Methylammonium Ions Research Articles

Synergistic Enhancement of Stability and Performance for Perovskite Solar Cells Using Fluorinated Benzoic Acids as Additives

The introduction of additives with specific functional groups is an important approach to extend the operational lifetime of perovskite solar cells (PSCs). Herein, the effect of the additives of benzoic acid (0F‐B), 4‐fluorobenzoic acid (1F‐B), and 2,3,4,5,6‐pentafluorobenzoic acid (5F‐B) on the performance and stability of MAPbI3‐based PSCs is systematically investigated. These additives can both chelate onto lead ions and form hydrogen bond with methyl ammonium ions. These combined interactions result in an increased activation energy for nucleation of perovskite crystals, thereby, increasing crystal size, reducing defect formation, improving electronic properties, as well as reducing ion migration. As a result, PSCs added with 5F‐B achieve the highest power conversion efficiency (PCE) of 20.50% with a narrow distribution compared to those PSC devices added with 1F‐B (19.25 %), 0F‐B (18.80 %), and pristine devices (18.53 %). Notably, 5F‐B‐added PSCs retain 80% of their initial PCE after ≈100‐day humidity test (at 25 °C and 50% relative humidity), 30‐day thermal stability test (at 85 °C in nitrogen environment), and 12‐day light illumination test (under continuous simulated solar radiation).

Importance of Low Humidity and Selection of Halide Ions of Octylammonium Halide in 2D–3D Perovskite Solar Cells Fabricated in Air

AbstractAir fabrication of efficient and stable perovskite solar cells with substantial reproducibility is of importance for commercialization of perovskite solar cells. There has been little consensus on which halide is the best selection for thin 2D perovskite layer on 3D perovskite when processing in ambient air. In this work, the influence of humidity and halide ions in alkylammonium halide (OAX)‐treated perovskite layer on photovoltaic performance is investigated. The authors find that a combination of high humidity and the presence of chloride ions can induce the deprotonation of methylammonium (MA+) ions. With X‐ray diffraction, cross‐sectional scanning electron microscope, Fourier‐transform infrared spectroscopy, the authors elucidate that the deprotonation reaction in the octylammonium chloride (OACl)‐treated perovskite layer makes MA+ ions (CH3NH3+) volatile MA (CH3NH2), which results in void volumes and defects in the bulk of perovskite layer as well as at the interface. The defective perovskite solar cells induced by the deprotonation reaction have the suppressed hole transfer at the interface between the perovskite layer and the hole transport layer, yielding the reduced internal quantum efficiency and JSC. Space charge‐limited current analysis reveals that the reduced VOC is caused by the high trap density estimated from VTFL of hole‐only devices.

Solvent-Free Mechanochemical Dense Pore Filling Yields CPO-27/MOF-74 Metal–Organic Frameworks with High Anhydrous and Water-Assisted Proton Conductivity

Proton-conducting solids operating in both anhydrous and humid conditions are of paramount importance for the development of fuel cells. We demonstrate that incorporating formamidinium or methylammonium ions in CPO-27/MOF-74 metal–organic frameworks renders them highly conductive under the two conditions. Highest proton conductivities reach 8 × 10–4 S/cm under anhydrous conditions, exceed 10–3 S/cm already at low relative humidity (30–50% RH) and low temperatures (25–60 °C), and reach 10–2 S/cm at 60 °C and 70% RH. The dense pore filling with the protic cations is enabled by solvent-free mechanochemistry and stoichiometric saturation of open metal sites (Mg, Ni) with thiocyanate ligands. DFT modeling reveals that high anhydrous proton conductivity is associated with the presence of water-free extended networks of hydrogen bonds along the [001] channel and together with electrochemical impedance measurements (EIS) shed light on the mechanism of proton transport.

