Moisture-Induced Non-Equilibrium Phase Segregation in Triple Cation Mixed Halide Perovskite Monitored by In Situ Characterization Techniques and Solid-State NMR

Environmental stability is a major bottleneck of perovskite solar cells.Only a handful of studies are investigating the effect of moisture on the structural degradation of the absorber.They mostly rely on ex situ experiments and on completely degraded samples,which restrict the assessment on initial and final stage.By combining in situ X-ray diffraction under controlled 85%relative humidity,and live observations of the water-induced degradation using liquid-cell transmission electron microscopy,we reveal two competitive degradation paths leading on one hand to the decomposition of state-of-theart mixed cation/anion(Cs_(0.05)(MA_(0.17)FA_(0.83))_(0.95)Pb(Br_(0.17)I_(0.83))_(3)(CsMAFA)into PbI_(2) through a dissolution/recrystallization mechanism and,on the other hand,to a non-equilibrium phase segregation leading to CsPb_(2)Br_(5) and a Cesium-poor/iodide-rich Cs_(0.05)-x(MA_(0.17)FA_(0.83))_(0.95)Pb(Br_(0.17-2y)I_(0.83)+2y)_(3) perovskite.This degradation mechanism is corroborated at atomic-scale resolution through solid-state ^(1)H and ^(133)Cs NMR analysis.Exposure to moisture leads to a film containing important heterogeneities in terms of morphology,photoluminescence intensities,and lifetimes.Our results provide new insights and consensus that complex perovskite compositions,though very performant as champion devices,are comparatively metastable,a trait that limits the chances to achieve long-term stability.

Water induced phase segregation in hydrocarbon proton exchange membranes

Proton exchange membranes(PEMs) are a key material for proton exchange membrane fuel cells(PEMFCs). Non-fluorinated hydrocarbon PEMs are low-cost alternatives to Nafion, but limited by the low proton conductivity, because of the weak phase segregation structure and narrow ion-transport channels.Various efforts have been taken to improve the performance of hydrocarbon PEMs, but mostly with complex methodologies. Here we demonstrate a simple, yet very efficient method to create phase segregation structure inside a typical hydrocarbon PEM, sulfonated poly(ether ether ketone)(SPEEK). By simply adding appropriate amounts of water into the DMF solvent, the resulting SPEEK membrane exhibits widened ion-transport channels, with the phase size of 2.7 nm, as indicated by both molecular dynamic(MD) simulations and transmission electron microscope(TEM) observations, and the proton conductivity is thus improved by 200%. These findings not only further our fundamental understanding of hydrocarbon PEMs, but are also valuable to the development of low-cost and practical fuel cell technologies.

Preventing inhomogeneous elemental distribution and phase segregation in mixed Pb-Sn inorganic perovskites via incorporating PbS quantum dots

Inhomogeneous Pb/Sn elemental distribution and the resulted phase segregation in mixed Pb-Sn halide perovskites would result in energy disorder(band structure and phase distribution disorder),which greatly limits their photovoltaic performance.Here,Pb S quantum dot has been synthesized and demonstrated as seeds for modulation crystallization dynamics of the mixed Pb-Sn inorganic perovskites,allowing an enhanced film quality and significantly suppressing phase segregation.With this additive power conversion efficiency of 8%and 6%is obtained under irradiation of full sunlight in planar and mesoporous structured solar cells in combination with CsPb_(0.5) Sn_(0.5)I_(2)Br inorganic perovskite,respectively.Our finding reveals exploring the actual Pb/Sn atoms location in perovskite structure and its influence on developing efficient and stable low-bandgap perovskite solar cells.

A-site phase segregation in mixed cation perovskite

Mixed cation strategy greatly benefits the enhancement of device performance and chemical stability.However,adverse impact also accompanies the mixed cation system simultaneously.It brings the compositional instability,wherein the homogeneous film is likely to segregate into multi-phases during the fabrication and ageing process,thus resulting in the efficiency reduction of perovskite solar cells(PSCs)devices.This review focuses on the cation induced phase segregation,and elucidates the segregation mechanisms from the perspectives of film formation and ageing process,respectively.Furthermore,the influence of cation segregation on device performance and operational stability are discussed.And based on these understandings,viable strategies are proposed for the design of phase-stable mixed composition halide perovskites and for suppressing segregation to benefit its development towards commercial applications.

