Low-Cost Solar Array Project Research Articles

Effect of thickness on silicon solar cell efficiency

Semiconductor material cost is one of the factors which determines the performance-cost ratio and economical feasibility of silicon solar cells for terrestrial power generation. Decreasing the cell thickness would lower the silicon material cost. The energy conversion efficiency of a back-surface field solar cell will have a peak as the silicon film thickness is reduced due to two opposing factors: 1) the open-circuit voltage increases and 2) the short-circuit current decreases with decreasing cell thickness. A computer-aided-design study on the dependence of this efficiency peak on the concentrations of the recombination and dopant impurities is presented in this paper. The illuminated current-voltage characteristics of over 100 cell designs were obtained using the transmission line circuit model to numerically solve the Shockley equations. Using an AM1 efficiency of 17 percent as a target value, which is the highest encapsulated silicon cell efficiency used in the Block IV modules of the Low-Cost Solar Array Project, it is shown that the efficiency versus thickness dependence has a broad maximum which varies less than 1 percent over more than a three-to-one range of cell thickness from 30 to 100 µm. Optical reflecting back surface will give only a slight improvement of AM1 efficiency, about 0.7 percent, in this thickness range. The sensitive dependence of efficiency on patchiness across the back-surface field low-high junction in thin cells is noted.

Effect of zinc impurity on silicon solar-cell efficiency

Zinc is a major residue impurity in the preparation of solar-grade silicon material by the zinc vapor reduction of silicon tetrachloride. This paper projects that in order to get a 17-percent AM1 cell efficiency for the Block IV module of the Low-Cost Solar Array Project,1the concentration of the zinc recombination centers in the base region of silicon solar cells must be less than 4 × 1011Zn/cm3in the p-base n+/p/p+ cell and 7 × 1011Zn/cm3in the n-Base p+/n/n+ cell for a base dopant impurity concentration of 5 × 1014atoms/cm3. If the base dopant impurity concentration is increased by a factor of 10 to 5 × 1015atoms/cm3, then the maximum allowable zinc concentration is increased by a factor of about two for a 17-percent AM1 efficiency. The thermal equilibrium electron and hole recombination and generation rates at the double-acceptor zinc centers are obtained from previous high-field measurements as well as new measurements at zero field described in this paper. These rates are used in the exact dc-circuit model to compute the projections.

Assessment of low-cost manufacturing process sequences

The Low-Cost Solar Array Project, sponsored by the U.S. Department of Energy, is managing an extensive research and development activity to reduce the cost of manufacturing photovoltaic solar arrays by a factor of approximately one hundred. The Solar Array Manufacturing Industry Costing Standards (SAMICS) methodology was developed to help meet this management objective by providing a systematic, standardized way to determine manufacturing costs. Due to space limitations and the fact that the detailed description is available internationally, this paper presents only a broad outline of SAMICS, and concentrates on its international applicability. The primary inputs for SAMICS are detailed descriptions of the manufacturing processes to be used: throughput rates, equipment costs, space requirements, personnel, materials, yields, and so forth. SAMICS also requires economic data, such as prices of materials, tax rates, inflation rates, and the desired financial return. From this information, SAMICS constructs a model of an industry made up of factories using specified sequences of those manufacturing processes, then calculates the annual costs for operating each factory at a given output rate. The annual costs are used to calculate a desired price for each product. To calculate the total annual costs, it is necessary to estimate indirect costs. These cannot be taken as a simple multiple of direct costs, because the appropriate multiple depends on the degree of automation, the structure of the work force, and other characteristics of the full-scale technology and cannot be reliably by examining the currently nascent solar array manufacturing industry. Accordingly, a considerably more detailed model was developed, and constitutes a major part of SAMICS' contribution to the state of the art. A side benefit is that SAMICS should be useful in estimation of product prices in many industries using de (not necessarily automated) production lines. SAMICS has been implemented by both a computer program (SAMIS) and, with some powerful simplifying assumptions, by an easily applied manual procedure (IPEG). It has been applied to the assessment of Low-Cost Solar Array Project progress, and it has been used as a tool in the selection of research and development priorities. (Over 200 process sequences have been costed by SAMIS at JPL alone; over dozen other organizations have also used it extensively.)