Molecularly Engineered Multifunctional Bridging Layer Derived from Dithiafulavene Capped Spiroxanthene for Stable and Efficient Perovskite Solar Cells.

This study introduces a novel approach centered around the design and synthesis of an interfacial passivating layer in perovskite solar cells (PSCs). This architectural innovation is realized through the development of a specialized material, termed dithiafulvene end-capped Spiro[fluorene-9,9'-xanthene], denoted by the acronym AF32. In this design architecture, dithiafulvene is thoughtfully attached to the spiroxanthene fluorene core with phenothiazine as the spacer unit, possessing multiple alkyl chains. AF32 passivates interfacial defects by coordinating the sulfur constituents of the phenothiazine and dithiafulvene frameworks to the uncoordinated Pb2+ cations on the surface of the perovskite film, and the alkyl chains construct a hydrophobic environment, preventing moisture from entering the hydrophilic perovskite layer and improving the long-term stability of PSCs. Furthermore, this conductive interlayer facilitates hole transport in PSCs due to its well-aligned molecular orbital levels. Such improvements translated into an enhanced power conversion efficiency (PCE) of 22.6% for the device employing 1.5 mg/mL AF32, and it maintained 85% of its initial PCE after more than 1800 h under ambient conditions [illumination and 45 ± 5% relative humidity (RH)]. This study not only marks progress in photovoltaic technology but also expands our understanding of manipulating interfacial properties for optimized device performance and stability.

Speciation of Lanthanide Metal Ion Dopants in Microcrystalline All-Inorganic Halide Perovskite CsPbCl3.

Lanthanides are versatile modulators of optoelectronic properties owing to their narrow optical emission spectra across the visible and near-infrared range. Their use in metal halide perovskites (MHPs) has recently gained prominence, although their fate in these materials has not yet been established at the atomic level. We use cesium-133 solid-state NMR to establish the speciation of all nonradioactive lanthanide ions (La3+, Ce3+, Pr3+, Nd3+, Sm3+, Sm2+, Eu3+, Eu2+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, Lu3+) in microcrystalline CsPbCl3. Our results show that all lanthanides incorporate into the perovskite structure of CsPbCl3 regardless of their oxidation state (+2, +3).

Improved Efficiency and Stability in 1,5-Diaminonaphthalene Iodide-Passivated 2D/3D Perovskite Solar Cells.

Engineering multidimensional two-dimensional/three-dimensional (2D/3D) perovskite interfaces as light harvesters has recently emerged as a potential strategy to obtain a higher photovoltaic performance in perovskite solar cells (PSCs) with enhanced environmental stability. In this study, we utilized the 1,5-diammonium naphthalene iodide (NDAI) bulky organic spacer for interface modification in 3D perovskites for passivating the anionic iodide/uncoordinated Pb2+ vacancies as well as facilitating charge carrier transfer by improving the energy band alignment at the perovskite/HTL interface. Consequently, the NDAI-treated 2D/3D PSCs showed an enhanced open-circuit voltage and fill factor with a remarkable power conversion efficiency (PCE) of 21.48%. In addition, 2D/3D perovskite devices without encapsulation exhibit a 77% retention of their initial output after 1000 h of aging under 50 ± 5% relative humidity. Furthermore, even after 200 h of storage in 85 °C thermal stress, the devices maintain 60% of their initial PCE. The defect passivation and interface modification mechanism were studied in detail by UV vis absorption, photoluminescence spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), solid-state NMR, space-charge-limited current (SCLC) mobility measurement, and impedance spectroscopy. This study provides a promising path for perovskite surface modification in slowing their degradation against external stimuli, providing a future direction for increasing the perovskite device efficiency and durability.

Probing the Low-Frequency Response of Impedance Spectroscopy of Halide Perovskite Single Crystals Using Machine Learning.

