Solar cells, which consist of two semiconductors with different band gaps, can achieve significantly higher levels of efficiency when used at the same time than the individual cells alone. This is because tandem cells use the solar spectrum more efficiently. In particular, conventional silicon solar cells primarily efficiently convert the infrared components of light into electrical energy, while certain perovskite compounds can effectively use the visible components of sunlight, making this a powerful combination.
At the beginning of 2020, a team led by Prof. Steve Albrecht from HZB broke the previous world record for tandem solar cells made of perovskite and silicon (28.0%, Oxford PV) and set a new world record with 29.15%. Compared to the highest certified and scientifically published efficiency (26.2% in DOI: 10.1126 / science.aba3433) this is a big step forward.
The new value was certified at Fraunhofer ISE and listed in the NREL table (press release here). Now the results have been published in the journal Science with a detailed explanation of the manufacturing process and the underlying physics.
“An efficiency of 29.15% is not only the record for this technology, it is also at the top of the entire category of emerging PV in the NREL table,” says Eike Kohnen, doctoral student in Albrecht’s team and joint lead author of the study. In addition, the new perovskite / silicon tandem cell is characterized by constant performance over more than 300 hours under constant exposure to air and simulated sunlight, without being protected by encapsulation. The team used a complex perovskite composition with a band gap of 1.68 eV and focused on optimizing the substrate interface.
Together with partners from Lithuania (the group of Prof. Vytautas Getautis) they developed an intermediate layer of organic molecules that autonomously arrange themselves into a self-organized monolayer (SAM). It consisted of a new carbazole-based molecule with methyl group substitution (Me-4PACz). This SAM was applied to the electrode and facilitated the flow of electrical charge carriers. “We first prepared the perfect bed, so to speak, on which the perovskite lies,” says Amran Al-Ashouri, who is also a member of Albrecht’s team and joint first author of the study.
The researchers then used a number of complementary investigation methods to analyze the various processes at the interfaces between the perovskite, SAM and the electrode: “In particular, we optimized the so-called fill factor, which depends on how many charge carriers are lost on the way out of the perovskite Cell, “explains Al-Ashouri.
As the electrons flow through the C60 layer in the direction of sunlight, the “holes” move through the SAM layer in the opposite direction into the electrode. “However, we observed that the extraction of holes is much slower than the electron extraction, which limits the fill factor,” says Al-Ashouri. However, the new SAM layer accelerated the hole transport considerably and at the same time contributes to improving the stability of the perovskite layer.
A combination of photoluminescence spectroscopy, modeling, electrical characterization and terahertz conductivity measurements enabled the different processes at the interface of the perovskite material to be differentiated and the origin of significant losses to be determined.
Many partners were involved in the project, including the Technical University of Kaunas / Lithuania, the University of Potsdam, the University of Ljubljana / Slovenia, the University of Sheffield / Great Britain and the Physikalisch-Technische Bundesanstalt (PTB), the HTW Berlin and the Technical University Berlin, at which Albrecht holds a junior professorship. The work on the individual perovskite and silicon cells took place in the HZB laboratories HySPRINT and PVcomB.
“Each partner brought their own expertise to the project so that we could achieve this breakthrough together,” says Albrecht. The maximum possible efficiency is already within reach: the researchers analyzed the two cells individually and calculated a maximum possible efficiency of 32.4% for this design. “We can certainly achieve over 30%,” says Albrecht.
Research report: http://dx.doi.org/10.1126/science.abd4016
Helmholtz Center Berlin For Materials And Energy
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