MINORITY CARRIER LIFE TIME IN SILICON SOLAR CELLS

Abstract

Sun has been a very reliable source of light and energy since ages. Its large scale-application-has gained much attention with the slow but steady fall of conventional energy resources. Its two universal applications in wide use are thermal and photovoltaic applications. Solar cells are the most fundamental frame work of the photovoltaic system for the electrical power generation. Silicon solar cells find a top.priority due to relative abundance of the silicon material and also due to its extensively well established technology. Most of the semiconductor devices work under non-equilibrium conditions and excess carriers generated by applied bias have a tendency to return to their thermal equilibrium value. This tendency is governed by two important parameters viz• life time and diffusion length of excess carriers. These are defined as the survival time and the distance traversed before recombination takes place. Efficiency of a solar cell depends upon a number of factors and one of these is minority carrier current which in its turn depends on diffusion length or life time of excess minority carriers, and from the point of view of efficient application of solar cells these parameters (diffusion length and life time of excess carriers) should have high values. Parameter of diffusion length or life time plays a decisive role in the determination of short ii- circuit current and efficiency of a device, and so from the device characterization point of view their correct knowledge is essential. Solar cells may in reality be. put to use under diff-erent illuminations and ambient temperatures with the device material having different dopant concentrations of impurities. So obviously for correct estimate of efficiency and also for optimum utilization under different practical situations one must have detailed understanding of the variation of life time or diffusion length of excess carriers with temperature, illumination and dopant concentration. These two parameters are intricately related through the relation L =,,STIDt, where L stands for diffusion length, for life time and D for diffusivity. ThUs one should expect least discrepancy in the values of life time evaluated directly or indirectly through diffusion length measurements. In this thesis, we begin with these views in our mind and provide a very consistent analysis for the evaluation of diffusion length from the steady state response and that of life time from transient response. We also discuss the ways to minimize the coupling of base and diffused regions while ma-ing parameters measurements in the base region, which is a significant region in the performance of a solar cell. Our thedey suggests least discrepancy to occur in to the direct and indirect techniques used in the measurements of life time of excess carriers. Also, we study the variation -iv- of life time with illumination, temperature and dopant concentration, using Schockley-Read-Hall theory for recombination centres. Physical significance of this study from the application point of view to silicon solar cells has bean discussed in our work. The case of a silicon solar cell with three regions, that is a BSF solar cell has been taken up for consideration and stress has been laid in the study of spectral response of device as dependent on lif time of excess carriers in three regions. This, in turn suggests an optical technique to evaluate the effective-ness of back surface field. In the following, a brief account of work presented in the thesis is given.