ABSTRACT:Recently, there has been an explosive growth in research based on hybrid lead-halide perovskites for photovoltaics owing to rapid improvements in efficiency. The advent of these materials for solar applications has led to widespread interest in understanding the key enabling properties of these materials. This has resulted in renewed interest in related compounds and a search for materials that may replicate the defect-tolerant properties and long lifetimes of the hybrid lead-halide perovskites. Given the rapid pace of development of the field, the rises in efficiencies of these systems have outpaced the more basic understanding of these materials. Measuring or calculating the basic properties, such as crystal/electronic structure and composition, can be challenging because some of these materials have anisotropic structures, and/or are composed of both heavy metal cations and volatile, mobile, light elements. Some consequences are beam damage during characterization, composition change under vacuum, or compound effects, such as the alteration of the electronic structure through the influence of the substrate. These effects make it challenging to understand the basic properties integral to optoelectronic operation. Compounding these difficulties is the rapid pace with which the field progresses. This has created an ongoing need to continually evaluate best practices with respect to characterization and calculations, as well as to identify inconsistencies in reported values to determine if those inconsistencies are rooted in characterization methodology or materials synthesis. This article describes the difficulties in characterizing hybrid lead-halide perovskites and new materials, and how these challenges may be overcome. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 challenges discussed. The focus in this article is on crystallography, composition measurements, photoemission spectroscopy and calculations on perovskites and new, related absorbers. We suggest how some of the important artifacts could be avoided and how the reporting for each technique could be streamlined between groups to ensure reproducibility as the field progresses.

The coefficient of thermal expansion ͑CTE͒, biaxial modulus, and stress of some amorphous semiconductors ͑a-Si:H, a-C:H, a-Ge:H, and a-GeC x :H͒ and metallic ͑Ag and Al͒ thin films were studied. The thermal expansion and the biaxial modulus were measured by the thermally induced bending technique. The stress of the metallic films, deposited by thermal evaporation ͑Ag and Al͒, is tensile, while that of the amorphous films deposited by sputtering ͑a-Si:H, a-Ge:H, and a-GeC x :H͒ and by glow discharge (a-C:H) is compressive. We observed that the coefficient of thermal expansion of the tetrahedral amorphous thin films prepared in this work, as well as that of the films reported in literature, depend on the network strain. The CTE of tensile films is smaller than that of their corresponding crystalline semiconductors, but it is higher for compressive films. On the other hand, we found out that the elastic biaxial modulus of the amorphous and metallic films is systematically smaller than that of their crystalline counterparts. This behavior stands for other films reported in the literature that were prepared by different techniques and deposition conditions. These differences were attributed to the reduction of the coordination number and to the presence of defects, such as voids and dangling bonds, in amorphous films. On the other hand, columnar structure and microcrystallinity account for the reduced elasticity of the metallic films.

The coefficient of thermal expansion (CTE) of hydrogenated amorphous carbon (a-C:H) was investigated as a function of the concentration of sp2 hybridization. The CTE, determined using the thermally induced bending technique, depends on the concentration of sp2 bonded carbon, increasing to the value of graphite as the sp2 concentration approaches 100%. By using a combination of the thermally induced bending technique and nanohardness measurements, we extract separately the Young’s modulus and Poisson’s ratio of the a-C:H films as function of the sp2 concentration.

High-quality amorphous hydrogenated germanium has been deposited using the diode rf glow discharge method out of a gas plasma of GeH4 and H2. The optical, electrical, and structural properties of this material have been extensively characterized. The optical and electrical properties are all consistent with material containing a low density of defect related states in the energy gap. In particular, this material has an ημτ=3.2×10−7 cm2/V, ratio of photocurrent to dark current of 1.3×10−1, and flux dependence of the photocurrent with γ=0.79 at 1.25 eV measured using photoconductivity, a μτ=4×10−8 cm2/V measured using time of flight, an Urbach energy of 51 meV and α at 0.7 eV of 8.3 cm−1 measured using photothermal deflection spectroscopy, a dangling bond spin density of 5×1016 cm−3 measured using electron spin resonance, photoluminescence with a peak energy position of 0.81 eV and full width at half maximum of 0.19 eV, an activation energy of 0.52 eV and σ0 of 6.1×103 (Ω cm)−1 measured using dark conductivity, and an E04 band gap of 1.24 eV measured by optical absorption. The structural measurements indicate a homogeneous material lacking any island/tissue and columnar structure when investigated using transmission and scanning electron microscopy, respectively. Hydrogen concentrations calculated from infrared and gas evolution measurements can only by reconciled by postulating a large quantity of unbonded hydrogen whose presence is confirmed using deuteron magnetic resonance. The bonded deuterium component, as seen in this film using DMR, has a spin-lattice relaxation time of the order of 4000 s. The differential scanning calorimetry measurement shows crystallization occurring at 421 °C and the presence of large compressive stresses has been confirmed using a bending-beam method. The experimental details necessary to interpret the quantities quoted here are set out in the text which follows. It is considered that the very good optical and electrical properties of this as yet unoptimized material are directly related to the structural properties detailed above.

Perovskite solar cells (PSCs) technology is now reaching its full potential in terms of power conversion efficiency, but still presenting problems related to long‐term stability under operating conditions. One of the most promising alternatives to PSCs is the layered PSCs (2D‐PSCs). Layered perovskites present a huge compositional variety, which can be used to directly tune photophysical characteristics that influence the operational mechanisms of the devices. This review addresses the structural organization of both the organic and inorganic sublattices, focusing on how the structure influences the quantum and dielectric confinement, phonons and charge carriers' dynamics, charge mobility, and structural defects. We discuss the relation between the structure‐properties of layered perovskites with the performance of solar cells. We, then, offer insights into how these characteristics have been controlled in the assembly of 2D‐PSCs to improve their efficiency and stability. We conclude by giving a perspective of future developments and open areas of exploration that might impact the progress of this rapidly growing technology.