UC Berkeley Researchers Offer New Insight on the Use of Perovskite Semiconductors and Their Limitations

University of California Berkeley scientists have created a blue light-emitting diode (LED) using halide perovskite – a new semiconductor material – that overcomes a major barrier to using these cheap and easy-to-make materials in electronic devices. 

During their research, the UC Berkeley scientists identified a characteristic of halide perovskites that may place a bar on them from being used as semiconductors and solar cells. However, this characteristic may make way for a whole new use for perovskites that surpasses the property of existing semiconductors. 

 

A graphic depicting the blue-emitting halid perovskite structural changes after being heated. Image used courtesy of The University of California, Berkeley.

 

An Opportunity Rather than a Limitation

In a paper published in the Science Advances journal, the UC Berkley scientist submitted their research and findings, demonstrating that environmental factors – chemicals, temperature, and humidity – cause the crystal’s structure to change. This disrupts their electronic and optical characteristics and renders them inherently unstable in applications where the physical and chemical environment cannot be closely controlled. 

Peidong Yang, project leader and director of the Kavli Energy NanoSciences Institute said, "Some people may say this is a limitation. For me, this is a great opportunity. This is new physics: a new class of semiconductors that can be readily reconfigured, depending on what sort of environment you put them in. They could be a really good sensor, maybe a really good photoconductor, because they will be very sensitive in their response to light and chemicals."

Today’s semiconductors are made of silicon or gallium nitride. These are extremely stable across a broad range of applications and environments due to the strong covalent bonds that hold their crystalline structures together. In contrast, bonds holding the halide perovskite crystals together are weaker ionic bonds – like those found in salt crystals – and this makes them both easier to make but more sensitive to environmental conditions such as temperature and humidity. 

Yang added, "This paper is not just about showing off that we made this blue LED… We are also telling people that we really need to pay attention to the structural evolution of perovskites during the device operation, any time you drive these perovskites with an electrical current, whether it is an LED, a solar cell or a transistor. This is an intrinsic property of this new class of semiconductor and affects any potential optoelectronic device in the future using this class of material."

 

Two different types of blue light-emitting perovskite crystals. Image used courtesy of The University of California, Berkely. 

 

The Blue Perovskite Development Process

It is difficult to manufacture blue-emitting perovskite diodes due to the method used for growing the crystals as a thin film. This promotes the formation of other crystal structures, each emitting at a different wavelength. When electrons move rapidly towards those crystals, it produces red light.

To prevent this, Yang and his team developed perovskite crystals in a single layer. Then, using a low-tech process to produce graphene, they used tape to remove a single uniform perovskite layer. When integrated into a live circuit, it emitted a blue light which differed in wavelength as the number of perovskite layers varied. Each layer is separated from one another by an organic molecular layer that allows easy separation and safeguarding of the surface. 

The team’s research also demonstrated that perovskites that emit blue light alter their emission colors alongside temperature, something which has potentially fascinating applications according to Yang. 

“We need to think in different ways of using this class of semiconductor,” he said. “We should not put halide perovskites into the same application environment as a traditional covalent semiconductor, like silicon. We need to realize that this class of material has intrinsic structural properties that make it ready to reconfigure. We should utilize that.”

The study was funded by the U.S. Department of Energy’s Basic Energy Sciences program.