USF Libraries
USF Digital Collections

Fabrication and analysis of Poly (3-hexylthiophene) interfaces via electrospray deposition and photoemission spectroscopy

MISSING IMAGE

Material Information

Title:
Fabrication and analysis of Poly (3-hexylthiophene) interfaces via electrospray deposition and photoemission spectroscopy
Physical Description:
Book
Language:
English
Creator:
Lyon, James
Publisher:
University of South Florida
Place of Publication:
Tampa, Fla.
Publication Date:

Subjects

Subjects / Keywords:
XPS
UPS
P3HT
Electronic structure
Orbital lineup
Dissertations, Academic -- Electrical Engineering -- Masters -- USF   ( lcsh )
Genre:
bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

Notes

Abstract:
ABSTRACT: P3HT (Poly(3-hexylthiophene)) is an organic polymer that shows promise as an active material in semiconducting electronics. It is important to study the electronic properties of this material in order to determine its efficacy in such devices. However, many current studies of thiophene only examine the oligomer, since it is a simpler material to investigate. In this study, several P3HT interfaces were analyzed to determine their electronic properties. The P3HT was deposited on Au, highly-ordered pyrolitic graphite (HOPG), and indium tin oxide (ITO) substrates via electrospray deposition. The depositions were performed in several steps, with x-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) measurements taken between each step without breaking the vacuum. The resulting series of spectra allowed orbital line-up diagrams to be generated for each interface, giving detailed analysis of the interfacial properties, including the charge injection barriers and interface dipoles. The results, when compared to similar oligomer-based investigations, show a difference in the orbital line-up between oligomeric and polymeric P3HT junctions.
Thesis:
Thesis (M.S.)--University of South Florida, 2007.
Bibliography:
Includes bibliographical references.
System Details:
System requirements: World Wide Web browser and PDF reader.
System Details:
Mode of access: World Wide Web.
Statement of Responsibility:
by James Lyon.
General Note:
Title from PDF of title page.
General Note:
Document formatted into pages; contains 60 pages.

Record Information

Source Institution:
University of South Florida Library
Holding Location:
University of South Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 001917301
oclc - 181592172
usfldc doi - E14-SFE0002031
usfldc handle - e14.2031
System ID:
SFS0026349:00001


This item is only available as the following downloads:


Full Text
xml version 1.0 encoding UTF-8 standalone no
record xmlns http:www.loc.govMARC21slim xmlns:xsi http:www.w3.org2001XMLSchema-instance xsi:schemaLocation http:www.loc.govstandardsmarcxmlschemaMARC21slim.xsd
leader nam Ka
controlfield tag 001 001917301
003 fts
005 20071119150836.0
006 m||||e|||d||||||||
007 cr mnu|||uuuuu
008 071119s2007 flu sbm 000 0 eng d
datafield ind1 8 ind2 024
subfield code a E14-SFE0002031
040
FHM
c FHM
035
(OCoLC)181592172
049
FHMM
090
TK145 (ONLINE)
1 100
Lyon, James.
0 245
Fabrication and analysis of Poly (3-hexylthiophene) interfaces via electrospray deposition and photoemission spectroscopy
h [electronic resource] /
by James Lyon.
260
[Tampa, Fla.] :
b University of South Florida,
2007.
3 520
ABSTRACT: P3HT (Poly(3-hexylthiophene)) is an organic polymer that shows promise as an active material in semiconducting electronics. It is important to study the electronic properties of this material in order to determine its efficacy in such devices. However, many current studies of thiophene only examine the oligomer, since it is a simpler material to investigate. In this study, several P3HT interfaces were analyzed to determine their electronic properties. The P3HT was deposited on Au, highly-ordered pyrolitic graphite (HOPG), and indium tin oxide (ITO) substrates via electrospray deposition. The depositions were performed in several steps, with x-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) measurements taken between each step without breaking the vacuum. The resulting series of spectra allowed orbital line-up diagrams to be generated for each interface, giving detailed analysis of the interfacial properties, including the charge injection barriers and interface dipoles. The results, when compared to similar oligomer-based investigations, show a difference in the orbital line-up between oligomeric and polymeric P3HT junctions.
502
Thesis (M.S.)--University of South Florida, 2007.
504
Includes bibliographical references.
516
Text (Electronic thesis) in PDF format.
538
System requirements: World Wide Web browser and PDF reader.
Mode of access: World Wide Web.
500
Title from PDF of title page.
Document formatted into pages; contains 60 pages.
590
Advisor: Rudy Schlaf, Ph.D.
653
XPS.
UPS.
P3HT.
Electronic structure.
Orbital lineup.
690
Dissertations, Academic
z USF
x Electrical Engineering
Masters.
773
t USF Electronic Theses and Dissertations.
4 856
u http://digital.lib.usf.edu/?e14.2031



PAGE 1

Fabrication and Analysis of Poly(3-hexylth iophene) Interfaces Using Electrospray Deposition and Photoemission Spectroscopy By James Lyon A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Electrical Engineering Department of Electrical Engineering College of Engineering University of South Florida Major Professor: Rudy Schlaf, Ph.D. Matthias Batzill, Ph.D. Hua Cao, Ph.D. Date of Approval: April 5, 2007 Keywords: XPS, UPS, P3HT, elect ronic structure, orbital lineup Copyright 2007, James Lyon

PAGE 2

Dedication To Dr. Schlaf for giving me the opportun ity to engage in research at the undergraduate level and for inspiring me to pursue a graduate degree.

PAGE 3

Acknowledgements I would like to thank Dr. Rudy Schlaf fo r allowing me the opportunity to engage in research, Dr. Martin Beerbom and Dr. Yeonjin Yi for their advice and instruction, and my family for their constant support. Financial suppor t from the National Science Foundation and the College of Engineering REU program is gratefully acknowledged.

PAGE 4

Table of Contents List of Figures....................................................................................................................iii Abstract....................................................................................................................... ........v Introduction................................................................................................................... ......1 Materials.............................................................................................................................5 HOPG..............................................................................................................................6 ITO..................................................................................................................................6 Polythiophene.................................................................................................................7 Interfaces.............................................................................................................................9 Metal-Metal Interfaces....................................................................................................9 Metal-Semiconductor Interfaces: Schottky Barriers.....................................................10 Interface Dipoles...........................................................................................................11 Metal-Polymer Interfaces..............................................................................................12 Electrospray................................................................................................................... ...15 Historical Background..................................................................................................16 Physical Basis...............................................................................................................17 Photoemission Spectroscopy............................................................................................20 Physical Basis...............................................................................................................20 PES Equipment.............................................................................................................24 X-ray Photoelectron Spectroscopy...............................................................................28 Ultraviolet Photoemission Spectroscopy......................................................................31 Experimental................................................................................................................... ..33 Experimental Method....................................................................................................33 Sample Preparation.......................................................................................................34 Au and HOPG Analysis Procedure...............................................................................35 ITO Analysis Procedure................................................................................................36 Results and Discussion.....................................................................................................37 P3HT/HOPG Results....................................................................................................37 P3HT/HOPG Discussion..............................................................................................39 P3HT/Au Results..........................................................................................................41 P3HT/Au Discussion....................................................................................................44 i

