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

A silicon solar cell was first patented in 1946 by Russell Ohl while working at Bell Labs, and publicly demonstrated in 1954 by Fuller, Chapin and Pearson at the same research institution; however, these early proposals were 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 dated October 4, 1960, by Hiroshi Mori, who worked for Hayakawa Denki Kogyo Kabushiki Kaisha (Hayakawa Electric Industry Co. Ltd.), which later became the Sharp Corporation. The proposed cell was a pnp bifacial structure with contact electrodes on two opposite edges.

The first demonstrations of bifacial solar cells and panels, however, were carried out as part of the Soviet space program on the military LEO space stations Salyut 3 (1974) and Salyut 5 (1976). 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, 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 pipedes with a maximum size of 1 mm x 1 mm x 1 mm, connected in series to achieve a density of 100 cells/cm². As with today's BSCs, they proposed the use of isotypic pp+ compounds 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 orbit of up to 34% compared to the monofacial panels used at the time, due to the Earth's albedo. During the Salyut 5 space station flight, a gain of 17–45% was observed through the use of bifacial panels (0.48 m² – 40 W).

Parallel to this Russian research, on the other side of the Iron Curtain, the semiconductor laboratory of the School of Telecommunication Engineering at the Technical University of Madrid, under the direction of Professor Antonio Luque, was independently conducting a comprehensive research program to develop industrially applicable bifacial solar cells. While Mori's patent and the VNIIT-KVANT spacecraft prototypes were based on tiny cells without a metal grid on their surface and were therefore complexly interconnected, more in the style of the microelectronic devices then in their infancy, Luque would file two Spanish patents 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 at the time and still is today, with metal grids on both surfaces. They considered both npp+ and pnp structures.

The development of BSCs in the semiconductor laboratory was approached in a three-pronged manner, resulting in three doctoral dissertations by Andrés Cuevas (1980), Javier Eguren (1981), and Jesús Sangrador (1982). The first two were supervised by Luque, and the third by Dr. Gabriel Sala from the same group. Cuevas's dissertation involved the construction of Luque's first patent from 1976, which, due to its transistor-like npn structure, was called the "transcell." Eguren's dissertation focused on demonstrating Luque's second patent from 1977, featuring an npp+ doping profile where the pp+ isotope junction is located adjacent to the back surface of the cell, creating what is commonly referred to as the "back surface field" (BSF) in solar cell technology. This work led to several publications and additional patents. In particular, the beneficial effect of reducing the p-doping in the base was significant, as the reduction in voltage at the emitter junction (front pn junction) was compensated by a voltage increase at the rear isotypic junction, while simultaneously allowing for a greater diffusion length of minority carriers, which increases current output under bifacial illumination. In Sangrador's dissertation and third development approach at the Technical University of Madrid, the so-called vertical, edge-illuminated multi-junction solar cell was proposed, in which p+nn+ cells are stacked and connected in series and illuminated from their edges. These are high-voltage cells that do not require a surface metal grid for current generation.

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

 

 

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