Power/Performance Bits: May 10

Improving three photovoltaic materials: CZTS, tungsten diselenide, and MoS2.


Non-toxic thin-films

A team at Australia’s University of New South Wales achieved the world’s highest efficiency using flexible solar cells that are non-toxic and cheap to make, with a record 7.6% efficiency in a 1cm2 area thin-film CZTS cell.

Unlike its thin-film competitors, CZTS cells are made from abundant materials: copper, zinc, tin and sulphur, and has none of the toxicity problems of its two thin-film rivals, CdTe (cadmium-telluride) and CIGS (copper-indium-gallium-selenide). Cadmium and selenium are toxic at even tiny doses, while tellurium and indium are extremely rare.

“This is the first step on CZTS’s road to beyond 20% efficiency, and marks a milestone in its journey from the lab to commercial product,” said Dr Xiaojing Hao of the Australian Centre for Advanced Photovoltaics at UNSW. “There is still a lot of work needed to catch up with CdTe and CIGS, in both efficiency and cell size, but we are well on the way.”

The new high-efficiency, low-toxicity solar cells developed by UNSW's Australian Centre for Advanced Photovoltaics. (Source: Robert Largent/UNSW)

The new high-efficiency, low-toxicity solar cells developed by UNSW’s Australian Centre for Advanced Photovoltaics. (Source: Robert Largent/UNSW)

“In addition to its elements being more commonplace and environmentally benign, we’re interested in these higher bandgap CZTS cells for two reasons,” said UNSW’s Professor Martin Green, a pioneer of photovoltaic research.

“They can be deposited directly onto materials as thin layers that are 50 times thinner than a human hair, so there’s no need to manufacture silicon ‘wafer’ cells and interconnect them separately,” he added. “They also respond better than silicon to blue wavelengths of light, and can be stacked as a thin-film on top of silicon cells to ultimately improve the overall performance.”

By being able to deposit CZTS solar cells on various surfaces, the team believes this puts them firmly on the road to making thin-film photovoltaic cells that can be rigid or flexible, and durable and cheap enough to be widely integrated into buildings to generate electricity from the sunlight that strikes structures such as glazing, façades, roof tiles and windows.

However, because CZTS is cheaper — and easier to bring from lab to commercialization than other thin-film solar cells, given already available commercialized manufacturing methods — applications are likely even sooner. UNSW is collaborating with a number of large companies to develop applications well before it reaches 20% efficiency – probably, Hao said, within the next few years.

Photoluminescence leap

Researchers from the National University of Singapore (NUS) developed a method to enhance the photoluminescence efficiency of tungsten diselenide.

The single-molecule-thick semiconductor is part of an emerging class of materials called transition metal dichalcogenides (TMDCs), which have the ability to convert light to electricity and vice versa, making them strong potential candidates for optoelectronic devices such as thin film solar cells, photodetectors flexible logic circuits and sensors. However, its atomically thin structure reduces its absorption and photoluminescence properties, thereby limiting its practical applications.

Typical photoluminescence enhancement from transition metal dichalcogenides is 100-fold, with recent enhancement of 1,000-fold achieved by simultaneously enhancing absorption, emission and directionality of the system.

A schematic of the light emission from a single crystal monolayer of tungsten diselenide flake on a gold substrate. Part of the triangular flake rests on the patterned region of the substrate consisting of sub-20 nm wide trenches. (Source: Andrew T. S. Wee)

A schematic of the light emission from a single crystal monolayer of tungsten diselenide flake on a gold substrate. Part of the triangular flake rests on the patterned region of the substrate consisting of sub-20 nm wide trenches. (Source: Andrew T. S. Wee)

However, by incorporating monolayers of tungsten diselenide onto gold substrates with nanosized trenches, the research team was able to enhance the nanomaterial’s photoluminescence by up to 20,000-fold.

“The key to this work is the design of the gold plasmonic nanoarray templates,” said Professor Andrew Wee of the Department of Physics at the NUS. “In our system, the resonances can be tuned to be matched with the pump laser wavelength by varying the pitch of the structures. This is critical for plasmon coupling with light to achieve optimal field confinement.”

MoS2 absorbency

A team at Rice University is working on increasing the light absorbency of molybdenum disulfide (MoS2).

“Basically, we want to understand how much light can be confined in an atomically thin semiconductor monolayer of MoS2,” said Isabell Thomann, assistant professor of electrical and computer engineering at Rice. “By using simple strategies, we were able to absorb 35% to 37% of the incident light in the 400- to 700-nanometer wavelength range, in a layer that is only 0.7 nanometers thick.”

(Source: Thomann Group/Rice University)

(Source: Thomann Group/Rice University)

“Squeezing light into these extremely thin layers and extracting the generated charge carriers is an important problem in the field of two-dimensional materials,” she said. “That’s because monolayers of 2-D materials have different electronic and catalytic properties from their bulk or multilayer counterparts.”

Thomann and her team used a combination of numerical simulations, analytical models and experimental optical characterizations. Using three-dimensional electromagnetic simulations, they found that light absorption was enhanced 5.9 times compared with using MoS2 on a sapphire substrate.

“If light absorption in these materials was perfect, we’d be able to create all sorts of energy-efficient optoelectronic and photocatalytic devices. That’s the problem we’re trying to solve,” Thomann said.