Nanostructures raise solar cell efficiency

Nanoparticles scatter incoming light
The latest design maximises photon absorption
Researchers are working to develop new devices that 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.
Past approach
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.
From the outside, the new optimized devices behave just like traditional thin-film solar cells. But inside, nanostructures enable the solar cells to circumvent an important trade-off 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.
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 per cent sunlight-to-electricity conversion efficiencies.
“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 per cent.
This is a very large improvement over the 31 per cent maximum theoretical efficiency for today’s solar cells with classic p-n junctions,” said Edward Yu, the Principal Investigator.

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, according to a University of California, San Diego, press release.
Long path
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