While investigating perovskite crystals, University of Groningen scientists made an observation that could make perovskite solar cells more efficient. According to the researchers it could also lead to new sensors for oxygen and water vapour.
Maria Antonietta Loi, Professor of Photophysics and Optoelectronics at the University of Groningen, says that photovoltaic cells based on hybrid perovskites were first introduced in 2009, and rapidly became as efficient as standard silicon solar cells. They now convert light into electricity at about 22% efficiency - the theoretical limit is about 33%.
However, part of the electric charge disappears into what are known as traps. This happens in both silicon and perovskite, and reduces the efficiency of photovoltaic cells. So it would be nice to know more about traps and how to avoid them.
A serendipitous observation by University of Groningen scientists provided new insight into hybrid perovskite traps. Researchers placed a crystal in a vacuum chamber to cool it down. While pumping out the air the researcher left on a laser that excites the crystal. This laser light produces electronic charges in the crystal, which emit light when they recombine.
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, the researchers noticed that when they let the air in again, the light emission was restored. So apparently, without air, most charges disappear into the traps.
Figure: University of Groningen - Electronic properties of MAPbBr3 single-crystal devices
(A) Structure of the single-crystal device for current-voltage (I-V) measurement. (B and C) Side-view and top-view of a single-crystal device. (D) I-V curves of the MAPbBr3 single-crystal device under laser illumination in air and vacuum. (E) Dark current of the MAPbBr3 single-crystal device in air and vacuum.
Researchers believed that atmospheric gases somehow blocked the activity of the 'charge eaters' in the crystals, so set out to investigate. This lead the researchers to expose crystals to different types of gas and discovered that oxygen and water vapour deactivated the traps, while gases such as nitrogen, carbon dioxide or argon had no effect. The next step was to localise the traps, which they did by using two different laser lights to excite either the surface or the interior of the crystals. This lead to the discovery that the traps were mainly on the surface.
The researchers assumed that there are positively charged groups of traps on the surface because of the crystal structure of the hybrid perovskites. The next step is to find a way to eliminate them. Water vapour 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 they succeed, they will further enhance the efficiency of perovskite solar cells. The number of traps in the material that used for these experiments was relatively low, but the researchers estimate that by eliminating them, they could go from an efficiency of 22% to 25% - equalling or surpassing that of crystalline silicon.
The researchers believe that there is another possible application for the findings because as the effect of oxygen and water vapour on perovskite is reversible, it could make an interesting sensor. Perovskite crystals inside sealed food packaging could detect the presence of harmful oxygen - by just shining a laser on the sensor, and see if it lights would indicate that the seal had been broken.
Ultrahigh sensitivity of methylammonium lead tribromide perovskite single crystals to environmental gases
Hong-Hua Fang | Sampson Adjokatse | Haotong Wei | Jie Yang | Graeme R. Blake | Jinsong Huang | Jacky Even | Maria Antonietta Loi
Science Advances 27 Jul 2016: | Vol. 2, no. 7, e1600534 | DOI: 10.1126/sciadv.1600534
One of the limiting factors to high device performance in photovoltaics is the presence of surface traps. Hence, the understanding and control of carrier recombination at the surface of organic-inorganic hybrid perovskite is critical for the design and optimization of devices with this material as the active layer. We demonstrate that the surface recombination rate (or surface trap state density) in methylammonium lead tribromide (MAPbBr3) single crystals can be fully and reversibly controlled by the physisorption of oxygen and water molecules, leading to a modulation of the photoluminescence intensity by over two orders of magnitude. We report an unusually low surface recombination velocity of 4 cm/s (corresponding to a surface trap state density of 108 cm−2) in this material, which is the lowest value ever reported for hybrid perovskites. In addition, a consistent modulation of the transport properties in single crystal devices is evidenced. Our findings highlight the importance of environmental conditions on the investigation and fabrication of high-quality, perovskite-based devices and offer a new potential application of these materials to detect oxygen and water vapor.