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Author(s): Kim, Min-cheol; Ahn, Namyoung; Cheng, Diyi; Xu, Mingjie; Pan, Xiaoqing; Kim, Suk Jun; Luo, Yanqi; Fenning, David P; Tan, Darren HS; Zhang, Minghao; Ham, So-Yeon; Jeong, Kiwan; Choi, Mansoo; Meng, Ying Shirley | Abstract: Perovskite solar cells have drawn much attention in recent years, owing to its world-record setting photovoltaic performances. Despite its promising use in tandem applications and flexible devices, its practicality is still limited by its structural instability often arising from ion migration and defect formation. While it is generally understood that ion instability is a primary cause for degradation, there is still a lack of direct evidence of structural transformation at the atomistic scale. Such an understanding is crucial to evaluate and pin-point how such instabilities are induced relative to external perturbations such as illumination or electrical bias with time, allowing researchers to devise effective strategies to mitigate them. Here, we designed an in-situ TEM setup to enable real-time observation of amorphization in double cation mixed perovskite materials under electrical biasing at 1 V. It is found that amorphization occurs along the (001) and (002) planes, which represents the observation of in-situ facet-dependent amorphization of a perovskite crystal. To reverse the degradation, the samples were heated at 50 oC and was found to recrystallize, effectively regaining its performance losses. This work is vital toward understanding fundamental ion-migration phenomena and address instability challenges of perovskite optoelectronics.

Summary The most important obstacle to widespread use of perovskite solar cells is their poor stability under operational stressors. Here, we systematically monitor the evolution of the photovoltaic performance of perovskite solar cells based on formamidinium-cesium lead iodide (FA0.9Cs0.1PbI3) for 600 h, under a series of controlled operational stressors. Although these devices exhibit reasonable thermal stability, their stability under illumination or stabilized power output (SPO) is far from commercial demands. Synchrotron-based nanoprobe X-ray fluorescence and X-ray-beam-induced current measurements reveal that current-blocking Cs-rich phases segregate during stress tests. The decrease in performance is in line with the increasing density of the Cs-rich clusters in area upon illumination. Theoretical calculations indicate that light-generated carriers provide the thermodynamic driving force for that phase segregation. Our findings correlate device performance to microscopic behavior and atomistic mechanisms and shed light on inhibiting the cation-dependent phase segregation during device operation.

Organic–inorganic hybrid perovskites have electronic and optoelectronic properties that make them appealing in many device applications1–4. Although many approaches focus on polycrystalline materials5–7, single-crystal hybrid perovskites show improved carrier transport and enhanced stability over their polycrystalline counterparts, due to their orientation-dependent transport behaviour8–10 and lower defect concentrations11,12. However, the fabrication of single-crystal hybrid perovskites, and controlling their morphology and composition, are challenging12. Here we report a solution-based lithography-assisted epitaxial-growth-and-transfer method for fabricating single-crystal hybrid perovskites on arbitrary substrates, with precise control of their thickness (from about 600 nanometres to about 100 micrometres), area (continuous thin films up to about 5.5 centimetres by 5.5 centimetres), and composition gradient in the thickness direction (for example, from methylammonium lead iodide, MAPbI3, to MAPb0.5Sn0.5I3). The transferred single-crystal hybrid perovskites are of comparable quality to those directly grown on epitaxial substrates, and are mechanically flexible depending on the thickness. Lead–tin gradient alloying allows the formation of a graded electronic bandgap, which increases the carrier mobility and impedes carrier recombination. Devices based on these single-crystal hybrid perovskites show not only high stability against various degradation factors but also good performance (for example, solar cells based on lead–tin-gradient structures with an average efficiency of 18.77 per cent). A solution-based lithography-assisted epitaxial-growth-and-transfer method is used to fabricate single-crystal hybrid perovskites on any surface, with precise control of the thickness, area and chemical composition gradient.