BT-MA0.6FA0.4PbI3–xClx Unsymmetrical Perovskite for Solar Cells with Superior Stability and PCE over 23%

Combining superior stability and high power conversion efficiency (PCE) is a challenge in 3D (three-dimensional) and 2D (two-dimensional) perovskite solar cell (PSC) research. Mitigating the limitations of the symmetrical 3D structure and 2D charge transmissive channel has also not been achieved. Analogous 2D unsymmetrical perovskite structure is an excellent strategy to establish a balance between stability and PCE. Here, the BT (1,2,4,5-benzenetetramine tetrahydrochloride)-MA0.6FA0.4PbI3–xClx analogous 2D unsymmetrical perovskite film is applied to improve the PCE and stability of PSCs. The BT groups substitute the formamidinium and methylammonium ions, interact with lead trihalide to form a bond, and prevent the formation of vacancy traps from amino groups. The high electron density and extensive delocalized electrons of π-conjugated heterocycles of BT enhance charge transport and separation. The chloride from BT provides a rich halogen environment, which further improves the quality of the perovskite film. The BT-MA0.6FA0.4PbI3–xClx PSC exhibits superior PCE and stability, simultaneously. The champion PCE is up to 23.33%, which is also a record PCE for MAPbI3–xClx PSCs. Compared with the pristine MA0.6FA0.4PbI3–xClx PSC, the stability of the optimized device is improved by 10–20% under different conditions. The enhanced performance is ascribed to the stable unsymmetrical structure, excellent charge transport, and high quality of the perovskite film.

Zwitterion‐Functionalized SnO2 Substrate Induced Sequential Deposition of Black‐Phase FAPbI3 with Rearranged PbI2 Residue

Black-phase formamidinium lead iodide (FAPbI3 ) with narrow bandgap and high thermal stability has emerged as the most promising candidate for highly efficient and stable perovskite photovoltaics. In order to overcome the intrinsic difficulty of black-phase crystallization and to eliminate the lead iodide (PbI2 ) residue, most sequential deposition methods of FAPbI3 -based perovskite will introduce external ions like methylammonium (MA+ ), cesium (Cs+ ), and bromide (Br- ) ions to the perovskite structure. Here a zwitterion-functionalized tin(IV) oxide (SnO2 ) is introduced as the electron-transport layer (ETL) to induce the crystallization of high-quality black-phase FAPbI3 . The SnO2 ETL treated with the zwitterion of formamidine sulfinic acid (FSA) can help rearrange the stack direction, orientation, and distribution of residual PbI2 in the perovskite layer, which reduces the side effect of the residual PbI2 . Besides, the FSA functionalization also modifies SnO2 ETL to suppress deep-level defects at the perovskite/SnO2 interface. As a result, the FSA-FAPbI3 -based perovskite solar cells (PSCs) exhibit an excellent power conversion efficiency of up to 24.1% with 1000 h long-term operational stability. These findings provide a new interface engineering strategy on the sequential fabrication of black-phase FAPbI3 PSCs with improved optoelectronic performance.

Room-temperature cost-effective in-situ grown MAPbBr3 crystals and their characterization towards optoelectronic devices

We report the in-situ, room-temperature synthesis of methylammonium lead bromide CH3NH3PbBr3 crystals using N-methyl formamide as a source of methylammonium (MA+) ions during the crystallization process to explore the structural, dielectric, and electronic properties of CH3NH3PbBr3 crystals for optoelectronic applications. Optical absorption and radio-luminescence measurements affirm the direct bandgap nature of the crystals. Impedance spectroscopy measurements with various applied AC voltages within the 20 Hz–10 MHz frequency range depict the influence of ionic motions on electrical transport across crystal planes. We have extracted electrical transport parameters in CH3NH3PbBr3 crystals from the Nyquist plots, which we found to be distinctly varied wherein two different AC voltage amplitude regimes, broadly for 10–50 mV and 100–500 mV AC voltage range.

