) in order to excite an electron from the valence band into the conduction band. The difference in energy between these photons and the silicon bandgap is converted into heat (via lattice vibrations, i.e., phonons) rather than into usable electrical energy. Most cells produce a voltage of 0.6–0.7 V, depending on the charge carrier concentration and bandgap, regardless of the surface area of the cell. The larger the cell is, more current it will produce. The theoretical limit of a solar cell using a p-n junction is described by the Shockley-Queisser limit, which states that a maximum efficiency of 33.7% can be reached for a solar cell with a single p-n junction [18]. This limit occurs because the devices are selective in the photons they absorb. Solar cells made with multiple layers of p-n junctions can overcome this limit, with a theoretical limit of 86.8% if the device is composed of an infinite number of layers.
3.1 Modelling I-V characteristics of a solar PV cell
The current-voltage (IV) and power-voltage (PV) characteristic equations of the photovoltaic cell can be described from the equivalent circuit shown in Figure 3.