Tandem Solar Cell Achieves Record Efficiency

A new tandem solar cell featuring monolithic perovskite and silicon has produced electricity with record efficiency, reports Helmholz-Berlin.

“Teams from the Helmholtz-Zentrum Berlin and École Polytechnique Fédérale de Lausanne, Switzerland, have been the first to successfully combine a silicon heterojunction solar cell with a perovskite solar cell monolithically into a tandem device.”

THZB reports this hybrid tandem cell has shown an efficiency of 18%, and while not the highest solar cell efficiency rating, this is the highest reported value for this particular  type of cell architecture.

Organic-inorganic perovskite materials are said to represent a great step forward in ongoing solar cell research. Increases in efficiency continue to be reported for perovskite solar cells, which can be manufactured from solution and cost-effectively printed on large surface areas.

tandem solar cell schichtaufbau
A heterojunction silicon cell provides the base for the tandem cell. A very thin layer of transparent tin dioxide was deposited on this bottom cell, followed by 500 nm of perovskite as well as 200 nm of spiro-OMeTAD hole-conductor material. Thin MO3 serves as a protective layer between this hole conductor and the transparent top electrode of ITO. Image: S. Albrecht / HZB

Explaining Randem Cell Technology

Perovskite layers are known to efficiently absorb light in the blue region of the spectrum. But combining these with silicon layers, which convert long-wavelength red and near-infrared light, has proven to be difficult.

HZB writes, “This is because for high-efficiency perovskite cells, it is usually required to coat the perovskite onto titanium dioxide layers that must be previously sintered at about 500 degrees Celsius. However, at such high temperatures, the amorphous silicon layers that cover the crystalline silicon wafer in silicon heterojunction degrades.”

The team behind this project was led by Professor Bernd Rech and Dr. Lars Korte from the HZB Institute for Silicon Photovoltaics, working in cooperation with HZB’s PVcomB and a group headed by Professor Michael Graetzel at the École Polytechnique Fédérale de Lausanne (EPFL).

This group is the first to have fabricated this kind of monolithic tandem cell,  depositing a layer of tin dioxide at low temperatures — to replace the standard titanium dioxide. A thin layer of perovskite was then spin-coated onto this intermediate layer and covered with hole-conductor material. In addition, a crucial element in the device architecture is the transparent top contact. Typically,  metal oxides are deposited by sputtering, but this would destroy the sensitive perovskite layer, as well as the hole-conductor material. Therefore, the team from HZB modified the fabrication process and incorporated a transparent protective layer.

Understanding Perovskites

A number of chemical researchers believe perovskites open the door to a new era of high-efficiency, low-cost solar cells. If this is true, the economic impact on the photovoltaics market and the solar industry will be felt worldwide.

As CleanTechnica’s Tina Casey puts it: “Perovskites are easily synthesized, and their distinctive crystalline structure makes them a perfect match for the development of efficient solar cells that can beat the current gold standard, which is silicon.”

18% and High Open-Circuit Voltage

According to the research, this tandem cell attained an 18% efficiency level — nearly 20% higher than the efficiency of individual cells. The open-circuit voltage is 1.78 volts. “At that voltage level, this combination of materials could even be used for the generation of hydrogen from sunlight,” said Dr. Steve Albrecht, lead author of the paper that has now appeared in the journal Energy & Environmental Science.

Additional light catching structures may increase efficiencies

Right now the perovskite-silicon tandem cell is being fabricated on a polished silicon wafer. Efficiencies may change dramatically, states Dr. Lars Korte, head of the silicon heterojunction solar cell group at the Institute for Silicon Photovoltaics, if the wafer surface is textured using light-trapping features, like random pyramids. “The efficiency might be increased further to 25 or even 30%,” she added.

Beyond the discovery stage comes integration into existing silicon technologies

More important than maximizing efficiencies will be how this technology is integrated into existing technologies, pointed out Professor Bernd Rech;

“Silicon technology currently dominates 90% of the market, which means there are many established production facilities for silicon cells. The perovskite layers could considerably increase the efficiency level. To achieve this, the fabrication techniques only need to be supplemented with a few more production steps. For that reason, our work is also extremely interesting for industry. However, the problems of long-term stability and the lead content of perovskite solar cells still need to be solved in future research.”

Images via HZB

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