Researchers at the University of Groningen provided new insight into hybrid perovskite traps - the loss of electric charges that happens in both silicon and perovskite, and reduces the efficiency of photovoltaic cells.
The new insight happened by chance. The researchers placed a perovskite crystal in a vacuum chamber in an attempt to cool it down and while pumping out the air, a laser was left on, that excited the crystal. This laser light produced electronic charges in the crystal, which emitted light when they recombined. In this instance the crystal should have emitted green light, but surprisingly, when the air was removed from around it, the green light disappeared too. However, when the air was let back in again, the light emission was restored. So apparently, without air, most charges disappear into the traps.
Atmospheric gases somehow blocked the activity of the 'charge eaters' in the crystals, so the researchers decided to investigate. They exposed crystals to different types of gas and discovered that oxygen and water vapor deactivated the traps, while gases such as nitrogen, carbon dioxide or argon had no effect. The next step was to localize the traps, which they did by using two different laser lights to excite either the surface or the interior of the crystals. They discovered that the traps were mainly on the surface.
We assume that there are positively charged groups of traps on the surface because of the crystal structure of the hybrid perovskites', explain the scientists. The next step is to find a way to eliminate them. Water vapor or oxygen work well, but in the long run they can damage the material, so they are not an option. The researchers are busy testing alternatives. If successful, they will further enhance the efficiency of perovskite solar cells.
There is another possible application for the findings. The scientists explain that as the effect of oxygen and water vapor on perovskite is reversible, it would make a viable sensor. Perovskite crystals inside sealed food packaging could detect the presence of harmful oxygen - just shine a laser on the sensor, and if it lights up you know the seal has been broken.