A bifacial solar cell (BSC) is a photovoltaic solar cell that can produce electrical energy from both front and rear side. In contrast, monofacial solar cells produce electrical energy only when photons are incident on their front side. Bifacial solar cells and solar panels (devices that consist of multiple solar cells) can improve the electric energy output and modify the temporal power production profile compared with their monofacial counterparts.
Efficiency of solar cells, defined as the ratio of incident luminous power to generated electrical power under one or several suns (1 sun = 1000W/m2), is measured independently for the front and rear surfaces for bifacial solar cells. 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 reduced soiling impact.
Usage In conventional installations, such as fixed-tilt equator-facing solar panels or panels mounted on solar trackers, bifacial solar cells allow additional energy production due to more effective use of albedo (reflectivity) irradiation. The albedo of the ground, the bifaciality factor of the solar panels (defined as the ratio of rear efficiency to the front efficiency subject to the same irradiance), and the technical installation parameters define how high this production boost is. In cases where additional irradiance on the rear side is lacking, such as common parallel-to-roof solar panels, bifaciality does not provide additional yield.
Another way to use bifaciality of solar cells is to install them vertically, so that the sides are facing east and west. This installation produces a two-humped production profile, where the electricity production peaks twice daily, in the morning and in the evening. Compared with a conventional solar energy production profile that has a single high production peak at noon, the vertical bifacial production profile allows the production of more electricity when consumption is typically high and also avoids overproduction of solar energy around noon. Especially in power systems with a high share of solar energy like Germany's, vertical bifacial solar cells allow mitigation of the "duck curve."
Vertical bifacial solar cells are especially attractive in high-latitude locations, where they can reach higher annual production than their monofacial counterparts. Utilizing vertical installation on agricultural land (agrivoltaics) allows for efficient land use, as sparse spacing of vertical solar panels can allow machinery to move between the rows. Moreover, vertical panels are less vulnerable to soiling than panels with lower tilt angles, which is beneficial in certain environments, like the desert. However, vertical bifacial solar cells are vulnerable to shading from surroundings: due to vertical installation and low solar elevation angle when the sun is at east or west, vertical solar panels require open space in the direction where the panel is facing.
History
Invention and first devices
A silicon solar cell was first patented in 1946 by Russell Ohl when working at Bell Labs and first publicly demonstrated at the same research institution by Calvin Fuller, Daryl Chapin, and Gerald Pearson in 1954; however, these first proposals were monofacial cells and not designed to have their rear face active. The first theoretical bifacial solar cell was proposed in a Japanese patent with a priority date of 4 October 1960, by Hiroshi Mori, when working for the company Hayakawa Denki Kogyo Kabushiki Kaisha (in English, Hayakawa Electric Industry Co. Ltd.), which later developed into nowadays Sharp Corporation. The proposed cell was a two-junction pnp structure with contact electrodes attached to two opposite edges.
However, the first demonstrations of bifacial solar cells and panels were carried out in the Soviet Space Program in the Salyut 3 (1974) and Salyut 5 (1976) LEO military space stations. These bifacial solar cells were developed and manufactured by Bordina et al. at the VNIIT (All Union Scientific Research Institute of Energy Sources) in Moscow that in 1975 became Russian solar cell manufacturer KVANT. In 1974 this team filed a US patent in which the cells were proposed with the shape of mini-parallelepipeds of maximum size 1mm × 1mm × 1mm connected in series so that there were 100 cells/cm2. As in modern-day BSCs, they proposed the use of isotype junctions pp+ close to one of the light-receiving surfaces. In Salyut 3, small experimental panels with a total cell surface of 24 cm2 demonstrated an increase in energy generation per satellite revolution due to Earth's albedo of up to 34%, compared to monofacial panels at the time. A 17–45% gain due to the use of bifacial panels (0.48m2 – 40W) was recorded during the flight of Salyut 5 space station.
Simultaneous to this Russian research, on the other side of the Iron Curtain, the Laboratory of Semiconductors at the School of Telecommunication Engineering of the Technical University of Madrid, led by Professor Antonio Luque, independently carries out a broad research program seeking the development of industrially feasible bifacial solar cells. While Mori's patent and VNIIT-KVANT spaceship-borne prototypes were based on tiny cells without surface metal grid and therefore intricately interconnected, more in the style of microelectronic devices which were at that time in their onset, Luque will file two Spanish patents in 1976 and 1977 and one in the United States in 1977 that were precursory of modern bifacials .
