“I believe that perovskite solar cells could be the invention that solves the global energy crisis,” says Farhana Aziz of the Advanced Membrane Technology Centre at the Universiti Teknologi Malaysia. She is well-placed to make such a startling judgement, having co-authored a review of the technology published in the journal Applied Materials Today.
The term perovskites refers to a vast range of mineral structures that share the same geometric arrangement as the ions in the natural perovskite mineral calcium titanate (CaTiO3). Replacing the calcium, titanium, and oxygen with ions of other elements, or with carbon-based ‘organic’ chemical groups, generates new hybrid perovskites with many useful properties, such as those needed for producing solar cells.
“We focus our review specifically on ways to control the crystal structures and increase efficiency by enhancing the transport of charge through the perovskite materials,” Aziz explains.
Aziz's interest in solar cells came naturally to her, as she is from Malaysia, where there is abundant sunshine all year round. Her focus on perovskites was stimulated some years ago by increasing excitement around the field, which led one prestigious journal to hail perovskite solar cells as one of the top ten breakthroughs of 2013. Global research interest and progress has continued to build steadily since then.
To act as a solar cell, a material must contain electrons that can be kicked out of place by the energy of sunlight, leaving regions appropriately called ‘holes’ behind. The electrons can be fed through a circuit, creating the useful electric current, before ultimately recombining with the holes, which themselves need to move through the materials for maximum efficiency.
Aziz and her co-authors explain that perovskites are proving especially suitable for this task, and can now almost match the energy-converting efficiency of conventional silicon-based solar cells. But the perovskites offer the great advantage of being simpler and less expensive to manufacture, showing potential to transform the economic feasibility of solar power.
By layering perovskites together with more conventional silicon structures, devices can also be made to capture frequencies of the solar spectrum that cannot otherwise be tapped.
The need to replace fossil fuel power sources with cheap and clean solar technologies becomes ever more urgent as concerns over global warming and air pollution increase.
Significant technical challenges do remain, principally in enhancing the chemical stability, controlling the crystalline layers during fabrication, and increasing the energy-converting efficiency of the perovskite solar cells.
The use of lead in many perovskites is also a problem, as this toxic element could escape into the environment as solar cells degrade or are discarded. Aziz reports that researchers are making good progress in efforts to create new lead-free perovskite solar cells.
It certainly seems likely that perovskites will be a significant part of global energy generation systems in the near future.
Aziz, F. et al.: "Towards high performance perovskite solar cells: A review of morphological control and HTM development," Applied Materials Today (2018)