
A bifacial solar cell (BSC) is any photovoltaic
solar cell
A solar cell, also known as a photovoltaic cell (PV cell), is an electronic device that converts the energy of light directly into electricity by means of the photovoltaic effect. 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
Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...
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 ), is measured independently for the front and rear surfaces for bifacial solar cells. The ''bifaciality factor'' (%) is defined as the ratio of rear efficiency to the front efficiency subject to the same irradiance.
The vast majority of solar cells today are made of
silicon
Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...
(Si). Silicon is a
semiconductor
A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...
and as such, its external electrons are in an interval of energies called the
valence band
In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in ...
and they completely fill the energy levels of this band. Above this valence band there is a forbidden band, or
band gap
In solid-state physics and solid-state chemistry, a band gap, also called a bandgap or energy gap, is an energy range in a solid where no electronic states exist. In graphs of the electronic band structure of solids, the band gap refers to t ...
, of energies within which no electron can exist, and further above, we find the
conduction band
In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in ...
. 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
Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...
atoms. Alternatively, impurification with
boron
Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three ...
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.
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.
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.
History of the bifacial solar cell
Invention and first devices
A silicon
solar cell
A solar cell, also known as a photovoltaic cell (PV cell), is an electronic device that converts the energy of light directly into electricity by means of the photovoltaic effect. was first patented in 1946 by
Russell Ohl
Russell Shoemaker Ohl (January 30, 1898 – March 20, 1987) was an American scientist who is generally recognized for patenting the modern solar cell (, "Light sensitive device").
Ohl was a notable semiconductor researcher prior to the invention ...
when working at
Bell Labs
Nokia Bell Labs, commonly referred to as ''Bell Labs'', is an American industrial research and development company owned by Finnish technology company Nokia. With headquarters located in Murray Hill, New Jersey, Murray Hill, New Jersey, the compa ...
and first publicly demonstrated at the same research institution by
Calvin Fuller
Calvin Souther Fuller (May 25, 1902 – October 28, 1994) was an American physical chemist at AT&T Bell Laboratories where he worked for 37 years from 1930 to 1967. Fuller was part of a team in basic research that found answers to physical ch ...
,
Daryl Chapin
Daryl Muscott Chapin (21 July 1906 – 19 January 1995) was an American physicist, best known for co-inventing solar cells in 1954 during his work at Bell Labs alongside Calvin S. Fuller and Gerald Pearson. For this, he was inducted into the Nat ...
, and
Gerald Pearson
Gerald L. Pearson (March 31, 1905 – October 25, 1987) was an American physicist whose work on silicon rectifiers at Bell Labs led to the invention of the solar cell. In 2008, he was inducted into the National Inventors Hall of Fame.
Biograph ...
in 1954; however, these first proposals were monofacial cells and not designed to have their rear face active. The first bifacial solar cell theoretically proposed is in a Japanese patent with a priority date 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
is a Japanese electronics company. It is headquartered in Sakai, Osaka, and was founded by Tokuji Hayakawa in 1912 in Honjo, Tokyo, and established as the Hayakawa Metal Works Institute in Abeno-ku, Osaka, in 1924. Since 2016, it is majority o ...
. The proposed cell was a
two-junction pnp structure with contact electrodes attached to two opposite edges.
However, first demonstrations of bifacial solar cells and panels were carried out in the
Soviet Space Program
The Soviet space program () was the state space program of the Soviet Union, active from 1951 until the dissolution of the Soviet Union in 1991. Contrary to its competitors (NASA in the United States, the European Space Agency in Western Euro ...
in the
Salyut 3
Salyut 3 (, also known as OPS-2 or Almaz 2Portree (1995).) was a Soviet
The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental coun ...
(1974) and
Salyut 5
Salyut 5 ( meaning ''Salute 5''), also known as OPS-3, was a Soviet space station. Launched in 1976 as part of the Salyut programme, it was the third and last Almaz space station to be launched for the Soviet military. Two Soyuz missions visited ...