PAGE 5

P3HT/ITO Results........................................................................................................48 P3HT/ITO Discussion...................................................................................................51 Conclusion........................................................................................................................55 P3HT/Au.......................................................................................................................55 P3HT/HOPG.................................................................................................................55 P3HT/ITO.....................................................................................................................56 References.........................................................................................................................57 ii

PAGE 6

List of Figures Figure 1: Band Diagram for Two IT O Films With Differing Oxygen Contents (from Reference [15])..................................................................7 Figure 2: Diagram of a Monomer in a Poly(3-hexylthioph ene) Chain (from Reference [16])............................................................................................8 Figure 3: Two Metals Before and After Contact........................................................9 Figure 4: A Metal and a Semiconductor Before and After Contact.........................11 Figure 5: Formation of a Dipole Between a Metal and an Organic Layer (from Reference [14]).........................................................................................13 Figure 6: Possible Factors Forming and Affecting Interfacial Dipole Layers (from Reference [14])...............................................................................14 Figure 7: Comparison of a Metal-Organic Interface a) With and b) Without the Shift in Vacuum Level (V L) Corresponding to a Dipole (from Reference [14])...............................................................................14 Figure 8: Process of Electrospray Depositi on Technique (from Reference [17])....16 Figure 9: Photoelectric Effect for a Model Atom.....................................................22 Figure 10: Photoelectric Effect fo r a Model Atom at 2p Orbital...............................23 Figure 11: Auger Effect for a Model Atom................................................................24 Figure 12: Photoemission Spectrosc opy Equipment Schematic (from Reference [33]).........................................................................................27 Figure 13: XPS Example, Spectrum of a Clean Gold Sample (from Reference [33]).........................................................................................29 Figure 14: Peak Shift Example, Lo wering of O1s Binding Energy After Exposure to X-ray Radiati on (from Reference [33])................................30 Figure 15: UPS Spectra of the P3HT/Au Interface....................................................32 iii

PAGE 7

Figure 16: S2p XPS Core Level Spectra for P3HT/HOPG Interface.........................38 Figure 17: UPS Spectra for P3HT/HOPG Interface...................................................39 Figure 18: Orbital Lineup of the P3HT/ HOPG Interface Constructed from the UPS and XPS Data Set.............................................................................41 Figure 19: XPS Core Level Spectra for P3HT/Au Interface......................................43 Figure 20: UPS Spectra for P3HT/Au Interface.........................................................44 Figure 21: Orbital Lineup of the P3HT/Au Interface Constructed from the UPS and XPS Data Set......................................................................................48 Figure 22: ITO work function after light intensity x-ray exposure............................49 Figure 23: IXPS Measurements of ITO Sample After Exposure to Standard Intensity X-rays.........................................................................................50 Figure 24: LIXPS and UPS Spectra for P3HT/ITO Interface....................................51 Figure 25: Orbital Lineup of the P3HT/I TO Interface Before and After UPS Exposure...................................................................................................54 iv

PAGE 8

Fabrication and Analysis of Poly(3-hexylthiophene) Interfaces Using Electrospray Deposition and Photoemission Spectroscopy James Lyon ABSTRACT P3HT (Poly(3-hexylthiophene)) is an orga nic polymer that shows promise as an active material in semiconducting electronics. It is important to study the electronic properties of this material in order to dete rmine its efficacy in such devices. However, many current studies of thiophene only exam ine the oligomer, si nce it is a simpler material to investigate. In this study, several P3HT interfaces were analyzed to determine their electronic properties. The P3HT was deposited on Au, highly-ordered pyrolitic graphite (HOPG), and indium tin oxide (ITO) subs trates via electrospray depos ition. The depositions were performed in several steps, with x-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) measurements taken between each step without breaking the vacuum. The resulting series of spectra allowed orbital line-up diagrams to be generated for each interface, giving detailed analysis of the interfacial properties, including the charge injection barriers and in terface dipoles. The results, when compared to similar oligomer-based investigations, show a difference in the orbital line-up between oligomeric and polymeric P3HT junctions. v

PAGE 9

Introduction The last decade has seen increased interest in the fabrication of polymer electronics. Organic/inorgani c interfaces have promising potential application in socalled plastic electronics, thanks to their material properties and low cost. The conjugated polymer poly(3-hexylthiophene) (P3H T) is a promising organic material for these devices, due to its relatively high dr ift mobility and semiconducting properties. Recent devices utilizing P3HT interfaces include organic solar cells [1, 2] and field-effect transistors [3, 4]. Electronic devices typically rely on ma terial interfaces for operation. These interfaces create an asymmetric junction which drives current in a particular direction. Different materials create interfaces with differing properties. It is necessary to investigate the properties of these junctions to determine which materials are optimal for a particular device. These investigations are frequently performed using Photoemission Spectroscopy. Photoemission spectroscopy (PES), which includes x-ray PES (XPS) and ultraviolet PES (UPS), is a surface science t echnique that provides information about the surface of a sample. XPS shows the chemical interactions of the interface and of the individual molecules themselves, while UPS allows determination of the many of the interface properties, including ch arge injection barriers and interface dipoles. It is a surface sensitive technique and is not sensitive to the bulk of the substrate, thus giving a 1

PAGE 10

view only of the surface and the newly create d interface. Its high surface sensitivity requires performance of the measurements in UHV to avoid contamination issues. Interfacial investigations using PES were first carried out by Waldrop and Grant on inorganic interfaces [5]. As organic mate rials began to be considered for use in electronic devices, this method was expanded to organic interfaces, such as polymer/metal junctions [6]. Polymeric interfaces, including ones utili zing P3HT, are difficult to fabricate, mainly due to the fragility of the polymers. Evaporation of the polymer in vacuum is ruled out because of the high molecular weight of polymers, making them nonvolatile even at high temperatures. Many investigator s have taken to creating polymeric films exsitu through the use of spin coating. Such a process was carried out by Salaneck et al. in their investigation of organic-metal interface s via UPS [6], and by Atreya et al. in a similar study that examined the interactions between evaporated metals and the polymer MEH-PPV [7]. Ex-situ creation of polymer ic films, however, e xposes the polymeric substrate to ambient conditions, introducing en vironmental contaminants. Evaporation of metal onto a polymeric substrate in-vacuum is a possibility, but th e evaporated, highlyenergetic metals atoms may damage the polymer film. To allow for in-situ evaporation of organic materials for interfacial invest igations, some researchers substituted the oligomer for the polymer when investigati ng the properties of organic interfaces, basing their actions on the apparent similarities between the oligomers and polymers of organic materials. Oligomers have a lower molecula r weight than their pol ymeric counterparts, and as such are easier to evaporate. Howeve r, assumptions about the similarities between oligomers and polymers may create challenges in the interpretation of the nature of the 2

