arXiv subjects

Yana Vaynzof

Publications and source records attributed to Yana Vaynzof.

61 records · Page 4Linked to original sources

arXiv2020

Improved evaluation of deep-level transient spectroscopy on perovskite solar cells reveals ionic defect distribution

One of the key challenges for future development of efficient and stable metal halide perovskite solar cells is related to the migration of ions in these materials. Mobile ions have been linked to the observation of hysteresis in the current--voltage characteristics, shown to reduce device stability against degradation and act as recombination centers within the band gap of the active layer. In the literature one finds a broad spread of reported ionic defect parameters (e.g. activation energies) for seemingly similar perovskite materials, rendering the identification of the nature of these species difficult. In this work, we performed temperature dependent deep-level transient spectroscopy (DLTS) measurements on methylammonium lead iodide perovskite solar cells and developed a extended regularization algorithm for inverting the Laplace transform. Our results indicate that mobile ions form a distribution of emission rates (i.e. a distribution of diffusion constants) for each observed ionic species, which may be responsible for the differences in the previously reported defect parameters. Importantly, different DLTS modes such as optical and current DLTS yield the same defect distributions. Finally the comparison of our results with conventional boxcar DLTS and impedance spectroscopy (IS) verifies our evaluation algorithm.

physics.app-ph↗

arXiv2019

Enhancing the Open-Circuit Voltage of Perovskite Solar Cells by up to 120 mV using π-Extended Phosphoniumfluorene Electrolytes as Hole Blocking Layers

Four π-extended phosphoniumfluorene electrolytes (π-PFEs) are introduced as hole-blocking layers (HBL) in inverted architecture planar perovskite solar cells (PVSCs) with the structure of ITO/PEDOT:PSS/MAPbI3/PCBM/HBL/Ag. The deep-lying highest occupied molecular orbital (HOMO) energy level of the π-PFEs effectively blocks holes, decreasing contact recombination. We demonstrate that the incorporation of π-PFEs introduces a dipole moment at the PCBM/Ag interface, resulting in a significant enhancement of the built-in potential of the device. This enhancement results in an increase in the open-circuit voltage of the device by up to 120 mV, when compared to the commonly used bathocuproine HBL. The results are confirmed both experimentally and by numerical simulation. Our work demonstrates that interfacial engineering of the transport layer/contact interface by small molecule electrolytes is a promising route to suppress non-radiative recombination in perovskite devices and compensate for a non-ideal energetic alignment at the hole-transport layer/perovskite interface.

physics.app-ph↗

arXiv2019

Effect of Ion Migration Induced Electrode Degradation on the Operational Stability of Perovskite Solar Cells

Perovskite-based solar cells are promising due to their rapidly improving efficiencies, but suffer from instability issues. Recently it has been claimed that one of the key contributors to the instability of perovskite solar cells is ion migration induced electrode degradation, which can be avoided by incorporating inorganic hole blocking layers (HBL) in the device architecture. In this work, we investigate the operational environmental stability of methylammonium lead iodide (MAPbI3) perovskite solar cells that contain either an inorganic or organic HBL, with only the former effectively blocking ions from migrating to the metal electrode. This is confirmed by X-ray photoemission spectroscopy measured on electrodes of degraded devices, where only electrodes of devices with an organic HBL show a significant iodine signal. Despite this, we show that when these devices are degraded under realistic operational conditions (i.e. constant illumination in a variety of atmospheric conditions), both types of devices exhibit nearly identical degradation behavior. These results demonstrate that contrary to prior suggestions, ion-induced electrode degradation is not the dominant factor in perovskite environmental instability under operational conditions.

physics.app-ph↗

arXiv2019

Quantifying the Damage Induced by XPS Depth Profiling of Organic Conjugated Polymers

