A review of polytypism in lead iodide

Tailoring Interface Energies via Phosphonic Acids to Grow and Stabilize Cubic FAPbI3 Deposited by Thermal Evaporation

Coevaporation of formamidinium lead iodide (FAPbI3) is a promising route for the fabrication of highly efficient and scalable optoelectronic devices, such as perovskite solar cells. However, it poses experimental challenges in achieving stoichiometric FAPbI3 films with a cubic structure (α-FAPbI3). In this work, we show that undesired hexagonal phases of both PbI2 and FAPbI3 form during thermal evaporation, including the well-known 2H-FAPbI3, which are detrimental for optoelectronic performance. We demonstrate the growth of α-FAPbI3 at room temperature via thermal evaporation by depositing phosphonic acids (PAc) on substrates and subsequently coevaporating PbI2 and formamidinium iodide. We use density-functional theory to develop a theoretical model to understand the relative growth energetics of the α and 2H phases of FAPbI3 for different molecular interactions. Experiments and theory show that the presence of PAc molecules stabilizes the formation of α-FAPbI3 in thin films when excess molecules are available to migrate during growth. This migration of molecules facilitates the continued presence of adsorbed organic precursors at the free surface throughout the evaporation, which lowers the growth energy of the α-FAPbI3 phase. Our theoretical analyses of PAc molecule-molecule interactions show that ligands can form hydrogen bonding to reduce the migration rate of the molecules through the deposited film, limiting the effects on the crystal structure stabilization. Our results also show that the phase stabilization with molecules that migrate is long-lasting and resistant to moist air. These findings enable reliable formation and processing of α-FAPbI3 films via vapor deposition.

Order-disorder phase transition driven by interlayer sliding in lead iodides

A variety of phase transitions have been found in two-dimensional layered materials, but some of their atomic-scale mechanisms are hard to clearly understand. Here, we report the discovery of a phase transition whose mechanism is identified as interlayer sliding in lead iodides, a layered material widely used to synthesize lead halide perovskites. The low-temperature crystal structure of lead iodides is found not 2H polytype as known before, but non-centrosymmetric 4H polytype. This undergoes the order-disorder phase transition characterized by the abrupt spectral broadening of valence bands, taken by angle-resolved photoemission, at the critical temperature of 120 K. It is accompanied by drastic changes in simultaneously taken photocurrent and photoluminescence. The transmission electron microscopy is used to reveal that lead iodide layers stacked in the form of 4H polytype at low temperatures irregularly slide over each other above 120 K, which can be explained by the low energy barrier of only 10.6 meV/atom estimated by first principles calculations. Our findings suggest that interlayer sliding is a key mechanism of the phase transitions in layered materials, which can significantly affect optoelectronic and optical characteristics.

In Situ and Operando Characterizations of Metal Halide Perovskite and Solar Cells: Insights from Lab-Sized Devices to Upscaling Processes

The performance and stability of metal halide perovskite solar cells strongly depend on precursor materials and deposition methods adopted during the perovskite layer preparation. There are often a number of different formation pathways available when preparing perovskite films. Since the precise pathway and intermediary mechanisms affect the resulting properties of the cells, in situ studies have been conducted to unravel the mechanisms involved in the formation and evolution of perovskite phases. These studies contributed to the development of procedures to improve the structural, morphological, and optoelectronic properties of the films and to move beyond spin-coating, with the use of scalable techniques. To explore the performance and degradation of devices, operando studies have been conducted on solar cells subjected to normal operating conditions, or stressed with humidity, high temperatures, and light radiation. This review presents an update of studies conducted in situ using a wide range of structural, imaging, and spectroscopic techniques, involving the formation/degradation of halide perovskites. Operando studies are also addressed, emphasizing the latest degradation results for perovskite solar cells. These works demonstrate the importance of in situ and operando studies to achieve the level of stability required for scale-up and consequent commercial deployment of these cells.

