Conventional photovoltaic cell
One of the earliest fabricated photovoltaic cells [1] is a cell on a wafer of a p-type semiconductor. A so-called n+ emitter layer was applied to the front surface of the wafer, usually made by doping phosphorus onto the surface. Electrodes (Ag) were applied to this surface and then the cell was passivated by a dielectric (e.g., \( SiN_{x} \)), which also acts as an anti-reflective layer. The back surface of the p-type Si semiconductor is coated with aluminum to form a junction with Si that counteracts minority charges from reaching the back surface ( Fig. 1 ).

Passivated Emitter and Rear Cell PERC is a passivated emitter and rear cell technology. With respect to standard cells, the PERC-type cell [2] has an additional dielectric layer that increases the efficiency of the cell by reflecting any light that reaches the back layer of the cell without generating an exciton. With this reflection, photons have a second chance to generate current.

A PERC-type cell can reduce electron recombination on the back surface by adding an additional dielectric layer between the silicon layers and the aluminum electrode, so that only the aluminum is in contact with a small portion of the cell area. The additional dielectric layers of \( SiO_{2} \) and \( Al_{2}O_{3} \) significantly reduce electron recombination on the surface, leading to an increase in cell efficiency.
In a PERC-type photovoltaic cell, as shown in Fig. 2, both the front and back surfaces are passivated by the dielectric. Small holes of the back dielectric layer are produced by a laser so that the electrode metal can contact the back surface of the cell. Compared to conventional Si photovoltaic cells, the PERC-type cell is able to improve the performance, mainly due to the additional passivating dielectric layer at the back. The additional back dielectric layer reflects photons back into the cell. This causes the optical path of the photon to be extended, resulting in greater light absorption and increased current generation.

PERT-type photovoltaic cells [3]
An innovative and even more electrically efficient solution, relative to PERC, is the Passivated Emitter Rear Cell Totally Diffused (PERT) photovoltaic cell. There are no holes in the back passivation layer of this cell and therefore it does not allow the electrons to escape, but forces them to bounce off the back of the cell and circulate in it again, giving the electrons a chance to get to the p-n junction. In PERT technology, the passivation layer puts up a barrier, preventing the electrons from escaping, causing them to enter the cell after bouncing off the back layer, increasing the energy yield. The following graphic illustrates this phenomenon ( Fig. 3 ).

PERT cells have a greater ability to absorb solar radiation and thus have a higher efficiency. Passivation of the back of the cell causes solar radiation to be reflected towards the active part of the cell, thus increasing the optical path of the ray in the cell. This is particularly important for radiation in the infrared region, since absorption in this region is characterized by a small extinction coefficient. In order to absorb radiation from the infrared spectral region ( \( \lambda \) > 800 nm to 1100 nm for silicon), the optical path of the beam must be lengthened, which translates into increased absorption and generation of more energy.
The use of an additional layer of silicon with doped boron allows the electrode to be cut off from the interference of light rays, since infrared radiation absorbed by the electrodes and not converted to electricity heats the cell structure lowering its efficiency. Therefore, the aim is to use this part of the solar energy. PERT cells are cells that use light also reflected from the Earth's surface and diffuse solar energy.

PERT cells are constructed using either p-type or n-type silicon wafers shown in Fig. 4. When the cell is constructed on a p-type silicon wafer, a scattering layer is formed by boron doping of the back layer of the p-type silicon wafer. The front layer, the emitter of the n-type wafer, is formed by diffusion of phosphorus. When the cell structure is based on an n-type silicon wafer, the emitter, or front layer, is obtained by diffusion of boron and the back layer is obtained by diffusion of phosphorus ( Fig. 4b). PERT-type cells do not exhibit significant light-induced degradation and can be adapted to bilateral cell structures.