The Perovskite-Info newsletter (November 23, 2021)

Researchers from Korea, led by Prof. Bongjin Simon Mun from Gwangju Institute of Science and Technology, have used ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and low energy electron diffraction (LEED) to investigate how fabrication conditions (annealing in an oxygen-rich environment and an oxygen deficit, low-pressure environment) for a particular perovskite material, SrTiO₃, affects its undoped surface and the resulting interfacial layer of the heterostructure.

Certain perovskites can be promising alternatives to silicon-based components for next generation electronic applications. Their structure makes them ideal for use as a base for growing oxide films to form heterostructures with unique electrical properties. The properties of these heterostructures depend on the charge transfer in the interfacial layer between the perovskite substrate and oxide overlayer. This charge transfer can be manipulated via either doping or through the fabrication process.

Researchers from Professor Lioz Etgar's group at The Hebrew University of Jerusalem have recently studied the effects of electron transport layers (ETLs) and hole transport layer (ETLs) on the performance of inverted perovskite-based SC structures.

They focused on the inverted architecture, where the ETL and the HTL from the solar cell structure are eliminated. Three main architectures of were studied: a fully inverted structure, an ETL-free structure, and a HTL-free structure.

A group of scientists, led by Prof. Yiqiang Zhan from Fudan University, has reported high-efficiency flexible perovskite solar cells (f-PSCs) by annealing a SnO₂ ETL in a rough vacuum at a low temperature (100 ℃), and peak efficiency reached 20.14%.

SnO₂ layers that have been prepared by this method have shown higher robustness and hydrophobicity in comparison with samples prepared in an air atmosphere and temperatures of 100 °C, leading to an improved ETL/perovskite interface connection and reducing defects in the SnO₂ /perovskite interface. The appropriate density of oxygen vacancies on the surface during this treatment can be responsible for higher conductivity, which is beneficial for charge transfer.

Three HZB teams, led by Prof. Christiane Becker, Prof. Bernd Stannowski and Prof. Steve Albrecht, have jointly managed to bring the efficiency of perovskite silicon tandem solar cells to a new record value of 29.80%. This result has been officially certified by Fraunhofer ISE CalLab and is documented in the NREL-charts.

The perovskite silicon tandem cell is based on two innovations: A nanotextured front side ( left) and a back side with dielectric reflector (right). © Alexandros Cruz /HZB

Several HZB groups have been working intensively since 2015 on both the perovskite semiconductors and silicon technologies and the combination of both into innovative tandem solar cells. In January 2020, HZB had achieved a record 29.15 % for a perovskite silicon tandem solar cell. Then, also in 2020, the company Oxford PV was able to announce a certified efficiency of 29.52% . Since then, the race for new records has been on. "An efficiency of 30% is like a psychological threshold for this fascinating new technology which could revolutionize the photovoltaic industry in the near future," explains Steve Albrecht, who is working on perovskite thin films at the HySPRINT lab at HZB. Bernd Stannowski, group leader for silicon technology, adds: "I would particularly emphasize the good cooperation between the different groups and institutes at HZB. This is how we managed to develop these new tandem solar cells entirely at HZB and once again get the world record."

Researchers from various universities and laboratories around the world have recently melted metal-organic frameworks (MOFs) and mixed them with perovskites to yield highly stable luminescent composites. The mixtures reportedly resist exposure to heat, air, and humidity.

Lead-halide perovskites, such as cesium lead iodide, naturally exhibit photoluminescence, says chemist Thomas D. Bennett of Cambridge University and the paper’s lead author. “However, this light-emitting phase is only stable at high temperatures, and its effects disappear when the material cools down,” he adds. In the new work, researchers preserved this photoluminescence using MOF glasses to trap this metastable phase at room temperature and, at the same time, encapsulate and protect the perovskite.

Researchers from the University of Cambridge, in collaboration with Cambridge's Cavendish Laboratory, the Diamond Light Source synchrotron facility in Didcot and the Okinawa Institute of Science and Technology in Japan, have used a suite of correlative, multimodal microscopy methods to visualize, for the first time, why perovskite materials are seemingly so tolerant of defects in their structure.

The impressive performance of perovskites is surprising, as the typical model for an excellent semiconductor is a very ordered structure, but the array of different chemical elements combined in perovskites creates a much 'messier' landscape. This heterogeneity causes defects in the material that lead to nanoscale 'traps', which reduce the photovoltaic performance of the devices. But despite the presence of these defects, perovskite materials still show efficiency levels comparable to their silicon alternatives. In fact, earlier research by the same group has shown the disordered structure can actually increase the performance of perovskite optoelectronics, and their latest work seeks to explain why.