Atomic layer deposition of PbCl2, PbBr2 and mixed lead halide (Cl, Br, I) PbXnY2-n thin films

Atomic layer deposition offers outstanding film uniformity and conformality on substrates with high aspect ratio features. These qualities are essential for mixed-halide perovskite films applied in tandem solar cells, transistors and light-emitting diodes. The optical and electronic properties of mixed-halide perovskites can be adjusted by adjusting the ratios of different halides. So far ALD is only capable of depositing iodine-based halide perovskites whereas other halide processes are lacking. We describe six new low temperature (≤100 °C) ALD processes for PbCl₂ and PbBr₂ that are crucial steps for the deposition of mixed-halide perovskites with ALD. Lead bis[bis(trimethylsilyl)amide]-GaCl₃ and -TiBr₄ processes yield the purest, crystalline, uniform and conformal films of PbCl₂ and PbBr₂ respectively. We show that these two processes in combination with a PbI₂ process from the literature deposit mixed lead halide films. The four less optimal processes revealed that reaction by-products in lead halide deposition processes may cause film etching or incorporate themselves into the film.

Unveiling the Interaction Mechanisms of Electron and X-ray Radiation with Halide Perovskite Semiconductors using Scanning Nanoprobe Diffraction

The interaction of high-energy electrons and X-ray photons with beam-sensitive semiconductors such as halide perovskites is essential for the characterization and understanding of these optoelectronic materials. Using nanoprobe diffraction techniques, which can investigate physical properties on the nanoscale, studies of the interaction of electron and X-ray radiation with state-of-the-art (FA_0.79 MA_0.16 Cs_0.05 )Pb(I_0.83 Br_0.17 )₃ hybrid halide perovskite films (FA, formamidinium; MA, methylammonium) are performed, tracking the changes in the local crystal structure as a function of fluence using scanning electron diffraction and synchrotron nano X-ray diffraction techniques. Perovskite grains are identified, from which additional reflections, corresponding to PbBr₂ , appear as a crystalline degradation phase after fluences of 200 e^- Å^{- 2} . These changes are concomitant with the formation of small PbI₂ crystallites at the adjacent high-angle grain boundaries, with the formation of pinholes, and with a phase transition from tetragonal to cubic. A similar degradation pathway is caused by photon irradiation in nano-X-ray diffraction, suggesting common underlying mechanisms. This approach explores the radiation limits of these materials and provides a description of the degradation pathways on the nanoscale. Addressing high-angle grain boundaries will be critical for the further improvement of halide polycrystalline film stability, especially for applications vulnerable to high-energy radiation such as space photovoltaics.

Highly Anisotropic Organometal Halide Perovskite Nanowalls Grown by Glancing-Angle Deposition

Polarizers are ubiquitous components in current optoelectronic devices as displays or photographic cameras. Yet, control over light polarization is an unsolved challenge, since the main drawback of the existing display technologies is the significant optical losses. In such a context, organometal halide perovskites (OMHP) can play a decisive role given their flexible synthesis with tunable optical properties such as bandgap and photoluminescence, and excellent light emission with a low non-radiative recombination rate. Therefore, along with their outstanding electrical properties have elevated hybrid perovskites as the material of choice in photovoltaics and optoelectronics. Among the different OMHP nanostructures, nanowires and nanorods have lately arisen as key players in the control of light polarization for lighting or detector applications. Herein, the fabrication of highly aligned and anisotropic methylammonium lead iodide perovskite nanowalls by glancing-angle deposition, which is compatible with most substrates, is presented. Their high alignment degree provides the samples with anisotropic optical properties such as light absorption and photoluminescence. Furthermore, their implementation in photovoltaic devices provides them with a polarization-sensitive response. This facile vacuum-based approach embodies a milestone in the development of last-generation polarization-sensitive perovskite-based optoelectronic devices such as lighting appliances or self-powered photodetectors.