Electrochemical impedance spectroscopy (EIS) has emerged as a versatile technique for characterization and analysis of metal halide perovskite solar cells (PSCs). The crucial information about ion migration and carrier accumulation in PSCs can be extracted from the low-frequency regime of the EIS spectrum. However, lengthy measurement time at low frequencies along with material degradation due to prolonged exposure to light and bias motivates the use of machine learning (ML) in predicting the low-frequency response. Here, we have developed an ML model to predict the low-frequency response of the halide perovskite single crystals. We first synthesized high-quality MAPbBr3 single crystals and subsequently recorded the EIS spectra at different applied bias and illumination intensities to prepare the dataset comprising 8741 datapoints. The developed supervised ML model can predict the real and imaginary parts of the low-frequency EIS response with an R2 score of 0.981 and a root mean squared error (RMSE) of 0.0196 for the testing set. From the ground truth experimental data, it can be observed that negative capacitance prevails at a higher applied bias. Our developed model can closely predict the real and imaginary parts at a low frequency (50 Hz-300 mHz). Thus, our method makes recording of EIS more accessible and opens a new way in using the ML techniques for EIS.

Rationalizing the Effect of Polymer-Controlled Growth of Perovskite Single Crystals on Optoelectronic Properties.

To improve and modulate the optoelectronic properties of single-crystal (SC) metal halide perovskites (MHPs), significant progress has been achieved. Polymer-assisted techniques are a great approach to control the growth rate of SCs effectively. However, the resultant optoelectrical properties induced by polymers are ambiguous and need to be taken into the consideration. In this study, we have synthesized methylammonium lead triiodide (MAPbI3) SCs using polyethylene glycol (PEG) and polystyrene (PS) polymers where PEG contains oxygen functionalities and PS does not. We studied the electrical properties of these SCs under dark and illumination conditions. It was observed that PEG-assisted SCs showed few defects with lower photocurrent as compared to the PS-assisted ones because of defect-mediated conductivity. The results are further verified by transient current response, responsivity, and capacitance-frequency measurements. The present study sheds light on the polymer selection for the growth of MHP SCs and their optoelectronic properties.

Interplay of Kinetic and Thermodynamic Reaction Control Explains Incorporation of Dimethylammonium Iodide into CsPbI3.

CsPbI3 is a promising material for optoelectronics owing to its thermal robustness and favorable bandgap. However, its fabrication is challenging because its photoactive phase is thermodynamically unstable at room temperature. Adding dimethylammonium (DMA) alleviates this instability and is currently understood to result in the formation of DMA x Cs1-x PbI3 perovskite solid solutions. Here, we use NMR of the 133Cs and 13C local structural probes to show that these solid solutions are not thermodynamically stable, and their synthesis under thermodynamic control leads to a segregated mixture of yellow one-dimensional DMAPbI3 phase and δ-CsPbI3. We show that mixed-cation DMA x Cs1-x PbI3 perovskite phases only form when they are kinetically trapped by rapid antisolvent-induced crystallization. We explore the energetics of DMA incorporation into CsPbI3 using first-principles calculations and molecular dynamics simulations and find that this process is energetically unfavorable. Our results provide a complete atomic-level picture of the mechanism of DMA-induced stabilization of the black perovskite phase of CsPbI3 and shed new light on this deceptively simple material.

Facile NaF Treatment Achieves 20% Efficient ETL-Free Perovskite Solar Cells.

Electron transporting layer (ETL)-free perovskite solar cells (PSCs) exhibit promising progress in photovoltaic devices due to the elimination of the complex and energy-/time-consuming preparation route of ETLs. However, the performance of ETL-free devices still lags behind conventional devices because of mismatched energy levels and undesired interfacial charge recombination. In this study, we introduce sodium fluoride (NaF) as an interface layer in ETL-free PSCs to align the energy level between the perovskite and the FTO electrode. KPFM measurements clearly show that the NaF layer covers the surface of rough underlying FTO very well. This interface modification reduces the work function of FTO by forming an interfacial dipole layer, leading to band bending at the FTO/perovskite interface, which facilitates an effective electron carrier collection. Besides, the part of Na+ ions is found to be able to migrate into the absorber layer, facilitating enlarged grains and spontaneous passivation of the perovskite layer. As a result, the efficiency of the NaF-treated cell reaches 20%, comparable to those of state-of-the-art ETL-based cells. Moreover, this strategy effectively enhances the operational stability of devices by preserving 94% of the initial efficiency after storage for 500 h under continuous light soaking at 55 °C. Overall, these improvements in photovoltaic properties are clear indicators of enhanced interface passivation by NaF-based interface engineering.

Tetrahedral M4(µ4-O) Motifs Beyond Zn: Efficient One-Pot Synthesis of Oxido-Amidate Clusters via a Transmetalation/Hydrolysis Approach.

