Alan D. F. Dunbar

Efficient planar heterojunction mixed-halide perovskite solar cells deposited via spray-deposition

We report the use of ultra-sonic spray-coating under ambient conditions as a deposition technique for the fabrication of planar heterojunction CH3NH3PbI3−xClx perovskite solar cells. We make a first optimization of processing parameter space using this deposition technique, and explore the role of the temperature of the substrate during spray-casting, the volatility of the casting solvent and the post deposition anneal on determining the efficiency of the resultant solar cells. We find that maximum device efficiency is correlated with the creation of dense films having a surface coverage above 85%. When such films are incorporated into a solar cell device, power conversion efficiencies of up to 11% are realized. These results demonstrate that spray-coating can be used in the large-area, low-cost manufacture of high efficiency solution-processed perovskite solar cells.

Depletion of PCBM at the Cathode Interface in P3HT/PCBM Thin Films as Quantified via Neutron Reflectivity Measurements

[*] Dr. A. J. Parnell, A. J. Pearson, Dr. P. A. Staniec, A. J. C. Dennison, Dr. H. Hamamatsu, Prof. D. G. Lidzey, Prof. R. A. L. Jones Department of Physics and Astronomy, The University of Sheffield Hicks Building, Hounsfield Road, Sheffield, S3 7RH (UK) E-mail: a.j.parnell@sheffield.ac.uk; d.g.lidzey@sheffield.ac.uk Dr. A. D. F. Dunbar Department of Chemical and Process Engineering, The University of Sheffield Sir Robert Hadfield Building, Mappin St, Sheffield S1 3JD (UK)

Detection of Volatile Organic Compounds Using Porphyrin Derivatives

Seven different porphyrin compounds have been investigated as colorimetric gas sensors for a wide range of volatile organic compounds. The porphyrins examined were the free base and Mg, Sn, Zn, Au, Co, and Mn derivatives of 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine. Chloroform solutions of these materials were prepared and changes in their absorption spectra induced by exposure to various organic compounds measured. The porphyrins that showed strong responses in solution were selected, and Langmuir-Blodgett films were prepared and exposed to the corresponding analytes. This was done to determine whether they are useful materials for solid state thin film colorimetric vapor sensors. Porphyrins that readily coordinate extra ligands are shown to be suitable materials for colorimetric volatile organic compound detectors. However, porphyrins that already have bound axial ligands when synthesized only show a sensor response to those analytes that can substitute these axial ligands. The Co porphyrin displays a considerably larger response than the other porphyrins investigated which is attributed to a switch between Co(II) and Co(III) resulting in a large spectral change.

In situ imaging and height reconstruction of phase separation processes in polymer blends during spin coating

Spin coating polymer blend thin films provides a method to produce multiphase functional layers of high uniformity covering large surface areas. Applications for such layers include photovoltaics and light-emitting diodes where performance relies upon the nanoscale phase separation morphology of the spun film. Furthermore, at micrometer scales, phase separation provides a route to produce self-organized structures for templating applications. Understanding the factors that determine the final phase-separated morphology in these systems is consequently an important goal. However, it has to date proved problematic to fully test theoretical models for phase separation during spin coating, due to the high spin speeds, which has limited the spatial resolution of experimental data obtained during the coating process. Without this fundamental understanding, production of optimized micro- and nanoscale structures is hampered. Here, we have employed synchronized stroboscopic illumination together with the high light gathering sensitivity of an electron-multiplying charge-coupled device camera to optically observe structure evolution in such blends during spin coating. Furthermore the use of monochromatic illumination has allowed interference reconstruction of three-dimensional topographies of the spin-coated film as it dries and phase separates with nanometer precision. We have used this new method to directly observe the phase separation process during spinning for a polymer blend (PS-PI) for the first time, providing new insights into the spin-coating process and opening up a route to understand and control phase separation structures.

Electron diffraction and Raman studies of the effect of substrate misorientation on ordering in the AlGaInP system

We report a systematic study of the effect of substrate orientation on the ordering in the AlGaInP system, including the Al0.52In0.48P lattice-matched ternary case. Four AlGaInP/GaAs/AlInP samples were grown by metalorganic vapor phase epitaxy under identical growth conditions on [100] substrates orientated 0°, 2°, 10° and 15° either towards the [110] or the [111] axis. The ordering in both the AlInP and the AlGaInP layers was studied by electron diffraction and the Raman scattering technique. The tendency for ordering decreased with increasing misorientation and is less for (AlxGa1−x)0.52In0.48P than for AlInP. The AlInP was found to spontaneously order even when grown after a completely disordered AlGaInP layer. The Raman results show features correlated to the electron diffraction results and hence we conclude that this technique constitutes a reliable nondestructive means of characterizing this system.

