We study the influence of electrical biasing on the modification of the chemical composition and electrical performance of perovskite solar cells (PSCs) by coupling electrochemical impedance spectroscopy (EIS) and scanning transmission X-ray microscopy (STXM) techniques. EIS reveals the formation of charge accumulation at the interfaces and changes in the resistive and capacitive properties. STXM study on PSCs after applying a strong electric field for a long biasing time indicates the breakdown of methylammonium (MA+) cation, promoting iodide ions to migrate and create defects at the interface. This complementary EIS and STXM study allows the suggestion of a degradation mechanism that includes the migration of iodide ions that leads to interface defects and subsequent degradation of solar cell performance. In addition, we study the evolution of the performance of PSCs under air. We observe an increased hysteresis index on current-voltage curves and fill factor reduction of the perovskite solar cells with aging in air. EIS measurements show the formation of a capacitive layer resulting from the accumulation of iodide ions through modification of the mobile ion concentration and ion mobility.

Metal-halide perovskites are complex materials with outstanding optoelectronic properties. Thus it is of interest to analyze these materials by using every available research tool. Synchrotron tools have played an important role in fundamental and applied research for decades. Many synchrotron-based hard X-ray tools are already providing effective feedback to the perovskite solar cell (PSC) research community. With several fourth-generation light sources up and running or under development, this contribution will continue to impact every aspect of scientific advancement including PSC research. Arguably, the contribution of soft X-rays in PSC research is relatively limited. In view of the developments in the synchrotron world and the fact that a multimethod approach, combining laboratory-based techniques as well as synchrotron-based techniques, is necessary to provide constructive feedback to the PSC community we present here a collection of arguments and procedures with the aim of highlighting the use of soft X-ray scanning transmission X-ray microscopy (STXM). Some aspects of these arguments are elaborated with STXM investigation of perovskite material formamidinium-methylammonium lead iodide (FA1−xMAxPbI3).

Organic–inorganic metal halide perovskites (MHPs) have recently been receiving a lot of attention due to their newfound application in optoelectronic devices, including perovskite solar cells (PSCs) which have reached power conversion efficiencies as high as 25.5%. However, the fundamental mechanisms in PSCs, including the correlation of degradation with the excellent optoelectrical properties of the perovskite absorbers, are poorly understood. In this paper, we have explored synchrotron-based soft X-ray characterization as an effective technique for the compositional analysis of MHP thin films. Most synchrotron-based studies used for investigating MHPs so far are based on hard X-rays (5–10 keV) which include various absorption edges (Pb L-edge, I L-edge, Br K-edge, etc.) but are not suited for the analysis of the organic component in these materials. In order to be sensitive to a maximum number of elements, we have employed soft X-ray-based scanning transmission X-ray microscopy (STXM) as a spectro-microscopy technique for the characterization of MHPs. We examined its sensitivity to iodine and organic components, aging, or oxidation by-products in MHPs to make sure that our suggested method is suitable for studying MHPs. Furthermore, methylammonium triiodide with different deposition ratios of PbI 2 and CH 3 NH 3 I (MAI), and different thicknesses, were characterized for chemical inhomogeneity at the nanoscale by STXM. Through these measurements, we demonstrate that STXM is very sensitive to chemical composition and homogeneity in MHPs. Thus, we highlight the utility of STXM for an in-depth analysis of physical and chemical phenomena in PSCs.

We describe the investigation of metal halide perovskite layers, particularly CH3NH3PbI3 used in photovoltaic applications, by soft X-ray scanning transmission X-ray microscopy (STXM). Relevant reference spectra were used to fit the experimental data using singular value decomposition. The distribution of key elements Pb, I, and O was determined throughout the layer stack of two samples prepared by wet process. One sample was chosen to undergo electrical biasing. Spectral data shows the ability of STXM to provide relevant chemical information for these samples. We found the results to be in good agreement with the sample history, both regarding the deposition sequence and the degradation of the perovskite material.

Lead-halide perovskite solar cells (PSCs) based on unusual semiconductors made with ions, have shown impressive improvement in photovoltaic performance in few years exceeding nowadays 25 % power conversion efficiency. However, PSCs suffer from a lack of stability and show significant hysteresis in current-voltage curves, which are impeding commercialization. We confirmed the importance of halide ion migration in the hysteresis effect which has direct consequences on device efficiency. Using impedance spectroscopy, in addition to the geometrical capacitance found at high frequency for fresh samples without bias, we observed a second capacitance at low frequency after ageing or under bias. This second capacitance is interpreted as a charge accumulation layer at interfaces, which can be promoted by the presence of grain boundaries. Through glow-discharge optical emission spectroscopy elemental depth profiles, we found that under dark conditions, iodide ions diffuse through the electron transport layer versus ageing time. These ions interact chemically with the front-end electrode after four weeks and form silver iodide.

