Bifacial solar cells
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.
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.
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.
[11][12] 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.
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".
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%).
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.
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.
At that time, the market of terrestrial photovoltaic power plants, to which Isofoton oriented its production, essentially consisted of demonstration projects.
[30] 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.
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.
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.
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.
[35] 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.
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.
[38] 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.
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.
[41] 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.
[42][43] 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.
[44] 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.
[47] 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.
Co-diffusion is one option to simplify this process, consisting in the pre-deposition and doping of boron and phosphorus on both sides of the cell simultaneously; however, it requires controlling there will be no cross-doping.
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.
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:[52][53]
Typically X represents one of the cell characteristic parameters such as the short circuit current Jsc, the peak power Pmax or the efficiency η.
