(a)
(b)
Figure 6: Band diagram of energy levels of the PN junction: (a) not illuminated, (b) illuminated (: Fermi energy level, : center of the forbidden gap, : valence band edge, : conduction band edge, : energy forbidden gap, : diffusion voltage, : photovoltaic voltage, : electric field intensity, : frequency, : Planck constant, and : coordinate perpendicular to the PN junction).