Mixed-halide based hybrid perovskite semiconductors have attracted tremendous attention as a promising candidate for high efficient photovoltaic and light-emitting devices. However, these advanced perovskite materials may undergo phase segregation under light illumination due to halide ion migration, affecting their optoelectronic properties. Here, we report photo-excitation induced phase segregation in triple-cation mixed-halide perovskite films that yields to red-shift in photoluminescence response. We demonstrate that photo-excitation induced halide ion migration leads to the formation of smaller-bandgap iodide-rich and larger-bandgap bromide-rich domains in the perovskite film, where the phase segregation rate is found to follow the excitation power-density as a power law. Results confirm that charge carrier lifetime increases with redshift in photoluminescence due to the trapping of photo-excited carriers in the segregated smaller-bandgap iodide-rich domains. Interestingly, we found that these photo-induced changes are fully reversible and thermally activated when the excitation power is turned off. A significant difference in activation energies for halide ion migration is observed during phase segregation and recovery process under darkness. Additionally, we have investigated the emission linewidth broadening as a function of temperature which is governed by the exciton-optical phonon coupling. The mechanism of photo-induced phase segregation is interpreted based on excitonphonon coupling strength in both mixed and demixed (segregated) states of perovskite film.

Perovskite-based solar cells (PSCs) have opened the possibility of cost-effective, high-efficiency photovoltaic conversion. However, their instabilities prevent them from commercialization. One of the instability triggers has been attributed to the mobile ions flowing into the carrier transport layer(s). To study the effect of this ionic migration, a numerical PSC model is developed, considering electronic and ionic mixed drift-diffusion transport both in the perovskite and the hole transport layer. The inverted PSC architecture, phenyl-C61-butyric acid methyl ester (PCBM)/perovskite/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) with two heterojunctions, is analyzed. The effect of the ionic migration on the performance of the PSCs has been analyzed by (1) the variation of the ionic mobile concentration and (2) the modification of the local trapping density. The current-voltage (J-V) and capacitance-voltage characteristics show that the electric field in the bulk can be screened by the ionic distribution modifying the effective built-in voltage. At high ionic concentrations, the electric field at the interfaces is also affected, hindering the charge extraction. The simulations show that the short circuit current is therefore strongly modified. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)

Understanding the role of defects in hybrid organic inorganic perovskites (HOIPs) is critically important to engineer the stability and performance of photovoltaic devices based on HOIPs. Recent reports on multi-cation compositions of general formula (A 1 ,A 2 ,A 3 ,A 4 )Pb(X 1 ,X 2 ,X 3 ) 3 , where the A sites can be occupied by a distribution of 2–4 metallic/organic cations and X sites with halide anions have shown stabilization effects against the well-known methyl ammonium lead triiodide (CH 3 NH 3 PbI 3 ), although the underlying mechanism is not fully elucidated. Herein, polycrystalline layers of 4APb(IBr) 3 perovskite, where A is occupied by a combination of Cs + (cesium ion), GA + (guanidinium), MA + (methylammonium), and FA + (formamidinium) ions were synthesized. To gain insight on the role of intrinsic defects, electron irradiation was used for introducing point defects in a controlled way in the quadruple-cation HOIPs. Our results show that the engineered defects in perovskites strongly influenced the absorption, photoluminescence, and time-resolved photoluminescence of these materials, probably due to introduction of additional energy levels that modify electronic and light emitting properties of the material. Furthermore, the irradiation-induced defects were found to strongly affect the aging behavior of HOIPs and modify their radiative recombination properties.

Ionic migration in halide perovskite materials is now well recognized to affect the electrical properties of perovskite solar cells. Native point defects such as vacancies are considered as respons...

