Three institutes in the ETH Domain are conducting research on so-called perovskite-based optoelectronics, such as solar cells, photodetectors, and light-emitting diodes (LEDs). In a project called AMYS, labs of EPFL, ETH Zurich and Empa have now joined forces for four years to explore new chemical compositions, but also simple and scalable low cost production methods.
Various solutions are available for converting sunlight directly into electricity. The best known are silicon solar cells, which are based on silicon single crystals. Solar cells of this type are relatively thick and fragile.
As a further variant, so-called thin-film solar cells have emerged, which are about 100 times thinner than crystalline silicon cells. This cell structure is flexible and can be vapor-deposited onto flexible substrates such as plastic films or metal foils. The thin-film cells made of the semiconductors gallium arsenide (GaAs), cadmium telluride (CdTe) or copper indium gallium sulfur selenium (CIGS), which have been known for some time, have now been joined by a new class: organic-inorganic perovskites. The term perovskite describes the common crystal structure of the materials in these thin films.
The AMYS project (Advanced Manufacturability of Hybrid Organic-inorganic Semiconductors for Large Area Optoelectronics), which was launched as part of the Strategic Focus Area Advanced Manufacturing (SFA-AM), is now trying to solve precisely these problems. What is needed is an industrial manufacturing method for perovskite thin-films, which have so far been produced mainly in "wet" processes in laboratories.
The tasks of the research partners are carefully distributed: Perovskite specialists Ayodhya N. Tiwari and Fan Fu’s team from Empa’s "Thin Films and Photovoltaics Laboratory" is looking for a flexible perovskite photodetector and solar cells; Chih-Jen Shih’s team from ETH Zurich’s Nanomaterials Engineering Research Group wants to build perovskite LEDs that produce light with particularly high color accuracy. And Christophe Ballif of EPFL and his team are on the hunt for particularly efficient tandem solar cells that consist of silicon on the bottom and a semi-transparent perovskite layer on top.
Sebastian Siol at Empa is helping him with this. He is a specialist in coating processes and in the analysis of industrially produced thin films. He will use automated high-throughput experiments to screen a large number of different chemical compositions and process parameters with the goal to create a "library" of promising perovskite mixtures. This will give Wolff and his colleagues in all working groups crucial tips on where to look. That speeds up the journey to the goal of cheap, stable and large-area optoelectronic devices with a manifold of applications. Empa researcher Fan Fu is a specialist in perovskites and is also part of the research consortium. He has taken on two tasks at once: On the one hand, he is looking for new photodetectors and solar cells based on perovskites. On the other, he also wants to find a "green path" for the industrial production of perovskite cells. "While the EPFL team has found a dry process, we at Empa have further developed the wet process," the researcher explains. "We no longer need any toxic solvents, which you can use in the lab but are a handicap in the industrial process. We now work with isopropanol - which is also used in every hair dresser’s store." Fan Fu now wants to adapt his wet process to industrial processes such as slot-die coating. His Empa colleague Sebastian Siol is also helping him in his search for the optimal process. He will map the test specimens from Fu’s test series and help find the optimal process parameters.
Fan Fu has a second project that he is pursuing as part of AMYS: Perovskite cells could potentially also serve as photodetectors in cameras or X-ray detectors for medical imaging - and would have two key advantages: they are far cheaper and easier to produce than the silicon camera chips commonly used today. And they are flexible and can adapt to body shapes. Fan Fu uses an example to explain how interesting this could become: "Blood oxygen and pulse rate sensors in smartwatches are partly based on optical detection of blood flow." With flexible, optical sensors, such readings could be obtained much cheaper and at the same time more accurately in the future, says Fu. "Measurement devices that rest directly on the skin are a key technology for future interaction between humans and machines."