solar cell

Graphene/silicon heterojunction solar cell with 18.8% efficiency

hotoluminescence intensity map of a 3 × 3 cm2 solar cell without (left) or with (right) GO:Nafion film. Image: Hebei University, Advanced Materials Interfaces, Creative Commons License CC BY 4.0

A Chinese-German research group developed the cell with an ink of graphene oxide (GO) mixed with Nafion that can be spin-coated on an n-type silicon wafer to form a high-quality passivating contact scheme. The GO:Nafion layer simultaneously creates a p–n junction with silicon and passivates the surface defects at the GO:silicon interface.

From pv magazine

An international research group has unveiled a heterojunction solar cell based on graphene-oxide (GO) and silicon with a large area of 5.5 cm2.

GO is a compound of carbon, oxygen and hydrogen that is obtained by treating graphite with oxidizers and acids. It consists of a single-layer sheet of graphite oxide that is commonly used to produce graphene-family nanomaterials for various applications, including electronics, optics, chemistry and energy storage.

The scientists developed an ink of GO mixed with Nafion that can be spin-coated on an n-type silicon wafer to form a high-quality passivating contact scheme. “Low interface recombination is provided by the Nafion and carrier selection by the GO,” they explained, noting that the passivation scheme also includes an electron-selective passivation contact comprising n-doped hydrogenated amorphous silicon with an indium tin oxide (ITO) overlayer aimed at improving light trapping and reducing surface recombination.

“Graphene was also shear force mixed in Nafion at a concentration of 8 mg mL–1,” they further explained. “Either of these inks were then spin-coated onto the back of the Si wafer, a thin film of Ag was evaporated on top of this and finally an electrical Ag paste was applied for encapsulation and to block the ingress of small quantities of water. Atomic force microscopy (AFM) revealed that the GO:Nafion layers completely covered the Si surface and a root mean roughness of 89 nm was recorded.”

According to the researchers, the GO:Nafion layer simultaneously creates a p–n junction with silicon and passivates the surface defects at the GO:Si interface. The graphene-silicon solar cell is reportedly able to achieve a power conversion efficiency of 18.8%. “Pseudo JV curve shows a pseudo fill factor of 80.6% without the serious resistance effect, and potentially predicts that an efficiency of 21.59% could be achieved with further optimization.”

They presented the solar cell in the paper “High-Efficiency Graphene-Oxide/Silicon Solar Cells with an Organic-Passivated Interface,” published in Advanced Materials Interfaces. The research team comprises academics from the Karlsruhe Institute of Technology (KIT) in Germany, the Hebei University in China, and Chinese module manufacturer Yingli Green Energy Holding Co., Ltd. “The scalable fabrication and good wettability of the GO:Nafion ink provides a favorable direction toward development of carbon-based PV in the future,” they concluded.


Large-area organic solar cell with 14.7% efficiency

Image: KIST

Scientists in Korea built an organic solar cell that is reportedly able to prevent aggregation in photoactive layers. The device could be used for applications in buildings, vehicles, and the Internet of Things.

From pv magazine

Scientists at the Korea Institute of Science and Technology (KIST) have fabricated an organic solar cell based on polymer additives that they claim are able to solve the performance degradation issue of large-area organic solar cells.

“The spin coating method, a solution process mainly used in the laboratory research stage, creates a uniform photoactive layer mixture as the solvent evaporates rapidly while the substrate rotates at a high speed,” the researchers explained. “However, the large-area, continuous solution process designed for industrial use caused solar cell performance deterioration because the solar cell material solution’s solvent evaporation rate was too slow. Consequently, unwanted aggregation between the photoactive materials can be formed.”

The Korean group designed the cell with ternary photoactive layers that contain polymer additives and, as a result, prevent aggregation in photoactive layers. It also engineered nano-level structure control to improve sunlight trapping.

The solar cell achieved a power conversion efficiency of 14.7%, which the academics said is 23.5% higher than that of a conventional binary system. The device was also able to retain 84% of its initial efficiency after 1,000 hours at a temperature of 85 C.

“We have gotten closer to organic solar cell commercialization by proposing the core principle of a solar cell material capable of high-quality, large-area solution processing,” said KIST researcher Hae Jung Son. “Commercialization through follow-up research will make eco-friendly self-sufficient energy generation possible that is easily applicable to exterior building walls and automobiles and also utilized as an energy source for mobile and IoT devices.”


Novel busbar-free cell design for shingled solar modules

Electrode patterns of a solar cell for a shingled PV module. Image: Sungkyunkwan University

South Korean scientists have fabricated a busbar-free solar cell for shingled modules that uses 60% less silver than its busbar counterparts. A module with the new cells had almost the same performance as a reference shingled panel built with a conventional cell design.

