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 (V_OC) 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.