Edge Raman enhancement at layered PbI2platelets induced by laser waveguide effect

As a two-dimensional (2D) layered semiconductor, lead iodide (PbI₂) has been widely used in optoelectronics owing to its unique crystal structure and distinctive optical and electrical properties. A comprehensive understanding of its optical performance is essential for further application and progress. Here, we synthesized regularly shaped PbI₂platelets using the chemical vapor deposition method. Raman scattering spectroscopy of PbI₂platelets was predominantly enhanced when the laser radiated at the edge according to Raman mapping spectroscopy. Combining the outcome of polarized Raman scattering spectroscopy and finite-difference time domain simulation analysis, the Raman enhancement was proven to be the consequence of the enhancement effects inherent to the high refractive index contrast waveguide, which is naturally formed in well-defined PbI₂platelets. Because of the enlarged excited area determined by the increased propagation length of the laser in the PbI₂platelet formed waveguide, the total Raman enhancements are acquired rather than a localized point enhancement. Finally, the Raman enhancement factor is directly related to the thickness of the PbI₂platelet, which further confirms the waveguide-enhanced edge Raman. Our investigation of the optical properties of PbI₂platelets offers reference for potential 2D layered-related optoelectronic applications.

Defects of monolayer PbI2: a computational study

PbI₂ is highly desired for optoelectronics, particularly as a crucial precursor in the preparation of hybrid perovskites. Multifarious defects of PbI₂ have been frequently observed in experiments; however, the origin and impact on its intrinsic properties are not clear currently. Hereby, a systematic study on the defects in PbI₂ was performed by first principles calculations. Likely defect structures in both 1T and 1H phases were predicted, and the corresponding formation energies at the neutral and charged states based on the pre-estimated chemical potential of I/Pb were both calculated. At the neutral states, the formation energy of V_I is relatively low, which is consistent with the experimental observation. However, at the charged states, V_Pb seems to be dominant regardless of the chemical potential and contributes more to the conductivity. Compared with the two-dimensional transition metal dichalcogenides, the relatively low formation energy of PbI₂ indicates its flexibility and the weak Pb-I interaction. Our DFT calculations bring new insights into understanding the influence of defects on its opto-electrical performance and instruct to suppress the formation of those defects that are deleterious to its performance.

A Sodium Chloride Modification of SnO2 Electron Transport Layers to Enhance the Performance of Perovskite Solar Cells

A sodium chloride modification was applied where different amounts of sodium chloride was physically blended in a tin oxide colloid solution to passivate the interface between the electron transport layer (ETL) and perovskite layer and improve the performance of perovskite solar cells. Sodium chloride-modified tin oxide was utilized as the electron transport material to fabricate perovskite solar cells. It was found that sodium chloride-modified tin oxide as an ETL could considerably enhance the performance of the device compared to pristine tin oxide. The power conversion efficiency of the perovskite solar cell displayed 8.8% remarkable improvement from 18.7 ± 0.4% to 20.3 ± 0.3% on average and 9.5% improvement from 18.9 to 20.7% in champion devices because of the considerable enhancement of the fill factor when 25 mM sodium chloride-modified tin oxide as the ETL was used in comparison with pristine tin oxide.

A combined molecular dynamics and experimental study of two-step process enabling low-temperature formation of phase-pure α-FAPbI3

It is well established that the lack of understanding the crystallization process in a two-step sequential deposition has a direct impact on efficiency, stability, and reproducibility of perovskite solar cells. Here, we try to understand the solid-solid phase transition occurring during the two-step sequential deposition of methylammonium lead iodide and formamidinium lead iodide. Using metadynamics, x-ray diffraction, and Raman spectroscopy, we reveal the microscopic details of this process. We find that the formation of perovskite proceeds through intermediate structures and report polymorphs found for methylammonium lead iodide and formamidinium lead iodide. From simulations, we discover a possible crystallization pathway for the highly efficient metastable α phase of formamidinium lead iodide. Guided by these simulations, we perform experiments that result in the low-temperature crystallization of phase-pure α-formamidinium lead iodide.