From pv magazine

Scientists at Sungkyunkwan University (SKKU) in South Korea have developed a busbar-free solar cell design for shingled solar modules to reduce silver (Ag) paste consumption. They designed the cell with a busbar-free electrode pattern, with the electrodes consisting of Ag fingers and busbars on the front side and aluminum (Al) electrodes and Ag busbars on the rear side.

“The new electrode pattern is expected to significantly reduce the manufacturing cost of high-density shingled photovoltaic modules,” researcher Jaehyeong Lee told pv magazine.

The fingers collect the current from the front side of the solar cell and the current flows to the busbar, where it can flow to the outside circuitry. On the cell’s rear side, the Al electrode moves the current to the silver pad, and the current can then flow through the soldered metal ribbon. The cell measures 156.75 mm × 156.75 mm and relies on 100 fingers, each with a width of 50 μm.

“The conventional electrode pattern has five front and rear busbars, each with a 1 mm width, while the cost-efficient design has no busbar,” the researchers said.

They used laser scribing and mechanical breaking to assemble the cells in cell strips and an electrically conductive adhesive from German manufacturer Henkel to bond the cells in strings, with each string containing 15 cell strips.

“The cell strips at both edges of the string have a pattern with busbars since the shingled string should be interconnected to manufacture a PV module,” they said, noting that the shingled strings were connected to each other via a tabbing and busing process. “The manufacturing method is the same as that of the conventional module, except for the laser scribing and ECA bonding processes to produce a shingled string.”

The research team assembled the strings in an experimental laminated and framed shingled PV module. After a series of measurements, they found that its performance was almost the same as that of a reference shingled panel built with a conventional cell design.

“The difference in the output power between the two types of modules is only 0.5 W, which could be negligible given the measurement error range,” they said, adding that the main advantage of the busbar-free cell is a 61.33% reduction in silver consumption.

The South Korean group described the cell design in “Busbar-free electrode patterns of crystalline silicon solar cells for high density shingled photovoltaic module,” which was recently published in Solar Energy Materials and Solar Cells. “The most important advantage of the busbar-free electrode pattern presented in this work is the cost reduction,” they said. 


Applying perovskite solar cells onto BIPV steel products

A perovskite solar cell developed at Swansea University. As part of the new partnership with Tata Steel, Swansea scientists will investigating applying perovskite solar cells onto steel products. Image: Swansea University

Swansea University will collaborate with Indian multinational Tata Steel to investigate perovskite solar cell materials that could be applied directly onto coated steel to make building-integrated PV components. The partnership will add to an “active buildings” project that the Welsh university has been running for several years.

From pv magazine

This week, Swansea University and Tata Steel launched a new collaboration that will see the university look to develop solar cell materials that can be applied directly to the latter’s coated steel products and used to make building components that also generate electricity.

Debashish Bhattacharjee, Vice President New Materials Business visited Swansea’s Bay Campus earlier this week to sign a memorandum of understanding for the three-year research project, which is part of a broader collaboration between the two, focused on reducing the environmental impacts of the steel industry.

“The future is about solar energy technology being built in, not added on afterwards. These printable solar cells can be built into the fabric of our homes, shops and offices, allowing them to generate the power they need, and more besides,” said Dave Worsley, head of materials science and engineering at Swansea University. We know the concept works as we’ve demonstrated it in our Active Buildings in sunny Swansea. This new collaboration with Tata Steel will enable us to develop its potential more quickly, identifying new types of steel products that actively work to generate electricity.”

The research will be conducted as part of Swansea’s “active buildings” project, which demonstrates a range of energy generation, storage, and efficiency technologies integrated into classroom and office buildings at the university campus.

Swansea University will lend its experience with perovskite materials and solar cell printing processes to the project, while Tata will focus on material supply chains, as well as adapting it steel coatings to have solar cells deposited on top of them. With the university located close to Swansea’s Port Talbot Steelworks, the two have collaborated on various projects in the past focused on both steel production and renewable energy.

“We are buoyant with the possibilities that the perovskite technology brings to the table – especially in integration to the building and construction solutions – across different value streams in Tata Steel,” said Sumitesh Das, UK R&D director for Tata Steel. “The combination of a ‘green’ solar technology with steel is a significant step in our net zero ambitions.”


Four-terminal multijunction approach for bifacial modules

A scheme of the 4T system. Image: Institute for Microelectronics and Microsystems

Scientists tested bifacial PV devices based on silicon and gallium arsenide cells connected in a four terminal layout. They note that the four terminal design offers significant advantages over more common two terminal devices, allowing for a 17% (relative) increase in efficiency thanks to better absorption of light reaching the rear side.

From pv magazine

A research team from the Institute for Microelectronics and Microsystems (IMM) of the Italian National Research Council (CNR) has developed a four-terminal bifacial multijunction PV device consisting of two mini modules based on gallium arsenide (GaAs) and a silicon heterojunction technology, respectively.

