The golden ticket for successful solar cell technology is efficiency: the amount of sunlight a solar cell can convert to electricity. With perovskites—a particular type of crystal-structured materials—scientists have been able make solar cells with efficiencies of 22 per cent, up from about 3 per cent in just a few years.
That kind of surge in efficiency is unlike any other material in the history of solar cell technology. That perovskites are poised to be low cost and easy to integrate into infrastructure makes them a game changer for the future of clean energy.
Excitement around organometal halide perovskite, the kind used in solar cells, is hard to miss among those in the field. But fundamental understanding of this mysterious material—and why it works so well—is limited.
Today, Scientific Reports published a study from Dal physicists that examines this perovskite, how it behaves when it absorbs light, and a special kind of energy that’s central to its efficiency.
“We’ve used a technique that hasn’t been applied to these materials before,” says Kimberley Hall, lead author of the study and Canada Research Chair in Ultrafast Science.
Ultrafast lasers and spectroscopy
Drs. Hall and colleague Ian Hill, with their team of graduate students from the Department of Physics and Atmospheric Science, used four wave mixing spectroscopy to study organometal halide perovskite. This technique, together with the application of ultrafast laser technology, allowed the team to observe what happens within a few femtoseconds—one quadrillionth of a second—after light is absorbed into the material.
The team was particularly interested in particles called excitons: electrons in an excited but bound state, as if “stuck” in an atom. But electrons have to be able to flow freely in a material to create a current that can be harvested for electricity. In perovskites, the bound electrons are eventually knocked free, allowing the electrical current necessary for a functioning solar cell.
“How hard it is to rip this electron away is a very important quantity and characteristic of a solar cell,” says Dr. Hall (left). “Unless you can break this thing apart with a pretty small amount of energy, you can’t make current.”
The energy responsible for freeing an electron is called the “exciton binding energy.” Previous attempts to measure it have ended in contradicting results. Perovskites are very easy to make, which is why they are low in cost, but they also have a lot of defects within their chemical makeup. These defects make it difficult to accurately measure a variety of material properties, including the binding energy.
Using their ultrafast lasers, the Dal research team was able to sift through the mess and differentiate defect-bound excitons from others. They’ve provided the clearest picture of exciton binding energy within perovskites to date.
Despite the defects, perovskite efficiency is still remarkable. Dr. Hill and Dr. Hall note the current industry standard silicon solar cell would never work as well with that many defects.
Source : Dalhousie University