Humidity controlled crystallization of thin CH3NH3PbI3 films for high performance perovskite solar cell

Control of crystallization of a solution‐processed perovskite layer is of prime importance for high performance solar cells. In spite of the negative effect of water on perovskite solar energy conversion in several previous works, we observed that humidity plays a critical role to develop a thin uniform, dense perovskite film with preferred crystals, in particular, in a device with architecture of ITO/PEDOT:PSS/CH₃NH₃PbI₃/ PC₇₁BM/LiF/Al fabricated by two‐step sequential spin‐coating process. Humidity controlled spin‐coating of CH₃NH₃I on the pre‐formed PbI₂ layer was the most influential process and systematic structural investigation as a function of humidity revealed that grains of CH₃NH₃PbI₃ perovskite crystals increase in size with their preferred orientation while film surface becomes roughened as the humidity increases. The performance of a device was closely related to the humidity dependent film morphology and in 40% relative humidity, the device exhibited the maximum power conversion efficiency of approximately 12% more than 10 times greater than that of a device fabricated at 20% humidity. The results suggest that our process with controlled humidity can be another efficient route for high performance and reliable perovskite solar cells. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Emission properties of sequentially deposited ultrathin CH3NH3PbI3/MoS2 heterostructures

Hybrid organic-inorganic perovskite materials have obtained considerable attention due to their exotic optoelectronic properties and extraordinarily high performance in photovoltaic devices. Herein, we successively converted the ultrathin PbI₂/MoS₂ into the CH₃NH₃PbI₃/MoS₂ heterostructures via CH₃NH₃I vapor processing. Atomic force microscopy (AFM)、Scanning electron microscopy (SEM) and X-ray photoemission spectroscopy (XPS) measurements prove the high-quality of the converted CH₃NH₃PbI₃/MoS₂. Both MoS₂ and CH₃NH₃PbI₃ related photoluminescence (PL) intensity quenching in CH₃NH₃PbI₃/MoS₂ implies a Type-II energy level alignment at the interface. Temperature-dependent PL measurements show that the emission peak position shifting trend of CH₃NH₃PbI₃ is opposite to that of MoS₂ (traditional semiconductors) due to the thermal expansion and electron-phonon coupling effects. The CH₃NH₃PbI₃/TMDC heterostructures are useful in fabricating innovative devices for wider optoelectronic applications.     

Nonhydrogenic exciton spectrum in perovskite CH3NH3PbI3

The excitons in the orthorhombic phase of the perovskite CH₃NH₃PbI₃ are studied using the effective mass approximation. The electron–hole interaction is screened by a distance‐dependent dielectric function, as described by the Haken potential or the Pollmann–Büttner potential. The energy spectrum and the eigenfunctions are calculated for both cases. The results show that the Pollmann–Büttner model, using the corresponding parameters obtained from ab initio calculations, provides better agreement with the experimental results.

     

Sonochemical synthesis of CH3NH3PbI3 perovskite ultrafine nanocrystal sensitizers for solar energy applications

The organic–inorganic hybrid perovskite CH₃NH₃PbI₃ is becoming an interesting material in the field of energy harvesting. This material is one of the cleanest and cheapest components in solar cells which is available in ample amounts. However, most of the previous research work was done on thin film of this material. In the present work we describe the preparation of a powder containing nanoparticles of CH₃NH₃PbI₃ using a sonochemical method. Characterization of the product was done by various methods, such as HRTEM, FTIR, PL, DLS and XRD. The particles were found to be highly crystalline (tetragonal crystal structure), polygonal in shape and having diameters of 10–40 nm.     

Hole‐extraction layer dependence of defect formation and operation of planar CH3NH3PbI3 perovskite solar cells

Three planar CH₃NH₃PbI₃ (MAPbI₃) solar cells having the same structure except a hole‐extraction layer (HEL) showed distinctive difference in operation characteristics. Analysis of frequency‐dependent capacitance and dielectric‐loss spectra of the three MAPbI₃ devices showed two types of recombination‐loss channels with different time constants that we attributed respectively to interface and bulk defects. Discrepancy in defect formation among the three devices with a HEL of PEDOT:PSS, NiO_x, or Cu‐doped NiO_x was not surprising because grain‐size distribution and crystalline quality of MAPbI₃ can be affected by surface energy and morphology of underlying HELs. We were able to quantify interface and bulk defects in these MAPbI₃solar cells based on systematic and simultaneous simulations of capacitance and dielectric‐loss spectra, and current–voltage characteristics by using the device simulator SCAPS.

