The current world record tandem photovoltaic cell provided stable performance for 300 hours even without encapsulation.
A HZB team has published a report in the journal Science on the development of its current world record of 29.15% efficiency for a tandem photovoltaic or solar cell made from perovskite & silicon. The tandem cell provided a stable performance for 300 hours even without encapsulation. To accomplish this, the group headed by Prof. Steve Albrecht investigated physical processes at the interfaces to enhance the transport of the charge carriers.
Solar cells consisting of two semiconductors with differing band gaps are able to have considerably higher efficiencies when utilized in tandem compared to the individual cells on their own. This is often because tandem cells use the solar spectrum more efficiently. Especially, conventional silicon solar cells primarily convert the infrared components of light efficiently into electricity while certain perovskite compounds can effectively utilize the visible components of light, making this a strong combination.
New Record 29.15%
In the beginning of 2020, a team head by Prof. Steve Albrecht at the HZB broke the previous record for tandem solar cells made from perovskite & silicon (28.0%, Oxford PV), setting a new record of 29.15%. Compared to the very best certified & scientifically published efficiency (26.2% in DOI: 10,1126/science.aba3433), this is often an enormous breakthrough. The new value has been certified at Fraunhofer ISE & listed in the NREL chart. Now, the results have been published in the journal Science with detailed explanation of the fabrication process & underlying physics.
Consistent Performance Over 300 Hours
“29.15% efficiency isn’t only the record for this technology but is at the very top of the whole Emerging PV category in the NREL chart,” says Eike Köhnen, PhD student on Albrecht’s team & shared first author of the study. Additionally, the new perovskite or silicon tandem cell is characterized by consistent performance during more than 300 hours under continuous exposure to air & simulated sunlight without being protected by encapsulation. The team utilized a complex perovskite composition with a 1.68 eV band gap & focused on optimizing the substrate interface.
Useful: Self Assembled Monolayer
With partners from Lithuania (the group of Prof. Vytautas Getautis), they developed an intermediate layer of organic molecules that arrange themselves autonomously into a Self-Assembled Monolayer (SAM). It consisted of a novel carbazole-based molecule with methyl group substitution (Me-4PACz). This SAM was applied to the electrode & facilitated the flow of the electrical charge-carriers. “We first prepared the perfect bed, so to talk, on which the perovskite lays on,” says Amran Al-Ashouri, a member of Albrecht’s team & shared first author of the study.
Fill Factor Optimized
The researchers then used a variety of complementary investigation methods to analyze the various processes at the interfaces between perovskite, SAM & the electrode: “In particular, we optimized what’s called the fill factor, which is influenced by how many charge carriers are lost on their way out of the perovskite top cell,” explains Al-Ashouri. While the electrons flow through the direction of sunlight through the C60 layer, the “holes” move in the other way through the SAM layer into the electrode. “However, we observed that the extraction of holes is far slower than electron extraction, which limited the fill factor,” says Al-Ashouri. However, new SAM layer considerably accelerated the hole transport and thus simultaneously contributes to improved stability of the perovskite layer.
Combination of methods
Through a combination of photoluminescence spectroscopy, modelling, electrical characterization, & terahertz conductivity measurements, it had been possible to differentiate the varied processes at the interface of the perovskite material & to determine the origin of significant losses.
Cooperations as a key to success
Many partners were involved in the project including Kaunas University of Technology/Lithuania, University of Potsdam, University of Ljubljana/Slovenia, University of Sheffield/UK & also the Physikalisch-Technische Bundesanstalt (PTB), HTW Berlin & Technische Universität Berlin where Albrecht holds a junior professorship. The work on the individual perovskite & silicon cells took place in the HZB labs HySPRINT & PVcomB, respectively. “Each partner brought their own special expertise to the project, so we were ready to achieve this breakthrough together,” says Albrecht. The utmost possible efficiency is already within reach: the researchers analyzed the 2 cells individually & calculated a maximum possible efficiency of 32.4% for this design. “We can certainly achieve over 30%,” says Albrecht.
