Researchers at the University of California, Los Angeles (UCLA) have succeeded in improving the performance and thermal stability of metal halide perovskite solar cells by adding an unexpected ingredient to them – caffeine. The technique could potentially play an important role in scaling up the production of these solar cells, they say.
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 though and is holding back the commercialization of PVSK solar cells.
PVSK film rapidly deteriorates
MA-based PVSKs are the best candidates in this respect since the tetragonal black phase of these materials is stable at low temperatures. At higher temperatures, however, the volatile MA organic cation means that the PVSK film rapidly deteriorates and precipitates out trigonal PbI2.
Worse still, the numerous under-coordinated ions in the PVSK (as is the case in most ionic crystals) mean that I– ions easily migrate though the polycrystalline grains and even out of the PVSK layer to interfere with the metal electrodes in a solar cell device when exposed to heat. This process produces defects that act as non-radiative recombination sites at grain boundaries, thus lowering the performance of the solar cell.
Finally, the randomly oriented PVSK crystallites may result in poor charge transport in the vertical direction. The random orientation comes as a consequence of the fast and uncontrollable growth of the PVSK film when it is synthesized.
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.”
And that is not all: since the caffeine is non-volatile, it remains inside the perovskite film and forms a “molecular lock” therein, allowing the molecule to strongly interact with Pb2+ions, he adds. “This interaction prohibits the degradation of the perovskite at high temperature. The enhanced crystallinity also suppresses ion migration, which contributes to the much-enhanced thermal stability too.”
PCE as high as 20.25%
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).
“Solar cells obviously work under sunlight, which of course heats them up, so they must be made thermally tolerant,” he adds. “Our result shows that caffeine can help the perovskite cells maintain over 85% of their original efficiency after 1300 hours of continuous heating at 85°C. We believe that our strategy could help push forward the commercialization of these materials in the future.”
The UCLA researchers, reporting their work in Joule, are now busy further investigating the chemical structure of their caffeine-containing PVSKs. “We are also looking for the best passivation agents for these materials to enhance their stability and efficiency even more. Finally, we will be focusing on fabricating high-quality large area samples from these films.”