BSC – Bifacial Solar Cells: History of the bifacial or two-surface solar cell

Published on: January 22, 2022 / Update from: February 24, 2023 - Author: Konrad Wolfenstein

History of the bifacial or two-surface solar cell - Image: Xpert.Digital / Sunward Art|Shutterstock.com

A silicon solar cell was first patented by Russell Ohl in 1946 while working at Bell Labs and publicly demonstrated by Fuller, Chapin, and Pearson at the same research facility in 1954; However, these initial proposals involved monofacial cells, the back of which was not intended to be active.

The first theoretically proposed bifacial solar cell can be found in a Japanese patent with a priority date of October 4, 1960 by Hiroshi Mori, who worked for the company Hayakawa Denki Kogyo Kabushiki Kaisha (Hayakawa Electric Industry Co. Ltd.), which later developed into today's Sharp Corporation . The proposed cell was a PNP dual cell structure with contact electrodes on two opposite edges.

However, the first demonstrations of bifacial solar cells and panels were carried out as part of the Soviet space program in the military LEO space stations Salyut 3 (1974) and Salyut 5 (1976). These bifacial solar cells were developed by Bordina et al. developed and manufactured at VNIIT (All Union Scientific Research Institute of Energy Sources) in Moscow, which became the Russian solar cell manufacturer KVANT in 1975. In 1974, this team filed a US patent proposing the cells in the form of mini-parallel pipes with a maximum size of 1 mm x 1 mm x 1 mm, connected in series to provide 100 cells/cm2 . As with today's BSCs, they proposed the use of isotypic compounds pp+ near one of the light-receiving surfaces. In Salyut 3, small experimental panels with a total cell area of ​​24 cm2 showed an increase in energy production per satellite revolution due to Earth's albedo by up to 34% compared to the monofacial panels of the time. During the flight of the Salyut 5 space station, a gain of 17-45% was observed using bifacial panels (0.48 m2 - 40 W).

In parallel with this Russian research, on the other side of the Iron Curtain, the semiconductor laboratory of the School of Telecommunication Engineering of the Technical University of Madrid, led by Professor Antonio Luque, is independently carrying out a comprehensive research program to develop industrially applicable bifacial solar cells. While Mori's patent and the VNIIT-KVANT prototypes in the spacecraft were based on tiny cells with no metal grid on the surface and were therefore intricately connected, more in the style of the microelectronic devices then in their early stages, Luque will have two Spanish patents in in 1976 and 1977 and one in the United States in 1977, which were precursors to modern bifacial cells. Luque's patents were the first to propose BSCs with one cell per silicon wafer, as was the case with monofacial cells then and still is today, with metal grids on both surfaces. They considered both the npp+ structure and the pnp structures.

The development of BSCs in the semiconductor laboratory was approached in a three-fold approach, resulting in three doctoral theses, authored by Andrés Cuevas (1980), Javier Eguren (1981) and Jesús Sangrador (1982), the first two supervised by Luque and the third by Dr. Gabriel Sala from the same group. Cuevas' doctoral work consisted of the construction of Luque's first patent in 1976, which was called a "transcell" due to its transistor-like npn structure. In his dissertation, Eguren addressed the demonstration of Luque's second patent from 1977 with an npp+ doping profile in which the pp+ isotopic junction lies adjacent to the back of the cell, creating what is commonly referred to in solar cell technology as " back surface field (BSF). This work led to several publications and additional patents. In particular, the beneficial effect of reducing p-doping in the base, where the reduction in voltage in the emitter junction (front pn junction) was compensated by a voltage increase in the rear isotypic junction, while at the same time allowing a higher diffusion length of minority carriers, which increases the power output with bifacial lighting. Sangrador's dissertation and third path of development at the Technical University of Madrid proposed the so-called vertical edge-lit multiple junction solar cell, in which p+nn+ are stacked and connected in series and illuminated from their edges, which are high-voltage cells that do not require a surface metal grid require electricity generation.

In 1979, the semiconductor laboratory was transformed into the Institute of Solar Energy (IES-UPM), which, with Luque as the first director, continued intensive research on bifacial solar cells until the first decade of the 21st century. For example, in 1994, two Brazilian graduate students at the Institute of Solar Energy, Adriano, developed and produced Moehlecke and Izete Zanesco, together with Luque, developed a bifacial solar cell that performed 18.1% on the front and 19.1% on the back; a bifacial record of 103% (at the time the record efficiency for monofacial cells was just under 22%).

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