Perovskite Solar Cells Surpass 20 % Efficiency

Prototype of the Perovskite solar cell (Source: Herzog / EPFL)

Prototype of the Perovskite solar cell (Source: Herzog / EPFL)

Michael Graetzel and his team from EPFL are pushing the limits of perovskite solar cell performance by exploring the best way to grow these crystals.  They 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. This is promising news for perovskite techno­logy that is already low cost and under industrial development.

However, high performance in perovskites 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. 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 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.

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.

For instance, spinning the cell when the ink is still wet flattens the ink and wicks 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. (Source: EPFL)

Reference: X. Li et al.: A vacuum flash–assisted solution process for high-efficiency large-area perovskite solar cells, Science, online 09 Jun 2016, DOI: 10.1126/science.aaf8060

Link: Photonics and Interfaces (M. Grätzel), Dept. of Chemistry and Chemical Engineering, Swiss Federal Institute of Technology Lausanne, Switzerland

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