Diaminobenzene Dihydroiodide‐MA0.6FA0.4PbI3−xClx Unsymmetrical Perovskites with over 22% Efficiency for High Stability Solar Cells

Abstract2D perovskites exhibit limited charge transfer and a stable unsymmetrical structure. Hence, a 2D perovskite solar cell (PSC) is more stable but less efficient than a 3D PSC. An effective combination of good stability and high power conversion efficiency (PCE) is desirable in PSCs. A novel diaminobenzene dihydroiodide‐MA0.6FA0.4PbI3−xClx analogous 2D unsymmetrical perovskite is designed to further enhance PSC performance. The two amino groups of diaminobenzene dihydroiodide (DD) function similarly as the amino groups of methylammonium and formamidinium ions. Therefore, diaminobenzene dihydroiodide can replace methylammonium and formamidinium and create better bonding interaction with the lead trihalide. The analogous 2D unsymmetrical perovskite not only possesses sufficient charge transfer but also exhibits high stability with an appropriate incorporation of DD. Noticeably, the champion device shows a PCE of 22.34%, setting a new record for an MAPbI3−xClx based PSC. The thermal, illumination, and environmental stability is enhanced by 20%–30%. The improved PCE and stability is attributed to better charge transfer and stable structure.

Frequency and temperature-dependent dielectric characteristics of lead-free Br doped perovskites (CH3NH3)3Bi2Cl9 and (CH3NH3)3Bi2BrxCl9-x

We report the synthesis and dielectric characteristics of lead-free methylammonium bismuth chloride (MABiCl) and Br doped methylammonium bismuth chloride (MABiBrCl) in powder form. The morphology of the samples showed growth of large flakes with large surface area. The band gaps of the perovskites are obtained from optical absorption data. The dielectric characteristics of the samples were measured in the frequency range 1 Hz to 1 MHz and temperature range 100–300 K. The data shows a complex electrical transport in these halide perovskite materials. The dielectric permittivity data shows the existence of a critical temperature at which the material undergoes a structural phase transition. The transition temperature is found to be 270 K and 266 K for MABiCl and MABiBrCl respectively. The results can be explained by certain change in orientation of methyl-ammonium (MA) ions as the temperature is increased.

FIRST-PRINCIPLE MODELING OF STRUCTURAL, ELECTRONIC, AND OPTICAL PROPERTIES OF CH3NH3PbBr3

Organic-inorganic hybrid perovskites have recently attracted attention due to their potential applications in solar cells, light-emitting, and photovoltaic devices. In this work, we focused on the study within the density functional theory of the structural, electronic, and optical properties of three temperature-dependent phases of perovskite CH 3 NH 3 PbBr 3 . We present basic parameters such as lattice parameters, electronic structures, effective charge masses, and optical absorption spectra to understand the fundamental properties of the investigated perovskite. The change in the properties of the studied crystalline phases was analyzed using various approximations for the exchange-correlation functionals. For our theoretical calculations, exchange−correlation functional was chosen in the GGA-PBE and GGA-PBEsol parametrizations, also with Hubbard correction to GGA (GGA-PBEsol+U). The U parameter was selected at 5 eV and applied to Br p-states. The calculation of the electronic properties of the CH 3 NH 3 PbBr 3 perovskite shows the change in the electronic spectra was taken into account when the spin-orbit coupling (SOC) was included in the calculations. The results of band structure calculations showed that cubic, tetragonal, and orthorhombic perovskite crystal phases have semiconductor properties with direct band gaps. The use of the GGA-PBEsol and GGA-PBEsol+U methods gives the results closest to the experimental ones. We observed a radical change in the electronic spectrum when the SOC effect is included in the calculations, which leads to a reduction in the bandgap. Further explanation of the electronic bandgap nature of CH 3 NH 3 PbBr 3 crystals is performed through the total (DOS) and partial density of state (PDOS) analysis. For all crystalline phases, considered perovskites showed a similar DOS distributions, which may be divided by three regions. The first region is formed by the methylammonium ion p-states, which forms narrow bands located deep about at −8eV. The second region at −6.0 – 0 eV within the valence band complex, characterizes with the largest contribution being observed from hybrid Br p-orbitals and Pb s-orbitals. The third region is placed from 2.44 to 7.0 eV for cubic, from 2.24 to 6.9 eV for tetragonal, and from 2.35 to 6.3 eV for orthorhombic phases, respectively. The conduction band minimum is mainly formed by hybridized states of the Pb p-orbitals and a small contribution of Br p-orbitals. Calculations of the values of the effective masses in the three main directions for all crystalline phases were also carried out. The optical properties show that the CH 3 NH 3 PbBr 3 perovskite has a good ability for photon absorption and demonstrates the possibility of its use in a wide temperature range. Key words : density functional theory, band gap, perovskite, electronic structure, density of states, optical properties