Simple Summary Metal-oxide nanomaterials enter cancer and normal cells even when not specifically targeted, and often interact with specific cellular structures and biological molecules solely due to their innate physical-chemical properties. This raises concerns for the use of nanoparticles, which can be alleviated only with rigorous studies of nanoparticle–cell interactions, studies independent of post-interaction labeling of nanomaterials. X-ray fluorescence microscopy is an imaging technique that quantifies and maps all chemical elements from the periodic table solely based on their native fluorescence excited by the incoming X-ray. We used two different instruments to interrogate the same sample in 3D at two different resolutions and determine heterogeneity of cell-to-cell interactions with nanomaterials, as well as subcellular nanoparticle distribution. This is the first example of multi-scale 3D X-ray fluorescence imaging. This work begins a new era of study on how nanoparticle-based therapies can be developed to be more predictable and safer for use. Abstract Research in cancer nanotechnology is entering its third decade, and the need to study interactions between nanomaterials and cells remains urgent. Heterogeneity of nanoparticle uptake by different cells and subcellular compartments represent the greatest obstacles to a full understanding of the entire spectrum of nanomaterials’ effects. In this work, we used flow cytometry to evaluate changes in cell cycle associated with non-targeted nanocomposite uptake by individual cells and cell populations. Analogous single cell and cell population changes in nanocomposite uptake were explored by X-ray fluorescence microscopy (XFM). Very few nanoparticles are visible by optical imaging without labeling, but labeling increases nanoparticle complexity and the risk of modified cellular uptake. XFM can be used to evaluate heterogeneity of nanocomposite uptake by directly imaging the metal atoms present in the metal-oxide nanocomposites under investigation. While XFM mapping has been performed iteratively in 2D with the same sample at different resolutions, this study is the first example of serial tomographic imaging at two different resolutions. A cluster of cells exposed to non-targeted nanocomposites was imaged with a micron-sized beam in 3D. Next, the sample was sectioned for immunohistochemistry as well as a high resolution “zoomed in” X-ray fluorescence (XRF) tomography with 80 nm beam spot size. Multiscale XRF tomography will revolutionize our ability to explore cell-to-cell differences in nanomaterial uptake.

Optimizing surface passivation Unproductive charge recombination at surface defects can limit the efficiency of hybrid perovskite solar cells, but these defects can be passivated by the binding of small molecules. Wang et al. studied three such small molecules—theophylline, caffeine, and theobromine—that bear both carbonyl and amino groups. For theophylline, hydrogen bonding of the amino hydrogen to surface iodide optimized the carbonyl interaction with a lead antisite defect and improved the efficiency of a perovskite cell from 21 to 22.6%. Science , this issue p. 1509

Using scanning X-ray diffraction microscopy, we investigate the relationship between residual strains from crystal growth in CsPbBr3 thin-film crystals, their stability, and local photoluminescence (PL). We find that out-of-plane compressive strain that arises from cooldown from crystallization is detrimental to material stability under X-ray irradiation. We also find that the optical PL red-shifts as a result of the out-of-plane compressive strain. The sensitivity of the PL peak position to strain suggests possible applications such as stress-sensitive sensors. Mosaicity, the formation of small misorientations in neighboring crystalline domains we observe in some CsPbBr3 single crystals, indicates the significant variations in the crystal quality that can occur even in single-crystal halide perovskites. The nanodiffraction results suggest that reducing local strains is a necessary path to enhance the stability of perovskite optoelectronic materials and devices from light-emitting diodes to high-energy de...

Degradation due to electron beam exposure has posed a challenge in the use of electron microscopy to probe halide perovskite materials and devices. In this study, the interaction between the electron beam and the perovskite across acceleration voltages and at low probe currents is investigated in a scanning electron microscope (SEM) by monitoring the electron-beam-induced current (EBIC) response in perovskite solar cells in a plan-view configuration. SEM probe conditions are identified where dozens of repeated scans over a single region of the perovskite solar cell induce minimal electronic degradation. Overall, the induced current response of the perovskite device is found to strongly depend upon the beam condition: Rapid decay occurs at high beam powers, the current activates at the lowest beam powers, and a newfound quasi-steady response is revealed at intermediate beam conditions. A quantitative window for the successful conduction of e-beam studies with minimal electronic degradation is revealed by evaluating induced current response over a wide range of perovskite devices, which invites broader use of SEM-based characterization techniques, including EBIC, as powerful techniques for correlative microscopy investigations.

