Formamidinium Lead Iodide Research Articles

Core–shell carbon-polymer quantum dot passivation for near infrared perovskite light emitting diodes

High-performance perovskite light-emitting diodes (PeLEDs) require a high quality perovskite emitter and appropriate charge transport layers to facilitate charge injection and transport within the device. Solution-processed n-type metal oxides represent a judicious choice for the electron transport layer (ETL); however, they do not always present surface properties and energetics compatible with the perovskite emitter. Moreover, the emitter itself exhibits poor nanomorphology and defect traps that compromise the device performance. Here, we modulate the surface properties and interface energetics between the tin oxide (SnO2) ETL with the perovskite emitter by using an amino functionalized difluoro{2-[1-(3,5-dimethyl-2H-pyrrol-2-ylidene-N)ethyl]-3,5-dimethyl-1H-pyrrolato-N}boron compound and passivate the defects present in the perovskite matrix with carbon-polymer core–shell quantum dots inserted into the perovskite precursor. Both these approaches synergistically improve the perovskite layer nanomorphology and enhance the radiative recombination. These properties resulted in the fabrication of near-infrared PeLEDs based on formamidinium lead iodide (FAPbI3) with a high radiance of 92 W sr−1 m−2, an external quantum efficiency (EQE) of 14%, reduced efficiency roll-off and prolonged lifetime. In particular, the modified device retained 80% of the initial EQE (T80) for 33 h compared to 6 h of the reference cell.

Effects of Alkaline Earth Metal Additives on Methylammonium‐Free Lead Halide Perovskite Thin Films and Solar Cells

Organic–inorganic lead halide perovskite solar cells are regarded as one of the most promising technologies for the next generation of photovoltaics due to their high power conversion efficiency (PCE) and simple solution manufacturing. Among the different compositions, the formamidinium lead iodide (FAPbI3) photoactive phase has a bandgap of 1.4 eV, which enables the corresponding higher PCEs according to the Shockley–Queisser limit. However, the photoactive crystal phase of FAPbI3 is not stable at room temperature. The most high‐performing compositions to date have reduced this problem by incorporating the methylammonium (MA) cation into the FAPbI3 composition, although MA has poor stability at high temperatures and in humid environments, which can limit the lifetime of FAxMA1−xPbI3 films. CsxFA1−xPbI3 perovskites are also explored, but despite better stability they still lag in performance. Herein, the additive engineering of MA‐free organic−inorganic lead halide perovskites using divalent cations Sr2+ and Ca2+to enhance the performances of CsxFA1−xPbI3 perovskite compositions is explored. It is revealed that the addition of up to 0.5% of Sr2+ and Ca2+ leads to improvements in morphology and reduction in microstrain. The structural improvements observed correlate with improved solar cell performances at low additive concentrations.

Capacitance spectroscopy of thin-film formamidinium lead iodide based perovskite solar cells

This work concerns the interpretation of capacitance spectroscopy results in perovskite-based solar cells. Based on the deep level transient spectroscopy and admittance spectroscopy results, we present arguments that the observable signals in perovskite-based solar cells come from anion migration rather than being a response from deep trap energy levels. The ion migration parameters, such as activation energy and ion concentration, are calculated and compared to theoretical values for different migration paths of ions in perovskites. Those parameters evolve with time, reflecting in the degradation of the cells, which we propose to link with a change in the anion migration path in perovskite. • The same signal is observed in DLTS and admittance spectroscopy. • Its activation energy is in the 0.3 eV–0.5 eV range and decays with time. • Time constants and the presence of large capacitance transients at +1 V point into the diffusion related origin of the signals. • Low activation energy, DLTS sign, comparison with other experiments and theoretical calculations point into anion diffusion. • The observed, dominating signal correlates with the increased shunt of the device.