(1976)
LEO
Leo is the Latin word for lion. It most often refers to:
* Leo (constellation), a constellation of stars in the night sky
* Leo (astrology), an astrological sign of the zodiac
* Leo (given name), a given name in several languages, usually mas ...
military
space station
A space station (or orbital station) is a spacecraft which remains orbital spaceflight, in orbit and human spaceflight, hosts humans for extended periods of time. It therefore is an artificial satellite featuring space habitat (facility), habitat ...
s. 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/cm
2. 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 cm
2 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 (0.48m
2 – 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 Iron Curtain was the political and physical boundary dividing Europe into two separate areas from the end of World War II in 1945 until the end of the Cold War in 1991. On the east side of the Iron Curtain were countries connected to the So ...
, the Laboratory of Semiconductors at the School of Telecommunication Engineering of the
Technical University of Madrid
The Technical University of Madrid or sometimes called Polytechnic University of Madrid (, UPM) is a public university, located in Madrid, Spain. It was founded in 1971 as the result of merging different Technical Schools of Engineering and Arc ...
, 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%).
Hiroshi Mori's first patented bifacial solar cell (1961).jpg, First page of Mori's1966 patent. In Fig. 1. p-layers (2-2') diffused on three sides of a bulk n-type silicon (1). Electrodes on both edges connect the p (4) and n (3) regions to the electric circuit. In Fig. 3. the cells are connected in series.
Bordina's bifacial solar cell patent.jpg, Drawing in Bordina's 1976 patent. Millimetric parallelipedic bifacial solar cells connected in series. In each mini-cell bulk material is p-type. Dashed lines are pn junctions and dotted lines isotype pp+. Diagonally striped lines left to right are metal electrodes and diagonally striped lines right to left is an insulator filler. 100 cells/cm2.
Luque's patent of the npp+ bifacial solar cell.jpg, Drawings in Luque's 1978 patent ES458514A1 of the npp+ cell bifacial solar cell. (a): n-type layer; (b):metal grids; (c): p+-type layer; (d) p-type wafer
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
Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...
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
whitewash
Whitewash, calcimine, kalsomine, calsomine, asbestis or lime paint is a type of paint made from slaked lime ( calcium hydroxide, Ca(OH)2) or chalk (calcium carbonate, CaCO3), sometimes known as "whiting". Various other additives are sometimes ...
ed. 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
Málaga (; ) is a Municipalities in Spain, municipality of Spain, capital of the Province of Málaga, in the Autonomous communities of Spain, autonomous community of Andalusia. With a population of 591,637 in 2024, it is the second-most populo ...
, 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
Andalusia
Andalusia ( , ; , ) is the southernmost autonomous communities of Spain, autonomous community in Peninsular Spain, located in the south of the Iberian Peninsula, in southwestern Europe. It is the most populous and the second-largest autonomou ...
n
Autonomous Community
The autonomous communities () are the first-level administrative divisions of Spain, created in accordance with the Spanish Constitution of 1978, with the aim of guaranteeing limited autonomy to the nationalities and regions that make up Sp ...
. It set sail with 45 shareholders, Luque as 1st
chairman
The chair, also chairman, chairwoman, or chairperson, is the presiding officer of an organized group such as a board, committee, or deliberative assembly. The person holding the office, who is typically elected or appointed by members of the gro ...
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
Iberdrola, S.A. () is a Spanish multinational electric utility company based in Bilbao, Spain. It has around 40,000 employees and serves around 30 million customers.
Subsidiary, Subsidiaries include ScottishPower (United Kingdom), Ava ...
, and an off-grid installation by 1988 also of 20kWp in the village of Noto Gouye Diama (
Senegal
Senegal, officially the Republic of Senegal, is the westernmost country in West Africa, situated on the Atlantic Ocean coastline. It borders Mauritania to Mauritania–Senegal border, the north, Mali to Mali–Senegal border, the east, Guinea t ...