Quantum cutting-induced near-infrared luminescence of Yb3+ and Er3+ in a layer structured perovskite film

Quantum cutting is an attractive optical phenomenon where one high-energy photon is converted into two low-energy photons, resulting in photoluminescence quantum yields (PLQYs) above 100%. In this report, we demonstrate a novel approach to enhance the quantum cutting energy transfer from an all-inorganic perovskite (CsPbCl₃) to ytterbium (Yb³⁺) and erbium (Er³⁺) ions as near-infrared (NIR) emitters by using the highly orientated crystalline film. Yb³⁺ ions are fixed in the neighborhood of the CsPbCl₃ lattice by preparing a one-to-one layer arrangement consisting of quasi-2D CsPbCl₃ perovskite and Yb³⁺ layers. The successful preparation of layer arrangements resulted in the highly sensitized luminescence of Yb³⁺ by CsPbCl₃ with NIR PLQYs exceeding 130%, which is attributed to quantum cutting. In addition, Er³⁺ luminescence at 1540 nm is acquired by the co-existence of Er³⁺ with Yb³⁺ in a layer, which is a result of the intralayer metal-to-metal energy transfer from Yb³⁺ activated by CsPbCl₃ via the interlayer quantum cutting process. The PLQY of Er³⁺ luminescence reaches to 12.6%, which is the highest value ever observed for Er³⁺ compounds, resulting from the efficient interlayer quantum cutting process over 100% and the following intralayer resonance metal to metal energy transfer with the efficiency over 80%.

One-step vapour phase growth of two-dimensional formamidinium-based perovskite and its hot carrier dynamics

Formamidinum lead iodide perovskite is one of the most promising materials for application in solar cells due to its narrow band gap and higher thermal stability. In this work, we demonstrate the facile synthesis of square-shaped formamidinium lead iodide single crystals on indium tin oxide (ITO) substrates using a one-step vapour phase deposition method. Formamidinium lead iodide-based two-dimensional layered perovskite crystals were successfully synthesized by controlling the deposition conditions. These crystals exhibited a blue-shifted photoluminescence (PL) compared to the conventional formaminium lead iodide perovskite crystals. Power law fittings of the excitation power dependent PL spectra revealed that Auger heating becomes dominant at high excitation densities. In addition, we observed an asymmetric broadening of the PL peak tail at the high energy side, indicating light emission from hot carriers even under steady-state illumination conditions. Phonon-bottleneck effect and Auger heating were considered as the main mechanisms for retardation of hot carrier cooling. Further analysis of the high energy tails using Maxwell-Boltzmann fitting revealed hot-carrier temperatures as high as 690 K. Our findings provide an important aspect of the synthetic approach of perovskites for their potential application in hot carrier solar cells.

The growth of methylammonium lead iodide perovskites by close space vapor transport

Vapor deposition processes have shown promise for high-quality perovskite solar cells with potential pathways for scale-up to large area manufacturing. Here, we present a sequential close space vapor transport process to deposit CH3NH3PbI3 (MAPI) perovskite thin films by depositing a layer of PbI2 then reacting it with CH3NH3I (MAI) vapor. We find that, at T = 100 °C and pressure = 9 torr, a ∼225 nm-thick PbI2 film requires ≥125 minutes in MAI vapor to form a fully-reacted MAPI film. Raising the temperature to 160 °C increases the rate of reaction, such that MAPI forms within 15 minutes, but with reduced surface coverage. The reaction kinetics can be approximated as roughly first-order with respect to PbI2, though there is evidence for a more complicated functional relation. Perovskite films reacted at 100 °C for 150 minutes were fabricated into solar cells with an SLG/ITO/CdS/MAPI/Spiro-OMeTAD/Au structure, and a device efficiency of 12.1% was achieved. These results validate the close space vapor transport process and serve as an advance toward scaled-up, vapor-phase perovskite manufacturing through continuous vapor transport deposition.

