Photoinduced Phase Separation Research Articles

Dissipative Particle Dynamics Study on the Phase Region of Spatial Gradient Materials Produced by Photoinduced Isomerization

AbstractSpatial gradient materials occupy an important research position in the field of functional materials with their unique porous structure. Gradient changes in pore size and density distribution have received extensive attention in the fields of biomimetic and smart materials. The gradient transition law is mathematically related to the driving force of isomerization reaction and component phase separation. In this study, a dissipative particle dynamics simulation is used to introduce photoisomerization reactions into the system. Lambert's law is used to construct a reaction model for the variation of light intensity with irradiation depth, and a gradient structure with a spatial transition law is obtained. The effects of the extinction coefficient ε, the initial reaction probability Pr0, and the interactions α(A,B) between the isomerized molecules as well as the viscosity on the formation of the gradient structure are investigated in detail. Furthermore, the mathematical proportionality between the size of the phase region and interfacial energy of the two phases is elucidated. This study provides preliminary computational insights into the factors affecting the photoinduced phase separation process of polymeric gradient materials. It may help to develop effective strategies to improve the phase separation and properties of polymer gradient materials in subsequent studies.

Pure‐Iodide Wide‐Bandgap Perovskites for High‐Efficiency Solar Cells by Crystallization Control

AbstractWide‐bandgap perovskite is a vital part of perovskite‐based tandem solar cells. Currently, wide‐bandgap perovskites are typically based on mixed‐halide (I/Br) materials, but suffer from photoinduced phase separation. The pure‐iodide formamidine/cesium (FA/Cs) based FAxCs1−xPbI3 perovskites with high Cs content are good candidates, whereas the control of crystallization is challenging due to the complex crystallization kinetics. Here, pure‐iodide FA0.5Cs0.5PbI3 wide‐bandgap perovskite solar cells is reported. As an acidic diammonium salt, methylenediaminium dichloride (MDACl2) is applied as an additive to control the whole crystallization process of perovskite films, including both nucleation and crystal growth. Starting from the solution chemistry, the MDACl2 additive with acidity and strong solvation properties can effectively regulate the chemical composition of perovskite precursor, thus inhibiting the growth of undesired 1D intermediates during the nucleation process. Besides, the incorporation of larger‐sized MDA2+ into the lattice compensates for the tolerance factor and accelerates the ion exchange reaction between FA+ and Cs+ in the crystal growth process. As a result, the crystallinity of the perovskite films is significantly improved, benefitting from the dual function of MDACl2. Finally, the efficiency of hole transport layer‐free carbon electrode‐based wide‐bandgap perovskite solar cells reaches 18.52%, which is the highest reported so far.

Halide Segregation in Mixed Halide Perovskites: Visualization and Mechanisms

Photoinduced halide segregation in mixed halide perovskites is an intriguing phenomenon and simultaneously a stability issue. In-depth probing this effect and unveiling the underpinning mechanisms are of great interest and significance. This article reviews the progress in visualized investigation of halide segregation, especially light-induced, by means of spatially-resolved imaging techniques. Furthermore, the current understanding of photoinduced phase separation based on several possible mechanisms is summarized and discussed. Finally, the remained open questions and future outlook in this field are outlined.

Development of a magnetic hybrid material capable of photoinduced phase separation of iron chloride by shape memory and photolithography

FeCl3 in the polymer matrix can be partially ordered using photolithography. Ordered FeCl3 has improved magnetic susceptibility.

Compositional Engineering of FA-Based Lead Halide Perovskitesfor Shear Coating Processes

