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%). 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 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 Besides Isofoton, some other PV manufacturers, however, specialized in space applications, reported developments of BSCs at a laboratory scale such as
COMSAT in 1980, Solarex in 1981 or
AEG Telefunken 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-grown
silicon nitride, 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, produced n+pn+ 4 cm2 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, 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, 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 (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 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 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 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 (later acquired by
Panasonic) 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 ==