Solar Cells with 30-year Lifetimes

A new transparency-friendly solar cell design could marry high efficiencies with 30-year estimated lifetimes, research led by the University of Michigan has shown. It may pave the way for windows that also provide solar power.

Solar energy is about the cheapest form of energy that mankind has ever produced since the industrial revolution,” said Stephen Forrest, Professor of Electrical Engineering, who led the research. “With these devices used on windows, your building becomes a power plant.”

While silicon remains king for solar panel efficiency, it isn’t transparent. For window-friendly solar panels, researchers have been exploring organic—or carbon-basedmaterials. The challenge for Forrest’s team was how to prevent very efficient organic light-converting materials from degrading quickly during use.

The strength and the weakness of these materials lie in the molecules that transfer the photogenerated electrons to the electrodes, the entrance points to the circuit that either uses or stores the solar power. These materials are known generally as “non-fullerene acceptors” to set them apart from the more robust but less efficient “fullerene acceptors” made of nanoscale carbon mesh. Solar cells made with non-fullerene acceptors that incorporate sulfur can achieve silicon-rivaling efficiencies of 18%, but they do not last as long.

The team, including researchers at North Carolina State University and Tianjin University and Zhejiang University in China, set out to change that. In their experiments, they showed that without protecting the sunlight-converting material, the efficiency fell to less than 40% of its initial value within 12 weeks under the equivalent of 1 sun’s illumination.

Non-fullerene acceptors cause very high efficiency, but contain weak bonds that easily dissociate under high energy photons, especially the UV [ultraviolet] photons common in sunlight,” said Yongxi Li, U-M assistant research scientist in electrical engineering and computer science and first author of the paper in Nature Communications.


Noise-cancelling Windows Halve Traffic Sounds

People living in cities with warm climates face a problem during summer months: keeping windows open for ventilation means letting in traffic sounds. A noise-cancelling device could solve this dilemma. Bhan Lam at the Nanyang Technological University in Singapore and his colleagues have created a device that can halve the noisiness of urban traffic, reducing the sound coming through an open window by up to 10 decibels.

To cancel out road noise, the researchers used 24 small loudspeakers and fixed these to the security grilles of a typical window in Singapore in an 8×3 grid. These grilles are a common feature across South-East Asia, says Lam. He adds that the spacing of the speakers was dependent on the frequency of the noise that they wanted to cancel out.

The researchers spaced each speaker 12.5 centimetres apart facing outwards and programmed them to emit sounds at the same frequency of noise detected by a sensor placed outside the window. The device was most successful at cancelling noise between frequencies of 300 and 1000 Hz, with up to a 50 per cent reduction in loudness for sounds within this range. It isn’t optimised for the noise from human voices, which have higher frequencies.

The effect is similar to the technology used in noise-cancelling headphones, which are often tuned specifically to cancel out the hum of aircraft engines”, says Lam. “The speakers the team used were only 4.5 centimetres in diameter – too small to cancel out noise at frequencies below 300 Hz. “A speaker needs to move a huge volume of air for low frequency sounds.

The team placed the window in a replica room and played road traffic, train and aircraft noise from another loudspeaker 2 metres away. The frequency of most of the noise from traffic and passing aircraft ranges from 200 to 1000 hertz. Large trucks and motorcycles tend to generate sounds on the lower end of the range, while the majority of the sound from  is around 1000 Hz.