Connecting two solar technologies is a win-win situation in terms of efficiency and stability

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While traditional silicon-based solar cells have had an unequivocal impact on building renewable energy resources around the world, it is becoming increasingly difficult to make incremental improvements in performance as devices approach the limits of their practical efficiency. This limitation has led scientists to search for new technologies that can be combined with silicon cells to unlock higher efficiencies.

Solar cells made of crystals called perovskite are one such technology that has quickly emerged as an attractive, low-cost addition, but perovskite cells are notoriously sensitive to changes in voltage — the shade from an overhanging tree branch or a nearby plant can blow out an entire unit in a matter of minutes.

Now, researchers from Princeton University and the King Abdullah University of Science and Technology (KAUST) have coupled the well-established silicon solar cell with an ascending perovskite into a tandem solar cell to not only boost overall efficiency, but also enhance stability. Results, reported in JouleShow that the connection protects the weak perovskite solar cell from voltage-induced breakdown while achieving efficiencies greater than either cell alone could achieve.

“Tandem solar cells have already demonstrated greater energy conversion efficiencies than silicon or perovskite solar cells alone,” said Barry Rand, research leader and professor of electrical and computer engineering at the Endlinger Center for Energy and the Environment. “We thought that in addition to their higher efficiency, tandem solar cells could also solve some of the stability challenges of perovskites by pairing them with silicon cells, which are much more stable.”

To test their hypothesis, the researchers built three strings of solar cells: one containing only silicon solar cells, one containing only perovskite solar cells, and one made up of tandem solar cells, connecting the two technologies in series. The researchers then shaded one of the cells in the string to simulate the partial shade conditions that the solar system would encounter at least once during its decades-long lifetime.

This partial darkening usually leads to perovskite degradation, as the still luminous cells force charge to flow through the shaded, inactive cell, rapidly weakening it and the entire module. On the other hand, silicon solar cells are more flexible to voltage fluxes, and can tolerate periods of partial shading with fewer problems.

As expected, the perovskite-only solar module rapidly deteriorated after partial shading, while the silicon solar module was only slightly affected. Interestingly, however, the tandem solar module was just as flexible as the silicon-only module, meaning that by linking the two solar technologies, the silicon cell was able to mask the fragility of the perovskite.

“When you combine two different materials to make a final product, it is usually the weakest link that determines the overall strength of the chain,” said co-author Stefan De Wolf, professor of materials science and engineering at KAUST. “But in this case, it is the stronger element that protects the weaker element.”

The researchers said their findings show that partial shading — which used to be a major barrier for perovskite modules alone — may be of little concern for string-connected tandem solar devices.

The team also said the results bode well for perovskites’ commercial prospects, as they imply that perovskites may have the greatest potential when deployed with silicon solar cells, for which there is already a mature manufacturing ecosystem. Rather than having to build out a competing manufacturing process, perovskites can be added to a commercially proven production process for silicon solar cells.

While the team noted that several challenges in addition to partial shading still need to be resolved before tandem solar cells achieve the life expectancy of commercial solar technologies, such as their poor heat tolerance, they said that tandem devices could enable solar research. further development thereafter. Silicon solar cells have reached the upper limits of energy conversion efficiency.

“If some of the other stability challenges can be solved, tandem solar cells can take an already successful commercial technology and make it even better,” Rand said. “Our results make a strong argument that tandem devices should be a comprehensive work area for future solar energy research.”

more information:
Zhaojian Xu et al., Reverse bias elasticity of perovskite/silicon tandem solar cells, Joule (2023). doi: 10.1016/j.joule.2023.07.017.

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