An insight into the analogies, state-of-the-art technologies, concepts, and prospects under the umbrella of perovskite materials (both inorganic-organic hybrid halide perovskites and ferroelectric perovskites) for future multifunctional energy conversion and storage devices is provided. Often, these are considered entirely different branches of research; however, considering them simultaneously and holistically can provide several new opportunities. Recent advancements have highlighted the potential of hybrid perovskites for high-efficiency solar cells. The intrinsic polar properties of these materials, including the potential for ferroelectricity, provide additional possibilities for simultaneously exploiting several energy conversion mechanisms such as the piezoelectric, pyroelectric, and thermoelectric effect and electrical energy storage. The presence of these phenomena can support the performance of perovskite solar cells. The energy conversion using these effects (piezo-, pyro-, and thermoelectric effect) can also be enhanced by a change in the light intensity. Thus, there lies a range of possibilities for tuning the structural, electronic, optical, and magnetic properties of perovskites to simultaneously harvest energy using more than one mechanism to realize an improved efficiency. This requires a basic understanding of concepts, mechanisms, corresponding material properties, and the underlying physics involved with these effects.

Mutual Insight on Ferroelectrics and Hybrid Halide Perovskites: A Platform for Future Multifunctional Energy Conversion.

Hybrid halide perovskites have become the “next big thing” in emerging semiconductor materials, as the past decade witnessed their successful application in high-performance photovoltaics. This resurgence has encompassed enormous and widespread development of the three-dimensional (3D) perovskites, spearheaded by CH3NH3PbI3. The next generation of halide perovskites, however, is characterized by reduced dimensionality perovskites, emphasizing the two-dimensional (2D) perovskite derivatives which expand the field into a more diverse subgroup of semiconducting hybrids that possesses even higher tunability and excellent photophysical properties. In this Perspective, we begin with a historical flashback to early reports before the “perovskite fever”, and we follow this original work to its fruition in the present day, where 2D halide perovskites are in the spotlight of current research, offering characteristics desirable in high-performance optoelectronics. We approach the evolution of 2D halide perovskites f...

Two-Dimensional Hybrid Halide Perovskites: Principles and Promises.

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.

State of the Art and Prospects for Halide Perovskite Nanocrystals.

Vibrational and vibronic relaxation and related energy redistribution play an important role in many photoinduced processes of molecules. In the condensed phase, in which collision-induced relaxation is of particular relevance, these phenomena occur on a very rapid time scale between several tens of femtoseconds and 100 ps (1, 2). Spectroscopy with ultrashort laser pulses has been widely applied to elucidate the dynamics and physical mechanisms of intraand intermolecular energy redistribution (3, 4). Indeed, time resolved experiments have provided extensive information not accessible by other experimental techniques. In this article, we present a review on vibrational and vibronic relaxation of large molecules in liquids. Photoinduced redistribution processes of molecules, which consist typically of 20 or more atoms, are considered. We discuss. femtoand picosecond spectroscopy of the intramolecular redistribution and the intermolecular energy transfer from vibrationally hot molecules to their (cold) surrounding. We focus on measurements of vibrational and vibronic lifetimes, as well as on studies of energy redis­ tribution and transport. However, we will not consider dephasing of vibranic or vibrational states, relaxation processes connected to the

VIBRATIONAL AND VIBRONIC RELAlXATION OF LARGE POL-yrATOMIC MOLECULES IN LIQUIDS

Strain engineering is a powerful tool with which to enhance semiconductor device performance1,2. Halide perovskites have shown great promise in device applications owing to their remarkable electronic and optoelectronic properties3–5. Although applying strain to halide perovskites has been frequently attempted, including using hydrostatic pressurization6–8, electrostriction9, annealing10–12, van der Waals force13, thermal expansion mismatch14, and heat-induced substrate phase transition15, the controllable and device-compatible strain engineering of halide perovskites by chemical epitaxy remains a challenge, owing to the absence of suitable lattice-mismatched epitaxial substrates. Here we report the strained epitaxial growth of halide perovskite single-crystal thin films on lattice-mismatched halide perovskite substrates. We investigated strain engineering of α-formamidinium lead iodide (α-FAPbI3) using both experimental techniques and theoretical calculations. By tailoring the substrate composition—and therefore its lattice parameter—a compressive strain as high as 2.4 per cent is applied to the epitaxial α-FAPbI3 thin film. We demonstrate that this strain effectively changes the crystal structure, reduces the bandgap and increases the hole mobility of α-FAPbI3. Strained epitaxy is also shown to have a substantial stabilization effect on the α-FAPbI3 phase owing to the synergistic effects of epitaxial stabilization and strain neutralization. As an example, strain engineering is applied to enhance the performance of an α-FAPbI3-based photodetector. A method of deposition of mixed-cation hybrid perovskite films as lattice-mismatched substrates for an α-FAPbI3 film is described, giving strains of up to 2.4 per cent while also stabilizing the metastable α-FAPbI3 phase for several hundred days.