First-principles calculations for fundamental and spectroscopic screening of hybrid perovskite (HC(NH2)2PbI3) formamidinium lead iodide

Formamidinium lead iodide [(HC(NH2)2PbI3] material has generated significant interest in their inherent properties and extensive scope of prospective applications. Here, we have successfully computed the structural, electronic, optical, thermal, thermoelectric and elastic properties with the framework of density functional theory over the PBE-sol and WC potential approximations using the WIEN2K code. These explorations show that this material is a direct bandgap semiconductor (1.30 eV), with high absorption coefficients and better thermal stability. The dielectric function and absorption spectra peaks reveal in the IR-visible-UV energy region show the significant future of this material to be used in photovoltaic (PV) devices. Thermal properties have been calculated by GIBBS2 at different temperatures (0–800 K) and pressures (0–8 GPa). Boltztrap with constant relaxation time approximation has been used for computing the thermoelectric parameters at different temperatures (300–800 K). Seebeck coefficient decreases with increasing temperature, electrical conductivity nearly constant in the whole temperature ranges, electrical thermal conductivity increases with increasing temperature, power factor and figure of merit's are also increasing with increasing temperature at a given electron and hole concentration (1018-1019 cm−3). We have also carried out the calculations of spectroscopic limited maximum efficiency (SLME) parameter (39.09%). Moreover, for mechanical stability, we have determined the elastic properties (C11, C12, C44), bulk modulus (B), Young's modulus (Y), shear modulus (G) and stability constants B/G ratio, and Poisson's ratio (ν). These computed results show that the FAPbI3 is a relevant material to be used as an alert substance in photovoltaic device fabrications and proposed outcomes are in good acceptance with the experimental and other theoretical data.

The Role of Thermal Fluctuations and Vibrational Entropy: A Theoretical Insight into the δ-to-α Transition of FAPbI3.

Formamidinium lead iodide as a typical organometal perovskite has attracted considerable interest due to its suitable electronic structure. However, the intrinsic mechanisms of its unwanted δ-to-α phase transition remain elusive. By combined first-principles calculations, lattice dynamics analysis, and molecular dynamics simulations, we assign the α phase to the highly dynamic tetragonal phase, with the high-symmetry cubic structure emerging as a dynamically unstable maximum in the system's potential energy landscape. Finite-temperature Gibbs free energy calculations confirm that the δ-to-α transition should be considered as a hexagonal-to-tetragonal transition in contrast to the previous hexagonal-to-cubic assignment. More importantly, phonon thermal property calculations indicate that the driving force of the process is the vibrational entropy difference. These results point out the dynamical nature of the α phase and the key role of the vibrational entropy in perovskite-related phase transitions, the harnessing of which is critical for the successful uptake of organometal perovskites in commercial applications.

Efficient and Stable FA‐Rich Perovskite Photovoltaics: From Material Properties to Device Optimization

AbstractThe perovskite photovoltaic field has developed rapidly within a decade. In particular, formamidinium (FA)‐rich perovskite allows a broad absorption spectrum, and is considered to be one of the most promising perovskite materials. Great progress has been achieved, and most recorded high‐efficient perovskite solar cells (PSCs) used the FA‐rich perovskite light absorption layer. However, the black α‐phase formamidinium lead iodide (FAPbI3) perovskite easily transforms into an undesirable δ‐phase at a low temperature. Thus, researchers have put a lot of effort into deeply understanding the phase transformation and stabilization mechanism of FA‐rich perovskite. Herein, the fundamental physical properties of FAPbI3 materials, including crystal structure, phase‐transition temperature, charge‐carrier dynamics, etc. are summarized, and establish a complete phase evolution with temperature by reviewing previous reports. The intrinsic and external factors are subsequently discussed for influencing the stability of FAPbI3 perovskite and the remarkable breakthroughs of FA‐rich PSCs in recent years are reviewed. Moreover, a series of strategies to stabilize FA‐rich perovskite is summarized, including but not limited to compositional engineering, passivating engineering, processing engineering, and strain engineering. Finally, several new viewpoints are provided for improving the efficiency and stability of PSCs. This review may be regarded as a reference project for preparing high‐efficient and stable perovskite photovoltaic devices.

