Transition metal dichalcogenides (TMDs) 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.
With this in mind, a group of scientists from Stanford University has built a TMD solar cell that is claimed to achieve a power per weight ratio on a par with well-established thin-film technologies such as cadmium telluride (CdTe); copper, indium, gallium and selenium (CIGS); amorphous silicon (a-Si); and III-V solar cells.
“By adopting novel device architecture and integration methods we achieved over 10 times higher power conversion efficiency and over 100 times higher power per weight, compared to previous demonstrations,” the research corresponding author, Koosha Nassiri Nazif, told pv magazine.
The cell was built with an ultrathin, lightweight and flexible polyimide (PI) substrate with a thickness of 5μm, transparent hole-collecting graphene contacts doped with molybdenum oxide (MoOx), and multi-layer tungsten diselenide (WSe2) absorbers measuring around 200nm. Furthermore, the U.S. group used passivation and anti-reflection coatings, and optically-reflective electron-collecting gold (Au) bottom contacts.
The use of graphene contacts is an important aspect of the cell technology as it mitigates Fermi-level pinning, which is a phenomenon that occurs in solar cells, and especially in TMD devices, when an energy barrier is created for electrons and holes by bending the bands at the semiconductor interface. The Fermi levels define the efficient conversion of the energy of radiation into electrochemical energy and Fermi-level pinning has been responsible for limiting the power conversion efficiency of the best TMD solar cells to around 2%.
Tested under standard solar radiation conditions, the device showed an efficiency of 5.1% and a power per weight return of 4.4 W/g, which the scientist said compares to a 0.7% efficiency and a power per weight of 0.04 W/g for the best device with a flexible structure built in previous research. Reducing the PI substrate thickness to 1 μm is projected to increase cell power per weight to 8.6 W/g. “According to realistic, detailed balance models developed for TMD photovoltaic cells, single-junction multi-layer TMDs can in principle achieve a power conversion efficiency of around 27%, with an optimized optical and electronic design,” they further explained.
According to the researchers, these cells have the potential to outperform organic and perovskite solar cells without the stability challenges that these two cell technologies present.
“Research efforts to scale up TMD growth to large areas would soon enable scalable and low-cost production of TMD photovoltaic cells, similar to other chalcogenide solar cells, such as CdTe and CIGS,” Nassiri Nazif told pv magazine. “The methods for scalable and large-area fabrication of TMDs already exist. However, there has not yet been any demonstration of large-area (>1cm2) TMD solar cells reported in the literature.”
Looking forward, the Stanford group said it wants to demonstrate large-area TMD solar cells made in a scalable fashion. Its findings can be found in the paper High-specific-power flexible transition metal dichalcogenide solar cells, which was recently published in nature communications.
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