Strain engineering and epitaxial stabilization of halide perovskites.

Metal-halide perovskites have long carrier diffusion lengths, low trap densities and high carrier mobilities, and are therefore of value in the development of photovoltaics and light-emitting diodes. However, the presence of thermally activated carriers in the materials leads to high noise levels, which limits their photodetection capabilities. Here, we show that ultrasensitive photodetectors can be created from single-crystalline nanowire arrays of layered metal-halide perovskites. A series of nanowires was fabricated in which layer-by-layer self-organization of insulating organic cations and conductive inorganic frameworks, along the nanowire length, creates high resistance in the interior of the crystals and high conductivity at the edges of the crystals. Using these structures, high-performance photodetection was achieved with responsivities exceeding 1.5 × 104 A W−1 and detectivities exceeding 7 × 1015 jones. Our state-of-the-art device performance originates from a combination of efficient free-carrier edge conduction and resistive hopping barriers in the layered perovskites. Photodetectors made from single-crystalline nanowire arrays of layered metal-halide perovskites exhibit detectivities of more than 7 × 1015 jones, due to a nanowire structure that combines high resistance in the interior of the crystals and high conductivity at the edges of the crystals.

Single-crystalline layered metal-halide perovskite nanowires for ultrasensitive photodetectors

Polarization-sensitive perovskite photodetectors are realized by crystallographically aligning 1D perovskite arrays. High-quality inorganic perovskite single crystals with crystallographic order are fabricated by strictly manipulating the dewetting process of organic solution, yielding photodetectors with high photoresponsivity and fast response speed.

Crystallographically Aligned Perovskite Structures for High-Performance Polarization-Sensitive Photodetectors.

Superwetting interfaces arising from the cooperation of surface energy and multiscale micro/nanostructures are extensively studied in biological systems. Fundamental understandings gained from biological interfaces boost the control of wettability under different dimensionalities, such as 2D surfaces, 1D fibers and channels, and 3D architectures, thus permitting manipulation of the transport physics of liquids, gases, and ions, which profoundly impacts chemical reactions and material fabrication. In this context, the progress of new chemistry based on superwetting interfaces is highlighted, beginning with mass transport dynamics, including liquid, gas, and ion transport. In the following sections, the impacts of the superwettability-mediated transport dynamics on chemical reactions and material fabrication is discussed. Superwettability science has greatly enhanced the efficiency of chemical reactions, including photocatalytic, bioelectronic, electrochemical, and organic catalytic reactions, by realizing efficient mass transport. For material fabrication, superwetting interfaces are pivotal in the manipulation of the transport and microfluidic dynamics of liquids on solid surfaces, leading to the spatially regulated growth of low-dimensional single-crystalline arrays and high-quality polymer films. Finally, a perspective on future directions is presented.

Superwettability-Based Interfacial Chemical Reactions.

Traditional optical organic vapor sensors with solvatochromic shift mechanisms have lower sensitivity due to weak intermolecular interactions Here, we report a general strategy to prepare a higher sensitivity optical organic vapor sensor through polymeric swelling-induced variation of fluorescent intensity We combine one-dimensional polymeric structures and aggregation-induced emission (AIE) molecules together to form a polymer/AIE microwires array as a sensor The prepared sensors based on different commercial polymers can successfully classify and identify various organic vapors Among them, the poly(vinyl butyral)/AIE microwires array can detect methanol vapor as low as 005% of its saturation vapor pressure According to the theory of like dissolves like, we further fabricate a polymer/AIE microwires array derived from designable polyethersulfones, through regulating their side chains, to distinguish similar organic vapors of benzene and toluene Both experimental and theoretical simulation results reveal that specific molecular interactions between the polyethersulfones and organic vapors can improve the specific recognition performance of the sensors

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Hanfei Gao

Papers

Single-crystalline layered metal-halide perovskite nanowires for ultrasensitive photodetectors

Crystallographically Aligned Perovskite Structures for High-Performance Polarization-Sensitive Photodetectors.

Superwettability-Based Interfacial Chemical Reactions.

Highly-sensitive optical organic vapor sensor through polymeric swelling induced variation of fluorescent intensity.

Stable α-CsPbI3 perovskite nanowire arrays with preferential crystallographic orientation for highly sensitive photodetectors