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# Layered Solar Cell Technology Exhibits Record Levels of Power-Conversion Efficiency

March 14, 2020 by Luke James

## Scientists from the University of Colorado Boulder (CU Boulder) have created a layered, low-cost solar cell technology that exhibits some of the highest levels of power-conversion efficiency to date.

Solar cells, which represent a $30 billion per year industry, have been enhanced by thirty-percent, according to Michael McGehee, a professor in the Department of Chemical and Biological Engineering and co-author of the research team’s paper. This was achieved by layering cells along with a unique combination of elements, and the CU Boulder team is hopeful that it will pave the way for a brighter future for solar power. ##### Perovskite/silicon solar cells. Image used courtesy of Dennis Schroeder / NREL. ### Layering Perovskite on Silicon For several years, silicon solar cells have been the go-to standard in the solar power industry. However, current silicon-based solar cells are only capable of converting between 18% and 21% of the sun’s energy into useful electricity on average, and they top-out at around 26.6%. In an attempt to make solar cells more efficient, the research team took a perovskite solar cell—a crystal structure that harvests higher energy photons—and layered it on top of one of these silicon solar cells, which capture photons in the infrared part of the light spectrum. Combined, the perovskite and silicon solar cells achieve an efficiency of 27%. This represents an increase of one-third of a silicon cell’s efficiency of 21%. #### Impact on Solar Panel Efficiency Although this may sound like a marginal increase, it actually represents a dramatic improvement. When solar cells are applied over a large surface area—for example in a solar panel—they lose a few “percentage points” worth of conversion potential. Therefore, the average efficiency of solar panels is lower than the actual maximum. This means that more solar panels need to be installed to achieve higher levels of efficiency, and this translates to a higher overall cost. By layering a perovskite solar cell on top of a silicon one, however, overall efficiency is dramatically improved without having to install as many panels to get a comparable amount of power. ##### Michael, McGehee, the lead researcher of the CU Boulder team that developed a low-cost solar cell which dramatically increases the efficiency of solar panels. Image used courtesy of CU Boulder. ### A More Affordable Solution This is not the first time that researchers have improved efficiency by layering solar cells. Tandem or multi-junction solar cells were developed way back in the 1970s and these can achieve efficiency levels of almost 50%. Unfortunately, these are very, very expensive—often coming with price tags as high as$80,000/m2—due to the way they are manufactured.

By taking a different approach, the CU Boulder research team has managed to create their cells at a cost more than 100 times lower. This “secret formula”, as McGehee put it, is down to the use of a unique triple-halide alloy of chlorine, bromine, and iodine.

### Considerations for a Growing Solar Power Market

In solar cells, there is an ideal bandgap—the space between energy levels—and bromine can be used to raise this. When used with iodine and exposed to light, however, these elements tend to not stay in place. And while previous studies have tried to use chlorine and iodine together, not enough chlorine has been able to fit in the perovskite crystal structure due to the elements’ varying particle sizes.

By using different amounts of bromine, chlorine, and iodine, however, the CU Boulder team has managed to shrink the crystal structure, allowing more chlorine to fit in and stabilize and improve the solar cell’s efficiency.

With the solar power market growing around 30% per year, efficiency, cost and longevity are major considerations for which new technologies will become mainstream. McGehee is optimistic about the potential of the team’s wide-bandgap perovskite solar cell, which exhibited minimal change in efficiency after 1,000 hours of intensive light and heat testing.