In Situ Dual-Interface Passivation Strategy Enables The Efficiency of Formamidinium Perovskite Solar Cells Over 25

Perovskite solar cells (PSCs) are promising candidates for next-generation photovoltaics owing to their unparalleled power conversion efficiencies (PCEs). Currently, approaches to further improve device efficiencies tend to focus on the passivation of interfacial defects. Although various strategies have been developed to mitigate these defects, many involve complex and time-consuming post-treatment processes, thereby hindering their widespread adoption in commercial applications. In this work, a concise but efficient in situ dual-interface passivation strategy is developed wherein 1-butyl-3-methylimidazolium methanesulfonate (MS) is employed as a precursor additive. During perovskite crystallization, MS can either be enriched downward through precipitation with SnO2 , or can be aggregated upward through lattice extrusion. These self-assembled MS species play a significant role in passivating the defect interfaces, thereby reducing nonradiative recombination losses, and promoting more efficient charge extraction. As a result, a PCE >25% (certified PCE of 24.84%) is achieved with substantially improved long-term storage and photothermal stabilities. This strategy provides valuable insights into interfacial passivation and holds promise for the industrialization of PSCs.

Trivalent Europium-Doped CsCl Quantum Dots for MA-Free Perovskite Solar Cells with Inherent Bandgap through Lattice Strain Compensation

Cesium-formamidinium (Cs-FA) perovskites have garnered widespread interest owing to their excellent thermal- and photostability in achieving stable perovskite solar cells (PSCs). However, Cs-FA perovskite typically suffers from Cs+ and FA+ mismatches, affecting the Cs-FA morphology and lattice distortion, resulting in an enlarged bandgap (Eg ). In this work, "upgraded" CsCl, Eu3+ -doped CsCl quantum dots, are developed to solve the key issues in Cs-FA PSCs and also exploit the advantage of Cs-FA PSCs on stability. The introduction of Eu3+ promotes the formation of high-quality Cs-FA films by adjusting the Pb-I cluster. CsCl:Eu3+ also offsets the local strain and lattice contraction induced by Cs+ , which maintains the inherent Eg of FAPbI3 and decreases the trap density. Finally, a power conversion efficiency (PCE) of 24.13% is obtained with an excellent short-circuit current density of 26.10 mA cm-2 . The unencapsulated devices show excellent humidity stability and storage stability, and an initial PCE of 92.2% within 500 h under continuous light illumination, and bias voltage conditions is achieved. This study provides a universal strategy to address the inherent issues of Cs-FA devices and maintain the stability of MA-free PSCs to satisfy future commercial criteria.

Archetype-Cation-Based Room-Temperature Ionic Liquid: Aliphatic Primary Ammonium Bis(trifluoromethylsulfonyl)imide as a Highly Functional Additive for a Hole Transport Material in Perovskite Solar Cells

Room-temperature ionic liquids (RTILs) have attracted significant attention owing to their unique nature and a variety of potential applications. The archetypal RTIL comprising an aliphatic primary ammonium was discovered over a century ago, but this cation is seldom used in modern RTILs because other bulky cations (e.g., quaternary ammonium-, pyridine-, and imidazole-based cations) are prominent in current major applications, such as electrolytes and solvents, which require low and/or reversible reactivities. However, although the design of materials should change according to the intended application, RTIL designs remain conventional even when applied in unexplored fields, limiting their functions. Herein, RTIL consisting of an archetypal aliphatic primary ammonium (i.e., n-octylammonium: OA) cation and a modern bis(trifluoromethylsulfonyl)imide (TFSI) anion is proposed and demonstrated as a highly functional additive for a 2,2',7,7'-tetrakis(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-OMeTAD), which is the most common hole transport material (HTM), in perovskite solar cells (PSCs). The OA-TFSI additive exhibits prominent functions via permanent reactions of the component ions with the PSC components, thus providing several advantages. The OA cations spontaneously and densely passivate the perovskite layer during the HTM deposition process, leading to both suppression of carrier recombination at the HTM/perovskite interface and hydrophobic perovskite surfaces. Meanwhile, the TFSI anions effectively improve the HTM function most likely via efficient stabilization of the Spiro-OMeTAD radical, enhancing hole collection properties in the PSCs. Consequently, PSC performances involving long-term stability were significantly improved using the OA-TFSI additive. Based on the present results, this study advocates that reconsidering the RTIL design, even when it differs from the current major designs yet is suitable for a target application, can provide functions superior to conventional ones. The insights obtained in this work will spur further study of RTIL designs and aid the development of the broad materials science field including PSCs.

