Organic-inorganic hybrid perovskites, which have the chemical formula (CH3NH3)PbX3 (where Pb is lead and X can be iodine, bromine or chlorine), are one of the most promising thin-film solar-cell materials around today thanks to the fact that they can absorb light over a broad range of solar-spectrum wavelengths. The power-conversion efficiency (PCE) of solar cells made from these materials has gone from just 3% to more than 22% in the last eight years, which means that their PCE is now comparable to that of silicon-based solar cells.

For such cells to be widely employed and commercialized, however, we need large-area (1 m2), uniformly high-quality perovskite films from which to make the devices. This is because perovskite-based cells cannot be easily scaled up. Indeed, their PCE decreases from more than 20% to about 10% when they are increased in size from 0.1 cm2 to 25 cm2.

Converting amine complex precursors to perovskite films

A team led by Liyuan Han in China and Michael Grätzel in Switzerland has now developed a new technique to produce large-area methylammonium lead halide (CH3NH3PbI3) perovskite films that relies on rapidly converting amine complex precursors (CH3NH3I·mCH3NH2 (where m is close to 3) and PbI2·nCH3NH2 (where n is close to 1) to perovskite films and then applying pressure to them.

The deposited films are free of pinholes and are highly uniform, say the researchers. “Our technique has the advantage that it does not require any toxic or irritating solvents like N,N-dimethylformamide, dimethyl sulphoxide (DMSO) or gamma-butyrolactone, unlike conventional methods to produce these cells,” says team member Xudong Yang. “It does not produce any waste either and no thermal annealing is required. The technique also works in air and at low temperatures, making it more cost-friendly and environmentally friendly overall.

And that is not all, the pressure-processing step at the end is better than the spin-coating method that is widely employed for depositing perovskite films, he, tells nanotechweb.org.

The film produced by the new technique is highly uniform over a large area (36.1 cm2) with only a 2% variation in film thickness and the grains in the material are around 0.8-1.0 microns in size, which is three to four times bigger than those in spin-coated processed film. The researchers succeeded in making a photovoltaic module with a PCE of 12.1% from such a film.

According to the researchers, reporting their work in Nature doi:10.1038/nature23877, the technique will be useful for growing perovskite crystals, which could greatly reduce so-called trap states and further enhance the photovoltaic performance of these materials. “It could be then used to produce low-cost optoelectronics devices, like light-emitting diodes or laser diodes on a large scale,” adds Han.