PAGE 11

polymeric junctions. Fujimoto et al. and Chandekar and Whitten have both investigated the differences in electronic structure between short and long-chain o ligomer films [8, 9]. Other researchers have focused on other non-vacuum techniques to investigate these interfaces, such as I/V curves. Electrospray thin film deposition is a fa brication process that eliminates these preparatory constraints. Elect rospray allows the direct injection of macromolecules from solution into high vacuum. It has been prim arily used in the past to introduce large molecules into mass spectrometry devices, and ha s only recently been considered for thin film deposition in vacuum [1012]. The injected polymers can be deposited on various substrates in several steps without breaki ng the vacuum and without damage to the polymer. This multi-step deposition process permits x-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectro scopy (UPS) measurements in between deposition steps. The thin films produced by electrospray deposition allow XPS and UPS to clearly characterize the resulting interface. Several previous investigations of P3HT interfaces examined only the finished interface, without examining its properties as it was grown [6, 7]. The studies presented here, in contrast, use the multi-step deposition technique in which the interface was analyzed at each step of deposition, allowing a detailed characterization of the junction during its formation. Clean samples were expos ed to the deposition process and then sent in-situ to an analysis chamber, where PE S was performed. The process was then repeated several times, each deposition step longer than the previous one. This multi-step process allowed evaluation of the electronic interface, detection of charging and other measurement-related phenome non, and a view of the subtle growth of 3

PAGE 12

interface characteristics such as band bendi ng. The combination of electrospray deposition to grow actual polymeric interfaces the multi-step deposition process, and insitu characterization makes these sets of expe riments unique in their ability to probe the exact details of the growth of these interfaces and their precise device characteristics. 4

PAGE 13

Materials Au (gold) is a transition metal element, so called because of its partially filled dsub-shell. D-orbital electrons have a hi gh likelihood of delocalizing within the metal lattice, and this gives gold and other transi tion metals a number of properties, such as high tensile strength, density, melting and boili ng points, and its characteristic color. Because of its high conductivity and resistance to corrosion, gold is frequently used as electrical contacts for va rious solid state devices. As a solid, gold forms a continuum of ener gy levels to create a valence band, as with other solids. The d-bands of gold c ontain more energy levels than its s and p orbitals within a narrower range [13]. This leads to high electron density levels at the energy levels where the d-bands lie, and consequently high, sharp peaks in golds valence band energy spectra. This feature can be cl early seen in Figure 19. The large peaks in the gold samples valence band correspond to de nsity spikes from the elements d-bands. Most metals possess a cloud of electron gas about their surface, creating a dipole that increases the work function of the metal [ 14]. When an organic material is deposited onto the metal, the cloud of electrons is pushed back into the surface of the metal, reducing the work function. It is the pushing back of the electronic cloud which results in the large interface dipoles found between gold and P3HT in this experiment. 5

PAGE 14

HOPG HOPG (highly ordered pyroli tic graphite) is one of the allotropes of carbon. It consists of sheets of hexagonally ordered car bons. HOPG has a relatively high electrical conductivity compared to diamond. This is due to the presence of delocalized pi orbital electrons across the hexagonal lat tices of the graphite sheets. The hybridized sp2 orbitals form covalent bonds between the carbon atoms w ithin a single sheet, while the sheets are held together by much weaker van der Waals forces, making layers ea sy to shear off one another and permitting the cleaving of HOPG sa mple in situ to achieve a contaminationfree surface. Partially due to its lack of d-orbital electrons, HOPG has relatively weak valence band emissions. This makes it an ideal substr ate for investigating the formation of P3HT valence band emissions as the P3HT is deposited. The HOMO cutoff position of P3HT is more easily seen on HOPG substrates. Cons equently, a more accurate P3HT ionization energy can be obtained, which can be used for the evaluation of experiments on more challenging substrates HOPG creates only weak interface dipoles wi th organic materials, due to the lack of primary bonding between the HOPG and the poly mer. The organic material adheres to the HOPG only through van der Waals forces circumventing the usual processes for creating dipoles. ITO Indium Tin Oxide (In 2 O 3 :Sn or ITO) is a mixture of indium oxide and tin oxide. Transparent and colorless, its high electrical conductivity makes it an ideal electrode for many devices, especially the transparent layer of solar cells. 6

PAGE 15

Since ITO is a degenerately n-doped materi al [15], its Fermi level can be expected to lie within the conduction band, and not in the bandgap, as found in moderately doped semiconductors. However, the surface of th e ITO contains a higher concentration of oxygen, due to the ITOs treatment in ambien t conditions. This excess of oxygen reduces the degeneration of the ITO, lowering its Fe rmi level to inside the band gap and thus increasing its work function. The high work function of such an ITO sample makes it a promising anode for electronic devices. Fi gure 1 shows a comparison between ITO films with a 0% and 10% oxygen content. Figure 1: Band Diagram for Two ITO Films With Differing Oxygen Contents (from Reference [15]) Polythiophene Polythiophene is an organic semic onducting polymer. Semiconducting polymers are a relatively recent phenomenon, having be en discovered at the end of the 1970s by Heeger and MacDiarmid, who synthesized a polyacetylene chain with a 12 orders of magnitude increase in conduc tivity upon charge-transfer oxi dative doping [16]. Their conductivity is highly anisotro pic, mainly along the chain of the polymer. Work on 7

PAGE 16

conducting polymers (CP) began to accelerate, with many new suitable organic materials researched. Researchers were interested in CP because of their advantages over traditional semiconducting materials: low co st, ease of processing, light weight, and resistance to corrosion. Thiophene is an aromatic, heterocycl ic compound. The non carbon element in the compound is sulfur, which forms one node of a ring of alternating single and double bonds with four atoms of carbon. The si ngle bonds are formed by sigma bonding between electrons in the plane of the car bon nuclei. The double bonds are formed via a sigma bond and a pi bond formed by an overlap of p-orbitals above and below the plane of the ring. These orbitals, since they are out of the plane of the nuclei, are free to interact and move as they wish, and hence become delocalized. In addition to these electrons in the pi bonds, the lone electron pairs on the su lfur atom also participate significantly, giving thiophene a high charge mobility and its semiconducting properties. This type of system is known as a conj ugate system: the single and double bonds delocalize the pi electrons to lower the ove rall energy of the molecule and increase stability. Figure 2 displays a diagram of a P3HT molecule. Figure 2: Diagram of a Monomer in a Poly(3-hexylthiophene) Chain (from Reference [16]) 8

PAGE 17

Interfaces Metal-Metal Interfaces In a metal, energy states are filled to the Fermi level of the sample, there is a charge transfer that occurs from the lower work function metal to the higher work function. Figure 3 shows two metals before a nd after contact. This happens because the electrons in the lower work function metal possess a higher energy than those in the lower work function metal. Upon contact, th e lower work functions electron tunnel over to the lower available energy states in the higher work function metal. This electron transfer reduces the total energy of the dual system, but at the same time negatively the higher work function metal with regard to the lower work function metal. This charge imbalance creates a contact voltage (or potentia l difference) between the two metals [14]. Figure 3: Two Metals Before and After Contact 9

PAGE 18

Metal-Semiconductor Interfaces: Schottky Barriers When a metal and a semiconductor are brought into contact, there is the possibility of the formation of a Schottky barr ier. A Schottky barrier is an interface with rectifying properties; that is, charge is able to flow easily in one direction, but is impeded when attempting to flow in the other direction. Figure 4 shows a metal and semiconductor before and after contact. In the Schottky barrier m odel, we assume that m > n the work function of the smaller is greater than that of a p-type se miconductor (the doping type of P3HT). This impels the holes in the VB of the semiconductor to tunnel to the higher energy states available in the metal. This creates a contact (built-in) potential, as with metal-metal contacts. This potential eventually becomes strong until to impede the further flow of charge, and the Fermi levels within the materials equilibrate. This charge transfer creates a depletion region in the semi conductor near to the metal. Since this region has been depleted of charge carriers, it constitutes a space charge layer (SCL), or a non-uniform internal field directed away fr om the surface of the metal. This space charge layer causes the bands of the semiconductor to bend, becau se the charges within the semiconductor are not free to move and redistribute the charge to keep the bands in the SC flat. Figure 4 shows a typical metal-semiconductor contact. The PE barrier for charge moving from the metal to the semiconductor is known as the Schottky barrier height b When the junction is biased so that the metal is connected to the negative terminal and the semiconductor is connected to the positive terminal, a voltage dr op occurs at the depletion region. This causes the bands of the semiconductor to shift downwards, while keeping b the same. The decrease in the steepne ss of the VB permits holes to overcome the potential barrier and move to the metal, creating a current. 10

