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Research interests

Carbon nanotubes

• Polymer-nanotube blends
• Photoluminescence Excitation (PLE) mapping
• Solution and film preparation

Ultrafast photophysical experiments

• Optical-pump terahertz-probe spectroscopy

Organic solar cells

• Heterojunction
• Dye-sensitised
• Carbon nanotubes as components

Publications

1. Structured organic-inorganic perovskite toward a distributed feedback laser,
   M Saliba, SM Wood, JB Patel, PK Nayak, J Huang, JA Alexander-webber, B Wenger, SD Stranks, MT Horantner, JTW Wang, RJ Nicholas, LM Herz, MB Johnston, SM Morris, HJ Snaith, MK Riede
   Adv. Mater., 28:923-929 (2016) [ abstract | pdf | doi:10.1002/adma.201502608 ]
   A general strategy for the in-plane structuring of organic–inorganic perovskite films is presented. The method is used to fabricate an industrially relevant distributed feedback (DFB) cavity, which is a critical step toward all-electrially pumped injection laser diodes. This approach opens the prospects of perovskite materials for much improved optical control in LEDs, solar cells, and also toward applications as optical devices.
2. Plasmonic-induced photon recycling in metal halide perovskite solar cells,
   M Saliba, W Zhang, VM Burlakov, SD Stranks, Y Sun, JM Ball, MB Johnston, A Goriely, U Wiesner, HJ Snaith
   Adv. Funct. Mater., 25:5038-5046 (2015) [ abstract | pdf | doi:10.1002/adfm.201500669 ]
   Organic-inorganic metal halide perovskite solar cells have emerged in the past few years to promise highly efficient photovoltaic devices at low costs. Here, temperature-sensitive core-shell Ag@TiO2 nanoparticles are successfully incorporated into perovskite solar cells through a low-temperature processing route, boosting the measured device efficiencies up to 16.3%. Experimental evidence is shown and a theoretical model is developed which predicts that the presence of highly polarizable nanoparticles enhances the radiative decay of excitons and increases the reabsorption of emitted radiation, representing a novel photon recycling scheme. The work elucidates the complicated subtle interactions between light and matter in plasmonic photovoltaic composites. Photonic and plasmonic schemes such as this may help to move highly efficient perovskite solar cells closer to the theoretical limiting efficiencies.
3. Enhanced amplified spontaneous emission in perovskites using a flexible cholesteric liquid crystal reflector,
   SD Stranks, SM Wood, K Wojciechowski, F Deschler, M Saliba, H Khandelwal, JB Patel, SJ Elston, LM Herz, MB Johnston, APHJ Schenning, MG Debije, MK Riede, SM Morris, HJ Snaith
   Nano Lett., 15:4935-4941 (2015) [ abstract | pdf | doi:10.1021/acs.nanolett.5b00678 ]
   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 urn from a 50 nm-thick film of CH3NH3PbI3 perovskite that is sandwiched within a cavity composed of a thin-film (similar to 7 mu m) 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 wavelength-tunable "mirror-less" single-mode lasers on flexible substrates, which could find use in applications such as flexible displays and friend or foe identification.
4. Efficient, semitransparent neutral-colored solar cells based on microstructured formamidinium lead trihalide perovskite,
   GE Eperon, D Bryant, J Troughton, SD Stranks, MB Johnston, T Watson, DA Worsley, HJ Snaith
   J. Phys. Chem. Lett., 6:129-138 (2015) [ abstract | pdf | doi:10.1021/jz502367k ]
   Efficient, neutral-colored semitransparent solar cells are of commercial interest for incorporation into the windows and surfaces of buildings and automobiles. Here, we report on semitransparent perovskite solar cells that are both efficient and neutral-colored, even in full working devices. Using the microstructured architecture previously developed, we achieve higher efficiencies by replacing methylammonium lead iodide perovskite with formamidinium lead iodide. Current voltage hysteresis is also much reduced. Furthermore, we apply a novel transparent cathode to the devices, enabling us to fabricate neutral-colored semitransparent full solar cells for the first time. Such devices demonstrate over 5% power conversion efficiency for average visible transparencies of almost 30%, retaining impressive color-neutrality. This makes these devices the best-performing single-junction neutral-colored semitransparent solar cells to date. These microstructured perovskite solar cells are shown to have a significant advantage over silicon solar cells in terms of performance at high incident angles of sunlight, making them ideal for building integration.
