2D solar cell design based on transition metal dichalcogenides promises 12.87% efficiency

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From pv magazine global

A group of researchers from the University of Pennsylvania have designed a 2D solar cell based on transition metal dichalcogenides (TMDCs) with a special superlattice structure that reportedly enables higher solar absorption levels.

TMDCs are two-dimensional materials with remarkable semiconducting properties and high optical absorption coefficients, which makes them suitable for the manufacture of semi-transparent and flexible solar cells with potential applications in aerospace, architecture, electric vehicles, and wearable electronics, where light weight, a high power-per-weight ratio, and flexibility are very desirable.

“I think people are slowly coming to the realization that 2D TMDCs are excellent photovoltaic materials, though not for terrestrial applications, but for applications that are mobile—more flexible, like space-based applications,” said lead author Deep Jariwala. “The weight of 2D TMDC solar cells is 100 times less than silicon or gallium arsenide solar cells, so suddenly these cells become a very appealing technology.”

The scientists built the cell with a monolayer absorber made of molybdenum disulfide (MoS2), a 3 nm insulator based on aluminum oxide (Al2O3) placed on a substrate made of Al2O3 and gold (Au), with the latter serving as a reflector. “The thickness of the Al2O3 layer has been optimized to enhance photocarrier generation,” they explained. “The active layer of the device is intrinsic and is 0.98 μm long, with silver and gold cathode and anode electrodes measuring 0.01 μm in length each.”

According to the researchers, the novelty of this cell consists of its superlattice structure, which they say can separate the layers of 2D TMDCs by a spacer or non-semiconductor layer. “Spacing out the layers allows you to bounce light many, many times within the cell structure, even when the cell structure is extremely thin,” said Jariwala, noting that the cell shows large exciton binding energies.

When tested in a series of simulations, the proposed cell design with separate contacts was able to reach a power conversion efficiency of 12.87%. As a way of comparison, real 2D TMDC solar cells were so far able to reach efficiencies of up to 6%.

“We were not expecting cells that are so thin to see a 12% value. Given that the current efficiencies are less than 5%, my hope is that in the next 4 to 5 years people can actually demonstrate cells that are 10% and upwards in efficiency,” the researchers stated.

They described the new cell technology in the paper “How good can 2D excitonic solar cells be?” published in device. “Our findings indicate that 2D TMDC-based PVs, when optimized for both optical and electronic design, exhibit superior performance compared with other thin-film materials in terms of power density, which is vital for many applications such as aerospace, remote sensing, and wearable technology,” they concluded.

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