Microsecond Carrier Lifetimes, Controlled p-Doping, and Enhanced Air Stability in Low-Bandgap Metal Halide Perovskites.

Mixed lead-tin halide perovskites have sufficiently low bandgaps (∼1.2 eV) to be promising absorbers for perovskite-perovskite tandem solar cells. Previous reports on lead-tin perovskites have typically shown poor optoelectronic properties compared to neat lead counterparts: short photoluminescence lifetimes (<100 ns) and low photoluminescence quantum efficiencies (<1%). Here, we obtain films with carrier lifetimes exceeding 1 μs and, through addition of small quantities of zinc iodide to the precursor solutions, photoluminescence quantum efficiencies under solar illumination intensities of 2.5%. The zinc additives also substantially enhance the film stability in air, and we use cross-sectional chemical mapping to show that this enhanced stability is because of a reduction in tin-rich clusters. By fabricating field-effect transistors, we observe that the introduction of zinc results in controlled p-doping. Finally, we show that zinc additives also enhance power conversion efficiencies and the stability of solar cells. Our results demonstrate substantially improved low-bandgap perovskites for solar cells and versatile electronic applications.EPSRC (EP/M005143/1 and DTP funding) Royal Society ER

Stabilized tilted-octahedra halide perovskites inhibit local formation of performance-limiting phases.

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

Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites.

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 devices1,2. This strong performance (albeit below the practical limits of about 30 per cent and 35 per cent, respectively3) is surprising in thin films processed from solution at low-temperature, a method that generally produces abundant crystalline defects4. Although point defects often induce only shallow electronic states in the perovskite bandgap that do not affect performance5, 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 performance6. The origin and distribution of these trap states are unknown, but they have been associated with light-induced halide segregation in mixed-halide perovskite compositions7 and with local strain8, both of which make devices less stable9. 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 process10,11, 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

Halide perovskites scintillators: unique promise and current limitations.

The widespread use of X- and gamma-rays in a range of sectors including healthcare, security and industrial screening is underpinned by the efficient detection of the ionising radiation. Such detector applications are dominated by indirect detectors in which a scintillating material is combined with a photodetector. Halide perovskites have recently emerged as an interesting class of semiconductors, showing enormous promise in optoelectronic applications including solar cells, light-emitting diodes and photodetectors. Here, we discuss how the same superior semiconducting properties that have catalysed their rapid development in these optoelectronic devices, including high photon attenuation and fast and efficient emission properties, also make them promising scintillator materials. By outlining the key mechanisms of their operation as scintillators, we show why reports of remarkable performance have already emerged, and describe how further learning from other optoelectronic devices will propel forward their applications as scintillators. Finally, we outline where these materials can make the greatest impact in detector applications by maximally exploiting their unique properties, leading to dramatic improvements in existing detection systems or introducing entirely new functionality

Strain analysis and engineering in halide perovskite photovoltaics.

Halide perovskites are a compelling candidate for the next generation of clean-energy-harvesting technologies owing to their low cost, facile fabrication and outstanding semiconductor properties. However, photovoltaic device efficiencies are still below practical limits and long-term stability challenges hinder their practical application. Current evidence suggests that strain in halide perovskites is a key factor in dictating device efficiency and stability. Here we outline the fundamentals of strain within halide perovskites relevant to photovoltaic applications and rationalize approaches to characterize the phenomenon. We examine recent breakthroughs in eliminating the adverse impacts of strain, enhancing both device efficiencies and operational stabilities. Finally, we discuss further challenges and outline future research directions for placing stress and strain studies at the forefront of halide perovskite research. An extensive understanding of strain in halide perovskites is needed, which would allow effective strain management and drive further enhancements in efficiencies and stabilities of perovskite photovoltaics

Nanoscale Chemical Heterogeneity Dominates the Optoelectronic Response of Alloyed Perovskite Solar Cells

Halide perovskites perform remarkably in optoelectronic devices including tandem photovoltaics. 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 visualisation 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 funneling 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.K.F. acknowledges a George and Lilian Schiff Studentship, Winton Studentship, the Engineering and Physical Sciences Research Council (EPSRC) studentship, Cambridge Trust Scholarship, and Robert Gardiner Scholarship. M.A. acknowledges funding from the Marie Skłodowska-Curie actions (grant agreement No. 841386) under the European Union’s Horizon 2020 research and innovation programme. S.M. and K.W.P.O. acknowledge EPSRC studentships. T.A.S.D. acknowledges a National University of Ireland Travelling Studentship. The authors acknowledge the Diamond Light Source (Didcot, Oxfordshire, UK) for providing beamtime at the I14 Hard X-ray Nanoprobe facility through proposals sp19023 and sp20420. S.D.S. acknowledges the Royal Society and Tata Group (UF150033). The work has received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (HYPERION - grant agreement no. 756962, SOLARX - grant agreement no. 758826).Y.H-C. thanks the Cambridge Trust for a studentship. The authors acknowledge the EPSRC (EP/R023980/1, EP/M006360/1) and the Winton Programme for the Physics of Sustainablity for funding. A.J.W. and K.M.D. acknowledge that this work was supported by the Femtosecond Spectroscopy Unit of the Okinawa Institute of Science and Technology Graduate University and JSPS Kakenhi Grant Number JP19K05637. The authors acknowledge the support for this work from the Imaging Section and Engineering Support Section of the Okinawa Institute of Science and Technology Graduate University

Composition Dictates Octahedral Tilt and Photostability in Halide Perovskites.