Probing the Role of Local Structure in Driving the Stability of Halide Perovskites CH3NH3PbX3

Organic–inorganic lead halide perovskites have rapidly emerged as one of the leading contenders in optoelectronic and photovoltaic technology, but they show stability issues. Among methylammonium (MA) lead bromide and MA lead iodide, the Br counterpart is reported to exhibit improved stability under ambient conditions. However, the underlying fundamentals of such an enhancement are not fully understood. Using temperature-dependent X-ray diffraction and extended X-ray absorption fine structure measurements, we find relatively reduced value of linear coefficient of thermal expansion, elevated Einstein temperature, and reduced dynamic disorder in MAPbBr3 compared to MAPbI3 which implies that a relatively rigid Pb–Br bond offers steric hindrance to the migration of bulky MA ion resulting in improved stability of MAPbBr3.

Methylammonium- and bromide-free perovskites enable efficient and stable photovoltaics

Hybrid perovskite solar cell (PSC) has attracted extensive research interest due to its rapid increase in efficiency, regarding as one of the most promising candidates for the next-generation photovoltaic technology. The certified power conversion efficiency of the devices based on formamidinium lead iodide (FAPbI3) perovskite has reached 25.5%, approaching the record of monocrystalline silicon solar cells. Unfortunately, the black α-phase FAPbI3 materials can spontaneously transform to non-optically active δ-phase at room temperature, which greatly hinder their photovoltaic application. In order to overcome this problem, various strategies, especially introducing methylammonium (MA+), caesium (Cs+) and bromide (Br-) ions into the materials, have been widely adopted. However, MA+ can largely reduce the thermal stability of the materials. Furthermore, the introduction of Br- can enlarge the materials’ bandgap, resulting in a reduced theoretical efficiency. Keeping these in mind, developing the strategies which without using MA+ and Br- is the inevitable trend. Here, we focus on the recent progresses of stabilizing α FAPbI3 without employing MA+ and Br-, and discuss the advantages of inorganic ions doping and dimensionality engineering to stabilized α FAPbI3. Meanwhile, in order to deeply understand the relationship between the semiconducting properties and device performance of the corresponding materials, we then summarize several significant strategies to suppress the non-radiation recombination, such as interface modification and trap passivation. Finally, we propose to develop more effective ‘A-site’ alternatives to stabilize α FAPbI3, which is expected to achieve high-efficient PSCs with long-term stability, facilitating its commercialization process.

Surface Stabilization of a Formamidinium Perovskite Solar Cell Using Quaternary Ammonium Salt.

Dimensionality engineering is an effective approach to improve the stability and power conversion efficiency (PCE) of perovskite solar cells (PSCs). A two-dimensional (2D) perovskite assembled from bulky organic cations to cover the surface of three-dimensional (3D) perovskite can repel ambient moisture and suppress ion migration across the perovskite film. This work demonstrates how the thermal stability of the bulky organic cation of a 2D perovskite affects the crystallinity of the perovskite and the optoelectrical properties of perovskite solar cells. Structural analysis of (FAPbI3)0.95(MAPbBr3)0.05 (FA = formamidinium ion, MA = methylammonium ion) mixed with a series of bulky cations shows a clear correlation between the structure of the bulky cations and the formation of surface defects in the resultant perovskite films. An organic cation with primary ammonium structure is vulnerable to a deprotonation reaction under typical perovskite-film processing conditions. Decomposition of the bulky cations results in structural defects such as iodide vacancies and metallic lead clusters at the surface of the perovskite film; these defects lead to a nonradiative recombination loss of charge carriers and to severe ion migration during operation of the device. In contrast, a bulky organic cation with a quaternary ammonium structure exhibits superior thermal stability and results in substantially fewer structural defects at the surface of the perovskite film. As a result, the corresponding PSC exhibits the PCE of 21.6% in a reverse current-voltage scan and a stabilized PCE of 20.1% with an excellent lifetime exceeding 1000 h for the encapsulated device under continuous illumination.

