Power/Performance Bits: April 29

Lithium-free flexible battery
A Rice University laboratory has flexible, portable and wearable electronics in its sights with the creation of a thin film for energy storage.

The researchers have developed a flexible material with nanoporous nickel-fluoride electrodes layered around a solid electrolyte to deliver battery-like supercapacitor performance that combines the best qualities of a high-energy battery and a high-powered supercapacitor without the lithium found in commercial batteries today.

The electrochemical capacitor is about a hundredth of an inch thick but can be scaled up for devices either by increasing the size or adding layers. They expect that standard manufacturing techniques may allow the battery to be even thinner.

In tests, the square-inch device was found to hold 76 percent of its capacity over 10,000 charge-discharge cycles and 1,000 bending cycles.

The team had set out to find a material that has the flexible qualities of graphene, carbon nanotubes and conducting polymers while possessing much higher electrical storage capacity typically found in inorganic metal compounds. Inorganic compounds have, until recently, lacked flexibility.

Nickel-fluoride electrodes around a solid electrolyte are an effective energy storage device that combines the best qualities of batteries and supercapacitors, according to Rice University researchers. The electrodes are plated onto a gold and polymer backing (which can be removed) and made porous through a chemical etching process. (Source: Rice University)

Compared with a lithium-ion device, the researchers explained that the structure is quite simple and safe. It behaves like a battery but the structure is that of a supercapacitor. If it is used as a supercapacitor, it can be charged quickly at a high current rate and discharged it in a very short time. But for other applications, it can be set up to charge more slowly and discharged slowly like a battery.

Trapping photons
In the quest to make sun power more competitive, Stanford University researchers are designing ultrathin solar cells that cut material costs. At the same time, they’re keeping these thin cells efficient by sculpting their surfaces with photovoltaic nanostructures that behave like a molecular hall of mirrors.

The researchers, who surveyed 109 recent scientific papers from teams around the world, said they want to make sure light spends more quality time inside a solar cell, and their overview revolves around a basic theme: looking at the many different ways researchers are trying to maximize the collisions between photons and electrons in the thinnest possible layers of photovoltaic materials. The goal is to reveal trends and best practices that will help drive developments in the field.

Solar energy can be harvested when photons of light collide with the electrons in a photovoltaic crystal and set them free. As loose electrons move through the crystal, they generate an electrical current. Today’s solar cells are already thin. They are made up of layers of photovoltaic materials, generally silicon, that average 150 to 300 micrometers, which is roughly the diameter of two to three human hairs. As engineers continue to shave down those dimensions they have to develop new nanoscale traps and snares to ensure that photons don’t simply whiz through their ultrathin solar cells before the electrical sparks can fly.

Their review provides a high level view of how scientists are trying to design structures to facilitate interactions between the infinitesimal instigators of solar current, the photons and the electrons.

Researchers face enormous challenges in trying to design nanostructures attuned to catch light. Sunlight consists of many colors. When we see a rainbow, what we see is the result of atmospheric moisture acting as a prism capable of separating light into its constituent colors. Creating different nanostructures to catch the pot of photons at the end of each color of the rainbow is part of what this research is about.

Nevertheless, scientists are already reporting some success with systems that use one-one hundredth as much photovoltaic material as today’s solar cells while getting 60 percent to 70 percent of the electrical output. The most common photovoltaic material is a refined form of silicon similar to that found in computer chips. This material accounts for 10 percent to 20 percent of a solar cell’s cost. Reducing those expenses 100-fold would therefore have a considerable effect on the overall cost-efficiency of solar energy production.

Decreasing material costs is only part of the push behind ultrathin solar. Another benefit is flexibility. Because of the thickness of the light-catching silicon layer, today’s solar cells must be kept rigid lest their crystal lattice be damaged and the flow of electrons disrupted.

The magic of molybdenite
After using it to develop a computer chip, flash memory device and photographic sensor, EPFL scientists have once again tapped into the electronic potential of molybdenite (MoS2) by creating diodes that can emit light or absorb it to produce electricity.

Molybdenite has a few surprises still up its sleeve. After having used it to build an electronic chip, a flash memory device and a photographic sensor, an EPFL professor and his team in the Laboratory of Nanoscale Electronics and Structures is continuing the study of this promising semiconductor and have demonstrated the possibility of creating light-emitting diodes and solar cells.

The scientists built several prototypes of diodes – electronic components in which voltage flows in only one direction – made up of a layer of molybdenite superposed on a layer of silicon. At the interface, each electron emitted by the MoS2 combines with a “hole” – a space left vacant by an electron – in the silicon. The two elements lose their respective energies, which then transforms into photons. This light production is caused by the specific properties of molybdenite; other semi-conductors would tend to transform this energy into heat.

Even better, by inversing the device, electricity can be produced from light. The principle is the same: when a photon reaches the molybdenite, it ejects an electron, thus creating a “hole” and generating voltage. Thus, the diode works like a solar cell, and has shown an efficiency of more than 4% in tests. Molybdenite and silicon are working in tandem here. The MoS2 is more efficient in the visible wavelengths of the spectrum, and silicon works more in the infrared range, thus the two working together cover the largest possible spectral range.

Next, the scientists want to study the possibility of building electroluminescent diodes and bulbs. This discovery could, above all, reduce the dissipation of energy in electronic devices such as microprocessors, by replacing copper wires used for transmitting data with light-emitters.