Perovskite solar cells: Stability lies at interfaces

Perovskite solar cells are developing fast but their lifetimes must be extended. Now, large-area printed perovskite solar modules have been shown to be stable for more than 10,000 hours under continuous illumination.

O rganic-inorganic halide perovskite solar cells benefit from high power conversion efficiency (now beyond 22%), ease of fabrication, and low cost. Despite these advantages, their operational stability and the materials toxicity remain of foremost concern. Stability issues appear in the halide perovskite itself but also in other constituent materials, as well as at interfaces between the various layers of the device. Correspondingly, several options have been proposed to reduce device degradation: inverting the solar cell structure to reduce reactivity, replacing organic semiconductors with oxides at interfaces to improve moisture and oxygen stability, or tuning the composition of the halide perovskite to stabilize its crystal structure and improve thermal strength 1 , for example. Among the different organicinorganic halide perovskite structures, 3D hybrid compounds currently lead to the most impressive power conversion efficiency. These compounds are characterized by their excellent photophysical properties, such as high absorbance and long excitation and charge diffusion lengths. However, 3D halide perovskites, which contain small organic cations, are moisture sensitive and this leads to rapid performance degradation. In contrast, layered 2D halide perovskite structures accommodate bulkier and more hydrophobic organic cations, resulting in superior moisture stability, making them more attractive for large-scale industrial implementation. Nevertheless, 2D halide perovskites show lower power conversion efficiencies, currently around 12% (ref. 2). Now, writing in Nature Communications, Mohammad Khaja Nazeeruddin and colleagues from Switzerland and Italy combine 3D and 2D halide perovskite structures using functional organic molecules, and report one year of operational stability for printed 10 cm × 10 cm photovoltaic modules fabricated in air 3 .
The key feature of the solar cells fabricated by Nazeeruddin and colleagues is found at the interfaces ( Fig. 1). They show that a 2D halide perovskite and the pure 3D halide perovskites can be bonded together at a 2D/3D interface by aminovaleric acid iodide. The protonated form of this small organic acid can coordinate on one hand with semiconductor oxides, like the TiO 2 used as the electron transport electrode, thanks to its carboxyl functional group (Fig. 1c,e), and on the other hand with various halide perovskite compounds, thanks to both its amine and carboxyl functional groups (Fig. 1d,e). The final 2D/3D material contains the best of both structures: the ease of fabrication and moisture stability of the 2D perovskite, and the panchromatic absorption and good photovoltaic properties of the 3D perovskite.

Stability lies at interfaces
Perovskite solar cells are developing fast but their lifetimes must be extended. Now, large-area printed perovskite solar modules have been shown to be stable for more than 10,000 hours under continuous illumination.  . c-e, The interface modification of interlayers is made through the aminovaleric acid iodide molecule, with carboxyl and amine anchoring groups. Some examples of carboxyl functional groups bonding with semiconducting (TiO 2 ) or inert scaffold (ZrO 2 ) metal oxides 6 (c) and schematic representation of one possible perovskite/perovskite interaction, the halogen to hydrogen bonding to 2D/3D perovskites 2,4 (d). M, metal; FTO, fluorine doped tin oxide; ETL, electrode transport layer; HTL, hole transport layer.   (1) (2) ) ) ) )

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an enhanced stability when compared to similar devices made only with the 3D compound. After 300 h of continuous illumination under one sun (at 45 °C, in an argon atmosphere, for encapsulated devices), the spin-coated 2D/3D perovskite-based solar cell shows a 30% decrease in its original efficiency value, while the solar cell made with only the 3D perovskite shows a 50% efficiency decrease 3 , demonstrating the beneficial and synergetic effect attained by interface engineering.
In a second step towards scalability and improved stability, the researchers avoid the use of unstable hole transport layers, like Spiro-OMeTAD. Instead, they employ a nanoparticulated TiO 2 thin film, followed by a coating of an inert layer of ZrO 2 nanoparticles used as a scaffold. It is finally completed with a carbon-based coating which acts as a back current collector and a water-retaining layer. The halide perovskite is introduced into the oxide thin films via infiltration. All the layers are fabricated by a combination of spray and screen printing, and the fabrication process is carried out completely in ambient atmosphere (Fig. 1b). This promising fabrication method, developed back in 2014 (ref. 6), has already generated devices with lifetime stability among the longest for perovskite solar cells under continuous illumination 6 . In the work reported by Nazeeruddin and colleagues, the researchers demonstrate the possibility of using this printing method with their new 2D/3D perovskite to fabricate 10 cm × 10 cm modules (47.6 cm 2 active area, ~10% efficiency). Device stability was tested under realistic conditions (employing ISOS standards) on encapsulated devices under continuous illumination and protected from ultraviolet light, achieving an impressive module stability of more than 10,000 h (more than 1 year).
While the molecular functionalization of interfaces in halide perovskite solar cells is not new, the study of their effect on device stability is unusual. Examples from the literature suggest that the stability of the solar cell is always enhanced after interface engineering, independently of the type of solar cell configuration, the type and duration of the stability test performed, or the organic functional molecule used (Fig. 2). What's more, improvement in stability is observed regardless of the interface being modified: the oxide/perovskite 8 , the perovskite/perovskite 2,3,5,6 , the perovskite/transport layer [7][8][9] and even the interface with the back metal electrode. Here, the work reported by Nazeeruddin and colleagues furthermore demonstrates that a single molecule with double-anchoring groups is able to modify multiple interfaces. It permits the functionalization of the halide perovskite, allowing the formation of a new and improved hybrid organic-inorganic 2D/3D perovskite interface where the functional groups interact through hydrogen-to-halogen bonding 4,5 (Fig. 1d) or by the interaction of the amine and the A sites in the perovskite structure 4 . It also permits the anchoring to the TiO 2 , through any of the possible carboxylate-metal binding modes (Fig. 1c), resulting in the presence of fewer defects and preventing charge recombination. But probably the most attractive aspect, from a technological point of view, is the possibility of also modifying the inert ZrO 2 scaffold via carboxylate bonding. Without the ZrO 2 scaffold, the stability of the device is inferior (Fig. 2, molecule 2) 3,6 . Indeed, it is specifically the combination of the dual functionalization, the 2D/3D perovskite structure and the bonding to the perovskites and the TiO 2 and ZrO 2 oxides that leads to the remarkable stability, with no loss in performance for more than 10,000 h (Fig. 2, molecule 9) 3 .
Given the rich chemistry of hybrid halide perovskites, the ample variety of organic modifiers with diverse anchoring groups, and the vast amount of oxides (semiconductors or scaffolds), the work from Nazeeruddin and colleagues suggests that developing matching sets of chemically bound materials will be of tremendous importance to combine their respective advantages. This work heralds the imminent and inexorable development of large-scale perovskite solar cells with competitive lifetimes, which may soon reach commercialization. ❐