The Fraunhofer Institute for Solar Energy Systems recently announced the development of a multijunction photovoltaic cell with 44.7% efficiency, breaking its own record of 43.6% set just four months prior. A full percentage-point increase in efficiency occurring in such a short time suggests that further improvements may develop rapidly.
Conventional photovoltaic cells used in flat-plate collectors convert about 15% of the sunlight that reaches them into electricity. Those single-junction cells are used in typical rooftop PV panels because they’re relatively inexpensive. One reason for their inefficiency is that they only absorb a narrow band of optical wavelengths. Multijunction cells consist of several layers, each tuned to a particular wavelength of light. As shown below, each layer has a peak efficiency at a certain wavelength, making the combination of layers in the multijunction cell efficient over a wider range of wavelengths. “External Quantum Efficiency” is the ratio of charge carriers (free electrons) to the number of photons that reach the cell. In other words, it’s a measure of how many electrons are “knocked free” by incoming photons.
The key to the improved efficiency is a new method of bonding semiconductor layers. According to Frank Dimroth, Department Head and Project Leader at Fraunhofer, “Besides improved materials and optimization of the structure, a new procedure called wafer bonding plays a central role. With this technology, we are able to connect two semiconductor crystals, which otherwise cannot be grown on top of each other with high crystal quality. In this way we can produce the optimal semiconductor combination to create the highest efficiency solar cells.”
Multijunction cells have higher efficiencies than single junction PV cells, but they’re much more expensive. You won’t see them on rooftops any time soon, but they are gaining wide acceptance in concentrated photovoltaic (CPV) solar farms, which use lenses or curved mirrors to concentrate sunlight onto very small but highly efficient PV cells.
CPV won’t be replacing flat-panel photovoltaics altogether. Flat-plate collectors can still generate energy under indirect light and on cloudy days, while CPV works primarily in direct sunlight, which requires very sunny locations and tracking systems. That’s okay though. There isn’t a single-technology solution to our energy situation. In a way, a multijunction PV cell serves as a nice metaphor for green energy production: different layers are tuned to particular wavelengths, just as different technologies are “tuned” to particular regions. Diversity is a good thing!
Images: Fraunhofer Institute for Solar Energy Systems