E02-JEM ARM 300CF
I14-Hard X-ray Nanoprobe
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Stuart
Macpherson
,
Tiarnan A. S.
Doherty
,
Andrew J.
Winchester
,
Sofiia
Kosar
,
Duncan N.
Johnstone
,
Yu-Hsien
Chiang
,
Krzysztof
Galkowski
,
Miguel
Anaya
,
Kyle
Frohna
,
Affan N.
Iqbal
,
Satyawan
Nagane
,
Bart
Roose
,
Zahra
Andaji-Garmaroudi
,
Kieran W. P.
Orr
,
Julia E.
Parker
,
Paul A.
Midgley
,
Keshav M.
Dani
,
Samuel D.
Stranks
Diamond Proposal Number(s):
[24111, 20420]
Abstract: Understanding the nanoscopic chemical and structural changes that drive instabilities in emerging energy materials is essential for mitigating device degradation. The power conversion efficiency of halide perovskite photovoltaic devices has reached 25.7% in single junction and 29.8% in tandem perovskite/silicon cells1,2, yet retaining such performance under continuous operation has remained elusive3. Here, we develop a multimodal microscopy toolkit to reveal that in leading formamidinium-rich perovskite absorbers, nanoscale phase impurities including hexagonal polytype and lead iodide inclusions are not only traps for photo-excited carriers which themselves reduce performance4,5, but via the same trapping process are sites at which photochemical degradation of the absorber layer is seeded. We visualise illumination-induced structural changes at phase impurities associated with trap clusters, revealing that even trace amounts of these phases, otherwise undetected with bulk measurements, compromise device longevity. The type and distribution of these unwanted phase inclusions depends on film composition and processing, with the presence of polytypes being most detrimental for film photo-stability. Importantly, we reveal that performance losses and intrinsic degradation processes can both be mitigated by modulating these defective phase impurities, and demonstrate that this requires careful tuning of local structural and chemical properties. This multimodal workflow to correlate the nanoscopic landscape of beam sensitive energy materials will be applicable to a wide range of semiconductors for which a local picture of performance and operational stability has yet to be established.
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May 2022
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E02-JEM ARM 300CF
I14-Hard X-ray Nanoprobe
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Diamond Proposal Number(s):
[25250, 20420]
Abstract: The interaction of high-energy electrons and X-ray photons with beam-sensitive semiconductors such as halide perovskites is essential for the characterisation and understanding of these optoelectronic materials. Using nano-probe diffraction techniques, which can investigate physical properties on the nanoscale, we perform studies of the interaction of electron and X-ray radiation with state-of-the-art (FA0.79MA0.16Cs0.05)Pb(I0.83Br0.17)3 hybrid halide perovskite films (FA, formamidinium; MA, methylammonium). We track the changes in the local crystal structure as a function of fluence using scanning electron diffraction and synchrotron nano X-ray diffraction techniques. We identify perovskite grains from which additional reflections, corresponding to PbBr2, appear as a crystalline degradation phase after fluences of ∼200 e–Å–2. These changes are concomitant with the formation of small PbI2 crystallites at the adjacent high-angle grain boundaries, with the formation of pinholes, and with a phase transition from tetragonal to cubic. A similar degradation pathway is caused by photon irradiation in nano-X-ray diffraction, suggesting common underlying mechanisms. Our approach explores the radiation limits of these materials and provides a description of the degradation pathways on the nanoscale. Addressing high-angle grain boundaries will be critical for the further improvement of halide polycrystalline film stability, especially for applications vulnerable to high-energy radiation such as space photovoltaics.
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Mar 2022
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E02-JEM ARM 300CF
I14-Hard X-ray Nanoprobe
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Tiarnan A. S.