     

Enhancing the efficiency of TiO2–perovskite heterojunction solar cell via evaporating Cs2CO3 on TiO2

In a TiO₂–perovskite heterojunction solar cell (TiO₂–PHSC), besides the perovskite CH₃NH₃PbX₃, TiO₂ as one side of the TiO₂/CH₃NH₃PbX₃ heterojunction also plays an important role in the photovoltaic effect. In order to improve the performance of the TiO₂–PHSC with the structure of glass/FTO/compact TiO₂/mesoporous TiO₂/CH₃NH₃PbI_3–xCl_x /poly‐TPD (poly(N,N ′‐bis(4‐butylphenyl)‐N,N ′‐bis(phenyl)benzidine))/Au, a 2 nanometer thick Cs₂CO₃ layer is thermally evaporated on the mesoporous TiO₂ layer. The short‐circuit current density (J_sc) raises from 17.7 mA cm^–2 to 18.9 mA cm^–2, the open‐circuit voltage (V_oc) from 0.81 V to 0.87 V, and the fill factor (FF) from 55.2% to 67.3%; as a result, the power conservation efficiency (PCE) increases from 8.0% to 11.1% under AM 1.5G solar illumination (100 mW cm^–2). Moreover, in a TiO₂–PHSC free of mesoporous TiO₂, where Cs₂CO₃ is evaporated on the compact TiO₂ layer, the J_sc, V_oc, FF and PCE values increase from 16.0 mA cm^–2, 0.83 V, 50.8% and 6.7% to 17.9 mA cm^–2, 0.90 V, 59.3%, and 9.5%, respectively. The reasons of the PCE increase for either the first kind of TiO₂–PHSC or the mesoporous‐TiO₂‐free TiO₂–PHSC with a nanometer‐thick Cs₂CO₃ layer on mesoporous TiO₂ or compact TiO₂ are discussed. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Ultrasonically sprayed-on perovskite solar cells-effects of organic cation on defect formation of CH3NH3PbI3 films

Methylammonium lead iodide (CH₃NH₃PbI₃) based perovskite having low degrees of the disorder is of great interest for optoelectronic and photovoltaic applications. In this work, a layer of CH₃NH₃PbI₃ was successfully prepared using an ultrasonically sprayed-nebulous method. Changes in structural and optical properties alongside with photo-induced charge separation and transportation behavior were systematically studied. The surface photovoltage spectra reveal a significant reduction of the density of deep defect states as the organic content was increased. It was observed that the measured values of Urbach energies decrease from 33.36 to 28.24 meV as the amount of organic content was increased to an optimum value. The best perovskite solar cells obtained using the sprayed-on approach exhibited a J_sc of 16.54 mA/cm², a V_oc of 0.99 V, and a FF of 62.4, resulting in an overall PCE of 10.09%.     

Modulation of strain,electric field and organic cation rotation on the band gap and electronic structures of organic-inorganic hybrid perovskite CH3NH3PbI3

Band gap modulation engineering is an important step in the application of optoelectronic materials. In this paper, the first-principles calculations were carried out to study the influence of strain, external electric field, spatial orientation of organic cation on the band gaps and electronic structures of organic-inorganic hybrid halide perovskites CH₃NH₃PbI₃. The results show that both the uniform strain and the tetragonal deformation can modulate the band gap obviously. The electric field of 0.2 V/Å is the critical point of the band gap modulation. The band gap increases when an electric field is applied from 0 to 0.2 V/Å. The electric field above 0.2 V/Å will cause the band gap to decrease. The spatial orientation of the organic cation also has modulation influence on the band gap of CH₃NH₃PbI₃, but has no effect on the direct semiconductor characteristics. The above results will be helpful to study the band gap modulation of other organic-inorganic hybrid halide perovskites.     