Summary Perovskite solar cells (PSCs) are one of the emerging solar cell technologies with high conversion efficiency. Several deposition methods had been applied for preparing their hole transport layer (HTL). However, there are few direct evidences to demonstrate whether HTL and its interfaces in PSCs have been influenced by the deposition methods. In this study, the 3D morphology of PSCs has been reconstructed by focused ion beam-scanning electron microscopy from the PSCs in which HTLs are deposited by different methods. The compositional distribution of HTLs is unveiled as well. All these associated layers and interfaces display obvious morphological and compositional differences that are attributed to the HTL components’ solubility differences in the precursor solvent. Our investigation demonstrates the PSCs that HTL fabricated by dynamic spin-coating method have higher efficiency, better film uniformity, and less interfacial roughness than the static spin-coating-based devices.

Improved understanding of the origins of instability during photovoltaic operation of perovskite solar cell materials must be established to overcome barriers to commercialization. In this study, we analyze the microscopic mechanisms of degradation in high-performing methylammonium free (FA0.9Cs0.1PbI3) perovskite solar cells (PSC) over 600 hours of operation under stressors inherent to PV operation, including heat, illumination, and a load while excluding atmospheric effects by testing in a water-and oxygen-free atmosphere. While the PSCs exhibit reasonable thermal stability, they show considerable performance loss under constant illumination or stable power output. Synchrotron-based nanoprobe X-ray fluorescence and X-ray beam induced current (XRF/XBIC) measurements reveal segregation of current-blocking Cs-rich phases during stress testing. The decrease in performance correlates with the resulting number density of the Cs-rich clusters, which varies by stress condition. These findings unveil cation-dependent instability in FA0.9Cs0.1PbI3 perovskites and provide a framework for understanding the energy landscape in alloy perovskites to guide the engineering of long-lived halide perovskite devices.

The electrochemical conversion of CO2 to hydrocarbons and alcohols for use as a renewable energy storage medium is a promising approach to CO2 utilization and energy sustainability. Herein, we demonstrate that the selectivity of an electrochemically reduced Cu(OH)2 nanowire catalyst toward C2–C3 compounds (ethylene, ethanol, and n-propanol) is systemically modified by surface morphology, which is governed by the electrolysis potential. The total Faradaic efficiency of CO2 reduction to C2–C3 compounds is found to be 38% at a moderate potential of −0.81 V vs RHE, and stable electrocatalytic performance is observed for 40 h of CO2 electrolysis. Electro- and physicochemical analyses indicate that the Cu(OH)2 nanowires are completely reduced to metallic Cu, forming a mesostructured catalyst after a few minutes of electrolysis. The shift in product selectivity is strongly correlated with this change in mesoscale catalyst morphology, offering additional dimensionality and multiple length scales for catalyst desi...

Compositional engineering related to the organic and inorganic cations (A-site) in halide perovskites solar cells has helped improve efficiency and long-term durability. However, this compositional complexity can lead to phase segregation that weakens the optoelectronic performance. Here, we show the halide distribution and cation distribution by means of synchrotron-based nanoprobe x-ray fluorescence. We find that the halide distribution homogenizes upon the addition of CsI and RbI precursors. The halide homogenization coincides with long-lived charge carrier decays. Additionally, we observe Rb-rich areas that phase segregate within the film and the Rb aggregates are identified to be recombination active using X-ray and E-beam induced current microscopy.

The poor stability of halide perovskites creates a barrier for common e-beam based microscopies. Thus, carefully optimized e-beam characterization conditions are needed before any meaningful measurement can be taken. In this study, we present a range of e-beam operating conditions for characterizing perovskite materials with minimal electronic damage by monitoring plan-view electron beam induced current (EBIC) of functional solar cell devices. We see a minimum of beam induced damage in plan-view operation the range of 7.5 to 9.5 kV at low currents (