PAGE 19

Reversing the bias causes the SC bands to shift further upward. The holes in the metal cannot overcome the Schottk y barrier height to reach the SC, and so no current flows. Figure 4: A Metal and a Semicond uctor Before and After Contact Interface Dipoles In both metal-metal and metal-semiconductor interfaces, a common vacuum level (VL) is usually assumed. However, this is generally not the case for most metal-organic contacts. Generally, a surface dipole exists at the interface betw een the metal and the organic material [14]. Around the surface of a metal, the electrons are free enough to form an electron cloud. This creates a surface dipole, si nce the area around the metal has become negatively charged by th e electron cloud, and the consequent lack of electrons inside the surface makes the bulk side positive. When a molecule comes into contact with the metal, a dipole forms between the two materials. This can be due to interfacial charge transfer, redistribution of the metals electron cloud, an inte rfacial chemical reaction, and other 11

PAGE 20

localized charge rearrangements. The pushi ng of the metals electrons back into its surface reduces the free metal dipole, thus re ducing the work function of the metal at the interface. This is displayed as an asymmetr y in the vacuum level between the metal and the organic layer. Figure 5 shows the fo rmation of a dipole between a metal and an organic layer after contact, while Figure 6 shows the various ways such a dipole could form. Such interface dipoles are important to c onsider when investigating the properties of an interface, as they have strong repercussions on the properties of the junction. A misalignment of the vacuum level between the metal and the organic material reduces the band bending at the organic materials surf ace, changing the inject ion barriers of the interface and thus altering the interfaces perf ormance when used in electronic devices. Figure 7 shows a comparison of a metal-organi c junction with and without an interfacial dipole, comparing the effects of the dipole on its band structure. Metal-Polymer Interfaces Polymeric materials differ in their electronic structures from semiconductors. Organic polymers are composed of polymeric chains held together by weak van der Waals attraction. This means that their core atomic orbitals (AOs) are still localized, but their valence levels interact to form delocali zed molecular orbitals (MOs). The density of these orbital levels can be fine-grained or more spaced apart, depending on the number of atoms in the polymer chain [14]. The highest MO in an organic polymer is known as the highest occupied molecular orbital, or HOMO, and the lowest energy level in the conduction band is known as the lowest unocc upied molecular orbital, or LUMO. 12

PAGE 21

Electrons in the valence band and lower conduction bands of a polymeric material are localized and can only travel along the path of the one dimensional polymeric chain, whereas those in the higher conduction band levels become delocalized and can freely travel from chain to chain. Figure 5: Formation of a Dipole Between a Metal and an Organic Layer(from Reference [14]) 13

PAGE 22

Figure 6: Possible Factors Forming and Aff ecting Interfacial Dipole Layers (from Reference [14]) Figure 7: Comparison of a Metal-Organic Interface a) With and b) Without the Shift in Vacuum Level (VL) Corresponding to a Dipole (from Reference [14]) 14

PAGE 23

Electrospray Electrospray is a method which allows di rect injection of macromolecules into high vacuum. It is useful for processes such as mass spectrometry and thin film depositions on substrates. Growing organic thin film s can be a challenging pro cess. Organic materials cannot be deposited by evaporation, because their high molecular weight makes them difficult to evaporate, and the fragility of the polymers is increased by the heat. Spin coating is a popular method, but must be do ne in ambient cond itions, exposing the subsequent film to environmental contaminants. Figure 8 shows a schematic of the Electro spray process. Electrospray works by creating small ionized droplets by application of an external electric field between a capillary and a counter electrode. The charged molecules within the solution move from liquid phase to gas phase during the process, where they are directed for use in a mass spectrometer or for thin film growth. During the process, an inert or nebulizing gas is sometimes used to increase flow rates and pe rmits use of solvents with higher surface tensions. 15

PAGE 24

Figure 8: Process of Electrospray Deposi tion Technique (from Reference [17]) Historical Background The first known reference to the electr ospray phenomenon was by W. Gilbert in the year 1600, in his book De Magnete, where he discussed the most recent discoveries in the then nascent fields of electricity and magnetism [18]. Nearly 200 years later, G.M. Bose [19] and Labbe J. A. Nollet [20] bot h noted a possible existence of electrospray in their experiments, the former while applyi ng electric potential to a glass capillary and the latter while experimenting w ith human blood and electricity. Until the 1960s, electrospray was mainly utilized as a painting technique, the process being described by Burton and Weigand. During the 1960s, the Taylor cone was first discovered by Sir Ge offrey Ingram Taylor [21]. In the 1980s, the groups of 16

PAGE 25

Fenn [22, 23] and Alexandrov [24] independently used electrospray to generate gas phase ions for mass spectrometry successfully for the first time. In 1987, Bruins et al. used a process known as pneumatically assisted elec trospray, or Ionspray, which first used a nebulizing, inert gas to assist in charge aerosol formation [ 25]. In 1988, Fenn et al. [26] achieved multiple charging, making it possible to analyze large molecules on mass spec machines with limited mass range, and greatly increasing interest in electrospray. A technique called Nanospray was developed by Wilm and Mann in the mid 1990s to generate electrospray using low flow rates (nl min -1 ) [27]. Physical Basis The process of ESI can be broken down into three steps: 1. Charge droplet formati on at the capillary tip. The charge is induced by an external applied electric field between the capillary tip and a counter electrode. Charge separation occurs, the charge depending on the bias of the capillar y, and if the field is strong enough, a Taylor cone will develop and protrude from the capillary, as well as a thin liquid jet, where small, charged drop lets are generated in a fine mist. During the charge droplet formation stage the Taylor cone can be observed. This cone is created by the rearrangement of ch arge produced by the external field. If the solution is conductive enough, the solutions cations will (in positive mode) move towards the fields strongest point, the tip of the capillary, while the anions will migr ate away from the surface. The counteracting of this electr ic force with the surface te nsion of the liquid will 17

PAGE 26

create a cone (the Taylor cone) at the capillary tip. When the external potential becomes high enough, a liqui d jet will emerge from the cone, ejecting charged droplets. The potent ial needed to generate a stable electrospray and the subsequent Taylor cone can be generated with this equation [28]: V on = A 1 (2 r c cos( )/ 0 )1/2ln(4d/r c ) Where V on is the electrospray onset voltage (V), A 1 is the dimensionless constant ~0.5-0.7, is the surface tension (Nm-1), r c is the capillary radius (m), is the half angle of the Taylor cone apex, 0 is the electrical permittivity in vacuum (C2N-1m-2), and d is the spray needle counter electrode distance (m) 2. Evaporation of solvent from the droplets. As the charged droplets are impelled forward by the external field, their radius will decrease as the solvent evaporates and is pumped out by vacuum pumps. At a point known as the Rayleigh limit, the Coulombic forces will overcome the liquids surface tension and the dr oplet will undergo fission, separating into several smaller droplets. This pr ocess can repeat, creating even smaller and more densely charged droplets. 3. Formation of gas phase ions. At a certain point in the el ectrospray process, the solvated ions transfer to gas phase ions. Two main theories have been proposed for this process: the 18