Long-Lived Exciton Coherence in Mixed-Halide Perovskite Crystals

Compositional engineering of the optical properties of hybrid organic-inorganic lead halide perovskites is crucial for the realization of efficient solar cells and light-emitting devices. We study the effect of band gap fluctuations on coherent exciton dynamics in a mixed FA0.9Cs0.1PbI2.8Br0.2 perovskite crystal by using photon echo spectroscopy. We reveal a narrow homogeneous exciton line width of 16 μeV at a temperature of 1.5 K. The corresponding exciton coherence time T2 = 83 ps is exceptionally long due to the localization of excitons at the scale of tens to hundreds of nanometers. From spectral and temperature dependences of the two- and three-pulse photon echo decay, we conclude that for low-energy excitons pure decoherence associated with elastic scattering on phonons is comparable with the exciton lifetime, while for excitons with higher energies, inelastic scattering to lower energy states via phonon emission dominates.

Synergistic Effect of Precursor and Interface Engineering Enables High Efficiencies in FAPbI3 Perovskite Solar Cells

Formamidinium lead iodide (FAPbI3)-based perovskite solar cells have gained immense popularity over the last few years within the perovskite research community due to their incredible opto-electronic properties and the record power conversion efficiencies (PCEs) achieved by the solar cells. However, FAPbI3 is vulnerable to phase transitions even at room temperature, which cause structural instability and eventual device failure during operation. We performed post-treatment of the FAPbI3 surface with octyl ammonium iodide (OAI) in order to stabilize the active phase and preserve the crystal structure of FAPbI3. The formation of a 2D perovskite at the interface depends on the stoichiometry of the precursor. By optimizing the precursor stoichiometry and the concentration of OAI, we observe a synergistic effect, which results in improved power conversion efficiencies, reaching the best values of 22% on a glass substrate. Using physical and detailed optical analysis, we verify the presence of the 2D layer on the top of the 3D surface of the perovskite film.

Homologous post-treatment strategy enabling phase-pure α-FAPbI3 films

Formamidinium lead triiodide (FAPbI3) is considered as the prospective light-absorbing layer on account of the close-to-ideal bandgap of the α-phase, wide optical absorption spectrum and good thermal stability. Therefore, how to realize δ to α-phase transition to obtain phase-pure α-FAPbI3 without additives is important for FAPbI3 perovskite films. Herein, a homologous post-treatment strategy (HPTS) without additives is proposed to prepare FAPbI3 films with pure α-phase. The strategy is processed along with dissolution and reconstruction process during the annealing. The FAPbI3 film has tensile strain with the substrate, and the lattice keeps tensile, and the film maintains in an α/δ hybrid phase. The HPTS process releases the tensile strain between the lattice and the substrate. The process of strain release realizes the phase transition from δ to α-phase during this process. This strategy can accelerate the transformation from hexagonal δ-FAPbI3 to cubic α-FAPbI3 at 120℃. As a result, the acquired α-FAPbI3 films exhibit better film quality in optical and electrical properties, accordingly achieving device efficiency of 19.34% and enhanced stability. This work explores an effective approach to obtain additive-free and phase-pure α-FAPbI3 films through a homologous post-treatment strategy to fabricate uniform high-performance α-FAPbI3 perovskite solar cells.

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-FAPbI_3, 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.

Ligand-Assisted Coupling Manipulation for Efficient and Stable FAPbI3 Colloidal Quantum Dot Solar Cells

For emerging perovskite quantum dots (QDs), understanding the surface features and their impact on the materials and devices is becoming increasingly urgent. In this family, hybrid FAPbI₃ QDs (FA: formamidium) exhibit higher ambient stability, near-infrared absorption and sufficient carrier lifetime. However, hybrid QDs suffer from difficulty in modulating surface ligand, which is essential for constructing conductive QD arrays for photovoltaics. Herein, assisted by an ionic liquid formamidine thiocyanate, we report a facile surface reconfiguration methodology to modulate surface and manipulate electronic coupling of FAPbI₃ QDs, which is exploited to enhance charge transport for fabricating high-quality QD arrays and photovoltaic devices. Finally, a record-high efficiency approaching 15 % is achieved for FAPbI₃ QD solar cells, and they retain over 80 % of the initial efficiency after aging in ambient environment (20-30 % humidity, 25 °C) for over 600 h.

