Scientists on both sides of the Atlantic Ocean are working on higher-efficiency perovskite-based solar cells. In the U.S., Solar-Tectic LLC (Briarcliff Manor, N.Y.) has produced silicon- and perovskite-based solar cells. Neuchâtel (located in Switzerland) scientists have done the same.

Perovskite is a material with a unique crystal structure that has attracted attention in the solar-cell universe. Improved solar cells use a perovskite-structured hybrid organic-inorganic material as a light-harvesting active layer. Metal halide perovskites provide a high absorption coefficient when using ultrathin films of around 500 nm to absorb the complete visible solar spectrum.

Perovskites take their name from the mineral, which was first discovered in the Ural mountains of Russia by Gustav Rose in 1839 and is named after Russian mineralogist L.A. Perovski (1792–1856). The general chemical formula for perovskite compounds is ABX3, where “A” and “B” are two cations of very different sizes, and “X” is an anion that bonds to both.

Perovskite solar cells hold an advantage over traditional silicon solar cells in the simplicity of their processing. Traditional silicon cells require expensive, multistep processes, conducted at high temperatures (>1000°C) in a high vacuum in special clean-room facilities. On the other hand, the organic-inorganic perovskite material can be manufactured with simpler wet chemistry techniques in a traditional lab environment.

Solar-Tectic LLC has obtained U.S. patents 9,978,532 “Maximizing the Power Conversion Efficiency of a Tin Perovskite/Silicon Thin-Film Tandem Solar Cell” and U.S. ser. 15/157,539 “Methods of Growing Heteroepitaxial Single Crystal or Large Grained Semiconductor Films and Devices Thereon.” They’re part of a “Tandem Series” of high-efficiency and cost-effective solar-cell technologies with the potential of surpassing the efficiencies of current thin-film solar-cell technologies like CdTe, CIGS, and a-Si, as well as other silicon technologies based on poly and monocrystalline wafers.

In Neuchâtel, researchers from EPFL and CSEM have combined silicon- and perovskite-based solar cells. Their resulting efficiency of 25.2% is said to be a record for this type of tandem cell. Their innovative, yet simple, manufacturing technique could be directly integrated into existing production lines, and efficiency could eventually rise above 30%.

EPFL is École polytechnique fédérale de Lausanne, a research institute and university in Lausanne, Switzerland, that specializes in natural sciences and engineering.  The Swiss Center for Electronics and Microtechnology (CSEM) is a research and development center (public-private partnership) specializing in microtechnology, nanotechnology, microelectronics, system engineering, photovoltaics, and communications technologies.

Silicon Cells

In the field of photovoltaic technologies, silicon-based solar cells now make up about 90% of the market. In terms of cost, stability, and efficiency (20-22% for a typical solar cell), they’re well ahead of any competition.

One solution is to place two different types of solar cells on top of each other to maximize the conversion of light rays into electrical power. These “double-junction” cells are being widely researched in the scientific community, but are expensive to make. The research teams from EPFL and CSEM have found an economically competitive solution, though.

They integrated a perovskite cell directly on top of a standard silicon-based cell, obtaining an efficiency of 25.2%. Their production method is promising, because it would add only a few extra steps to the current silicon-cell production process, and the cost would be reasonable. Their research has been published in Nature Materials.

Perovskite-on-Silicon

Perovskite’s unique properties have ramped up research into its use in solar cells over the last few years. In the space of nine years, the efficiency of these cells has risen by a factor of six. By implementing perovskite, high conversion efficiency now can be achieved at a potentially limited production cost.

Silicon’s pyramids are covered with perovskite. (Courtesy of EPFL)

In tandem cells, perovskite complements silicon. It converts blue and green light more efficiently, while silicon is better at converting red and infrared light. “By combining the two materials, we can maximize the use of the solar spectrum and increase the amount of power generated. The calculations and work we have done show that a 30% efficiency should soon be possible,” say Florent Sahli and Jérémie Werner, the Nature Material article’s main authors.

Creating an effective tandem structure by superposing the two materials is no easy task. “Silicon’s surface consists of a series of pyramids measuring around 5 microns, which trap light and prevent it from being reflected. In addition, silicon’s surface texture makes it hard to deposit a homogeneous film of perovskite,” explains Quentin Jeangros, who co-authored the Nature Materials paper. When the perovskite is deposited in liquid form, as it usually is, it accumulates in the valleys between the pyramids while leaving the peaks uncovered, leading to short circuits.

Scientists from EPFL and CSEM have circumvented that problem by using evaporation methods to form an inorganic base layer that fully covers the pyramids. That layer is porous, enabling it to retain the liquid organic solution, which is then added using a thin-film deposition technique called spin-coating. The researchers subsequently heat the substrate to a relatively low temperature of 150°C to crystallize a homogeneous film of perovskite on top of the silicon pyramids.