Recently Organic-inorganic halide perovskite (OIHP) solar cells have become one of the most promising next-generation photovoltaic materials due to their excellent electrical and optical properties. Formamidinium lead triiodide (HC(NH2)2PbI3, FAPbI3) exhibits the narrowest band gap (~1.45 eV) among Pb-based halide perovskites, which is closer to Shockley-Queisser limit than that (~1.55 eV) of Methylammonium Lead Iodide (CH3NH3PbI3, MAPbI3). Therefore, FAPbI3 has been widely adopted for recently developed highly efficient photovoltaic cells. In addition, FAPbI3 has better stability than MAPbI3, which can be easily decomposed at a high temperature or humid environment. The spin-coating has been widely adopted to make FAPbI3-based solar cells, whereas the large-area coating technologies have not been extensively studied, yet.This study presents the compositional engineering research on the polymorph control of formamidinium-based lead halide perovskites for the large-area film coating. The polymorph control of FAPbI3 has been one of the most important issues since its first appearance in photovoltaic applications. Non-perovskite structure (δ-FAPbI3) is thermodynamically more stable than the three-dimensional perovskite structure (α-FAPbI3). We adopted the shear coating method to make perovskite layers and controlled the polymorphism through A-site mixing. MA, commonly used A-site molecule, is excluded because MA can be a weak spot at the humid condition. Br mixing is not applied due to the photo-induced phase separation issue.Our results reveal that double-cation mixing (Cs-FA or Rb-FA) is not enough and triple-cation mixing (Rb-Cs-FA) cannot be avoided to make robust α-FAPbI3 for the shear coating process. We present the ternary phase diagram of (Rb-CS-FA)PbI3 to show the process window based on the XRD analysis of mixed perovskite films. The photovoltaic cell shows 15.8% power conversion efficiency (PCE) at the best compositional mixing point, which is the best PCE for FA-based perovskite solar cells fabricated by shear coating method without MA-Br mixing.

Photoinduced phase separation in the lead halides is a polaronic effect.

We present a perspective on recent observations of the photoinduced phase separation of halides in multi-component lead-halide perovskites. The spontaneous phase separation of an initial homogeneous solid solution under steady-state illumination conditions is found experimentally to be reversible, stochastic, weakly dependent on morphology, yet strongly dependent on composition and thermodynamic state. Regions enriched in a specific halide species that form upon phase separation are self-limiting in size, pinned to specific compositions, and grow in number in proportion to the steady-state carrier concentration until saturation. These empirical observations of robustness rule out explanations based on specific defect structures and point to the local modulation of an existing miscibility phase transition in the presence of excess charge carriers. A model for rationalizing existing observations based on the coupling between composition, strain, and charge density fluctuations through the formation of polarons is reviewed.

In Situ TEM Monitoring of Phase-Segregation in Inorganic Mixed Halide Perovskite.

Photoinduced phase separation, which limits the available band gap energies for photovoltaic applications, was reported for a range of mixed-halide perovskites. A microscopic understanding of the phase separation mechanism is still lacking but may be beneficial to rationalize limitations as well as enable the design of phase-stable perovskite semiconductors. In this letter, electron-beam-induced phase separations and transformations were investigated in a small crystallite of CsPb(Br0.8I0.2)3 by means of in situ high-resolution imaging in a transmission electron microscope. The acquired time series was evaluated using principal and independent component analysis to classify the structural change during the illumination by the electron beam. A more iodine-rich phase with the approximate composition of CsPb(Br0.6I0.4)3 was found to form at the edges of the particle, while a ternary pure bromide phase of CsPbBr3 remained at its center. These results provide an atomistic picture of in-grain phase segregation into iodide-rich phases at grain boundaries and bromide-rich phases in the interior of the grain.

Photo-induced actuator using temperature and light dual responsive azobenzene containing ion gel in ionic liquid

A series of well-defined random copolymers comprising butyl acrylate (BA) and 4-phenylazophenylmethacrylate (AzoMA) (P(AzoMA-r-BA)) are prepared successfully by reversible addition fragmentation chain transfer (RAFT) polymerization. P(AzoMA-r-BA)s show LCST-type phase transition in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfone)amide ([C4mim][NTf2]). LCST depends on the photoisomerization state of azobenzene, as well as on the AzoMA composition in the random copolymers. LCST of (P(cis-AzoMA-r-BA) is significantly higher than that of P(trans-AzoMA-r-BA), because cis-AzoMA and trans-AzoMA behave as solvato-philic and solvato-phobic comonomer, respectively. At a bistable temperature, photo-induced phase separation is completely reversible. Secondly, based on this phenomenon, a thermo-/photo- responsive ion gel (BA-AzoMA ion gel) can be prepared by free radical polymerization of BA and AzoMA using ethylene glycol dimethacrylate (EGDMA) as crosslinker in [C4mim][NTf2]. BA-AzoMA ion gel shows high temperature contraction and low temperature expansion behavior, due to LCST-type phase behavior of polymer system consisting of AzoMA and BA. Contraction temperature of BA-AzoMA ion gel depends on photoisomerization state of the azobenzene group in polymers. At a bistable temperature, photo-induced expansion-contraction is reversible. Finally, a Photo-induced actuator can be realized using BA-AzoMA ion gel at suitable temperatures, featuring reversible bending by alternate irradiating with visible and UV light. The bending behavior is also demonstrated.