Hybride perovskite solar cells (PSCs) have rapidly emerged as a promising candidate for the next generation photovoltaics with power conversion efficiencies (PCEs) attaining 22%. Low temperature solution processing, low cost raw material and relative insensitivity to intrinsic point defects are some of the attractive qualities of this emerging class of devices. But one of the major obstacles for the commercialization of PSCs lies in the long-term stability of the perovskite films subjected to different environmental conditions such as temperature, humidity and illumination. In this work, we focused on experimental evidence of halide ion migration in CH$_3$ NH$_3$ PbI$_{3-x}$ Cl$_x$ based solar cells and its effect on current-voltage hysteresis for which various mechanisms have been proposed. The inverted planar structure adopted for the PSCs was: glass/ITO/PEDOT:PSS/perovskite/PCBM/Ag. The perovskite thin films were deposited by 1-step spin-casting process and the organic PEDOT:PSS (hole-transporting layer) and PCBM (electron-transporting layer) layers were deposited by spin-coating process. Firstly the PCE under 1 sun equivalent illumination reached 12.7% for the best cell of a series of 10 samples with an active area of 0.28 cm$^2$. The J-V hysteresis effect was small (less than 2.5%) between the reverse and the forward direction, consistent with the results reported in the literature. Secondly, using glow discharge optical emission spectrometry (GD-OES), a spectrochemical technique allowing direct determination of major and trace elements, we have shown that halide ions (I-and Cl-) migrate inside the perovskite films under an applied bias in both directions, the time of migration being typically 2 min. Furthermore no migration of lead and nitrogen ions was observed in the same time scale. Thirdly we observed the hysteresis of current-voltage characteristics under dark conditions (thus without any photo-generated carriers) versus voltage scanning rate and temperature. The activation energy value of 0.253 eV derived from the Nernst-Einstein relation above 264 K, for which the perovskite phase is tetragonal, indicates that the conduction is dominated by the ions (instead of electrons for conventional semiconductors) and furthermore confirms that the conduction is ascribed to the migration of anion vacancies, which is well known in the perovskite-type halides such as CsPbCl$_3$ or CsPbBr$_3$. These experiments prove that there is a direct link between halide ion migrations in CH$_3$NH$_3$PbI$_{3-x}$Cl$_x$ based perovskite thin films and current-voltage hysteresis.

Hybride perovskite solar cells (PSCs) have rapidly emerged as a promising candidate for the next generation photovoltaics with power conversion efficiencies (PCEs) attaining 22%. Low temperature solution processing, low cost raw material and relative insensitivity to intrinsic point defects are some of the attractive qualities of this emerging class of devices. But one of the major obstacles for the commercialization of PSCs lies in the long-term stability of the perovskite films subjected to different environmental conditions such as temperature, humidity and illumination [1]. Several technological approaches have been proposed to overcome the instability problem [2-3]. In this work, we focused on experimental evidence of halide ion migration in CH3NH3PbI3-xClx based solar cells and its effect on current-voltage hysteresis for which various mechanisms have been proposed in the literature. In this study we consider hybrid perovskite crystals as imperfect ionic crystals with defects of the Frenkel type (with an equal number of vacancies and interstitials of the same ion) and we propose an explanation for the origin of the current-voltage hysteresis under dark. The inverted planar structure adopted for the PSCs was: glass/ITO/PEDOT:PSS/perovskite/PCBM/Ag. The perovskite thin films were deposited by 1-step spin-casting process and the organic PEDOT:PSS (hole-transporting layer) and PCBM (electron-transporting layer) layers were deposited by spincoating process. Firstly the PCE under 1 sun equivalent illumination reached 12.7% for the best cell of a series of 10 samples with an active area of 0.28 cm. The J-V hysteresis effect was small (less than 2.5%) between the reverse and the forward directions, consistent with the results reported in the literature. Secondly, using glow discharge optical emission spectrometry (GD-OES), a spectrochemical technique allowing direct determination of major and trace elements, we have shown that halide ions (I and Cl ) migrate inside the perovskite films under an applied bias in both directions, the time of migration being typically 2 min. Furthermore no migration of lead and nitrogen ions was observed in the same time scale [4]. Thirdly we observed the hysteresis of current-voltage characteristics under dark conditions (thus without any photo-generated carriers) versus voltage scanning rate and temperature. The activation energy value of 0.253 eV derived from the Nernst-Einstein relation above 264 K, for which the perovskite phase is tetragonal, indicates that the conduction is dominated by the ions (instead of electrons for conventional semiconductors) and furthermore confirms that the conduction is ascribed to the migration of halide ion vacancies, which is well known in the perovskite-type halides such as CsPbCl3 or CsPbBr3 [5]. These experiments prove that there is a direct link between halide ion migration in CH3NH3PbI3-xClx based perovskite thin films and current-voltage hysteresis under dark conditions.

In recent decades, the development of organic-inorganic hybrid perovskite solar cells (PSCs) has been increasing very quickly due to their high initial efficiency and low-cost process. However, key points such as crystal growth mechanisms, current-voltage hysteresis, and instability remain still unexplained or misunderstood. Among several possibilities, ionic migration in PSCs has been suggested to explain the hysteresis effect. However, direct experimental evidence of ionic migration under operation or measurement conditions of PSCs is still missing. This work shows directly the ionic migration of halogen components (I- and Cl-) of a CH3NH3PbI3-xClx perovskite film under an applied bias using glow discharge optical emission spectrometry (GD-OES). Furthermore, no migration of lead and nitrogen ions is observed on a polarization time scale less than 2 min. The ratio of fixed to mobile iodide ions is deduced from the evolution of the GD-OES profile lines as a function of the applied bias. The average length of iodide and chloride ion migration is deduced from the experimental results.

