Perovskite / Silicon Tandem Solar Cells at a magic threshold of 30% efficiency

Tandem solar cell structure in 3D. Credit: Eike Koehnen / HZB

The current world record tandem solar cell offered a stable performance for 300 hours, even without encapsulation.

A HZB team has published a report in the journal Science, developing an efficiency of 29.15% of the current world record for a tandem solar cell made of perovskite and silicon. The tandem cell offered a stable performance for 300 hours, even without encapsulation. To achieve this, the team led by Professor Steve Albrecht investigated the physical processes at the interfaces to improve the transport of cargo carriers.

Solar cells made up of two semiconductors with different band gaps can achieve considerably greater efficiency when used in tandem with each cell. This is because tandem cells use the solar spectrum more efficiently. In particular, ordinary silicon solar cells mainly convert infrared components of light efficiently into electrical energy, some perovskite compounds can use the impressive components of sunlight efficiently, making it a powerful combination.

New record 29.15%

In early 2020, a team led by Professor Steve Albrecht broke the previous world record in HZB, perovskite and silicon solar cell tandems (28.0%, Oxford PV), setting a new world record of 29.15%. Compared to the highest certified and scientifically published efficiency (26.2% DOI: 10.1126 / science.aba3433), it is a huge step forward. The new value is certified in the Fraunhofer ISE and is shown in the NREL table. Now, the results have been published in the journal Science with a detailed explanation of the manufacturing process and the underlying physics.

Consistent performance over 300 hours

“29.15% efficiency is not only the record for this technology, it is at the top of the entire Emerging PV category in the NREL table,” says Eike Köhnen, PhD student at the Albrecht team and lead author of the study. In addition, the new perovskite / silicon tandem cell has a continuous performance for more than 300 hours without protection through encapsulation in air exposure and sunlight simulation. The group used a complex perovskite composition with a 1.68 eV band gap and focused on optimizing the substrate interface.

Useful: self-assembled monolayer

Together with their Lithuanian partners (Professor Vytautas Getautis ’team) they developed an intermediate layer of organic molecules to transform themselves into a self-assembled monolayer (SAM). It was formed by a new molecule based on carbazole, with the substitution of the methyl group (Me-4PACz). This SAM was applied to the electrode and facilitated the flow of electric charge carriers. “Based on Perovskite, we prepared the perfect bed, so to speak,” says Amran Al-Ashouri, a member of the Albrecht team and also the first author of the study.

Optimized compliance factor

The researchers used a number of complementary research methods to study the different processes at the interfaces between perovskite, SAM and electrode: “In particular, we optimized what we call the filler factor, which affects the loss of charge carriers when perovskite exits the upper cell,” Al-Ashouri explained. While electrons flow in the direction of sunlight through the C60 layer, the “holes” move in the opposite direction across the SAM layer at the electrode. “However, we have seen that hole extraction is much slower than electron extraction, which limits the filling factor,” says Al-Ashouri. However, the new SAM layer greatly accelerated the transport of holes and at the same time helps to improve the stability of the perovskite layer.

Combination of methods

Through a combination of photoluminescence spectroscopy, modeling, electrical characterization, and measurements of terahertz conductivity, it was possible to distinguish several processes at the interface of the perovskite material and determine the origin of significant losses.

Cooperation as the key to success

Many partners participated in the project, including Kaunas University of Technology / Lithuania, Potsdam University, Ljubljana / Slovenia University, Sheffield University / UK, and Physikalisch-Technische Bundesanstalt (PTB), HTW Berlin and Technische Berlin University, where Albrecht is a tenured professor. den. Work on individual perovskite and silicon cells was performed in HZB laboratories HySPRINT and PVcomB, respectively. “Each partner brought their own unique experience to the project, so we made this breakthrough together,” says Albrecht. The highest possible efficiency is already available: the researchers analyzed the two cells individually and calculated a maximum possible efficiency of 32.4% for this design. “We can definitely get over 30%,” says Albrecht.

Reference: Amran Al-Ashouri, Eike Köhnen, Bor Li, Artiom Magomedov, Hannes Hempel, Pietro Caprioglio, José A. Márquez, Anna Belen Morales Vilches “Improved extraction of holes with monolithic perovskite / silicon tandem solar cell> 29% efficiency”. , Ernestas Kasparavicius, Joel A. Smith, Nga Phung, Dorothee Menzel, Max Grischek, Lukas Kegelmann, Dieter Skroblin, Christian Gollwitzer, Tadas Malinauskas, Marko Jošt, Gašper Matic, Bernd Rech, Rutger Schlatmann, Marko Topic, Lars Korte, Antonio, Bernd Stannowski, Dieter Neher, Martin Stolterfoht, Thomas Unold, Vytautas Getautis and Steve Albrecht, 11 December 2020, Science.
DOI: 10.1126 / science.abd4016

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