) 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
Abengoa, S.A. was a Spanish multinational company in the green infrastructure, energy and water sectors. The company was founded in 1941 by Javier Benjumea Puigcerver and José Manuel Abaurre Fernández-Pasalagua, and was based in Seville, Spain. ...
or
Alcatel
Alcatel SA was a French industrial conglomerate active between 1963 and 2006. It has roots to ''Compagnie Générale d’Electricité'' (CGE), a conglomerate founded in 1898 as an early state owned cable and telephone equipment company that lat ...
or banks such as
BBVA
Banco Bilbao Vizcaya Argentaria, S.A. (), better known by its initialism BBVA, is a Spanish multinational financial services company based in Bilbao, with operative offices in Madrid. It is one of the largest financial institutions in the world, ...
. 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

Besides Isofoton, some other PV manufacturers, however, specialized in space applications, reported developments of BSCs at a laboratory scale such as
COMSAT
Communications Satellite Corporation (COMSAT) is a global telecommunications company based in the United States.
By 2007, it had branches in Brazil, Argentina, Colombia, Mexico, Peru, Venezuela and several other countries in the Americas. Alt ...
in 1980, Solarex in 1981 or
AEG Telefunken
Telefunken was a German radio and television producer, founded in Berlin in 1903 as a joint venture between Siemens & Halske and the '' Allgemeine Elektrizitäts-Gesellschaft'' (AEG) ("General electricity company").
Prior to World War I, the comp ...
in 1984. During the late 1980s and the 1990s research and improvement of bifacial solar cell technology continued. In 1987 Jaeger and Hezel at ISFH (Institute for Solar Energy Research in Hamelin) successfully produced a new BSC design based on a single junction n
+p, in which the rear contact was replaced by a metal grid and all intermetallic surfaces were passivated with
PECVD
Plasma-enhanced chemical vapor deposition (PECVD) is a chemical vapor deposition process used to deposit thin films from a gas state (vapor) to a solid state on a substrate (materials science), substrate. Chemical reactions are involved in the pr ...
-grown
silicon nitride
Silicon nitride is a chemical compound of the elements silicon and nitrogen. (''Trisilicon tetranitride'') is the most thermodynamically stable and commercially important of the silicon nitrides, and the term ″''Silicon nitride''″ commonly re ...
, this resulting in 15% and 13.2% under front and rear illumination respectively. In this way, these devices presented a Metal Insulator Semiconductor-Inversion Layer (MIS-IL) front junction. Ten years later, the same research group replaced this MIS layer with a diffused pn junction to produce BSC laboratory devices with 20.1% front and 17.2% rear efficiencies. In 1997, Glunz et al., at the
Fraunhofer Institute for Solar Energy Systems
The Fraunhofer Institute for Solar Energy Systems ISE (or Fraunhofer ISE) is an institute of the Fraunhofer-Gesellschaft. Located in Freiburg, Germany, the Institute performs applied scientific and engineering research and development for all a ...
, produced n
+pn
+ 4 cm
2 devices with 20.6% front and 20.2% rear conversion efficiencies. This was a double junction cell (one of the junctions not connected or "floating") with the metal grid only on the rear surface, i.e. operating an interdigitated back contact (IBC) solar BSC and with the floating front junction performing as passivation. By 1997,
SunPower
SunPower Corporation is an American provider of photovoltaic solar energy generation systems and battery energy storage products, primarily for residential customers. The company, headquartered in San Jose, California, was founded in 1985 b ...
, by then the solar cell manufacturer producing the highest efficiency cells through its back contact design, published research by a team led by its founder,
Richard Swanson
Richard Swanson (born 1945) is an American electrical engineer and businessman, retired founder of SunPower, a solar photovoltaic cell manufacturer.
Biography
Swanson was born in Davenport, Iowa in 1945.
He graduated in 1969 with Bachelor o ...
, on a back contact BSC with front efficiency of 21.9% and rear efficiency of 13.9%. A prototype series of cells and modules were produced but never made it to mass production.