FAPbI3 Perovskite Solar Cells: From Film Morphology Regulation to Device Optimization

Organic–inorganic hybrid perovskite solar cells (PSCs) have attracted great attentions due to their rapid increase of power conversion efficiency (PCE). Although the highest PCE of PSCs (25.7%) has been achieved via using formamidinium lead iodide (FAPbI3) with a suitable bandgap, there is still a lack of systematic analysis on FAPbI3‐based PSCs toward high stability and high efficiency. Herein, the progress in FAPbI3 films and achievements in their high‐efficiency and long‐term stability PSCs are comprehensively reviewed. First, the progress from the aspects of morphology, defect, dimension, and strain for FAPbI3 film optimization is summarized and then the development of FAPbI3 PSCs in both efficiency and stability is discussed. Then, the methods to improve the FAPbI3 film quality by morphology control, defect passivation, dimensional regulation, and strain engineering, as well as strategies to optimize the device structure and interface layers, which are critical to promote device stability and efficiency, are evaluated. Finally, the outlook and strategies for realizing commercialized FAPbI3 PSCs with high efficiency and long lifetime are discussed.

Synchronous Passivation of Defects with Low Formation Energies via Terdentate Anchoring Enabling High Performance Perovskite Solar Cells with Efficiency over 24%

AbstractThe ionic nature endows halide perovskites with intrinsic interfacial defects in the formed polycrystalline films, thus imposing the challenge of synchronously passivating these defects with low formation energies that directly account for the unsatisfied performance of perovskite solar cells (PSCs). By virtue of the theoretically proven capability of a three to four times enhancement of the formation energy of each defect of Pb‐I antisite (PbI) and iodine vacancy (VI), a new passivation molecule of 1,10‐phenanthrolin‐5‐amine (PAA) is intentionally explored to synchronously passivate the dual defects. The pronounced passivation effect is experimentally verified by the sharp enhancement of the open‐circuit voltage in ternary PSCs from the original 1.118 up to 1.207 V, as well as the construction of PAA‐modified formamidinium lead iodide PSCs with a champion efficiency up to 24.06%, thus providing a universal alternative of addressing interfacial charge carrier dynamics and operational stability of PSCs that are bothered by the multiple interfacial defects.

Metal Halide Perovskites Demonstrate Radiation Hardness and Defect Healing in Vacuum.

Herein, we subject formamidinium lead iodide films to oxygen-containing gases (flowing O2 or free diffusion of lab atmosphere), inert gases (flowing He, Ar, or N2), and vacuum. Our films are irradiated by Cu Kα X-rays and held at 75 °C while X-ray diffraction is recorded. Under all gas conditions, we observe a reproducible 1.1 ± 0.5 Å3 perovskite lattice contraction from an initial unit cell volume of 256.5 ± 0.8 Å3 concurrent with continuous perovskite loss and lead iodide growth. Oxygen-containing gases increase the reaction rates without materially altering perovskite structural changes. Under the same temperature and irradiation conditions in vacuo, a self-healing reaction is observed, exhibited by a reproducible (0.9 ± 0.3 Å3) lattice expansion and stabilization of the perovskite. Interactions between the perovskite, defects, and minority phases are simulated by generalized gradient approximation Perdew-Burke-Ernzerhof (GGA-PBE) density functional theory. Lattice contraction indicates an increase in the concentration of Schottky defects─pairs of formamidinium and iodine vacancies. Under irradiation in every atmospheric condition, a solid solution of Schottky defects with a concentration of several percent diffuses and precipitates forming lead iodide and consuming the defects. In the presence of ionized gases, this framework is modified to include the continual loss of formamidinium and iodine ions from the perovskite forming Schottky defects.

Thermodynamic Study of Formamidinium Lead Iodide (CH5N2PbI3) from 5 to 357 K.