PAGE 27

charged residue mechanism (CRM) and th e ion evaporation mechanism (IEM). It has been suggested that both these processes can occur, but for differing analyte types. The two methods of gas phase ion formation have been debated since the 1960s. The Charged Residue Model (CRM), by Dole et al. [29], reasons that the droplets keep undergoing fission into smaller and smaller droplets until only one macromolecule pe r drop remained. The evaporation of the rest of the solvent leaves the macr omolecule intact and isolated. The Ion Evaporation Model (IEM) by Iribane and Thomson [30] proposed that when the droplets have shrunk to a certain size, the ionic macromolecules would be emitted directly from the droplets. 19

PAGE 28

Photoemission Spectroscopy Photoemission (or photoelectron) spectrosc opy (PES) is an an alysis procedure whereby monochromatic photons impinge upon a sample, ionizing electrons. This process of ionization, known as the photoelectric effect, allows a detailed investigation of a materials electronic propert ies and chemical composition. The emitted electrons have a kinetic energy equal to the energy of the photons minus the ener gy required to ionize them. The electrons are collected in an analyzer, where a count of the number of electrons with a particular kinetic energy is performed. This produces a spectrum which can be used to characterize a sample. Two types of PES were used in th ese experiments: xray photoemission spectroscopy (XPS) and ultrav iolet photoemission spectroscopy (U PS). The soft x-rays used for XPS are in the 200-2000 eV range and can be used to examine the core energy levels of a sample, while UPS, using narrow lin e width UV light in the 10-45 eV range, is used to examine the valence bands with high resolution. Physical Basis The photoelectric effect was first obse rved by Alexandre Edmond Becquerel in 1839, by exposing an electrode in a conductive solution to light Despite this, Heinrich Hertz is generally recognized to be the di scoverer of the phenomenon (giving the effect a third, though obsolete nomenclature: The Hertz effect). His experiment consisted of two arcs, a driving and secondary one. The secondary arc was observed to be more 20

PAGE 29

pronounced when it was not shielded from the driving arc by a pane of glass. Substituting the glass shield for a quartz one produced no noticeable difference in the arc. Hertz concluded that ultraviolet light from the driving arc increased the arc length by assisting the electrons in jumping across the gap. The glass pane absorbed the UV and hence reduced the arc length, while the quartz, which does not absorb UV, did not affect it. Hertz made no further effort to explain the results. It was Einstein who first proposed an e xplanation of the eff ect, using the newly formulated rules of quantum mechanics. It was observed that hi gher intensity light increased the current of the emitted electrons, whereas higher energy did not. Einsteins explanation of the photoelectric effe ct in 1905 [31] was part of his annus mirabilis burst of papers, during which he also revealed speci al relativity and the equivalence of matter and energy. When a photon impinges on an electron, it imparts all its energy to the electron (Einstein concluded definitely th at a photon is not able to transfer a por tion of its energy). If the energy received by the electron is greater than the work function of the material the electron resides in, the electron is ionized an d ejected out from the material. The energy of the photon striki ng the electron is E = h (1.) where E is the energy, h is plancks constant, and is the frequency of light. The binding energy, BE, of the electron is the energy required to move an electron from its initial position to the Fermi level of the solid. The Fermi level is located in between the high occupied molecular orbital (HOMO) and the vacuum level. The work function, of the material is the energy difference between the Fermi level and the vacuum level. When a 21

PAGE 30

photon impinges on an electron, the electron must overcome first its BE and then the materials in order to escape into vacuum. If it manages to ionize, the remaining energy of the photon is considered to be the electrons kinetic energy, KE. Thus, KE = h BE (2.) Figure 9: Photoelectric Effect for a Model Atom Figure 9 shows the photoelec tric effect schematically. In this presentation, the work function of the material is included in the binding energy. The loss of the electron produces a positively charged hole is represented by a white circle. The KE of the ionized electron is shown on th e right to be the photonic en ergy after the loss of the energy required to ionize the electron. 22

PAGE 31

Figure 10: Photoelectric Effect fo r a Model Atom at 2p Orbital Figure 10 displays the photon striking a lo wer BE electron, this one in the 2p orbital. Since the electron has a lower BE, more of the energy of the photon is transformed into the ionized electron KE. When an electron is emitted from a sample it leaves a hole behi nd in its orbital. This hole may be filled by another electr on dropping down from a lower BE. The lowering of this secondary electrons ener gy causes the electron to emit a photon, which in turn may impinge upon another electron and ionize it. This emitted electron is known as an Auger electron. Since the Auger electr on receives its energy from another electron, its energy is independent of the original photon energy, and instead depends on the 23

PAGE 32

difference in energy between the secondary electrons BE before and after dropping to a lower orbital. Figure 11 shows the process of Auger electron creation. Line A shows an electron dropping from its initia l state to the orbital of a pr eviously ionized electron. Its release of energy upon dropping has the effect of ionizing a second electron into the vacuum level, line B. Since Auger electrons are referenced by the orbitals involved in the process, the electr on would be called a KL 23 L 23 or KLL electron. Figure 11: Auger Effect for a Model Atom PES Equipment PES equipment consists of a fixed energy radiation source, a sample, an analyzer, an electron detector, and a high vacuum en vironment. The radiation source varies depending on the type of PES us ed. Other devices may be used to enhance analysis, such 24

PAGE 33

as an electron gun and a voltage generator to bias the sample and improve electron collection. Figure 12 shows an exam ple schematic of a PES system. The photon source is typically an X-ray gun for XPS or an ultraviolet (UV) gas discharge lamp for UPS. The x-rays produ ced by the x-ray gun in this study came from Mg (h = 1235.6 eV). The UV source produced He I (h = 21.22 eV) photons. The analyzer acts as a band pass filter, allowing only electrons of a certain energy range through. This is done via tunable magne tic or electrics fields Those outside the set range will be deflected and absorbed by the analyzer walls. The analyzer used in these experiments was a spherical deflection analyzer (SDA). It consists of two concentr ic hemispheres, as shown in Figure 12. The analyzer is arranged to be able to colle ct the electron released from the sample by the photoelectric effect. The analyzer is preset to allow th e desired electrons in. Before reaching the analyzer, the electrons must first pass through a physical apertu re. This is settable to different sizes. The smaller the aperture size, the lower the intensity of the electrons that reaches the analyzer. While the resolution is hi gher, the aperture is typically set to a low setting for UPS, due to the high intensity of UV produced by the discharge lamp, which in turn generates a large numb er of ionized electrons. A t oo high intensity of electrons can damage the analyzer. XPS can be sa fely used with higher aperture settings. The electrons must pass another barrier be fore final entry into the analyzer: the retardation stage. This retardation is essentially a high pass filter, removing low energy electrons which tend to increas e noise. The retardation energy for the electrons is constantly adjusted to allow electrons of different energies through and produce a full spectrum of energy levels. 25