During these days, with PV module cost being almost the only driver towards a wider embracement of solar electricity – as has happened ever after – and despite their attractiveness and the large research effort carried out, the added complexity of BSCs precluded its adoption for large-scale production as had only previously been achieved by Isofoton. Niche applications where BSCs presented competitive advantages were proposed and demonstrated, even to the point of involving some pilot productions. For example, sun shading bifacial PV modules in facades or carports. A celebrated application demonstration was the one by Nordmann et al. in 1997, consisting of a 10 kW PV noise barrier along a north-south-oriented 120m tranche of the A1 motorway in
Wallisellen
Wallisellen is a Municipalities of Switzerland, municipality in the district of Bülach (district), Bülach in the Cantons of Switzerland, canton of Zürich (canton), Zürich in Switzerland, and belongs to the Glatt Valley (German: ''Glattal'').
...
(north of Zurich). BSC cells here were manufactured by German companies ASE (later RWE Schott Solar GmbH) and Kohlhauer based on a system patent by TNC Energie Consulting, and this application has since been abundantly replicated.
With the turn of the millennium, paths towards the industrial production of BSC cells and modules started to be laid again. In 2000, Japanese manufacturer
Hitachi
() is a Japanese Multinational corporation, multinational Conglomerate (company), conglomerate founded in 1910 and headquartered in Chiyoda, Tokyo. The company is active in various industries, including digital systems, power and renewable ener ...
released results of its research in BSCs with another transistor-like n
+pn
+ cell with 21.3% front and 19.8% rear efficiency. By 2003 Hitachi had developed BSC module technology that was licensed in 2006 to the US company Prism Solar. In 2004 a team led by
Prof. Andrew Blakers at the
Australian National University
The Australian National University (ANU) is a public university, public research university and member of the Group of Eight (Australian universities), Group of Eight, located in Canberra, the capital of Australia. Its main campus in Acton, A ...
published its first results on the so-called Sliver BSC technology, that had taken the design route previously proposed by Mori and also realized by IES-UPM by Sangrador and Sala, i.e., a stack of laterally connected bifacial cells requiring no metal grids, however, by then with more advanced means with which thousands of cells were micromachined out of one p-type silicon wafer. The technology was later transferred to
Origin Energy
Origin Energy Ltd is an ASX listed public company with headquarters in Sydney. It is a major integrated electricity generator, and electricity and natural gas retailer. It operates Eraring Power Station, Australia's largest coal-fired power ...
that planned large-scale manufacturing for the Australian market by 2008, but finally this never occurred due to price pressure from Chinese competition. In 2012
Sanyo
is a former Japanese electronics manufacturer founded in 1947 by Toshio Iue, the brother-in-law of Kōnosuke Matsushita, the founder of Matsushita Electric Industrial, now known as Panasonic. Iue left Matsushita Electric to start his own bu ...
(later acquired by
Panasonic
is a Japanese multinational electronics manufacturer, headquartered in Kadoma, Osaka, Kadoma, Japan. It was founded in 1918 as in Fukushima-ku, Osaka, Fukushima by Kōnosuke Matsushita. The company was incorporated in 1935 and renamed and c ...
) successfully launches industrial production of bifacial PV modules, based on its
HIT (Heterojunction with Intrinsic Thin layer) technology. By 2010,
ECN releases results on its research on BSCs, based on the by then classical p
+nn
+ Back Surface Field BSC. This technology, dubbed n-PASHA, was transferred to the leading Chinese PV manufacturer Yingli by 2012, that began to commercialize them under the brand name Panda. Yingli was at that time the no.1 PV producer holding 10% of the world's shipments, and this technology transfer by ECN can be considered a milestone in the ultimate coming of age of BSCs, in which the technology is picked up by the, by then, mighty Chinese manufacturers mainly responsible for the steep decrease experienced PV prices since the beginning of the 2010s.
By 2020, the ENF Solar directory of solar companies lists 184 producers of bifacial solar panels, and according to the International Technology Roadmap for Photovoltaics, they held a 20% share of the overall PV market and its forecast is that this share will rise to 70% by 2030. When looking back on the development history of the BSC, it seems clear that fully industrializing the monofacial PV solar cells and the development of its nowadays booming market, was a necessary condition for BSCs to become a next step in the advancement of PV solar cell technology, with a solar market and industry that can thus make the most of its performance advantages.