A DFT Study of Alkaline Earth Metal-Doped FAPbI(3) (111) and (100) Surfaces

Density functional theory calculations have been performed to study the effect of replacing lead by alkaline earth metals on the stability, electronic and optical properties of the formamidinium lead triiodide (FAPbI₃) (111) and (100) surfaces with different terminations in the form of FAPb_1-xAE_xI₃ structures, where AE is Be, Mg or Ca. It is revealed that the (111) surface is more stable, indicating metallic characteristics. The (100) surfaces exhibit a suitable bandgap of around 1.309 and 1.623 eV for PbI₅ and PbI₆ terminations, respectively. Increases in the bandgaps as a result of Mg- and Ca-doping of the (100) surface were particularly noted in FAPb_0.96Ca_0.04I₃ and FAPb_0.8Ca_0.2I₃ with bandgaps of 1.459 and 1.468 eV, respectively. In the presence of Be, the band gap reduces critically by about 0.315 eV in the FAPb_0.95Be_0.05I₃ structure, while increasing by 0.096 eV in FAPb_0.96Be_0.04I₃. Optimal absorption, high extinction coefficient and light harvesting efficiency were achieved for plain and doped (100) surfaces in the visible and near UV regions. In order to improve the optical properties of the (111)-PbI₃ surface in initial visible areas, we suggest calcium-doping in this surface to produce FAPb_0.96Ca_0.04I₃, FAPb_0.92Ca_0.08I₃, and FAPb_0.88Ca_0.12I₃ structures.

Ligand exchange engineering of FAPbI3 perovskite quantum dots for solar cells

Formamidinium lead triiodide (FAPbI₃) perovskite quantum dots (PQDs) show great advantages in photovoltaic applications due to their ideal bandgap energy, high stability and solution processability. The anti-solvent used for the post-treatment of FAPbI₃ PQD solid films significantly affects the surface chemistry of the PQDs, and thus the vacancies caused by surface ligand removal inhibit the optoelectronic properties and stability of PQDs. Here, we study the effects of different anti-solvents with different polarities on FAPbI₃ PQDs and select a series of organic molecules for surface passivation of PQDs. The results show that methyl acetate could effectively remove surface ligands from the PQD surface without destroying its crystal structure during the post-treatment. The benzamidine hydrochloride (PhFACl) applied as short ligands of PQDs during the post-treatment could fill the A-site and X-site vacancies of PQDs and thus improve the electronic coupling of PQDs. Finally, the PhFACl-based PQD solar cell (PQDSC) achieves a power conversion efficiency of 6.4%, compared to that of 4.63% for the conventional PQDSC. This work provides a reference for insights into the surface passivation of PQDs and the improvement in device performance of PQDSCs.

Controlling Intrinsic Quantum Confinement in Formamidinium Lead Triiodide Perovskite through Cs Substitution

Lead halide perovskites are leading candidates for photovoltaic and light-emitting devices, owing to their excellent and widely tunable optoelectronic properties. Nanostructure control has been central to their development, allowing for improvements in efficiency and stability, and changes in electronic dimensionality. Recently, formamidinium lead triiodide (FAPbI₃) has been shown to exhibit intrinsic quantum confinement effects in nominally bulk thin films, apparent through above-bandgap absorption peaks. Here, we show that such nanoscale electronic effects can be controlled through partial replacement of the FA cation with Cs. We find that Cs-cation exchange causes a weakening of quantum confinement in the perovskite, arising from changes in the bandstructure, the length scale of confinement, or the presence of δ_H-phase electronic barriers. We further observe photon emission from quantum-confined regions, highlighting their potential usefulness to light-emitting devices and single-photon sources. Overall, controlling this intriguing quantum phenomenon will allow for its suppression or enhancement according to need.

Room-Temperature Synthesis of Air-Stable Near-Infrared Emission in FAPbI3 Nanoparticles Embedded in Silica

Hybrid organic-inorganic and all-inorganic metal halide perovskite nanoparticles (PNPs) have shown their excellent characteristics for optoelectronic applications. We report an atmospheric process to embed formamidinium CH(NH2)2PbI3 (FAPbI3) PNPs in silica protective layer at room temperature (approximately 26 °C) employing (3-aminopropyl) triethoxysilane (APTES). The resulting perovskite nanocomposite (PNCs) achieved a high photoluminescence (PL) quantum yield of 58.0% and good stability under atmospheric moisture conditions. Moreover, the PNCs showed high PL intensity over 1 month of storage (approximately 26 °C) and more than 380 min of PNCs solutions in DI water. The studied near-infrared (NIR) light-emitting diode (LED) combined a NIR-emitting PNCs coating and a blue InGaN-based chip that exhibited a 788 nm electroluminescence spectrum of NIR-LEDs under 2.6 V. This may be a powerful tool to track of muscle and disabled patients in the detection of a blood vessel.

