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Instruments / Technical Area	Publication Details	Published
I14-Hard X-ray Nanoprobe	Revealing nanomechanical domains and their transient behavior in mixed‐halide perovskite films Ioanna Mela , Chetan Poudel , Miguel Anaya , Geraud Delport , Kyle Frohna , Stuart Macpherson , Tiarnan A. S. Doherty , Anna Scheeder , Samuel D. Stranks , Clemens F. Kaminski Advanced Functional Materials DOI: 10.1002/adfm.202100293 Diamond Proposal Number(s): [20420] Show Abstract ▼ Abstract: Halide perovskites are a versatile class of semiconductors employed for high performance emerging optoelectronic devices, including flexoelectric systems, yet the influence of their ionic nature on their mechanical behavior is still to be understood. Here, a combination of atomic‐force, optical, and compositional X‐ray microscopy techniques is employed to shed light on the mechanical properties of halide perovskite films at the nanoscale. Mechanical domains within and between morphological grains, enclosed by mechanical boundaries of higher Young's Modulus (YM) than the bulk parent material, are revealed. These mechanical boundaries are associated with the presence of bromide‐rich clusters as visualized by nano‐X‐ray fluorescence mapping. Stiffer regions are specifically selectively modified upon light soaking the sample, resulting in an overall homogenization of the mechanical properties toward the bulk YM. This behavior is attributed to light‐induced ion migration processes that homogenize the local chemical distribution, which is accompanied by photobrightening of the photoluminescence within the same region. This work highlights critical links between mechanical, chemical, and optoelectronic characteristics in this family of perovskites, and demonstrates the potential of combinational imaging studies to understand and design halide perovskite films for emerging applications such as photoflexoelectricity.	Mar 2021
I09-Surface and Interface Structural Analysis	Elucidating and mitigating degradation processes in perovskite light‐emitting diodes Zahra Andaji-garmaroudi , Mojtaba Abdi-jalebi , Felix U. Kosasih , Tiarnan Doherty , Stuart Macpherson , Alan R. Bowman , Gabriel J. Man , Ute B. Cappel , Hakan Rensmo , Caterina Ducati , Richard H. Friend , Samuel D. Stranks Advanced Energy Materials 11 DOI: 10.1002/aenm.202002676 Diamond Proposal Number(s): [22668] Show Abstract ▼ Abstract: Halide perovskites have attracted substantial interest for their potential as disruptive display and lighting technologies. However, perovskite light‐emitting diodes (PeLEDs) are still hindered by poor operational stability. A fundamental understanding of the degradation processes is lacking but will be key to mitigating these pathways. Here, a combination of in operando and ex situ measurements to monitor the performance degradation of (Cs0.06FA0.79MA0.15)Pb(I0.85Br0.15)3 PeLEDs over time is used. Through device, nanoscale cross‐sectional chemical mapping, and optical spectroscopy measurements, it is revealed that the degraded performance arises from an irreversible accumulation of bromide content at one interface, which leads to barriers to injection of charge carriers and thus increased nonradiative recombination. This ionic segregation is impeded by passivating the perovskite films with potassium halides, which immobilizes the excess halide species. The passivated PeLEDs show enhanced external quantum efficiency (EQE) from 0.5% to 4.5% and, importantly, show significantly enhanced stability, with minimal performance roll‐off even at high current densities (>200 mA cm−2). The decay half‐life for the devices under continuous operation at peak EQE increases from <1 to ≈15 h through passivation, and ≈200 h under pulsed operation. The results provide generalized insight into degradation pathways in PeLEDs and highlight routes to overcome these challenges.	Nov 2020
E02-JEM ARM 300CF I14-Hard X-ray Nanoprobe	Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites Tiarnan A. S. Doherty , Andrew J. Winchester , Stuart Macpherson , Duncan N. Johnstone , Vivek Pareek , Elizabeth M. Tennyson , Sofiia Kosar , Felix U. Kosasih , Miguel Anaya , Mojtaba Abdi-jalebi , Zahra Andaji-garmaroudi , E. Laine Wong , Julien Madéo , Yu-hsien Chiang , Ji-sang Park , Young-kwang Jung , Christopher E. Petoukhoff , Giorgio Divitini , Michael K. l. Man , Caterina Ducati , Aron Walsh , Paul A. Midgley , Keshav M. Dani , Samuel D. Stranks Nature 580, 360 - 366 DOI: 10.1038/s41586-020-2184-1 Diamond Proposal Number(s): [19023, 19793] Show Abstract ▼ Abstract: Halide perovskite materials have promising performance characteristics for low-cost optoelectronic applications. Photovoltaic devices fabricated from perovskite absorbers have reached power conversion efficiencies above 25 per cent in single-junction devices and 28 per cent in tandem devices. This strong performance (albeit below the practical limits of about 30 per cent and 35 per cent, respectively) is surprising in thin films processed from solution at low-temperature, a method that generally produces abundant crystalline defects. Although point defects often induce only shallow electronic states in the perovskite bandgap that do not affect performance, perovskite devices still have many states deep within the bandgap that trap charge carriers and cause them to recombine non-radiatively. These deep trap states thus induce local variations in photoluminescence and limit the device performance. The origin and distribution of these trap states are unknown, but they have been associated with light-induced halide segregation in mixed-halide perovskite compositions and with local strain, both of which make devices less stable. Here we use photoemission electron microscopy to image the trap distribution in state-of-the-art halide perovskite films. Instead of a relatively uniform distribution within regions of poor photoluminescence efficiency, we observe discrete, nanoscale trap clusters. By correlating microscopy measurements with scanning electron analytical techniques, we find that these trap clusters appear at the interfaces between crystallographically and compositionally distinct entities. Finally, by generating time-resolved photoemission sequences of the photo-excited carrier trapping process, we reveal a hole-trapping character with the kinetics limited by diffusion of holes to the local trap clusters. Our approach shows that managing structure and composition on the nanoscale will be essential for optimal performance of halide perovskite devices.	Apr 2020
E02-JEM ARM 300CF	Heterogeneity at multiple length scales in halide perovskite semiconductors Elizabeth M. Tennyson , Tiarnan A. S. Doherty , Samuel D. Stranks Nature Reviews Materials 44 DOI: 10.1038/s41578-019-0125-0 Diamond Proposal Number(s): [19793] Show Abstract ▼ Abstract: Materials with highly crystalline lattice structures and low defect concentrations have classically been considered essential for high-performance optoelectronic devices. However, the emergence of high-efficiency devices based on halide perovskites is provoking researchers to rethink this traditional picture, as the heterogeneity in several properties within these materials occurs on a series of length scales. Perovskites are typically fabricated crudely through simple processing techniques, which leads to large local fluctuations in defect density, lattice structure, chemistry and bandgap that appear on short length scales (<100 nm) and across long ranges (>10 μm). Despite these variable and complex non-uniformities, perovskites maintain exceptional device efficiencies and are, as of 2018, the best-performing polycrystalline thin-film solar cell material. In this Review, we highlight the multiple layers of heterogeneity ascertained using high-spatial-resolution methods that provide access to the relevant length scales. We discuss the impact that the optoelectronic variations have on halide perovskite devices, including the prospect that it is this very disorder that leads to their remarkable power-conversion efficiencies.	Jul 2019

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