Publication status: PublishedFunder: Gonville and Caius College, CambridgeFunder: Cambridge Philosophical Society; doi: http://dx.doi.org/10.13039/100013858Funder: British Spanish SocietyFunder: Sir Richard Stapley Educational Trust; doi: http://dx.doi.org/10.13039/501100016406Funder: Rank Prize FundFunder: Ernest Oppenheimer FundFunder: Schmidt Science FellowshipsFunder: Taiwan Cambridge ScholarshipFunder: D.M.P. FoundationFunder: St John's College, CambridgeFunder: Royal Academy of Engineering; doi: http://dx.doi.org/10.13039/501100000287Funder: Diamond Light Source; doi: http://dx.doi.org/10.13039/100011889Halide perovskites are excellent candidate materials for use in solar cell, LED, and detector devices, in part because their composition can be tuned to achieve ideal optoelectronic properties. Empirical efficiency optimization has led the field toward compositions rich in FA (formamidinium) on the A-site and I on the X-site, with additional small amounts of MA (methylammonium) or Cs A-site cations and Br X-site anions. However, it is not clear how and why the specific compositions of alloyed, that is, mixed component, halide perovskites relate to photo-stability of the materials. Here, this work combines synchrotron grazing incidence wide-angle X-ray scattering, photoluminescence, high-resolution scanning electron diffraction measurements and theoretical modelling to reveal the links between material structure and photostability. Namely, this work finds that increased octahedral titling leads to improved photo-stability that is correlated with lower densities of performance-harming hexagonal polytype impurities. These results uncover the structural signatures underpinning photo-stability and can therefore be used to make targeted changes to halide perovskites, bettering the commercial prospects of technologies based on these materials

Imaging Light-Induced Migration of Dislocations in Halide Perovskites with 3d Nanoscale Strain Mapping.

In recent years, halide perovskite materials have been used to make high performance solar cell and light-emitting devices. However, material defects still limit device performance and stability. Here, we use synchrotron-based Bragg Coherent Diffraction Imaging to visualise nanoscale strain fields, such as those local to defects, in halide perovskite microcrystals. We find significant strain heterogeneity within MAPbBr3 (MA = CH3 NH3 + ) crystals in spite of their high optoelectronic quality, and identify both 〈100〉 and 〈110〉 edge dislocations through analysis of their local strain fields. By imaging these defects and strain fields in situ under continuous illumination, we uncover dramatic light-induced dislocation migration across hundreds of nanometers. Further, by selectively studying crystals that are damaged by the X-ray beam, we correlate large dislocation densities and increased nanoscale strains with material degradation and substantially altered optoelectronic properties assessed using photoluminescence microscopy measurements. Our results demonstrate the dynamic nature of extended defects and strain in halide perovskites, which will have important consequences for device performance and operational stability. This article is protected by copyright. All rights reserved

Imaging Light-Induced Migration of Dislocations in Halide Perovskites with 3d Nanoscale Strain Mapping.

Funder: King Abdullah University of Science and Technology; doi: http://dx.doi.org/10.13039/501100004052Funder: U.S. Department of Energy; doi: http://dx.doi.org/10.13039/100000015Funder: Office of Science; doi: http://dx.doi.org/10.13039/100006132Funder: HORIZON EUROPE European Research Council; doi: http://dx.doi.org/10.13039/100019180Funder: China Scholarship Council; doi: http://dx.doi.org/10.13039/501100004543Funder: British Spanish SocietyFunder: Sir Richard Stapley Educational Trust; doi: http://dx.doi.org/10.13039/501100016406Funder: Rank Prize FundFunder: Winton Sustainability FundFunder: George and Lilian Schiff FoundationFunder: Ernest Oppenheimer Early Career FellowshipFunder: Schmidt Science FellowshipIn recent years, halide perovskite materials have been used to make high-performance solar cells and light-emitting devices. However, material defects still limit device performance and stability. Here, synchrotron-based Bragg coherent diffraction imaging is used to visualize nanoscale strain fields, such as those local to defects, in halide perovskite microcrystals. Significant strain heterogeneity within MAPbBr3 (MA = CH3 NH3 + ) crystals is found in spite of their high optoelectronic quality, and both 〈100〉 and 〈110〉 edge dislocations are identified through analysis of their local strain fields. By imaging these defects and strain fields in situ under continuous illumination, dramatic light-induced dislocation migration across hundreds of nanometers is uncovered. Further, by selectively studying crystals that are damaged by the X-ray beam, large dislocation densities and increased nanoscale strains are correlated with material degradation and substantially altered optoelectronic properties assessed using photoluminescence microscopy measurements. These results demonstrate the dynamic nature of extended defects and strain in halide perovskites, which will have important consequences for device performance and operational stability

Research data supporting "Local Nanoscale Phase Impurities are Degradation Sites in Halide Perovskites"

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.Royal Society (UF150033) European Research Council (756962) European Commission Horizon 2020 (H2020) Marie Sklodowska-Curie actions (841136) European Commission Horizon 2020 (H2020) Marie Sklodowska-Curie actions (841386) Engineering and Physical Sciences Research Council (EP/S030638/1) EPSRC (EP/T02030X/1) EPSRC (2127077) EPSRC (EP/V012932/1) Engineering and Physical Sciences Research Council (EP/R023980/1