Power/Performance Bits: Jan. 26

New switchable material

Two MIT researchers developed a thin-film material whose phase and electrical properties can be switched between metallic and semiconducting simply by applying a small voltage. The material then stays in its new configuration until switched back by another voltage. The discovery could pave the way for a new kind of nonvolatile memory.

The findings involve the thin-film material strontium cobaltite (SrCoOx), which has two different structures depending on how many oxygen atoms per unit cell it contains. When more oxygen is present, SrCoOx forms the tightly-enclosed, cage-like crystal structure of perovskite, whereas a lower concentration of oxygen produces the more open structure of brownmillerite.

The researchers found that the material can be flipped between the two forms with the application of a very tiny amount of voltage — just 30 millivolts. And, once changed, the new configuration remains stable until it is flipped back by a second application of voltage.

This diagram shows how an electrical voltage can be used to modify the oxygen concentration, and therefore the phase and structure, of strontium cobaltites. Pumping oxygen in and out transforms the material from the brownmillerite form (left) to the perovskite form (right). (Source: Bilge Yildiz and Qiyang Lu/MIT)

Previous work with strontium cobaltites relied on changes in the oxygen concentration in the surrounding gas atmosphere to control which of the two forms the material would take. The basic principle was developed within the last year by scientists at Oak Ridge National Laboratory. “While interesting, this is not a practical means for controlling device properties in use,” said MIT associate professor Bilge Yildiz.

“Voltage modifies the effective oxygen pressure that the material faces,” Yildiz added. To make that possible, the researchers deposited a very thin film of the material (the brownmillerite phase) onto a yttrium-stabilized zirconia substrate. In that setup, applying a voltage drives oxygen atoms into the material. Applying the opposite voltage has the reverse effect.

Ongoing research will focus on a better understanding of the electronic properties of the material in its different structures and extending the approach to other oxides of interest for memory and energy applications.

Improved by defects

Theoretical research from the Energy Department’s National Renewable Energy Laboratory (NREL) resulted in what may seem like a paradoxical outcome – certain defects in silicon solar cells may actually improve their performance.

Specifically, simulations to add impurities to layers adjacent to the silicon wafer in a solar cell suggest defects with properly engineered energy levels can improve carrier collection out of the solar cell, or improve surface passivation of the absorber layer.

Finding the right defect was key to the process. To promote carrier collection through the tunneling SiO2 layer, the defects need to have energy levels outside the Si bandgap but close to one of the band edges in order to selectively collect one type of photocarrier and block the other. In contrast, for surface passivation of Si by aluminum oxide (Al2O3), without carrier collection, a beneficial defect is deep below the valence band of silicon and holds a permanent negative charge. The simulations removed certain atoms from the oxide layers adjacent to the Si wafer, and replaced them with an atom from a different element, thereby creating a “defect.” For example, when an oxygen atom was replaced by a fluorine atom it resulted in a defect that could possibly promote electron collection while blocking holes.

Schematic of a ‘good’ defect (red cross), which helps collection of electrons from photo-absorber (n-Si), and blocks the holes, hence suppresses carriers recombination. (Source: NREL)

While more research is needed in order to determine which defects would produce the best results, the principles are applicable to other materials and devices: a recent study by the same authors showed that the addition of oxygen could improve the performance of two-dimensional semiconductors. For solar cells and photoanodes, engineered defects could possibly allow thicker, more robust carrier-selective tunneling transport layers or corrosion protection layers that might be easier to fabricate.

Boosting perovskite-based solar cells

Some of the most promising solar cells today use light-harvesting films made from perovskites. However, perovskite-based solar cells require expensive “hole-transporting” materials, whose function is to move the positive charges that are generated when light hits the perovskite film.

But researchers at EPFL saw a problem: there are currently only two hole-transporting materials available for perovskite-based solar cells, and both types are quite costly to synthesize, adding to the overall expense of the solar cell.

To address this, the team developed a molecularly engineered hole-transporting material, dissymmetric fluorene–dithiophene (FDT), which could bring costs down while keeping efficiency up to competitive levels. Tests showed that the efficiency of FDT rose to 20.2% – higher than the other two alternatives.

3-D illustration of FDT molecules on a surface of perovskite crystals. (Source: Sven M. Hein/EPFL)

“The best performing perovskite solar cells use hole transporting materials, which are difficult to make and purify, and are prohibitively expensive, costing over €300 per gram preventing market penetration,” said EPFL’s Mohammad Nazeeruddin. “By comparison, FDT is easy to synthesize and purify, and its cost is estimated to be a fifth of that for existing materials – while matching, and even surpassing their performance.”