Caffeine boosts perovskite solar cells
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| IS THE WORLD READY FOR MORMON-UNFRIENDLY PHOTOVOLTAICS? |
Organic-inorganic hybrid halide perovskites (PVSKs) have an ABX3 structure, where A is caesium, methylammonium (MA) or formamidinium (FA), B is lead or tin and X is chlorine, bromine or iodine. They are a promising alternative to silicon for making solar cells thanks to their unique photoelectric properties that lend themselves well to a host of applications. They are also cheaper and more flexible than solar cells made from silicon and are easier to manufacture (from solution-based precursors, for example, rather than solid crystals).
Researchers have managed to increase the power conversion efficiency (PCE) of these materials from just 3.8% (in 2009) to over 23%. This makes their performance comparable to that of established technologies such as silicon, GaAs and CdTe. Despite this impressive advance, the long-term stability of PVSK is still rather poor...At higher temperatures, however, the volatile MA organic cation means that the PVSK film rapidly deteriorates and precipitates out trigonal PbI2.
Caffeine boost
A team led by Yang Yang at the Department of Materials Science and Engineering at UCLA has now shown that 1,3,7-trimethylxanthine – a commodity chemical containing two conjugated carboxyl groups and better known by its common name caffeine – improves the performance and thermal stability of solar cells based on MAPbI3 when added directly to the perovskite film. The technique also works well for the perovskite CsFAMAPbI3. This type of perovskite contains a black phase of Cs that is thermodynamically unfavourable at room temperature.
The idea of adding caffeine to these materials started out as a joke over morning coffee, recalls team member Jingjing Xue. “One day, as we were discussing perovskite solar cells, our colleague Rui Wang said: ‘if we need coffee to boost our energy, then what about perovskites?’”
Improved electronic properties and a “molecular lock”
“We found that caffeine strongly interacts with the perovskite precursors thanks to its lone electron pairs of carbonyl groups – an effect that we confirmed with Fourier transform infrared spectroscopy,” explains Wang. “This strong interaction enhances the activation energy for the crystallization of the perovskite and retards its crystallization. This results in a perovskite film with a preferred orientation and improved electronic properties that is more efficient at converting light into electricity.”
The superior crystallinity of the PVSK films containing caffeine boast reduced defect densities and better vertical charge transport, allowing for a champion PCE as high as 20.25% (compared to 17% for films without caffeine), he tells Physics World. The caffeine-containing devices are also thermally stable for over 1300 hours at 85°C (compared to just 150 hours for films without caffeine).READ M
