Gas-Induced Formation/Transformation of Organic–Inorganic Halide Perovskites

The recently discovered gas-induced formation/transformation (GIFT) of organic–inorganic halide perovskites (OIHPs) represents unprecedented phenomena. The GIFT phenomena have not only revealed surprising properties of the fascinating OIHP materials but also showed tremendous promise in various applications, including solar cells, optoelectronics, sensors, and beyond. After briefly reviewing recent progress in the exploration and understanding of GIFT, a perspective on the future research directions in this area is provided herein. Also provided is a discussion on the significance of future mechanistic studies in unraveling the rich chemistry and materials science underlying GIFT and in exploring more potential applications

Exceptional Grain Growth in Formamidinium Lead Iodide Perovskite Thin Films Induced by the δ‑to‑α Phase Transformation

The annealing of fine-grained (∼0.175 μm) “yellow” δ-FAPbI₃ nonperovskite thin films at 100 °C in dimethyl sulfoxide vapor for a few minutes results in exceptional grain growth (∼5 μm) induced by their transformation to the desirable “black” α-FAPbI₃ perovskite phase. Mechanistic insights into this unprecedented phenomenon are provided

Exceptional Morphology-Preserving Evolution of Formamidinium Lead Triiodide Perovskite Thin Films via Organic-Cation Displacement

Here we demonstrate a radically different chemical route for the creation of HC­(NH₂)₂PbI₃ (FAPbI₃) perovskite thin films. This approach entails a simple exposure of as-synthesized CH₃NH₃PbI₃ (MAPbI₃) perovskite thin films to HC­(NH)­NH₂ (formamidine or FA) gas at 150 °C, which leads to rapid displacement of the MA⁺ cations by FA⁺ cations in the perovskite structure. The resultant FAPbI₃ perovskite thin films preserve the microstructural morphology of the original MAPbI₃ thin films exceptionally well. Importantly, the myriad processing innovations that have led to the creation of high-quality MAPbI₃ perovskite thin films are directly adaptable to FAPbI₃ through this simple, rapid chemical-conversion route. Accordingly, we show that efficiencies of perovskite solar cells fabricated with FAPbI₃ thin films created using this route can reach ∼18%

Direct Observation of Ferroelectric Domains in Solution-Processed CH₃NH₃PbI₃ Perovskite Thin Films

A new generation of solid-state photovoltaics is being made possible by the use of organometal-trihalide perovskite materials. While some of these materials are expected to be ferroelectric, almost nothing is known about their ferroelectric properties experimentally. Using piezoforce microscopy (PFM), here we show unambiguously, for the first time, the presence of ferroelectric domains in high-quality β-CH₃NH₃PbI₃ perovskite thin films that have been synthesized using a new solution-processing method. The size of the ferroelectric domains is found to be about the size of the grains (∼100 nm). We also present evidence for the reversible switching of the ferroelectric domains by poling with DC biases. This suggests the importance of further PFM investigations into the local ferroelectric behavior of hybrid perovskites, in particular <i>in situ</i> photoeffects. Such investigations could contribute toward the basic understanding of photovoltaic mechanisms in perovskite-based solar cells, which is essential for the further enhancement of the performance of these promising photovoltaics

Mapping the Photoresponse of CH₃NH₃PbI₃ Hybrid Perovskite Thin Films at the Nanoscale

Perovskite solar cells (PSCs) based on thin films of organolead trihalide perovskites (OTPs) hold unprecedented promise for low-cost, high-efficiency photovoltaics (PVs) of the future. While PV performance parameters of PSCs, such as short circuit current, open circuit voltage, and maximum power, are always measured at the macroscopic scale, it is necessary to probe such photoresponses at the nanoscale to gain key insights into the fundamental PV mechanisms and their localized dependence on the OTP thin-film microstructure. Here we use photoconductive atomic force microscopy spectroscopy to map for the first time variations of PV performance at the nanoscale for planar PSCs based on hole-transport-layer free methylammonium lead triiodide (CH₃NH₃PbI₃ or MAPbI₃) thin films. These results reveal substantial variations in the photoresponse that correlate with thin-film microstructural features such as intragrain planar defects, grains, grain boundaries, and notably also grain-aggregates. The insights gained into such microstructure-localized PV mechanisms are essential for guiding microstructural tailoring of OTP films for improved PV performance in future PSCs

Crystal Morphologies of Organolead Trihalide in Mesoscopic/Planar Perovskite Solar Cells

