green-techZONE Products for the week of June 2, 2003

Why Does SunPower's Back Contact Cell, The A-300, Have Superior Performance?
White Paper provided by SunPower as a companion piece to Lee Goldberg's product review.

A generic single crystal wafered silicon solar cell has an efficiency of about 14% but there are various features which can be added to improve that number. In the optimum case cell efficiencies in the 23-25% range are obtained for high-performance laboratory cells which are usually very expensive to manufacture. SunPower's backside contact cell incorporates a number of efficiency enhancing features in a very cost effective manner, resulting in over 20% efficiency. The efficiency enhancing features include:

1. In a conventional cell structure, the n-type top layer must perform several functions. It collects free electrons generated in the base p-type wafer; it transports these electrons laterally to the metal contact grid; and it provides electrical contact to the grid. Also, holes generated in the n-type layer must diffuse to the base without recombining and in order to perform all of these tasks adequately the layer design is necessarily a compromise in each area. For example, the lateral transport is improved by making the layer thick and heavily doped. This, however, degrades diffusion resulting in an efficiency loss compared to a situation where these constraints did not exist. In a backside contact cell, these compromises do not exist.
2. Having the metal grid on the top causes shadowing loss which increasse if a larger metal coverage is used to reduce the cell resistance loss. A balance must be found which usually results in about 5% shadowing loss and 5% resistance loss. This compromise also does not exist in a backside contact cell.
3. In a conventional cell the metallic backside contact covers the entire back surface. This surface absorbs unlucky minority carriers (electrons in the p-type base.) In SunPower's cell the back surface is covered with a passivating layer of silicon dioxide which does not cause this loss. The metal contacts the cell through small holes in the backside passivating layer.
4. About 10% of the photons hitting the surface of a solar cell actually pass through the silicon wafer and reach the backside. In a conventional cell these are lost. In SunPower's cell they are reflected off the back metal contact, which is made very reflective, and then trapped within the cell by the textured top surface. They bounce back and forth until they are eventually absorbed, producing the desired electron-hole pair, a phenomenon is termed "light trapping."

On the negative side most of the photon absorption and carrier generation occurs near the top of the cell. The minority carriers must diffuse to the back of the cell to be collected and this requires high-quality silicon that contains few defects that would trap carriers and cause recombination.

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