5. Optical properties and limiting photocurrent of thin-film perovskite solar cells,
   JM Ball, SD Stranks, MT Horantner, S Huttner, W Zhang, EJW Crossland, I Ramirez, M Riede, MB Johnston, RH Friend, HJ Snaith
   Energy Environ. Sci., 8:602-609 (2015) [ abstract | pdf | doi:10.1039/C4EE03224A ]
   Metal-halide perovskite light-absorbers have risen to the forefront of photovoltaics research offering the potential to combine low-cost fabrication with high power-conversion efficiency. Much of the development has been driven by empirical optimisation strategies to fully exploit the favourable electronic properties of the absorber layer. To build on this progress a full understanding of the device operation requires a thorough optical analysis of the device stack providing a platform for maximising the power conversion efficiency through a precise determination of parasitic losses caused by coherence and absorption in the non-photoactive layers. Here we use an optical model based on the transfer-matrix formalism for analysis of perovskite-based planar heterojunction solar cells using experimentally determined complex refractive index data. We compare the modelled properties to experimentally determined data and obtain good agreement revealing that the internal quantum efficiency in the solar cells approaches 100percent. The modelled and experimental dependence of the photocurrent on incidence angle exhibits only a weak variation{,} with very low reflectivity losses at all angles highlighting the potential for useful power generation over a full daylight cycle.
6. Solution deposition-conversion for planar heterojunction mixed halide perovskite solar cells,
   P Docampo, FC Hanusch, SD Stranks, M Doblinger, JM Feckl, M Ehrensperger, NK Minar, MB Johnston, HJ Snaith, T Bein
   Adv. Energy Mater., 4:1400355 (2014) [ abstract | pdf | doi:10.1002/aenm.201400355 ]
   The alkylammonium metal trihalide perovskite absorbers first used in working photovoltaic devices were based on liquid elec- trolyte sensitized solar cells. Introduced by Kojima et al., the devices exhibited a starting point power conversion efficiency of 3.8% and, with further work, they were quickly improved to reach over 6%.[1] It was not until a solid-state configuration was employed, however, that high device efficiencies were achieved.[2] Initial results were reported at 9% for perovskite sensitized titania-based devices[2b] and further improvements were simultaneously achieved in a “meso-superstructured” configuration by replacing the mesoporous TiO2 scaffold with an electronically inactive mesoporous Al2O3 layer, exhibiting device efficiencies of over 12%.[2c,3] Some of the key advantages for this material system over other competing device concepts are that they are compatible with solution-processing tech- niques and can be fully processed at low temperatures, thus enabling their use in flexible device applications.[4]
7. An ultrafast carbon nanotube terahertz polarisation modulator,
   CJ Docherty, SD Stranks, SN Habisreutinger, HJ Joyce, LM Herz, RJ Nicholas, MB Johnston
   J. Appl. Phys., 115:203108 (2014) [ abstract | pdf | doi:10.1063/1.4879895 ]
   We demonstrate ultrafast modulation of terahertz radiation by unaligned optically pumped single-walled carbon nanotubes. Photoexcitation by an ultrafast optical pump pulse induces transient terahertz absorption in nanowires aligned parallel to the optical pump. By controlling the polarisation of the optical pump, we show that terahertz polarisation and modulation can be tuned, allowing sub-picosecond modulation of terahertz radiation. Such speeds suggest potential for semiconductor nanowire devices in terahertz communication technologies.
8. Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells,
   GE Eperon, SD Stranks, C Menelaou, MB Johnston, LM Herz, HJ Snaith
   Energy Environ. Sci., 7:982-988 (2014) [ abstract | pdf | doi:10.1039/C3EE43822H ]
   Perovskite-based solar cells have attracted significant recent interest, with power conversion efficiencies in excess of 15% already superceding a number of established thin-film solar cell technologies. Most work has focused on a methylammonium lead trihalide perovskites, with a bandgaps of ~1.55 eV and greater. Here, we explore the effect of replacing the methylammonium cation in this perovskite, and show that with the slightly larger formamidinium cation, we can synthesise formamidinium lead trihalide perovskites with a bandgap tunable between 1.48 and 2.23 eV. We take the 1.48 eV-bandgap perovskite as most suited for single junction solar cells, and demonstrate long-range electron and hole diffusion lengths in this material, making it suitable for planar heterojunction solar cells. We fabricate such devices, and due to the reduced bandgap we achieve high short-circuit currents of >23 mA cm-2, resulting in power conversion efficiencies of up to 14.2%, the highest efficiency yet for solution processed planar heterojunction perovskite solar cells. Formamidinium lead triiodide is hence promising as a new candidate for this class of solar cell.
9. Dependence of Dye Regeneration and Charge Collection on the Pore-Filling Fraction in Solid-State Dye-Sensitized Solar Cells,
   CT Weisspfennig, DJ Hollman, C Menelaou, SD Stranks, HJ Joyce, MB Johnston, HJ Snaith, LM Herz
   Adv. Funct. Mater., 24:668--677 (2014) [ abstract | pdf | doi:10.1002/adfm.201301328 ]
   Solid-state dye-sensitized solar cells rely on effective infiltration of a solid-state hole-transporting material into the pores of a nanoporous TiO2 network to allow for dye regeneration and hole extraction. Using microsecond transient absorption spectroscopy and femtosecond photoluminescence upconversion spectroscopy, the hole-transfer yield from the dye to the hole-transporting material 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) is shown to rise rapidly with higher pore-filling fractions as the dye-coated pore surface is increasingly covered with hole-transporting material. Once a pore-filling fraction of ≈30% is reached, further increases do not significantly change the hole-transfer yield. Using simple models of infiltration of spiro-OMeTAD into the TiO2 porous network, it is shown that this pore-filling fraction is less than the amount required to cover the dye surface with at least a single layer of hole-transporting material, suggesting that charge diffusion through the dye monolayer network precedes transfer to the hole-transporting material. Comparison of these results with device parameters shows that improvements of the power-conversion efficiency beyond ≈30% pore filling are not caused by a higher hole-transfer yield, but by a higher charge-collection efficiency, which is found to occur in steps. The observed sharp onsets in photocurrent and power-conversion efficiencies with increasing pore-filling fraction correlate well with percolation theory, predicting the points of cohesive pathway formation in successive spiro-OMeTAD layers adhered to the pore walls. From percolation theory it is predicted that, for standard mesoporous TiO2 with 20 nm pore size, the photocurrent should show no further improvement beyond an ≈83% pore-filling fraction.
10. Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber,
    SD Stranks, GE Eperon, G Grancini, C Menelaou, MJ Alcocer, T Leijtens, LM Herz, A Petrozza, HJ Snaith
    Science, 342:341--344 (2013) [ abstract | pdf | doi:10.1126/science.1243982 ]
    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 (CH3NH3PbI(3-x)Cl(x)) 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.
11. Novel carbon nanotube-conjugated polymer nanohybrids produced by multiple polymer processing,
    SD Stranks, SN Habisreutinger, B Dirks, RJ Nicholas
    Adv. Mater., 25:4365-4371 (2013) [ abstract | pdf | doi:10.1002/adma.201205250 ]
    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.
