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Recent Projects

A selection of recent and ongoing projects are described below:

Impact of Microstructure on the Electron-hole Interaction in Metal Halide Perovskites

Collaborators: UNSW; Laboratoire National des Champs Magnetiques Intenses, Toulouse; Monash University; University of Warsaw; MIT; University of Oxford; University of Cambridge

Despite the remarkable progress in the performance of devices based on the metal halide perovskite semiconductor family, there is still a lack of consensus on their fundamental photophysical properties. Here, using magneto-optical transmission spectroscopy we elucidate the impact of the microstructure on the Coulomb interaction between photo-created electron-hole pairs in methylammonium lead triiodide (MAPbI3) and the triple-cation lead mixed-halide composition, Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)(Cs: Cesium, MA: methylammonium, FA: formamidinium) by investigating thin films with a wide range of grain sizes from tens of nanometers to microns. At low temperatures, in which thermal fluctuations of the interactions are frozen and the rotational disorder of the organic cation is negligible, the exciton binding energy and reduced effective mass of carriers remain effectively unchanged with grain size. We conclude that the microstructure plays a negligible role in the Coulomb interaction of the photo-created electron-hole pairs, in contrast to previous reports. This renewed understanding of the relationship between these fundamental electronic properties and the microstructure is critical for future fundamental studies and improving device design.

Relevant publications:

  • Soufiani, A.ng, Z.; Young, T.; Miyata, A.; Surrente, A.; Pascoe, A.; Galkowski, K.; Abdi-Jalebi, M.; Brenes, R.; Urban, J.; Zhang, N.; Bulović, V.; Portugall, O.; Cheng, Y.-B.; Nicholas, R. J.; *Ho-Baillie, A.; Green, M. A.; *Plochocka, P.; *Stranks, S. D., Impact of microstructure on the electron–hole interaction in lead halide perovskitesEnergy Environ. Sci.2017101358-1366
  • Miyata, A.; Mitioglu, A.; Plochocka, P.; Portugal, O.; Wang, J. T. W.; Stranks, S. D.; Snaith, H. J.; Nicholas, R. J., Direct Measurement of the Exciton Binding Energy and Effective Masses for Charge carriers in Organic-Inorganic Tri-halide PerovskitesNat. Physics, 201511582-587
  • Galkowski, K.; Mitioglu, A.; Miyata, A.; Plochocka, P.; Portugal, O.; Eperon, G. E.; Wang, J. T. W.; Sterogiopoulos, T.; Stranks, S. D.; Snaith, H. J.; Nicholas, R. J., Determination of the exciton binding energy and effective masses for methylammonium and formamidinium lead tri-halide perovskite semiconductorsEnergy. Environ. Sci., 20169962-970
BEgrainsize

Plot showing that the exciton binding energy and effective mass is invariant with grain size.


Direct–indirect character of the bandgap in methylammonium lead iodide perovskite

Collaborators: Delft University of Technology, MIT, University of Cambridge

Metal halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) are generating great excitement due to their outstanding optoelectronic properties, which lend them to application in high-efficiency solar cells and light-emission devices. However, there is currently debate over what drives the second-order electron–hole recombination in these materials. Here, we propose that the bandgap in CH3NH3PbI3 has a direct–indirect character. Time-resolved photo-conductance measurements show that generation of free mobile charges is maximized for excitation energies just above the indirect bandgap. Furthermore, we find that second-order electron–hole recombination of photo-excited charges is retarded at lower temperature. These observations are consistent with a slow phonon-assisted recombination pathway via the indirect bandgap. Interestingly, in the low-temperature orthorhombic phase, fast quenching of mobile charges occurs independent of the temperature and photon excitation energy. Our work provides a new framework to understand the optoelectronic properties of metal halide perovskites and analyse spectroscopic data.

Relevant publications:

  • Hutter, E. M.; Osherov, A.; Gelvez-Rueda, M. C.; Bulovic, V.; Grozema, F. C.; *Stranks, S. D.; *Savenije, T. Direct-Indirect Bandgap Character of the Band Gap in Methylammonium Lead Iodide Perovskite, Nat. Mater., 2017, 16115-120 

nmater

Left: Proposed indirect-direct bandgap character, right: activation energy of charges thermally-activated back to the direct band.


