ConspectusA new generation of semiconducting materials based on metal halide perovskites has recently been launched into the scientific spotlight, exhibiting outstanding optoelectronic properties and providing promise for the development of efficient optical devices. As a vivid example, solar cells made from these materials have quickly reached conversion efficiencies exceeding 25%, now on par with well-established technologies, like silicon. Their widespread success is due, in part, to a unique ability to retain high-quality optoelectronic performance while being easily solution-processed into thin films. This feature is what defines them as a brand-new class of optoelectronic materials, with the ability to compete with traditional semiconductors requiring higher processing costs, like the III-Vs or II-IVs. However, the interesting photophysics of metal halide perovskites come with a catch; their soft ionic lattice promotes complex thermal-induced phase transitions and a high capacity for postsynthetic compositional changes, e.g., halide anion exchange. Such dynamic behavior has ultimately made understanding several important structure-property relationships ambiguous and obstructed a clear path toward commercialization due to inherent phase instability.Our aim in this Account is to highlight the fundamental aspects of metal halide perovskites that dictate a stable crystal structure and enable efficient anion exchange, through the lens of thermodynamic preference and phase formation energies. Taking the all-inorganic CsPbI3-xBrx system as a suitable case study, we focus on several ways in which its thermodynamically unstable perovskite structure can be maintained at room temperature and elucidate the restructuring pathways taken during destabilization. In addition, we will discuss the origin and mechanisms of postsynthetic anion exchange in CsPbX3 (X = I, Br, Cl) perovskites, with emphasis made toward direct visualization using in situ optical microspectroscopy and arriving at quantitative results. For several notable features of halide perovskites dealt with in this Account, e.g., strain stabilization, nonperovskite phase restructuring pathway, and lattice anion diffusion, we attempt to rationalize them using state-of-the-art materials modeling techniques.It is within this spirit that we not only modify a broad range of properties existing within metal halide perovskites but also regulate them for enhanced material functionality. For example, controlling partial phase changes and local replacement of halide composition in CsBX3 (B = Pb, Sn and X = I, Br, Cl) nanowires can facilitate the formation of optoelectronic heterojunctions, due to the abrupt change in local crystal structure and the correlated transition in optoelectronic properties. From this combined perspective, metal halide perovskites appear as highly dynamic systems, whereby structural and compositional modifications have a large impact on the underlying phase stability and optoelectronic properties. Thus, we highlight several scientific aspects important to the fundamental understanding of metal halide perovskites, ranging from the underlying mechanism and kinetics through which phase destabilization and anion exchange take place, to tuning the thermodynamic energy landscape using external stimuli. We anticipate that providing a clear perspective for these topics will help deepen our knowledge of the nature of ionic semiconductors and provide the stimulus required to build new research directions toward utilizing halide perovskites within versatile optoelectronic devices.

For polymer solar cells (PSCs), the mixture of polymer donors and small-molecule acceptors (SMAs) is fine-tuned to realize a favorable kinetically trapped morphology and thus a commercially viable device efficiency. However, the thermodynamic relaxation of the mixed domains within the blend raises concerns related to the long-term operational stability of the devices, especially in the record-holding Y-series SMAs. Here, a new class of dimeric Y6-based SMAs tethered with differential flexible spacers is reported to regulate their aggregation and relaxation behavior. In their polymer blends with PM6, it is found that they favor an improved structural order relative to that of Y6 counterpart. Most importantly, the tethered SMAs show large glass transition temperatures to suppress the thermodynamic relaxation in mixed domains. For the high-performing dimeric blend, an unprecedented open circuit voltage of 0.87 V is realized with a conversion efficiency of 17.85%, while those of regular Y6-base devices only reach 0.84 V and 16.93%, respectively. Most importantly, the dimer-based device possesses substantially reduced burn-in efficiency loss, retaining more than 80% of the initial efficiency after operating at the maximum power point under continuous illumination for 700 h. The tethering approach provides a new direction to develop PSCs with high efficiency and excellent operating stability.

The black perovskite phase of CsPbI₃ is promising for optoelectronic applications; however, it is unstable under ambient conditions, transforming within minutes into an optically inactive yellow phase, a fact that has so far prevented its widespread adoption. Here we use coarse photolithography to embed a PbI₂-based interfacial microstructure into otherwise-unstable CsPbI₃ perovskite thin films and devices. Films fitted with a tessellating microgrid are rendered resistant to moisture-triggered decay and exhibit enhanced long-term stability of the black phase (beyond 2.5 years in a dry environment), due to increasing the phase transition energy barrier and limiting the spread of potential yellow phase formation to structurally isolated domains of the grid. This stabilizing effect is readily achieved at the device level, where unencapsulated CsPbI₃ perovskite photodetectors display ambient-stable operation. These findings provide insights into the nature of phase destabilization in emerging CsPbI₃ perovskite devices and demonstrate an effective stabilization procedure which is entirely orthogonal to existing approaches.

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.