A Synergistic Precursor Regulation Strategy for Scalable Fabrication of Perovskite Solar Cells

The nucleation and crystallization processes within solution‐processed deposition is key for obtaining perovskite films with high quality. Herein, the synergistic effect of Cs+, ligand solvent (tetramethylene sulfoxide, TMSO), and methylammonium ions (MA+) on nucleation and crystallization procedures of preparing perovskites films is studied. The results reveal that producing α‐phase nuclei in the intermediate‐phase film (a perovskite film formed by removing the solvent before annealing) is the key factor for obtaining α‐phase‐pure perovskites, wherein Cs+ is necessary for reducing the formation energy of the α‐phase. In addition, the nonvolatile ligand (TMSO) is used to inhibit rapid nucleation, because it can form a stable intermediate phase. Furthermore, the introduction of the volatile MA+ cations compound (MACl) combined with Cs+ contributed to the direct formation of α‐phase nuclei in the intermediate‐phase film, and ion exchange (between FA+ and MA+) occurs during the subsequent annealing process, which leads to the formation of large‐grain perovskite film with good crystallinity. Maximum power conversion efficiencies of 22.08% and 16.88% are obtained via this strategy for a perovskite solar cell on a small area (active area: 0.09 cm2) and a perovskite solar module (aperture area: 12.92 cm2), respectively.

An ab initio anharmonic approach to IR, Raman and SFG spectra of the solvated methylammonium ion.

The methylammonium ion (CH3NH3+, or noted as MA-H+) is one of the smallest organic ammonium ions that play important roles in organic-inorganic halide perovskites. Despite the simple structure, the vibrational spectra of MA-H+ exhibit complicated features in the 3 μm region which are sensitive to the solvation environment. In the present work, we have applied the ab initio anharmonic algorithm at the CCSD/aug-cc-pVDZ level to simulate the IR and Raman spectra of the solvated methylammonium ion, MA-H+⋯X3, where X denotes the solvent molecules, to understand the Fermi resonance mechanism in which the overtones of NH bending modes are coupled with the fundamentals of NH stretching modes. The spectral features of the solvated clusters with proper solvent species resemble those observed in the perovskite crystal, indicating that they have similar solvation environments and hydrogen bond interactions. Therefore, a linkage between the gas-phase cluster models and the condensed-phase materials can be established, and our simulations and anharmonic analyses help in interpreting the spectral assignments of the observed IR and Raman spectra of perovskites reliably. Furthermore, we have extended this approach to the SFG spectra to demonstrate the selective appearance of bands depending on both the beam polarization configurations and the symmetry of vibrational modes.

Fine-tuning interlayer spacing in MoS2 for enriching 1T phase via alkylated ammonium ions for electrocatalytic hydrogen evolution reaction

An environmentally benign pathway for commercial production of molecular hydrogen can be made by electrocatalytic water splitting, which requires highly active catalyst like Pt. Nowadays, 1T MoS2 is recognized as a good alternate to Pt in hydrogen evolution reaction (HER) by virtue of its dense active sites, hydrophilicity and enhanced electronic conductivity. However the formation of such phase is still challenging as being 2H is the most stable phase of MoS2. Here, a simple hydrothermal approach is followed, wherein alkylated ammonium ion introduction between layers of MoS2, was employed for 1T phase enriched MoS2 formation, aiming to elucidate the influence of the intercalated ions on the interlayer expansion as well as the selective formation of the 1T phase. Methyl ammonium ion was found to be the most apt for 1T phase enrichment as over 70% was achieved and the resultant MoS2 exhibits magnificent HER activity with a low Tafel slope of 56 mV/dec and required just 270 mV overpotential to attain a current density of 10 mA cm−2 with robust durability. The optimum variations in interlayer, rich 1T phase formation are the key findings of this study. This investigation opens up new strategy i.e., interlayer expansion induced enrichment of 1T phase via alkylated ammonium ions will be highly useful for other 2D layered materials in near future for energy related HER applications.

