Requirements of Electrical Contacts to Photovoltaic Solar Cells
- PDF / 1,704,303 Bytes
- 12 Pages / 420.48 x 639 pts Page_size
- 6 Downloads / 232 Views
Requirements of Electrical Contacts to Photovoltaic Solar Cells T.A. Gessert and T.J. Coutts Solar Energy Research Institute, 1617 Cole Blvd., Golden, CO 80401 USA.
ABSTRACT The importance of contacts to photovoltaic solar cells is often underrated mainly because the required values of specific contact resistance and metal resistivity are often thought to be relatively modest compared with those associated with very large scale integration (VLSI) applications. However, due to the adverse environmental conditions experienced by solar cells, and since many of the more efficient cells are economically advantageous only when operated under solar concentration, the requirements for solar cell contacts are sometimes more severe. For example, at one-sun operation, the upper limit in specific contact resistance is usually taken to be 10-2 Q-cm 2 . However, at several hundred suns, this value should be reduced to less than 10 -4 0-cm2 . Additionally, since grid line fabrication often relies on economical plating processes, porosity and contamination issues can be expected to cause reliability and stability problems once the device is fabricated. It is shown that, in practice, these metal resistivity issues can be much more important than issues relating to specific contact resistance and that the problem is similar to that of providing stable, low resistance interconnects in VLSI. This paper is concerned with the design and fabrication of collector grids on the front of the solar cells and, although the discussion is fairly general, it will center on the particular material indium phosphide. This Ir-V material is currently of great importance for space application because of its resistance to the damaging radiation experienced in space.
INTRODUCTION A solar cell exploits the photovoltaic (PV) effect and delivers electrical power to an external load when illuminated by a suitable light source. Since sunlight is a relatively dilute source of energy at the surface of the earth, the electrical output per unit area of a PV device is quite small; thus large solar cell areas are normally required to generate significant amounts of power. At present, the maximum reported conversion efficiency for a Si based solar cell is slightly over 23% [1]. Although Si based solar cells are becoming increasingly cost effective for many power generation applications, the most efficient solar cells (although not the most commonly used), with efficiencies approaching 35%, are those based on the II-V binary, ternary and quatemary alloys involving GaAs and InP [2]. However, these materials are expensive, when compared with Si, and thus the relative cost of generating the energy economically becomes a critical issue. Often, cost effectiveness will only be possible when the cells are operated under optical concentration. This has a two-fold advantage in that it not only minimizes the amount of the costly materials, but also, due to the electrical characteristics of the PV device, causes the junction efficiency actually to increase with concentration.
Data Loading...