In the present study, the molar heat capacity of solid formamidinium lead iodide (CH5N2PbI3) was measured over the temperature range from 5 to 357 K using a precise automated adiabatic calorimeter. In the above temperature interval, three distinct phase transitions were found in ranges from 49 to 56 K, from 110 to 178 K, and from 264 to 277 K. The standard thermodynamic functions of the studied perovskite, namely the heat capacity C°p(T), enthalpy [H0(T) − H0(0)], entropy S0(T), and [G°(T) − H°(0)]/T, were calculated for the temperature range from 0 to 345 K based on the experimental data. Herein, the results are discussed and compared with those available in the literature as measured by nonclassical methods.

Surface Defect Formation and Passivation in Formamidinium Lead Triiodide (FAPbI3) Perovskite Solar Cell Absorbers

Formamidinium lead iodide based hybrid perovskite materials with improved efficiency and stability still lack well-understood surface defect formation mechanisms. Controlling the surface termination and defects has the potential to improve the performance of both conventional 3D and latterly reduced-dimensional perovskites photovoltaics. Here, we characterized the termination and all possible defect formations in FAPbI3 surface by the first-principles calculations. We found that, among the surfaces we considered, FAI-termination exhibits the most stable surface with a high defect tolerance. The PbI2-terminated surface is also found to be relatively stable; however, certain defects, such as electron-donating FA-interstitial and Pb-interstitial defects, can create deep-level stable charge-traps, potentially limiting the optoelectronic performance. We further investigate the surface treatment on these deep defects by model small molecule additives. We found that benzene additive with delocalized electron distribution can effectively passivate the deep FA-interstitial and Pb-interstitial defects by electron donating to the surface defect through charge-transfer.

Partial replacement of B-site cation to stabilize the optically active cubic phase of FAPbI3 for optoelectronic applications

Due to the unique and indispensable electronic, physical and chemical properties, halide perovskites are being widely used in optoelectronic applications like solar cells and LEDs. However, the stability of halide perovskites has always been an alarming issue towards the development of these devices on an industrial scale. Specifically, Formamidinium Lead Iodide (FAPbI3), the most suitable organic halide perovskite for solar cells is very unstable due to environmental factors like humidity, temperature and ultraviolet radiation which led to poor-performing devices. Moreover, FAPbI3 is more stable in its photo-inactive hexagonal (δ-FAPbI3) phase. For photovoltaic devices, only the optically active cubic phase (α-FAPbI3) is suitable due to its proper band gap (∼1.45 eV). In this communication, we have partially substituted the Pb2+ cation in FAPbI3 with Tin (Sn2+) and Manganese (Mn2+). The structural and optical properties are analyzed using X-Ray Diffraction, Ultra-violet Diffuse Reflectance Spectroscopy and Photoluminescence (PL) spectroscopy. The results show that the α-FAPbI3 phase can be retained with successful incorporation of Sn2+ and Mn2+ at the B-site of FAPbI3 structure.

A Novel Fabrication Approach for Improving the Efficiency of FAPbI3 Based Perovskite Solar Cells

The newly emerging perovskite solar cells (PSCs) embody excellent properties that are making them an attractive in photovoltaic (PV) technology. However, some shortcomings are impeding their commercial success. These include stability, efficiency, shelf life and operational life time. In this study, we attempted to overcome these shortcomings by exploring the fabrication by a modified spin coating approach where the dispensing of anti-solvent was optimized. Using Methyl Ammonium Formamidinium Lead Iodide as light harvester, several PSC devices were fabricated. For comparison, similar devices were also synthesized using a commonly used standard procedure. The devices were characterized for their I-V and P-V response using full sun solar simulator. Our results revealed that the fabrication approach based on optimized dispensing of anti-solvent resulted in considerable improvements in the efficiency with low production cost. In future, the approach can be adopted to overcome some of the problems associated with the current PSCs fabrication approaches.

Mechanochemistry Advances High-Performance Perovskite Solar Cells.