12. Production of high-purity single-chirality carbon nanotube hybrids by selective polymer exchange,
    SD Stranks, AMR Baker, JA Alexander-Webber, B Dirks, RJ Nicholas
    Small, 9:2245-2249 (2013) [ abstract | pdf | doi:10.1002/smll.201202434 ]
13. Optimizing the Energy Offset between Dye and Hole-Transporting Material in Solid-State Dye-Sensitized Solar Cells,
    CT Weisspfennig, MM Lee, J Teuscher, P Docampo, SD Stranks, HJ Joyce, H Bergmann, I Bruder, DV Kondratuk, MB Johnston, HJ Snaith, LM Herz
    J. Phys. Chem. C, 117:19850-19858 (2013) [ abstract | pdf | doi:10.1021/jp405734f ]
    The power-conversion efficiency of solid-state dye-sensitized solar cells can be optimized by reducing the energy offset between the highest occupied molecular orbital (HOMO) levels of dye and hole-transporting material (HTM) to minimize the loss-in-potential. Here, we report a study of three novel HTMs with HOMO levels slightly above and below the one of the commonly used HTM 2,2′,7,7′- tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) to systematically explore this possibility. Using transient absorption spectroscopy and employing the ruthenium based dye Z907 as sensitizer, it is shown that, despite one new HTM showing a 100% hole-transfer yield, all devices based on the new HTMs performed worse than those incorporating spiro-OMeTAD. We further demonstrate that the design of the HTM has an additional impact on the electronic density of states present at the TiO2 electrode surface and hence influences not only hole- but also electron-transfer from the sensitizer. These results provide insight into the complex influence of the HTM on charge transfer and provide guidance for the molecular design of new materials.
14. Nanoengineering Coaxial Carbon Nanotube--Dual-Polymer Heterostructures,
    SD Stranks, C Yong, JA Alexander-Webber, C Weisspfennig, MB Johnston, LM Herz, RJ Nicholas
    ACS Nano, 6:6058--6066 (2012) [ abstract | pdf | doi:10.1021/nn301133v ]
    We describe studies of new nanostructured materials consisting of carbon nanotubes wrapped in sequential coatings of two different semiconducting polymers, namely, poly(3- hexylthiophene) (P3HT) and poly(9,90-dioctylfluorene-co-benzothiadiazole) (F8BT). Using absorption spectroscopy and steady-state and ultrafast photoluminescence measurements, we demonstrate the role of the different layer structures in controlling energy levels and charge transfer in both solution and film samples. By varying the simple solution processing steps, we can control the ordering and proportions of the wrapping polymers in the solid state. The resulting novel coaxial structures open up a variety of new applications for nanotube blends and are particularly promising for implementation into organic photovoltaic devices. The carbon nanotube template can also be used to optimize both the electronic properties and morphology of polymer composites in a much more controlled fashion than achieved previously, offering a route to producing a new generation of polymer nanostructures.
15. Ultrafast Charge Separation at a Polymer-Single-Walled Carbon Nanotube Molecular Junction,
    SD Stranks, C Weisspfennig, P Parkinson, MB Johnston, LM Herz, RJ Nicholas
    Nano Lett., 11:66-72 (2011) [ abstract | pdf | doi:10.1021/nl1036484 ]
    We have investigated the charge photogeneration dynamics at the interface formed between single-walled carbon nanotubes (SWNTs) and poly(3-hexylthiophene) (P3HT) using a combination of femtosecond spectroscopic techniques. We demonstrate that photoexcitation of P3HT forming a single molecular layer around a SWNT leads to an ultrafast (~430 fs) charge transfer between the materials. The addition of excess P3HT leads to long-term charge separation in which free polarons remain separated at room temperature. Our results suggest that SWNT-P3HT blends incorporating only small fractions (1%) of SWNTs allow photon-to-charge conversion with efficiencies comparable to those for conventional (60:40) P3HT−fullerene blends, provided that small-diameter tubes are individually embedded in the P3HT matrix.