Atomic structure and defect dynamics of monolayer lead iodide nanodisks with epitaxial alignment on graphene

Lead Iodide (PbI2) is a large bandgap 2D layered material that has potential for semiconductor applications. However, atomic level study of PbI2 monolayer has been limited due to challenges in obtaining thin crystals. Here, we use liquid exfoliation to produce monolayer PbI2 nanodisks (30-40 nm in diameter and > 99% monolayer purity) and deposit them onto suspended graphene supports to enable atomic structure study of PbI2. Strong epitaxial alignment of PbI2 monolayers with the underlying graphene lattice occurs, leading to a phase shift from the 1 T to 1 H structure to increase the level of commensuration in the two lattice spacings. The fundamental point vacancy and nanopore structures in PbI2 monolayers are directly imaged, showing rapid vacancy migration and self-healing. These results provide a detailed insight into the atomic structure of monolayer PbI2, and the impact of the strong van der Waals interaction with graphene, which has importance for future applications in optoelectronics.

Vapor Phase Selective Growth of Two-Dimensional Perovskite/WS2 Heterostructures for Optoelectronic Applications

Organic-inorganic hybrid perovskites have attracted increased interest owing to their exceptional optoelectronic properties and promising applications. Monolayers of transition metal dichalcogenides (TMDCs), such as tungsten disulfide (WS₂), are also intriguing because of their unique optoelectronic properties and their atomically thin and flexible structures. Therefore, the combination of these different types of materials is very attractive in terms of fundamental science of interface interaction, as well as for the realization of ultrathin optoelectronic devices with high performance. Here, we demonstrate the controlled synthesis of two-dimensional (2D) perovskite/WS₂ heterostructures by an all vapor-phase growth approach. This involves the chemical vapor deposition (CVD) growth of monolayer WS₂, followed by the vapor-phase selective deposition of 2D PbI₂ onto the WS₂ with the successive conversion of PbI₂ to organic-inorganic perovskite (CH₃NH₃PbI₃). Moreover, the selective growth of the perovskite on prepatterned WS₂ enables the direct synthesis of patterned heterostructures, avoiding any damage to the perovskite. The photodetectors utilizing the perovskite/WS₂ heterostructure show increased responsivities compared with isolated thin perovskite obtained by conventional solution methods. The integration of 2D perovskite with TMDCs opens a new avenue to fabricate advanced devices by combining their unique properties and overcoming current processing difficulties of perovskites.

Pb clustering and PbI2 nanofragmentation during methylammonium lead iodide perovskite degradation

Studying defect formation and evolution in MethylAmmonium lead Iodide (MAPbI₃) perovskite layers has a bottleneck in the softness of the matter and in its consequent sensitivity to external solicitations. Here we report that, in a polycrystalline MAPbI₃ layer, Pb-related defects aggregate into nanoclusters preferentially at the triple grain boundaries as unveiled by Transmission Electron Microscopy (TEM) analyses at low total electron dose. Pb-clusters are killer against MAPbI₃ integrity since they progressively feed up the hosting matrix. This progression is limited by the concomitant but slower transformation of the MAPbI₃ core to fragmented and interconnected nano-grains of 6H-PbI₂ that are structurally linked to the mother grain as in strain-relaxed heteroepitaxial coupling. The phenomenon occurs more frequently under TEM degradation whilst air degradation is more prone to leave uncorrelated [001]-oriented 2H-PbI₂ grains as statistically found by X-Ray Diffraction. This path is kinetically costlier but thermodynamically favoured and is easily activated by catalytic species.

Methylammonium lead halide perovskites have great potential for optoelectronic applications but are prone to degradation. Here the authors show that degradation can occur through clustering of Pb containing defects at the grain boundaries while concurrent formation of PbI₂ blocks further reactions, suggesting a strategy for passivation.