PAGE 34

Now the electrons enter the hemispherica l analyzer, where they are transported and filtered via the application of an electric field between the two plates. This voltage is kept at a user-defined constant to determin e the variance of electron energies that are permitted through, and hence adjusts the resolution of the scan. An electron detector on the other side of the analyzer then measures the intensity of the electrons at each energy level. The electron detector used in this study is an electron multiplier tube, although micro-cha nnel detectors are also common in PES systems. The electron detector creates a cascade of electrons produced by the initial electrons exiting the analyzer. This is accomplished by a voltage applied in the detector, giving the initial electrons addi tional energy. These electrons strike the detector wall, creating several more electrons for each init ial electron. The process repeats until a cascade of electrons has been created, whic h reach the end of the detector and are measured. It should be noted that the work function of the sample is not needed to determine the binding energy of the emitted electrons. Due to a common ground connection, the Fermi levels of the sample and spectrometer are equilibrated, their equilibration confirmed via calibration. Thus, the measur ed electrons have a binding energy given by: KE = h BE s where s is the work function of the spectrometer. 26

PAGE 35

Figure 12: Photoemission Spectroscopy Equipment Schematic (f rom Reference [33]) The sample is grounded via contact with a sample holder to prevent charging. Lack of grounding of the sample will preven t replacement of the electrons emitted during the emission process, and will leave the sample positively charged, thus ostensibly increasing the binding energy of the electrons as exposure time increases. This ground is 27

PAGE 36

also used as a reference level for the Fermi equilibration of the sample and spectrometer, as described above. The spectroscopy process is performed in a high vacuum environment to sustain the integrity of a samples surface composition during measurements. A high vacuum environment will allow the sample to remain contamination-free for up to several days if the pressure is low enough. By contrast, even a few seconds spent in ambient conditions will cause the sample to accumulate contamin ants on its surface that will significantly affect the measurements. X-ray Photoelectron Spectroscopy X-rays are ideal for probing the core levels of the molecules in a sample due to their high energies. Each element contains electrons at characteri stic binding energies that can be measured. Evaluation of the full range of electron energies emitted by the xrays allows a detailed spectrum of these characteristic energies, and allows a determination of which elements are presen t in the sample, depending on the orbital peaks present. Given similar ionization cros s sections, a higher peak implies a greater number of the element within the sample. On ly electrons with binding energies smaller than the x-rays energy value can be probed, as those with higher binding energies will not receive enough energy to leave the sample. Figure 13 shows an example of a cleanly sputtered Au sample characterized by XPS. The resulting peaks ar e all characteri stic of Au. Electrons within the same orbital may produce different binding energies, depending on the configuration of the electrons These configurations are not equally 28

PAGE 37

probable, and so the re sult is a cluster of peaks within the same general area which are not symmetric to each other and have differi ng intensity. The split configuration is always the same for each orbital in an elemen t. One of golds spin doublets can be seen in Figure 13 between 300 and 400 eV. Figure 13: XPS Example, Spectrum of a Clean Gold Sample (from Reference [33]) 29

PAGE 38

Changes to a samples molecular prope rties can be determined by noting the alterations occurring to the samples peaks, including shifts to lo wer or higher binding energies and changes in peak intensity. Figur e 14 shows an example of a O1s peak shift and lowering of intensity. Figure 14: Peak Shift Example, Lowering of O1s Binding Energy After Exposure to X-ray Radiation (from Reference [33]) A secondary technique utilizing XPS is low intensity XPS (LIXPS). LIXPS is a measurement procedure utilizing a greatly reduced photon flux from standard XPS (1.2% of standard XPS photon flux) [34]. LIXPS is used to measure the secondary edges of samples. Since the high intensity of UPS may lead to charging of the sample while measuring secondary edges, LIXPS is perfor med before and after each UPS measurement to determine whether charging occurred duri ng UPS operation. LIXPS may also be used 30

PAGE 39

in lieu of standard UPS on light-sensitive sa mples, such as ITO. A bias voltage is typically applied to separate analyzer and sample secondary edges and to collect emitted electrons more efficiently. Ultraviolet Photoemi ssion Spectroscopy Ultraviolet photoemission sp ectroscopy is used to analyze the valence bands of a sample and determine its work function through analysis of the secondary edge. The gas discharge lamp typically used for UPS provi des a narrow line width of radiation to the gass discrete energy structure, and al so allows a large photon flux, permitting the process a high resolution and a high ra tion of signal to noise. He I is the typical photon used, with an energy of 21.22 eV. UPS is generally used to investigate such properties as the highest occupied molecular orbital (HOMO) and charge injection ba rriers of a sample. Because of the source of the photons from UPS is a gas, the atoms emit photons in a narrow band of energies, increasing resoluti on of the resulting spectra and narrowing the peak widths. A bias voltage is typically used in conjuncti on with UPS to increase the magnitude of the secondary edge and to sepa rate analyzer and sample secondary edges. Figure 15 shows the UPS spectra of the P3HT/Au interface. The bottom spectra of each of the panels show bare Au, with each successive layer representing a deposition step of P3HT. The center panel displays the complete UPS spectra. The left panel shows the normalized secondary edge of the sample. The large shift after the first deposition is most likely related to the formation of an interface dipole, while the later, more gradual shifts can be attributed to either band bending or charging of the sample. The rightmost panel shows the valence bands of the sample. For the bare Au, large peaks can be seen 31

PAGE 40

which represent the d-bands of the gold. As more P3HT is deposited, these peaks attenuate and P3HT-related characteristic emission arise. Figure 15: UPS Spectra of the P3HT/Au Interface 32

PAGE 41

Experimental Three main experiments were performed to investigate the electronic properties of various P3HT interfaces. P3HT was deposited on three substrates: Au, HOPG, and ITO. The ITO experiment was performed differ ently from the others due to special consideration of the ITO substr ate. These experiments will be discussed at length in the following sections, with the ITO experiment explained in detail following the first two. Experimental Method All experiments were performed in an ultra high vacuum (UHV) system, shown in Figure X. The system was composed of f our chambers: a fast entry lock, a preparation chamber, a deposition chamber, and an analysis chamber. The system was acquired from SPECS (Berlin, Germany). The base pressures of the chambers were 1x10 -10 for the analysis chamber, 1x10 -8 for the preparation chamber, and 1x10 -7 for the entry and deposition chambers. The system was pumped with turbomolecular/rotary pumps in combination with ion pumps. The prepar ation chamber contained a SPECS IQE 11/35 ion source for Ar + ion sputtering. The analysis ch amber was equipped with a SPECS non-monochromated XR 50 dual x-ray gun for XPS, a SPECS UVS 10/35 ultraviolet source for UPS, and a SPECS Phoibos 100 hemi spherical analyzer. Igor Pro software (Wavemetrics, Inc.) was used for all evaluation, graphing, and curve fitting. The P3HT was prepared in a toluene so lution at 0.1 mg/ml and kept wrapped in an aluminum cover to avoid exposure to light. The P3HT ha d a weight-average 33