Doherty
,
Dominik
Kubicki
,
Stuart
Macpherson
,
Young-Kwang
Jung
,
Duncan
Johnstone
,
Affan
Iqbal
,
Dengyang
Guo
,
Kyle
Frohna
,
Mohsen
Danaie
,
Elizabeth
Tennyson
,
Satyawan
Nagane
,
Anna
Abfalterer
,
Miguel
Anaya
,
Yu-Hsien
Chiang
,
Phillip
Crout
,
Francesco Simone
Ruggeri
,
Sean
Collins
,
Clare
Grey
,
Aron
Walsh
,
Paul
Midgley
,
Samuel
Stranks
Diamond Proposal Number(s):
[20420, 24111]
Abstract: There is currently substantial interest in stabilizing the simple ternary FAPbI3 perovskite because of its near-optimal band gap and superior thermal stability compared to methylammonium-based materials.1 The key challenge of FAPbI3 is the thermodynamic instability of the polymorph required for efficient light harvesting. Without additives, the black photoactive α-polymorph is only stable above ca. 160°C. At room temperature, it is metastable and rapidly transitions to the non-perovskite yellow polymorph. The stabilization of the black polymorph at room temperature can be achieved, for example, by adding a small amount of the pernicious MA through use of methylammonium chloride (in conjunction with formamidinium formate),2 methylammonium thiocyanate,3 or methylammonium formate.4 We have developed a new stabilization strategy which does not involve the addition of MA.5 Instead, it uses a surface-templating agent (EDTA) which modifies the material without incorporating into the structure. We use a combination of scanning electron diffraction (SED) and nuclear magnetic resonance spectroscopies (NMR, NQR) to identify the atomic-level mechanism of action of EDTA in this role. We find that it templates the structure by inducing a small octahedral tilt, only resolvable with local characterization techniques, and imparts remarkable phase stability by arresting transitions to low-dimensional polymorphs. This octahedral tilt engineering strategy is remarkably universal, and we show that it is the intrinsic stabilization mechanism in the state-of-the-art FA-rich mixed-cation materials.
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Feb 2022
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E02-JEM ARM 300CF
I14-Hard X-ray Nanoprobe
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Tiarnan A. S.
Doherty
,
Satyawan
Nagane
,
Dominik J.
Kubicki
,
Young-Kwang
Jung
,
Duncan N.
Johnstone
,
Affan N.
Iqbal
,
Dengyang
Guo
,
Kyle
Frohna
,
Mohsen
Danaie
,
Elizabeth M.
Tennyson
,
Stuart
Macpherson
,
Anna
Abfalterer
,
Miguel
Anaya
,
Yu-Hsien
Chiang
,
Phillip
Crout
,
Francesco Simone
Ruggeri
,
Sean M.
Collins
,
Clare P.
Grey
,
Aron
Walsh
,
Paul A.
Midgley
,
Samuel D.
Stranks
Diamond Proposal Number(s):
[20420, 24111]
Abstract: Efforts to stabilize photoactive formamidinium (FA)–based halide perovskites for perovskite photovoltaics have focused on the growth of cubic formamidinium lead iodide (α-FAPbI3) phases by empirically alloying with cesium, methylammonium (MA) cations, or both. We show that such stabilized FA-rich perovskites are noncubic and exhibit ~2° octahedral tilting at room temperature. This tilting, resolvable only with the use of local nanostructure characterization techniques, imparts phase stability by frustrating transitions from photoactive to hexagonal phases. Although the bulk phase appears stable when examined macroscopically, heterogeneous cation distributions allow microscopically unstable regions to form; we found that these transitioned to hexagonal polytypes, leading to local trap-assisted performance losses and photoinstabilities. Using surface-bound ethylenediaminetetraacetic acid, we engineered an octahedral tilt into pure α-FAPbI3 thin films without any cation alloying. The templated photoactive FAPbI3 film was extremely stable against thermal, environmental, and light stressors.
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Dec 2021
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I14-Hard X-ray Nanoprobe
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Kyle
Frohna
,
Miguel
Anaya
,
Stuart
Macpherson
,
Jooyoung
Sung
,
Tiarnan A. S.
Doherty
,
Yu-Hsien
Chiang
,
Andrew J.
Winchester
,
Kieran W. P.
Orr
,
Julia E.
Parker
,
Paul D.
Quinn
,
Keshav M.
Dani
,
Akshay
Rao
,
Samuel D.