One-dimensional facile growth of MAPbI3 perovskite micro-rods

One-dimensional microrods (4–5 mm) of PbI₂ and CH₃NH₃PbI₃ (MAPbI₃) with unique structural and morphological properties have been grown at room temperature. X-ray diffraction (XRD) patterns of both types of micro-rods exhibit a hexagonal system (P3̄m1(164) space group) with 2H polytype structures. In the case of PbI₂, the atomic composition of the microcrystals indicates the formation of pure phases of PbI₂, however, energy-dispersive X-ray spectroscopy (EDX) of MAPbI₃ indicates the existence of intermediate phases due to the addition of MAI. FTIR results reveal the existence of a strong interaction between C–H and N–H groups in the crystals which has been cross validated by Raman spectroscopic analysis. The morphological studies performed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirm the crack free morphology of PbI₂ and MAPbI₃ micro-rods with a porous structure. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) studies show that the addition of MAI in the PbI₂ reduced the weight loss and the decomposition temperature has been increased by 1.5 °C as well. The growth of these unique one-dimensional micro-rods signifies a novel concept in perovskite synthesis for solar cells and optoelectronic applications.

One-dimensional microrods (4–5 mm) of PbI₂ and CH₃NH₃PbI₃ (MAPbI₃) with unique structural and morphological properties have been grown at room temperature.

Low Threshold Fabry-Pérot Mode Lasing from Lead Iodide Trapezoidal Nanoplatelets

Lead Iodide (PbI₂ ) is a layered semiconductor with direct band gap holding great promises in green light emission and detection devices. Recently, PbI₂ planar lasers are demonstrated using hexagonal whispering-gallery-mode microcavities, but the lasing threshold is quite high. In this work, lasing from vapor phase deposition derived PbI₂ trapezoidal nanoplatelets (NPs) with threshold that is at least an order of magnitude lower than the previous value is reported. The growth mechanism of the trapezoidal NPs is explored and attributed to the synergistic effects of van der Waals interactions and lattice mismatching. The lasing is enabled by the population inversion of n = 1 excitons and the optical feedback is provided by the Fabry-Pérot oscillation between the side facets of trapezoidal NPs. The findings not only advance the understanding of growth and photophysics mechanism of PbI₂ nanostructures but also provide ideas to develop low threshold ultrathin lasers.

CH3NH3PbI3 grain growth and interfacial properties in meso-structured perovskite solar cells fabricated by two-step deposition

Although the two-step deposition (TSD) method is widely adopted for the high performance perovskite solar cells (PSCs), the CH₃NH₃PbI₃ perovskite crystal growth mechanism during the TSD process and the photo-generated charge recombination dynamics in the mesoporous-TiO₂ (mp-TiO₂)/CH₃NH₃PbI₃/hole transporting material (HTM) system remains unexploited. Herein, we modified the concentration of PbI₂ (C_(PbI2)) solution to control the perovskite crystal properties, and observed an abnormal CH₃NH₃PbI₃ grain growth phenomenon atop mesoporous TiO₂ film. To illustrate this abnormal grain growth mechanism, we propose that a grain ripening process is taking place during the transformation from PbI₂ to CH₃NH₃PbI₃, and discuss the PbI₂ nuclei morphology, perovskite grain growing stage, as well as Pb:I atomic ratio difference among CH₃NH₃PbI₃ grains with different morphology. These C_(PbI2)-dependent perovskite morphologies resulted in varied charge carrier transfer properties throughout the mp-TiO₂/CH₃NH₃PbI₃/HTM hybrid, as illustrated by photoluminescence measurement. Furthermore, the effect of CH₃NH₃PbI₃ morphology on light absorption and interfacial properties is investigated and correlated with the photovoltaic performance of PSCs.