Luque's patents were the first to propose BSCs with one cell per silicon wafer, as was by then the case of monofacial cells and so continues to be, with metal grids on both surfaces. They considered both the npp+ structure and the pnp structures. Development of BSCs at the Laboratory of Semiconductors was tackled in a three-fold approach that resulted in three PhD theses, authored by Andrés Cuevas (1980), Javier Eguren (1981) and Jesús Sangrador (1982), the first two having Luque as doctoral advisor while Dr. Gabriel Sala, from the same group, conducted the third. Cuevas' thesis consisted of constructing the first of Luque's patents, the one of 1976, that due to its npn structure similar to that of a transistor, was dubbed the "transcell". Eguren's thesis dealt with the demonstration of Luque's 2nd patent of 1977, with a npp+ doping profile, with the pp+ isotype junction next to the cell's rear surface, creating what is usually referred as a back surface field (BSF) in solar cell technology. This work gave way to several publications and additional patents.
In particular, the beneficial effect of reducing p-doping in the base, where reduction of voltage in the emitter junction (front p-n junction) was compensated by voltage increase in the rear isotype junction, while at the same time enabling higher diffusion length of minority carriers that increases the current output under bifacial illumination. Sangrador's thesis and third development route at the Technical University of Madrid, proposed the so-called vertical multijunction edge-illuminated solar cell in which p+nn+ where stacked and connected in series and illuminated by their edges, this being high voltage cells that required no surface metal grid to extract the current. In 1979 the Laboratory for Semiconductors became the Institute for Solar Energy (IES-UPM), that having Luque as the first director, continued intense research on bifacial solar cells well until the first decade of the 21st century, with remarkable results. For example, in 1994, two Brazilian PhD students at the Institute of Solar Energy, Adriano Moehlecke and Izete Zanesco, together with Luque, developed and produced a bifacial solar cell rendering 18.1% in the front face and 19.1% in the rear face; a record bifaciality of 103% (at that time record efficiency for monofacial cells was slightly below 22%).
The first bifacial solar cell factory: Isofoton
Of the three BSC development approaches carried out at the Institute of Solar Energy, it was that of Eguren's thesis, the npp+, the one that gave the best results. On the other hand, it was found that bifacial solar cells could deliver up to 59% more power yearly when installed with a white surface at their back, which enhanced the sun's reflected radiation (albedo radiation) going into the cells' rear face. It could have been expected this finding to happen easier in Spain, where houses, especially rural ones are, in the south, frequently whitewashed. Hence, a spin-off company was founded to manufacture bifacial solar cells and modules, based on the npp+ development, to commercially exploit their enhanced power production when suitably installed with high albedo surfaces behind, whether ground or walls.
Founded in 1981 it was named Isofotón (because its cells singularly used all isotropic photons) and established in Málaga, Luque's hometown. Its initial capital came from family and friends (e.g. most of the employees and research staff of the Institute of Solar Energy) plus some public capital from an industrial development fund, SODIAN, owned by the Andalusian Autonomous Community. It set sail with 45 shareholders, Luque as 1st chairman and co-CEO, together with his brother Alberto, a seasoned industrial entrepreneur, and having Javier Eguren as CTO. Eguren and Sala led the technology transfer from the Institute of Solar Energy to Isofoton. By 1983 Isofoton's factory in Málaga had a manufacturing capacity of 330 kW/yr. of bifacial modules (with a 15 people net headcount) at a time when the global market of photovoltaics was in the range of 15 MW. At that time, the market of terrestrial photovoltaic power plants, to which Isofoton oriented its production, essentially consisted of demonstration projects. Thus, early landmarks of Isofoton's production were the 20kWp power plant in San Agustín de Guadalix, built in 1986 for Iberdrola, and an off-grid installation by 1988 also of 20kWp in the village of Noto Gouye Diama (Senegal) funded by the Spanish international aid and cooperation programs.
As Isofotón matured, its early shareholding structure of individuals was replaced by big technology and engineering corporations as Abengoa or Alcatel or banks such as BBVA. Upon Alcatel's entry as a major shareholder in 1987 the decision was taken to switch production to more conventional monofacial photovoltaic cells, based on licensed technology from US PV manufacturer Arco Solar, this being the end of Isofoton as the world's first and until then, only bifacial cell manufacturer. However, Isofoton still continued to forge ahead successfully and between 2000 and 2005 it ranked consistently among the world's top 10 photovoltaic manufacturers. In 2015 it filed for bankruptcy when, as almost all of the other European and Western PV manufacturers of its time, it could not withstand the competitive pressure of the new wave of Chinese PV manufacturers.
Later progress until mass production