The Impact of Phase Retention on the Structural and Optoelectronic Properties of Metal Halide Perovskites

Collaborators: MIT; Delft University of Technology; CNRS-Toulouse; University of Oxford; University of Cambridge

Metal halide perovskites have come to the forefront of the research community due to their excellent performance in optoelectronic devices. However, the extent to which their soft structural properties, resulting from their ionic nature, influence the optoelectronic performance is unclear. Here, we use temperature-dependent X-Ray Diffraction (XRD) measurements to study the low-temperature tetragonal-orthorhombic phase transition in thin films and powders of CH3NH3PbI3. We find there is a strong hysteresis in the phase transition temperature when cooling compared to heating. Intriguingly, we find that the phase transition hysteresis effects manifest themselves in other important properties including photoluminescence, absorption and photo-conductance, suggesting an intimate relationship between structure, substrate and optoelectronic performance. We use micro-photoluminescence measurements to reveal that the origin of the hysteretic effect is the coexistence of microscopic inclusions of both phases even above and below the nominal phase transitions. Furthermore, cooling/heating cycling through the tetragonal-orthorhombic phase transition induces a change in the texture of the films and a fusing together of smaller grains into larger grains, resulting in a dramatic improvement in emission properties and revealing strong links between crystal orientation and non-radiative decay losses. Our work will guide control of perovskite fabrication towards further improved optoelectronic properties and devices.

Relevant publications:

  • Osherov, A.; Hutter, E. M.; Galkowski, K.; Brenes, R.; Maude, D. K.; Nicholas, R. J.; Plochocka, P.; Bulović, V.; Savenije, T. J.; *Stranks, S. D. The Impact of Phase Retention on the Structural and Optoelectronic Properties of Metal Halide PerovskitesAdv. Mater.,2016, 28, 10757-10763 phase

d-spacing extracted from XRD measurements when cycling through temperature to show hysteresis in the phase. Inset: Micro-PL showing emitting inclusions of each phase.


Photo-Induced Halide Redistribution in Organic-Inorganic Perovskite Films

Collaborators: University of Washington, University of Oxford, MIT, University of Cambridge

Organic-inorganic perovskites such as CH3NH3PbI3 are promising materials for a variety of optoelectronic applications, with certified power conversion efficiencies in solar cells already exceeding 21%. Nevertheless, state-of-the-art films still contain performance-limiting non-radiative recombination sites and exhibit a range of complex dynamic phenomena under illumination that remain poorly understood. Here, we use a unique combination of confocal photoluminescence (PL) microscopy and chemical imaging to correlate the local changes in photophysics with composition in CH3NH3PbI3 films under illumination. We demonstrate that the photo-induced “brightening” of the perovskite PL can be attributed to an order-of-magnitude reduction in trap state density. By imaging the same regions with time-of-flight secondary-ion-mass spectrometry (ToF-SIMS), we correlate this photobrightening with a net migration of iodine. Our work provides visual evidence for photo-induced halide migration in triiodide perovskites and reveals the complex interplay between charge carrier populations, electronic traps, and mobile halides which collectively impact optoelectronic performance.

Relevant publications:

  •  deQuilettes, D. W.; Zhang, W.; Burlakov, V. M.; Graham, D. J.; Leijtens, T.; Bulovic, V.; Ginger, D. S.; Snaith, H. J., *Stranks, S. D.  Photo-Induced Halide Redistribution in Organic-Inorganic Perovskite FilmsNat. Commun., 2016, 7, 11683

Top panel: Map showing iodine counts in a film illuminated in the region highlighted by the red dashed line, showing a depletion of iodine in the illuminated region and an enrichment in the adjacent region. These local chemical changes correspond to a rise in photoluminescence intensity of the same spot (bottom panel).


Impact of microstructure on local carrier lifetime in perovskite solar cells

Collaborators: University of Washington, University of Oxford

The remarkable performance of hybrid perovskite photovoltaics is attributed to their long carrier lifetimes and high photoluminescence (PL) efficiencies. High-quality films are associated with slower PL decays, and it has been claimed that grain boundaries have a negligible impact on performance.We used confocal fluorescence microscopy correlated with scanning electron microscopy to spatially resolve the PL decay dynamics from films of nonstoichiometric organic-inorganic perovskites, CH3NH3PbI3(Cl). The PL intensities and lifetimes varied between different grains in the same film, even for films that exhibited long bulk lifetimes. The grain boundaries were dimmer and exhibited faster nonradiative decay. Energy-dispersive x-ray spectroscopy showed a positive correlation between chlorine concentration and regions of brighter PL, whereas PL imaging revealed that chemical treatment with pyridine could activate previously dark grains.

