“27plus6”: lighthouse efficiencies for perovskite/silicon tandem solar cells
The recently launched research project “27plus6” is intended to set a new benchmark for the entire solar cell industry. The name says it all: the researchers want to achieve a lighthouse efficiency of 27 percent with silicon technology. In addition, they want to extend these solar cells to form a tandem, i.e., to apply a second solar cell made of perovskite. In this way they want to increase the utilization of the solar spectrum, ultimately achieving efficiencies of 33 percent, explains project manager Sascha Wolter from the Institute for Solar Energy Research in Hameln (ISFH). In addition to the ISFH, the Karlsruhe Institute of Technology (KIT) and Centrotherm International AG as well as other supporting partners are involved in the project.
The targeted efficiency of 33 percent will initially only be achieved in laboratory arrangements. But the research is also relevant for industrially manufactured solar cells, emphasizes Helge Haverkamp, Head of Development at Centrotherm: “For us plant manufacturers, the integration of our processes into the production of these record solar cells offers an excellent advertising platform in international competition”. According to Haverkamp, current processes from the mass production of silicon solar cells could also be further developed and optimized on the sensitive platform of the high-efficiency solar cell. Project funding of 3.2 million euros from the German Federal Ministry of Economics (BMWi) thus supports the innovative strength of the companies in the strategically important renewable energy market.
In order to speed up industrial implementation, the aim is not only to achieve lighthouse efficiencies under standard test conditions, but also to demonstrate a significant increase in energy yield. The real solar spectrum changes during the course of the day and year, which makes it difficult for tandem solar cells to adapt the current of the two sub-cells when they are connected in the typical 2-terminal configuration with one electrical contact each on the front and back. In 27plus6, cells with three terminals are therefore developed, which are characterized by greater robustness against the changing solar spectrum during the course of the day. “In combination with an innovative interconnection concept, solar modules with cells of this type offer a higher energy yield,” emphasizes Ulrich Paetzold, Research Group Leader at KIT.
On the one hand, the ISFH contributes to the project its expertise in the development of back-contacted Si solar cells with passivating contacts, which led to an efficiency record for silicon solar cells of 26.1% already in 2018. On the other hand, the ISFH will further develop the conformal deposition of perovskite absorbers on textured surfaces by co-evaporation. Pioneering work on 3-terminal tandem solar cells has been done by ISFH in cooperation with the US-American National Renewable Energy Laboratory. The ISFH coordinates the project.
KIT contributes its expertise in the field of perovskite cell development to the project. This includes, for example, the implementation of “two-dimensional” perovskite crystal structures under the contact layers in order to passivate the “three-dimensional” perovskite absorber by means of the resulting offset in the energy band structure. The perovskite solar cell recently demonstrated by the KIT with 20.7% at a band gap of 1.7 eV, which is ideal for perovskite / silicon tandem application, also offers an excellent starting point for the project.
Centrotherm is contributing its processes for the deposition of layers for passivating contacts to the project, without which the targeted efficiencies on silicon could not be achieved. The processes will be further developed in the project and optimized with a view to industrial production.
The effect of lighthouses in PV technology is well documented: The aforementioned p-type Si world record of 26.1%, which has existed since 2018 and is based on the pioneering work of ISFH and MBE on passivating contacts, has led to the broad adoption of this technology. Currently Centrotherm and other equipment manufacturers are transferring the technology to mass production.
The project aims to demonstrate a significant increase in energy yield compared to today’s Si solar cells, which could reduce the cost of generating electricity through PV to below the already low level of 4 €ct/kWh for the German energy market. Due to its high efficiency per unit area, PV could also open up special applications such as integration in electric vehicles.
More information: https://www.imt.kit.edu/paetzold.php
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