How to Image Solar Cells in 3D

Using two-photon tomography, carrier lifetimes are mapped in polycrystalline CdTe photovoltaic devices. (Source: LBL)

Using two-photon tomography, carrier lifetimes are mapped in polycrystalline CdTe photovoltaic devices. (Source: LBL)

Next-gene­ration solar cells made of super-thin films of semi­conducting material hold promise because they’re relatively inex­pensive and flexible enough to be applied just about anywhere. Researchers are working to drama­tically increase the effi­ciency at which thin-film solar cells convert sunlight to elec­tricity. But it’s a tough challenge, partly because a solar cell’s sub­surface realm – where much of the energy-con­version action happens – is inaccessible to real-time, nonde­structive imaging. It’s difficult to improve processes you can’t see.

Now, scientists from the Department of Energy’s Lawrence Berkeley National Labo­ratory have developed a way to use optical micro­scopy to map thin-film solar cells in 3-D as they absorb photons. The method images opto­electronic dynamics in materials at the micron scale. This is small enough to see indi­vidual grain boundaries, substrate interfaces, and other internal obstacles that can trap excited electrons and prevent them from reaching an electrode, which saps a solar cell’s efficiency. So far, scientists have used the technique to better understand why adding a specific chemical to solar cells made of cadmium telluride improves the solar cells’ performance.

“To make big gains in photovoltaic efficiency, we need to see what’s happening throughout a working photovoltaic material at the micron scale, both on the surface and below, and our new approach allows us to do that,” says Edward Barnard, a principal scientific engineering associate at the Molecular Foundry. While fabri­cating new solar cell materials at the Molecular Foundry, the team found that standard optical techniques couldn’t image the inner-workings of the materials, so they developed the new technique to obtain this view. Next, scientists from the National Renewable Energy Labo­ratory came to the Molecular Foundry and used the new method to study CdTe solar cells.

To develop the approach, the scientists modified two-photon micro­scopy so that it can be applied to bulk semi­conductor materials. The method uses a highly focused laser beam of infrared photons that penetrate inside the photo­voltaic material. When two low-energy photons converge at the same pinpoint, there’s enough energy to excite electrons. These electrons can be tracked to see how long they last in their excited state, with long-lifetime electrons appearing as bright spots in micro­scopy images. In a solar cell, long-lifetime electrons are more likely to reach an electrode. In addition, the laser beam can be systema­tically repositioned throughout a test-sized solar cell, creating a 3-D map of a solar cell’s entire opto­electronic dynamics.

The method has already shed light on the benefits of treating CdTe solar cells with cadmium chloride, which is often added during the fabri­cation process. Scientists know cadmium chloride improves the efficiency of CdTe solar cells, but its effect on excited electrons at the micron scale is not well under­stood. Studies have shown that the chlorine ions tend to pile up at grain boundaries, but how this changes the lifetime of excited electrons is unclear.

Thanks to the new imaging technique, the researchers discovered the cadmium chloride treatment increases the lifetime of excited electrons at the grain boundaries, as well as within the grains themselves. This is easily seen in 3-D images of CdTe solar cells with and without the treatment. The treated solar cell lights up much more uniformly throughout the material, both in the grains and the spaces in between. “Scientists have known that cadmium chloride passi­vation improves the lifetime of electrons in CdTe cells, but now we’ve mapped at the micron scale where this improvement occurs,” says Barnard.

The new imaging technique could help scientists make more informed decisions about improving a host of thin-film solar cell materials in addi­tion to CdTe, such as perov­skite and organic compounds. “Researchers trying to push photo­voltaic effi­ciency could use our technique to see if their strategies are working at the micro­scale, which will help them design better test-scale solar cells—and even­tually full-sized solar cells for rooftops and other real-world applications,” he says. (Source: LBL)

Reference: E. S. Barnard et al.: 3D Lifetime Tomography Reveals How CdCl2 Improves Recombination Throughout CdTe Solar Cells, Adv. Mat., online 15 November 2016; DOI: 10.1002/adma.201603801

Link: Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, USA

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