Retarding Crystallization during Facile Single Coating of NaCl-Incorporated Precursor Solution for Efficient Large-Area Uniform Perovskite Solar Cells

We demonstrated crystallization retardation of CH₃NH₃PbI₃ thin film during single coating of precursor solution by simple addition of NaCl. NaCl was codissolved into a precursor mixture solution containing PbI₂ and methylammonium iodide (MAI). Dissolved NaCl interacted with the PbI₂ in solution and produced a stable intermediate phase, which was converted to a full-coverage uniform perovskite absorber layer via reaction with MAI during a single spin-coating. The resulting planar-structure perovskite solar cell made from NaCl-supplemented precursor solution showed a 48% improvement in power conversion efficiency (PCE) (maximum value 15.16%) over the device fabricated without the additive. Our NaCl-supplemented single coating represents an easy approach to effectively obtain highly reproducible uniform performance at an overall position in 5 cm × 5 cm sized cells (divided into 20 subcells with an active area of 0.06 cm²) with average PCEs of 12.00 ± 0.48%.

Formation Dynamics of CH3NH3PbI3 Perovskite Following Two-Step Layer Deposition

Hybrid metal-halide perovskites have emerged as a leading class of semiconductors for optoelectronic devices because of their desirable material properties and versatile fabrication methods. However, little is known about the chemical transformations that occur in the initial stages of perovskite crystal formation. Here we follow the real-time formation dynamics of MAPbI3 from a bilayer of lead iodide (PbI2) and methylammonium iodide (MAI) deposited through a two-step thermal evaporation process. By lowering the substrate temperature during deposition, we are able to initially inhibit intermixing of the two layers. We subsequently use infrared and visible light transmission, X-ray diffraction, and photoluminescence lifetime measurements to reveal the room-temperature transformations that occur in vacuum and ambient air, as MAI diffuses into the PbI2 lattice to form MAPbI3. In vacuum, the transformation to MAPbI3 is incomplete as unreacted MAI is retained in the film. However, exposure to moist air allows for conversion of the unreacted MAI to MAPbI3, demonstrating that moisture is essential in making MAI more mobile and thus aiding perovskite crystallization. These dynamic processes are reflected in the observed charge-carrier lifetimes, which strongly fluctuate during periods of large ion migration but steadily increase with improving crystallinity.

Pressure-induced phase transitions of lead iodide

High-pressure phase transitions of lead iodide have been proposed up to 200 GPa through the theoretically swarm-intelligent global structure searches.

Polarized Raman spectra of phosphorene in edge and top view measuring configurations

We demonstrate the anisotropic features of the A_1g, B_2gand A_2gvibrational Raman modes of phosphorene by performing for the first time a comparison of two orthogonal polarized Raman scattering studies in edge and top view measuring configurations.

Crystallization Dynamics of Organolead Halide Perovskite by Real-Time X-ray Diffraction

We analyzed the crystallization process of the CH3NH3PbI3 perovskite by observing real-time X-ray diffraction immediately after combining a PbI2 thin film with a CH3NH3I solution. A detailed analysis of the transformation kinetics demonstrated the fractal diffusion of the CH3NH3I solution into the PbI2 film. Moreover, the perovskite crystal was found to be initially oriented based on the PbI2 crystal orientation but to gradually transition to a random orientation. The fluctuating characteristics of the crystallization process of perovskites, such as fractal penetration and orientational transformation, should be controlled to allow the fabrication of high-quality perovskite crystals. The characteristic reaction dynamics observed in this study should assist in establishing reproducible fabrication processes for perovskite solar cells.

Solvent engineering towards controlled grain growth in perovskite planar heterojunction solar cells

We report an effective solvent engineering process to enable controlled perovskite crystal growth and a wider window for processing uniform and dense methyl ammonium lead iodide (MAPbI3) perovskite films. Planar heterojunction solar cells fabricated with this method demonstrate hysteresis-free performance with a power conversion efficiency around 10%. The crystal structure of an organic-based Pb iodide intermediate phase is identified for the first time, which is critical in controlling the crystal growth and optimizing thin film morphology.