In situ NMR Investigation of the Photoresponse of Perovskite Crystal

Summary Hybrid perovskites are outstanding candidates for use in optoelectronic devices. Nonetheless, hybrid perovskites present numerous fundamental issues that are not yet fully understood. This study seeks to address this problem by reporting on the occurrence of ultraslow dynamic orientation changes in the methylammonium (MA) cations and the corresponding changes in the inorganic lattice of MAPbI3 perovskite crystals occurring over a time scale of 100 s under continuous illumination based on in situ single-crystal 2H nuclear magnetic resonance and in situ X-ray diffraction results. The demonstrated ultraslow dynamic changes in MAPbI3 are far slower than elementary molecular motions typically having time scales of femtoseconds and picoseconds and have never been reported previously. The ultraslow dynamic changes have been correlated with the reduction of band gap and the increase of absorption coefficient of MAPbI3, providing the molecular origins of the stabilization process in the photoresponse of the MAPbI3 device.

Bioinspired Molecular Bridging in a Hybrid Perovskite Leads to Enhanced Stability and Tunable Properties

AbstractHybrid perovskites demonstrate high potential in optoelectronic applications. Their main drawback is their low stability under humid conditions. In this paper, one of nature's strategies is implemented—the incorporation of amino acids into the crystal lattice—in order to improve the stability of methylammonium lead bromide (MAPbBr3) in water, and to tune its structure, as well as its optical and thermal properties. The amino acid lysine, which possesses two NH3+ groups, is incorporated into the hybrid unit cell, by substituting two methylammonium ions and serves as a “molecular bridge”. This incorporation induces a decrease in the lattice parameter of the host, accompanied with an increase in its bandgap and noticeable changes in its morphology. Furthermore, a substantial decrease in the thermal expansion coefficient of MAPbBr3 and a shift of its cubic‐to‐tetragonal phase transformation temperature are observed. Two different modes of incorporation are identified, which depend on the conditions of crystallization. These modes dictate the level of lysine incorporation and the magnitude of MAPbBr3 bandgap changes. Notably, lysine incorporation strongly increases the perovskite stability in water. This study demonstrates a unique and promising approach to tune the properties and improve the stability of hybrid perovskites via this novel bioinspired route.

Hybrid Perovskites with Larger Organic Cations Reveal Autocatalytic Degradation Kinetics and Increased Stability under Light

Hybrid organic-inorganic perovskites have shown incredible promise as active materials for photovoltaic devices, but their instability to light remains a significant roadblock in realizing these applications. Changing the organic cation has been shown to affect light-induced degradation. As a strategy for increasing the stability of these materials, we replaced varying percentages of methylammonium ion in the archetypical methylammonium lead iodide (MAPbI3) hybrid organic-inorganic perovskite with three significantly larger organic ammonium cations: imidazolium, dimethylammonium, and guanidinium. We were able to synthesize hybrid organic-inorganic perovskites with the same 3D perovskite structure as MAPbI3 with substitution of the larger ions as high as 20-30%. These substituted hybrid organic-inorganic perovskites retained similar optoelectronic properties. We discovered that the light-induced degradation in MAPbI3 and its substituted derivatives is autocatalytic, and we calculated rate coefficients for the degradation. All of the substituted hybrid organic-inorganic perovskites showed light-induced degradation slower than that of MAPbI3, up to a 62% decrease in degradation rate coefficient, at all substitution percentages up to 20%. This work provides evidence that a high percentage of a variety of large ammonium cations can be substituted into the hybrid organic-inorganic perovskite lattice without compromising its desirable optoelectronic properties. Insight into the autocatalytic mechanism of light-induced degradation will be valuable for designing additional strategies to improve the stability of hybrid organic-inorganic perovskites. We also offer insights into how factors other than size, such as hydrogen bonding, influence the stability of the materials. Overall, we have shown that substitution of methylammonium ion for the much larger imidazolium, dimethylammonium, and guanidinium cations in MAPbI3 is a valid strategy for creating stable hybrid organic-inorganic perovskite derivatives by slowing the rate of light-induced degradation.

