The design of the top contact involves not only the minimization of the finger and busbar resistance, but the overall reduction of losses associated with the top contact. These include resistive losses in the emitter, resistive losses in the metal top contact and shading losses. The critical features of the top contact design which determine how the magnitude of these losses are the finger and busbar spacing, the metal height-to-width aspect ratio, the minimum metal line width and the resistivity of the metal. These are shown in the figure below.
Impact of Finger Spacing on Emitter Resistance
An important factor in top contact design is that of resistive losses in the emitter. As shown in the Emitter Resistance page, the power loss from the emitter depends on the cube of the line spacing, and therefore a short distance between the fingers is desirable for a low emitter resistance.
The grid resistance is determined by the resistivity of the metal used to make the metal contact, the pattern of the metalization and on the aspect ratio of the metalization scheme. A low resistivity and a high metal height-to-width aspect ratio are desirable in solar cells, but in practice are limited by the fabrication technology used to make the solar cell.
Shading losses are caused by the presence of metal on the top surface of the solar cell which prevents light from entering the solar cell. The shading losses are determined by the transparency of the top surface, which, for a planar top surface, is defined as the fraction of the top surface covered by metal. The transparency is determined by the width of the metal lines on the surface and on the spacing of the metal lines. An important practical limitation is the minimum linewidth associated with a particular metalization technology. For identical transparencies, a narrow line-width technology can have closer finger spacing, thus reducing the emitter resistance losses.
While a multitude of top contacting schemes exist, for practical reasons most top surface metalization patterns are relatively simple and highly symmetrical. A symmetrical contacting scheme can be broken down into unit cells and several broad design rules can be determined. It can be shown 1 that:
- the optimum width of the busbar, WB, occurs when the resistive loss in the busbar equals its shadowing loss;
- a tapered busbar has lower losses than a busbar of constant width; and
- the smaller the unit cell, the smaller finger width, WF , and the smaller the finger spacings, S, the lower the losses.
A program for designing and developing the front surface grid pattern is available at the PV Lighthouse Metal Grid Calculator
- 1. a. b. , «Optimizing Solar Cell Performance by Simultaneous Consideration of Grid Pattern Design and Interconnect Configurations», 13th IEEE Photovoltaic Specialists Conference. Washington, D.C., USA, pp. 1-8, 1978.