While zinc µ4-oxido-centered complexes are widely used as versatile precursors and building units of functional materials, the synthesis of their analogues based on other transition metals is highly underdeveloped. Herein, we present the first efficient systematic approach for the synthesis of homometallic [M4(µ4-O)L6]-type clusters incorporating divalent transition-metal centers, coated by bridging monoanionic organic ligands. As a proof of concept, we prepared a series of charge-neutral metal-oxido benzamidates, [M4(µ4-O) (NHCOPh)6] (M = Fe, Co, Zn), including iron(II) and cobalt(II) clusters not accessible before. The resulting complexes were characterized using elemental analysis, FTIR spectroscopy, magnetic measurements, and single-crystal X-ray diffraction. Detailed structural analysis showed interesting self-assembly of the tetrahedral clusters into 2D honeycomb-like supramolecular layers driven by hydrogen bonds in the proximal secondary coordination sphere. Moreover, we modeled the magnetic properties of new iron (II) and cobalt (II) clusters, which display a general tendency for antiferromagnetic coupling of the µ4-O/µ-benzamidate-bridged metal centers. The developed synthetic procedure is potentially easily extensible to other M(II)-oxido systems, which will likely pave the way to new oxido clusters with interesting optoelectronic and self-assembly properties and, as a result, will allow for the development of new functional materials not achievable before.

Cyclodextrin-Templated Co(II) Grids: Symmetry Control over Supramolecular Topology and Magnetic Properties.

While inherent complexation properties and propensity for self-organization of cyclodextrins (CDs) render them potentially promising scaffolds of magnetic materials, this research area is still at an embryonic stage. We report on the synthesis and structure characterization of a new sandwich-type complex, [(α-CD)2Co3Li6(H2O)9] (α-1), which represents a smaller analogue of the previously characterized [(γ-CD)2Co4Li8(H2O)12] (γ-1) cluster. A comprehensive structural analysis of α-1 and a careful reinvestigation of γ-1 reveal how the symmetry of CD ligands determines the molecular composition and supramolecular arrangements of Co/Li sandwich-type complexes. Furthermore, the first comparative studies of the magnetic properties in this type of system point to subtle differences in the magnetic behavior of both compounds. The sandwich-type complexes α-1 and γ-1 exhibit field-induced slow magnetic relaxation, defining a new family of magnetic materials with a pillared grid-like supramolecular structure composed of weakly interacting CoII centers forming an SMM.

Thiocyanate-Passivated Diaminonaphthalene-Incorporated Dion-Jacobson Perovskite for Highly Efficient and Stable Solar Cells.

Two-dimensional (2D) metal halide perovskites have recently emerged as promising photovoltaic materials due to their superior ambient stability and rich structural diversity. However, power conversion efficiencies (PCEs) of the 2D perovskites solar cells (PSCs) still lag behind their three-dimensional (3D) counterpart, particularly due to the anisotropy in the charge carrier mobility and inhomogeneous energy landscape. A promising alternative is Dion-Jacobson (D-J) phase quasi-2D perovskite, where the bulky organic diammonium cations are introduced into inorganic frameworks to remove the weak van der Waals interactions between interlayers and to improve the open-circuit voltage (Voc). Although the D-J phase 2D perovskite shows a homogeneous energy landscape and better charge transport, their poor crystallinity and existence of higher trap states remain a major challenge for the development of high-efficiency solar cells device. To address this issue, here, we report the eclipsed D-J phase 2D perovskite using 1,5-diaminonaphthalene cation and subsequently treated the film with ammonium thiocyanate (NH4SCN) additive to further improve the film crystallinity, out-of-plane orientation, and carrier mobility. We observe that 2 mol NH4SCN surface treatment in NDA-based D-J phase perovskite leads to better film morphology and improved crystallinity, as confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Time-resolved photoluminescence (TRPL) spectroscopy and steady-state space charge limited current (SCLC) mobility measurement reveal a significant reduction of trap-assisted nonradiative recombination and improvement of carrier mobility in the thiocyanate-passivated perovskite. Consequently, the PCE of the NH4SCN-treated (NDA)(MA)3(Pb)4(I)13 perovskite device enhanced nearly 46% from 10.3 to 15.08%. We have further studied intensity-dependent J-V characteristics, which demonstrate the reduction of ideality factor, confirming the effective suppression of trap-assisted nonradiative recombination, consistent with the transient PL results. Electrochemical impedance spectroscopy (EIS) confirms the improved charge carrier transport in NH4SCN additive-treated devices. Interestingly, our additive-engineered unsealed perovskite devices retained 75% of their initial efficiency after 1000 h of continuous storage under 60% relative humidity. This study opens up the strategy for developing high-efficiency and stable 2D perovskite solar cells.