Spatial heterogeneities in the chemical makeup of thin film photovoltaic devices are pivotal in determining device efficiency. We report the in-plane spatial distribution and degree of chlorine incorporation in organic–inorganic lead halide perovskite absorbers by means of nondestructive synchrotron-based nanoprobe X-ray fluorescence. The presence of chlorine is positively identified in CH3NH3PbI3 films synthesized with Cl-containing precursors and as an impurity in some films synthesized with nominally Cl-free precursors. The impurity may be introduced from precursors or as contaminants during film synthesis. The films formed from Cl-containing precursors contain roughly an order of magnitude higher amount of chlorine, with Cl:I values greater than 0.02 found whether Cl is present in either the organic or the inorganic precursor for both one- and two-step fabrication processes. A spatial variation in the Cl incorporation is observed within single particles and as well as between particles within a given ...

Grain boundaries play a key role in the performance of thin-film optoelectronic devices and yet their effect in halide perovskite materials is still not understood. The biggest factor limiting progress is the inability to identify grain boundaries. Noncrystallographic techniques can misidentify grain boundaries, leading to conflicting literature reports about their influence; however, the gold standard - electron backscatter diffraction (EBSD) - destroys halide perovskite thin films. Here, this problem is solved by using a solid-state EBSD detector with 6000 times higher sensitivity than the traditional phosphor screen and camera. Correlating true grain size with photoluminescence lifetime, carrier diffusion length, and mobility shows that grain boundaries are not benign but have a recombination velocity of 1670 cm s(-1), comparable to that of crystalline silicon. Amorphous grain boundaries are also observed that give rise to locally brighter photoluminescence intensity and longer lifetimes. This anomalous grain boundary character offers a possible explanation for the mysteriously long lifetime and record efficiency achieved in small grain halide perovskite thin films. It also suggests a new approach for passivating grain boundaries, independent of surface passivation, to lead to even better performance in optoelectronic devices.

In this study, we use nanoprobe X-ray diffraction to measure structural variations in thin halide perovskite single crystals including mosaicity, tilt and strain. Using X-ray diffraction at 100-nm spatial resolution offers high sensitivity to structural variations within thin film materials. Here, we examine the crystal structural changes under X-ray irradiation toward understanding in detail the determining factors in this process including chemistry of materials, dose, and dose rate.

Hybrid perovskite materials are promising for application as the active layers in solar cells due to their excellent optoelectronic properties. Improved understanding of the origins of hysteresis in perovskite materials must be established to overcome barriers to commercialization. In this study, we use a lateral device geometry to bias a methylammonium lead bromide thin film single crystal. Upon biasing, the halide concentration varies laterally within the crystal, as observed by means of nondestructive synchrotron-based nanoprobe X-ray fluorescence (Nano-XRF). The direct observation of a partially reversible bromine re-distribution under bias is revealed experimentally. The origins of device hysteresis and switchable effects can be partially explained by the local elemental information available from Nano-XRF.

Quasi-2D perovskite photovoltaics have been shown to have better stability than their 3D counterparts but the effects of the A-site cation on charge collection have not been fully clarified in these systems. In this study, the nanoscale carrier collection properties within quasi-2D bromide based lead perovskites (n=3 and 40) are found to be uncorrelated with local variations in halide stoichiometry, in contrast to the 3D perovskite. The local elemental distribution is quantified by means of synchrotron-based nanoprobe X-ray fluorescence (Nano-XRF), while the local carrier collection is simultaneous collected via X-ray beam induced current (XBIC).

Interfacial interactions between conjugated polymers and carbon nanotubes are pivotal in determining the device performance of nanotube-based polymer electronic devices. Here, we report on interfacial structures and crystallization kinetics of poly(3-hexylthiophene) (P3HT) in the presence of single-walled carbon nanotubes (SWNTs) in anisole by means of transmission electron microscope (TEM) and ultraviolet-visible (UV-vis) absorption spectroscopy. Confined on SWNT surfaces, the P3HT forms nanofibril crystals perpendicular to the long axis of SWNTs. The equilibrium dissolution temperature of the P3HT crystals in anisole is determined to be 381 ± 10 K according to the Hoffman-Weeks extrapolation approach. Upon cooling, the polymer solution spontaneously undergoes a time-dependent chromism. Various kinetics factors such as crystallization temperature, concentration, and SWNT loading have been investigated. It is found that the growth rate (G) of the crystals scales with concentration (C) as G ∝ C(1.70±0.16). The Avrami model is utilized to analyze the nucleation mechanism and the Avrami exponents vary between 1.0 and 1.3. The Lauritzen-Hoffman theory is applied to study the chain-folding process. The fold surface free energy is calculated to be (5.28-11.9) × 10(-2) J m(-2). It is evident that the addition of 0.30 wt % SWNTs reduces the fold surface free energy by 55.6%.

