Crystals form the basis for the penetrating icy blue glare of car headlights and now they could be fundamental to the future in solar energy technology? Crystals are at the heart of diodes. Not the kind you might find in quartz, formed naturally, but manufactured to form alloys, such as indium gallium nitride or InGaN. This alloy forms the light emitting region of LEDs, for illumination in the visible range, and of laser diodes (LDs), in the blue-UV range.

The atomic arrangement at a relaxed InGaN/GaN interface. Research at ASU and Georgia Tech show layer-bylayer crystal growth may lead to record-breaking efficiencies in photovoltaic solar cell technology. (Photo: Arizona State University)

Research into making better crystals with high crystalline quality, light emission efficiency and luminosity is also at the heart of studies being done at Arizona State University by research scientist Alec Fischer and doctoral candidate Yong Wei in professor Fernando Ponce’s group in the Department of Physics. The ASU group, in collaboration with a scientific team led by professor Alan Doolittle at the Georgia Institute of Technology, has just revealed the fundamental aspect of a new approach to growing InGaN crystals for diodes, which promises to move photovoltaic solar cell technology toward record-breaking efficiencies.

The InGaN crystals are grown as layers in a sandwich-like arrangement on sapphire substrates. Typically, researchers have found that the atomic separation of the layers varies; a condition that can lead to high levels of strain, breakdowns in growth and fluctuations in the alloy’s chemical composition.

According to Ponce, easing the strain and increasing the uniformity in the composition of InGaN is very desirable, but difficult to achieve. Growth of these layers is similar to trying to smoothly fit together two honeycombs with different cell sizes, where size difference disrupts a periodic arrangement of the cells.

The scientists developed an approach where pulses of molecules were introduced to achieve the desired alloy composition. The method, developed by Doolittle, is called metal-modulated epitaxy. Essentially, it allows an atomic, layerby- layer growth of the material.

Analysis of the atomic arrangement and the luminosity at the nanoscale level was performed by Fischer and Wei. Their results showed that the films grown with the epitaxy technique had almost ideal characteristics and revealed that the unexpected results came from the strain relaxation at the first atomic layer of crystal growth. Doolittle’s group was able to assemble a final crystal that is more uniform and whose lattice structures match up, resulting in a film that resembles a perfect crystal. The luminosity was also like that of a perfect crystal.

The ASU and Georgia Tech team’s elimination of these two seemingly insurmountable defects (non-uniform composition and mismatched lattice alignment) ultimately means that LEDs and solar photovoltaic products can now be developed that have much higher, efficient performance. The researchers believe that, although they are still a ways off from recordsetting solar cells, this breakthrough could have immediate and lasting impact on light emitting devices and could potentially make the second most abundant semiconductor family, III-Nitrides, a real factor in the solar cell field.

For more information, visit https://asunews.asu.edu/20131025-solar-cell-efficiency-boost