Researchers from the University of Amsterdam and NWO-Institute AMOLF have examined the efficiency gain offered by perovskite/silicon tandem solar cells containing several semiconductors with diverse energy gaps, with a spectrum splitter added between the top and bottom terminals.
This design allows the tandem solar cells to be responsive to a wider region of the sunlight's spectrum. However, such cells usually deal with ineffective light trapping and management due to parasitic light absorption in inactive layers and reflection between layers. Various studies have looked into these issues, yet the idea of spreading sunlight in the tandem subcells with controlled spectral splitting was not adequately investigated.
"In PV research, every digit that can be gained in cell efficiency is crucial. Adding a spectrum splitting interlayer can help us harvest the maximum current and voltage by splitting the incident sunlight into low- and high-energy spectral bands and distributing them effectively among the top and bottom cells", said Albert Polman, Study Lead, NWO-Institute AMOLF.
Polman observes that adding spectral splitters can also pave the way to higher throughput in fabrication, economical solar cells and lower toxicity per unit cell area. Moreover, they can increase the range of tandem cells for applications in the photoelectrochemical splitting of water.
Polman's team analyzed two types of spectral splitters, namely "Lambertian" and "planar" splitters, in their research. The planar splitter behaves like a mirror, reflecting the light right back to the top cell to allow it to be captured there.
On the contrary, the Lambertian splitter reflects light in an angular manner so that it has to complete a much longer path via the solar cell, and hence can be captured without difficulty.
Based on their calculations, when a spectral splitter is present, industrially valid top cells with energy gaps of 1.7 eV and higher demonstrated a 5-to-6 percent gain in absolute efficiency in the limit of infinite thickness for a 500-nm thick top cell.
Their predictions also showed that selecting a Lambertian spectral splitter over a planar splitter considerably increases the efficiency enhancement.
Although the study is theoretical, the results seem economically appealing. As long as there is no substantial splitting performance loss with upscaling, and the production steps are of comparable costs, the efficiency gain per layer of the perovskite/silicon coupled with a Lambertian spectral splitter is inside an economically competitive range.