One-dimensional excitons in inorganic–organic self-organized quantum-wire crystals [NHC(I)=NH]PbI and [CHSC(=NH)NH]PbI

Optical properties of lead-iodide-based one-dimensional perovskite-type crystals [NH₂C(I)=NH₂]₃PbI₅ and [CH₃SC(=NH₂)NH₂]₃PbI₅ have been investigated theoretically and experimentally. The electronic and excitonic structures are studied based on group theoretical consideration and first-principle band calculation. Strong one-dimensional anisotropy of the optical absorption spectra, large Stokes shifts (1.0 eV) and huge exchange energies (70 meV) indicate that the excitons in these crystals are one-dimensional Frenkel excitons.     

Multistep spin–spray deposition of large-grain-size CH3NH3PbI3 with bilayer structure for conductive-carbon-based perovskite solar cells

Large-grain-size and void-free CH₃NH₃PbI₃ films with bilayer structure are fabricated by spin-coating a PbI₂ layer onto a mesoporous TiO₂ layer and sequentially spraying CH₃NH₃I (methylammonium iodide, MAI) multilayers. The sprayer is controlled by a homemade three-axis computer numerical control machine; thus, the substrates are coated by successive parallel passes achieved by moving the nozzle. Spray deposition at the optimal spray rate and substrate temperature produces a large-grain-size and void-free methylammonium lead iodide (MAPbI₃) bilayer structure. The mesoporous TiO₂ layer plays an important role in electron transport by preventing the return of electrons to the perovskite layer and decreasing the contact resistance at the perovskite/compact TiO₂/fluorine tin oxide interface. When the films are incorporated into a solar cell device with a conductive carbon counter electrode, a maximum power conversion efficiency of 10.58% is realised.     

钙钛矿/银电极界面降解和离子迁移的原位研究

本文对CH_3NH_3PbI_3钙钛矿层与Ag电极之间的界面降解和离子迁移过程进行了全面地研究.利用原位光电子能谱检测于段,发现了Ag电极会诱导钙钛矿层的降解,导致PbI_2和AgI物种的形成以及Pb~(2+)离子在界面处还原成金属Pb物种.I 3d谱峰强度的反常增强为碘离子从CH_3NH_3PbI_3下表面迁移到Ag电极提供了直接的实验证据.此外,Ag电极和钙钛矿层接触会在CH_3NH_3PbI_3/Ag界面处诱导0.3 eV的界面偶极,这可能进一步促进碘离子扩散迁移,导致钙钛矿层的分解和Ag电极的侵蚀.     

Charge carrier decay and diffusion in organic–inorganic CH3NH3PbI3–xClx perovskite based solar cell

In recent years, organic–inorganic lead halides attracted widespread interest, mainly due to their impressive photoconversion properties and low‐cost solution processing. In this study, we employed small amplitude transient photovoltage and photocurrent spectroscopy to investigate charge transport and recombination properties of perovskite CH₃NH₃PbI_3–xCl_x solar cell under realistic light harvesting conditions (<1 sun). Cell structure resembles outlay commonly found in organic photovoltaics, with perovskite absorber being sandwiched between two thin layers of organic polymers. Tested device displayed high power conversion efficiency (10.3%), good fill factor and negligible hysteresis effect. Fundamental device parameters were characterized at various open‐circuit voltages (V_oc) by examination of small voltage and current perturbations created by the low intensity pulsed laser excitations. The obtained results exhibit long charge carrier lifetimes and fast charge transport over the full range of applied optical bias, as well as remarkable diffusion lengths exceeding 1 μm. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Photoelectrochemistry on TiO2/Ti anodes as a tool to increase the knowledge about some photo‐oxidation mechanisms in CH3CN

Through current efficiency measurements, obtained from the photoelectrochemical oxidation at TiO₂ (rutile)/Ti anodes, further mechanistic information has been obtained regarding the TiO₂ photosensitized oxidation of benzylic alcohols, ethers and 1,2‐diols in CH₃CN. In deaerated medium, two electrons are captured by the semiconductor from all the considered substrates (one from the substrate, the second from the intermediate benzylic radical). In contrast, in aerated CH₃CN, the number of TiO₂‐captured electrons can be reduced to one because, depending on its oxidizability, the benzylic radical can be competitively captured by oxygen. Copyright © 2008 John Wiley & Sons, Ltd.     