Interfacial interactions between donor and acceptor molecules are determinative to the device performance of hybrid photovoltaics. However, the dynamic process of such interactions remains largely obscure. In this work, we report the kinetic behavior of solution crystallization of poly(3-hexylthiophene) (P3HT) in anisole in the presence of ZnO nanowires by means of ultraviolet-visible absorption spectroscopy. ZnO nanowires are surface-modified by covalently attaching aliphatic and aromatic ligands to enhance the miscibility and interfacial interactions between P3HT and ZnO nanowires. Upon cooling the hot solution to room temperature, a significant time-dependent chromism occurs spontaneously. Analysis of the kinetics of isothermal solution crystallization across a range of crystallization temperature displays that the growth rate of the crystals scales with polymer concentration as R ∝ C1.6 for both the control P3HT and P3HT with ZnO nanowires. The Lauritzen–Hoffman theory of secondary nucleation is utilized to analyze the kinetic behavior of crystallization, and the fold surface free energies of the crystals of P3HT in anisole are calculated to be 6.6–10.3 × 10−2 J m−2. It is found that the addition of surface-modified ZnO nanowires decreases the fold surface free energy by 21.5% and 43.8%, respectively, for aliphatic and aromatic ligands.

Spatial heterogeneities in the chemical makeup of thin film photovoltaic devices are pivotal in determining device efficiency. In this study, the presence of chlorine is identified in perovskite films synthesized with Cl-containing precursors by means of nanoprobe X-ray fluorescence (Nano-XRF). Additionally, a spatial variation in the Cl incorporation is observed within a given film, and the standard deviation of Cl: I ratio across the film is large. Using Nano-XRF, the Cl incorporation in methylammonium lead iodide perovskite films can be manipulated by precursor stoichiometry ratio.

The personal ultraviolet (UV) dosimeter is a useful measurement tool to prevent UV induced dermal damages; however, conventional digital dosimeters are either bulky or require external power sources. Here, a wearable, colorimetric UV film dosimeter that provides color transition, from purple to transparent, is reported to indicate the UV dose. The film dosimeter is made of a purple photodegradable dye ((2Z,6Z)-2,6-bis(2-(2,6-diphenyl-4H-thiopyran-4-ylidene)ethylidene)cyclohexanone or DTEC) blended in low density polyethylene film. The DTEC film discolored 3.3 times more under the exposure of UV light (302 nm) than visible light (543 nm), and a UV bandpass filter is developed to increase this selectivity to UV light. The DTEC film completely discolors to transparency in 2 h under an AM 1.5 solar simulator, suggesting the potential as an indicator for individuals with types I-VI skin to predict interventions to avoid sunburn. Finally, the DTEC film is integrated with the UV bandpass filter on a wristband to function as a wearable dosimeter for low cost and convenient monitoring of sunlight exposure.

Author(s): Luo, Yanqi; Aharon, Sigalit; Stuckelberger, Michael; Magana, Ernesto; Lai, Barry; Bertoni, Mariana I; Etgar, Lioz; Fenning, David P