Relevant publications:

  • deQuilettes, D. W.; Vorpahl, S. M.; Stranks, S. D.; Nagaoka, H.; Eperon, G. E.; Ziffer, M. E.; Snaith, H. J.; Ginger, D. S., Impact of Microstructure on Local Carrier Lifetime in Perovskite Solar CellsScience, 2015, 348683-686

Photoluminescence map of a perovskite film overlaid over a scanning electron microscope (SEM) image


Enhanced Amplified Spontaneous Emission in Perovskites Using a Flexible Cholesteric Liquid Crystal Reflector

Collaborators: University of Oxford, University of Cambridge, Eindhoven University of Technology

Organic−inorganic perovskites are highly promising solar cell materials with laboratory-based power conversion efficiencies already matching those of established thin film technologies. Their exceptional photovoltaic performance is in part attributed to the presence of efficient radiative recombination pathways, thereby opening up the possibility of efficient light-emitting devices. Here, we demonstrate optically pumped amplified spontaneous emission (ASE) at 780 nm from a 50 nm-thick film of CH3NH3PbI3 perovskite that is sandwiched within a cavity composed of a thin-film (∼7 μm) cholesteric liquid crystal (CLC) reflector and a metal back-reflector. The threshold fluence for ASE in the perovskite film is reduced by at least two orders of magnitude in the presence of the CLC reflector, which results in a factor of two reduction in threshold fluence compared to previous reports. We consider this to be due to improved coupling of the oblique and out of-plane modes that are reflected into the bulk in addition to any contributions from cavity modes. Furthermore, we also demonstrate enhanced ASE on flexible reflectors and discuss how improvements in the quality factor and reflectivity of the CLC layers could lead to single-mode lasing using CLC reflectors. Our work opens up the possibility of fabricating widely wavelengthtunable “mirror-less” single-mode lasers on flexible substrates, which could find use in applications such as flexible displays and friend or foe identification.

Relevant publications:

  • Stranks, S. D.; Wood, S. M.; Wojciechowski, K.; Deschler, F.; Saliba, M.; Khandelwal, H.; Patel, J. B.; Elston, J. S.; Herz, L. M.; Johnston, M. B.; Schenning, A. P. H. J.; Debije, M. G.; Riede, M. K.; Morris, S. M.; Snaith, H. J. Enhanced Amplified Spontaneous Emission in Perovskites using a Liquid Crystal ReflectorNano Lett., 2015, 15, 4935-4941

Optically-pumped amplified spontaneous emission (ASE) from a thin perovskite film deposited on a flexible cholesteric liquid crystal reflector


Recombination kinetics in organic-inorganic perovskites

Collaborators: University of Oxford, Delft University of Technology

Organic-inorganic perovskites are attracting increasing attention for their use in high performance solar cells. Nevertheless, a detailed understanding of charge generation, interplay of excitons and free charge carriers, and recombination pathways, crucial for further device improvement, remains incomplete. Here, we present an analytical model describing both equilibrium properties of free charge carriers and excitons in the presence of electronic subgap trap states and their time evolution after photoexcitation in CH3NH3PbI3−xClx. At low fluences the charge-trapping pathways limit the photoluminescence quantum efficiency, whereas at high fluences the traps are predominantly filled and recombination of the photogenerated species is dominated by efficient radiative processes. We show experimentally that the photoluminescence quantum efficiency approaches 100% at low temperatures and at high fluences, as predicted by our model. Our approach provides a theoretical framework to understand the fundamental physics of perovskite semiconductors and to help in designing and enhancing the material for improved optoelectronic device operation.