Why cannot all divalent cations completely substitute the Pb cations of CH3NH3PbI3 perovskite?

Organic-inorganic hybrid CH₃NH₃PbI₃ perovskite has a great potential for applications in low-cost photovoltaic devices. However, the doped and substitution of Pb sites in CH₃NH₃PbI₃ has not been widely reported. In this article, a quantum mechanical model was applied to determine why all divalent cations cannot substitute the Pb cations of CH₃NH₃PbI₃ perovskite. The evaluation was performed by comparison the model with experimental results. On this basis, we carefully examined 42 types of cations and identified only nine kinds of cations including Ca²⁺, Sr²⁺, Sc²⁺, Ti²⁺, V²⁺, Y²⁺, Zr²⁺, Nb²⁺ and Sn²⁺ for doped into Pb sites. In these cases, it is expected that the corresponding compound would be single phase. Finally, an analysis was performed based on first principle, and the results indicate that divalent cations substituting the Pb sites modify the band structure and influence the performance of perovskite-based photostatics.     

Theoretical study on the reactions of CH3NHNH2 with ground state O(3P) atom and excited state O(1D) atom

The reaction mechanisms of methylhydrazine (CH₃NHNH₂) with O(³P) and O(¹D) atoms have been explored theoretically at the MPW1K/6-311+G(d,p), MP2/6-311+G(d,p), MCG3-MPWPW91 (single-point), and CCSD(T)/cc-pVTZ (single-point) levels. The triplet potential energy surface for the reaction of CH₃NHNH₂ with O(³P) includes seven stable isomers and eight transition states. When the O(³P) atom approaches CH₃NHNH₂, the heavy atoms, namely N and C atoms, are the favourable combining points. O(³P) atom attacking the middle-N atom in CH₃NHNH₂ results in the formation of an energy-rich isomer (CH₃NHONH₂) followed by migration of O(³P) atom from middle-N atom to middle-H atom leading to the product P6 (CH₃NNH₂+OH), which is one of the most favourable routes. The estimated major product CH₃NNH₂ is consistent with the experimental measurements. Reaction of O(¹D) + CH₃NHNH₂ presents different features as compared with O(³P) + CH₃NHNH₂. O(¹D) atom will first insert into C–H2, N1–H4, and N2–H5 bonds barrierlessly to form the three adducts, respectively. There are two most favourable paths for O(¹D) + CH₃NHNH₂. One is that the C–N bond cleavage accompanied by a concerted H shift from O atom to N atom (mid-N) leads to the product P_I (CH₂O + NH₂NH₂), and the other is that the N–N bond rupture along with a concerted H shift from O to N (end-N) forms P_IV (CH₃NH₂ + HNO). The similarities and discrepancies between two reactions are discussed.     

Vibrational analysis of ethylamines: Trans and gauche forms

Starting from force constant values calculated by an ab initio MO method ($\text{4-31G(N}{}^1\text{)}$), and by adjusting the diagonal elements, a practical force constant matrix (F) has been reached which could explain the observed infrared and Raman spectra (in the frequency range lower than 2000 cm^?1) of the gauche form of the ethylamine CH₃CH₂NH₂ molecule and five isotopic species CH₃¹³CH₂NH₂, CH₃CH₂¹⁵NH₂, CH₃CD₂NH₂, CH₃CH₂ND₂, and CD₃CD₂NH₂. The F matrix for the trans form of ethylamine was constructed by transferring ab initio $\text{4-31G(N}{}^1\text{)}$ values and by revising diagonal elements with conversion factors whose values are equal to the corresponding values of gauche form. A nearly complete set of assignments was achieved of the vibrational bands of ethylamines, observed so far in the spectral range 2000–100 cm^?1. In matrix isolation spectroscopy, two bands assignable to the NH₂ wagging vibrations of gauche and trans forms have been found at 775 and 782 cm^?1, respectively, for CH₃CH₂NH₂. They are at 768 and 774 cm^?1, respectively, for CD₃CD₂NH₂. From the intensity changes of these bands observed on changing the nozzle temperature in the matrix formation, the energy difference ΔE (gauche-trans) of these two conformers has been estimated to be 100 ± 10 cm^?1.     