Paintable Encapsulated Body-Temperature-Responsive Photonic Reflectors with Arbitrary Shapes

A temperature-responsive photonic coating on a flexible substrate was prepared by a photoinduced phase-separation process. In this coating a low molecular weight cholesteric liquid crystal (Ch-LC) mixture was encapsulated between the substrate and an in situ formed protective polymer top layer. The photonic coating showed a blue-shift of the photonic reflection band of 100 nm by heating from 22 to 23 °C due to the close proximity to the smectic to cholesteric phase transition and an overall 330 nm blue-shift while heating from 22 to 45 °C. Hence, the red coating turned green upon contact with skin within seconds. Furthermore, the coating structure and composition were investigated in detail, revealing a thick top coat. The adhesion of the coating was improved by providing trays on the substrate (by etching or 3D printing), resulting in a link between arbitrary-shaped substrates and the protective polymer top layer. These bendable coatings could be of interest for sensors, anticounterfeit labels, or custom...

Controlled Shape and Porosity of Polymeric Colloids by Photo-Induced Phase Separation.

The shape and porosity of polymeric colloids are two properties that highly influence their ability to accomplish specific tasks. For micro-sized colloids, the control of both properties was demonstrated by the photo-induced phase separation of droplets of NOA81—a thiol-ene based UV-curable adhesive—mixed with acetone, water, and polyethylene glycol. The continuous phase was perfluoromethyldecalin, which does not promote phase separation prior to UV activation. A profound influence of the polymer concentration on the particle shape was observed. As the photo-induced phase separation is triggered by UV radiation, polymerization drives the extracted solution out of the polymeric matrix. The droplets of the extracted solution coalesce until they form a dimple correlated to the polymer concentration, significantly changing the shape of the formed solid colloids. Moreover, control could be gained over the porosity by varying the UV intensity, which governs the kinetics of the reaction, without changing the chemical composition; the number of nanopores was found to increase significantly at higher intensities.

Formation mechanism of asymmetric porous polymer films by photoinduced phase separation in the presence of solvent

ABSTRACTPhotoinduced phase separation in the presence of a nonreactive volatile solvent provides a simple, light‐tunable, and cost‐effective route for fabrications of porous polymer films. Here, we show that the short‐time (<10 s) ultraviolet irradiation of a photoreactive solution film promotes particular asymmetries in the composition and porosity across the thickness. Confocal Raman spectroscopy revealed that the compositional asymmetry is maximum at a certain critical light intensity. We discuss the film formation mechanism based on the light‐driven diffusion of the solvent and monomer. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47867.

Kinetically Controlled Photoinduced Phase Separation for Hybrid Radical/Cationic Systems

Controlling phase separation in polymer systems has shown significant promise in combining properties of different components into an integrated polymer system. In this work, we investigate the eff...

Recent advances in photo-stability of lead halide perovskites

Lead halide perovskites, which can be grown by solution process, are very suitable for various optoelectronic applications, such as in solar cells, light-emitting diodes, lasers and photo-detectors, due to their outstanding electrical and optical properties. However, lead halide perovskites are unstable under continuous light illumination, which severely degrades the performance and lifetime of optoelectronic devices based on them. Therefore, the photo-stability of perovskites and the mechanisms have received more and more attention. In this article we review the main phenomena in perovskites, induced by continuous light illumination, namely photo curing, photo dissociation, photo-induced phase separation, and photo-induced phase transition. The proposed mechanisms for the instability from the perspectives of defect states, ion migration, thermodynamics and chemical bonds, respectively, are also introduced. Finally, the complexity presented in the studying of photo-stability, and the issues should be addressed in the future are also briefly discussed.

