A simple theory of back surface field (BSF) solar cells

Abstract

A theory of an n-p-p+ junction is developed, entirely based on Shockley’s depletion layer approximation. Under the further assumption of uniform doping the electrical characteristics of solar cells as a function of all relevant parameters (cell thickness, diffusion lengths, etc.) can quickly be ascertained with a minimum of computer time. Two effects contribute to the superior performance of a BSF cell (n-p-p+ junction) as compared to an ordinary solar cell (n-p junction). The sharing of the applied voltage among the two junctions (the n-p and the p-p+ junction) decreases the dark current and the reflection of minority carriers by the builtin electric field of the p-p+ junction increases the short-circuit current. The theory predicts an increase in the open-circuit voltage (VOC) with a decrease in cell thickness. Although the short-circuit current decreases at the same time, the efficiency of the cell is virtually unaltered in going from a thickness of 200 μm to a thickness of 50 μm. The importance of this fact for space missions where large power-to-weight ratios are required is obvious.