The effect of atomic hydrogen assisted pre-treatment on the growth of vertically aligned carbon nanotubes using hot-filament chemical vapor deposition was investigated. Iron nanoparticle catalysts were formed on an aluminum oxide support layer by spraying of iron chloride salt solutions as catalyst precursor. It is found that pre-treatment time and process temperature tune the density as well as the shape and the structure of the grown carbon nanotubes. An optimum pre-treatment time can be found for the growth of long and well aligned carbon nanotubes, densely packed to each other. To provide insight on this behavior, the iron catalytic nanoparticles formed after the atomic hydrogen assisted pre-treatment were analyzed by atomic force microscopy. The relations between the size and the density of the as-formed catalyst and the as-grown carbon nanotube's structure and density are discussed.

Easily degradable organic photovoltaic (OPV) devices based on flexible polymer substrates require barrier films exhibiting extremely low permeation rates for water vapor. We report the barrier properties of hard/soft alternating layer structures where a soft PMMA layer is sandwiched between two hard SiNx layers. The interfaces between the hard/soft layers hinder the propagation of pinholes from one single-layer to the other and thus have an effect on the overall barrier properties of the whole system. We focus our attention on the effect of increasing number of interfaces on the final barrier properties. The encapsulation barrier properties of these layers are assessed using the electrical calcium test. A minimum water vapor transmission rate (WVTR) of ~2×10−4g/m2·day is reported for 7 hard/soft interfaces, which has also high potential for flexible barrier applications.

The reliability and stability are key issues for the commercial utilization of organic photovoltaic devices based on flexible polymer substrates. To increase the shelf-lifetime of these devices, transparent moisture barriers of silicon nitride (SiN x ) films are deposited at low temperature by hot wire CVD (HW-CVD) process. Instead of the conventional route based on organic/inorganic hybrid structures, this work defines a new route consisting in depositing multilayer stacks of SiN x thin films, each single layer being treated by argon plasma. The plasma treatment allows creating smoother surface and surface atom rearrangement. We define a critical thickness of the single layer film and focus our attention on the effect of increasing the number of SiN x single-layers on the barrier properties. A water vapor transmission rate (WVTR) of 2 × 10−4 g/(m2·day) is reported for SiN x multilayer stack and a physical interpretation of the plasma treatment effect is given.

In this work we produce and study the flexible organic-inorganic hybrid moisture barrier layers for the protection of air sensitive organic opto-electronic devices. The inorganic amorphous silicon nitride layer (SiNx:H) and the organic PMMA [poly (methyl methacrylate)] layer are deposited alternatingly by using hot wire chemical vapor deposition (HW-CVD) and spin-coating techniques, respectively. The effect of organic-inorganic hybrid interfaces is analyzed for increasing number of interfaces. We produce highly transparent (similar to 80% in the visible region) hybrid structures. The morphological properties are analyzed providing a good basis for understanding the variation of the water vapor transmission rate (WVTR) values. A minimum WVTR of 4.5 x 10(-5) g/m(2) day is reported at the ambient atmospheric conditions for 7 organic/inorganic interfaces. The hybrid barriers show superb mechanical flexibility which confirms their high potential for flexible applications. (C) 2014 Elsevier B.V. All rights reserved.

Hot-wire chemical vapour deposition (CVD) was applied to grow zinc oxide (ZnO)-based transparent conducting oxide films. Indium (In)-, gallium (Ga)-, and aluminium (Al)-doped ZnO films were deposited at 400 °C on sapphire-R, Si (100) and glass substrates using a cold wall pulsed liquid injection CVD system containing nichrome wires installed in front of the substrate holder. Zn, In, Al 2,2,6,6-tetramethyl-3,5-heptanedionates, and Ga 3,5-pentanedionate dissolved in 1,2-dimethoxyethane were used as precursors. Hall measurements were performed to evaluate the resistivity, carrier concentration, and carrier mobility in doped ZnO films grown on sapphire substrates at wire currents of 6 A and 9 A. The influence of the dopant type, doping level, substrate, and wire heating current on crystallinity and the electrical and optical properties of the films was investigated and discussed. The best electrical properties were obtained for Al- and Ga-doped films grown at 9 A wire current (resistivity ≈ 1 × 10− 3 Ωcm, carrier mobility ≈ 50 cm2 V− 1 s− 1 and carrier concentration ≈ 1 × 1020 cm− 3). The films exhibited a high transmittance in the mid-infrared region (≈ 90% at 2.5 μm). Additional annealing of the films at 400 °C in a mixture of Ar and hydrogen (10%) resulted in the increase in carrier concentration and mobility and in the reduction of film resistivity.