Instability of Formamidinium Lead Iodide (FAPI) Deposited on a Copper Oxide Hole Transporting Layer (HTL)

Copper oxide appears to be a promising candidate for a hole transport layer (HTL) in emerging perovskite solar cells. Reasons for this are its good optical and electrical properties, cost-effectiveness, and high stability. However, is this really the case? In this study, we demonstrate that copper oxide, synthesized by a spray-coating method, is unstable in contact with formamidinium lead triiodide (FAPI) perovskite, leading to its decomposition. Using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible (UV-vis) spectrophotometry, we find that the entire copper oxide diffuses into and reacts with the FAPI film completely. The reaction products are an inactive yellow δ-FAPI phase, copper iodide (CuI), and an additional new phase of copper formate hydroxide (CH2CuO3) that has not been reported previously in the literature.

Self-Assembly of Delta-Formamidinium Lead Iodide Nanoparticles to Nanorods: Study of Memristor Properties and Resistive Switching Mechanism

In the quest for advanced memristor technologies, this study introduces the synthesis of delta-formamidinium lead iodide (δ-FAPbI3 ) nanoparticles (NPs) and their self-assembly into nanorods (NRs). The formation of these NRs is facilitated by iodide vacancies, promoting the fusion of individual NPs at higher concentrations. Notably, these NRs exhibit robust stability under ambient conditions, a distinctive advantage attributed to the presence of capping ligands and a crystal lattice structured around face-sharing octahedra. When employed as the active layer in resistive random-access memory devices, these NRs demonstrate exceptional bipolar switching properties. A remarkable on/off ratio (105 ) is achieved, surpassing the performances of previously reported low-dimensional perovskite derivatives and α-FAPbI3 NP-based devices. This enhanced performance is attributed to the low off-state current owing to the reduced number of halide vacancies, intrinsic low dimensionality, and the parallel alignment of NRs on the FTO substrate. This study not only provides significant insights into the development of superior materials for memristor applications but also opens new avenues for exploring low-dimensional perovskite derivatives in advanced electronic devices.

Lattice Strain Regulation Enables High-Performance Formamidinium Perovskite Photovoltaics

Formamidinium lead iodide (FAPbI3 ) perovskite possesses an ideal optical bandgap and is a potential material for fabricating the most efficient single-junction perovskite solar cells (PSCs). Nevertheless, large formamidinium (FA) cations result in residual lattice strain, which reduces the power conversion efficiency (PCE) and operational stability of PSCs. Herein, the modulation of lattice strain in FAPbI3 crystals via a π-conjugated organic amine, i.e., 4-pyrene oxy butylamine (PYBA), is proposed. PYBA pairs at the grain boundary serve as a template for the crystallization of FAPbI3 perovskite, thereby inducing a highly oriented crystal and a pure α-phase film. The PYBA pairs with strong π-π interactions provide a solid fulcrum for external compression strain, thus compensating for the inherent tension strain of FAPbI3 crystals. The strain release elevates the valence band of the perovskite crystals, thereby decreasing the bandgap and trap density. Consequently, the PYBA-regulated FAPbI3 PSC achieves an excellent PCE of 24.76%. Moreover, the resulting device exhibits improves operational stability and maintains over 80% of its initial PCE after 1500 h under maximum power point tracking conditions.

Formamidinium Lead Iodide Perovskite Thin Films Formed by Two-Step Sequential Method: Solvent-Morphology Relationship

Thin films made of formamidinium lead iodide (FAPbI₃) perovskites prepared by a two-step sequential deposition method using various solvents for formamidinium iodide (FAI) - isopropanol, n-butanol and tert-butanol, were studied with the aim of finding a correlation between morphology and solvent properties to improve film quality. They were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) and their photophysical properties were studied by means of absorption and photoluminescence (PL) spectroscopies. XRD patterns, absorption and PL spectra proved α-phase formation for all selected solvents. An excessive amount of PbI₂ found in perovskite films prepared with n-butanol indicates incomplete conversion. Thin film morphology, such as grain and crystallite size, depended on the solvent. Using tert-butanol, thin films with a very large grain size of up to several micrometers and with preferred crystallite orientation were fabricated. The grain size increased as follows: 0.2-0.5, 0.2-1 and 2-5 µm for isopropanol, n-butanol and tert-butanol, respectively. A correlation between the grain size and viscosity, electric permittivity and polarizability of the solvent could be considered. Our results, including fabrication of perovskite films with large grains and fewer grain boundaries, are important and of interest for many optoelectronic applications.