Relevant publications:

  • Stranks, S. D.; Burlakov, V. M.; Leijtens, T.; Ball, J. M.; Goriely, A.; Snaith, H. J. Recombination Kinetics in Organic-Inorganic Perovskites: Excitons, Free Charge, and Subgap StatesPhys. Rev. Applied, 2014, 2, 3.
  • Hutter, E. M.; Eperon, G. E.; Stranks, S. D.; Savenije, T. J., Charge Carriers in Planar and Meso-Structured Organic-Inorganic Perovskites: Mobilities, Lifetimes and Concentrations of Trap States. J. Phys. Chem. Lett., 2015,63082-3090

Understanding the fluence-dependent photoluminescence decays from a perovskite film by invoking a model including recombination via sub-gap trap states


Determining the diffusion length in organic-inorganic perovskites

Collaborators: University of Oxford, IIT Milan

Organic-inorganic perovskites have shown promise as high-performance absorbers in solar cells, first as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures. Here, we report transient absorption and photoluminescence-quenching measurements to determine the electron-hole diffusion lengths, diffusion constants, and lifetimes in mixed halide (CH3NH3PbI3-xClx) and triiodide (CH3NH3PbI3) perovskite absorbers. We found that the diffusion lengths are greater than 1 micrometer in the mixed halide perovskite, which is an order of magnitude greater than the absorption depth. In contrast, the triiodide absorber has electron-hole diffusion lengths of ~100 nanometers. These results justify the high efficiency of planar heterojunction perovskite solar cells and identify a critical parameter to optimize for future perovskite absorber development.

Relevant publications:

  • Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding One Micrometer in an Organometal Trihalide Perovskite AbsorberScience2013342, 341-34

A. Scanning electron microscope cross-section image of a thin perovskite film with a hole-quenching Spiro-OMeTAD layer. B. Photoluminescence decays for perovskite films with non-quenching PMMA or with hole-quenching Spiro-OMeTAD or electron-quenching fullerene PCBM, with fits to the data shown with the solid lines


Novel Carbon Nanotube-Conjugated Polymer Nanohybrids Produced By Multiple Polymer Processing

Collaborators: University of Oxford, Umea University

We describe two methods in which we manipulate the binding of multiple conjugated polymers to single-walled carbon nanotubes (SWNTs) to produce new and novel nanostructures. One method first utilizes the selective binding of poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) to a narrow distribution of semiconducting SWNTs and then uses a polymer exchange to transfer this purity to other nanotube-polymer combinations, using technologically useful polymers such as poly(3-hexylthiophene) (P3HT) and poly(9,9′-dioctylfluorene-co-benzothiadiazole) (F8BT) as first examples. The other method involves controlling the competitive binding of P3HT and F8BT to SWNTs to produce coaxial nanostructures consisting of both polymers simultaneously bound in ordered layers. We show that these two simple solution-processing techniques can be carried out sequentially to afford new dual-polymer nanostructures comprised of a semiconducting SWNT of a single chirality. This allows the favorable properties of both polymers and purified semiconducting SWNTs to be implemented into potentially highly efficient organic photovoltaic devices.

Relevant publications:

  • *Barbero, D. R.; *Stranks, S. D. Functional Single-Walled Carbon Nanotubes and Nanoengineered Networks for Organic- and Perovskite-Solar-Cell ApplicationsAdv. Mater.2016, 28, 9668–9685
  • Stranks, S. D.; Habisreutinger, S. N.; Dirks, B.; Nicholas, R. J. Novel Carbon Nanotube-Conjugated Polymer Nanohybrids Produced By Multiple Polymer ProcessingAdv. Mater., 2013, 25, 4365–4371
  • Stranks, S. D.; Baker, A. M. H.; Alexander-Webber, J. A.; Dirks, B.; Nicholas, R. J. Production of High Purity Single Chirality Carbon Nanotube Hybrids by Selective Polymer ExchangeSmall, 2013, 9, 2245-2249
  • Stranks, S. D.; Yong, C-K.; Alexander-Webber, J. A.; Weisspfennig, C.; Johnston, M. B.; Herz, L. M.; Nicholas, R. J. Nano-engineering Coaxial Carbon Nanotube-Dual Polmer HeterostructuresACS Nano, 201266058-6066
  • Stranks, S. D.; Parkinson, P.; Weisspfennig, C.; Johnston, M. B.; Herz, L. M.; Nicholas, R. J. Ultrafast Charge Separation at a Polymer−Single-Walled Carbon Nanotube Molecular JunctionNano Lett., 20111166–72

 

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