A solution concentration dependent transition from self-stratification to lateral phase separation in spin-cast PS:d-PMMA thin films

Thin films with a rich variety of different nano-scale morphologies have been produced by spin casting solutions of various concentrations of PS:d-PMMA blends from toluene solutions. During the spin casting process specular reflectivity and off-specular scattering data were recorded and ex situ optical and atomic force microscopy, neutron reflectivity and ellipsometry have all been used to characterise the film morphologies. We show that it is possible to selectively control the film morphology by altering the solution concentration used. Low polymer concentration solutions favour the formation of flat in-plane phase-separated bi-layers, with a d-PMMA-rich layer underneath a PS-rich layer. At intermediate concentrations the films formed consist of an in-plane phase-separated bi-layer with an undulating interface and also have some secondary phase-separated pockets rich in d-PMMA in the PS-rich layer and vice versa. Using high concentration solutions results in laterally phase-separated regions with sharp interfaces. As with the intermediate concentrations, secondary phase separation was also observed, especially at the top surface.

Templated-assembly of conducting antimony cluster wires

Wires with meso- and nanoscale widths have been fabricated using a novel assembly technique based on the deposition of Sb clusters on templated surfaces. The template elements are V-grooves etched into the surface of a Si wafer. It is demonstrated that the clusters bounce or slide to the apex of the V-grooves, without significant fragmentation, and that this motion is the underlying mechanism behind the formation of the wires. The flow rate of inert gas into the cluster growth chamber controls the average velocity of the clusters and the morphology and width of the wires. Sb cluster-assembled wires with lengths over 150 µ ma nd widths down to 100 nm have been assembled from ∼40 nm diameter Sb clusters. Electrical contacts to the wire sh av eb een achieved via lithographic alignment of NiCr/Au contacts to the V-grooves prior to deposition.

Alkylamine Sensing Using Langmuir-Blodgett Films of n-Alkyl-N-phenylamide-Substituted Zinc Porphyrins

Two porphyrin compounds, zinc(II) 5,10,15,20-tetrakis(3,5,5-trimethyl- N-phenylhexanamide)porphyrin and zinc(II) 5,10,15,20-tetrakis(2,2-dimethyl- N-phenylpropanamide)porphyrin, have been investigated as possible candidates for the detection of alkylamines. UV-visible spectroscopy has shown that their solution absorption spectra are significantly modified upon interaction with a range of organic analytes, including acetic acid, butanone, ethylacetate, hexanethiol, octanal, octanol, alkylamines, and trimethylphosphite. Large spectral changes are observed for the family of alkylamines as a result of the specific affinity between zinc and the amine moiety. Langmuir-Blodgett (LB) films of the porphyrins have been fabricated in order to assess their solid-state sensing capability toward amines. The surface pressure-area (Pi- A) isotherms reveal a clear three-phase Langmuir film behavior and show that these monolayer films may be compressed to a relatively high surface pressure ( approximately 40-50 mN m (-1)). The isotherm data alongside molecular modeling suggest a relatively flat orientation of the porphyrin rings of both compounds: that is, a mutually parallel alignment of the plane of the porphyrin ring and that of the water surface. LB films deposited at 15 mN m (-1) have been exposed to alkylamine vapor (carried by N 2). A red shift and increase in intensity of the Soret band absorbance is observed which can be reversed by flowing pure N 2 over the gently heated sample (60 degrees C) after exposure. Primary amines were expected to invoke the greatest sensing response due to (i) their larger association constants with these porphyrins compared to secondary and tertiary amines and (ii) the ease of diffusion of amines which is expected to follow the order primary > secondary > tertiary due to the steric hindrance arising from the bulky secondary and tertiary amines. However, the magnitude of the absorbance change is largest for exposure to the secondary amines, dipropylamine and dibutylamine, for both porphyrins, compared to primary and tertiary amines. This trend follows that observed when the amines were added to solutions of the porphyrins. The rate of response of the porphyrin LB films falls as the molecular weight of the diffusing alkylamine increases. Furthermore, a greater rate of response is observed for the phenylhexanamide porphyrin compared to the phenylpropanamide porphyrin due to its lower molecular density within the LB film and therefore more porous structure.

Tuning free base tetraphenylporphyrins as optical sensing elements for volatile organic analytes

Changes in the visible absorption spectra of ten free base porphyrin compounds in solution and thin film form upon exposure to a range of volatile organic compounds have been investigated. The functional groups attached to the phenyl rings varied in their electron donating/withdrawing strength. These porphyrins were studied with a view to determining their suitability for use in the fabrication of an optical based gas sensor. It has been found that the sensitivity of these sensor materials depends upon the electron donating/withdrawing strength of the functional groups attached to the phenyl rings.

The effect of strain field seeding on the epitaxial growth of Ge islands on Si(001)

The effect of strain, due to a buried, nominally 6 ML Ge quantum dot layer, upon the growth of subsequent Ge layers grown by gas source molecular beam epitaxy has been investigated. A series of samples were grown at 700 °C with a nominally 6 ML Ge layer followed by a 30 nm Si spacer and then a second, thinner Ge layer. In each sample, the thickness of the second Ge layer was varied (2, 3, and 4 ML). Atomic force microscopy shows that in the second Ge layer islands form at thicknesses below the established critical thickness for this material system. This is confirmed by transmission electron microscopy images which also show the quantum dots in the second layers are stacked above those in the first layer, the island growth in the thin Ge layer being seeded by the strain field from the buried Ge islands. Photoluminescence results show a luminescence feature attributed to the strain-controlled quantum dots in the thin Ge layer. This band has properties similar to the frequently observed Ge dot luminescence ...