Temperature Dependent Photoinduced Reversible Phase Separation in Mixed-Halide Perovskite

Even though tandem solar cells comprised of mixed-halide perovskites CH3NH3Pb(I1–xBrx)3 were expected to have much higher efficiency, the observation that they undergo photoinduced phase separation/demixing put forth a limitation to their possible utility. Herein, using temperature dependent photoluminescence studies, we show that the stated photoinduced phase separation occurs only in a narrow temperature range and above a particular bromine concentration. Our observation of the disappearance of phase separation at elevated temperatures suggests the possibility that these tandem solar cells may indeed work better at elevated temperatures. Further, we provide the first experimental proof for the demixing transition temperature as predicted by Bischak et al. and also observe that demixing and remixing temperatures are pinned to crystallographic phase transition temperatures. Longer carrier lifetime of iodide-rich clusters is observed confirming the strong electron–phonon interaction (polaronic effect) whic...

Tunable Polaron Distortions Control the Extent of Halide Demixing in Lead Halide Perovskites.

Photoinduced phase separation in mixed halide perovskites emerges from their electro-mechanical properties and high ionic conductivities, resulting in photoinduced I--rich charge carrier traps that diminish photovoltaic performance. Whether photoinduced phase separation stems from the polycrystalline microstructure or is an intrinsic material property has been an open question. We investigate the nanoscale photoinduced behavior of single-crystal mixed Br-/I- methylammonium (MA+) lead halide perovskite (MAPb(Br xI1- x)3) nanoplates, eliminating effects from extended structural defects. Even in these nanoplates, we find that phase separation occurs, resulting in I--rich clusters that are nucleated stochastically and stabilized by polarons. Upon lowering the electron-phonon coupling strength by partially exchanging MA+ for Cs+, a phase-separated steady state is not reached, nevertheless transient I- clustering still occurs. Our results, supported by multiscale modeling, demonstrate that photoinduced phase separation is an intrinsic property of mixed halide perovskites, the extent and dynamics of which depends on the electron-phonon coupling strength.

Ion-Migration Inhibition by the Cation-π Interaction in Perovskite Materials for Efficient and Stable Perovskite Solar Cells.

Migration of ions can lead to photoinduced phase separation, degradation, and current-voltage hysteresis in perovskite solar cells (PSCs), and has become a serious drawback for the organic-inorganic hybrid perovskite materials (OIPs). Here, the inhibition of ion migration is realized by the supramolecular cation-π interaction between aromatic rubrene and organic cations in OIPs. The energy of the cation-π interaction between rubrene and perovskite is found to be as strong as 1.5 eV, which is enough to immobilize the organic cations in OIPs; this will thus will lead to the obvious reduction of defects in perovskite films and outstanding stability in devices. By employing the cation-immobilized OIPs to fabricate perovskite solar cells (PSCs), a champion efficiency of 20.86% and certified efficiency of 20.80% with negligible hysteresis are acquired. In addition, the long-term stability of cation-immobilized PSCs is improved definitely (98% of the initial efficiency after 720 h operation), which is assigned to the inhibition of ionic diffusions in cation-immobilized OIPs. This cation-π interaction between cations and the supramolecular π system enhances the stability and the performance of PSCs efficiently and would be a potential universal approach to get the more stable perovskite devices.

Photoinduced phase separation with local structural ordering in organic molecular conductors

In this work, polarized pump-probe spectroscopy was carried out to investigate the effects of a structural ordering of molecules on photoinduced phase separation (PIPS) in the organic conductors $\ensuremath{\kappa}\ensuremath{-}{(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF})}_{2}X$ [$X=\mathrm{Cu}[\mathrm{N}{(\mathrm{CN})}_{2}]\mathrm{Br} (\ensuremath{\kappa}\ensuremath{-}Br)$ and $\mathrm{Cu}{(\mathrm{NCS})}_{2}$ ($\ensuremath{\kappa}$-NCS)]. We found that the anisotropic response for the probe polarization appeared at around ${T}_{\mathrm{g}}$, where the glasslike structural transition occurs. The anisotropy can be a result of a transient destruction of the local ordering of molecules, indicating a connection between the glasslike transition and PIPS. Moreover, we found that the PIPS response gradually develops with decreasing temperature in $\ensuremath{\kappa}$-Br, whereas it steeply increases in $\ensuremath{\kappa}$-NCS. This qualitative difference suggests that the structural ordering caused by a PIPS is more crucial in $\ensuremath{\kappa}$-NCS than in $\ensuremath{\kappa}$-Br.

Partially Reversible Photoinduced Chemical Changes in a Mixed-Ion Perovskite Material for Solar Cells.