Stranks
Diamond Proposal Number(s):
[19023, 20420]
Abstract: Halide perovskites perform remarkably in optoelectronic devices. However, this exceptional performance is striking given that perovskites exhibit deep charge-carrier traps and spatial compositional and structural heterogeneity, all of which should be detrimental to performance. Here, we resolve this long-standing paradox by providing a global visualization of the nanoscale chemical, structural and optoelectronic landscape in halide perovskite devices, made possible through the development of a new suite of correlative, multimodal microscopy measurements combining quantitative optical spectroscopic techniques and synchrotron nanoprobe measurements. We show that compositional disorder dominates the optoelectronic response over a weaker influence of nanoscale strain variations even of large magnitude. Nanoscale compositional gradients drive carrier funnelling onto local regions associated with low electronic disorder, drawing carrier recombination away from trap clusters associated with electronic disorder and leading to high local photoluminescence quantum efficiency. These measurements reveal a global picture of the competitive nanoscale landscape, which endows enhanced defect tolerance in devices through spatial chemical disorder that outcompetes both electronic and structural disorder.
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Nov 2021
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I14-Hard X-ray Nanoprobe
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Sofiia
Kosar
,
Andrew J.
Winchester
,
Tiarnan A. S.
Doherty
,
Stuart
Macpherson
,
Christopher E.
Petoukhoff
,
Kyle
Frohna
,
Miguel
Anaya
,
Nicholas S.
Chan
,
Julien
Madéo
,
Michael K. L.
Man
,
Samuel D.
Stranks
,
Keshav M.
Dani
Diamond Proposal Number(s):
[19023]
Open Access
Abstract: With rapidly growing photoconversion efficiencies, hybrid perovskite solar cells have emerged as promising contenders for next generation, low-cost photovoltaic technologies. Yet, the presence of nanoscale defect clusters, that form during the fabrication process, remains critical to overall device operation, including efficiency and long-term stability. To successfully deploy hybrid perovskites, we must understand the nature of the different types of defects, assess their potentially varied roles in device performance, and understand how they respond to passivation strategies. Here, by correlating photoemission and synchrotron-based scanning probe X-ray microscopies, we unveil three different types of defect clusters in state-of-the-art triple cation mixed halide perovskite thin films. Incorporating ultrafast time-resolution into our photoemission measurements, we show that defect clusters originating at grain boundaries are the most detrimental for photocarrier trapping, while lead iodide defect clusters are relatively benign. Hexagonal polytype defect clusters are only mildly detrimental individually, but can have a significant impact overall if abundant in occurrence. We also show that passivating defects with oxygen in the presence of light, a previously used approach to improve efficiency, has a varied impact on the different types of defects. Even with just mild oxygen treatment, the grain boundary defects are completely healed, while the lead iodide defects begin to show signs of chemical alteration. Our findings highlight the need for multi-pronged strategies tailored to selectively address the detrimental impact of the different defect types in hybrid perovskite solar cells.
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Sep 2021
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I14-Hard X-ray Nanoprobe
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Elizabeth M.
Tennyson
,
Kyle
Frohna
,
William K.
Drake
,
Florent
Sahli
,
Terry
Chien-Jen Yang
,
Fan
Fu
,
Jérémie
Werner
,
Cullen
Chosy
,
Alan R.
Bowman
,
Tiarnan A. S.
Doherty
,
Quentin
Jeangros
,
Christophe
Ballif
,
Samuel D.
Stranks
Diamond Proposal Number(s):
[20420]
Open Access
Abstract: Halide perovskite/crystalline silicon (c-Si) tandem solar cells promise power conversion efficiencies beyond the limits of single-junction cells. However, the local light-matter interactions of the perovskite material embedded in this pyramidal multijunction configuration, and the effect on device performance, are not well understood. Here, we characterize the microscale optoelectronic properties of the perovskite semiconductor deposited on different c-Si texturing schemes. We find a strong spatial and spectral dependence of the photoluminescence (PL) on the geometrical surface constructs, which dominates the underlying grain-to-grain PL variation found in halide perovskite films. The PL response is dependent upon the texturing design, with larger pyramids inducing distinct PL spectra for valleys and pyramids, an effect which is mitigated with small pyramids. Further, optimized quasi-Fermi level splittings and PL quantum efficiencies occur when the c-Si large pyramids have had a secondary smoothing etch. Our results suggest that a holistic optimization of the texturing is required to maximize light in- and out-coupling of both absorber layers and there is a fine balance between the optimal geometrical configuration and optoelectronic performance that will guide future device designs.
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May 2021
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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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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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