlGaAs solar cells grown by droplet epitaxy, exploring the potential of this materials system for the realization of intermediate band photovoltaic devices. In the first chapter a general introduction to the field of solar energy is given, outlining the reasons why this research has been performed. The physics of the photovoltaic cell is briefly explained in its most important points, to give the reader clear understanding of what is presented in the following chapters. Intermediate band devices are presented in the second chapter. The theoretical foundations presented do not aim at constituting an exhaustive explanation of the theory underlying intermediate band solar cells, but the scope is again to give clear understanding of the characterization of the quantum dot devices reported in the following chapters. A survey of the state of the art in the field is given, pointing out the differences with our technology. The initial part of my Ph.D. work was spent in developing the technology to design and grow (Al)GaAs photovoltaic devices, as well as the characterization techniques required to understand the behavior of such devices. In chapter 3 the method developed to design the solar cell structure is illustrated, and in chapter 5 the experimental setup used for characterization is presented, along with the measurements on the single junction devices realized during this work. Chapter 4 is dedicated to the description of the growth and fabrication methods used to grow the samples reported here. The development of the fabrication technology proceeded in close contact with the characterizations of the devices, in order to optimize the process. Finally in chapter 6 the results on quantum dot photovoltaic cells are reported: the key working principles of intermediate band devices have been demonstrated with our materials system, and this, to the knowledge of the author, is the first time that strain free quantum dot solar cells are reported of intermediate band behavior. The role of defects in the AlGaAs matrix is explained in connection with both the optical and electrical characterizations presented.