In situ active guanidinium salts interaction promotes facet orientation and crystallization for efficient and stable inverted perovskite solar cells

A research team from Tianjin University of Technology, Shandong Univeristy and Shantou University has developed a novel method to enhance the performance and durability of inverted perovskite solar cell (PSC). By introducing guanidinium salt into the fabrication process, the team successfully controlled crystal growth and orientation, leading to solar cells with higher efficiency (25.85%), improved charge transport and exceptional stability under humid and thermal conditions. Moreover, extensive testing demonstrated that the devices retain over 95% of their original efficiency after thousands of hours of operation and exposure to challenging environments.

This work offers a reliable approach to fabricating highly oriented, low-defect perovskite films, accelerating the development and commercialization of efficient and stable PSCs.

Formamidine (FA)-based perovskite solar cells have attracted significant interest from both academia and industry due to their outstanding photoelectric properties and strong potential for commercialization. However, polycrystalline perovskite films fabricated via low-temperature solution processing often suffer from random crystal orientation and uncontrolled crystallization. These issues lead to structural imperfections and poor phase stability, ultimately limiting device performance and operational lifetime. Therefore, developing new strategies that simultaneously regulate crystallization kinetics and strengthen intermolecular interactions is essential for improving the efficiency and durability of perovskite photovoltaics.

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The Solution: The researchers reported an in-situ modulation strategy for the fabrication of highly efficient and stable inverted perovskite solar cells. The fabrication process involves several steps to achieve high-quality perovskite films. First, a precursor solution is prepared by dissolving formamidinium iodide (FAI), lead iodide (PbI₂), and the guanidinium-based additive (ClFACl) in a solvent (DMF). During preparation, the additive undergoes an in-situ reaction with FAI, generating active guanidine species (FA-Gua), which play a crucial role in dictating the crystallization behavior. Next, the precursor solution is deposited onto an indium tin oxide (ITO) substrate a coated with a hole-transport layer using spin-coating. During the coating process, the solution spreads uniformly, and solvent evaporation induces nucleation and crystal growth. The presence of FA-Gua interacts with the (001) facets of the forming crystals via coordination bonds, promoting directional growth and larger crystal sizes with fewer grain boundaries.

Following deposition, the films are subjected to controlled thermal annealing to facilitate solvent removal and complete crystallization. The in situ formed FA-Gua molecules strongly passivate surface defects and grain boundaries through hydrogen bonding and coordination interactions, significantly enhancing the morphological quality and stability of the perovskite layer.

Finally, the perovskite film is integrated into a complete photovoltaic device by sequentially layering charge transport materials and electrodes. This process results in films with larger grain sizes, higher crystallinity, suppressed defect states, and improved environmental resistance, ultimately advancing the development of next-generation perovskite photovoltaics.

The treated devices gave an optimized power conversion efficiency (PCE) of 25.85%, along with an excellent open-circuit voltage (V_OC) of 1.21 V and an improved filling factor (FF) of 82.93%. In addition, the unencapsulated ClFACl-treated devices retained 95% of the initial PCE after 2000 h storage. These findings provide a new perspective to preparing highly facet-oriented and stable perovskite films for construction of efficient and stable PSCs.

The Future: Future research will focus on developing more efficient control strategies to further improve the performance and stability of perovskite solar cells, with an emphasis on compatibility with scalable industrial manufacturing.

This work proposes an in-situ modulation strategy by the active guanidinium salts (FA-Gua). Thanks to the in-situ reaction between chloroformamidine hydrochloride (ClFACl) additive and formamidine hydroiodide (FAI), on the one hand, the transformation of intermediate phase to α-FAPbI₃ is promoted, which induces the selective crystal growth along the (001) plane at the same time. On the other hand, the strengthened intermolecular interactions maintains the stability of the ɑ-FAPbI₃ perovskite and restrains the degradation of the as-prepared perovskite films under humid and thermal treatments. The devices with high orientation and crystallization of perovskite films deliver an champion power conversion efficiency (PCE) of 25.85%. Furthermore, the optimized device maintains excellent stability even under humid and thermal environmental conditions.

The Impact: This research not only provides a reliable method for preparing high orientation and low defect density high-quality perovskite films, but also has natural scalability, thus opening up a path for the industrial-scale production of the new generation of high-performance photovoltaic products.

The research has been recently published in the online edition of Materials Futures, a prominent international journal in the field of interdisciplinary materials science research.

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Reference: Jiajia Du, Yilin Chang, Le Liu*, Zhibin Yu, Qinglin Du, Wenfeng Yang, Yuan Qiu, Fushen Lu*, Tonggang Jiu* and Huanqi Cao*. In situ active guanidinium salts interaction promotes facet orientation and crystallization for efficient and stable inverted perovskite solar cells.  DOI: 10.1088/2752-5724/ae0c77.