PHYSICS OF SOLAR CELLS AND IV CURVES

In this course we describe the fundamental structure of solar cells at the atomic level and how that structure results in a one-way flow of electrons out of a cell when exposed to sunlight. We explain about p-type and n-type doping and how photons of light create hole-electron pairs. We then go on to describe the energy band theory of silicon and how at the junction of p-type and n-type regions there is an electrostatic field that sweeps out photo-generated electrons, and importantly how that field changes from conditions of Isc to Voc, and how this results in the exponential shape of the typical I-V curve that we all take for granted. To help explain the I-V curve shape we also present the ‘equivalent circuit’ of a solar cell. We finish with a discussion of how I-V curves are added for series and parallel circuits, and how local shading results in shaded cells ‘dragging down’ the curves of unshaded cells, and how the operation of bypass diodes reduces this effect.