Towards Understanding the 2D Passivation of 3D Perovskites Through Structure-Activity Relationships

Perovskite solar cells (PSCs) became the “next big thing” in solar energy research due to wide possibility of cost-effective fabrication approaches, high efficiencies and its undeniable match with silicon towards tandem solar cells. Unfortunately, reproducibility and long-term stability for PSCs are not even comparable with the commercial silicon technology. Several factors, both external and internal, affect the stability of PSCs, hence effective encapsulation cannot be the single remedy. Interface engineering is one of the critical aspects for enhanced performance and stability, and surface passivation with 2D perovskites have emerged as one of the leading approaches. Several large cations have been utilized in the literature for 2D/3D PSCs with promising results. However, due to the delicate nature of perovskites, different perovskite formulations used in these studies and the differences in device architecture makes it impossible to have clear structure-activity relationship.<br/>In our ongoing efforts for deeper understanding of the 2D/3D perovskite interface, we designed and synthesized several large ammonium cations based on PEAI (phenylethylammonium iodide) with different substituents at different positions. All these salts (12 different) were then utilized in 2D/3D perovskites solar cells with same perovskite formulation and same device architecture. Among these salts several have showed solid performance and significantly improved stability. For example, <i>o</i>-OMe-PEAI, a methoxy substituted PEAI salt, utilized 2D/3D perovskite solar cells revealed PCEs as high as 23.34% and the device treated with <i>o</i>-OMe-PEAI maintained 95% of its initial yield while the reference device only retained 59% (<i>Cell Rep. Phys. Sci., </i><b>2023</b>, <i>4</i>, 101380). Another derivative, <i>m</i>-CPEAI, a chlorine substituted PEAI utilized devices showed a high PCE of 23.16%, and showed remarkable stability where after 1000 hours of continuous illumination, <i>m</i>-ClPEAI-treated devices retained 87% of initial efficiency compared to the 36% retained by the reference (<i>Adv. Energy Mater.</i>, <b>2023</b>, <i>13</i>, 2370184).<br/>Detailed DFT analyses showed that <i>o</i>-OMe-PEAI incorporated cells demonstrate favorable formation energy and desired vertical orientation along with optimal surface coating which resulted in the improved performance. However, similar DFT analyses revealed that lower formation energies, but the higher interfacial dipoles achieved by <i>m</i>-ClPEAI was responsible for the enhanced performance. The results clearly demonstrated in addition to the chemical identity, the positioning of the substituents have a major effect on the performance of the resulting devices and the beneficial effects resulted on the interface can arise from different physical phenomenon which needs to be incorporated into future design strategies.<br/>Shorter chain ammonium cations such as phenylmethylammonium iodide (PMAI) passivates the 3D perovskite surface but do not actually for 2D-perovskite lattice. However, both performance and stability enhancements were demonstrated. In a recent study, we showed that five-membered heterocyclic aromatics (furan, thiophene, selenophene) substituted PMAI salts passivates the perovskite surface effectively and results in increased performance. The effect of substitution was also clear here where thiophene substituted derivative outperformed the counterparts (Reference PCE: 20.63%, thiophene: 22.93%, selenophene: 21.45% and furan: 21.87%). The stability enhancement was also significant where the reference cell had a 50% loss of efficiency after 1250 hours, whereas thiophene-based PAI treated cells retained nearly all their initial performance.<br/>We are now creating a larger library of ethylammonium and methylammonium based cations with electronically distinct substituents and positional geometries to device a general set of design rules that will enable us to design next generation of material towards highly stable commercial 2D/3D PSCs.