Beyond Strain: Controlling the Surface Chemistry of CsPbI₃ Nanocrystal Films for Improved Stability against Ambient Reactive Oxygen Species

Abstract

Colloidal halide perovskite nanocrystals (NCs) have the possibility of easy scale-up due to their batch synthesis and have demonstrated excellent optoelectronic properties. In particular, perovskite NCs have remarkably high photoluminescence quantum yields in solution and as thin films and impressive open circuit voltages in photovoltaic devices. Despite these promising results, little work has been done to understand the stability of CsPbI 3 NCs for optoelectronic device applications. It has been previously shown that the ligands impart tensile surface strain, which stabilizes the black three-dimensional (3D) perovskite phase against phase degradation, making CsPbI 3 NCs some of the most structurally robust inorganic halide perovskites to date. However, understanding exactly how CsPbI 3 NCs degrade under ambient conditions is critical. We demonstrate that the degradation mechanism of NCs is unique from, and 2 orders of magnitude slower than, their polycrystalline thin-film counterparts. Under specific conditions, CsPbI 3 NC films show a compositional instability instead of the phase instability seen in large grain CsPbI 3 . This is mediated through reactions with superoxide and other reactive oxygen species, which are initiated from surface defect states, O 2 and light. We then use this mechanistic insight to identify multiple strategies to prolong the lifetimes of CsPbI 3 NC films, by going beyond surface strain to mitigate key surface chemistries. We demonstrate that (1) minimizing the number of surface defects (2) using an alkylammonium bromide ligand surface treatment and (3) encapsulation with an oxygen scavenging layer all increase NC film lifetimes by inhibiting various steps in the photo-oxidation degradation reaction.

Surface Chemistry Engineering of Perovskite Quantum Dots: Strategies, Applications, and Perspectives

“…[ 169 ] Interestingly, it was found that the commonly used hole transport material Spiro‐OMeTAD could act as an efficient oxygen scavenger to react with the diffused oxygen before initiating the decomposition of the underneath PQD thin films. Thus, the Spiro‐OMeTAD‐encapsulated PQD film can retain 90% of the initial absorptance after 170 h under illumination in ambient conditions (RH 45%), while the control sample showed a fast color decay within 10 h. [ 169 ] Physically, polymer encapsulants such as, poly(ethylene‐ co ‐vinyl acetate) and polydimethyl siloxane can isolate the working areas of PQD devices from external environment, but their high manufacturing cost remains a concern.…”

Quantum Dots for Photovoltaics: A Tale of Two Materials

“…[145] Luther and coworkers demonstrated that the degradation of CsPbI 3 QDs is 2 orders of magnitude slower than the CsPbI 3 bulk film. [274] In addition, they reported that the compositional instability of CsPbI 3 QDs is dominant, where surface defect states initiate and accelerate the oxidation of QDs to form superoxide and other oxidants. It is well recognized in the community that the complicated structural and phase degradation remains an obstacle for PVK QDs.…”

CsPbI3 perovskite quantum dot solar cells: opportunities, progress and challenges

“…Once the superoxide gets access to these vulnerable sites, the structure collapses by propogating chain reactions of the superoxide and ROS. 158 These flaws and surface imperfections can be treated by different strategies for the purpose to achieve long-term stability. 46,54,149 However, the PCE and stability of CsPbI 3 QDs are unacceptable for researchers to compare with counterpart solar cells at the commercial level.…”

“…62,157 It is ascertained from the literature that under coordinated sites of fluctuates device performance and stability. 158,161 Thirdly, the superoxide (O 2 -) can react with CO 2 and 32 produced carbonate (CO 3 2-) and water. Then CO 3 2can produce PbCO 3 and CsCO 3 by reaction with the under coordinated sites of Pb 2+ and Cs + .…”

“…(e, f) Schematic illustration of the phase change of solid QDs film in an ambient environment supported by UV-vis absorption spectra and XRD results 110. (g) Photo-oxidation strategy of CsPbI 3 QDs involving different processes, which is labelled by A, B, C, D and E, respectively 158. (h, i) Stability of CsPbI 3 QD solar cell under ambient environment after post-treatment of 2-MP and 4-MP as well as inserted images of fresh and aged solar cell devices 110.…”