An investigation of dark current and photocurrent superposition in photovoltaic devices

Abstract

In the design and analysis of photovoltai cells, a principle of superposition of light and dark currents is usually assumed to apply. This principle states that the current flowing in an illuminated device subject to a bias V is given by the superposition of the short circuit photocurrent and the current that would flow at bias V in the dark. Using a straightforward modification of Shockley's theory of the pn junction diode, the validity of the superposition principle is established here for Si and GaAs homojunction cells exposed to one sun illumination. The argument commences by demonstrating that the quasi-fermi potentials are essentially constant across the depletion region for a device exposed to one sun illumination and biased at a reasonable operating point. Proceeding in this way, it is found that superposition applies even when recombination and photogeneration in the depletion region contribute substantially to both the dark current and the photocurrent. The theoretical argument is confirmed by direct numerical solution of the basic semiconductor equations.