Power/Performance Bits: April 17

Flexible LCDs; potassium for perovskites; lithium-sulfur batteries.


Flexible LCDs
Researchers at Donghua University and Hong Kong University of Science and Technology developed a flexible, optically rewriteable LCD for paperlike displays.

The team estimates it will be cheap to produce, perhaps only costing $5 for a 5-inch screen.

Optically rewriteable LCDs, like conventional LCDs, are structured like a sandwich, with a liquid crystal filling between two plates. Unlike conventional liquid crystals where electrical connections on the plates create the fields required to switch individual pixels from light to dark, optically rewritable LCDs coat the plates with special molecules that realign in the presence of polarized light and switch the pixels.

This removes the need for traditional electrodes, reduces the structure’s bulk and allows more choices in the type and thickness of plates. Consequently, optically rewritable LCDs are thinner than traditional LCDs, at less than half a millimeter thick, can be made from flexible plastic, and weigh only a few grams. “It’s only a little thicker than paper,” said Jiatong Sun of Donghua University in China.

Optically rewritable LCDs are durable and cheap to manufacture because of their simple structure. Moreover, like an electronic paper screen in an e-book, energy is only required to switch display images or text, not to sustain the images on the screen.

Combined flexible blue optically rewritable LCD. (Source: Zhang et al. / AIP)

To make the screen flexible, the team had to create a new spacer design. Spacers create the separation of the plastic or glass plates. “We put spacers between glass layers to keep the liquid crystal layer uniform,” Sun said. Spacers are used in all LCDs to determine the thickness of the liquid crystal. A constant thickness is necessary for good contrast ratio, response time and viewing angle. However, when plates bends, it forces the liquid crystal away from the impact site and leaves sections of the screen blank and so alterations in spacer design are critical to prevent liquid crystal in flexible LCDs from moving excessively.

Ultimately, a meshlike spacer worked best to prevent the liquid crystal from flowing when the LCD was bent or hit.

The team also improved the color rendering of the screen. Previously, optically rewritable LCDs had only been able to display two colors at a time. Now, their optically rewritable LCD simultaneously displays the three primary colors. They achieved this by placing a special type of liquid crystal behind the LCD, which reflected red, blue and green.

To make this into a commercial product, the team wants to improve the resolution of the flexible optically rewritable LCD. “Now we have three colors but for full color we need to make the pixels too small for human eyes to see,” said Sun.

Potassium for perovskites
Researchers at the University of Cambridge, Uppsala University, Delft University of Technology, and University of Sheffield found that adding potassium iodide to perovskite solar cells extended their efficiency.

Tiny defects in the crystalline structure of perovskites, called traps, can cause electrons to get ‘stuck’ before their energy can be harnessed. Another issue is that ions can move around in the solar cell when illuminated, which can cause a change in the bandgap.

“So far, we haven’t been able to make these materials stable with the bandgap we need, so we’ve been trying to immobilize the ion movement by tweaking the chemical composition of the perovskite layers,” said Sam Stranks, from Cambridge’s Cavendish Laboratory. “This would enable perovskites to be used as versatile solar cells or as colored LEDs, which are essentially solar cells run in reverse.”

Atomic scale view of the perovskite crystal structure forming (‘self-assembling’). The potassium ions (in red) are decorating the surfaces of the structures to heal defects and immobilizing the excess halides. (Source: Matt Klug)

In the study, the researchers altered the chemical composition of the metal halide perovskite layers by adding potassium iodide to perovskite inks, which then self-assemble into thin films. The technique is compatible with roll-to-roll processes. The potassium iodide formed a ‘decorative’ layer on top of the perovskite which had the effect of ‘healing’ the traps so that the electrons could move more freely, as well as immobilizing the ion movement, which makes the material more stable at the desired bandgap.

“Potassium stabilizes the perovskite bandgaps we want for tandem solar cells and makes them more luminescent, which means more efficient solar cells,” said Stranks. “It almost entirely manages the ions and defects in perovskites.”

The perovskite and potassium devices showed good stability in tests, and were 21.5% efficient at converting light into electricity, which is similar to the best perovskite-based solar cells. Tandem cells made of two perovskite layers with ideal bandgaps have a theoretical efficiency limit of 45% and a practical limit of 35% – both of which are higher than the current practical efficiency limits, 29%, for silicon.

Improving lithium-sulfur batteries
Researchers at the University of Texas at Dallas developed a high-powered lithium-sulfur battery they say could extend battery life over lithium-ion batteries. Lithium-sulfur batteries have the potential to be less expensive to make, weigh less, store almost twice the energy of lithium-ion batteries and be better for the environment.

“A lithium-sulfur battery is what most of the research community thinks is the next generation of battery,” said Kyeongjae “K.J.” Cho, professor of materials science and engineering at UT Dallas. “It has a capacity of about three to five times higher than lithium-ion batteries, meaning if you are used to a phone lasting for three hours, you can use it for nine to 15 hours with a lithium-sulfur battery.”

Unfortunately, sulfur is a poor electrical conductor and can become unstable over just several charge-and-recharge cycles. Electrodes breaking down is another reason lithium-sulfur batteries aren’t mainstream.

Attempts have been made to improve lithium-sulfur batteries by putting lithium metal on one electrode and sulfur on the other. However, lithium metal often is too unstable, and sulfur too insulating. The scientists discovered a technology that produced a sulfur-carbon nanotube substance that created more conductivity on one electrode, and a nanomaterial coating to create stability for the other.

The researchers discovered that molybdenum creates a material that adjusts the thickness of the coating when combined with two atoms of sulfur, a coating thinner than the silk of a spiderweb. They found it improved stability and compensated for poor conductivity of sulfur, thus allowing for greater power density and making lithium-sulfur batteries more commercially viable.

The team plans for further develop the battery, said Cho. “We are taking this to the next step and will fully stabilize the material, and bring it to actual, practical commercial technology.”

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