Nanostructures Will Raise Thin-Film Solar Cell Efficiency

Thanks to nanostructures that scatter and channel light, University of California, San Diego electrical engineers are working toward thin-film “single junction” solar cells with the potential for nearly 45 percent sunlight-to-electricity conversion efficiencies. This effort to break the theoretical limit of 31 percent efficiency for conventional single junction cells recently received a big funding boost from the U.S. Department of Energy’s Solar America program.

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In November 2007, a team of Jacobs School electrical engineers led by professor Edward Yu won an $885,000 grant from the U.S. DOE to further develop their thin-film and nanowire solar cell devices that incorporate nanostructures, including semiconductor quantum wells and photon-scattering nanoparticles.

The new devices could lead to big gains in thin-film solar cell efficiency by increasing both the number of photons thin-film solar cells absorb and the number of excited electrons the same devices collect.

“The most recent estimate of the maximum power conversion efficiency – under normal illumination conditions – that one can expect with our new thin-film solar cell approach is approximately 45 percent. This is a very large improvement over the 31 percent maximum theoretical efficiency for today’s solar cells with classic p-n junctions,” said Edward Yu, the Principal Investigator on the three-year DOE grant.

Electrical engineering professors Paul Yu and Deli Wang are co-PIs on the project.

From the outside, the optimized devices behave just like traditional thin-film solar cells. But inside, the nanostructures enable the solar cells to circumvent an important tradeoff that has stymied past attempts to incorporate quantum wells into thin-film solar cells in order to boost device efficiency. Quantum wells can increase solar cell efficiency by raising photon absorption by lowering the energy band gap.

In the past, engineers have tried to add quantum wells to thin-film solar cell devices by stacking several quantum-well layers to achieve a high probability of absorption of low-energy photons. This approach, however, can be counter productive because electron-hole pairs get stuck in the quantum wells, making it impossible for them to generate current for the device.

The UC San Diego engineers are using nanoparticles to scatter incoming light into paths within the quantum well region – paths that run parallel to the p-n junction. This gives photons more time to be absorbed without having to stack the quantum wells to a thickness that makes it hard for electrons and holes to escape.

“Our devices have a much thinner stack of quantum wells, which means the extra photons that are absorbed are much more likely to make it out of the quantum wells and generate current,” explained Edward Yu. “This enables high photon absorption efficiency, high electron and hole collection efficiency – and therefore also high voltage – to be achieved simultaneously.”

In the UCSD approach, the photons are provided with a long path along the quantum wells and the carriers have a short path to the electrode. This design maximizes photon absorption while minimizing a major drain on device efficiency in solar cells – electron-hole recombination.

“We have already demonstrated the basic concepts in thin-film devices. I think it will take a few years to see how far this approach can be pushed to achieve really high efficiency because there are many aspects that have yet to be optimized,” said Yu.