Publications by Alexandra Ramadan

The Structure of VOPc on Cu(111): Does V=O Point Up, or Down, or Both?

Journal of Physical Chemistry C (2018)

PJ Blowey, RJ Maurer, LA Rochford, DA Duncan, JH Kang, DA Warr, AJ Ramadan, TL Lee, PK Thakur, G Costantini, K Reuter, DP Woodruff

© 2018 American Chemical Society. The local structure of the nonplanar phthalocyanine, vanadyl phthalocyanine (VOPc), adsorbed on Cu(111) at a coverage of approximately one-half of a saturated molecular layer, has been investigated by a combination of normal-incidence X-ray standing waves (NIXSW), scanned-energy mode photoelectron diffraction (PhD), and density-functional theory (DFT), complemented by scanning tunnelling microscopy (STM). Qualitative assessment of the NIXSW data clearly shows that both "up" and "down" orientations of the molecule (with V=O pointing out of, and into, the surface) must coexist on the surface. O 1s PhD proves to be inconclusive regarding the molecular orientation. DFT calculations, using two different dispersion correction schemes, show good quantitative agreement with the NIXSW structural results for equal co-occupation of the two different molecular orientations and clearly favor the many body dispersion (MBD) method to deal with long-range dispersion forces. The calculated relative adsorption energies of the differently oriented molecules at the lowest coverage show a strong preference for the "up" orientation, but at higher local coverages, this energetic difference decreases, and mixed orientation phases are almost energetically equivalent to pure "up"-oriented phases. DFT-based Tersoff-Hamann simulations of STM topographs for the two orientations cast some light on the extent to which such images provide a reliable guide to molecular orientation.

Impact of Bi3+ Heterovalent Doping in Organic-Inorganic Metal Halide Perovskite Crystals.

Journal of the American Chemical Society 140 (2018) 574-577

PK Nayak, M Sendner, B Wenger, Z Wang, K Sharma, AJ Ramadan, R Lovrinčić, A Pucci, PK Madhu, HJ Snaith

Intrinsic organic-inorganic metal halide perovskites (OIHP) based semiconductors have shown wide applications in optoelectronic devices. There have been several attempts to incorporate heterovalent metal (e.g., Bi3+) ions in the perovskites in an attempt to induce electronic doping and increase the charge carrier density in the semiconductor. It has been reported that inclusion of Bi3+ decreases the band gap of the material considerably. However, contrary to the earlier conclusions, despite a clear change in the appearance of the crystal as observed by eye, here we show that the band gap of MAPbBr3 crystals does not change due the presence of Bi3+ in the growth solution. An increased density of states in the band gap and use of very thick samples for transmission measurements, erroneously give the impression of a band gap shift. These sub band gap states also act as nonradiative recombination centers in the crystals.

Unravelling the Improved Electronic and Structural Properties of Methylammonium Lead Iodide Deposited from Acetonitrile

CHEMISTRY OF MATERIALS 30 (2018) 7737-7743

AJ Ramadan, NK Noel, S Fearn, N Young, M Walker, LA Rochford, HJ Snaith

Unveiling the Influence of pH on the Crystallization of Hybrid Perovskites, Delivering Low Voltage Loss Photovoltaics

Joule 1 (2017) 328-343

NK Noel, M Congiu, AJ Ramadan, S Fearn, DP McMeekin, JB Patel, MB Johnston, B Wenger, HJ Snaith

