Bulky ammonium iodide and in-situ formed 2D Ruddlesden-Popper layer enhances the stability and efficiency of perovskite solar cells

2D Phase Formation on 3D Perovskite: Insights from Molecular Stiffness

Several studies have demonstrated that low-dimensional structures (e.g., two-dimensional (2D)) associated with three-dimensional (3D) perovskite films enhance the efficiency and stability of perovskite solar cells. Here, we aim to track the formation sites of the 2D phase on top of the 3D perovskite and to establish correlations between molecular stiffness and steric hindrance of the organic cations and their influence on the formation and crystallization of 2D/3D. Using cathodoluminescence combined with a scanning electron microscopy technique, we verified that the formation of the 2D phase occurs preferentially on the grain boundaries of the 3D perovskite. This helps explain some passivation mechanisms conferred by the 2D phase on 3D perovskite films. Furthermore, by employing in situ grazing-incidence wide-angle X-ray scattering, we monitored the formation and crystallization of the 2D/3D perovskite using three cations with varying molecular stiffness. In this series of molecules, the formation and crystallization of the 2D phase are found to be dependent on both steric hindrance around the ammonium group and molecular stiffness. Finally, we employed a 2D/3D perovskite heterointerface in a solar cell. The presence of the 2D phase, particularly those formed from flexible cations, resulted in a maximum power conversion efficiency of 21.5%. This study provides insight into critical aspects related to how bulky organic cations' stiffness and steric hindrance influence the formation, crystallization, and distribution of 2D perovskite phases.

Elevated Efficiency and Stability of Hole-Transport-Layer-Free Perovskite Solar Cells Triggered by Surface Engineering

Surface engineering is one of the important strategies to enhance the power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). Herein, 2-chloro-1,3-dimethylimidazolidinium hexafluorophosphate (CIP) was introduced into PSCs to passivate the defects of the perovskite films. There are many F atoms in CIP molecules that have strong electronegativity and hydrophobicity. F groups can interact with Pb2+ defects, inhibit interface recombination, improve the interaction between the CIP ionic liquid and perovskite film, and reduce the defect density of perovskites, thus improving the stability of perovskite devices. Density functional theory calculation reveals that CIP can interact with uncoordinated Pb2+ in perovskites through coordination, reduce the defects of perovskite films, and inhibit nonradiation recombination. The ITO/SnO2/MAPbI3/CIP/carbon devices without hole transport layers possessed the highest PCE of 17.06%. Moreover, the unencapsulated device remains at 98.18% of the initial efficiency stored in 30-40% relative humidity for 850 h. This strategy provides an effective reference for enhancing the performance of PSCs.

Polarized Molecule 4-(Aminomethyl) Benzonitrile Hydrochloride for Efficient and Stable Perovskite Solar Cells

The rapid development of perovskite solar cells (PSCs) makes it one of the most competitive photovoltaic devices in the field of new energy. However, the suboptimal performance and poor stability caused by numerous defects are still the main factors limiting the development of PSCs. Herein, a polarized molecule additive of 4-(aminomethyl) benzonitrile hydrochloride (AMBNCl) is introduced into perovskite. Owing to its special polar electron density distribution, -C≡N group, -NH3+ terminal, and Cl- ions, the modification of AMBNCl can improve the quality of perovskite crystal growth, passivate the defects of Pb2+, adjust the energy level array between the perovskite layer and hole-transport layer, and alleviate the carrier nonradiative recombination. As a result, the AMBNCl-modified device achieves a champion efficiency of 23.52%. The unpacked device still maintained 91.2% of its original efficiency after storing in an air environment (RH ∼40%, 25 °C) for 50 days.