Silicon has underpinned the consumer electronics boom – the Valley didn’t get its name for nothing – and currently plays a crucial role in the rollout of solar power.
It’s stable, cheap and good at turning electricity into sunlight. But an international research collaboration between Chinese, French and Japanese scientists has found a stable material that could knock silicon from its perch.
In a paper published in the journal Science, teams from Shanghai Jiao Tong University, EPFL in Lausanne and the Okinawa Institute of Science and Technology (OIST) demonstrated an inorganic perovskite material, called CsPbl3, is capable of reaching high levels of efficiency at converting solar energy into electricity.
Perovskites are gaining popularity for use in solar panels due to their high efficiency and low cost, but stabilising them has been a challenge. "We are pleased with results suggesting that CsPbI3 can compete with industry-leading materials," says Professor Yabing Qi, head of OIST's Energy Materials and Surface Sciences Unit, who led on the surface science aspect of the study. "From this preliminary result we will now work on boosting the material's stability – and commercial prospects."
CsPbI3 is often studied in its alpha phase, a well-known configuration of the crystal structure appropriately known as the dark phase because of its black colour. This phase is particularly good at absorbing sunlight. Unfortunately, it is also unstable – and the structure rapidly degrades into a yellowish form, less able to absorb sunlight.
In this study, the researchers studied the crystal in its beta phase – a less understood arrangement that is more stable, but has shown relatively low efficiency. This results from cracks that can sometimes emerge in thin-film solar cells, which induce the loss of electrons into adjacent layers and means they can’t flow as electricity.
To heal these cracks, the team treated CsPbI3 crystals with a choline iodide solution, which also optimised the interface between layers in the solar cell, known as energy level alignment.
"Electrons naturally flow to materials with lower potential energy for electrons, so it is important that the adjacent layers' energy levels are similar to CsPbI3," says Dr. Luis K. Ono, a co-author from Qi's lab. "This synergy between layers results in fewer electrons being lost –and more electricity being generated."
The OIST team used ultraviolet photoemission spectroscopy to investigate the energy level alignment, and found that repairing the naturally emerging cracks led to an increase in conversion efficiency from 15 to 18 per cent.
Although that seems small, it could bring CsPbI3 into competition with silicon as a key component of solar panels. The next step for the team will be to improve its stability, cost and efficiency.
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