Raman spectroscopy study of the Fermi resonance between a local vibration and the two‐phonon spectrum in PbI2 crystal

The problem of complex Fermi resonance (FR) involving a local vibration is considered. It is shown for the first time that in the PbI₂ crystal, owing to the features of phonon bands, the following two effects superpose on each other: (1) the traditional FR between the fundamental Ω(A_1g) vibration and the two‐phonon states (TPS) arising from the 2Ω(E_u) vibrations, and (2) the FR between the impurity vibration and the same TPS band. It is shown theoretically that in the case of such complex FR the shape of the spectrum strongly depends on the impurity concentration, anharmonic constants and the phonon band structure. An analysis of experimental data and comparison with theory is made. Copyright © 2008 John Wiley & Sons, Ltd.     

Trapping levels in PbI2

A first investigation on trapping levels in PbI₂, performed by the Thermally Stimulated Current (TSC) technique, is presented. Three hole trapping centers are evidenced at 0.12, 0.29 and 0.59 eV above the valence band, with densities ranging between 8 × 10¹⁴ and 5 × 10¹⁶ cm^?3 and capture cross-sections between 8 × 10^?21 and 3 × 10^?17 cm². The center at 0.59 eV is likely responsible for the relatively short trapping time in PbI₂, as determined with nuclear techniques. By using a particular method, the behaviour of hole drift mobility along the layers as a function of temperature is determined for the first time. Finally, the presence of a spurious peak, not sensitive to irradiation, is reported and discussed.     

Adsorption and diffusion behaviors of H2, H2S,NH3, CO and H2O gases molecules on MoO3 monolayer: A DFT study

Molybdenum trioxide (MoO₃) with α-phase is a promising material for gas sensing because of its high sensitivity, fast response and thermodynamic stability. To probe the mechanism of superior gas detection ability of MoO₃ monolayer, the adsorption and diffusion of H₂, H₂S, NH₃, CO and H₂O molecules on two-dimensional (2D) MoO₃ layer are studied via density functional theory (DFT) calculations. Based on calculated adsorption energies, density of states, charge transfer, diffusion barriers and diffusion coefficient, MoO₃ shows a superior sensitive and fast response to H₂ and H₂S than CO, NH₃, H₂O, which is consistent with experimental conclusions. Moreover, the response of MoO₃ to H₂S and H₂ will be obviously enhanced at high gas concentration, and the incorporation of H₂ and H₂S results in an obvious increasing in DOS near Fermi level. Our analysis provides a conceptual foundation for future design of MoO₃-based gas sensing materials.     

A versatile sonication-assisted deposition–reduction method for preparing supported metal catalysts for catalytic applications

This work aims to develop a rapid and efficient strategy for preparing supported metal catalysts for catalytic applications. The sonication-assisted reduction–precipitation method was employed to prepare the heterogeneous mono- and bi-metallic catalysts for photocatalytic degradation of methyl orange (MO) and preferential oxidation (PROX) of CO in H₂-rich gas. In general, there are three advantages for the sonication-assisted method as compared with the conventional methods, including high dispersion of metal nanoparticles on the catalyst support, the much higher deposition efficiency (DE) than those of the deposition–precipitation (DP) and co-precipitation (CP) methods, and the very fast preparation, which only lasts 10–20 s for the deposition. In the AuPd/TiO₂ catalysts series, the AuPd(3:1)/TiO₂ catalyst is the most active for MO photocatalytic degradation; while for PROX reaction, Ru/TiO₂, Au–Cu/SBA-15 and Pt/γ-Al₂O₃ catalysts are very active, and the last one showed high stability in the lifetime test. The structural characterization revealed that in the AuPd(3:1)/TiO₂ catalyst, Au–Pd alloy particles were formed and a high percentage of Au atoms was located at the surface. Therefore, this sonication-assisted method is efficient and rapid in the preparation of supported metal catalysts with obvious structural characteristics for various catalytic applications.