MINORITY CARRIER DIFFUSION LENGTH AND DEFECT PASSIVATION BY HYDROGEN IN POLYCRYSTALLINE SILICON SOLAR CELLS

Abstract

The sun has been radiating energy over 500 million years and is expected to continue for at least the next 50 million years. The rate at which energy is received upon a unit surface area perpendicular to the sun's direction in the free space at the earth's mean distance from the sun is known as 'solar monstant'. This solar constant had traditionally been expressed in Langleys per minute (1 langley = 1 Cal/cm2). Although the estimated value of the solar constant over the years had varied the recently accepted value is 1.940 L/min (or 1353 Watt/meter21. Its large scale application has gained much attention due 'to its two very important applications viz., thermal and photovoltaic. The direct conversion of solar radiation into electrical power by means of photovoltaic cells appears more promising as a result of the recent work on silicon p-n junction solar cells. Because the radiant energy is used as electrical energy without first being converted to heat, the theoretical conversion efficiency is quite high. Silicon has been the most commonly used material for solar cells. It is of interest to note that silicon is the second most common element on the earth's surface. Also,, as it is the basic element used in the entire electronic industry its properties and technology are fairly well understood. Silicon can be used in iii cells these parameters should have high values, and, from the device characterization point of view their correct knowledge is essential. These two parameters are directly related through the relation L =1.171 where D is the diffu-sivity of the minority carriers. In polycrystalline silicon solar cells the photo-voltaic conversion efficiency from solar energy to electri-cal energy is lowered due to the defects present in the silicon material -and of these the most detrimental defect is the grain-boundary. The grain-boundaries are the regions where many intrinsic defects like, point defects, volume defects, dislocations etc. and extrinsic defects like segragated impurities are concentrated. In the present work we have studied the dependence of the spectral response in polycrystalline silicon solar cells as a function of the grain-bdundary recombination velocity, grain size and the bulk minority carrier diffusion length. Also, we have experimentally passivated the defects at the grain-boundaries and the intra-grain defects by thermal annealing in Mole-cular Hydrogen, using different annealing temperatures. The defects have also been passivated with the help of mono-atomic hydrogen produced by a hydrogen plasma. Measurements of the minority carrier diffusion length showed better results for the hydrogenated polycrystalline silicon solar cells in comparison to the unhydrogenated polycrystalline silicon solar cells, as expected, due to the passivation of the dangling bards present at the grain-boundariers and intra -grain defects. iv In Chapter 1 we have given an introduction of the photovoltaic devices and its material growth. A review of various defects, their introduction process in the material and their effect on photovoltaic parameters for the polycrystalline silicon solar cells is also given in this chapter. Chapter 2 provides discussion on the minority carrier diffusion length and grain-boundaries with the physics of carrier recombination process. It also reviews the various methods adopted by different workers for passivating the defects in polycrystalline silicon solar cells. The chapter finally defines the main aims and objectives of our present work. The different experinntal equipments and system used for our experi- mental investigations are described in Chapter 3, particular with regard to the techniques adopted for diffusion length measurement, thermal annealing in molecular hydrogen and hydrogen plasma annealing• Chapter 4 presents a comprehensive mathematical formulation to explain the recombination loss of current due to grain-boundaries in polycrystalline silicon solar cells. It is assumed that the grains are fairly large and column,?r- in nature and the n+ -p junction is normal to the grain-boundary plane. Due to the presence of grain-boundary recombination centres there is a flow of minority carriers towards the grain-boundaries when the cell is illuminated since the grain-boundaries give an attractive potential for minority carriers (or known as trap levels).