The scientists say these top-performing facets could hold the secret to highly efficient solar cells, although more research is needed.<
Solar cells made from compounds that have the crystal structure of the mineral perovskite have captured scientists' imaginations to produce low cost and high efficient devices. The efficiency at which perovskite solar cells convert photons to electricity has increased rapidly over the past few years more than any other material to date. It started at 3% in 2009—when researchers first began exploring the material’s photovoltaic capabilities—to 22 percent today. This is in the ballpark of the efficiency of silicon solar cells.
Now scientists from the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have published research that claims to have discovered a potential secret to dramatically boosting the efficiency of perovskite solar cells hidden in the nanoscale peaks and valleys of the crystalline material. A surprising characteristic of a perovskite solar cell that could be exploited for even higher efficiencies, possibly up to 31%.
By using photoconductive atomic force microscopy, the scientists mapped two properties on the active layer of the solar cell that relate to its photovoltaic efficiency. The maps revealed a bumpy surface composed of grains about 200 nanometers in length, and each grain has multi-angled facets like the faces of a gemstone.
Unexpectedly, the scientists discovered a large difference in energy conversion efficiency between facets on individual grains. They found poorly performing facets adjacent to highly efficient facets, with some facets approaching the material’s theoretical energy conversion limit of 31%.
The scientists say these top-performing facets could hold the secret to highly efficient solar cells, although more research is needed.
For the past two years, scientists at the nearby Joint Center for Artificial Photosynthesis have been making thin films of perovskite-based compounds, and studying their ability to convert sunlight and CO2 into useful chemicals such as fuel. Switching gears, they created pervoskite solar cells composed of methylammonium lead iodide. They also analysed the cells' performance at the macroscale.
The scientists also made a second set of half cells that didn’t have an electrode layer. They packed eight of these cells on a thin film measuring 1 cm2. These films were analysed at the Molecular Foundry, where researchers mapped the cells’ surface topography at a resolution of 10nm. They also mapped two properties that relate to the cells' photovoltaic efficiency: photocurrent generation and open circuit voltage.
This was performed using a state-of-the-art atomic force microscopy technique, developed in collaboration with Park Systems, which utilises a conductive tip to scan the material’s surface. The method also eliminates friction between the tip and the sample. This is important because the material is so rough and soft that friction can damage the tip and sample, and cause artefacts in the photo-current.
Figure: Lawrence Berkeley National Laboratory - Atomic force microscopy image of the grainy surface of a perovskite solar cell
This atomic force microscopy image of the grainy surface of a perovskite solar cell reveals a new path to much greater efficiency. Individual grains are outlined in black, low-performing facets are red, and high-performing facets are green. A big jump in efficiency could possibly be obtained if the material can be grown so that more high-performing facets develop. (Credit: Berkeley Lab)
The resulting maps revealed an order of magnitude difference in photocurrent generation, and a 0.6-volt difference in open circuit voltage, between facets on the same grain. In addition, facets with high photocurrent generation had high open circuit voltage, and facets with low photocurrent generation had low open circuit voltage.
The Molecular Foundry is a DOE Office of Science User Facility located at Berkeley Lab. The Joint Center for Artificial Photosynthesis is a DOE Energy Innovation Hub led by the California Institute of Technology in partnership with Berkeley Lab.
The research was supported in part by the Department of Energy’s Office of Science.
Sibel Y. Leblebici | Linn Leppert | Yanbo Li | Sebastian E. Reyes-Lillo | Sebastian Wickenburg | Ed Wong | Jiye Lee | Mauro Melli | Dominik Ziegler | Daniel K. Angell | D. Frank Ogletree | Paul D. Ashby | Francesca M. Toma | Jeffrey B. Neaton | Ian D. Sharp | Alexander Weber-Bargioni
Nature Energy 1, Article number: 16093 (2016) | doi:10.1038/nenergy.2016.93
Received: 04 December 2015 | Accepted: 26 May 2016 | Published online: 04 July 2016
Photovoltaic devices based on hybrid perovskite materials have exceeded 22% efficiency due to high charge-carrier mobilities and lifetimes. Properties such as photocurrent generation and open-circuit voltage are influenced by the microscopic structure and orientation of the perovskite crystals, but are difficult to quantify on the intra-grain length scale and are often treated as homogeneous within the active layer. Here, we map the local short-circuit photocurrent, open-circuit photovoltage, and dark drift current in state-of-the-art methylammonium lead iodide solar cells using photoconductive atomic force microscopy. We find, within individual grains, spatially correlated heterogeneity in short-circuit current and open-circuit voltage up to 0.6 V. These variations are related to different crystal facets and have a direct impact on the macroscopic power conversion efficiency. We attribute this heterogeneity to a facet-dependent density of trap states. These results imply that controlling crystal grain and facet orientation will enable a systematic optimization of polycrystalline and single-crystal devices for photovoltaic and lighting applications.