PAGE 42

molecular weight (M w ) of 19 398 with a polydispersity index (M w /M n ) of 1.59 as determined by gel permeation chromatography (GPC) based on polystyrene standards. 100nm thick Au films were created via therma l evaporation on Si wafers. The ITO and HOPG substrates were purchased from Mi kromasch USA (ZYA quality), and X, respectively. A 5 V bias was applied to the samples during UPS and LIXPS to separate the analyzer and sample secondary edges. Mg K (h = 1235.6 eV) radiation was used for the XPS measurements, and He I (h = 21.22 eV) for the UPS measurements. The PES measurements were analyzed via a SPE CS Phoibos 100 hemispherical analyzer. Peaks were fit using a GaussLorentzian profile, as outl ined by Kojima and Kurahashi [35]. Work function and HOMO cutoffs were determined by fitting a line tangent to the curve of the cutoff and evaluating its intersec tion with the energy axis of the spectra. Analyzer broadening was corrected by adding 0.1 eV to the fitted cutoff values. Sample Preparation For each experiment, the substrates were sonicated in acetone, iso-propanol, and methanol, and then dried. The substrates were then mounted onto a sample holder using conductive silver epoxy. For th e Au and ITO samples, a sm all spot of silver epoxy provided an electrical contact between the sample and the holder to allow a path to ground. Once inside the UHV system, the HOP G sample was cleaved to provide a contamination-free surface. 34

PAGE 43

Au and HOPG Analysis Procedure Each sample was placed into the fast entry load lock. After stabilization of pressure, the sample was moved to the prepar ation chamber, where it was sputtered with Ar + ions for 30-60 minutes. The sample was then transferred to the analysis chamber. LIXPS was performed on the sample, followed by UPS and then another LIXPS investigation. The LIXPS experiments were performed to ensure no charging occurred during the high intensity UPS i nvestigation. Following this, XPS was performed, first as a survey scan on the entire energy range availa ble to XPS, and then as intense, focused investigations on orbital peaks of particular im portance to the analysis of the sample and the eventual P3HT deposition. After this initial analysis, the sample was transferred to th e deposition chamber, where it was prepped for deposition of P3HT. A laser crosshair was utilized for correct positioning of the sample. A syringe filled with P3HT solution wa s placed onto a syri nge pump, where it was injected into the electrospray chamber through a capillary. The injection rates were 10 ml/h for the Au and HOPG samples and 4 ml/h for the ITO sample. The electrospray capillary was biased at -3.5 kV relative to the grounded chamber. N 2 gas was pumped into the electrospray enclosure around the or ifice to prevent exposure to ambient contamination. After initial deposition, the sample was tr ansferred back to th e analysis chamber, where the previous PES steps were again performed. This procedure was performed several times, with seven depositions of P3HT solution on the Au substrate (for a total of 6.35 ml) and five depositions on the HOPG substrate (for a total of 3.15 ml). 35

PAGE 44

ITO Analysis Procedure It has been observed that high intensity UV and x-rays significantly reduce the work function of ITO substrates [34, 36]. Due to this limitation, only LIXPS was performed on the ITO substrate during P3HT deposition. The substrate also was not sputter cleaned, as this proce dure also affects changes in the work function. Upon first loading and after each deposition, LIXPS was performed on the sample. These were done as quickly as possible to prevent unnecessary exposure to x-rays. A total of seven depositions were performed, resulting in a tota l of 6.45 ml of P3HT solution injected into the electrospray chamber. After the fina l LIXPS measurement, UPS was performed to examine the effects of high intensity UV light on the ITO work function. To measure the effects of x-ray exposure on ITO, two more ITO substrates were examined. The first was exposed to LIXPS 24 times, the measurements being performed as the exposure took place. The second was exposed to high intensity x-rays. Each exposure time was double the previous one, with the first exposure time set to 30 seconds. A total of six exposures were performed, w ith LIXPS performed between each exposure to evaluate the secondary edge shif ts that occurred during exposures. 36

PAGE 45

Results and Discussion P3HT/HOPG Results Figure 15 shows the S2p core level spectra of the P3HT/HOPG thin film. From first to last P3HT deposition, the S2p doublet shifts from a starti ng position of 163.88 eV to 164.0 eV, a total of 0.12 eV, most likely re lated to band bending at the interface. The magnitude of the peak also increases as P3HT is deposited, from the sulfur atom present inside the chain of a thiophene monomer. Figure 16 displays the UPS spectra of the P3HT/HOPG junction. The center panel displays the complete UPS spectra. The rightmost panel shows a normalized close up of the secondary cutoff. The secondary edge shifts from 16.60 eV (vacuum-cleaved HOPG) to 16.77 eV, indicating a reduction in the work function 0.17 eV, most likely related to band bending and a small interface di pole (small due to the stable nature of HOPG). The rightmost UPS panel shows the va lence band spectra of the interface, with background removed. A transition from HOPG to P3HT-dominant emissions reveals a weak P3HT HOMO shoulder of 0.41 eV. 37

PAGE 46

Figure 16: S2p XPS Core Level Spectra for P3HT/HOPG Interface 38

PAGE 47

Figure 17: UPS Spectra fo r P3HT/HOPG Interface P3HT/HOPG Discussion The UPS and XPS data of the P3HT/HOPG interface can be analyzed to construct an orbital lineup diagram of the junction. The UPS data in Figure 16 show the shift in the secondary edge as P3HT is deposited. A very gradual shift throughout the deposition process implies a lack of strong charging and a small or negligible interface dipole. Since the weak emission of the S 2p peak makes it an unreliable benchmark for band bending, the secondary edge shift is used to dete rmine the amount of band bending, showing a shift of 0.17 eV from first to last deposition. This shift includes both band bending and 39

PAGE 48

interface dipole, however. The dipole can be estimated by taking the work function of the pristine HOPG ( HOPG = 4.61 eV) and comparing it to the work functions of the first two depositions, which yielded values of 4.57 and 4.53 eV. This implies an interface dipole of between 0.04 and 0.08 eV, which when subtracted from the total work function shift of 0.17 eV yields band bending values of 0.09-0.14 eV. The average of these values (0.12 eV) is in perfect agreement with the shift of the S 2p peak. The ionization energy of the P3HT was determined by adding the work function of the 3.15 ml layer with the HOMO cuto ff position (0.41 eV), yi elding a value of 4.44 eV + 0.41 eV = 4.85 eV This is in reasonable agreem ent with the ionization energy value found in the P3HT/Au experiment. This value was used for the io nization energy of the P3HT/ITO experiment, because of the difficulty in evaluating the ionization energy in the ITO experiment. The hole injection barrier at the interf ace can be determined by subtracting the band bending (0.12 eV) from the HOMO cutoff position (0.41 eV), yielding a value of 0.29 eV. The electron injection barrier e = 1.41 was determined by subtracting the hole injection barrier from a previously discovered optical band gap of E g = 1.7 eV, determined by UV-visible spectroscopy [X]. Th is should be taken as a lower limit for the electron injection barrier, as the optical band gap (by the exciton binding energy) is smaller than the polaron band gap, which should be used for the band gap. The resulting data gathered and analyzed was used to c onstruct an orbital li neup diagram of the interface, Figure 17. 40

PAGE 49

Figure 18: Orbital Lineup of the P3HT/HOP G Interface Constructed from the UPS and XPS Data Set P3HT/Au Results Figure 18 shows the XPS spectra of th e P3HT/Au interface co re levels. The bottom spectra represent the bare, sputtered Au substrate. Ea ch of the following spectra was measured after each P3HT deposition step. The graph shows the C1s, S2p, and Au4f core levels, from left to right. A small carbon peak can be seen in the C1s spectrum before deposition. This is most likely rela ted to contamination of hydrocarbons that failed to be removed during sputtering. After the first deposition, two peaks emerge. One is the initial carbon peak of the nascen t P3HT thin film. The other appears to be residual toluene solvent adsorbed onto the substrate during the deposition sequence. 41

