Web Date: April 14, 2014
Promising Solar Cell Materials Also Emit Laser Light
Perovskites have recently become a hot topic in photovoltaics research. Scientists in the U.K. now show that the materials, known to be inexpensive and efficient at converting light to electricity, also can be used to make lasers (J. Phys. Chem. Lett. 2014; DOI: 10.1021/jz5005285). The researchers demonstrated that a perovskite can convert 70% of absorbed light into emitted light. This remarkably high luminescent efficiency is critical for light-emitting devices.
The advance points to the possibility of cheap lasers that could be used in many applications, such as communications, manufacturing, and consumer electronics, says Henry J. Snaith, a physicist at Oxford University who has pioneered work in perovskite solar cells.
Perovskites, a large class of crystalline materials, combine the best features of two major types of photovoltaic materials. Like silicon, they have high light-to-electricity conversion efficiencies. Yet they are inexpensive and easy to prepare in solution, just like organic photovoltaic materials.
In March, scientists at the Swiss Federal Institute of Technology, Lausanne, reported for the first time that perovskites could emit laser light (Nat. Mater. 2014, DOI: 10.1038/nmat3911). They observed spontaneous light emission from perovskite crystals consisting of two-dimensional layers of metal halides separated by organic materials.
In the new study, Snaith, Richard H. Friend at Cambridge University, and their colleagues tested the lasing abilities of the perovskite CH3NH3PbI3-xClx, which is a three-dimensional crystal. The 3-D structure makes it a better semiconductor, making it more suitable for light-emitting devices, Snaith says.
The researchers deposited a thin film of the perovskite onto a glass surface and then measured its luminescent efficiency—the ratio of photons emitted to photons absorbed. They found that when the material absorbs light, it generates electrons and positively charged vacancies in the crystals called holes within one picosecond. “Electrons and holes recombine either by emitting light or through a nonradiative decay pathway, like heat,” Snaith explains. Nonradiative decay can occur at defects in the crystal. The researchers speculate that because the crystal has very few defects, effectively 70% of the electron-hole pairs formed recombine to produce photons, resulting in the material’s high luminescent efficiency.
To make a laser, the researchers sandwiched the perovskite film between a gold mirror and a device called a Bragg reflector, which reflects only certain wavelengths. This forms a so-called optical cavity needed to bounce light back and forth to produce laser light. When the team excites the crystal with 0.4-ns-long pulses of green laser light at a high enough intensity, the laser emits near-infrared light with a wavelength around 760 nm.
Snaith says that they can easily tune the produced light from infrared to ultraviolet wavelengths. The team thinks the most promising application for the perovskite lasers will be efficient light-emitting diodes. To make such devices, Snaith says, they will need to find a way to excite the materials electrically.
Yang Yang, a materials scientist at the University of California, Los Angeles, says that because high luminescent efficiency corresponds to high light-to-electricity conversion efficiency, these materials could lead to thin-film photovoltaic devices that are “the dream of the solar cell community.” The new work “shows a bright future for perovskites in solar cell and LED applications,” he says.
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