Bifacial active photovoltaic panels are those whose cells absorb light from both sides, i.e., front and back (made with bifacial technology). Bifacial panels have two active sides and are covered with tempered glass or transparent film on both sides ( Fig. 1 ). With this technology, more photons reach the panel, so the energy produced is greater than that obtained from traditional, single-sided panels, by up to \( 30\% \).

A bifacial solar cells used in double-sided panels are mostly structures:

• based on a p-type crystalline silicon wafer and with a p+-p-n+ cell structure,
• based on an n-type crystalline silicon wafer and with a p+-n-n+ cell structure,
• based on a silicon heterojunction, made with internal thin films of amorphous silicon.

Solar rays reflect off different substrates at different percentages, depending on the type of substrate ( Fig. 2 ). For the effectiveness of double-sided panels, a key factor is the so-called albedo coefficient, which shows the ability to reflect light from a given surface. The brighter the surface, the more light it can reflect, resulting in more energy being extracted from the back surface of the panel. For very bright surfaces, the albedo coefficient can be up to \( 80\% \).

White roof membrane and snow have the highest albedo coefficient.
Obtaining additional energy from the back surface of a double-sided panel, therefore, depends on the substrate, its high albedo coefficient, and in addition on the angle of the panel and the height of the location above the reflecting surface.
The load and compound resistance of double-sided glass-fire-glass panels is higher than single-sided panels. Double glazing has a higher mechanical strength, and also the resistance of the glass to chemical compounds is higher than for EVA film. The price of such a panel is also higher.
There are also panels produced that have the cells pulled apart, giving the possibility for special applications. Double glazing provides stability to the cells, which gives the possibility to use this type of construction for roofing terraces, shelters or inserting in large areas covered with glass and requiring shading. In this way, double-sided panel can be used not only as an energy producer, but also for other utility and aesthetic purposes.

Polish producer Hanplast Energy has started production of photovoltaic cells with HPERC technology. The developed cell is an intermediate solution between cells made in PERC and HIT technologies. The technology is based on n-type monocrystalline HJT cells. The cells are connected using SmartWire Connection Technology (SWCT). The manufacturer claims a panel efficiency of \( 23-24\% \), which is up to \( 10\% \) higher efficiency than for conventional panels, maintaining \( 87.25\% \) of the panel's initial power output after 25 years of operation, and a higher energy yield of up to \( 30\% \). This is all thanks to the use of bifacial cell technology ( Fig. 3 ) [1], [2], [3].