STUDY OF POLYCRYSTALLINE SILICON SOLAR CELLS

Abstract

In this thesis, we begin with a fundamental and con- . sistent analysis of the grain boundary phenomena in poly-crystalline semiconductorsiin general and silicon,in parti-cular, under equilibrium and non-equilibrium conditions,. using depletion approximation. Also, we study the physics of electronic,conduction mechanism across the grain boundary utilizing the results of the preceding analysis. Ultimately, non-equilibrium calculations of the grain boundary barrier height taken from the first study has been combined with a two-dimensional self-consistent analysis for the excess minority carrier concentrations determined in the base region of the polycrystalline silicon solar cell to predict the grain boundary effective surface recombination velocity and the grain boundary effects on current-voltage character-istics of polysilicon solar cells. In the following, a brief account of the work presented in the thesis is given. In Chapter 1, the evolution of photovoltaic device research, highlighting the various improvements made in the understanding of the underlying physics, culminating in the emergence of low-cost efficient polycrystalline silicon solar cells, has been summarized. In passing, a brief account of the basic solar cell physics is also given. Finally, the recent investigations in the field of polycrystalline photovoltaic material and deVices have been reviewed. These investiga-tions,centre around the understanding of the role of grain boundaries in low-cost photovoltaic devices and exploring the possibilities of improving the device performance, e.g. by passivating the grain boundary recombination states through hydrogenation. Chapter 2 begins with an emphasis on the importance of the defect studies in semiconductor materials and in device development. A short exposure to the various defect types present in the semiconductor crystals is made. Finally,: a review of the electronic properties of the polycrystalline silicon, starting with the poineering work of Taylor et al., providing a gateway to the systematic work on the blocking behavior of grain boundaries upto the recent investiga-tions resulting in the understanding of various phenomena, viz. dopant segregation, carrier trapping and thermionic emission, to explain the transport parameters of polycrystal-line Si films or bulk slices, is given. Also, the recent state of understanding of the minority carrier recombina-tion at the grain boundary in polycrystalline semiconductors is highlighted. In the last section of this chapter, the aims and objectives of this study are presented along with the plan of the thesis....