Publications by Alexandra Ramadan


Revealing the stoichiometric tolerance of lead tri-halide perovskite thin-films

Chemistry of Materials American Chemical Society (ACS) (2019) acs.chemmater.9b02639

AJ Ramadan, M Ralaiarisoa, F Zu, LA Rochford, B WENGER, N Koch, H SNAITH


Overcoming zinc oxide interface instability with a methylammonium-free perovskite for high performance solar cells

Advanced Functional Materials Wiley (2019) 1900466

K Schutt, P Nayak, A Ramadan, B Wenger, Y-H Lin, H Snaith

Perovskite solar cells have achieved the highest power conversion efficiencies on metal oxide n‐type layers, including SnO2 and TiO2. Despite ZnO having superior optoelectronic properties to these metal oxides, such as improved transmittance, higher conductivity, and closer conduction band alignment to methylammonium (MA)PbI3, ZnO is largely overlooked due to a chemical instability when in contact with metal halide perovskites, which leads to rapid decomposition of the perovskite. While surface passivation techniques have somewhat mitigated this instability, investigations as to whether all metal halide perovskites exhibit this instability with ZnO are yet to be undertaken. Experimental methods to elucidate the degradation mechanisms at ZnO–MAPbI3 interfaces are developed. By substituting MA with formamidinium (FA) and cesium (Cs), the stability of the perovskite–ZnO interface is greatly enhanced and it is found that stability compares favorably with SnO2‐based devices after high‐intensity UV irradiation and 85 °C thermal stressing. For devices comprising FA‐ and Cs‐based metal halide perovskite absorber layers on ZnO, a 21.1% scanned power conversion efficiency and 18% steady‐state power output are achieved. This work demonstrates that ZnO appears to be as feasible an n‐type charge extraction layer as SnO2, with many foreseeable advantages, provided that MA cations are avoided.


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.


Enhancing the Charge Extraction and Stability of Perovskite Solar Cells Using Strontium Titanate (SrTiO<inf>3</inf>) Electron Transport Layer

ACS Applied Energy Materials (2019)

M Neophytou, M De Bastiani, N Gasparini, E Aydin, E Ugur, A Seitkhan, F Moruzzi, Y Choaie, AJ Ramadan, JR Troughton, R Hallani, A Savva, L Tsetseris, S Inal, D Baran, F Laquai, TD Anthopoulos, HJ Snaith, S De Wolf, I McCulloch

© 2019 American Chemical Society. Charge transport layers strongly influence the performance of perovskite solar cells (PSCs). To date, compact layers and mesoporous scaffolds of titanium dioxide have emerged as good electron transport layers (ETL), enabling record power conversion efficiencies (PCE). However, these ETLs require sintering above 400 °C, which excludes them from low-temperature applications such as flexible devices and silicon-heterojunction tandems. Furthermore, instability of TiO2 under prolonged exposure to sunlight appears to be a critical issue. Here, we present the promising characteristics of low-temperature processed strontium titanate (STO) as an ETL to realize PSCs with 19% PCE. STO is a wide bandgap transparent inorganic perovskite. Compared with other low-temperature processed interlayers, STO reduces the parasitic absorption in the ultraviolet and visible range, improves the electron transport, and greatly increases the stability of the devices, retaining ∼80% of their initial efficiency after 1000 h of constant white light illumination.


Revealing the stoichiometric tolerance of lead tri-halide perovskite thin-films - Raw Data

(2019)

A Ramadan, M Ralaiarisoa, F Zu, L Rochford, B Wenger, N Koch, H Snaith

Raw data associated with publication


Oxidative passivation of metal halide perovskites

Joule Cell Press 3 (2019) 2716-2731

J Godding, A Ramadan, Y-H Lin, K Schutt, HJ Snaith, B Wenger

Metal halide perovskites have demonstrated extraordinary potential as materials for next-generation optoelectronics including photovoltaics and light-emitting diodes. Nevertheless, our understanding of this material is still far from complete. One remaining puzzle is the phenomenon of perovskite “photo-brightening”: the increase in photoluminescence during exposure to light in an ambient atmosphere. Here, we propose a comprehensive mechanism for the reactivity of the archetypal perovskite, MAPbI3, in ambient conditions. We establish the formation of lead-oxygen bonds by hydrogen peroxide as the key factor leading to perovskite photo-brightening. We demonstrate that hydrogen peroxide can be applied directly as an effective “post-treatment” to emulate the process and substantially improve photoluminescence quantum efficiencies. Finally, we show that the treatment can be incorporated into photovoltaic devices to give a 50 mV increase in open-circuit voltage, delivering high 19.2% steady-state power conversion efficiencies for inverted perovskite solar cells of the mixed halide, mixed cation perovskite FA0.83Cs0.17Pb(I0.9Br0.1)3.


Impact of Bi3+ heterovalent doping in organic-inorganic metal halide perovskite crystals

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

P Nayak, M Sendner, B Wenger, Z Wang, K Sharma, A Ramadan, R Lovrinčić, A Pucci, PK Madhu, H 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 American Chemical Society 30 (2018) 7737–7743-

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

Perovskite-based photovoltaics are an emerging solar technology with lab scale device efficiencies of over 22 %, and significant steps are being made toward their commercialization. Conventionally high efficiency perovskite solar cells are formed from high boiling point, polar aprotic solvent solutions. Methylammonium lead iodide (CH3NH3PbI3) films can be made from a range of solvents and blends; however, the role the solvent system plays in determining the properties of the resulting perovskite films is poorly understood. Acetonitrile (ACN), in the presence of methylamine (MA), is a viable nontoxic solvent for fabrication of CH3NH3PbI3 photovoltaic devices with efficiencies &gt;18 %. Herein we examine films prepared from ACN/MA and dimethylformamide (DMF) and scrutinize their physical and electronic properties using spectroscopy, scanning probe imaging, and ion scattering. Significant differences are observed in the chemistry and electronic structure of CH3NH3PbI3 films made with each solvent, ACN/MA produces films with superior properties resulting in more efficient photovoltaic devices. Here we present a holistic and complete understanding of a high performance perovskite material from an electronic, physical, and structural perspective and establish a robust toolkit with which to understand and optimize photovoltaic perovskites.


Unveiling the influence of pH on the crystallization of hybrid perovskites, felivering low voltage loss photovoltaics

Joule Cell Press 1 (2017) 328-343

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

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 MAPbI3xClx 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.


Solution-processed cesium hexabromopalladate(IV), Cs2PdBr6, for optoelectronic applications

Journal of the American Chemical Society 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

ADVANCED FUNCTIONAL MATERIALS 26 (2016) 1188-1196

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

JOURNAL OF PHYSICAL CHEMISTRY C 120 (2016) 4448-4452

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

JOURNAL OF MATERIALS CHEMISTRY C 4 (2016) 348-351

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

ADVANCED MATERIALS INTERFACES 2 (2015) ARTN 1400540

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

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