© 2017 Elsevier Inc. Impressive power conversion efficiencies coupled with the relative ease of fabrication have made perovskite solar cells a front runner for next-generation photovoltaics. Although perovskite films and optoelectronic devices have been widely studied, relatively little is known about the chemistry of the precursor solutions. Here, we present a study on the hydrolysis of N,N-dimethylformamide, correlating how pH changes related to its degradation affect the crystallization of MAPbI3−xClx perovskite films. By careful manipulation of the pH, and the resulting colloid distribution in precursor solutions, we fabricate perovskite films with greatly improved crystallinity, which when incorporated into photovoltaic devices reproducibly yield efficiencies of over 18%. Extending this method to the mixed cation, mixed halide perovskite FA0.83MA0.17Pb(I0.83Br0.17)3, we obtain power conversion efficiencies of up to 19.9% and open-circuit voltages of 1.21 V for a material with a bandgap of 1.57 eV, achieving the lowest yet reported loss in potential from bandgap to a VOC of only 360 mV. Metal halide perovskites have shown tremendous promise in optoelectronic devices and are of particular interest as absorber materials in solar cells, having achieved remarkable power conversion efficiencies in a staggeringly short period of time. Although improvements in deposition techniques have greatly increased the quality of perovskite films and have allowed perovskite solar cells to dominate the class of emerging photovoltaic technologies, relatively little focus has been placed on understanding the chemistry of the precursor solutions. Here, we elucidate how the hydrolysis and thermal decomposition of N,N-dimethylformamide, the most commonly used solvent for perovskites, has far-reaching effects on the crystallization and optoelectronic properties of perovskite films and show how controlling the degradation of this solvent allows us to achieve record low voltage losses in highly efficient perovskite solar cells. The degradation of N,N-dimethylformamide results in the formation of formic acid and dimethylamine. The changes in pH that occur as a result of this solvent degradation can be correlated to changes in the colloid concentration in perovskite precursor solutions. By tuning the pH and hence colloid concentration of these solutions, we improve the crystallization and optoelectronic quality of the perovskite films, resulting in solar cells with a record low loss in potential from bandgap to VOC of 360 mV.

Solution-Processed Cesium Hexabromopalladate(IV), Cs2PdBr6, for Optoelectronic Applications.

Journal of the American Chemical Society 139 (2017) 6030-6033

N Sakai, AA Haghighirad, MR Filip, PK Nayak, S Nayak, A Ramadan, Z Wang, F Giustino, HJ Snaith

Lead halide perovskites are materials with excellent optoelectronic and photovoltaic properties. However, some hurdles remain prior to commercialization of these materials, such as chemical stability, phase stability, sensitivity to moisture, and potential issues due to the toxicity of lead. Here, we report a new type of lead-free perovskite related compound, Cs2PdBr6. This compound is solution processable, exhibits long-lived photoluminescence, and an optical band gap of 1.6 eV. Density functional theory calculations indicate that this compound has dispersive electronic bands, with electron and hole effective masses of 0.53 and 0.85 me, respectively. In addition, Cs2PdBr6 is resistant to water, in contrast to lead-halide perovskites, indicating excellent prospects for long-term stability. These combined properties demonstrate that Cs2PdBr6 is a promising novel compound for optoelectronic applications.

Processing Solvent-Dependent Electronic and Structural Properties of Cesium Lead Triiodide Thin Films.

The journal of Physical Chemistry Letters (2017) 4172-4176

AJ Ramadan, LA Rochford, S Fearn, HJ Snaith

Cesium lead triiodide (CsPbI3) is an attractive material for photovoltaic applications due to its appropriate band gap, strong optical absorption, and high thermal stability. However, the perovskite phase suffers from moisture induced structural instability. Previous studies have utilized a range of solvent systems to establish the role of solvent choice in structural instabilities. Despite this, effects of different solvents on the electronic structure of this material have not been compared. We report substantial chemical and compositional differences in thin films of CsPbI3 prepared from a range of solvent systems. We confirm via X-ray diffraction thin films formed from DMF, DMSO, and a mixture of these solvent systems share the same crystal structure. However, secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and low energy ion scattering measurements reveal significant differences between films processed via different solvent systems. Our findings reveal the critical impact solvents have upon compositional stoichiometry and thin-film morphology.

