Caffeine Significantly Boosts the Performance of Solar Cells

A new study by California NanoSystems Institute at UCLA and Solargiga Energy in China revealed that caffeine improves the thermal stability of perovskite solar cells, which are viewed as the future of solar power.

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“The boiling point of caffeine is 300°C, which is higher than the operational temperature of solar cells, so it seemed like a possible candidate,” Rui Wang, a UCLA graduate student, a co-first author of the Joule journal-published study, stated in 2018.

To test whether caffeine would improve the device’s thermal stability, Wang and other team members made a custom perovskite film - by mixing dimethylformamide, methylammonium iodide and lead iodide to create a liquid solution - added caffeine and poured the solution onto indium tin oxide glass to form a black layer of perovskite.

The carbon atoms double bonded to oxygen atoms – known as carbonyl groups – in caffeine to create a “molecular lock.”

The group incorporated the newly formed film into a solar cell, placed it on a plate heated to 85°C to test its ability to withstand high temperatures.

“Solar cells need high thermal stability since they are constantly exposed to sunlight, which warms up the devices,” UCLA’s Carol and Lawrence E. Tannas Jr. Professor of Engineering, Yang Yang, said.

The researchers found that the device retained thermal stability for more than 1,300 hours or about 55 days while preserving 86% of the energy it took in — a measure called power conversion efficiency.

“Parts of caffeine’s chemical structure were forming very strong binding with the lead ions and stabilizing the crystals,” Jingjing Xue, a UCLA graduate student and another co-first author of the study, stated. “The molecular lock between caffeine and lead also slowed down the growth of perovskite crystals, allowing them to align into an orientation that is beneficial for electric charge transfer.”

The team used a transmittance electron microscope to analyze how the new film’s crystalline structure evolved, confirming that there was indeed a molecular lock between the caffeine and the lead ions.

“The molecular lock may help push perovskite solar cells to commercialization in the future,” Yang said. “Caffeine is the first compound we identified, but there may be others that can work even more efficiently.”

So far, perovskite solar cells are trending to cost less to produce than the current widely used silicon solar cells as well as be more energy-efficient. The perovskite solar cells are also easier to manufacture because they can be fabricated from solution-based precursors as opposed to solid crystal ingots.  

However, the viability of perovskite solar cells requires further review since the material currently faces a challenge withstanding sustained heat from sunlight.

But the research on perovskite solar cells only dates back to the early 2010s, while its silicon solar cells counterpart - with which it has already proved to be a near-equal competitor - have been studied for over four decades. 

“While perovskites are an attractive option for solar cells, the materials degrade and become less stable over time. We need them to last 20 to 30 years like traditional solar cells,” Yang explained.

The research was supported by the Air Force Office of Scientific Research, the Office of Naval Research, the University of California Advanced Solar Technologies Institute, the Natural Science Foundation of China, Suzhou Nano Science and Technology’s Collaborative Innovation Center, Jiangsu Higher Education Institutions’ Priority Academic Program Development and Jinzhou Solargiga Energy.