Brightening thiocyanate-anion layered perovskite through internal stress modulated nano phase segregation

Thiocyanate-anion(SCN−)two-dimensional(2D)layered perovskite with internal stress-controlled nano phase segregation has been firstly demonstrated as a promising material system for luminescence applications.An interesting energy band structure is found as well as charge transfer process caused by nano phase segregation,which provide an alternative route to overcome the indirect-bandgap luminescence limit of SCN layered perovskites.It is revealed that,within the SCN layered framework,the segregated nano phases exist in a quantum well form,possessing much higher carrier localization and second-order radiative recombination abilities.With the help of internal stress modulation,these advantages can be significantly enhanced and finally contribute to high luminescence performances in visible-red regions.This work provides more potential opportunities for 2D layered perovskite materials in the future optoelectronic applications.

New understanding of phase segregation of bimetallic nanoalloys

In a recent online publication of Science,Professor Peter Strasser of the Technical University of Berlin,Germany,and his collaborators reported element-specific anisotropic growth of Pt and Ni in shaped Pt alloy synthesis[1].They showed that the surface steps in the Pt3Ni concave hexapod alloy formed in the initial stage of the synthesis were crucial in the segregation of an M-rich(M=Ni,Co,

Suppressed light-induced phase transition of CsPbBr_(2)I: Strategies, progress and applications in the photovoltaic field

The rapid rise in the power conversion efficiency(PCE)of CsPbBr_(2)I-based perovskite solar cells(PSCs),from 4.7%in 2016 to 11.08%in 2020,render it a promising material for use in photovoltaic devices.However,the phase stability and current hysteresis caused by photo-induced phase segregation in CsPbBr_(2)I represent major obstacles to further improvements in the PCE for such devices.In this review,we describe the basic structure and optical properties of CsPbBr_(2)I,and systematically elaborate on the mechanism of the phase transition.We then discuss the strategies in progress to suppress phase transition in CsPbBr_(2)I,and their potential application in the photovoltaic field.Finally,challenges and application prospects for CsPbBr2I PSCs are summarized in the final section of this article.

Relieving segregation in twin-roll cast Mg-8Al-2Sn-1Zn alloys via controlled rolling

Twin-roll casting(TRC)of Mg alloys with high Al content has unique advantages and broad development prospects.However,the severe segregation of high-alloyed Mg caused by TRC technology is difficult to solve at present,which restricts its practical application.In this work,the homogenization of coarse segregated phases in a twin-roll cast Mg-8Al-2Sn-1Zn(ATZ821)alloy was successfully achieved by a method combining solid-solution heat treatment and controlled rolling,which is suitable for industrial-scale production.In addition,the evolution of the microstructure was studied in detail.It was found that phases dynamically precipitated and dissolved repeatedly during the hot rolling process,and the evolution of entire second phases can be classified into three stages,i.e.,the net dissolution,balancing and net precipitation stages.During the process,the large-size chain-like eutectic phases evolved into a fine and nearly spherical(∼0.4μm)morphology that was uniformly dispersed in the Mg-matrix due to the thermodynamic coupling and the Ostwald ripening effect.Moreover,a fine grain microstructure(∼4μm)was obtained.The formidable segregation problem of twin-roll cast Mg alloys with high Al content was solved,which is an important finding for the industrial application of high-alloyed twin-roll cast sheets.©2020 Published by Elsevier B.V.on behalf of Chongqing University.

