Complex molecules on surfaces: in-situ electrospray deposition and photoelectron spectroscopy

Temperton, Robert H. (2017) Complex molecules on surfaces: in-situ electrospray deposition and photoelectron spectroscopy. PhD thesis, University of Nottingham.

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Furthering our understanding of next generation hight harvesting devices has the potential to revolutionise energy storage and production. This thesis discusses the use of surface science techniques, principally soft X-ray photoelectron spectroscopies, to study two types of molecular solar cell: dye sensitised solar cells and organic photovoltaic devices. Much of this work relies on electrospray deposition, which is becoming a well established technique allowing the in-situ deposition of fragile, non-volatile molecules in high vacuum environments. This thesis explores the potential use of electrospray deposition to build model photovoltaic devices, including bi-layer and bulk heterojucntion structures, to extend the use of the technique beyond the simple "molecule on a surface" type of experiment that have previously dominated the high-vacuum electrospray deposition literature.

Firstly this thesis presents the development of two instruments that aim to aid the characterisation of electrospray. A novel ultra-fast exposure microscopy system is presented that used a pulsed laser and purpose-designed cell containing a fluorescent dye to produce short (<20 ns) incoherent light pulses. Images of electrospray ionisation, highlighting the capabilities of the imaging system, are presented. Preliminary images showing the unusual behaviour when the liquid properties are severely changed by adding a high concentration of salt are also presented. We also include some proof-of-concept data, collected using a home-built image charge detection instrument, to measure velocity distributions of molecular ions that are landing on the surface. This instrument has led the way to a more refined image charge detection instrument that will be of great benefit to future studies.

A series of three experiments are presented that highlight the capability and suitability of high vacuum deposition to produce the types of structures used in molecular photovoltaic devices. Time of flight secondary ion mass spectrometry was used to image a bi-layer structure, produced using electrospray from two molecules dissolved in the same solvent, showing there is little mixing between the layers. These structures are challenging to make using processes like spin casting where solvent compatibility results in mixing of the films. Two electrospray sources were used to co-deposit two molecules simultaneously from immiscible solvents, which is again not possible using standard techniques. Finally a complex polymer blend was deposited using electrospray, from a single solution, and analysed using X-ray photoelectron spectroscopy (XPS) finding evidence of preferential deposition of one species.

Bi-isonicotionic acid (the ligand of many popular organometallic photo-sensitisers) has been deposited onto Ag(111) in-situ via sublimation. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) have been used to obtain chemical information and map out the density of states for the system. We find the LUMO (lowest unoccupied molecular orbital) lies below the Fermi level of the silver surface allowing ultra-fast bi-directional charge transfer between the molecule and surface. This was probed using resonant photoelectron spectroscopy (RPES) by observation of super-spectator and super-Auger decay mechanisms from the core excited state.

In the final experimental chapter, films of phenyl-C61-butyric acid methyl ester (PCBM) and Poly(3-hexylthiophene-2,5-diyl) (P3HT), common acceptor and donor molecules used in organic photovoltaic devices, were successfully deposited in-situ in UHV using electrospray deposition on the Au(111) surface. Bi-layer heterojunction and bulk heterojunction structures of the two molecules were also produced using electrospray. Again, XPS, XAS and RPES were used to investigate chemical interactions, map out the density of states for the molecular systems and probe charge transfer between the molecules and surfaces. For PCBM, some interesting chemical behaviour regarding an oxygen group on the ligand is observed. Measurements also show the LUMO of PCBM lies below the surface Fermi level, but surprisingly no evidence of super-spectator or super-Auger decay is found. RPES was unable to show evidence of charge transfer between the two molecules in a bi-layer heterojunction structure. Finally, careful ionisation potential calibration of XPS spectra were used to try and infer the position of the valence band maximum of the two molecules when brought together in a heterojunction.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: O'Shea, James N.
Beton, P.H.
Keywords: Electrospray deposition ; Photoemission spectroscopy ; X-ray photoelectron spectroscopy (XPS) ; X-ray absorption spectropscoscopy (XAS) Resonant photoelectron spectroscopy (RPES) Organic Photovoltaics Bi-isonicotinic acid Short-exposure video High speed video
Subjects: Q Science > QC Physics > QC350 Optics. Light, including spectroscopy
Faculties/Schools: UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID: 48056
Depositing User: Temperton, Robert
Date Deposited: 15 Jan 2018 11:02
Last Modified: 15 Jan 2018 11:05

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