Organometal halide perovskites are highly promising materials for photovoltaic applications, yet their rapid degradation remains a significant challenge. Here, the light-induced structural degradation mechanism of methylammonium lead iodide (MAPbI3) perovskite films and devices is studied in low humidity environment using X-Ray Diffraction, Ultraviolet-Visible (UV-Vis) absorption spectroscopy, Extended X-ray Absorption Fine Structure spectroscopy, Fourier Transform Infrared spectroscopy, and device measurements. Under dry conditions, the perovskite film degrades only in the presence of both light and oxygen, which together induce the formation of halide anions through donation of electrons to the surrounding oxygen. The halide anions generate free radicals that deprotonate the methylammonium cation and form the highly volatile CH3NH2 molecules that escape and leave pure PbI2 behind. The device findings show that changes in the local structure at the TiO2 mesoporous layer occur with light, even in the absence of oxygen, and yet such changes can be prevented by the application of UV blocking layer on the cells. Our results indicate that the stability of mp-TiO2-MAPbI3 photovoltaics can be dramatically improved with effective encapsulation that protects the device from UV light, oxygen, and moisture.

Organolead bromide CH3NH3PbBr3 perovskite nanocrystals (PNCs) with green photoluminescence (PL) have been synthesized using two different aliphatic ammonium capping ligands, octylammonium bromide (OABr) and octadecylammonium bromide (ODABr), resulting in PNC–OABr and PNC–ODABr, respectively. Structural studies by X-ray diffraction (XRD) and transmission electron microscopy (TEM) determined that the PNCs exhibit cubic phase crystal structure with average particle size dependent on capping ligand (3.9 ± 1.0 nm for PNC–OABr and 6.5 ± 1.4 nm for PNC–ODABr). The exciton dynamics of PNCs were investigated using femtosecond transient absorption (TA) techniques and singular value decomposition global fitting (SVD-GF), which revealed nonradiative recombination on the picosecond time scale mediated by surface trap states for both types of PNCs. The PL lifetime of the PNCs was measured by time-resolved photoluminescence (TRPL) spectroscopy and fit with integrated SVD-GF to determine the radiative as well as nonradiative lifetimes on the nanosecond time scale. Finally, a simple model is proposed to explain the optical and dynamic properties of the PNCs with emphasis on major exciton relaxation or electron–hole recombination processes. The results indicate that the use of capping ligand OABr resulted in PNCs with a high PL quantum yield (QY) of ∼20% (vs fluorescein, 95%), which have interesting optical properties and are promising for potential applications including photovoltaics, detectors, and light-emitting diodes (LEDs).

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Organometal halide (OMH) perovskites are highly promising for photovoltaic (PV) and other applications. However, their instability toward environmental factors such as humidity presents a major challenge in their potential commercial use. In this study, we developed a method to modify the surface of CH 3 NH 3 PbI 3 perovskite films by spin coating oleic acid (OA) to create a water resistant layer that results in enhanced stability and PV performance. The OA-surface passivated perovskites were studied using FT-IR spectroscopy, UV−vis absorption spectroscopy, and X-ray diffraction (XRD). The samples were aged in dark humid air at ∼76% relative humidity (RH) for 4 weeks. The surface passivated films showed minimal signs of decomposition, and the PV devices showed better performance than the unpassivated devices. A possible explanation is the carboxyl group (−COO − ) of OA binds to surface Pb 2+ and/or CH 3 NH 3 + to both passivate these surface defect sites, resulting in the formation of a thin layer of OA with their hydrophobic tail away from the perovskite film surface that effectively prevents water molecules from reaching the perovskite.

Organic–inorganic perovskite materials in the form of nanocrystals and thin films have received enormous attention recently because of their unique optoelectronic properties such as high absorption coefficient, narrow and tunable emission bandwidth, high photoluminescence quantum yield, long exciton lifetime, and balanced charge transport properties. These properties have found applications in a number of important fields, including photovoltaic solar cells, light-emitting diodes, photodetectors, sensors, and lasers. However, the stability of the materials and devices is strongly affected by several factors such as water moisture, light, oxygen, temperature, solvent, and other materials in contact such as metal oxides used in devices. Defects, particularly those related to surface states, play a critical role in the stability as well as the performance of the perovskites. Various surface modification and defect passivation strategies have been developed to enhance stability and improve performance. We review some recent progress in the development of synthetic approaches to produce high-quality nanostructured and bulk film perovskites with controlled properties and functionalities. We also highlight the degradation mechanism and surface passivation approaches to address the issue of instability. To help gain deeper fundamental insight into mechanisms behind degradation and surface passivation, relevant properties, including structural, optical, electronic, and dynamic, are discussed and illustrated with proposed models.

Solar energy production requires an environmentally friendly, efficient, and stable light absorber. Perovskite solar cells (PSCs) have emerged as excellent candidate materials for photovoltaic (PV) energy conversion because of their low cost, ease of fabrication, flexibility, and versatility. However, typical high‐efficiency PSCs, like methylammonium lead iodide (MAPbI3), rely on the use of the chemical lead (Pb), whose toxicity and detrimental impact on human health and the environment raise strong concerns. Therefore, less toxic substitutes that produce the same high performance as MAPbI3 are of interest to enable a wide deployment of PSCs. Hence, much effort has focused on replacing Pb with other elements, such as tin (Sn), bismuth (Bi), antimony (Sb), and germanium (Ge) in perovskite materials, with the dual objectives of maintaining the superior optoelectronic properties of the perovskite and reducing its toxicity. In this review, the structure, optoelectronic properties, as well as recent advances in device performance of Pb‐free PSCs are highlighted. Moreover, the stability challenges of Pb‐free perovskite are detailed, and strategies to overcome them are discussed. Finally, a conclusion and outlook towards Pb‐free perovskite photovoltaics (PV) is provided. © 2021 Society of Chemical Industry (SCI).

Solvent mixtures for making uniform, crystalline methylammonium lead iodide (MAPbI3) perovskite films are provided. The solvents dimethyl sulfoxide (DMSO), γ‐butyrolactone (GBL), and 1‐methyl‐2‐pyrrolidinone (NMP) are examined as well as their binary combinations to prepare solution‐sheared perovskite films at higher than 150 °C temperatures. Characterization methods such as imaging, X‐ray diffraction, grazing‐incidence wide‐angle X‐ray scattering, and photoluminescence (PL) to evaluate the crystal size, quality, and ordering that determine the viability of these films for device applications are explored and then a perovskite solar cell is made to test the stability of the most promising film. Excellent macroscopic crystals, over 1 mm in length, are achieved using a solvent mixture containing equal volumes of DMSO and NMP. These superior 3D MAPbI3 films, which retain solvent–perovskite intermediate phases and exhibit high crystalline ordering, are essential for high‐performing and stable optoelectronic devices.