Stabilization of mixed-halide lead perovskites under light by photothermal effects

Mixed-halide lead perovskites(MHLPs) are semiconductor materials with bandgaps that are tunable across the visible spectrum and have seen promising applications in photovoltaics and optoelectronics.However, their segregation into phases with enriched halide components, under resonant light illumination and/or electric field, have hindered their practical applications. Herein, we demonstrate the stabilization of the MHLP photoluminescence(PL) peak as a function of their excitation intensities.This effect is associated with the phase segregation of MHLPs and their subsequent remixing by photothermal heating. We conclude that the balance between these opposing processes dictates the equilibrium PL peak of the MHLPs. The findings in this work could serve as a potential approach to obtain MHLP with stable emission peaks under operating conditions.

Organic additives in all-inorganic perovskite solar cells and modules:from moisture endurance to enhanced efficiency and operational stability

Power conversion efficiency(PCE) of perovskite solar cells(PSC) has been skyrocketed to certified 25.5% owing to their improved and tunable optoelectronic properties. Although, various strategies have been adopted to date regarding PCE and stability enhancement within PSC technology, certain instability factors(moisture, heat, light) are hindering their commercial placement. Recently, all-inorganic PSCs got hype in the photovoltaic research community after they attained PCE > 20% and due to their significant endurance against heat and light mishmashes, but there only left moisture sensitivity as the only roadblock for their industrial integration. Here, we review the recent progress of additive inclusion into allinorganic(CsPbX_(3)) PSCs to stabilize their intrinsic structure and to withstand the performance limiting factors. We start with the detailed description of chemical instability of different perovskite compositions, phase segregation, and how organic molecules and dyes help to repair the structural defects to improve the overall PCE and stability of PSCs. Moisture endurance as a result of chemical passivation through organic additives, low-dimensional inorganic PSCs to enhance device stability and scalable fabrication of CsPbX_(3) PSCs are also reviewed. The challenges of module degradation and design implications with proposed strategies and outlook are interpreted in the ending phrases of this review.

Mechanical behavior of LC_4 alloy in semisolid state at high volume fractions of solid

The mechanical behavior of LC 4 alloy in the semisolid state at high volume fractions of solid has been studied through unconstrictive compressing test. The results show that peak stress mainly depends on grain boundary’s cohesion and instantaneous strain rate sensitivity in the semisolid state, which is similar to that in the solid state. Analyses on microstructures and status of compressive stress of specimen demonstrate that segregation of liquid solid phase is mainly affected by strain rate and deformation temperature. There are mainly two kinds of flow in liquid phase: either from the region with relatively large hydrostatic compressive stress to the region with relatively small hydrostatic compressive stress or from the grain boundaries perpendicular to the compression axis to the grain boundaries with a certain directional angle to the compression direction. Based on the above results, compressive deformation mechanism mainly depends on deformation temperature, strain rate and stress state.

Relationship between Localization of PBSU in Interlamellar/Interfibrillar Regimes and Double Peaks in DSC/SAXS in its Blend with PVDF

In this work,phase segregation and localization of PBSU have been investigated with the combination of SAXS and DSC in its blend with PVDF.After stepwise crystallization of PVDF and PBSU,there are double melting peaks of PBSU in DSC and double scattering peaks in SAXS.It has been demonstrated that double peaks can be attributed to the localization of PBSU in interlamellar/interfibrillar region in pre-formed PVDF crystal framework.In the case of low content of PBSU in blend,PBSU is trapped into the interlamellar region of PVDF crystals,resulting in the alternating lamellae crystal of them and the first peak(with low-q)in SAXS.The enhanced confinement effect produces thinner PBSU lamellae,corresponding to the lower melting temperature in DSC.Upon increasing its content in blend,some PBSU segregates in interfibrillar regions in addition to the enrichment in interlamellar regions of PVDF crystal framework.The larger space and higher concentration of PBSU in interfibrillarregions contribute to periodic lamellae structure of PBSU with higher thickness,which is the reason for the second peak(with high-q)in SAXS and DSC.Our results not only clarify the relationship between localization of PBSU in interlamellar/interfibrillar regions and double peaks in DSC/SAXS,but also provide a novel strategy to detect the interlamellar and interfibrillar segregation of low-T_(m) component in miscible crystalline/crystalline blend.