Growth of Large Crystalline Grains of Vanadyl-Phthalocyanine without Epitaxy on Graphene


AJ Marsden, LA Rochford, D Wood, AJ Ramadan, ZPL Laker, TS Jones, NR Wilson

Selecting Phthalocyanine Polymorphs Using Local Chemical Termination Variations in Copper Iodide


AJ Ramadan, I Hancox, S Huband, CC Parkins, SAF Bon, M Walker, S Fearn, CF McConville, TS Jones, LA Rochford

Organic/inorganic epitaxy: commensurate epitaxial growth of truxenone on Cu (111)

RSC ADVANCES 6 (2016) 17125-17128

AJ Ramadan, CB Nielsen, S Holliday, TS Jones, I McCulloch, LA Rochford

The effect of fluorination on the surface structure of truxenones

RSC ADVANCES 6 (2016) 67315-67318

LA Rochford, AJ Ramadan, S Holliday, TS Jones, CB Nielsen

Film formation of non-planar phthalocyanines on copper(I) iodide

RSC ADVANCES 6 (2016) 95227-95231

AJ Ramadan, S Fearn, TS Jones, S Heutz, LA Rochford

The morphology and structure of vanadyl phthalocyanine thin films on lithium niobate single crystals


AJ Ramadan, LA Rochford, J Moffat, C Mulcahy, MP Ryan, TS Jones, S Heutz

Ordered growth of vanadyl phthalocyanine (VOPc) on an iron phthalocyanine (FePc) monolayer.

Physical chemistry chemical physics : PCCP 17 (2015) 29747-29752

LA Rochford, AJ Ramadan, DP Woodruff, S Heutz, TS Jones

The growth and characterisation of a non-planar phthalocyanine (vanadyl phthalocyanine, VOPc) on a complete monolayer (ML) of a planar phthalocyanine (Iron(II) phthalocyanine, FePc) on an Au(111) surface, has been investigated using ultra-high vacuum (UHV) scanning tunnelling microscopy (STM) and low energy electron diffraction (LEED). The surface mesh of the initial FePc monolayer has been determined and shown to correspond to an incommensurate overlayer, not commensurate as previously reported. Ordered islands of VOPc, with (1 × 1) epitaxy, grow on the FePc layer at submonolayer coverages. The individual VOPc molecules occupy sites directly atop the underlying FePc molecules, indicating that significant intermolecular bonding must occur. It is proposed that this interaction implies that the V[double bond, length as m-dash]O points down into the surface, allowing a Fe-O bond to form. The detailed appearance of the STM images of the VOPc molecules is consistent with previous studies in other VOPc growth studies in which this molecular orientation has been proposed.

The influence of polar (0001) zinc oxide (ZnO) on the structure and morphology of vanadyl phthalocyanine (VOPc)

RSC ADVANCES 5 (2015) 65949-65952

AJ Ramadan, LA Rochford, MP Ryan, TS Jones, S Heutz

Structural Templating in a Nonplanar Phthalocyanine Using Single Crystal Copper Iodide


LA Rochford, AJ Ramadan, DS Keeble, MP Ryan, S Heutz, TS Jones

Exploring high temperature templating in non-planar phthalocyanine/copper iodide (111) bilayers


AJ Ramadan, LA Rochford, DS Keeble, P Sullivan, MP Ryan, TS Jones, S Heutz

Selective nucleation of iron phthalocyanine crystals on micro-structured copper iodide.

Physical chemistry chemical physics : PCCP 16 (2014) 25404-25408

LA Rochford, AJ Ramadan, S Heutz, TS Jones

Morphological and structural control of organic semiconductors through structural templating is an efficient route by which to tune their physical properties. The preparation and characterisation of iron phthalocyanine (FePc)-copper iodide (CuI) bilayers at elevated substrate temperatures is presented. Thin CuI(111) layers are prepared which are composed of isolated islands rather than continuous films previously employed in device structures. Nucleation in the early stages of FePc growth is observed at the edges of islands rather than on the top (111) faces with the use of field emission scanning electron microscopy (FE-SEM). Structural measurements show two distinct polymorphs of FePc, with CuI islands edges nucleating high aspect ratio FePc crystallites with modified intermolecular spacing. By combining high substrate temperature growth and micro-structuring of the templating CuI(111) layer structural and morphological control of the organic film is demonstrated.