Current bifacial solar cells

Several in-depth reviews on bifacial solar cells and their technology elements cover the current state-of-the-art. They summarize the most common BSC designs currently being marketed and then provide a review of their technological aspects.
BSC types in the market
Various bifacial PV modules with different architectures for their BSCs are currently available in the PV market. These include Passivated Emitter Rear Contact (PERC), Passivated Emitter Rear Locally-diffused (PERL), Passivated Emitter Rear Totally diffused (PERT), Heterojunction with Intrinsic Thin-layer (HIT), Interdigitated Back Contact (IBC).
PERT
* Efficiency: 19.5–22% (front), 17–19% (rear)
* Bifaciality: 80–90%
* Mostly commercialized (e.g.
Yingli
Yingli (), formally Yingli Green Energy Holding Company Limited () - . Yingli Green Energy Holding Company Limited, known as "Yingli Solar," is a solar panel manufacturer. Yingli Green Energy's manufacturing covers the photovoltaic value chain ...
,
Trina
Katrina Laverne Kearse (née Taylor; born December 3, 1974), is an American rapper. She rose to prominence in the late 1990s for her collaborations with Trick Daddy on the singles " Nann Nigga", " Shut Up", and " Take It to da House". In 2000, ...
,
LG) on the n-type c-Si wafer due to longer carrier lifetime than p-type and absence of boron in the bulk material avoiding light induced degradation (LID).
PERL
* Efficiency: 19.8% (front)
* Bifaciality: 80–90%
* Mainly based on p-type c-Si wafer
* Boron is locally diffused into contact areas at the rear side
PERC
* Efficiency: 19.4–21.2% (front), 16.7–18.1% (rear)
* Bifaciality: 70–80%
* Mostly commercialized (e.g.
JA Solar
JA Solar Holdings Co., Ltd. is a solar development company founded in Yangpu district, Shanghai. They design, develop, manufacture and sell solar cell and solar module products and are based in the People’s Republic of China. The company is a ...
,
LONGi
Longi ( Sicilian: ''Lonci'') is a ''comune'' (municipality) in the Metropolitan City of Messina in the Italian region Sicily, located about east of Palermo and about west of Messina.
Longi borders the following municipalities: Alcara li F ...
,
Trina
Katrina Laverne Kearse (née Taylor; born December 3, 1974), is an American rapper. She rose to prominence in the late 1990s for her collaborations with Trick Daddy on the singles " Nann Nigga", " Shut Up", and " Take It to da House". In 2000, ...
) e.g. on the p-type c-Si wafer
IBC
* Efficiency: 23.2%
* Bifaciality: 70–80%
* Mainly based on n-type c-Si wafer
* No metal grid contact on the front side
HIT
* Efficiency: 24.7%
* Bifaciality: 95–100%
* Mostly commercialized (e.g.
Panasonic
is a Japanese multinational electronics manufacturer, headquartered in Kadoma, Osaka, Kadoma, Japan. It was founded in 1918 as in Fukushima-ku, Osaka, Fukushima by Kōnosuke Matsushita. The company was incorporated in 1935 and renamed and c ...
,
Hanergy
Hanergy Holding Group Ltd. (Hanergy) is a Chinese multinational company headquartered in Beijing. The company is focusing on thin-film solar value chain, including manufacturing and solar parks development. It also owns the Jinanqiao Hydroelectr ...
) on n-type c-Si wafer
Technology aspects
Silicon wafers have traditionally been used as cell substrates, although other materials have been proposed and proven. The thickness of the substrate has an essential impact on material costs; thinner wafers mean savings, but at the same time, they make handling more difficult and costly or impact the throughput. Also, thinner substrates can improve efficiency due to the reduction of bulk
recombination.
