Unleashing perovskites’ potential for solar cells

Perovskites — a broad category of compounds that share a certain crystal structure — have attracted a great deal of attention as potential new solar-cell materials because of their low cost, flexibility, and relatively easy manufacturing process. But much remains unknown about the details of their structure and the effects of substituting different metals or other elements within the material. Now, researchers using the U.S. Department of Energy’s (DOE’s) Advanced Photon Source (APS) have been able to decipher a key aspect of the behavior of perovskites made with different formulations: With certain additives there is a kind of “sweet spot” where sufficient amounts will enhance performance and beyond which further amounts begin to degrade it. The findings were detailed in the journal Science.
Conventional solar cells made of silicon must be processed at temperatures above 1,400 degrees Celsius, using expensive equipment that limits their potential for production scale-up. In contrast, perovskites can be processed in a liquid solution at temperatures as low as 100 degrees, using inexpensive equipment. What’s more, perovskites can be deposited on a variety of substrates, including flexible plastics, enabling a variety of new uses that would be impossible with thicker, stiffer silicon wafers.

>Read more on the Advanced Photon Source (APS) website

Image: Perovskite-based solar cells are flexible, lightweight, can be produced cheaply, and could someday bring down the cost of solar energy. Shown here is the type of perovskite solar cell measured at the CNM/XSD Hard X-ray Nanoprobe at the APS.
Credit: Rob Felt

Spin-momentum locking in cuprate high-temperature superconductors

The results open a new chapter in the mystery of high-temperature superconductors, suggesting that new, unexplored interactions and mechanisms might be at play.

In the world of superconductors, “high temperature” means that the material can conduct electricity without resistance at temperatures higher than expected, but still far below room temperature. Within this special class of high-temperature superconductors (HTSCs), cuprates—consisting of superconducting CuO2 layers separated by spacer layers—are some of the best performers, generating interest in these materials for potential use in super-efficient electrical wires that can carry power without any loss of electron momentum.

A new spin on cuprate HTSCs

Two kinds of electron interactions have been known to give rise to novel properties in new materials, including superconductors. Scientists who study cuprate superconductors have focused on just one of those interactions: electron correlation—electrons interacting with each other. The other kind of electron interaction found in exotic materials is spin-orbit coupling—the way in which an electron’s magnetic moment interacts with atoms in the material.

>Read more on the Advanced Light Source website

Image: Chris Jozwiak, Alessandra Lanzara, Kenneth Gotlieb, and Chiu-Yun Lin.
Credit: Peter DaSilva/Berkeley Lab

New insight into high-temperature superconductors

Researchers have found evidence for an acoustic plasmon or “sound wave”, which has been predicted for layered systems and suggested to play a role in mediating high temperature superconductivity.

When electrical current propagates through a conducting material, energy dissipates due to the conductor’s electrical resistance. In a superconductor, however, the resistance can vanish completely if the material is cooled to extremely low temperatures. Such dissipationless supercurrent would be highly desirable for a plethora of electronic and technological applications, and has spawn decades of intense research dedicated to find materials with superconducting properties at elevated temperatures.

While all superconducting materials reported until the 1980’s had to be cooled below 30 K, the game changed in 1986, when the first superconductors based on copper oxide materials were discovered. These so-called high-temperature superconductors are composed of stacked layers of copper-oxygen planes and some show zero electrical resistance well above 100 K. By understanding the mechanisms mediating superconductivity in the copper oxides, the scientific community hopes to become able to devise novel materials that show zero resistance even at room temperature. However, a comprehensive understanding of these mechanisms has yet remained elusive. Nonetheless, superconductors are used already today in some technological applications, such as magnetic resonance imaging devices in the field of medicine. Future applications of room temperature superconductors could revolutionize the fields of electrical power storage and transmission, and enable rapid public transport by magnetically levitated trains.

>Read more on the European Synchrotron website

Image: Overview of the beamline ID32 at the ESRF.
Credits: P. Jayet

Lattice Coupling conspires in the correlated cuprate high-Tc superconductivity

For the cuprate high temperature superconductivity (high-Tc) research over the past three decades, the biggest challenge is to identify the relevant low energy degrees of freedom that are critical to formulating the correct theoretical model for high-Tc superconductivity. The main difficulty lies in the closeness between various relevant energy scales. For low energy processes that are comparable to the superconducting gap energy ∆sc, there are the spin exchange energy J, the lattice vibration (phonon) energy Ωph, and the van Hove singularity energy E(π,0). However, anomalous isotope effects on Tc and superfluid density in the cuprates cannot be captured by traditional phonon-mediated superconductivity theories. Historically, a purely electronic Hamiltonian – the Hubbard model – was widely regarded to encapsulate all the core physics of the high-Tc phenomena.

In a recent paper published in Science, scientists from Stanford University and from Stanford Institute for Materials and Energy Sciences (SIMES), in collaboration with material scientists from Japan and theoreticians from Japan, the Netherlands, and Berkeley, reinstated the substantial role of the lattice vibration in the cuprate high-Tc superconductivity – however, in a subtle way that is highly intertwined with the electronic correlations. They finely straddled 18 differently hole-doped high-Tc compound Bi2Sr2CaCu2O8+δ within 8% change of hole carrier concentration, a doping range where Tc evolves from 47 K to 95 K through a putative quantum critical point, around which the electronic correlation effect experiences a sudden change. Then systematic experiments were carried out using the angle-resolved photoemission spectroscopy (ARPES) facility at SSRL Beam Line 5-4. Here, the high-resolution ARPES end station provided critical information of both the superconducting gap and the electron-lattice coupling.

>Read more on the Stanford Synchrotron Radiation Lightsource website

Image: Intertwined growth of the superconductivity and the electron-phonon coupling tuned by the hole concentration. The red line is an illustration of the Tc in Bi-2212 (Tcmax = 95 K). The blue shade and line represent the single-layer Bi-2201 system, where the coupling to the B1g mode is weak and T max is only 38 K. The yellow ball represents the optimally doped tri-layer Bi-2223 where Tcmax is 108 K. The top-right inset shows the intertwined relation between the pseudogap and the EPC under strong electronic correlation. The Madelung potential and the lattice stacking along the c-axis are schematically depicted for the single- layer, bi-layer and tri-layer systems. The dark grey blocks represent the CuO2n- plane, and the light grey blocks represent the charge reservoir layers (Ca2+, SrO, BiO+). The orange dots mark the CuO2n- planes that experience to the first order a non-zero out-of-plane electric field.
Credit: Science, doi: 10.1126/science.aar3394