Snaith group

Abstract:

Lead Mixed Halide Perovskites (LMHPs), CsPbBrI₂, have attracted significant interest as promising candidates for wide bandgap absorber layers in tandem solar cells due to their relative stability and red-light emission with a bandgap ∼1.7 eV. However, these materials segregate into Br-rich and I-rich domains upon continuous illumination, affecting their optical properties and compromising the operational stability of devices. Herein, we track the microscopic processes occurring during halide segregation by using combined spectroscopic measurements at room and cryogenic temperatures. We also evaluate a passivation strategy to mitigate the halide migration of Br/I ions in the films by overcoating with cyclic olefin copolymer (COC). Our results explain the correlation between grain size, intensity dependencies, phase segregation, activation energy barrier, and their influence on photoinduced carrier lifetimes. Importantly, COC treatment increases the lifetime charge carriers in mixed halide thin films, improving efficient charge transport in perovskite solar cell applications.

Abstract:

Pseudo-halide (PH) anion engineering has emerged as a surface passivation strategy of interest for perovskite-based optoelectronics; but until now, PH anions have led to insufficient defect passivation and thus to undesired deep impurity states. The size of the chemical space of PH anions (>10⁶ molecules) has so far limited attempts to explore the full family of candidate molecules. We created a machine learning workflow to speed up the discovery process using full-density functional theory calculations for training the model. The physics-informed machine learning model allowed us to pinpoint promising molecules with a head group that prevents lattice distortion and anti-site defect formation, and a tail group optimized for strong attachment to the surface. We identified 15 potential bifunctional PH anions with the ability to passivate both donors and acceptors, and through experimentation, discovered that sodium thioglycolate was the most effective passivant. This strategy resulted in a power-conversion efficiency of 24.56% with a high open-circuit voltage of 1.19 volts (24.04% National Renewable Energy Lab-certified quasi-steady-state) in inverted perovskite solar cells. Encapsulated devices maintained 96% of their initial power-conversion energy during 900 hours of one-sun operation at the maximum power point.

Abstract:

Current improvement in perovskite solar cells (PSCs) has been achieved by interface engineering and fine-tuning of charge-selective contacts. In this work, we report three novel molecules that can form self-assembled layers (SAMs) as an alternative to the most commonly used p-type contact material, PTAA. Two of these molecules have bidentate anchoring groups (MC-54 and MC-55), while the last one is monodentate (MC-45). Besides the PTAA comparison, we also compared those two types of molecules and their effect on the solar cell's performance. Devices fabricated with MC-54 and MC-55 showed a remarkable field factor (about 80%) and a better current density, leading to higher efficient solar cells in comparison to MC-45 and PTAA. Moreover, mono- and bidentate present higher stability and reproducibility in comparison to PTAA.