February 2019

Researchers at MIT, ETH Zurich and Empa have made major steps toward finding a photon source with constant, predictable, and steady characteristics. In the quest to develop practical computing and communications devices based on the principles of quantum physics, such a source of individual particles of light is extremely desirable. The study involves using perovskites to make light-emitting quantum dots.

Scanning Transmission Electron Microscope image (STEM) of single perovskite quantum dots

The ability to produce individual photons with precisely known and persistent properties, including a wavelength, or color, that does not fluctuate at all, could be useful for many kinds of proposed quantum devices. Since each photon would be indistinguishable from the others in terms of its quantum-mechanical properties, it could be possible, for example, to delay one of them and then get the pair to interact with each other, in a phenomenon called interference.

Researchers from HZB, Oxford University, Technical University Berlin and Oxford PV have shown that the infrared reflection losses in tandem cells processed on a flat silicon substrate (such as perovskite/silicon tandem cells) can be significantly reduced by using an optical interlayer, consisting of nanocrystalline silicon oxide. Based on this, the team managed to achieve impressive efficiency and reported that the best tandem device in this work reached a certified conversion efficiency of 25.2%.

a) Cross-section of the simulated monolithic perovskite/SHJ tandem cell (layer thicknesses and morphological features not to scale). b) Cross-sectional SEM image of the top region of the tandem device.

Perovskite/silicon tandem solar cells are attractive for their potential for boosting cell efficiency beyond the crystalline silicon (Si) single-junction limit. However, the relatively large optical refractive index of Si, in comparison to that of transparent conducting oxides and perovskite absorber layers, often results in significant reflection losses at the internal junction between the cells in monolithic (two-terminal) devices. Therefore, light management is crucial for improving photocurrent absorption in the Si bottom cell.

Brazilian state-run oil major Petroleo Brasileiro, or Petrobras, has signed a partnership deal with Centro de Inovacoes CSEM Brazil to develop materials for the production of printed and flexible solar cells based on perovskite technology.

Under the research and development (R&D) agreement, Petrobras will invest BRL 23.77 million (USD 6.2 million/EUR 5.6 million) over a 30-month period. The main goal is the production of a prototype module and obtaining the know-how needed to make the industrial production of solar films with perovskite technology technically and economically viable.

A team of scientists from Washington University in St. Louis, Oak Ridge National Laboratory and University of Missouri studied the structure and properties of the commonly occurring planar defects at the atomic scale of lead halide perovskite.

When these materials are made, defects can occur where different crystals meet, known as grain boundaries. In conventional semiconductors, these defects can decrease their electrical conductivity and the solar energy-to-electricity conversion efficiency; however, in lead-halide perovskites, there are differing experimental reports on the activity of grain boundaries. In some cases, they are found to be harmful, while in other cases they either have no impact on performance or are even beneficial. But, to date, no one understood why. The research team in this work set out to discover these reasons.

Researchers from Rutgers University, University of Minnesota and University of Texas at Dallas in the U.S have discovered a new type of electric field effect that can control light emission from perovskite devices.

The electric field effect usually refers to the modulation of electrical conductivity in a semiconductor by means of an applied voltage to a gate electrode and forms the basis of modern digital electronics. In a conventional field effect transistor (FET), the conductivity of a semiconductor layer can be turned on or off or gradually ramped up or down. Now, the research team has found that the photoluminescence (PL) of a perovskite device can be modulated in a similar manner. 'Our work reports a novel type of field effect in which PL, rather than conductivity, is tuned by an 'electric knob' ' the gate voltage,' explains Vitaly Podzorov, who led the research.

A recent joint-research co-led by City University of Hong Kong (CityU) and Shanghai University has developed an efficient fabrication approach for all-inorganic perovskite films with better optical properties and stability, enabling the development of high color-purity and low-cost perovskite LEDs with a high operational lifetime.

The team has found that using cesium trifluoroacetate (TFA) as the cesium source in the one-step solution coating, instead of the commonly used cesium bromide (CsBr), enables fast crystallization of small-grained CsPbBr₃ perovskite crystals, forming the smooth and pinhole-free perovskite films. This is because the interaction of TFA- anions with Pb2+ cations in the CsPbX3 precursor solution greatly improves the crystallization rate of perovskite films and suppresses surface defects.

A team of researchers from UC San Diego, Georgia Institute of Technology, Purdue University, MIT and Argonne National Laboratory has reported new findings on perovskites, that could pave the way to developing low-cost, high-efficiency solar cells. Using high-intensity X-ray mapping, they explain why adding small amounts of cesium and rubidium salt improves the performance of lead-halide perovskites.

'Perovskites could really change the game in solar. They have the potential to reduce costs without giving up performance. But there's still a lot to learn fundamentally about these materials,' said David Fenning, a professor of nanoengineering at the University of California San Diego and co-senior author of the study. 'We're looking deeper into some of the state-of-the-art chemistries to understand what drives perovskite performance and why they work so well.'

Researchers at the Germany-based Helmholtz Center Berlin (HZB) have announced a thin-film solar cell made of perovskite and copper-indium-gallium-selenide (CIGS) with an efficiency of 21.6%.

The HZB researchers said they used a simple, robust production process suitable for scaling up. Rutger Schlatmann, director of the HZB's Institute PVcomB, spoke of an 'enormous step in the direction of commercial production'. The HZB team's tandem cell could theoretically reach an efficiency of more than 30%, according to the researchers.