Molecular engineering strategy boosts efficiency of inverted perovskite solar cells

Solar cells, devices that can directly convert radiation emitted from the sun into electricity, have become increasingly widespread and are contributing to the reduction of greenhouse gas emissions worldwide. While existing silicon-based solar cells have attained good performances, energy engineers have been exploring alternative designs that could be more efficient and affordable.

Perovskites, a class of materials with a characteristic crystal structure, have proved to be particularly promising for the development of low-cost and energy-efficient solar energy solutions. Recent studies specifically highlighted the potential of inverted perovskite solar cells, devices in which the extraction charge layers are arranged in the reverse order compared to traditional designs.

Inverted perovskite solar cells could be more stable and easier to manufacture on a large-scale than conventional perovskite-based cells. Nonetheless, most inverted cells developed so far were found to exhibit low energy-efficiencies, due to the uncontrolled formation of crystal grains that can produce defects and adversely impact the transport of charge carriers generated by sunlight.

Researchers at Huazhong University of Science and Technology recently devised a new molecular engineering strategy to control the crystallization of perovskite materials in inverted solar cells. This promising approach, outlined in a paper published in Nature Energy, entails mixing special naphthalene-based molecules into perovskites, to ensure that they grow more uniformly.

“Formamidinium and cesium metal halide perovskites enable high efficiency in inverted perovskite solar cells, but uncontrolled crystallization limits their performance,” wrote Qisen Zhou, Guoyu Huang and their colleagues in their paper. “We regulate the nucleation and growth of the perovskite through aromatic interactions between naphthalene ammonium salts and naphthalenesulfonates.”

Essentially, the researchers mixed naphthalene-based molecules into the perovskite solution to control the formation and growth of perovskite crystals. They found that the resulting perovskite films were uniform and had very few defects, which is highly favorable for the development of inverted solar cells.

“The ammonium groups of the naphthalene ammonium salts occupy the formamidinium site, while the sulfonate groups of the naphthalenesulfonates coordinate with lead ions,” explained the authors. “Their naphthalene moieties form tight aromatic stacking adjacent to the [PbI₆]⁴⁻ octahedra. These interactions promote ordered out-of-plane crystallization along the (100) plane, enhancing defect passivation and carrier transport.”

Zhou, Huang and his colleagues used the uniform perovskite films they created to fabricate inverted perovskite solar cells. They then tested the performance, efficiency and stability of these cells under continuous illumination.

“We achieve a power conversion efficiency of 27.02% (certified 26.88%) for inverted solar cells,” wrote the researchers. “Encapsulated devices retain 98.2% of their initial efficiency after 2,000 h of maximum power point tracking under continuous illumination in ambient air. Furthermore, we demonstrate a certified steady-state efficiency of 23.18% for inverted mini-modules with an aperture area of 11.09 cm² and a certified efficiency of 29.07% for all-perovskite tandem solar cells.”

The initial results gathered by this research team are highly promising, highlighting the promise of their molecular engineering approach for the development of energy-efficient inverted perovskite solar cells. In the future, their strategy could be further refined to achieve additional efficiency gains and used to realize high-quality perovskite films with varying compositions.

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