Comparative study of vapor- and solution-crystallized perovskite for planar heterojunction solar cells

Organometal halide perovskite (CH3NH3PbI3) could be crystallized by exposing PbI2 to either CH3NH3I solution or CH3NH3I vapor. Though high performance was achieved in both approaches, it was still not clear which approach would be more desirable for device performance in principle. Herein, we addressed this issue by investigating the influence of crystallization condition on perovskite morphology, and subsequently on device performances. We found that vapor-crystallized perovskite devices demonstrated smoother surface morphology, better light absorption, lower charge recombination, and thus much higher conversion efficiency than solution-crystallized devices, which would give some useful enlightenment to develop high-performance planar perovskite solar cells.

Metal electrode-free perovskite solar cells with transfer-laminated conducting polymer electrode

We report perovskite solar cells with a new device structure that employ highly conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (

PEDOT

PSS) as the top electrode replacing commonly used metal electrodes. The

PEDOT

PSS top electrode is prepared from its aqueous solution through a transfer-lamination technique rather than direct spin-coating, which converts the CH(3)NH(3)PbI(3) into PbI(2). Perovskite solar cells with the structure of glass/FTO/c-TiO(2)/m-TiO(2)/CH(3)NH(3)PbI(3)/spiro-OMeTAD/PEDOT:PSS yield a maximum open-circuit voltage (V(OC)) of 1.02 V, and a maximum power conversion efficiency (PCE) of 11.29% under AM1.5 100 mW/cm(2) illumination. The whole device was fabricated in air without high-vacuum deposition which simplifies the processing and lowers the threshold of both scientific research and industrial production of perovskite solar cells.

Formation of high-quality perovskite thin film for planar heterojunction solar cells

Solution processing of flat, uniform film can be realized via judicious choice of solvents of the sequential deposition technique which generates mirror-like perovskite film with resemble surface roughness to that from vapor assisted method.

Solvent engineering for high-performance inorganic-organic hybrid perovskite solar cells

Organolead trihalide perovskite materials have been successfully used as light absorbers in efficient photovoltaic cells. Two different cell structures, based on mesoscopic metal oxides and planar heterojunctions have already demonstrated very impressive advances in performance. Here, we report a bilayer architecture comprising the key features of mesoscopic and planar structures obtained by a fully solution-based process. We used CH3NH3 Pb(I(1-x)Br(x))3 (x = 0.1-0.15) as the absorbing layer and poly(triarylamine) as a hole-transporting material. The use of a mixed solvent of γ-butyrolactone and dimethylsulphoxide (DMSO) followed by toluene drop-casting leads to extremely uniform and dense perovskite layers via a CH3NH3I-PbI2-DMSO intermediate phase, and enables the fabrication of remarkably improved solar cells with a certified power-conversion efficiency of 16.2% and no hysteresis. These results provide important progress towards the understanding of the role of solution-processing in the realization of low-cost and highly efficient perovskite solar cells.

Termination Dependence of Tetragonal CH3NH3PbI3 Surfaces for Perovskite Solar Cells

We investigated the termination dependence of structural stability and electronic states of the representative (110), (001), (100), and (101) surfaces of tetragonal CH3NH3PbI3 (MAPbI3), the main component of a perovskite solar cell (PSC), by density functional theory calculations. By examining various types of PbIx polyhedron terminations, we found that a vacant termination is more stable than flat termination on all of the surfaces, under thermodynamic equilibrium conditions of bulk MAPbI3. More interestingly, both terminations can coexist especially on the more probable (110) and (001) surfaces. The electronic structures of the stable vacant and PbI2-rich flat terminations on these two surfaces largely maintain the characteristics of bulk MAPbI3 without midgap states. Thus, these surfaces can contribute to the long carrier lifetime actually observed for the PSCs. Furthermore, the shallow surface states on the (110) and (001) flat terminations can be efficient intermediates of hole transfer. Consequently, the formation of the flat terminations under the PbI2-rich condition will be beneficial for the improvement of PSC performance.