Substance and shadow of formamidinium lead triiodide based solar cells.

The current decade has witnessed a surge of progress in the investigation of methyl ammonium lead iodide (MAPbI3) perovskites for solar cell fabrication due to their intriguing electro-optical properties, despite the intrinsic degradation of the material that has restricted its commercialisation. As a promising alternative, solar cells based on its formamidinium analogue, FAPbI3, are currently being actively pursued for having demonstrated a certified efficiency of 24.4%, while the room-temperature conversion to a non-perovskite δ-phase impedes its further commercialisation, and strategies have been adopted to overcome this phase instability. An in-depth and real-time understanding of microstructural relationships with optoelectronic properties and their underlying mechanisms using operando in situ spectroscopic techniques is paramount. Thus, the design and development of a new process, data driven methodology, characterization and evaluation protocols for perovskite absorber layers and the fabricated devices is a judicious research direction. Here, in this perspective, we shed light on the compositional, surface engineering and crystallization kinetics manipulations for FAPbI3, followed by a proposition for unified testing protocols, for scalling of devices from the lab to the market.

Synthetic Control on Structure/Dimensionality and Photophysical Properties of Low Dimensional Organic Lead Bromide Perovskite.

Low dimensional lead halide perovskites have attracted huge research interest due to their structural diversity and remarkable photophysical properties. The ability to controllably change dimensionality/structure of perovskites remains highly challenging. Here, we report synthetic control on structure/dimensionality of ethylenediammonium (ED) lead bromide perovskite from a two dimensionally networked (2DN) sheet to a one dimensionally networked (1DN) chain structure. Intercalation of solvent molecules into the perovskite plays a crucial role in directing the final dimensionality/structure. This change in dimensionality reflects strongly in the observed differences in photophysical properties. Upon UV excitation, the 1DN structure emits white light due to easily formed " self-trapped" excitons. 2DN perovskites show band edge blue emission (∼410 nm). Interestingly, Mn2+ incorporated 2DN perovskites show a highly red-shifted Mn2+ emission peak at ∼670 nm. Such a long wavelength Mn2+ emission peak is unprecedented in the perovskite family. This report highlights the synthetic ability to control the dimensionality/structure of perovskite and consequently its photophysical properties.

Lead-sulfur interaction induced damp and water stability in pure formamidinium lead triiodide.

Research efforts in various multitudes have been demonstrated to stabilize methylammonium (MA)- and bromide (Br)-free formamidinium lead triiodide (FAPI) perovskite thin films. Despite these commendable efforts, pure FAPI perovskite thin film is prone to critical phase-transition issues due to its thermodynamically stable non-perovskite phase (2H). Here, in this work, we propose a rational additivization strategy to overcome this challenge. Our multifunctional ammonium salt containing a sulfur heteroatom shifts the thermodynamic stability from the 2H phase to an intermediate phase closer to the cubic phase. Along with the high crystallinity, micron-sized grains with preferred (00h) facet orientation stem the Pb…S interaction to offer exceptional stability against high relative humidity, direct water incursion, and shelf-life aging. Our findings through experimental and theoretical studies substantiate the role of Pb…S interaction in stabilizing the perovskite cubic phase and the stoichiometric distribution of elemental components.

Probing proton diffusion as a guide to environmental stability in powder-engineered FAPbI3 and CsFAPbI3 perovskites.

Formamidinium lead iodide-based solar cells show promising device reliability. The grain imperfection can be further suppressed by developing powder methodology. The water uptake capability is critical for the stability of α-formamidinium lead triiodide (FAPbI3) thin films, and elucidating the migration of hydrogen species is challenging using routine techniques such as imaging or mass spectroscopy. Here, we decipher the proton diffusion to quantify indirect monitoring of H migration by following the N-D vibration using transmission infrared spectroscopy. The technique allows a direct assessment of the perovskite degradation associated with moisture. The inclusion of Cs in FAPbI3, reveals significant differences in proton diffusion rates, attesting to its impact. CsFAPbI3's ability to block the active layer access by water molecules is five times higher than α-FAPbI3, which is significantly higher than methylammonium lead triiodide (MAPbI3). Our protocol directly probes the local environment of the material to identify its intrinsic degradation mechanisms and stability, a key requirement for optoelectronic applications.

Microstrain and Urbach Energy Relaxation in FAPbI3-Based Solar Cells through Powder Engineering and Perfluoroalkyl Phosphate Ionic Liquid Additives.

Structural and electronic imperfections are the origin of defects and lead to nonradiative recombination that is detrimental to fabricating efficient perovskite solar cells. Here, we propose a powder engineering methodology for α-FAPbI3 as a precursor material. Our developed methodology of α-FAPbI3 synthesis mitigates the notorious structural and electronic imperfections evidenced by a significant decline in the microstrain and Urbach energy as compared to reported δ-FAPbI3 powder and conventional precursor routes. In addition to the performance enhancement in photovoltaics, our engineered powder showed remarkable thermal and moisture stability along with cost-effectiveness through the employment of low-grade PbI2. Further, through additive engineering, with the use of ultrahydrophobic perfluoroalkyl phosphate anion-based ionic liquids, the microstrain and Urbach energy achieved the lowest values of 1.67 × 10-4 and 12.47 meV, respectively, as a result of defect passivation and a semi-ionic F-Pb interaction that stabilizes the surface.