Post-Treating the Precursor Intermediate Film by a Cooling Stage for Fabricating Efficient Formamidinium-Based Perovskite Solar Cells

Formamidinium lead iodide (FAPbI₃) is a category of perovskite material with an ideal band gap and high thermal stability, which can be efficiently prepared by two-step spin-coating. Spin-coating organic salts and transforming intermediate phase at the second step involves a components' reaction and state transition, thus playing a crucial role in the film quality formed afterward and optoelectronic properties of the fabricated perovskite solar cells (PSCs). In this paper, a cooling stage (CO) is used to post-treat the as-prepared precursor after the second spin-coating. The procedure of intermediate phase transferring to other state is found to be retarded; hence, the appearing velocity of perovskite nucleation is decreased. As a result, components react more adequately and larger perovskite grains with fewer defects are obtained; charge transport as well as carrier recommbination behaviors are therefore optimized. The PSCs based on the CO process achieved a champion power conversion efficiency (PCE) of 21.51% with enhanced stability. Moreover, CO treatment is observed to be beneficial for improving the film quality of perovskite in large-area preparation, which we anticipate can be further extended to the commercialized application of PSCs.

A Small-Molecule "Charge Driver" enables Perovskite Quantum Dot Solar Cells with Efficiency Approaching 13

Halide perovskite colloidal quantum dots (CQDs) have recently emerged as a promising candidate for CQD photovoltaics due to their superior optoelectronic properties to conventional chalcogenides CQDs. However, the low charge separation efficiency due to quantum confinement still remains a critical obstacle toward higher-performance perovskite CQD photovoltaics. Available strategies employed in the conventional CQD devices to enhance the carrier separation, such as the design of type-Ⅱ core-shell structure and versatile surface modification to tune the electronic properties, are still not applicable to the perovskite CQD system owing to the difficulty in modulating surface ligands and structural integrity. Herein, a facile strategy that takes advantage of conjugated small molecules that provide an additional driving force for effective charge separation in perovskite CQD solar cells is developed. The resulting perovskite CQD solar cell shows a power conversion efficiency approaching 13% with an open-circuit voltage of 1.10 V, short-circuit current density of 15.4 mA cm^-2 , and fill factor of 74.8%, demonstrating the strong potential of this strategy toward achieving high-performance perovskite CQD solar cells.

Direct Laser Writing of δ- to α-Phase Transformation in Formamidinium Lead Iodide

Organolead halide perovskites are increasingly considered for applications well beyond photovoltaics, for example, as the active regions within photonic devices. Herein, we report the direct laser writing (DLW: 458 nm cw-laser) of the formamidinium lead iodide (FAPbI₃) yellow δ-phase into its high-temperature luminescent black α-phase, a remarkably easy and scalable approach that takes advantage of the material's susceptibility to transition under ambient conditions. Through the DLW of α-FAPbI₃ tracks on δ-FAPbI₃ single-crystal surfaces, the controlled and rapid microfabrication of highly luminescent structures exhibiting long-term phase stability is detailed, offering an avenue toward the prototyping of complex perovskite-based optical devices. The dynamics and kinetics of laser-induced δ- to α-phase transformations are investigated in situ by Raman microprobe analysis, as a function of irradiation power, time, temperature, and atmospheric conditions, revealing an interesting connection between oxygen intercalation at the surface and the δ- to α-phase transformation dynamics, an insight that will find application within the wider context of FAPbI₃ thermal phase relations.

