University working to create nanostructures to raise thin-film solar cell efficiency

Professor Edward Yu, the principal investigator on the grant explained that the most recent estimate of the maximum power conversion efficiency – under normal illumination conditions – that one can expect with this approach is approximately 45%, which is a very large improvement over the 31% maximum theoretical efficiency for today's solar cells with classic p-n junctions.

By Ann Steffora Mutschler, Senior Editor -- EDN, 5/16/2008

Researchers at the University of California, San Diego (UCSD) are working to create thin-film “single junction” solar cells with 45% sunlight-to-electricity conversion efficiencies by using nanostructures that scatter and channel light.

UCSD recently received a big funding boost from the U.S. Department of Energy (DOE)’s Solar America program for this work that aims to break the theoretical limit of 31% efficiency for conventional single junction cells.

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

Professor Yu, the principal investigator on the grant explained in a statement that the most recent estimate of the maximum power conversion efficiency – under normal illumination conditions – that one can expect with this approach is approximately 45%, which is a very large improvement over the 31% maximum theoretical efficiency for today’s solar cells with classic p-n junctions.

UCSD electrical engineering professors Paul Yu and Deli Wang are co-principal investigators 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.

Specifically, the UCSD engineers are using nanoparticles to scatter incoming light into paths within the quantum well region – paths that run parallel to the p-n junction, which 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. This enables high photon absorption efficiency, high electron and hole collection efficiency – and therefore also high voltage – to be achieved simultaneously,” Yu said in a statement.

Further, in this approach, the photons are provided with a long path along the quantum wells and the carriers have a short path to the electrode that 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,” Yu concluded.

In other solar efficiency work, Monday, SunPower said it has developed a full-scale solar cell prototype with 23.4% efficiency.