"Monolithic multiple solar cells are regarded as the hope for the further development of today's dominant silicon solar cells, because they can realize significantly higher efficiencies for the conversion of sunlight into electricity. We consider efficiencies of 36% possible, which clearly outweighs the physical limit of a pure silicon solar cell of 29,4%, "explains Dr. Andreas Bett, Director of Fraunhofer ISE. The high efficiency makes it possible to generate more power per area and thus to save materials for solar cells and module materials - an important aspect for the sustainability of photovoltaics.

For the highly efficient multi-junction solar cell, layers of III-V semiconductors just a few micrometers thick are applied to a silicon solar cell. The different layers absorb different spectral ranges of the sunlight in order to use it optimally: gallium indium phosphide between 300 - 660 nm (visible light), aluminum gallium arsenide between 600 - 840 nm (near infrared light) and silicon between 800 - 1200 nm (longer wave light). In this way, the efficiency of silicon solar cells can be increased significantly. Since, like a normal silicon solar cell of today, they each have a contact on the front and back, the solar cells can be easily integrated into solar modules.

Bonded multiple solar cell: 34,1% efficiency

For the monolithic multi-junction solar cell, the method of direct wafer bonding known from microelectronics is used. In a first step, the III-V layers are deposited on a gallium arsenide substrate. Subsequently, the surfaces are deoxidized in a chamber under high vacuum with the aid of an ion beam and pressed together under pressure. The atoms of the III-V semiconductor layers form bonds with the silicon and form a unit. The stacked sub-cells of GaInP, AlGaAs and silicon are interconnected by tunnel diodes. Subsequently, the GaAs substrate is wet-chemically removed and a nanostructured backside contact as well as an antireflective coating and a contact grid on the front side are applied.

"Compared to previous results, the deposition conditions have been further improved and a new cell structure for the uppermost cell of gallium indium phosphide introduced, which converts the visible light even better. With 34,1%, the cell demonstrates the tremendous potential of this technology, "explains Dr. Frank Dimroth, Head of Department III-V Photovoltaics and Concentrator Technology at Fraunhofer ISE. The previous world record for this cell class was 33,3%.

Multiple solar cell with directly deposited semiconductor layers: 24,3% efficiency

Another possibility for the realization of multiple solar cells is the direct deposition of the III-V semiconductor layers (GaInP / GaAs) on the silicon solar cell. This process requires significantly fewer process steps than wafer bonding and avoids the use of the more expensive GaAs substrate, which is why it is advantageous for industrial implementation of the technology. However, the atomic structure must be very well controlled so that the gallium and phosphorus atoms at the silicon interface occupy the correct lattice sites. Defects in the semiconductor layers can also impair the efficiency of the solar cells. "Here we have made significant progress - the power generation in the three subcells hardly suffers from these defects, so we were able to realize 24,3% efficiency for this technology for the first time in the world," Dr. Frank Dimroth. "The potential is similar to that of the wafer-bound cell, and here we have some development work to do in the coming years to demonstrate this." In December 2018, Fraunhofer ISE presented such a solar cell with an efficiency record of 22,3%.

On the way to industrial mass production of monolithic multiple solar cells, the Fraunhofer ISE researchers see challenges in particular in a cost-effective process for the production of III-V semiconductor layers. Here, direct growth on silicon is currently the most promising approach. However, methods are also being investigated in which the GaAs substrates are recycled many times after the transfer of the semiconductor layers to silicon. For cost-effective throughput in solar cell production, new systems must also be developed to achieve deposition on larger substrates and in less time. These are approaches that researchers at ISE will follow in the coming years.

The work on the wafer-bonded solar cell is funded by the Federal Ministry for Economic Affairs and Energy (project PoTaSi, FKz. 0324247). The work on the directly grown cell, which involved partners Aixtron SE, TU Ilmenau and Philipps-Universität Marburg, was supported by the Federal Ministry of Education and Research (MehrSi project, FKz. 03SF0525A).

»In order to realize a CO2-free energy supply in all sectors, we must massively advance the expansion of photovoltaics, even beyond rooftops and open spaces. In the future, solar modules will be even more integrated into our already built environment, for example in vehicles as well, "explains Dr. med. Andreas Bett, Director of Fraunhofer ISE.

For the integration of the photovoltaic into the solar roof, the Freiburg researchers are relying on the shingle connection: the monocrystalline silicon solar cells are arranged overlapping and connected in a bonding process with a conductive adhesive. In this way, no inactive areas are created by cell gaps, the module area can be used as much as possible for power generation and offers a homogeneous, aesthetic overall picture. Furthermore, lower resistance losses and the elimination of shading by resting cell connectors as well as a particularly high shading tolerance ensure up to two percent higher module efficiency compared to conventional solar modules. The solar cell matrix is ​​laminated in a film laminator between the glasses of a commercially available, spherically curved panorama car roof. With the help of a specially manufactured mold, lamination can also be carried out in a conventional laminator.

Another special feature of the solar roof: The solar cells are completely hidden by an individual color coating and thus invisible. The loss of efficiency due to the Morpho-Color® glass coating is only seven percent relative. The Morpho Butterfly-inspired effect is achieved through special surface textures that allow high color saturation with good viewing angle stability. "The color options are almost infinite," says Dr. Martin Heinrich, Head of PV for Mobility at Fraunhofer ISE. The functionality of the solar roof corresponds to that of a standard metal car roof: The solar cells convert incoming solar radiation into electricity, which helps to reduce overheating in the car. Due to the shingle connection, the module voltage is higher than with a conventional module, which makes it easier to transform the voltage to the battery voltage. The large thermal and mechanical loads on traffic routes can also compensate well for the glued shingle cells.

The integrated solar cells of the PV car roof have an output of approx. 210 W / m² and can supply sustainable power for a daily 10 km car route on a sunny summer day. This corresponds to an annual renewal of the journey by about 10% or an equivalent consumption reduction. The calculation is based on the unshaded solar radiation in Freiburg im Breisgau, a consumption of the electric car of 17 kWh on 100 km and an annual driving performance of 15.000 km. Also for consumers who could otherwise limit the range of an electric vehicle (eg air conditioning, heating), the solar power can be used. The Fraunhofer ISE sees potential for further extension of the range in the integration of photovoltaics into additional vehicle areas.

Fraunhofer ISE, in collaboration with several forwarding companies, has already investigated the potential that lies dormant on vehicle roofs in a measuring campaign 2016-2017: 6 trucks were equipped with insolation and temperature sensors and GPS and their routes were recorded in the eastern US and in central and southern Europe. For Europe, 5000-7000 kilowatt-hours of annual power generation potential have been determined on a typical truck roof, equivalent to driving 5000-7000 kilometers. In the planned Citizen Science project "PV2Go", the researchers at Fraunhofer ISE want to determine the irradiation potential for cars with the support of interested car owners.

Source and images: Fraunhofer / ISE