Crystal-modifying agent piracetam provides scalable strategy for high-efficiency all-perovskite tandem solar cells

All-perovskite tandem solar cells (TSCs) are a class of solar cells comprised of two or more sub-cells that absorb light with different wavelengths, all of which are made of perovskites (i.e., materials with a characteristic crystal structure known to efficiently absorb light). These solar cells have been found to be highly promising energy solutions, as they could convert sunlight into electricity more efficiently than existing silicon-based solar cells.

Despite their potential, most all-perovskite TSCs developed to date only perform well when they are small and their performance rapidly declines as their size increases. This has ultimately prevented them from being manufactured and deployed on a large-scale.

Researchers at Wuhan University and other institutes in China recently introduced a new strategy for enhancing the performance of all-perovskite TSCs irrespective of their size, which could in turn contribute to their future commercialization. Their proposed approach for fabricating these cells, outlined in a paper published in Nature Nanotechnology, entails the use of piracetam, a chemical additive that can help to control the initial phase of crystal formation (i.e., nucleation) in wide-bandgap perovskites.

“All-perovskite TSCs offer exceptional performance and versatile applicability,” wrote Shiqiang Fu, Shun Zhou and their colleagues in their paper. “However, a significant challenge persists in bridging the power conversion efficiency (PCE) gap between small- and large-area (>1 cm²) devices, which presents a formidable barrier to the commercialization of all-perovskite TSCs. We introduce a specialized crystal-modifying agent, piracetam, tailored for wide-bandgap perovskites, homogenizing top wide-bandgap subcells and enabling the construction of efficient large-area TSCs.”

Piracetam, a synthetic compound with the molecular formula C₆H₁₀N₂O₂, was first introduced in the 1960s. As part of their study, the researchers used this compound as a crystal-modifying agent, or more specifically to control the growth of perovskite crystals and eliminate undesirable residual compounds, ultimately improving the performance of all-perovskite solar cells.

“Piracetam, featuring amide and pyrrolidone moieties, initially modulates perovskite nucleation, resulting in large-sized grains, preferred (110) orientation, enhanced crystallinity and uniform optoelectronic properties,” explained Fu, Zhou and their colleagues. “During the subsequent annealing process, it further eliminates residual PbI₂ and facilitates the formation of one-dimensional (Pi)PbI₃ (Pi = piracetam) perovskite nanoneedles at the grain boundaries and surfaces.”

The researchers used their proposed approach to synthesize smoother and high-quality wide-bandgap perovskite films, exhibiting fewer defects and an improved crystallinity. They then used these films to create both small and large TSCs, which they evaluated in a series of tests.

“Single-junction 1.77 eV-bandgap solar cells achieve a certified open-circuit voltage of 1.36 V and a PCE of 20.35%,” wrote Fu, Zhou, and their colleagues. “Furthermore, our monolithic two-terminal all-perovskite TSCs, with aperture areas of 0.07 cm² and 1.02 cm², yield PCEs of 28.71% (stabilized 28.55%, certified 28.13%) and 28.20% (stabilized 28.05%, certified 27.30%), respectively, demonstrating a minimal PCE loss of 0.51% when transitioning from small-area to large-area devices.

“In addition, piracetam demonstrates broad applicability across different perovskite compositions, increasing the PCE from 23.56% to 25.71% for single-junction 1.56 eV-bandgap counterparts.”

The results of this recent study highlight the potential of piracetam as a crystal-modifying agent to improve the quality of wide-bandgap perovskite films for solar cells. In the future, the approach employed by Fu, Zhou and his colleagues could contribute to the advancement of all-perovskite TSCs, boosting their performance as they are scaled up and facilitating their widespread adoption.

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