On-demand Lewis base formation strategy boosts efficiency and stability of perovskite solar cells

Solar cells based on perovskites, materials with a characteristic crystal structure first unveiled in the mineral calcium titanate (CaTiO₃), have emerged as a promising alternative to conventional silicon-based photovoltaics. A key advantage of these materials is that they could yield high power conversion efficiencies (PCEs), yet their production costs could be lower.

Perovskite films can exist in different structural forms, also referred to as phases. One is the so-called α-phase (i.e., a photoactive black phase), which is the most desirable phase for the efficient absorption of light and the transport of charge carriers. The δ-phase, on the other hand, is an intermediate phase characterized by a different atom arrangement and reduced photoactivity.

Researchers at the University of Toledo, Northwestern University, Cornell University and other institutes recently introduced a new strategy to control the crystallization process in perovskite-based solar cells, stabilizing the δ-phase while facilitating their transition to the α-phase. Their proposed approach, outlined in a paper in Nature Energy, enables the formation of Lewis bases on perovskites on demand to optimize crystallization, which can enhance the efficiency and stability of solar cells.

“In the fabrication of FAPbI₃-based perovskite solar cells, Lewis bases play a crucial role in facilitating the formation of the desired photovoltaic α-phase,” wrote Sheng Fu, Nannan Sun and their colleagues in their paper.

“However, an inherent contradiction exists in their role: they must strongly bind to stabilize the intermediate δ-phase, yet weakly bind for rapid removal to enable phase transition and grain growth. To resolve this conflict, we introduced an on-demand Lewis base molecule formation strategy.”

To reliably control the crystallization process during the fabrication of perovskites for photovoltaics, Fu, Sun and their colleagues used organic salts that contained Lewis acids. These salts deprotonate to produce Lewis bases at desired times, yet they can also be converted back into salts and easily removed once they serve their purpose.

The researchers assessed the potential of their proposed strategy in a series of tests and found that they enabled the optimal crystallization of α-phase FAPbI₃ perovskite films. Their approach was found to enhance the quality of the films, ensuring that A-site cations were uniformly and vertically distributed while also yielding larger grain sizes and fewer voids at buried interfaces.

The team used the perovskite films they created using their method to fabricate solar cells and then also tested the performance of these cells. Their results were very encouraging, as the cells attained good power conversion efficiencies that were maintained after their continuous operation for long periods of time.

“Perovskite solar cells incorporating semicarbazide hydrochloride achieved an efficiency of 26.1%, with a National Renewable Energy Laboratory-certified quasi-steady-state efficiency of 25.33%,” wrote Fu, Sun and their colleagues. “These cells retained 96% of their initial efficiency after 1,000 h of operation at 85 °C under maximum power point tracking. Additionally, mini-modules with an aperture area of 11.52 cm² reached an efficiency of 21.47%.”

The team’s Lewis-base formation strategy could soon be applied to other perovskite materials, potentially contributing to the advancement of perovskite-based solar cells and their future real-world deployment. As part of their study, Fu, Sun and their colleagues used the Lewis acid-containing salt semicarbazide hydrochloride, yet their approach can be replicated using any other Lewis-acid containing salt that exhibits a low acid dissociation constant.

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