Most photovoltaic cells are made of silicon, an element that is at the heart of all modern electronics. Silicon is special because of the arrangement of its electrons—it has four out of the possible eight electrons in its outermost shell. This means that it makes perfect covalent bonds with four other silicon atoms, forming a lattice structure.
Silicon solar cells are the most broadly utilized of all solar cell due to their high photo-conversion efficiency even as single junction photovoltaic devices. Besides, the high relative abundance of silicon drives their preference in the PV landscape.
Instead, it is free to move inside the silicon structure. A solar cell consists of a layer of p-type silicon placed next to a layer of n-type silicon (Fig. 1). In the n-type layer, there is an excess of electrons, and in the p-type layer, there is an excess of positively charged holes (which are vacancies due to the lack of valence electrons).
When sunlight strikes a solar cell, electrons in the silicon are ejected, which results in the formation of “holes”—the vacancies left behind by the escaping electrons. If this happens in the electric field, the field will move electrons to the n-type layer and holes to the p-type layer.
The ultimate efficiency of a silicon photovoltaic cell in converting sunlight to electrical energy is around 20 per cent, and large areas of solar cells are needed to produce useful amounts of power. The search is therefore on for much cheaper cells without too much of a sacrifice in efficiency.
A solar cell in its most fundamental form consists of a semiconductor light absorber with a specific energy band gap plus electron- and hole-selective contacts for charge carrier separation and extraction. Silicon solar cells have the advantage of using a photoactive absorber material that is abundant, stable, nontoxic, and well understood.