Scientists from Gwangju Institute of Science and Technology (South Korea), Hanwha Solution (South Korea), Korea Research Institute of Chemical Technology (South Korea), Imperial College London, Stony Brook University and U.S. Brookhaven National Laboratory have developed a new material processing protocol to boost the operational stability of planar hybrid perovskite solar cells.

Vacuum and solvent process for removing ionic defects imageA schematic showing the vacuum and solvent process for removing the ionic defects that reduce the performance of hybrid perovskite solar cells.

Typically, thin-film devices are made in solution by sandwiching the active light-absorbing material in between top and bottom metal electrical contacts (electrodes) and organic semiconductor interlayers, which enhance the extraction of electrical currents to the contacts. In this case, before putting the final electrode on top, the scientists put the device in vacuum. In prior experiments, the team had noticed that removing and then redepositing the top electrode and interlayer reduced burn-in loss, a rapid decrease in efficiency at the beginning of light illumination. They subsequently confirmed that the high-vacuum environment used to deposit the electrode had contributed to this reduction. During vacuum curing, loose ions emerge from the perovskite and concentrate at the top interlayer. In a second processing step, the scientists used a chemical solvent to selectively wash away this top layer.

“When these hybrid perovskites decompose, they start to leach negatively charged ions of iodine,” said lead author Hyungcheol Back, a research scientist at GIST and Hanwha Solutions. “These ions can move around and accumulate at the interface between the active light-absorbing perovskite and metal electrode to form an insulating layer, making the device less conductive.”

With the developed processing protocol, the centimetre-size device maintained more than 80 percent of its initial efficiency (18.8 percent) under standard operational testing conditions of continuous illumination or heat (applied separately) for 1000 hours. Moreover, the device did not exhibit burn-in loss, which is prevalent among hybrid perovskite solar cells.

To understand the improvement in stability, the GIST scientists performed characterization studies with process-modified and unmodified devices at the Center for Functional Nanomaterials (CFN) and National Synchrotron Light Source II (NSLS-II)—both U.S. Department of Energy (DOE) Office of Science User Facilities at Brookhaven National Laboratory.

At the Complex Materials Scattering (CMS) beamline, operated in partnership between CFN and NSLS-II, they conducted grazing-incidence wide-angle x-ray scattering experiments leveraging the CFN Advanced UV and X-ray Probes Facility. In this technique, the x-ray beam grazes the sample at very shallow angles, which can be adjusted to probe the surface or bulk of a thin film at molecular length scales. According to the scattering patterns, the perovskite surface becomes more crystalline after loose ions are removed through the vacuum and solvent washing process.

To further confirm these results, the scientists performed x-ray photoemission spectroscopy depth profiling at the CFN Proximal Probes Facility. The energy spectra of electrons ejected from the process-modified sample after it was irradiated with x-rays revealed that a smaller concentration of ions existed at the surface relative to the unmodified sample surface.

“The complementary tools at the CFN and NSLS-II allowed us to find out why the intrinsic stability of the material was enhanced,” said co-author Chang-Yong Nam, a materials scientist in the CFN Electronic Nanomaterials Group and an adjunct professor in the Department of Materials Science and Chemical Engineering at Stony Brook University. “We should be able to apply this understanding to improve the performance of not only this material but also other hybrid perovskites, which share similar stability issues.”

Ultimately, the GIST team would like to apply their processing method to large-area (inch-size) device fabrication and develop engineering solutions for the commercial manufacturing of highly efficient large-area hybrid perovskite solar cells.

“Our results exemplify the type of synergy that a close international collaboration can create toward a scientific understanding critical to advancing important energy technologies like hybrid perovskite solar cells,” said Lee. “The capabilities at the CFN and NSLS-II and the support received from scientists including Kevin Yager and Xiao Tong were instrumental. We look forward to future opportunities to conduct research at these user facilities.”