In our paper, we reported a previously unobserved Raman-like band centered at around 550 cm-1 [e.g., Figs. 2(a) and 3(a)] and found that it diminished after the material degraded. Our latest study has revealed that the assignment of the 550 cm-1 band as a Raman band was erroneous, and this Raman-like peak was in fact the result of stray light of the unusually strong photoluminescence (PL) signal, peaked at around 770 nm, in the pristine sample. The strongest band-edge PL intensity was on the order of (0.5-1.0) × 105 cps [e.g., Figs. 2(b) and 3(d)] at the same or comparable power as the Raman measurements. Assuming a typical stray-light suppression ratio of 10-4 to 10-5 for a single grating spectrometer (an exact value is not available from the manufacturer of the instrument), the stray-light level would be comparable to that of the observed "Raman" peak typically on the order of 1 cps. In our new study, this broad band was eliminated after adding an appropriate short-pass filter to block the band-edge PL signal. The broad band at around 1340 cm-1, as shown in Fig. 5(a), has the same origin as the 550 cm-1 band. The observed anticorrelation between the signals of around 100 cm-1 and 550 cm-1 was the result of structure degradation that enhanced the Raman signal near 100 cm-1 and, in the meantime, reduced the PL signal near the perovskite bandgap at around 770 nm but appeared at 550 cm-1 as stray light. The wrong interpretation of 550 cm-1 does not affect the primary conclusion, i.e., the multiple-stage structure transformation and the interpretation of other results.We regret this error and intend to write a paper to report the latest findings in detail in the near future. The effort of Jose F. Castaneda, a new Ph.D. student in Zhang's group, has led to the finding of this error.

By performing spatially resolved Raman and photoluminescence spectroscopy with varying excitation wavelength, density, and data acquisition parameters, we achieve a unified understanding towards the spectroscopy signatures of the organic-inorganic hybrid perovskite, transforming from the pristine state (CH3NH3PbI3) to the fully degraded state (i.e., PbI2) for samples with varying crystalline domain size from mesoscopic scale (approximately 100 nm) to macroscopic size (centimeters), synthesized by three different techniques. We show that the hybrid perovskite exhibits multiple stages of structure transformation occurring either spontaneously or under light illumination, with exceptionally high sensitivity to the illumination conditions (e.g., power, illumination time, and interruption pattern). We highlight four transformation stages (stages I-IV, with stage I being the pristine state) along either the spontaneous or photoinduced degradation path exhibiting distinctly different Raman spectroscopy features at each stage, and point out that previously reported Raman spectra in the literature reflect highly degraded structures of either stage III or stage IV. Additional characteristic optical features of partially degraded materials under the joint action of spontaneous and photodegradation are also given. This study offers reliable benchmark results for understanding the intrinsic material properties and structure transformation of this unique category of hybrid materials, and the findings are pertinently important to a wide range of potential applications where the hybrid material is expected to function in greatly different environment and light-matter interaction conditions.

Metal halide perovskite solar cells (PSCs) are infamous for their batch-to-batch and lab-to-lab irreproducibility in terms of stability and performance. Reproducible fabrication of PSCs is a critical requirement for market viability and practical commercialization. PSC irreproducibility plagues all levels of the community; from institutional research laboratories, start-up companies, to large established corporations. In this work, the critical function of atmospheric humidity to regulate the crystallization and stabilization of formamidinium lead triiodide (FAPbI₃) perovskites is unraveled. It is demonstrated that the humidity content during processing induces profound variations in perovskite stoichiometry, thermodynamic stability, and optoelectronic quality. Almost counterintuitively, it is shown that the presence of humidity is perhaps indispensable to reproduce phase-stable and efficient FAPbI₃-based PSCs.

Advances in renewable and sustainable energy technologies critically depend on our ability to design and realize materials with optimal properties. Materials discovery and design efforts ideally involve close coupling between materials prediction, synthesis and characterization. The increased use of computational tools, the generation of materials databases, and advances in experimental methods have substantially accelerated these activities. It is therefore an opportune time to consider future prospects for materials by design approaches. The purpose of this Roadmap is to present an overview of the current state of computational materials prediction, synthesis and characterization approaches, materials design needs for various technologies, and future challenges and opportunities that must be addressed. The various perspectives cover topics on computational techniques, validation, materials databases, materials informatics, high-throughput combinatorial methods, advanced characterization approaches, and materials design issues in thermoelectrics, photovoltaics, solid state lighting, catalysts, batteries, metal alloys, complex oxides and transparent conducting materials. It is our hope that this Roadmap will guide researchers and funding agencies in identifying new prospects for materials design.