Temperature-Assisted Crystal Growth of Photovoltaic α-Phase FAPbI3 Thin Films by Sequential Blade Coating

Formamidinium lead triiodide (FAPbI₃) exhibits the smallest band gap among lead halide perovskites, which is more desirable for solar cell applications compared to methylammonium-based counterparts. However, it remains a big challenge to prepare phase-pure α-FAPbI₃ in addition to controlling the crystal morphology during film formation. Herein, we developed a temperature-assisted crystal growth to prepare high-quality thin films of α-FAPbI₃ by sequential blade coating. It is found that depositing organic cation FAI at elevated temperatures facilitates the growth of α-FAPbI₃, which otherwise yields mainly a yellow δ-phase at room temperature. In parallel, the crystal morphology of the perovskite films can be effectively manipulated by taking advantage of the porous structure of PbI₂. Solar cells prepared with the blade-coated α-FAPbI₃ yield a champion efficiency of 18.41%, which is among the highest values for FAPbI₃-only solar devices. These results suggest that two-step sequential blade deposition offers a viable approach to fabricate high-quality α-FAPbI₃ films for optoelectronic applications.

Precursor Engineering of Vapor-Exchange Processes for 20%-Efficient 1 cm2 Inverted-Structure Perovskite Solar Cells

Due to mass diffusion issues, it is challenging to prepare black-phase thick formamidinium-based perovskite (FAPbI₃) films via vapor approaches. Precursor engineering is employed here to overcome the dilemma of thorough reaction and black-phase stabilization of FAPbI₃ in a sequential vapor approach. For the first time, FAPbBr₃ was used as an additive in the precursor to promote the formation of FAPbI₃ perovskite. To balance off the increased crystallization degree of precursor films due to the addition of FAPbBr₃, CsI dissolved in dimethyl sulfoxide (DMSO) was further added. It is indicated that the simultaneous incorporation of FAPbBr₃ and CsI-DMSO successfully accelerated the formation rate of perovskite and inhibited the formation of FAPbI₃ yellow phase. The power conversion efficiency of the as-prepared devices of different areas (0.1125 or 1 cm²) reached 20%, the first report of large-area 20%-efficiency PSCs based on a vapor approach, highlighting its applicability to large-area manufacture in the future. Furthermore, when blade coating is used in preparing the precursor film, the efficiency reached 19%. When the precursor film was prepared by dip coating, we could prepare conformal FAPbI₃ coatings on carbon fibers, suggesting possible future applications in fabricating wearable PSCs.

Stability of Precursor Solution for Perovskite Solar Cell: Mixture (FAI + PbI2) versus Synthetic FAPbI3 Crystal

We report here on the stability of a precursor solution for perovskite solar cells. Solution was aged at ambient conditions for 4 weeks, where two different precursor solutions were prepared by dissolving FAI and PbI₂ in DMSO/DMF solvent (precursor mixture solution) and the synthesized single crystalline α-FAPbI₃ in the same solvent (single crystal solution). Perovskite films were prepared by depositing fresh or aged solutions at weekly intervals. Photovoltaic parameters were hardly altered by aging the single crystal solution, while power conversion efficiency was gradually decreased with aging time for the precursor mixture solution mainly due to the decreased photocurrent density and fill factor. Solution pH was changed from basic to acidic due to HI formed by aging the precursor mixture solution, which prevents the formation of α-phase of FAPbI₃. For the single crystal solution, basic conditions remained unchanged by aging. In addition, the presence of δ-phase in the annealed perovskite films was found to have negative influence on the long-term stability. It is thus important to maintain the pH of the precursor solution to avoid aging effects and remove δ-phase in the annealed film for device stability.

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.

A Study of Inverted-Type Perovskite Solar Cells with Various Composition Ratios of (FAPbI₃)1-x(MAPbBr₃)x

This work presents mixed (FAPbI₃)_1−x(MAPbBr₃)_x perovskite films with various composition ratios, x (x = 0–1), which are formed using the spin coating method. The structural, optical, and electronic behaviors of the mixed (FAPbI₃)_1−x(MAPbBr₃)_x perovskite films are discussed. A device with structure glass/indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/mixed perovskite/C₆₀/BCP/Ag was fabricated. The mixed perovskite film was an active light-harvesting layer. PEDOT:PSS was a hole transporting layer between the ITO and perovskite. Both C₆₀ and bathocuproine (BCP) were electron transporting layers. MAPbBr₃ was added to FAPbI₃ with a composition ratio of x = 0.2, stabilizing the perovskite phase, which exhibited a uniform and dense morphology. The optimal device exhibited band matching with C₆₀, resulting in a low series resistance (R_sh) and a high fill factor (FF). Therefore, the device with composition (FAPbI₃)_1−x(MAPbBr₃)_x and x = 0.2 exhibited outstanding performance.

Single Crystal Formamidinium Lead Iodide (FAPbI3): Insight into the Structural, Optical, and Electrical Properties

5 mm-scale large FAPbI 3 single crystals and corresponding photoconductive properties are shown. The phase transition of FAPbI3 between the α-phase and δ-phase is studied. The carrier mobility is 4.4 cm(2) V(-1) s(-1) with a lifetime of 484 ns in the bulk of the single crystal. Finally, photodetectors based on single-crystal FAPbI3 are demonstrated.

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.