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Instruments / Technical Area	Publication Details	Published
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
I07-Surface & interface diffraction	Controlling the growth kinetics and optoelectronic properties of 2D/3D lead–tin perovskite heterojunctions Edoardo Ruggeri , Miguel Anaya , Krzysztof Galkowski , Geraud Delport , Felix Utama Kosasih , Anna Abfalterer , Sebastian Mackowski , Caterina Ducati , Samuel D. Stranks Advanced Materials DOI: 10.1002/adma.201905247 Diamond Proposal Number(s): [17223] Open Access Show Abstract ▼ Abstract: Halide perovskites are emerging as valid alternatives to conventional photovoltaic active materials owing to their low cost and high device performances. This material family also shows exceptional tunability of properties by varying chemical components, crystal structure, and dimensionality, providing a unique set of building blocks for new structures. Here, highly stable self‐assembled lead–tin perovskite heterostructures formed between low‐bandgap 3D and higher‐bandgap 2D components are demonstrated. A combination of surface‐sensitive X‐ray diffraction, spatially resolved photoluminescence, and electron microscopy measurements is used to reveal that microstructural heterojunctions form between high‐bandgap 2D surface crystallites and lower‐bandgap 3D domains. Furthermore, in situ X‐ray diffraction measurements are used during film formation to show that an ammonium thiocyanate additive delays formation of the 3D component and thus provides a tunable lever to substantially increase the fraction of 2D surface crystallites. These novel heterostructures will find use in bottom cells for stable tandem photovoltaics with a surface 2D layer passivating the 3D material, or in energy‐transfer devices requiring controlled energy flow from localized surface crystallites to the bulk.	Nov 2019
I07-Surface & interface diffraction	A highly emissive surface layer in mixed‐halide multication perovskites Zahra Andaji-garmaroudi , Mojtaba Abdi-jalebi , Dengyang Guo , Stuart Macpherson , Aditya Sadhanala , Elizabeth M. Tennyson , Edoardo Ruggeri , Miguel Anaya , Krzysztof Galkowski , Ravichandran Shivanna , Kilian Lohmann , Kyle Frohna , Sebastian Mackowski , Tom J. Savenije , Richard H. Friend , Samuel D. Stranks Advanced Materials DOI: 10.1002/adma.201902374 Diamond Proposal Number(s): [17223] Open Access Show Abstract ▼ Abstract: Mixed‐halide lead perovskites have attracted significant attention in the field of photovoltaics and other optoelectronic applications due to their promising bandgap tunability and device performance. Here, the changes in photoluminescence and photoconductance of solution‐processed triple‐cation mixed‐halide (Cs0.06MA0.15FA0.79)Pb(Br0.4I0.6)3 perovskite films (MA: methylammonium, FA: formamidinium) are studied under solar‐equivalent illumination. It is found that the illumination leads to localized surface sites of iodide‐rich perovskite intermixed with passivating PbI2 material. Time‐ and spectrally resolved photoluminescence measurements reveal that photoexcited charges efficiently transfer to the passivated iodide‐rich perovskite surface layer, leading to high local carrier densities on these sites. The carriers on this surface layer therefore recombine with a high radiative efficiency, with the photoluminescence quantum efficiency of the film under solar excitation densities increasing from 3% to over 45%. At higher excitation densities, nonradiative Auger recombination starts to dominate due to the extremely high concentration of charges on the surface layer. This work reveals new insight into phase segregation of mixed‐halide mixed‐cation perovskites, as well as routes to highly luminescent films by controlling charge density and transfer in novel device structures.	Sep 2019
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
I07-Surface & interface diffraction	Layered mixed tin-lead hybrid perovskite solar cells with high stability Daniel Ramirez , Kelly Schutt , Zhiping Wang , Andrew J. Pearson , Edoardo Ruggeri , Henry J. Snaith , Samuel D. Stranks , Franklin Jaramillo Acs Energy Letters DOI: 10.1021/acsenergylett.8b01411 Diamond Proposal Number(s): [17223] Show Abstract ▼ Abstract: Tin-lead mixed metal hybrid perovskites with tunable band gaps are attractive candidates to be used as the low-band gap cell in high efficiency tandem solar cells. Nevertheless, perovskites containing tin have a greater propensity to degrade due to the fast oxidation of Sn2+ to Sn4+, which is a restrictive factor in the development of these materials. Although significant improvements are achieved with Pb:Sn mixed-metal perovskites, in comparison to neat Sn perovskites, the intrinsic instability may still pose a threat to long-term operation. For neat Pb perovskites, two dimensional (2D) hybrid perovskites, where n layers of inorganic material are separated by a long chain organic cation, generally exhibit greater stability but have lower photovoltaic performance characteristics, motivating the study of 2D/3D mixed dimension systems to realize both high efficiency and stability. In this report we demonstrate such optimal compromise between performance and stability using formamidinium, cesium and t-butylammonium as A-site cations with Pb:Sn mixed metal low band gap perovskites. As determined by film structure measurements, the optimised 2D perovskite phases facilitate improved luminescence efficiency, which we infer to be via surface defect site passivation. Perovskite solar cells based on n = 4 and n = 5 lead-tin perovskites achieved power conversion efficiencies of up to 9.3% and 10.6%, respectively and correspondingly retained 47% and 29% of their initial efficiency during storage in nitrogen for 2000 hours. A similar stability trend for n = 4 over n = 5 was also observed for unencapsulated devices during continuous operation under combined air atmosphere and temperature for 10 hours, resulting in improved stability over the 3D lead-tin counterpart.	Aug 2018
