The multi-institution cooperation led by scientists from Cambridge University and the Graduate School of Okinawa University of Science and Technology (OIST) has recently achieved breakthrough results. They have discovered "efficiency defects" in the potential materials for the next generation of solar cells and flexible LEDs-perovskite Roots.

 

Over the past decade, perovskite, a variety of materials with specific crystal structures, has become a promising alternative to silicon solar cells because they are cheaper and more environmentally friendly to manufacture, while achieving comparable levels of efficiency .

 

However, solar cells made of perovskite materials often show significant performance loss and instability. To date, most research has focused on methods to eliminate these losses, but their actual physical causes are still unknown.

 

In a paper published recently in the journal Nature, researchers from the team of Dr. Sam Stranks of the Department of Chemical Engineering and Biotechnology at Cambridge University and Cavendish Laboratory and Professor Keshav Dani of OIST in Japan femtosecond Researchers in the spectroscopy department have identified the source of the problem. Their findings can simplify efforts to increase the efficiency of perovskite applications and bring them closer to mass-market production.

 

Generally speaking, when light shines on a perovskite solar cell or electricity passes through a perovskite LED, the electrons are excited and transition to a higher energy state. The negatively charged electrons stay behind a space called a hole, which then has a relatively positive charge. Both excited electrons and holes can move through the perovskite material and therefore act as charge carriers.

 

However, in perovskite, certain types of defects occur, and the current-carrying carriers are stuck. The trapped electrons and holes recombine and heat their energy loss instead of converting them into useful electricity or light, which greatly reduces the efficiency and stability of solar panels and LEDs.

 

So far, little is known about the causes of these "traps", partly because their behavior seems to be very different from the defects in traditional solar cell materials.

 

In 2015, Dr. Stranks's group published a paper in Science, which studied the luminescence of perovskite and revealed the performance of perovskite in absorbing or emitting light. They found the material to be very heterogeneous. Dr. Stranks described: "There are large areas that are bright and luminous, while other areas are actually dark. These dark areas correspond to the power loss of solar cells or LEDs. But the cause of power loss is always a mystery. , Especially because the perovskite is very resistant to defects. "

 

Due to the limitations of standard imaging techniques, the research team was unable to tell whether the darker areas were caused by a large trap site or many smaller traps, so it was difficult to determine why they only formed in certain areas. By late 2017, OIST's team of Professor Dani published a paper in Nature Nanotechnology, where they took a set of images showing how electrons behave in semiconductors after absorbing light. Professor Dani said: "By observing how the charge moves in the material or device after light irradiation, we can find a lot of things. For example, you can see where the charge is captured." "However, these losses are difficult to visualize. , Because they move very fast-on a time scale of one millionth of a billionth of a second; and at a very short distance, about one billionth of a meter.

 

So the team of Dr. Stranks and the team of Professor Dani formed a cooperation to see if they can jointly solve the problem of visualization of dark areas in the perovskite.

 

For the first time, OIST's team used a technique called light emission electron microscopy (PEEM) on perovskite. They used ultraviolet light to detect the material and form an image from the emitted electrons.

 

When they looked at the material, they found that the dark areas contained "traps", about 10-100 nanometers in length, which were clusters of trap sites with smaller atomic sizes. These trap clusters are unevenly distributed throughout the perovskite material, which explains the uneven luminescence phenomenon discovered in Dr. Stranks' early research.

 

Interestingly, when the researchers superimposed the image of the trap location on the image showing the grains of the perovskite material, they found that the trap clusters were formed only at specific locations, at the boundary between certain grains.

 

To understand why this phenomenon occurs only at certain grain boundaries, the research team collaborated with the team of Professor Paul Midgley of the Department of Materials Science and Metallurgy at Cambridge University, which uses a technique called scanning electron diffraction To create detailed images of the perovskite crystal structure. Professor Midgley's team used the electron microscope setup in the diamond light source synchrotron's ePSIC facility, which has dedicated equipment for imaging radiation-sensitive materials such as perovskite.

 

"Because these materials are very sensitive to light beams, you can use typical techniques to detect local crystal structures at these length scales, thereby rapidly changing the material when viewed," explains Tiarnan Doherty of Dr. Stranks. Research team and co-lead author. "Instead, we can use very low exposure doses, so we can prevent injuries."

 

"Through the work of OIST, we know the location of the trap cluster. On the ePSIC, we scanned around the same area to view the local structure. We were able to quickly identify unexpected changes in the crystal structure around the trap location."

 

The team found that trap clusters are only formed at the nodes, the structure of the material at the nodes is slightly deformed, and the area at the nodes is the original structure.

 

Dr. Strands said: "We have these regular inlaid materials in perovskite. Most of the grains are high-quality and original. This is the structure we expect." "But every once in a while, you will get A slightly distorted particle, and the chemical nature of the particle is uneven. What is really interesting is that what initially puzzled us is that it is not the distorted particle that is the trap, but when that particle meets a primitive particle; It was formed at that node. "

 

Based on the understanding of the nature of this "trap", OIST's team also used customized PEEM instruments to visualize the dynamics of the charge carrier trap process that occurs in the perovskite material. One of the unique features of the PEEM device is that it can image ultra-fast processes-as short as femtoseconds. Later, the researchers found that the capture process is mainly controlled by the charge carriers that diffuse into the trap cluster.

 

These findings represent a major breakthrough in bringing perovskite into the solar market. "We still don't know why the traps are gathered there, but now we know that they are indeed formed there, and only there." "This is exciting because it means that we now know what to increase the calcium titanium Mine performance. We need to target those heterogeneous phases or somehow get rid of these combinations. "

 

The team's research focused on a specific perovskite structure. Now, scientists will investigate whether the cause of these trap clusters is common in all perovskite materials.