Microstructural Defects Improve Cadmium Telluride Solar Cell Conductivity

Scientists studying cadmium telluride-based solar cells, which are a promising substitute for silicon-based cells, have found that microscopic "fault lines" present within and between the material’s crystals function as conductive pathways that enable the flow of electric current.

The details of the study conducted at the U.S. Department of Energy's Brookhaven National Laboratory and the University of Connecticut have been published in the journal Nature Science. The study demonstrates how cadmium telluride can be turned into a suitable material to generate solar power through a common processing technique. It can also suggest a method for designing solar devices that are more efficient than silicon-based devices.

If you look at semiconductors like silicon, defects in the crystals are usually bad.

Eric Stach, Physicist, Brookhaven Lab's Center for Functional Nanomaterials (CFN)

Stach explained that slight shifts in the alignment of atoms or misplaced atoms trap the particles that carry electricity - positively charged "holes" that get left behind and become mobile when photons in the sunlight knock loose the electrons, or negatively charged electrons.

Solar cells separate positive and negative charges, which are then run through a circuit. The current produced is used to power satellites, cities and houses. The flow of charges is interrupted by defects, which affect the efficiency of the solar cells.

However, the scientists found that with cadmium telluride, instead of acting as traps the boundaries between "planar defects" and individual crystals - fault-like misalignments in the atom arrangement - function as pathways for conductivity.

This unexpected connection was first noticed by members of Bryan Huey's team at the Institute of Materials Science at the University of Connecticut. Justin Luria and Yasemin Kutes, who were exploring the effects of a chloride solution treatment that immensely improves the conductive properties of cadmium telluride, studied solar cells before and after the treatment in a unique way.

Earlier studies that explored the surface of a solar cell have used a conducting atomic force microscope for the purpose. Equipped with a fine probe much sharper than the head of a pin, the microscope scans across the hills and valleys of a material's surface to observe its topographic features, and at the same time measures location specific conductivity.

This technique is used by scientists to study how the performance of a solar cell is affected by the surface features at the nanoscale.

No effort has been taken to measure the most significant part of the solar cell that lies beneath the surface. Kutes and Luria developed a new approach to achieve this.

They acquired hundreds of sequential images after removing one nanoscale layer of the material every time, to scan through the material’s thickness. These layer-by-layer images were then used to build a 3D, high resolution tomographic map of the solar cell, which is similar to a computed tomography (CT) brain scan.

Everyone using these microscopes basically takes pictures of the 'ground,' and interprets what is beneath. It may look like there's a cave, or a rock shelf, or a building foundation down there. But we can only really know once we carefully dig, like archeologists, keeping track of exactly what we find every step of the way—though, of course, at a much, much smaller scale.

Bryan Huey, University of Connecticut

The free flow of current through the fault-like defects and crystal boundaries of the cadmium telluride solar cells was revealed in the CT-AFM maps. In comparison to the samples that were not treated with the solution, and had few defects, no connectivity and less conductivity, the samples treated with the chloride solution had more defects, higher density of defects and high degree of connectivity.

The team suspected that the defects were planar defects that are caused by stacking arrangements within the crystals or shifts in atomic alignments. However, the CT-AFM is not capable of revealing structural details at an atomic scale. In order to get this information, the team approached Stach who is the head of the electron microscopy group of CFN, a DOE Office of Science User Facility.

"Having previously shared ideas with Eric, it was a natural extension of our discovery to work with his group," Huey said.

Said Stach, "This is the exact type of problem the CFN is set up to handle, providing expertise and equipment that university researchers may not have to help drive science from hypothesis to discovery."

CNF physicist Lihua Zhang used UConn's results and a transmission electron microscope (TEM) to study how the cadmium telluride’s atomic scale features post chloride solution treatment are related to the conductivity maps. The atomic structures of the defects as revealed by the TEM images confirm that they are caused by certain changes in the stacking sequence of atoms in the sample.

The images also confirmed that the planar defects linked various grains in the crystal and created high-conductivity pathways that facilitate electron and hole movement.

"When we looked at the regions with good conductivity, the planar defects linked from one crystal grain to another, forming continuous pathways of conductance through the entire thickness of the material," said Zhang. "So the regions that had the best conductivity were the ones that had a high degree of connectivity among these defects."

The researchers believe that the chloride treatment causes more connectivity as well as more defects, but further research is necessary to definitively determine the important effects of the chloride solution treatment.

Stach states that using electron microscopy and CTAFM technique in tandem yields a “clear winner” in the pursuit for cost-effective and efficient alternatives for silicon-based solar cells that have reached their efficiency levels.

There is already a billion-dollar-a-year industry making cadmium telluride solar cells, and lots of work exploring other alternatives to silicon. But all of these alternatives, because of their crystal structure, have a higher tendency to form defects. This work gives us a systematic method we can use to understand if the defects are good or bad in terms of conductivity. It can also be used to explore the effects of different processing methods or chemicals to control how defects form. In the case of cadmium telluride, we may want to find ways to make more of these defects, or look for other materials in which defects improve performance.

Eric Stach, Physicist, Brookhaven Lab's Center for Functional Nanomaterials (CFN)

The DOE Office of Science and the DOE Office of Energy Efficiency and Renewable Energy (EERE) including its SunShot Initiative supported the endeavor. Andrew Moore from the Colorado State University provided the cadmium telluride samples used in the study.

Video Credit: BrookhavenLab/Youtube.com