I09-Surface and Interface Structural Analysis	Maximizing and stabilizing luminescence from halide perovskites with potassium passivation Mojtaba Abdi-jalebi , Zahra Andaji-garmaroudi , Stefania Cacovich , Camille Stavrakas , Bertrand Philippe , Johannes M. Richter , Mejd Alsari , Edward P. Booker , Eline M. Hutter , Andrew J. Pearson , Samuele Lilliu , Tom J. Savenije , Hakan Rensmo , Giorgio Divitini , Caterina Ducati , Richard H. Friend , Samuel D. Stranks Nature 555, 497 - 501 DOI: 10.1038/nature25989 Diamond Proposal Number(s): [15841] Show Abstract ▼ Abstract: Metal halide perovskites are of great interest for various high-performance optoelectronic applications. The ability to tune the perovskite bandgap continuously by modifying the chemical composition opens up applications for perovskites as coloured emitters, in building-integrated photovoltaics, and as components of tandem photovoltaics to increase the power conversion efficiency. Nevertheless, performance is limited by non-radiative losses, with luminescence yields in state-of-the-art perovskite solar cells still far from 100 per cent under standard solar illumination conditions. Furthermore, in mixed halide perovskite systems designed for continuous bandgap tunability (bandgaps of approximately 1.7 to 1.9 electronvolts), photoinduced ion segregation leads to bandgap instabilities. Here we demonstrate substantial mitigation of both non-radiative losses and photoinduced ion migration in perovskite films and interfaces by decorating the surfaces and grain boundaries with passivating potassium halide layers. We demonstrate external photoluminescence quantum yields of 66 per cent, which translate to internal yields that exceed 95 per cent. The high luminescence yields are achieved while maintaining high mobilities of more than 40 square centimetres per volt per second, providing the elusive combination of both high luminescence and excellent charge transport. When interfaced with electrodes in a solar cell device stack, the external luminescence yield—a quantity that must be maximized to obtain high efficiency—remains as high as 15 per cent, indicating very clean interfaces. We also demonstrate the inhibition of transient photoinduced ion-migration processes across a wide range of mixed halide perovskite bandgaps in materials that exhibit bandgap instabilities when unpassivated. We validate these results in fully operating solar cells. Our work represents an important advance in the construction of tunable metal halide perovskite films and interfaces that can approach the efficiency limits in tandem solar cells, coloured-light-emitting diodes and other optoelectronic applications.	Mar 2018
I22-Small angle scattering & Diffraction	Atmospheric Influence upon Crystallization and Electronic Disorder and Its Impact on the Photophysical Properties of OrganicInorganic Perovskite Solar Cells Sandeep Pathak , Alessandro Sepe , Aditya Sadhanala , Felix Deschler , Amir Haghighirad , Nobuya Sakai , Karl C. Goedel , Samuel D. Stranks , Nakita Noel , Michael Price , Sven Hüttner , Nicholas A. Hawkins , Richard H. Friend , Ullrich Steiner , Henry J. Snaith Acs Nano 9 (3), 2311 - 2320 DOI: 10.1021/nn506465n Diamond Proposal Number(s): [8459] Show Abstract ▼ Abstract: Recently, solution-processable organic–inorganic metal halide perovskites have come to the fore as a result of their high power-conversion efficiencies (PCE) in photovoltaics, exceeding 17%. To attain reproducibility in the performance, one of the critical factors is the processing conditions of the perovskite film, which directly influences the photophysical properties and hence the device performance. Here we study the effect of annealing parameters on the crystal structure of the perovskite films and correlate these changes with its photophysical properties. We find that the crystal formation is kinetically driven by the annealing atmosphere, time and temperature. Annealing in air produces an improved crystallinity and large grain domains as compared to nitrogen. Lower photoluminescence quantum efficiency (PLQE) and shorter photoluminescence (PL) lifetimes are observed for nitrogen annealed perovskite films as compared to the air-annealed counterparts. We note that the limiting nonradiative pathways (i.e., maximizing PLQE) is important for obtaining the highest device efficiency. This indicates a critical impact of the atmosphere upon crystallization and the ultimate device performance.	Mar 2015

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