Ecole Polytechnique Fédérale de Lausanne researchers are pushing the limits of perovskite solar cell performance by exploring the best way to grow these crystals.

On the other side of the world scientists at Hong Kong Polytechnic University report they have successfully developed perovskite-silicon tandem solar cells with the world’s highest power conversion efficiency of 25.5%.

First a look at the Swiss effort where Michael Graetzel and his team found that, by briefly reducing the pressure while fabricating perovskite crystals, they were able to achieve the highest performance ever measured for larger-size perovskite solar cells, reaching over 20% efficiency and matching the performance of conventional thin-film solar cells of similar sizes. The Swiss team’s results have been published in Science.

While this is promising news for perovskite technology that is already low cost and under industrial development, the high performance in pervoskites does not necessarily herald the doom of silicon-based solar technology. Safety issues still need to be addressed regarding the lead content of current perovskite solar-cell prototypes in addition to determining the stability of actual devices.

The layering perovskites on top of silicon to make hybrid solar panels may actually boost the silicon solar-cell industry. Efficiency could exceed 30%, with the theoretical limit being around 44%. The improved performance would come from harnessing more solar energy: the higher energy light would be absorbed by the perovskite top layer, while lower energy sunlight passing through the perovskite would be absorbed by the silicon layer.

Graetzel is known for his transparent dye-sensitized solar cells. It turns out that the first perovskite solar cells were dye-sensitized cells where the dye was replaced by small perovskite particles. His lab’s latest perovskite prototype, roughly the size of an SD memory card, looks like a piece of glass that is darkened on one side by a thin film of perovskite. Unlike the transparent dye-sensitized cells, the perovskite solar cell is opaque.

Meanwhile in Hong Kong the scientists report they have successfully developed perovskite-silicon tandem solar cells with the world’s highest power conversion efficiency of 25.5%.

The research team in the Department of Electronic and Information Engineering led by Professor Charles Chee Surya, Clarea Au Endowed Professor in Energy, has recently made this world record with innovative means to enhance energy conversion efficiency. With this innovation, it is estimated that solar energy can be generated at cost of HK$2.73/W, compared with HK$3.9/W at present generated by existing silicon solar cells available in the market.

Because there are different wavelengths for solar energy, a combination of different materials for making solar cells would work best for energy absorption. For example, methylammonium lead tri-halide perovskite and silicon solar cells can form a complementary pair. With the perovskite solar cell functioning as a top layer, it can harvest the short wavelength photons while the bottom layer coated with silicon is designed to absorb the long wavelength photons.

PolyU’s research team maximizes efficiency by making use of this feature with three innovative approaches. Firstly, the team discovered a chemical process – low-temperature annealing process in dry oxygen to reduce the impact made by perovskite defects. Secondly, the team fabricated a tri-layer of molybdenum trioxide / gold / molybdenum trioxide with optimized thickness of each layer, making it highly transparent for light to go into the bottom silicon layer under perovskite layer.

Finally, by mimicking the surface morphology of the rose petals, a haze film, developed by Dr Zijian Zheng of PolyU Institute of Textiles and Clothing, was applied as the top layer of the solar panel to trap more light. All three innovative approaches help enhance energy conversion efficiency.

Professor Shen Hui of Sun Yat-sen University and Shun De SYSU Institute for Solar Energy, who excelled in the fabrication high-efficiency silicon cells, was responsible for the design and fabrication of the bottom silicon cell.

The folks in Hong Kong are on a roll with a lab prototype testing.

But don’t count the Swiss out.

Perovskite solar cells first appeared in 2009 with an efficiency of just 3.8%. With the outstanding photovoltaic properties, perovskite solar cell has become a subject of vigorous research for sustainable power generation, with researchers around the world finding new ways to increase its energy conversion efficiency. It has currently established itself as one of the most promising solar cell materials.

To make a perovskite solar cell, the scientists must grow crystals that have a special structure, called “perovskite” after Russian mineralogist Lev Perovski who discovered it.

The scientists first dissolve a selection of compounds in a liquid to make some “ink.” They then place the ink on a special type of glass that can conduct electricity. The ink dries up, leaving behind a thin film that crystallizes on top of the glass when mild heat is applied. The end result is a thin layer of perovskite crystals.

The tricky part is growing a thin film of perovskite crystals so that the resulting solar cell absorbs a maximum amount of light. Scientists are constantly looking for smooth and regular layers of perovskite with large crystal grain size in order to increase photovoltaic yields.

One example is spinning the cell when the ink is still wet flattening the ink and wicking off some of the excess liquid, leading to more regular films. A new vacuum flash technique used by Graetzel and his team also selectively removes the volatile component of this excess liquid. At the same time, the burst of vacuum flash creates seeds for crystal formation, leading to very regular and shiny perovskite crystals of high electronic quality.

Both of these teams are progressing with quite a bit of quality in the material performance. The numbers are looking good. There is still that lurking lead issue, but as more experience is gained and more creativity and innovation comes in, perovskite will get not just better, but commercially viable as well.


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