A prerequisite for commercializing perovskite photovoltaics is to develop a swift and eco-friendly synthesis route, which guarantees the mass production of halide perovskites in the industry. Herein, a green-solvent-assisted mechanochemical strategy is developed for fast synthesizing a stoichiometric δ-phase formamidinium lead iodide (δ-FAPbI3 ) powder, which serves as a high-purity precursor for perovskite film deposition with low defects. The presynthesized δ-FAPbI3 precursor possesses high concentration of micrometer-sized colloids, which are in favor of preferable crystallization by spontaneous nucleation. The resultant perovskite films own preferred crystal orientations of cubic (100) plane, which is beneficial for superior carrier transport compared to that of the films with isotropic crystal orientations using "mixture of PbI2 and FAI" as precursors. As a result, high-performance perovskite solar cells with a maximum power conversion efficiency of 24.2% are obtained. Moreover, the δ-FAPbI3 powder shows superior storage stability for more than 10 months in ambient environment (40± 10% relative humidity), being conducive to a facile and practical storage for further commercialization.

Combined Precursor Engineering and Grain Anchoring Leading to MA‐Free, Phase‐Pure, and Stable α‐Formamidinium Lead Iodide Perovskites for Efficient Solar Cells

Abstractα‐Formamidinium lead iodide (α‐FAPbI3) is one of the most promising candidate materials for high‐efficiency and thermally stable perovskite solar cells (PSCs) owing to its outstanding optoelectrical properties and high thermal stability. However, achieving a stable form of α‐FAPbI3 where both the composition and the phase are pure is very challenging. Herein, we report on a combined strategy of precursor engineering and grain anchoring to successfully prepare methylammonium (MA)‐free and phase‐pure stable α‐FAPbI3 films. The incorporation of volatile FA‐based additives in the precursor solutions completely suppresses the formation of non‐perovskite δ‐FAPbI3 during film crystallization. Grains of the desired α‐phase are anchored together and stabilized when 4‐tert‐butylbenzylammonium iodide is permeated into the α‐FAPbI3 film interior via grain boundaries. This cooperative scheme leads to a significantly increased efficiency close to 21 % for FAPbI3 perovskite solar cells. Moreover, the stabilized PSCs exhibit improved thermal stability and maintained ≈90 % of their initial efficiency after storage at 50 °C for over 1600 hours.

Extended X-ray absorption fine structure (EXAFS) of FAPbI3 for understanding local structure-stability relation in perovskite solar cells

Perovskite solar cells (PSCs) employing formamidinium lead iodide (FAPbI3) have shown high efficiency. However, operational stability has been issued due to phase instability of α phase FAPbI3 at ambient temperature. Excess precursors in the perovskite precursor solution has been proposed to improve not only power conversion efficiency (PCE) but also device stability. Nevertheless, there is a controversial issue on the beneficial effect on PCE and/or stability between excess FAI and excess PbI2. We report here extended X-ray absorption fine structure (EXAFS) of FAPbI3 to study local structural change and explain the effect of excess precursors on photovoltaic performance and stability. Perovskite films prepared from the precursor solution with excess PbI2 shows better stability than those from the one with excess FAI, despite similar PCE. A rapid phase transition from α phase to non-perovskite δ phase is observed from the perovskite film formed by excess FAI. Furthermore, the (Pb-I) bond distance evaluated by the Pb LIII-edge EXAFS study is increased by excess FAI, which is responsible for the phase transition and poor device stability. This work can provide important insight into local structure-stability relation in the FAPbI3-based PSCs.

