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Revisiting the Theory of Photocurrent in Solar Cells

The built-in potential of p-n junctions plays a pivotal role in charge separation, a fundamental process underlying the photovoltaic effect.However, conventional classical theories of photovoltaic behavior in p-n junctions often neglect its quantitative influence. In this work, we revisit the classical framework and derive an improved analytical expression for photocurrent by incorporating more accurate boundary conditions. Our analysis reveals that the photocurrent comprises two distinct components: the conventional forward photocurrent and a previously unrecognized backward photocurrent, which depends on the built-in potential and the applied voltage. The theoretical analysis predicts that, under specific forward-bias conditions, these two components may partially or completely cancel each other. This prediction was experimentally verified by optical lock-in measurements performed on a commercial silicon solar cell. These findings provide new insights into the fundamental mechanisms governing photovoltaic devices and suggest potential pathways for performance optimization.