Azahomofullerenes as New n-Type Acceptor Materials for Efficient and Stable Inverted Planar Perovskite Solar Cells.

Fullerene derivatives with a strong electron-accepting ability play a crucial role in enhancing both the performance and stability of perovskite solar cells (PSCs). However, most of the used fullerene molecules are based on [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), which limits the device performance due to difficulties in preparing high-quality and uniform thin films. Herein, solution-processable azahomofullerene (AHF) derivatives (abbreviated as AHF-1 and AHF-2) are reported as novel and effective electron-transport layers (ETLs) in p-i-n planar PSCs. Compared to the control PCBM ETL-based PSCs, the devices based on AHFs exhibit higher photovoltaic performances, which is attributed to the enhanced charge-transport properties and improved layer morphology leading to a maximum power conversion efficiency (PCE) of 20.21% in the case of the device based on AHF-2 ETL. Besides, due to the preferable energy band alignment with the perovskite layer, reduced trap states, and suppressed charge recombination, the device with AHF-2 ETL exhibits significantly suppressed hysteresis and improved stability under both ambient and thermal conditions.

Recent Progress in Growth of Single-Crystal Perovskites for Photovoltaic Applications.

The growth of high-quality single-crystal (SC) perovskite films is a great strategy for the fabrication of defect-free perovskite solar cells (PSCs) with photovoltaic parameters close to the theoretical limit, which resulted in high efficiency and superior stability of the device. Plenty of growth methods for perovskite SCs are available to achieve a maximum power conversion efficiency (PCE) surpassing 21% for SC-based PSCs. However, there is still a lot of room to further push the efficiency by considering new crystal growth techniques, interface engineering, passivation approaches, and additive engineering. In this review, we summarize the recent progress in the growth of SC-based perovskite films for the fabrication of high-efficiency and stable PSCs. We describe the impact of SC growth of perovskite films and their quality on the device performance and stability, compared with the commonly used polycrystalline perovskite films. In the last section, the challenges and potential of SCs in PSCs are also covered for future development.

Halide Mixing and Phase Segregation in Cs2AgBiX6 (X = Cl, Br, and I) Double Perovskites from Cesium-133 Solid-State NMR and Optical Spectroscopy.

All-inorganic double perovskites (elpasolites) are a promising potential alternatives to lead halide perovskites in optoelectronic applications. Although halide mixing is a well-established strategy for band gap tuning, little is known about halide mixing and phase segregation phenomena in double perovskites. Here, we synthesize a wide range of single- and mixed-halide Cs2AgBiX6 (X = Cl, Br, and I) double perovskites using mechanosynthesis and probe their atomic-level microstructure using 133Cs solid-state MAS NMR. We show that mixed Cl/Br materials form pure phases for any Cl/Br ratio while Cl/I and Br/I mixing is only possible within a narrow range of halide ratios (<3 mol % I) and leads to a complex mixture of products for higher ratios. We characterize the optical properties of the resulting materials and show that halide mixing does not lead to an appreciable tunability of the PL emission. We find that iodide incorporation is particularly pernicious in that it quenches the PL emission intensity and radiative charge carrier lifetimes for iodide ratios as low as 0.3 mol %. Our study shows that solid-state NMR, in conjunction with optical spectroscopies, provides a comprehensive understanding of the structure-activity relationships, halide mixing, and phase segregation phenomena in Cs2AgBiX6 (X = Cl, Br, and I) double perovskites.

Changes in the Electrical Characteristics of Perovskite Solar Cells with Aging Time.

The last decade has witnessed the impressive progress of perovskite solar cells (PSCs), with power conversion efficiency exceeding 25%. Nevertheless, the unsatisfactory device stability and current-voltage hysteresis normally observed with most PSCs under operational conditions are bottlenecks that hamper their further commercialization. Understanding the electrical characteristics of the device during the aging process is important for the design and development of effective strategies for the fabrication of stable PSCs. Herein, electrochemical impedance spectroscopical (IS) analyses are used to study the time-dependent electrical characteristics of PSC. We demonstrate that both the dark and light ideality factors are sensitive to aging time, indicating the dominant existence of trap-assisted recombination in the investigated device. By analyzing the capacitance versus frequency responses, we show that the low-frequency capacitance increases with increasing aging time due to the accumulation of charges or ions at the interfaces. These results are correlated with the observed hysteresis during the current-voltage measurement and provide an in-depth understanding of the degradation mechanism of PSCs with aging time.

