Bifacial solar cells

A bifacial solar cell (BSC) is any photovoltaic solar cell that can produce electrical energy when illuminated on either of its surfaces, front or rear. In contrast, monofacial solar cells produce electrical energy only when photons impinge on their front side. Bifacial solar cells can make use of albedo radiation, which is useful for applications where a lot of light is reflected on surfaces such as roofs. The concept was introduced as a means of increasing the energy output in solar cells. Efficiency of solar cells, defined as the ratio of incident luminous power to generated electrical power under one or several suns (1 sun = 1000W/m^{2 [2]}), is measured independently for the front and rear surfaces for bifacial solar cells. The bifaciality factor (%) is defined as the ratio of rear efficiency in relation to the front efficiency subject to the same irradiance.^[3]

The vast majority of solar cells today are made of silicon (Si). Silicon is a semiconductor and as such, its external electrons are in an interval of energies called the valence band and they completely fill the energy levels of this band. Above this valence band there is a forbidden band, or band gap, of energies within which no electron can exist, and further above, we find the conduction band. The conduction band of semiconductors is almost empty of electrons, but it is where valence band electrons will find accommodation after being excited by the absorption of photons. The excited electrons have more energy than the ordinary electrons of the semiconductor. The electrical conductivity of Si, as described so far, called intrinsic silicon, is exceedingly small. Introducing impurities to the Si in the form of phosphorus atoms will provide additional electrons located in the conduction band, rendering the Si n-type, with a conductivity that can be engineered by modifying the density of phosphorus atoms. Alternatively, impurification with boron or aluminum atoms renders the Si p-type, with a conductivity that can also be engineered. These impurity atoms retrieve electrons from the valence band leaving the so-called "holes" in it, that behave like virtual positive charges.^[4]

Si solar cells are usually doped with boron, so behaving as a p-type semiconductor and have a narrow (~0.5 microns) superficial n-type region. Between the p-type region and the n-type region the so-called p-n junction is formed, in which an electric field is formed which separates electrons and holes, the electrons towards the n-type region at the surface and the holes towards the p-type region. Under illumination an excess of electron-hole pairs are generated, because more electrons are excited. Thus, a photocurrent is generated, which is extracted by metal contacts located on both faces of the semiconductor. The electron-hole pairs generated by light falling outside the p-n junction are not separated by the electric field, and thus the electron-hole pairs end up recombining without producing a photocurrent.^[4] The roles of the p and n regions in the cell can be interchanged. Accordingly, a monofacial solar cell produces photocurrent only if the face where the junction has been formed is illuminated. Instead, a bifacial solar cell is designed in such a way that the cell will produce a photocurrent when either side, front or rear, is illuminated.

BSCs and modules (arrays of BSCs) were invented and first produced for space and earth applications in the late 1970s, and became mainstream solar cell technology by the 2010s. It is foreseen that it will become the leading approach to photovoltaic solar cell manufacturing by 2030 due to the shown benefits over monofacial options including increased performance, versatility, and reduce soiling impact.^[5]