Researchers have succeeded in increasing the power conversion efficiency of solar cells made from organometallic trihalide perovskite semiconductors from just 3% to over 20% in the last five years. These materials, which have the formula (CH3NH3)PbX3 (where Pb is lead and X can be iodine, bromine or chlorine), could be real alternatives to other types of photovoltaic material.

When perovskites absorb sunlight, electrons and holes are generated. These charge carriers are then broken apart and transferred to different transport materials (such as nanostructured TiO2) that are coated on top of the perovskite layer. Two types of transport material are required – one for the electrons and another one for the holes. The materials then carry the charges to separate electrodes that generate both current and voltage to extract power from the solar cell device.

Poor long-term stability

Although perovskites boast a number of unique properties – such as being able to absorb light over a broad range of solar spectrum wavelengths – they do suffer from poor long-term stability. Michael Grätzel of the EPFL in Lausanne and colleagues at the Wuhan National Laboratory for Optoelectronics may now have gone a long way in overcoming this problem with their new technique to crosslink neighbouring perovskite grain surfaces.

The researchers employed a bifunctional phosphonic acid-ammonium compound called butylphosphonic acid 4-ammonium chloride, or 4-ABPACl, to chemically modify the grain surface of the perovskite CH3NH3PbI3 in a one-step spin-coating deposition process. Subsequent X-ray diffraction and scanning tunnelling electron microscopy (STEM) measurements showed that the added 4-ABPACl molecules act as crosslinks between neighbouring grains or crystals of the perovskite materials thanks to hydrogen bonding of the ABPACl’s terminal groups (-PO(OH)2 and –NH3+) to the perovskite surface.

Solution-processed solar cells withstand higher temperatures

The ABPACl works in two ways. The first by allowing perovskite crystals to grow within a mesoporous TiO2 scaffold and the second by creating a capping coating over the perovskite layers to make them smoother.

Solar cells made using the new solution processing technique can withstand temperatures of 85°C for 350 hours in the absence of sunlight and retain 90% of their initial performance when exposed to artificial sunlight at 45°C for one week. To compare, control devices – that is, those made from pristine perovskites without the ABPACl additive - only retain 70% of their initial performance under the same conditions.

The additive also enhances the material’s photovoltaic performance from 8.8 to 16.7% as well as its resistance to moisture, say the researchers, who report their work in Nature Chemistry doi:10.1038/nchem.2324.

For more on the latest developments on perovskite solar cells, visit the Nanotechnology collection Focus on Perovskite Solar Cells.