Progress has been announced in the quest to overcome one of the biggest obstacles to cheap solar power. A tandem solar cell exceeding 25 percent efficiency has been proven possible, but the long-run potential is much higher.
Perovskite cells have shaken up solar research. They are potentially very cheap to make, and while their efficiency doesn't yet match the best silicon cells, records have been broken faster than for any previous cell-type, and they show impressive adaptability. The main drawback is their stability – perovskite cells decay in many environments.
So when Oxford University Professor Henry Snaith, one of the inventors of perovskite cells, announced a new version in Science this week, the most important part of the announcement is the reference to the product being “photostable.”
Water has been particularly damaging to previous perovskite cells, but Snaith told IFLScience: “If you submerse the films in water they will still degrade, but the new material is much more stable to heating in a humid environment than the previous materials. Once properly encapsulated in a cell this issue should not be critical.”
Snaith achieved this step forward by adding cesium to the perovskite crystals, resulting in a cell that combines stability with 17 percent efficiency in turning sunlight into electricity. This is far from record-breaking, but impressive for a cell that has the potential to be made very cheaply. However, the potential is far greater.
Solar cells have “bandgaps”, which represent the energy they can extract from a photon. A high bandgap prevents the cell from collecting lower energy photons. A low bandgap allows the cell to collect more photons, but wastes much of the potential of the most energetic forms of light.
Multijunction cells resolve this conundrum by stacking semi-transparent layers with high bandgaps above layers with lower bandgaps. This has allowed for some remarkably efficient cells, but only by making cells too expensive for most purposes.
Snaith placed a layer of his cesium perovskite film with a bandgap of 1.74 electron volts on top of a conventional silicon cell, which has lower values. On its own the silicon cell was 19.2 percent efficient, but the layered pair could manage 25.2 percent.
The paper concludes: “Considering further minor improvements in the perovskite, optical management and integration, and choice of [silicon] rear cell, it is feasible that this system could deliver up to 30 percent efficiency.” Snaith told IFLScience the additional cost should be around 5 percent, for a 20 to 40 percent performance boost. The fact that perovskite's bandgap can be tuned means there is potential, once the problems of large-scale production have been solved, to pile two perovskite layers with different bandgaps on top of a silicon cell.
Efficiencies this high would be a huge boost to solar power wherever space is limited. Even where space is not a restriction, more efficient cells at an only slightly increased cost would reduce installation costs, leaving fossil fuels unable to compete on price, even if environmental effects are ignored.