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Metal halide perovskite crystals grown on close-packed titanium dioxide colloidal monolayers exhibit hopper-like 3D morphologies, with growth initially directed vertically from the substrate before transitioning to the parallel direction. 

Solution-processable organic materials for emerging electronics can generally be divided into two classes of semiconductors, organic small molecules and polymers. The theoretical thermodynamic limits of device performance are largely determined by the molecular structure of these compounds, and advances in synthetic routes have led to significant progress in charge mobilities and light conversion and light emission efficiencies over the past several decades. Still, the uncontrolled formation of out-of-equilibrium film microstructures and unfavorable polymorphs during rapid solution processing remains a critical bottleneck facing the commercialization of these materials. This tutorial review provides an overview of the use of nanoconfining scaffolds to impose order onto solution-processed semiconducting films to overcome this limitation. For organic semiconducting small molecules and polymers, which typically exhibit strong crystal growth and charge transport anisotropy along different crystallographic directions, nanoconfining crystallization within nanopores and nanogrooves can preferentially orient the fast charge transport direction of crystals with the direction of current flow in devices. Nanoconfinement can also stabilize high-performance metastable polymorphs by shifting their relative Gibbs free energies via increasing the surface area-to-volume ratio. Promisingly, such nanoconfinement-induced improvements in film and crystal structures have been demonstrated to enhance the performance and stability of emerging optoelectronics that will enable large-scale manufacturing of flexible, lightweight displays and solar cells. 

Metal halide perovskites (MHPs) are frontrunners among solution-processable materials for lightweight, large-area and flexible optoelectronics. These materials, with the general chemical formula AMX 3 , are structurally complex, undergoing multiple polymorph transitions as a function of temperature and pressure. In this review, we provide a detailed overview of polymorphism in three-dimensional MHPs as a function of composition, with A = Cs + , MA + , or FA + , M = Pb 2+ or Sn 2+ , and X = Cl − , Br − , or I − . In general, perovskites adopt a highly symmetric cubic structure at elevated temperatures. With decreasing temperatures, the corner-sharing MX 6 octahedra tilt with respect to one another, resulting in multiple polymorph transitions to lower-symmetry tetragonal and orthorhombic structures. The temperatures at which these phase transitions occur can be tuned via different strategies, including crystal size reduction, confinement in scaffolds and (de-)pressurization. As discussed in the final section of this review, these solid-state phase transformations can significantly affect optoelectronic properties. Understanding factors governing these transitions is thus critical to the development of high-performance, stable devices. 

For organic semiconductor crystals exhibiting anisotropic charge transport along different crystallographic directions, nanoconfinement is a powerful strategy to control crystal orientation by aligning the fast crystallographic growth direction(s) with the unconfined axis(es) of nanoconfining scaffolds. Here, design rules are presented to relate crystal morphology, scaffold geometry, and orientation control in solution-processed small-molecule crystals. Specifically, organic semiconductor triisopropylsilylethynyl pyranthrene needle-like crystals with a dimensionality of n = 1 and perylene platelike crystals with n = 2 were grown from solution within nanoconfining scaffolds comprising cylindrical nanopores with a dimensionality of m = 1, representing one unconfined dimension along the cylinder axis, and those comprising nanopillar arrays with a dimensionality of m = 2. For m = n systems, native crystal growth habits were preserved while the crystal orientation in n = m direction(s) was dictated by the geometry of the scaffold. For n ≠ m systems, on the other hand, orientation control was restricted within a single plane, either parallel or perpendicular to the substrate surface. Intriguingly, control over crystal shape was also observed for perylene crystals grown in cylindrical nanopores ( n > m ). Within the nanopores, crystal growth was restricted along a single direction to form a needle-like morphology. Once growth proceeded above the scaffold surface, the crystals adopted their native growth habit to form asymmetric T-shaped single crystals with concave corners. These findings suggest that nanoporous scaffolds with spatially-varying dimensionalities can be used to grow single crystals of complex shapes. 

Abstract Optoelectronic properties of anisotropic crystals vary with direction requiring that the orientation of molecular organic semiconductor crystals is controlled in optoelectronic device active layers to achieve optimal performance. Here, a generalizable strategy to introduce periodic variations in the out‐of‐plane orientations of 5,11‐bis(triisopropylsilylethynyl)anthradithiophene (TIPS ADT) crystals is presented. TIPS ADT crystallized from the melt in the presence of 16 wt.% polyethylene (PE) forms banded spherulites of crystalline fibrils that twist in concert about the radial growth direction. These spherulites exhibit band‐dependent light absorption, photoluminescence, and Raman scattering depending on the local orientation of crystals. Mueller matrix imaging reveals strong circular extinction (CE), with TIPS ADT banded spherulites exhibiting domains of positive or negative CE signal depending on the crystal twisting sense. Furthermore, orientation‐dependent enhancement in charge injection and extraction in films of twisted TIPS ADT crystals compared to films of straight crystals is visualized in local conductive atomic force microscopy maps. This enhancement leads to 3.3‐ and 6.2‐times larger photocurrents and external quantum efficiencies, respectively, in photodetectors comprising twisted crystals than those comprising straight crystals.