Elucidation of the role of guanidinium incorporation in single-crystalline MAPbI3 perovskite on ion migration and activation energy.

Ion migration plays a significant role in the overall stability and power conversion efficiency of perovskite solar cells (PSCs). This process was found to be influenced by the compositional engineering of the A-site cation in the perovskite crystal structure. However, the effect of partial A-site cation substitution in a methylammonium lead iodide (MAPbI3) perovskite on the ion migration process and its activation energy is not fully understood. Here we study the effect of a guanidinium (GUA) cation on the ion transport dynamics in the single crystalline GUAxMA1-xPbI3 perovskite composition using temperature-dependent electrochemical impedance spectroscopy (EIS). We find that the small substitution of MA with GUA decreases the activation energy for iodide ion migration in comparison to pristine MAPbI3. The presence of a large GUA cation in the 3D perovskite structure induces lattice enlargement, which perturbs the atomic interactions within the perovskite lattice. Consequently, the GUAxMA1-xPbI3 crystal exhibits a higher degree of hysteresis during current-voltage (J-V) measurements than the single-crystalline MAPbI3 counterpart. Our results provide the fundamental understanding of hysteresis, which is commonly observed in GUA-based PSCs and a general protocol for in-depth electrical characterization of perovskite single crystals.

Local Structure and Dynamics in Methylammonium, Formamidinium, and Cesium Tin(II) Mixed-Halide Perovskites from 119Sn Solid-State NMR.

Organic-inorganic tin(II) halide perovskites have emerged as promising alternatives to lead halide perovskites in optoelectronic applications. While they suffer from considerably poorer performance and stability in comparison to their lead analogues, their performance improvements have so far largely been driven by trial and error efforts due to a critical lack of methods to probe their atomic-level microstructure. Here, we identify the challenges and devise a 119Sn solid-state NMR protocol for the determination of the local structure of mixed-cation and mixed-halide tin(II) halide perovskites as well as their degradation products and related phases. We establish that the longitudinal relaxation of 119Sn can span 6 orders of magnitude in this class of compounds, which makes judicious choice of experimental NMR parameters essential for the reliable detection of various phases. We show that Cl/Br and I/Br mixed-halide perovskites form solid alloys in any ratio, while only limited mixing is possible for I/Cl compositions. We elucidate the degradation pathways of Cs-, MA-, and FA-based tin(II) halides and show that degradation leads to highly disordered, qualitatively similar products, regardless of the A-site cation and halide. We detect the presence of metallic tin among the degradation products, which we suggest could contribute to the previously reported high conductivities in tin(II) halide perovskites. 119Sn NMR chemical shifts are a sensitive probe of the halide coordination environment as well as of the A-site cation composition. Finally, we use variable-temperature multifield relaxation measurements to quantify ion dynamics in MASnBr3 and establish activation energies for motion and show that this motion leads to spontaneous halide homogenization at room temperature whenever two different pure-halide perovskites are put in physical contact.

Atomic Layer Deposition of an Effective Interface Layer of TiN for Efficient and Hysteresis-Free Mesoscopic Perovskite Solar Cells.

Perovskite solar cells (PSCs) have experienced outstanding advances in power conversion efficiencies (PCEs) by employing new electron transport layers (ETLs), interface engineering, optimizing perovskite morphology, and improving charge collection efficiency. In this work, we study the role of a new ultrathin interface layer of titanium nitride (TiN) conformally deposited on a mesoporous TiO2 (mp-TiO2) scaffold using the atomic layer deposition method. Our characterization results revealed that the presence of TiN at the ETL/perovskite interface improves the charge collection as well as reduces the interface recombination. We find that the morphology (grain size) and optical properties of the perovskite film deposited on the optimized mp-TiO2/TiN ETL are improved drastically, leading to devices with a maximum PCE of 19.38% and a high open-circuit voltage (Voc) of 1.148 V with negligible hysteresis and improved environmental (∼40% RH) and thermal (80 °C) stabilities.