Interfacial interactions between the polymer and graphene are pivotal in determining the reinforcement efficiency in the graphene-enhanced polymer nanocomposites. Here, we report on the dynamic process of graphene-induced oriented interfacial crystals of isotactic polypropylene (iPP) in the single fiber polymer composites by means of polarized optical microscopy (POM) and scanning electron microscopy (SEM). The graphene fibers are obtained by chemical reduction of graphene oxide fibers, and the latter is produced from the liquid crystalline dispersion of graphene oxide via a wet coagulation route. The lamellar crystals of iPP grow perpendicular to the fiber axis, forming an oriented transcrystalline (TC) interphase surrounding the graphene fiber. Various factors including the diameter of graphene fibers, crystallization temperature, and time are investigated. The dynamic process of polymer transcrystallization surrounding the graphene fiber is studied in the temperature range 124-132 °C. The Lauritzen-Hoffman theory of heterogeneous nucleation is applied to analyze the transcrystallization process, and the fold surface free energy is determined. Study into microstructures demonstrates a cross-hatched lamellar morphology of the TC interphase and the strong interfacial adhesion between the iPP and graphene. Under appropriate conditions, the β-form transcrystals occur whereas the α-form transcrystals are predominant surrounding the graphene fibers.

Optoelectronic devices based on hybrid perovskites have demonstrated outstanding performance within a few years of intense study. However, commercialization of these devices requires barriers to their development to be overcome, such as their chemical instability under operating conditions. To investigate this instability and its consequences, the electric field applied to single crystals of methylammonium lead bromide (CH3 NH3 PbBr3 ) is varied, and changes are mapped in both their elemental composition and photoluminescence. Synchrotron-based nanoprobe X-ray fluorescence (nano-XRF) with 250 nm resolution reveals quasi-reversible field-assisted halide migration, with corresponding changes in photoluminescence. It is observed that higher local bromide concentration is correlated to superior optoelectronic performance in CH3 NH3 PbBr3 . A lower limit on the electromigration rate is calculated from these experiments and the motion is interpreted as vacancy-mediated migration based on nudged elastic band density functional theory (DFT) simulations. The XRF mapping data provide direct evidence of field-assisted ionic migration in a model hybrid-perovskite thin single crystal, while the link with photoluminescence proves that the halide stoichiometry plays a key role in the optoelectronic properties of the perovskite.

We report on a series of experiments on large-area ordered patterns of graphene oxide on solid substrates deposited from aqueous dispersions by directed drop evaporation. The aqueous dispersion of graphene oxide exhibits phase transitions from isotropic to liquid crystalline nematic phases via a biphasic region with increasing concentration. In the single nematic phase, schlieren textures accompanied by oriented bands are frequent. Drying of drops in each phase results in deposition covering the whole drop base. The dynamic process of drop drying is analyzed based on the weight loss, radius change, and texture change over time. It is found that the radial bands develop in the nematic drops in the vicinity of the receding of the contact line and subsequently transform into birefringent stripes after drying. Study into the structure and morphology of the stripes reveals anisotropic wrinkling of graphene oxide sheets. The nature of stripe orientation is strongly dependent on the local nematic order at the dewetting water front. Various macroscopic patterns with different stripe orientations including radial spokes, spider webs, and parallel stripes have been generated by tuning the nematic order of drops.

The journal of physical chemistry / C 124(33), 17949 - 17956 (2020). doi:10.1021/acs.jpcc.0c04645, Synchrotron micro- and nanoprobe beamlines have demonstrated great potential to advance photovoltaic devices. Most importantly, their small X-ray spot size has enabled the direct correlation of electrical performance with elemental composition at subgrain resolution for a variety of polycrystalline solar cells. Whereas the bulk of most inorganic semiconductors is stable under the high X-ray flux of focused X-ray beams, semiconductors with organic components are prone to a variety of degradation mechanisms. This is particularly critical to evaluate for the emerging organometal halide perovskite solar cells. Here, we investigate the effects of hard X-rays on the nanoscale performance and elemental distribution of these solar cells. We show that their composition does not change during common operando and in situ measurements at synchrotron nanoprobes. However, we found a significant X-ray-induced electronic degradation of solar cells with methylammonium lead iodide absorbers. Time- and dose-dependent measurements unveiled two characteristic degradation time constants on the order of 12 and 200 s that are independent of the X-ray flux. On the basis of heat and dose simulations, we attribute the fast decay to the dose-driven creation of recombination centers, while the slow decay is compatible with the observation of compositional changes. Finally, we detail how degradation-induced measurement artifacts can be outrun and showcase the high correlation of the X-ray-beam-induced current with the iodine and lead distribution., Published by Soc., Washington, DC