While monofacial cells require only one
diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
step when forming their single p-n junction, BSCs require two p-n junctions with different
dopant
A dopant (also called a doping agent) is a small amount of a substance added to a material to alter its physical properties, such as electrical or optics, optical properties. The amount of dopant is typically very low compared to the material b ...
s which increase the number of high temperature processes in the manufacturing and, therefore its cost. Co-diffusion is one option to simplify this process, consisting in the pre-deposition and doping of
boron
Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three ...
and
phosphorus
Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...
on both sides of the cell simultaneously; however, it requires controlling there will be no cross-doping. Another cost-saving option is to build the p-n junctions using
ion implantation
Ion implantation is a low-temperature process by which ions of one element are accelerated into a solid target, thereby changing the target's physical, chemical, or electrical properties. Ion implantation is used in semiconductor device fabrica ...
instead of diffusion.
As in monofacial cells, front contacts in BSCs cells are mainly silver screen printed that become, due to the silver content, one of its important cost elements. Research is conducted to replace screen printed silver contacts with copper-plated contacts,
TCOs, or aluminum. However, the most feasible so far has been to reduce the amount of screen printing paste by using front busbar-less solar cells with very thin contact wires.
In BSCs recombination at the metal-semiconductor interface in the rear surface is reduced when compared to monofacial cells, due to the former restricting this interface to that of the rear surface metal grid. However, passivation of silicon surfaces is still needed and its area extended by that of the rear surface. Again the target is to reduce the temperature of the manufacturing processes involved. Traditionally passivation was obtained by thermal oxidation (
SiO2); however, this requires over 1000C temperature. Currently, silicon surface passivation is achieved by putting silicon nitride (
SiNx) on both sides of the cell by means of plasma-enhanced chemical vapor deposition (
PECVD
Plasma-enhanced chemical vapor deposition (PECVD) is a chemical vapor deposition process used to deposit thin films from a gas state (vapor) to a solid state on a substrate (materials science), substrate. Chemical reactions are involved in the pr ...
), which requires 400C. Lower deposition temperatures of ~225C can be achieved by passivating with hydrogenated
amorphous silicon
Amorphous silicon (a-Si) is the non-crystalline form of silicon used for solar cells and thin-film transistors in LCDs.
Used as semiconductor material for a-Si solar cells, or thin-film silicon solar cells, it is deposited in thin films onto ...
, a-Si:H.
Bifacial solar cell performance parameters
The efficiency of BSCs is usually determined by means of independent efficiency measurements of the front and rear sides under one sun. Sometimes, the BSC is characterized using its ''equivalent efficiency,'' defined as the efficiency of a monofacial cell able to render the same power per unit area as the bifacial cell at the same test conditions. Alternatively, the equivalent efficiency has been defined as the sum of the front and rear side efficiencies weighted by the relative amounts of irradiance on both sides.
Another related parameter is the ''Bifaciality Factor,'' defined as the ratio of the front and rear efficiencies when illuminated and measured independently:
Also specific to BSCs is the ''Separation Rate'', that intends to measure the ''Bifacial Illumination Effect'' predicted by McIntosh et al. in 1997 by which, the electrical output of BSCs operating under bifacial illumination would not necessarily equal the sum of the front-only and rear-only electrical output, i.e. it is not merely a linear combination of the monofacial characteristics:
Typically ''X'' represents one of the cell characteristic parameters such as the short circuit current ''J
sc'', the peak power ''P''
max or the efficiency ''η.'' Furthermore, to characterize BSC operation under simultaneous front and rear irradiation, the irradiance gain, ''g'', defined as:
so that
and a bifacial ''1.x'' ''Efficiency'' can be defined as the efficiency obtained under a simultaneous irradiance of a certain amount on the front face and ''x'' times this amount on the rear side of the BSC. Then the actual gain of a BSC with respect to a monofacial one can be expressed through the ''Gain-Efficiency Product,'' which is the product of the irradiance gain ''g'' and the bifacial ''1.x Efficiency.''
See also
*
Vertical axis solar tracker
External links
Up-to-date information on bifacial solar panels(Open source)
References
{{Reflist, refs=
Energy conversion
Semiconductor devices
Photovoltaics