Two-surface, bifacial or bifacial solar cells – interesting information about solar modules

The vast majority of solar cells today are made from silicon. Silicon is a semiconductor and as such its outer electrons are located in an energy interval called the valence band, completely filling the energy levels of that band. Above this valence band is a forbidden band or energy gap in which no electron can exist, and further up is the conduction band. This conduction band is almost empty of electrons, but it is where the valence band electrons are housed after being excited by the absorption of photons. These electrons have more energy than the ordinary electrons of the semiconductor. The electrical conductivity of the Si described so far, which is referred to as intrinsic silicon, is extremely low. A slight contamination with phosphorus atoms results in additional electrons in the conduction band, making the silicon n-type and giving it a conductivity that can be influenced by changing the density of the phosphorus atoms. Alternatively, contamination with boron or aluminum atoms can cause the Si to become p-type and have a conductivity that can also be influenced. These impurity atoms absorb electrons from the valence band and leave so-called “holes” in it that behave like virtual positive charges. Si solar cells are typically doped with boron to behave like a p-type semiconductor and have a narrow (~0.5 micron) superficial n-type region. The so-called pn junction is formed between the two, in which an electric field is created that splits electrons and holes, the electrons to the surface and the holes to the interior. In this way, a photocurrent is generated, which is dissipated through metal contacts on both sides. The light falling away from the pn junction is not split, and the electron-hole pairs created eventually recombine and do not produce a photocurrent. The roles of the p and n regions in the cell can be reversed, as explained here.

Accordingly, a monofacial solar cell only generates a photocurrent when the surface where the junction was formed is illuminated.

A bifacial solar cell, on the other hand, is designed so that the cell is active on both sides and generates photocurrent when either side - front or back - is illuminated.

Additional power generation gains: Compared to P solar cells, N solar cells tend to significantly increase efficiency. Bifacial solar cells will have a broader application perspective due to the bifacial generation capacity and higher system efficiency, and are particularly suitable for snowier areas and distributed generation systems such as roofs, fences and sound barriers.

The cell backside efficiency can reach more than 19%, and the incident backlights can be used to improve the generation capacity of the system, with the unit area capacity increase up to 10%~30%.

With the glass module with bifacial cell technology, the light is captured on both the front and back of the module. Increasing the use of light increases the efficiency of the module. Up to 360 Wp total power can be achieved via the active rear of the module (290 Wp only at the front / total 320 - 360 Wp).

The efficiency gain depends on the radiation situation (atmosphere and background).

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