“Our device was not designed to achieve the maximum possible power conversion efficiency, but represents a prototype to study the applicability of the 4T multijunction approach and its compatibility with bifacial modules,” research co-author Andrea Scuto, told pv magazine. “It is suitable to illustrate the advantages of the 4T architecture including the bifacial technology already in production in Italy,” he added referring to the module production of Enel Green Power, which also partnered in the research.

In the paper “Outdoor performance of GaAs/bifacial Si heterojunction four-terminal system using optical spectrum splitting,” published in Solar Energy, the scientists explained that in the 4T design the bottom and top cells are not directly stacked and connected in series, but are connected in parallel. This architecture reportedly offers the advantage of considerable intrinsic robustness to variations in the solar spectrum.

“For such reason, the 4T configuration is much more robust than the tandem to changes of solar spectrum or to the bifaciality use,” the researchers explained. “4T devices also have a higher degree of flexibility for the choice of materials that best absorb the incident spectrum since no direct cell stacking and short-circuit current matching is necessary. Only the voltage matching of the bottom and top PV module has to be performed.”

The prototype was built with a dichroic mirror on the front of the device that splits the incident light. In this way, the GaAs mini-module is able to transform the visible portion of the solar spectrum, while the bifacial SHJ cell collects the incident infrared light on the front, and the light reflected from the ground on the rear of the device.

The system was tested in outdoor conditions and, throughout a full day, it exhibited a power conversion efficiency ranging from 21.3% to 25.4%, with the efficiency raising by 17% (relative) when it went from monofacial to bifacial operation. “We have acquired irradiance data and monitored the spectral distribution of the solar light during the day, observing a variation of 44% of the albedo and a significant shift of the solar spectrum towards infrared wavelengths,” the Italian group stated.

“The present system cannot effectively harvest diffuse light in the blue part of the solar spectrum due to the dichroic mirror, and the supporting structure increases the encumbrance of the system: for these reasons the device shouldn’t be considered for an industrial application. But the concept may be applied to more effective designs both in terms of form factor and cost” the authors specified.


Eco-friendly solvent for a 16.7% perovskite solar cell

A researcher at ZSW working on perovskite solar cells. Image: ZSW

Scientists in Germany looked to eliminate the use of toxic solvents in the production of perovskite solar cells, replacing them with a more environmentally material called dimethyl sulfoxide (DMSO) which has so far proved difficult to integrate into processes suitable for large-scale production. The group demonstrated a scalable blade coating process using DMSO as the only solvent, and reached cell efficiencies close to those achieved using more toxic substances.

From pv magazine

Since their manufacture requires considerably less energy than crystalline silicon solar products, perovskite solar cells are already seen by many as part of a much greener future for the solar industry. However, The use of toxic materials in solvents is one source of environmental concerns as perovskites move closer to commercial production.

The precursor materials for the perovskite solar cell have to be dissolved in a solvent to be evenly applied to a substrate. These solvents most often contain dimethylformamide (DMF), which is toxic to humans and the environment. In production, its safe treatment and disposal would lead to increased costs for the solar cells in the end.

Scientists led by Germany’s Center for Solar Energy and Hydrogen Research Baden Württemberg (ZSW) searched for a material with rare of properties of being both non-toxic and suitable as an industrial solvent, settling on dimethyl sulfoxide (DMSO), which has already been used in conjunction with other solvents in perovskite cell production, but is more challenging to use by itself.

“DMSO actually looks to be unsuitable for this coating process. It is a solvent with high surface tension and viscosity, which leaves an uneven layer deposited on the solar cell,” the group explained. “DMSO also makes it difficult to control the crystallization process, which often results in small perovskite crystals and a cell that generates less solar energy.”

Blade coating

The group’s experiments with DMSO are detailed in the paper One-Step Blade Coating of Inverted Double-Cation Perovskite Solar Cells from a Green Precursor Solvent, published in Applied Energy Materials.

By modifying the film formation and drying processes they were able to overcome these problems, “We used a surfactant made of silicon oxide nanoparticles to coat the perovskite solar cell and adapted the drying process,” said ZSW scientist Jan-Philipp Becker. “With these two improvements, the process now produces uniform layers with large crystallites.”

Using a blade coating process to apply the dissolved cell material to a glass substrate, the group achieved 16.7% efficiency, a strong result compared to the 16.9% it achieved using DMF as a solvent. The cells produced in this study measured 0.24 cm².

The group’s next challenge will be to demonstrate larger devices produced through the same process, and it is now targeting mini modules measuring 30 cm x 30 cm. “These new research findings are an important milestone on the path to industrial production,” said Becker. “Now we will further optimize the manufacturing process and produce larger modules.”