I14-Hard X-ray Nanoprobe
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Diamond Proposal Number(s):
[20420]
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.
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Mar 2021
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I09-Surface and Interface Structural Analysis
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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
Diamond Proposal Number(s):
[22668]
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.
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Nov 2020
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Anna
Abfalterer
,
Javad
Shamsi
,
Dominik J.
Kubicki
,
Christopher N.
Savory
,
James
Xiao
,
Giorgio
Divitini
,
Weiwei
Li
,
Stuart
Macpherson
,
Krzysztof
Gałkowski
,
Judith L.
Macmanus-driscoll
,
David O.
Scanlon
,
Samuel D.
Stranks
Open Access
Abstract: Optoelectronic devices based on lead halide perovskites are processed in facile ways, yet are remarkably efficient. There are extensive research efforts investigating lead-free perovskite and perovskite-related compounds, yet there are challenges to synthesize these materials in forms that can be directly integrated into thin film devices rather than as bulk powders. Here, we report on the colloidal synthesis and characterization of lead-free, antifluorite Cs2ZrX6 (X = Cl, Br) nanocrystals that are readily processed into thin films. We use transmission electron microscopy and powder X-ray diffraction measurements to determine their size and structural properties, and solid-state nuclear magnetic resonance measurements reveal the presence of oleate ligand, together with a disordered distribution of Cs surface sites. Density functional theory calculations reveal the band structure and fundamental band gaps of 5.06 and 3.91 eV for Cs2ZrCl6 and Cs2ZrBr6, respectively, consistent with experimental values. Finally, we demonstrate that the Cs2ZrCl6 and Cs2ZrBr6 nanocrystal thin films exhibit tunable, broad white photoluminescence with quantum yields of 45% for the latter, with respective peaks in the blue and green spectral regions and mixed systems exhibiting properties between them. Our work represents a critical step toward the application of lead-free Cs2ZrX6 nanocrystal thin films into next-generation light-emitting applications.
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Nov 2020
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I07-Surface & interface diffraction
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Baodan
Zhao
,
Yaxiao
Lian
,
Linsong
Cui
,
Giorgio
Divitini
,
Gunnar
Kusch
,
Edoardo
Ruggeri
,
Florian
Auras
,
Weiwei
Li
,
Dexin
Yang
,
Bonan
Zhu
,
Rachel A.
Oliver
,
Judith L.
Macmanus-driscoll
,
Samuel D.
Stranks
,
Dawei
Di
,
Richard H.
Friend
Diamond Proposal Number(s):
[17223]
Abstract: Light-emitting diodes based on halide perovskites have recently reached external quantum efficiencies of over 20%. However, the performance of visible perovskite light-emitting diodes has been hindered by non-radiative recombination losses and limited options for charge-transport materials that are compatible with perovskite deposition. Here, we report efficient, green electroluminescence from mixed-dimensional perovskites deposited on a thin (~1 nm) lithium fluoride layer on an organic semiconductor hole-transport layer. The highly polar dielectric interface acts as an effective template for forming high-quality bromide perovskites on otherwise incompatible hydrophobic charge-transport layers. The control of crystallinity and dimensionality of the perovskite layer is achieved by using tetraphenylphosphonium chloride as an additive, leading to external photoluminescence quantum efficiencies of around 65%. With this approach, we obtain light-emitting diodes with external quantum efficiencies of up to 19.1% at high brightness (>1,500 cd m−2).
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Oct 2020
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E02-JEM ARM 300CF
I14-Hard X-ray Nanoprobe
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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
Diamond Proposal Number(s):
[19023, 19793]
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.
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Apr 2020
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[17223]
Open Access
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.
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Nov 2019
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I07-Surface & interface diffraction
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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
Diamond Proposal Number(s):
[17223]
Open Access
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.
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Sep 2019
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E02-JEM ARM 300CF
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Diamond Proposal Number(s):
[19793]
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.
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Jul 2019
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[17223]
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.
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Aug 2018
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I09-Surface and Interface Structural Analysis
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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
Diamond Proposal Number(s):
[15841]
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.
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Mar 2018
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