In the absence of light, the PN jun

In the absence of light, the PN junction is in equilibrium and no current flows through the PN junction. When sunlight hits the surface of the solar cell, the balance of the PN junction is disturbed, resulting in a photocurrent flowing from the n region to the p region 1, and a photocurrent flowing from the p region to the n region, dark current opposite to photocurrent I. We define the direction of the photocurrent as the reference positive direction, therefore, the volt-ampere characteristics of an ideal PN junction solar cell can be expressed as