Photoexcitation dynamics in solution-processed formamidinium lead iodide perovskite thin films for solar cell applications

Formamidinium lead iodide (FAPbI₃) is a newly developed hybrid perovskite that potentially can be used in high-efficiency solution-processed solar cells. Here, the temperature-dependent dynamic optical properties of three types of FAPbI₃ perovskite films (fabricated using three different precursor systems) are comparatively studied. The time-resolved photoluminescence (PL) spectra reveal that FAPbI₃ films made from the new precursor (a mixture of formamidinium iodide and hydrogen lead triiodide) exhibit the longest lifetime of 439 ns at room temperature, suggesting a lower number of defects and lower non-radiative recombination losses compared with FAPbI₃ obtained from the other two precursors. From the temperature-dependent PL spectra, a phase transition in the films is clearly observed. Meanwhile, exciton-binding energies of 8.1 and 18 meV for the high- and low-temperature phases are extracted, respectively. Importantly, the PL spectra for all of the samples show a single peak at room temperature, whereas at liquid-helium temperature the emission features two peaks: one in higher energy displaying a fast decay (0.5 ns) and a second red-shifted peak with a decay of up to several microseconds. These two emissions, separated by ~18 meV, are attributed to free excitons and bound excitons with singlet and triplet characters, respectively.

On the Role of Interfaces in Planar-Structured HC(NH2 )2 PbI3 Perovskite Solar Cells

Planar-structured HC(NH2 )2 PbI3 (FAPbI3 ) perovskite solar cells were prepared via a two-step deposition process. To investigate the role of interface, the perovskite morphology was intentionally modified by varying HC(NH2 )2 I concentration. Surface and grain sizes of the deposited FAPbI3 became rougher and larger as the HC(NH2 )2 I concentration decreased from 58.2 to 40.7 mM. Average photocurrent was improved but photovoltage deteriorated slightly with decreasing concentration. Consequently, the average efficiency was improved from 7.82 % to 10.70 % and the best efficiency of 12.17 % was obtained at 40.7 mM. Photoluminescence (PL) at TiO2 /FAPbI3 interface was reduced with decreasing concentration, which was, however, reversed at FAPbI3 /spiro-MeOTAD one. By correlating PL data and the photovoltaic performance, we concluded that the TiO2 /perovskite interface plays a crucial role in determining photocurrent while the perovskite/spiro-MeOTAD interface is important in governing photovoltage.

Stabilization of the Trigonal High-Temperature Phase of Formamidinium Lead Iodide

Formamidinium lead iodide (FAPbI3) has the potential to achieve higher performance than established perovskite solar cells like methylammonium lead iodide (MAPbI3), while maintaining a higher stability. The major drawback for the latter material is that it can crystallize at room temperature in a wide bandgap hexagonal symmetry (P63mc) instead of the desired trigonal (P3m1) black phase formed at a higher temperature (130 °C). Our results show that employing a mixture of MAI and FAI in films deposited via a two-step approach, where the MAI content is <20%, results in the exchange of FA molecules with MA without any significant lattice shrinkage. Additionally, we show with temperature-dependent X-ray diffraction that the trigonal phase exhibits no phase changes in the temperature range studied (25 to 250 °C). We attribute the stabilization of the structure to stronger interactions between the MA cation and the inorganic cage. Finally, we show that the inclusion of this small amount of MA also has a positive effect on the lifetime of the photoexcited species and results in more efficient devices.

High-efficiency perovskite solar cells based on the black polymorph of HC(NH2)2 PbI3

Perovskite solar cells with power conversion efficiencies exceeding 16% at AM 1.5 G one sun illumination are developed using the black polymorph of formamidnium lead iodide, HC(NH2)2 PbI3 . Compared with CH3 NH3 PbI3 , HC(NH2 )2 PbI3 extends its absoprtion to 840 nm and shows no phase transition between 296 and 423 K. Moreover, a solar cell based on HC(NH2 )2 PbI3 exhibits photostability and little I-V hysteresis.