Reducing energy barrier of δ-to-α phase transition for printed formamidinium lead iodide photovoltaic devices

Recent progress in perovskite photovoltaics has witnessed a growing interest in formamidinium lead iodide (FAPbI3), primarily due to its high efficiency potential and excellent stability. However, the high energy barrier of δ-to-α phase transition presents a major hurdle to fabricate phase-pure α-FAPbI3 layers. Here, we report a two-step phase transition process to deposit high-quality photovoltaic α-FAPbI3 films by printing method. This is realized by judicious selection of a Lewis base N-methyl-2-pyrrolidone (NMP) and its counter Lewis acid, which enables the regulation of intermediary phase to reduce the energy barrier. With fine tuning the phase transition pathway, phase-pure and stable α-FAPbI3 perovskite films are obtained, which yield solar devices with a champion efficiency of 21.35%. The printed mini-modules with active areas of 12.32 cm2 and 55.44 cm2 are also fabricated, giving efficiencies of 17.07% and 14.17%, respectively. This work provides new insights of α-FAPbI3 crystallization for constructing efficient and stable printed photovoltaic devices.

Propylammonium Chloride Additive for Efficient and Stable FAPbI3 Perovskite Solar Cells

AbstractTo achieve high efficiency perovskite solar cells (PSCs) based on α‐phase formamidinium lead iodide (FAPbI3), addition of methylammonium chloride (MACl) in the precursor solution is commonly used, mainly because of phase stability and improvement of grain size and crystallinity. However, the instability of MA in the perovskite limits the device long‐term stability. In this report, n‐propylammonium chloride (PACl) is proposed as an alternative to MACl for more stable and efficient FAPbI3‐based PSCs. Perovskite grain size is increased after addition of PACl. Unlike the MA cation, the propylammonium cation passivates the grain boundary rather than being incorporated into the perovskite lattice due to larger ionic size, which minimizes the change in bandgap. Carrier lifetime is significantly increased by more than five times from 405 to 2110 ns with the PACl additive with negligible trap‐mediated recombination, while only four times longer carrier lifetime is observed by MACl additive. As a result, a power conversion efficiency over 22.2% is achieved by 20 mol% PACl additive, which is one of the best efficiencies among the MA‐free and Br‐free PSCs. In addition, stability against moisture is much better for PACl than for MACl due to an in situ formed barrier at the bulk perovskite.

Defect suppression and energy level alignment in formamidinium-based perovskite solar cells

The vast majority of high-performance perovskite solar cells (PSCs) are based on a formamidinium lead iodide (FAPbI3)-dominant composition. Nevertheless, the FA-based perovskite films suffer from undesirable phase transition and defects-induced non-ideal interfacial recombination, which significantly induces energy loss and hinders the improvement of device performance. Herein, we employed 4-fluorophenylmethylammonium iodide (F-PMAI) to modulate surface structure and energy level alignment of the FA-based perovskite films. The superior optoelectronic films were obtained with reduced trap density, pure α-phase FAPbI3 and favorable energy band bending. The lifetime of photogenerated charge carriers increased from 489.3 ns to 1010.6 ns, and a more “p-type” perovskite film was obtained by the post-treatment with F-PMAI. Following this strategy, we demonstrated an improved power conversion efficiency of 22.59% for the FA-based PSCs with an open-circuit voltage loss of 399 mV.

Interfacial engineering of quasi-2-D formamidinium lead iodide nanosheets for perovskite solar cell by mechanochemical approach

Solvent-free synthetic protocols for the formation of nanoarchitectures continues to attract a great attention amoung the wide class of synthetic chemists in order to overcome the harsh reaction conditions. In the current work we demonstrate, for the first time, solvent-free synthesis of dodecylammonium formamidinium lead iodide (DDA)2 [FAPbI3]n−1PbI4, a quasi-two-dimensional (2D) red-light-emitting perovskite nanosheet, where DDA is dodecylammonium, a long chain ammonium cation, and FA is a formamidinium cation. By varying the amount of DDA, we controlled the dual emission of quasi-2D perovskite comprising of both 2D and three-dimensional perovskite layers. The optical properties have manifested the effect of DDA on the structure of perovskite nanosheets. As a proof-of-concept demonstration the material has been explored in perovskite solar cell. The compound containing least amount of DDA exhibited open-circuit voltage (VOC) of 0.961 V, short-circuit current density (JSC) of 23.41 mA cm−2, and PCE of 15.23% under one-sun illumination conditions.