Organometal trihalide perovskite semiconductors, which have the formula (CH3NH3)PbX3, where Pb is lead and X can be iodine, bromine or chlorine) were first employed as the light absorbing component in so-called dye-sensitized solar cells in 2009. In these devices, the perovskites were coated onto the surface of a film made of titanium dioxide (TiO2) nanoparticles.

When the perovskite layer absorbs light, electrons and holes are generated. These charge carriers are subsequently transferred to different transport materials – TiO2 for the electrons and to another material for the holes. The transport materials then carry the charges to separate electrodes, and a voltage is produced. These solar cells have light-converting efficiencies of between 12 and 15% thanks to the large amount of perovskite packed into the TiO2 film.

Simplifying the device structure

Two teams of researchers at Oxford, led by Henry Snaith and Michael Johnston of the Clarendon Laboratory, have now shown that perovskites not only strongly absorb light but also transport both electrons and holes. This new discovery means that the nanostructured architecture previously used in the dye-sensitized solar cells is no longer necessary, which simplifies the device structure no end. Indeed, in the new device, the light-absorbing perovskites are simply sandwiched between electron and hole-selective electrodes – a set-up that is, in fact, the same as that used in conventional planar p-i-n solar cells.

"Our devices have a high solar-to-electric power efficiency of 15.4% and a large ‘open circuit’ voltage of 1.07 V – all in a solar cell in which the absorbing perovskite layer is only 330 nm thick," says Johnston. "This means that we only need a tiny amount of perovskite material to make a solar cell with good properties." In contrast, conventional crystalline silicon cells are much thicker (0.15 mm wafers are typically used) and the voltage produced by these cells is only about 0.7 V under open circuit conditions.

"Little is known about the photophysics of these materials, which I think is quite exciting – this is a rapidly evolving field," he told nanotechweb.org. "The fact that we can make such good solar cells using a conventional planar p-i-n architecture indicates that the charge carrier diffusion lengths (the distances electrons and holes travel before recombining) are long, and that these carriers last a long time in perovskites. That we can fabricate an efficient device without complex mesostructuring – as was previously the case with solar cells made from this material – also shows that perovskite is very good at both absorbing light and transporting photogenerated charge."

According to the researchers, these perovskite-based devices should be cheap to make using processes that are compatible with existing solar cell manufacturing infrastructures. And since they absorb light in a different part of the electromagnetic spectrum to silicon, the two materials might be used together in so-called tandem cells in which a silicon device would be placed underneath a perovskite one. "Here, the perovskite top cell would absorb higher energy photons and lower bandgap silicon lower energy ones," explains Johnston. Such a cell could be more efficient that one made from either silicon or perovskite on its own.

Perovskites work as bulk semiconductors

Richard Friend of the Cavendish Laboratory at Cambridge University, who was not involved in this work, says that this research began out of the Oxford’s team initial interest in dye-sensitized solar cells. These devices are considered to be ‘excitonic’ photovoltaics that require a large surface area for charge separation between electron-accepting TiO2 and the adsorbed dye layer. He says that the team’s new discovery is "remarkable" because it proves that these perovskites work as bulk semiconductors.

"Last year, this group already reported that the lead iodide perovskite structure described in this work, formed with an organic semiconductor hole transporter, could produce a power conversion efficiency above 10%. The new paper in Nature reports efficiencies of 15% in a very straightforward layer-by-layer structure deposited by very simple evaporation and solution processing techniques. It is unprecedented to see such rapid progress in performance – with less than a year of development, the material is now close to the efficiency of cadmium telluride (that has been studied for several decades)."

Spurred on by its initial results, the Oxford researchers are now busy optimizing film deposition parameters and device design. "I think we will see the efficiencies of these devices climbing higher in the near future," says Johnston. "Investigations into the fundamental photophysics of the perovskite layers will be particularly interesting and will also help us accelerate the optimization process."

The present results are published in Nature.

Further reading

Imprinted nanovoids trap light in solar cells (Sep 2012)
Thin-film solar: low-cost synthesis of CZTS nanocrystals (Jul 2011)
Layered structures for next-generation solar cells (Jul 2012)
Solution processing makes good solar cells (Jul 2011)