How Light is Converted to Electric Current
Electric current is produced by electrons moving within a conducting medium.
Electrons are diminutive, negatively charged particles, which circle around the positively charged atomic nucleus. Most solar cells consist of silicon atoms. If light in form of photons impinges on the silicon atom’s valence electrons, the electrons’ circular path around the nucleus changes: it becomes wider. If respective photons bear enough energy, the electrons are spun out of their orbit and released. They leave atoms deficient of one electron – featuring so called “holes”. Consequentially, light exposure will cause numerous free, negatively electrons and positively charged holes. However, this alone does not produce any electric current, since respective electrons will automatically occupy the next available hole and unite with atoms deficient of electrons. In order to prevent this and thus utilise these electrons as electric current, the silicon is equipped with an electrical field.
In order to generate this field, foreign or impurity atoms are deliberately incorporated with the sun-facing silicon layer (doping). Usually, phosphor serves for this purpose. Phosphor features 5 valence electrons, while silicon has only 4 of them. Phosphor thus features one more valence electron in its electron shell. Since this extra electron cannot be integrated with the silicon lattice, it moves free. This creates an excess of electrons and thus the solar cell’s n-layer.
The silicon on the solar cell’s backside is charged with atoms that lack one valence electron in their shell – such as e.g. boron. This is where the solar cell’s p-layer is formed.
Our cell now features two layers: on its top we find the negatively charged n-layer; its bottom side houses the positively charged p-layer. At the junction between these two layers (boundary layer), the p-layer attracts electrons from the n-layer. This “electron theft” causes reverse charge within the boundary layer: the p-layer is charged negatively, while the n-layer features a positive charge. However, the respective reverse charge exclusively applies to the boundary layer.
To both sides of the solar cell, a current collector made from highly conductive metal is applied. If light impinges on the solar cell, the electrons – spun out of their orbit by photons – are drawn towards the negative pole, as within the boundary layer the negative pole features a positive charge and thus attracts electrons. The boundary layer’s reverse charge enables us to cheat the electrons and draw them to the “wrong side”. Thus, electrons accumulate around the minus pole, which strives to repel them as its n-layer is negatively charged. If both poles are then connected to each other, the electrons migrate from the negative pole – which features an excess of electrons – to the positive pole with its electron deficiency. On their way from negative to positive pole, these electrons will drive all respective DC consumers or charge batteries.