Improved the Performance and Stability at High Humidity of Perovskite Solar Cells by Mixed Cesium-Metylammonium Cations

Perovskite solar cells have a great potential as competitor of silicon solar cells which have been dominated the market of solar cells since last decade, due to a tremendous improvement of their power conversion efficiency (PCE). Recently, a PCE of perovskite solar cells above 23% have been obtained. Moreover, perovskite solar cells can be fabricated using simple solution methods, therefore, the whole cost production of solar cells is less than half of silicon solar cells. However, their low stability in thermal and high humidity hinder them to be produced and commercially used to replace silicon solar cells. Many efforts have been done to improve both PCE and stability, including mixed inorganic-organic cations, mixed halide anions, improvement of perovskite morphology or crystallinity and using small molecules for passivation of defect in perovskite. In this paper, we used mixed cesium-methylammonium to improve both PCE and stability of perovskite solar cells. Cesium was used due to its smaller ionic radius than methylammonium (MA) ions, therefore, the crystal structure of perovskite is not distorted. Moreover, perovskite cesium-lead-bromide (CsPbBr3) are more stable than that of MAPbBr3 and doping cesium increased light absorption in perovskite MAPbBr3. We studied the effect of mixed cesium-MA on the PCE and stability at high humidity (>70%). The percentage of cesium was varied at 0%, 5%, 10%, 15% and 20%. The perovskite solar cells have monolithic hole-transport layer free (HTL-free) structure using carbon as electrode. This structure was used due simple and low cost in processing of solar cells. Our results showed that by replacing 10% of MA ions with Cs ions, both PCE and stability at high humidity are improved.

Structural phase transitions and photoluminescence mechanism in a layer of 3D hybrid perovskite nanocrystals

Although structural phase transitions in single-crystal hybrid methyl-ammonium (MA) lead halide perovskites (MAPbX3, X = Cl, Br, I) as a function of temperature are common phenomena, they have never been observed in the corresponding nanocrystals. Here, we demonstrate that two-photon-excited photoluminescence (PL) spectroscopy is capable of monitoring structural phase transitions in MAPbX3 nanocrystals because nonlinear susceptibilities govern the incident light absorption rates. We provide experimental evidence that the orthorhombic-to-tetragonal structural phase transition in a single layer of 20-nm-sized 3D MAPbBr3 nanocrystals is spread out within the T ∼ 70 K–140 K temperature range. This structural phase instability is believed to arise because, unlike in single-crystal MAPbX3, free rotations of MA ions in the corresponding nanocrystals are no longer restricted by a long-range MA dipole order. The resulting configurational entropy loss can be even enhanced by the interfacial electric field arising due to charge separation at the MAPbBr3/ZnO heterointerface, extending the structural phase instability range from T ∼ 70 K–230 K. We conclude that weak sensitivity of conventional one-photon-excited PL spectroscopy to structural phase transitions in 3D MAPbX3 nanocrystals results from structural phase instability and hence from negligible distortions of PbX6 octahedra. In contrast, the intensity of two-photon-excited PL and electric-field-induced one-photon-excited PL show higher sensitivity since nonlinear susceptibilities are involved. We also show that room-temperature PL may originate from the radiative recombination of the optical-phonon vibrationally excited polaronic excitons with energies might exceed the ground-state Fröhlich polaron and Rashba energies due to optical-phonon bottleneck.