16. Electronic and Mechanical Modification of Single-Walled Carbon Nanotubes by Binding to Porphyrin Oligomers,
    SD Stranks, JK Sprafke, HL Anderson, RJ Nicholas
    ACS Nano, 5:2307–2315 (2011) [ abstract | pdf | doi:10.1021/nn103588h ]
    We report on the noncovalent binding of conjugated porphyrin oligomers to small diameter single-walled carbon nanotubes (SWNTs) and highlight two remarkable observations. First, the binding of the oligomers to SWNTs is so strong that it induces mechanical strain on the nanotubes in solution. The magnitudes of the strains are comparable to those found in solid-state studies. Comparable strains are not observed in any other SWNT−supramolecular complexes. Second, large decreases in polymer band gap with increasing length of the oligomer lead to the formation of a type-II heterojunction between long chain oligomers and small-diameter nanotubes. This is demonstrated by the observation of enhanced red-shifts for the nanotube interband transitions. These complexes offer considerable promise for photovoltaic devices.
17. Noncovalent Binding of Carbon Nanotubes by Porphyrin Oligomers,
    JK Sprafke, SD Stranks, JH Warner, RJ Nicholas, HL Anderson
    Angew. Chem.-Int. Edit., 50:2313–2316 (2011) [ abstract | pdf | doi:10.1002/anie.201007295 ]
    Porphyrin oligomers bind strongly to single-walled carbon nanotubes, and debundle multitube aggregates. The strength of this interaction increases sharply with the number of porphyrin units in the oligomer, and the affinity is greatest for chiral (7,5) and (8,6) tubes. Quantitative information on these noncovalent recognition processes was obtained from UV/Vis/NIR absorption (see picture) and fluorescence titrations.
18. Two-step purification of pathogenesis-related proteins from grape juice and crystallization of thaumatin-like proteins,
    S van Sluyter, M Marangon, SD Stranks, KA Neilson, Y Hayasaka, PA Haynes, RI Menz, EJ Waters
    J. Agric. Food Chem., 57:11376-11382 (2009) [ abstract | pdf | doi:10.1021/jf902365r ]
    Grape thaumatin-like (TL) proteins and chitinases play roles in plant-pathogen interactions and can cause protein haze in white wine unless removed prior to bottling. A two-step method is described that highly purified hundreds of milligrams of TL proteins and chitinases from two juices by strong cation exchange (SCX) and hydrophobic interaction chromatography (HIC). The method was fast and separated isoforms of TL proteins and chitinases from within the same juice, in most cases to >97% purity. The isolated proteins were identified by peptide nanoLC-MS/MS and crystallized using a high-throughput screening method. Crystals from three protein fractions produced high-resolution X-ray crystallography data.
19. Model for amorphous aggregation processes,
    SD Stranks, H Ecroyd, S van Sluyter, EJ Waters, JA Carver, L von Smekal
    Phys. Rev. E, 80:051907 (2009) [ abstract | pdf | doi:10.1103/PhysRevE.80.051907 ]
    The amorphous aggregation of proteins is associated with many phenomena, ranging from the formation of protein wine haze to the development of cataract in the eye lens and the precipitation of recombinant proteins during their expression and purification. While much literature exists describing models for linear protein aggregation, such as amyloid fibril formation, there are few reports of models which address amorphous aggregation. Here, we propose a model to describe the amorphous aggregation of proteins which is also more widely applicable to other situations where a similar process occurs, such as in the formation of colloids and nanoclusters. As first applications of the model, we have tested it against experimental turbidimetry data of three proteins relevant to the wine industry and biochemistry, namely, thaumatin, a thaumatinlike protein, and alpha-lactalbumin. The model is very robust and describes amorphous experimental data to a high degree of accuracy. Details about the aggregation process, such as shape parameters of the aggregates and rate constants, can also be extracted.