Researchers at the Chinese Academy of Sciences have used Prussian blue nanoparticles to stabilise perovskite films, reaching 26.9 per cent cell efficiency and a certified 22.9 per cent on a 756 square centimetre submodule.
A team at the Chinese Academy of Sciences has pushed perovskite solar cell efficiency to 26.9 per cent. It did so by adding Prussian blue nanoparticles to the film that absorbs light. The researchers published the results in Science on 24 September. An independent test certified the cell at 26.2 per cent.
Meanwhile, a much larger submodule reached a certified 22.9 per cent. That result is the more telling one for industry.
Prussian blue is an iron-based compound that absorbs visible light strongly. Scientists have tried it before as a light absorber in dye-sensitised solar cells, with limited results. This time, the team used it to tackle the instability that has long held perovskites back.
How the nanoparticles work
The study centres on formamidinium lead iodide, a widely studied perovskite material. Its films tend to form defects, and ions drift through them under an electric field. Both problems erode performance and shorten a device’s working life.
During film formation, the nanoparticles act as templates, so the crystals grow more evenly. They also favour the alpha phase, the crystal structure that absorbs light and generates electricity efficiently. In addition, they convert metallic lead and neutral iodine defects back into their ionic forms.
Their rigid framework also traps mobile ions, particularly caesium. As a result, the energy ions need to migrate rose from 0.89 to 1.25 electronvolts. That makes ion movement harder and helps protect the films against electrical, thermal and light-induced damage.
“The main advance of our study is the use of a structurally compatible Prussian blue framework,” said Chong Liu, the study’s corresponding author. Liu said the framework “remains functional during both film formation and device operation”.
Gains in both cell designs
The team tested the approach in conventional and inverted cell designs. Conventional cells improved from 24.2 to 26.1 per cent. Inverted cells, meanwhile, rose from 26.0 to 26.9 per cent.
The gains held in larger devices too. Conventional minimodules measuring 6cm by 6cm reached 23.4 per cent, against 20.1 per cent for untreated reference devices. The 30cm by 30cm inverted submodule, with an aperture area of 756 square centimetres, recorded 22.8 per cent stabilised efficiency.
Size is important here. Perovskite records usually come from tiny lab cells, and efficiency tends to fall as devices grow. So a certified result on a submodule carries more weight for manufacturers. It shows the gains survive when the film covers a far bigger area.
Stability under heat and humidity
Durability remains the main hurdle for any perovskite solar cell. The team ran damp heat tests at 85°C and 85 per cent relative humidity. After 2,500 hours, conventional minimodules kept more than 90 per cent of their starting efficiency. They also kept more than 80 per cent after 2,200 hours of constant running at 65°C.
Accelerated ageing tests on the inverted submodules projected a T80 lifetime of around seven years at 35°C under constant light. T80 is the time a device takes to lose a fifth of its initial efficiency. However, the researchers note that they extrapolated this figure from accelerated tests. They haven’t yet shown it in long-term operation.
Outdoors, a treated submodule ran for five months in Zhuhai, China. Over that period, it showed no discernible decline in performance relative to a commercial silicon reference device.
A step towards commercial modules
Perovskites sit alongside a range of newer solar materials in development, such as the semi-transparent solar windows unveiled at UCL. To compete commercially, though, a perovskite solar cell has to last decades in the field. Seven projected years at 35°C still falls well short of that.
Even so, the work moves on two fronts at once. It lifts efficiency and it slows the decay that has held back commercial perovskite panels. The team also sees the results as evidence for a wider class of materials.
“Our findings highlight the potential of open-framework materials to improve solar cell efficiency, enable large-scale production and enhance the long-term reliability of perovskite photovoltaics,” Liu said.
