Abstract
ConspectusThe rediscovery of the halide perovskite class of compounds and, in particular, the organic and inorganic lead halide perovskite (LHP) materials and lead-free derivatives has reached remarkable landmarks in numerous applications. First among these is the field of photovoltaics, which is at the core of today’s environmental sustainability efforts. Indeed, these efforts have born fruit, reaching to date a remarkable power conversion efficiency of 25.2% for a double-cation Cs, FA lead halide thin film device. Other applications include light and particle detectors as well as lighting. However, chemical and thermal degradation issues prevent perovskite-based devices and particularly photovoltaic modules from reaching the market. The soft ionic nature of LHPs makes these materials susceptible to delicate changes in the chemical environment. Therefore, control over their interface properties plays a critical role in maintaining their stability. Here we focus on LHP nanocrystals, where surface termination by ligands determines not only the stability of the material but also the crystallographic phase and crystal habit. A surface analysis of nanocrystal interfaces revealed the involvement of Brønsted type acid–base equilibrium in the modification of the ligand moieties present, which in turn can invoke dissolution and recrystallization into the more favorable phase in terms of minimization of the surface energy. A large library of surface ligands has already been developed showing both good chemical stability and good electronic surface passivation, resulting in near-unity emission quantum yields for some materials, particularly CsPbBr3. However, most of those ligands have a large organic tail hampering charge carrier transport and extraction in nanocrystal-based solid films.The unique perovskite structure that allows ligand substitution in the surface A (cation) sites and the soft ionic nature is expected to allow the accommodation of large dipoles across the perovskite crystal. This was shown to facilitate electron transfer across a molecular linked single-particle junction, creating a large built-in field across the junction nanodomains. This strategy could be useful for implementing LHP NCs in a p–n junction photovoltaic configuration as well as for a variety of electronic devices. A better understanding of the surface propeties of LHP nanocrystals will also enable better control of their growth on surfaces and in confined volumes, such as those afforded by metal–organic frameworks, zeolites, or chemically patterened surfaces such as anodic alumina, which have already been shown to significantly alter the properties of in-situ-grown LHP materials.
Highlights
Lead-halide perovskite (LHP) film-based optoelectronic applications such as solar cells, radiation detectors, and LEDs have reached impressive performance in the past decade
oleic acid (OA)/OLA-capped CsPbBr3−xIx exchanged with oleylammonium iodide and lead undecenoate resulted in only low-density coverage of oleylammonium bromide on the surface and rapid morphological degradation as the iodide ratio increased, in contrast to similar CsPbBr3 NCs tested.[52]
Colloidal LHP NCs have an advantage over perovskite thin films in terms of production costs and material synthesis versatility, there are inherent challenges concerning their ligand passivation
Summary
Lead-halide perovskite (LHP) film-based optoelectronic applications such as solar cells, radiation detectors, and LEDs have reached impressive performance in the past decade. When A sites are occupied by groups that are too large, such as the long-chain alkyl aminmmonium cations, LHPs turn into the 2D layered structure (i.e., Ruddlesden−Popper perovskites)[12] to form quantum well superlattices, providing an additional degree of freedom to tune the intrinsic physical features, including the optical band gap, exciton binding energy, and dielectric constant.[13,14] In this Account, we highlight the role of molecular surface ligands in colloidal LHP nanocrystals, beginning with a discussion of their role in synthesis and crystal phase control, followed by their effect on the electronic properties at heterojunctions between LHPs and other semiconductors through the example of a CsPbBr3−CdSe hybrid structure
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