Synergistic ligand exchange and UV curing of PbS quantum dots for effective surface passivation.

Lead sulfide (PbS) quantum dots (QDs) are promising materials in solution-processed photovoltaic (PV) devices due to their tunable bandgap and low-cost processing. Replacing the long oleic acid ligands of the as-synthesized QDs with shorter ligands is a key step for making functional QD PVs with correctly tuned band energies and reduced non-radiative recombination centers. In this work, we study the effect of ultraviolet (UV) treatment of PbS QD layers on the QD surface states during ligand exchange. We demonstrate that this straightforward approach effectively reduces the surface trap states and passivates the surface of QDs. We find that UV treatment reduces the density of hydroxyl groups attached to the QD surface and improves the bonding of short ligands to the QD surface. Multiple analyses show the reduction of nonradiative recombination centers for the UV-treated sample. The power conversion efficiency (PCE) of our optimized PbS QD device reached 10.7% (vs. 9% for the control device) and was maintained above 10% after 230 h of constant illumination.

Mechanoperovskites for Photovoltaic Applications: Preparation, Characterization, and Device Fabrication.

Hybrid organic-inorganic metal halide perovskites (MHPs) have emerged as excellent absorber materials for next generation solar cells owing to their simple solution-processed synthesis and high efficiency. This breakthrough in photovoltaics along with an accompanying impact in light-emitting applications prompted a renaissance of interest in the broad family of MHPs. Notably, the optoelectronic properties and the photovoltaic parameters of MHPs are highly sensitive to the adopted synthetic strategy. The preparation of MHPs has commonly relied on solution-based methods requiring elevated temperatures for homogeneity of reaction mixtures. While the solution-based approach is relatively versatile, it faces challenges such as limitations in compositional engineering of MHPs or their long-term storage among others. Therefore, there is a continuous great challenge to develop efficient synthetic strategies affording various high-quality MHP materials for numerous technological optoelectronic applications. In the past decade, mechanochemistry has appeared as a green alternative to traditional synthesis. This solid-state, re-emerging efficient synthetic methodology mediated by direct absorption of mechanical energy is growing explosively across organic and inorganic chemistry and materials science. In this Account, we describe our shared interest in the productive use of mechanical force in chemistry of MHPs, as well as assembly of the respective solar cell devices. We highlight the milestones achieved by our groups along with the seminal contributions by other groups. In particular, we demonstrate that mechanochemistry efficiently allows the formation of various phase pure hybrid lead and lead-free halide perovskite compositions (called hereafter "mechanoperovskites"). The progress in solvent-free solid-state synthesis is greatly enhanced by the integration of advanced methods of solid-state analysis like powder X-ray diffraction (pXRD), solid-state nuclear magnetic resonance (ss-NMR) and UV-vis spectroscopies, and we aim to illustrate this ongoing integration through appropriate examples. Furthermore, we show that thin films based on mechanoperovskites have the advantage of providing a higher degree of control of the stoichiometry and higher reproducibility, stability, and material phase purity. The impact of using powdered mechanoperovskite as a precursor for thin film formation on the electrochemical and photovoltaic properties of the solar cells is also discussed. Finally, our view of current challenges and future directions in this emerging interdisciplinary area of research is provided.

Atomic-Level Microstructure of Efficient Formamidinium-Based Perovskite Solar Cells Stabilized by 5-Ammonium Valeric Acid Iodide Revealed by Multinuclear and Two-Dimensional Solid-State NMR.

Chemical doping of inorganic-organic hybrid perovskites is an effective way of improving the performance and operational stability of perovskite solar cells (PSCs). Here we use 5-ammonium valeric acid iodide (AVAI) to chemically stabilize the structure of α-FAPbI3. Using solid-state MAS NMR, we demonstrate the atomic-level interaction between the molecular modulator and the perovskite lattice and propose a structural model of the stabilized three-dimensional structure, further aided by density functional theory (DFT) calculations. We find that one-step deposition of the perovskite in the presence of AVAI produces highly crystalline films with large, micrometer-sized grains and enhanced charge-carrier lifetimes, as probed by transient absorption spectroscopy. As a result, we achieve greatly enhanced solar cell performance for the optimized AVA-based devices with a maximum power conversion efficiency (PCE) of 18.94%. The devices retain 90% of the initial efficiency after 300 h under continuous white light illumination and maximum-power point-tracking measurement.