“Until now, the standard approach for making a perovskite/silicon tandem cell was to level off the pyramids of the silicon cell, which decreased its optical properties and therefore its performance, before depositing the perovskite cell on top of it. It also added steps to the manufacturing process,” says Florent Sahli.

The new type of tandem cell is highly efficient and directly compatible with monocrystalline silicon-based technologies, which benefit from long-standing industrial expertise and are already being produced profitably. “We are proposing to use equipment that is already in use, just adding a few specific stages. Manufacturers won’t be adopting a whole new solar technology, but simply updating the production lines they are already using for silicon-based cells,” explains Christophe Ballif, head of EPFL’s Photovoltaics Laboratory and CSEM’s PV-Center.

Research continues on in efforts to further increase efficiency and give the perovskite film more long-term stability. Although the team has made a breakthrough, there’s still work to be done before their technology can be adopted commercially.

The research has been financed by Nano-Tera.ch’s Synergy project, the Swiss Federal Office of Energy (grant SI/501072-01), the Swiss National Science Foundation through the Sinergia Episode project (CRSII5_171000) and the NRP70 Energy Turnaround PV2050 project (407040), and the European Union through the Horizon 2020 innovation and research program (CHEOPS project 653296).

Perovskite Patents

In June 2018, Solar-Tectic LLC (“ST”) announced that two U.S. patent applications for perovskite thin-film solar cells have been granted. One patent covers all kinds of perovskites. The inventors are Ashok Chaudhari, Founding Manager of Solar-Tectic, and the late Dr. Praveen Chaudhari, a renowned materials physicist.

Recently, perovskite materials have gained much attention as a promising solution to the long-standing problem of solar-cell efficiency, which is of primary importance in today’s solar-panel industry. And while numerous reports of perovskite/silicon (wafer) tandem solar cells (and extensive intellectual property) have surfaced, not one has mentioned a perovskite/crystalline silicon thin-film tandem solar cell, until Solar-Tectic LLC’s breakthrough publication.

Wafer-sized bottom poly- and monocrystalline silicon layers in PERC, PERL, HIT, HJ, or perovskite/silicon tandem cells are typically 200-280 microns thick, whereas ST’s thin-film crystalline inorganic bottom layers can be as thin as 20-30 microns with the same or similar efficiency. Moreover, they can be processed at much lower temperatures, thereby significantly reducing costs of production.

The top perovskite layer is less than only 1 micron—an ultra-thin film—and a thin-film crystalline silicon (CSiTF) bottom layer decouples the need for a silicon wafer. If the price of polysilicon rises, less silicon material usage will be an additional cost savings.

Tandem perovskite solar cells are capable in theory of 45% efficiency, though ST has set a more realistic 30% efficiency goal, higher than the best silicon wafer technologies such as PERC, PERL, HIT, HJ cells with 25-26.6% efficiencies. The efficiencies of today’s solar cells generally range from 14%-25%. A cost-effective 30% efficient solar cell with a simple design would revolutionize the solar energy industry by dramatically reducing the balance of system (BoS) costs, thereby significantly lessening the need for fossil-fuel-generated electricity. Silicon-wafer technology based on polycrystalline or monocrystalline silicon, which is 90% of today’s market, would become obsolete.

Importantly, the entire ST process is environmentally friendly: Non-toxic tin (Sn) or gold (Au) is used to deposit the crystalline silicon thin-film material for the bottom layer in the tandem/heterojunction configuration as well as in the top, perovskite layer. The more commonly used toxic lead (Pb) isn’t used in the perovskite. The manufacturing methods employed in this technology—sputtering or electron beam evaporation—are conventional and similar to those used in today’s thin-film solar cell industry, and, importantly, in the display industry, where there’s much overlap and potential for synergy.

The breakthrough patents correspond to a “Tandem Series” of solar-cell technologies launched by ST. That includes a variety of different proven semiconductor photovoltaic materials (i.e., III-V, CZTS, a-Si, etc.) for the top layer on silicon (or germanium) bottom layer, on various substrates such as cheap soda-lime glass. A paper reporting a successful step in this approach was recently published. Last year, ST announced the first patent ever granted for this perovskite/silicon thin-film tandem approach.

A patent for a copper-oxide thin-film tandem solar cell was also granted to ST (US 9,997,661) in June 2018, thereby expanding the IP portfolio of the tandem series.

R&D of the new perovskite thin-film tandem technology began last year at Blue Wave Semiconductors, Inc., under the direction of Dr. Ratnakar D. Vispute.

Reference:

F. Sahli, J. Werner, B. A. Kamino, M. Bräuninger, R. Monnard, B. Paviet-Salomon, L. Barraud, L. Ding, J. J. Diaz Leon, D. Sacchetto, G. Cattaneo, M. Despeisse, M. Boccard, S. Nicolay, Q. Jeangros, B. Niesen, and C. Ballif, “Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency,” Nature Materials, 2018. DOI: 10.1038/s41563-018-0115-4.