X-ray photoemission spectroscopy (XPS) depth profiling using monoatomic Ar+ ion etching sources is a common technique that allows for the probing of the vertical compositional profiles of a wide range of materials. In polymer-based organic photovoltaic devices, it is commonly used to study compositional variations across the interfaces of the organic active layer with charge extraction layers or electrodes, as well as the vertical phase separation within the bulk-heterojunction active layer. It is generally considered that the damage induced by the etching of organic layers is limited to the very top surface, such that the XPS signal (acquired from the top ~10 nm of the layer) remains largely unaffected, allowing for a reliable measurement of the sample composition throughout the depth profile. Herein, we investigate a range of conjugated polymers and quantify the depth of the damage induced by monoatomic etching for Ar+ ion energies ranging from 0.5 keV to 4 keV using argon gas cluster ion beam depth profiling. The results demonstrate that even when etching with the lowest available monoatomic ion energy for as little as 3 s, the damaged polymer material extends deeper into the bulk than the XPS probing depth. We show that the damaged material distorts the compositional information obtained by XPS, resulting in erroneous depth profiles. Furthermore, we propose that only gas cluster ion beam etching sources should be used for depth profiling of organic conjugated polymers, as those induce significantly less damage and maintain the compositional information throughout the entire profile.

physics.app-ph↗

arXiv2019

Fractional Deviations in Precursor Stoichiometry Dictate the Properties, Performance and Stability of Perovskite Photovoltaic Devices

The last five years have witnessed a remarkable progress in the field of lead halide perovskite materials and devices. Examining the existing body of literature reveals staggering inconsistencies in the reported results among different research groups with a particularly wide spread in the photovoltaic performance and stability of devices. In this work we demonstrate that fractional, quite possibly unintentional, deviations in the precursor solution stoichiometry can cause significant changes in the properties of the perovskite layer as well as in the performance and stability of perovskite photovoltaic devices. We show that while the absorbance and morphology of the layers remains largely unaffected, the surface composition and energetics, crystallinity, emission efficiency, energetic disorder and storage stability are all very sensitive to the precise stoichiometry of the precursor solution. Our results elucidate the origin of the irreproducibility and inconsistencies of reported results among different groups as well as the wide spread in device performance even within individual studies. Finally, we propose a simple experimental method to identify the exact stoichiometry of the perovskite layer that researchers can employ to confirm their experiments are performed consistently without unintentional variations in precursor stoichiometry.

physics.app-ph↗

arXiv2017

Preparation of hierarchical C@MoS2@C sandwiched hollow spheres for Lithium ion batteries

Hierarchical C@MoS2@C hollow spheres with the active MoS2 nanosheets being sandwiched by carbon layers have been produced by means of a modified template method. The process applies polydopamine (PDA) layers which inhibit morphology change of the template thereby enforcing the hollow microsphere structure. In addition, PDA forms complexes with the Mo precursor, leading to an in-situ growth of MoS2 on its surface and preventing the nanosheets from agglomeration. It also supplies the carbon that finally sandwiches the 100-150 nm thin MoS2 spheres. The resulting hierarchically structured material provides a stable microstructure where carbon layers strongly linked to MoS2 offer efficient pathways for electron and ion transfer, and concomitantly buffer the volume changes inevitably appearing during the charge-discharge process. Carbon-sandwiched MoS2-based electrodes exhibit high specific capacity of approximately 900 mA h g-1 after 50 cycles at 0.1 C, excellent cycling stability up to 200 cycles, and superior rate performance. The versatile synthesis method reported here offers a general route to design hollow sandwich structures with a variety of different active materials.

cond-mat.mtrl-sci↗

arXiv2017

A facile synthesis method and electrochemical studies of hierarchical structured MoS2/C-nanocomposite

A uniformly coated MoS2/Carbon-nanocomposite with three-dimensional hierarchical architecture based on carbonized bacterial cellulose (CBC) nanofibers is synthesized by a facile one-step hydrothermal method followed by thermal annealing at 700 °C in Ar atmosphere. Strong hydrogen bonds between the Mo precursor and the BC nanofibers are found to be crucial for the in-situ growth of MoS2 nanosheets on the nanofibers during hydrothermal process. The unique structure was maintained and the connection between MoS2 and nanofibers were strengthened in the sintering process, leading to an improved stability of the resulting nanocomposite upon electrochemical cycling. The low-cost and environmentally friendly 3D web-like structure enables binder-free and carbon-free electrodes for lithium-ion batteries, which exhibit high specific discharge capacities up to 1140 mAh/g at the C-rate of 1 C without significant capacity fading for over 50 cycles. The porous conductive hierarchical structure of the composite endows excellent rate performance by avoiding the aggregation of the MoS2 nanosheets and by accommodating mechanical stress which appears upon electrochemical cycling.

cond-mat.mtrl-sci↗