While even tandem solar cells consisting of two different cell types have yet to prove themselves commercially, those in the research segment can often look a little further into the future. And their work gives us an early idea of what future solar technology could look like.
Stacking two or more light-absorbing materials on top of each other will most likely be a prominent feature – such devices have a theoretical efficiency limit of 68%, as opposed to around 33% for a single-junction solar cell. Indeed, cells comprising two active layers have been very well-investigated and are already far along the road to commercialization.
Combining three or more active layers into one cell is less explored. Efficiencies close to the 40% mark have been achieved with III-V materials, but for all perovskite devices, the efficiency record previously stood at 6.7%.
Now, scientists led by Eindhoven University of Technology in the Netherlands have developed a process that allowed them to fabricate three distinct perovskite layers, and combined these into a device that reached 16.8% conversion efficiency.
The process is described in the paper 16.8% Monolithic all-perovskite triple-junction solar cells via a universal two-step solution process, published in nature communications. By making changes to the precursor solution, the group was able to fabricate three different perovskite materials with complementary bandgap- narrow at 1.23 electron-volts (eV), mid at 1.57 eV and wide at 1.73 eV.
A tandem cell comprising the wide and narrow perovskites achieved 19.5% efficiency. In the triple-junction cell, efficiency is limited by the current of the narrow bandgap layer and parasitic absorption from the electrodes. It is in the wide bandgap perovskite, however, that the researchers identify a need for improvement. “…further improving the PCE of all-perovskite triple-junction solar cells would require an efficient ~2 eV wide bandgap perovskite, which enables a more balanced light absorption in each absorber layer,” the researchers state. “However, such wide bandgap PSCs with low open circuit voltage deficit has not been reported to date.”
The group notes that, though there is a need for more work to get around this limitation, it’s approach presents a reproducible method to produce multiple perovskite layers using a single process.
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