The crystal morphology of organolead trihalide perovskite (OTP) light absorbers can have profound influence on the perovskite solar cells (PSCs) performance. Here we have used a combination of conventional transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), in cross-section and plan-view, to characterize the morphologies of a solution-processed OTP (CH₃NH₃PbI₃ or MAPbI₃) within mesoporous TiO₂ scaffolds and within capping and planar layers. Studies of TEM specimens prepared with and without the use of focused ion beam (FIB) show that FIBing is a viable method for preparing TEM specimens. HRTEM studies, in conjunction with quantitative X-ray diffraction, show that MAPbI₃ perovskite within mesoporous TiO₂ scaffold has equiaxed grains of size 10–20 nm and relatively low crystallinity. In contrast, the grain size of MAPbI₃ perovskite in the capping and the planar layers can be larger than 100 nm in our PSCs, and the grains can be elongated and textured, with relatively high crystallinity. The observed differences in the performance of planar and mesoscopic-planar hybrid PSCs can be attributed in part to the striking differences in their perovskite-grain morphologies

Simultaneous Evolution of Uniaxially Oriented Grains and Ultralow-Density Grain-Boundary Network in CH₃NH₃PbI₃ Perovskite Thin Films Mediated by Precursor Phase Metastability

Solution-processed organic–inorganic halide perovskite (OIHP) thin films typically contain fine, randomly oriented grains and a high-density grain-boundary network, which are unfavorable for key film functions including charge transport and environmental stability. Here, we report a new chemical route for achieving CH₃NH₃PbI₃ (MAPbI₃) OIHP thin films comprising large, uniaxially oriented grains and an ultralow-density grain-boundary network. This route starts with a new metastable liquid-state precursor phase, MAPbI₃·MACl·<i>x</i>CH₃NH₂, which converts to metastable MAPbI₃·MACl and then to MAPbI₃ OIHP upon stepwise release of volatile CH₃NH₂ and MACl. Perovskite solar cells made via this route show high power conversion efficiency of up to 19.4%, with significantly enhanced environmental stability

Hybrid Perovskite Quantum Nanostructures Synthesized by Electrospray Antisolvent–Solvent Extraction and Intercalation

Perovskites based on organometal lead halides have attracted great deal of scientific attention recently in the context of solar cells and optoelectronic devices due to their unique and tunable electronic and optical properties. Herein, we show that the use of electrospray technique in conjunction with the antisolvent–solvent extraction leads to novel low-dimensional quantum structures (especially 2-D nanosheets) of CH₃NH₃PbI₃- and CH₃NH₃PbBr₃-based layered perovskites with unusual luminescence properties. We also show that the optical bandgaps and emission characteristics of these colloidal nanomaterials can be tuned over a broad range of visible spectral region by compositional tailoring of mixed-halide (I- and Br-based) perovskites

Earth-Abundant Nontoxic Titanium(IV)-based Vacancy-Ordered Double Perovskite Halides with Tunable 1.0 to 1.8 eV Bandgaps for Photovoltaic Applications

The possibility of lead (Pb) contamination and the volatility of the organic cations in the prevailing Pb-based organic-inorganic perovskite (HP) light absorbers are the two key issues of concern in the emerging perovskite solar cells (PSCs). The majority of the Pb-free HP candidates that are being explored for PSCs either suffer from instability issues and have unfavorable defect properties or have unsuitable bandgaps for PSC applications. We report the prediction of a promising new family of all-inorganic HPs based on the nontoxic, earth-abundant, ultrastable Ti­(IV) for use in PSCs. We show that the Ti-based HPs possess a combination of several desirable attributes, including suitable bandgaps, excellent optical absorption, benign defect properties, and high stability. In particular, we show experimentally that representative members of the Ti-based HP family, Cs₂TiI_<i>x</i>Br_6–<i>x</i>, have bandgaps that can be tuned between the ideal values of 1.38 and 1.78 eV for single-junction and tandem photovoltaic applications, respectively

Additive-Modulated Evolution of HC(NH₂)₂PbI₃ Black Polymorph for Mesoscopic Perovskite Solar Cells

Formamidinium lead triiodide (HC­(NH₂)₂PbI₃ or FAPbI₃) is gaining increasing interest in the field of mesoscopic perovskite solar cells (PSCs) for its broader light absorption compared with the more widely studied CH₃NH₃PbI₃ (MAPbI₃). Because FAPbI₃ has two polymorphs (“black” α-FAPbI₃ and “yellow” δ-FAPbI₃) at ambient temperature, where α-FAPbI₃ is the desirable photoactive perovskite phase, it becomes particularly important to suppress the formation of the nonperovskite δ-FAPbI₃ for achieving high efficiency in FAPbI₃-based mesoscopic PSCs. In this study, we demonstrate that the judicious use of low-volatility additives in the precursor solution assists in the evolution of α-FAPbI₃ through the formation of non-δ-FAPbI₃ intermediate phases, which then convert to α-FAPbI₃ during thermal annealing. The underlying mechanism involved in the additive-modulated evolution of α-FAPbI₃ upon mesoporous TiO₂ substrates is elucidated, which suggests guidelines for developing protocols for the fabrication efficient FAPbI₃-based mesoscopic PSCs