Metal halide perovskites have emerged as materials of high interest for solar energy-to-electricity conversion, and in particular, the use of mixed-ion structures has led to high power conversion efficiencies and improved stability. For this reason, it is important to develop means to obtain atomic level understanding of the photoinduced behavior of these materials including processes such as photoinduced phase separation and ion migration. In this paper, we implement a new methodology combining visible laser illumination of a mixed-ion perovskite ((FAPbI3)0.85(MAPbBr3)0.15) with the element specificity and chemical sensitivity of core-level photoelectron spectroscopy. By carrying out measurements at a synchrotron beamline optimized for low X-ray fluxes, we are able to avoid sample changes due to X-ray illumination and are therefore able to monitor what sample changes are induced by visible illumination only. We find that laser illumination causes partially reversible chemistry in the surface region, including enrichment of bromide at the surface, which could be related to a phase separation into bromide- and iodide-rich phases. We also observe a partially reversible formation of metallic lead in the perovskite structure. These processes occur on the time scale of minutes during illumination. The presented methodology has a large potential for understanding light-induced chemistry in photoactive materials and could specifically be extended to systematically study the impact of morphology and composition on the photostability of metal halide perovskites.

Electro-optical and dielectric characterization of submicrometer-sized PDLC films

This report concerns thin films (25 µm thickness) of polymer-dispersed liquid crystal (PDLC) composites having submicrometer-sized nematic liquid crystal domains. Such PDLC films were prepared by performing photo-induced phase separation of mixed soft material composed from the room-temperature nematic liquid crystal 4-n-heptyl cyanobiphenyl (7CB) and photo-curable polymer NOA-65. Being of interest for electro-optical (EO) and other applications driven by alternating-current electric field, the produced composites were studied by EO measurements and complex impedance spectroscopy. For the examined soft-solid composite material with a large content of submicro-confined nematic, was obtained information about the dielectric permittivity and molecular dynamics, useful for practice.

Enhancement of conversion efficiency for an organic semiconductor laser based on a holographic polymer dispersed liquid crystal

In this paper, we report a high-conversion-efficiency organic semiconductor distributed feedback laser. The gain layer of the laser device is made from poly (2-methoxy-5-(20-ethylhexyloxy) p-phenyl-enevinylene) (MEH-PPV), and the holographic polymer dispersed liquid crystal (HPDLC) grating is used as the external light feedback layer. Thus the parameters of the laser device can be modulated independently. The solution of MEH-PPV in xylene (6 mg·mL-1) is deposited on the bottom glass substrate by spin-coating (2000 r/min). The MEH-PPV layer thickness is controlled at (80±2) nm by the spin-coating rate and confirmed by the Dektak profilometer. The HPDLC is made by the photo-induced phase separation method. To determine the orientations of LC molecules, the diffraction efficiency of each sample is measured by a He-Ne laser. The diffraction efficiency is defined as the diffracted light intensity in the first order divided by the incident light intensity. If p light diffraction efficiency (ηp) is much larger (smaller) than s light diffraction efficiency (ηs), it can be thought of as a symbol of a fairly good alignment of LC along the grating vector (grating grooves). When the period of HPDLC grating is larger than 450 nm, ηp is greater than ηs, and the averaged orientation of liquid crystal molecules is aligned along the grating vector direction, i.e., orthogonal to the holographic plane. For feedback light propagating along the grating vector, the refractive index modulation is dependent on the difference between the polymer refractive index np and the ordinary refractive index no of phase-separated LC. These two values are very close to each other, thus the effective light feedback for lasing output is not high. However, when the period of HPDLC grating is smaller than 450 nm, ηs is greater than ηp, and the orientation of phase-separated LC is altered. The refractive index modulation of feedback light originates from the difference between the polymer refractive index np and the extraordinary refractive index ne of phase-separated LC, thus the refractive index modulation can be improved and the HPDLC layer can provide better light feedback. The lasing threshold is 0.70 μJ/pulse, and the conversion efficiency is 2.5% for the sample with a grating period of 593 nm. However, the lasing threshold is lowered to 0.18 μJ/pulse, and the conversion efficiency increases to 6.4% for the sample with a grating period of 395 nm. These results show that the output lasing performance can be improved by using small period grating, since it has bigger refractive index in the grating vector direction (the lasing feedback direction). The laser performance of sample with small grating period is improved in some aspects such as threshold energy, conversion efficiency to some extent compared with those reported previously.