Power/Performance Bits: Dec. 1

Hiding wires from the sun

There’s a problem with most solar cells: the electricity-carrying metal wire grid on top prevents sunlight from reaching the semiconductor below. A team from Stanford University tackled this problem, discovering a way to hide the reflective upper contact and funnel light directly to the semiconductor below.

For the study, the researchers placed a 16-nanometer-thick film of gold on a flat sheet of silicon. The gold film was riddled with an array of nanosized square holes, but to the eye, the surface looked like a shiny, gold mirror.

The perforated gold film covered 65% of the silicon surface and reflected, on average, 50% of the incoming light. Until they immersed the film in a solution of hydrofluoric acid and hydrogen peroxide, which caused the gold film to sink into the substrate and created silicon nanopillars in the holes of the film.

Within seconds, the silicon pillars grew to a height of 330 nanometers, transforming the shiny gold surface to a dark red, a signal that the metal was no longer reflecting light.

Silicon nanopillars funnel light through a gold metal contact to a sheet of silicon underneath. (Source: Vijay Narasimhan/Stanford)

“As soon as the silicon nanopillars began to emerge, they started funneling light around the metal grid and into the silicon substrate underneath,” Stanford graduate student Vijay Narasimhan explained, comparing the nanopillar array to a colander in a kitchen sink. “When you turn on the faucet, not all of the water makes it through the holes in the colander,” he said. “But if you were to put a tiny funnel on top of each hole, most of the water would flow straight through with no problem. That’s essentially what our structure does: The nanopillars act as funnels that capture light and guide it into the silicon substrate through the holes in the metal grid.”

“Solar cells are typically shaded by metal wires that cover 5-to-10 percent of the top surface,” Narasimhan said. “In our best design, nearly two-thirds of the surface can be covered with metal, yet the reflection loss is only 3 percent. Having that much metal could increase conductivity and make the cell far more efficient at converting light to electricity.”

The research team plans to test the design on a working solar cell and assess its performance in real-world conditions.

Self-healing gel for circuits

Researchers from The University of Texas at Austin developed a self-healing gel that repairs and connects electronic circuits. The material has high conductivity and strong mechanical and electrical self-healing properties, which could be of use in the development of flexible electronics, biosensors and batteries.

Previously, self-healing materials have relied on application of external stimuli such as light or heat to activate repair. “In the last decade, the self-healing concept has been popularized by people working on different applications, but this is the first time it has been done without external stimuli,” said mechanical engineering assistant professor Guihua Yu, who developed the gel. “There’s no need for heat or light to fix the crack or break in a circuit or battery, which is often required by previously developed self-healing materials.”

Self-repaired supergel supports its own weight after being sliced in half. (Source: UT Austin)

The team created the self-healing gel by combining two gels: a self-assembling metal-ligand gel that provides self-healing properties and a polymer hydrogel that is a conductor. Yu believes the self-healing gel would not replace the typical metal conductors that transport electricity, but could be used as a soft joint, joining other parts of the circuit.

“This gel can be applied at the circuit’s junction points because that’s often where you see the breakage,” he said. “One day, you could glue or paste the gel to these junctions so that the circuits could be more robust and harder to break.”

The team is also looking into other applications, including medical applications and energy storage, where it holds potential to be used within batteries to better store electrical charge.

Defect-free monolayer MoS2

Monolayer semiconductors hold promise in the development of transparent LED displays, ultra-high efficiency solar cells, photo detectors and nanoscale transistors. Unfortunately, the films are notoriously riddled with defects, killing their performance.

A research team, led by engineers at UC Berkeley and Lawrence Berkeley National Laboratory, found a way to fix these defects in monolayer molybdenum disulfide by dipping a sample in an organic superacid called bistriflimide. The chemical treatment led to a dramatic 100-fold increase in the material’s photoluminescence quantum yield, a ratio describing the amount of light generated by the material versus the amount of energy put in. The greater the emission of light, the higher the quantum yield and the better the material quality.

“Traditionally, the thinner the material, the more sensitive it is to defects,” said principal investigator Ali Javey, UC Berkeley professor and a faculty scientist at Berkeley Lab. “This study presents the first demonstration of an optoelectronically perfect monolayer, which previously had been unheard of in a material this thin.”

A MoS2 monolayer semiconductor shaped into a Cal logo. The image on the left shows the material before it was treated with superacid. On the right is the monolayer after treatment. (Source: Matin Amani/UC Berkeley)

The researchers added that the efficiency of an LED is directly related to the photoluminescence quantum yield so, in principle, one could develop high-performance LED displays that are transparent when powered off and flexible using the “perfect” optoelectronic monolayers produced in this study.

This treatment also has potential for transistors. As devices in computer chips get smaller and thinner, defects play a bigger role in limiting their performance. According to Javey, “the defect-free monolayers developed here could solve this problem in addition to allowing for new types of low-energy switches.”