Power/Performance Bits: Sept. 1

Cooling sensors with lasers
Researchers at the University of Washington developed a way to cool a solid semiconductor sensor component with an infrared laser. The laser was able to cool the solid semiconductor by at least 20 degrees C, or 36 F, below room temperature.

The device uses a cantilever, similar to a diving board, that can oscillate in response to thermal energy at room temperature. Such devices could be used for optomechanical sensors, where their vibrations can be detected by a laser. But that laser also heats the cantilever, which dampens its performance.

“Historically, the laser heating of nanoscale devices was a major problem that was swept under the rug,” said Peter Pauzauskie, a UW professor of materials science and engineering and a senior scientist at the Pacific Northwest National Laboratory. “We are using infrared light to cool the resonator, which reduces interference or ‘noise’ in the system. This method of solid-state refrigeration could significantly improve the sensitivity of optomechanical resonators, broaden their applications in consumer electronics, lasers and scientific instruments, and pave the way for new applications, such as photonic circuits.”

The researchers note that using a laser to cool the resonator in a sensor is a much more targeted approach to improve sensor performance compared to trying to cool an entire sensor.

The experimental setup used a nanoribbon of cadmium sulfide that extended from a block of silicon and would naturally undergo thermal oscillation at room temperature. At the end, the team placed a tiny ceramic crystal containing ytterbium ions. When the team focused an infrared laser beam at the crystal, the impurities absorbed a small amount of energy from the crystal, causing it to glow in light that is shorter in wavelength than the laser color that excited it. This “blueshift glow” effect cooled the ceramic crystal and the semiconductor nanoribbon it was attached to.

“These crystals were carefully synthesized with a specific concentration of ytterbium to maximize the cooling efficiency,” said Xiaojing Xia, a UW doctoral student in molecular engineering.

To measure the cooling, the team observed changes to the oscillation frequency of the nanoribbon. “The nanoribbon becomes more stiff and brittle after cooling — more resistant to bending and compression. As a result, it oscillates at a higher frequency, which verified that the laser had cooled the resonator,” said Pauzauskie.

The team also observed that the light emitted by the crystal shifted on average to longer wavelengths as they increased laser power, which also indicated cooling.

Using these two methods, the researchers calculated that the resonator’s temperature had dropped by as much as 20 degrees C below room temperature. The refrigeration effect took less than 1 millisecond and lasted as long as the excitation laser was on.

The team said potential applications include quantum sensors and scientific instruments.

Efficient large perovskite solar cell
Researchers from Nanyang Technological University Singapore built perovskite solar mini modules with the highest yet recorded power efficiency for a perovskite-based device larger than 10 cm2.

The team used thermal co-evaporation, a common industrial coating technique and used in the creation of OLED screens, to fabricate solar cell modules of 21 cm2 with power conversion efficiencies of 18.1%.

“The best-performing perovskite solar cells have so far been realized in the laboratory at sizes much smaller than 1 cm2, using a solution-based technique, called spin-coating,” said Annalisa Bruno, senior scientist at the Energy Research Institute at NTU. “However, when used on a large surface, the method results in perovskite solar cells with lower power conversion efficiencies. This is due to the intrinsic limitations that include defects and lack of uniformity over large areas, making it challenging for industrial fabrication methods.”

Utilizing the same technique, the researchers then fabricated colored semi-transparent versions of the perovskite solar cells and mini modules, which achieved similar measures of power conversion efficiency across a range of different colors.

“The solar mini modules can be used on facades and windows in skyscrapers, which is not possible with current silicon solar panels as they are opaque and block light. Building owners will be able to incorporate semi-transparent colored solar cells in the architectural designs to harvest even more solar energy without compromising the aesthetic qualities of their buildings,” said Subodh Mhaisalkar, professor and Associate Vice President (Strategy & Partnerships) at NTU.

Next, the team plans to work on a tandem solar cell integrating perovskite and silicon cells to further improve performance.

Roll-to-roll solar
Researchers at Swansea University recorded the highest efficiency yet for roll-to-roll printed perovskite solar cells. The four layers of slot-die coated perovskite solar cells reached 12.2% efficiency.

Slot-die coating is a pre-metered technique, which means the wet film thickness can be controlled before coating. The researchers note that it is also highly efficient in material usage, with minimal loss of material compared with spray coating or screen printing.

The team used an acetonitrile-based system, which results in better coatings due to due to low viscosity and low surface tension. Along with this, a ternary blend of high workplace exposure limit solvents was introduced, replacing chlorobenzene for the deposition of hole transport material.

“Perovskite solar cells aim to increase the efficiency and lower the cost of traditional solar energy generation. They have the potential to be highly efficient and relatively cheap to manufacture, so the aim is to improve fabrication methods for upscaling,” said Rahul Patidar of Swansea’s SPECIFIC Innovation and Knowledge Centre. “This study signifies the next step towards commercialization.”

The complete solar cell requires coating five layers. In this case, four layers were coated using slot-die coating and the top contact was put on using thermal evaporation. Slot-die coating the fifth (top) contact without destroying any layers underneath has not yet been achieved, but the team says that solving this would enable the manufacture of a fully roll-to-roll printed PSC.