PAGE 50

During the last three depositions, the C1s peak shifts to the left 0.11 eV (determined by peak fitting), which may be an indication of band bending. The S2p and Au4f show similar shifts. The Au4f peaks also attenuat e during deposition as th e gold is covered by the P3HT film. Figure 19 shows the UPS spectra of the P3HT/Au interface. The central panel shows the spectra in their entirety. To the left, a normalized close up of the interface secondary edge is presented. A strong shift is present after the first deposition, which is most likely related to an electric dipole formed from the compression of the electron cloud around the metal surface. After this initia l jump, a slower shift to the right can be seen, more likely due to band bending rather th an charging because the shift saturates as more P3HT is deposited. The rightmost pane l displays the valence bands spectra. The sharp peaks visible in the vale nce band are the Au samples d-orbitals. As the P3HT thin film grows, the Au valence ba nd features attenuate and P3HT HOMO features arise. The HOMO cutoff of the final P3HT thin film is measured to be 0.7 eV. 42

PAGE 51

Figure 19: XPS Core Level Spect ra for P3HT/Au Interface 43

PAGE 52

Figure 20: UPS Spectra for P3HT/Au Interface P3HT/Au Discussion Evaluation of the P3HT/Au photoemission spectroscopy measurements gives us a detailed look at the electronic properties of the interface. The UPS spectra, shown in Figure 19, display the work function and th e HOMO value of the junction before and after each deposition. The spectrum of the sputtered Au (bottom spectrum) is characteristic of Au UPS emissions, with prominent peaks representing the d-bands of the Au. New features arise on top of the gold peaks as the P3HT is deposited. These 44

PAGE 53

features have been related ea rlier to bonding orbitals of the alkyl ligand s (bands >5 eV), sulfur related localized states (peak at ~3.8 eV), and back bone related localized states (shoulder at ~1 eV) [37, 38]. After the first deposition, the secondary edge of the interface experiences a strong shift to the left. This shift is most likel y related to an interface dipole, caused by the electron gas surrounding the metal being pus hed into the metal surface by the P3HT overlayer and thus reducing the metal work function [39]. The XPS spectra shown in Figure 18 s how the C1s, S2p, and Au4f peaks of the interface during P3HT deposition. The bot tom C1s spectrum shows a weak signal present before P3HT deposition, probably rela ted to ambient contamination missed by the sputter cleaning. After the fi rst deposition (0.15 ml), a two component peak arises, the lower energy peak corresponding to a sma ll amount of Toluene solvent coadsorbed during deposition. The P3HT C1 s peak continues to strengthe n after each de position, and shifts slightly during the last three depositi ons a total of 0.11 eV. This is due to the occurrence of band bending as the Fermi leve ls of the polymer layer and substrate equilibrate. A similar shift is seen for the S2p peak. Charging is ruled out because the shifting saturates instead of accelerates, and no similar shift is seen in the UP spectra, where much higher magnitudes of photon inte nsity are used. No x-ray damage is presumed because none of the XPS peaks show typical shape degradation after severe xray exposure [40]. Using the information garnered from the XPS and UPS spectra, an electronic interface diagram can be constructed. Figur e X shows a summary of the evaluation. The hole injection barrier h was determined by subtracting the band bending of the polymer 45

PAGE 54

at the interface V b from the fitted HOMO cutoff of the 6.35 ml spectrum, giving an injection barrier of h = 0.7 eV 0.11 eV = 0.59 eV This evaluated barrier is considerably la rger than previously reported studies of the P3HT/Au interface [41], which determined a barrier of 0.3 eV. This difference may result from different preparat ion methods or the presence of ambient contamination at the interface [42]. The secondary cutoff of the 6.35 ml spectrum was determined to be at 17.02 eV (corrected for analyzer broade ning), yielding an ionization energy E ion of 4.90 eV. This was generated by finding the work function from the secondary edge and adding the HOMO cutoff value to it. The electron injection barrier was e determined using a pr eviously calculated optical energy gap of 1.7 eV [ 43]. Subtracting the hole injec tion barrier from this value yielded the result e = E g h = 1.7 eV 0.59 eV = 1.11 eV This is a lower limit for the electron injecti on barrier since the optical gap is typically smaller than the polaron gap, which is the correct gap to use to account for the exciton binding energy. The interface dipole eD was ca lculated by subtracting the hole injection barrier from the ionization energy of the polymer and then subtracting this total from the work function of the Au. Thus, eD = Au (E ion h ) = 5.31 eV (4.89 eV 0.59 eV) = 1.01 eV This is a sizable inte rface dipole, and shows a strong ch emical reaction between the Au and the P3HT. 46

PAGE 55

Comparison of these data with si milar oligomer-based thiophene/Au investigations yields interes ting conclusions. A paper by Fujimoto et al. analyzes the results of HOMO density of state changes with changes in oligomer chain length [44]. Comparison with their UP data on octithi ophene bears a close similarity with our polymer measurements, the primary differen ce being a more spread out HOMO cutoff on the polythiophene, suggesting a stronger disp ersion of the HOMO band due to its longer chain length. Chandekar and Whitten [8] have reported an invers e relationship between the oligothiophene chain length and the hole injection barrier values i.e. a higher chain length results in a smaller injection barrier. Their data on the sexithiophene oligomer show a hole injection barrier of 1.1 eV, significantly larger th an the measured 0.7 eV of the final deposition step in the polymer expe riments. They also report an ionization energy of 5.2 eV, a difference of 0.6 eV fr om our reported ionization energy. This suggests a symmetrical increase in thei r hole and electron barriers from ours. Another study done by Schwieger et al. [45] reports a more significant difference between the the oligomer and polymer inte rfaces structures, reporting a 1.2 eV work function shift during depositi on of a sexithiophene overla yer, suggesting a larger interface dipole. Their data s hows a hole injection barrier of 1 eV, a measurement similar to the one in reference 8. 47

PAGE 56

Figure 21: Orbital Lineup of the P3HT/Au Interface Constructed from the UPS and XPS Data Set P3HT/ITO Results Figure 21 shows the results of the ITO work func tion shifts when exposed to LIXPS. A total of 24 measurements were performed, in succession and as quickly as possible to keep the time between measuremen ts consistent. The work function of the ITO was reduced by 0.18 eV during the course of the experiment. Figure 22 displays the ITO work functi on shifts when exposed to standard intensity x-rays, as measured by LIXPS. Seven measurements were performed, each one after a period of x-ray expos ure double the previous time, beginning with a 30 second exposure. The leftmost panel shows the shif t of the secondary edge caused by the x-ray exposure, while the right panel shows a graph of x-ray exposure time versus work 48

PAGE 57

function of the ITO sample. A total shift of 0.53 eV was observed over the course of the exposures. Figure 23 shows the results of LIXPS measurements performed on an ITO sample upon which P3HT was deposited. A total of seven depositions were performed, between which LIXPS measurements were carried out. After the final LIXPS measurement, a UPS measurement was performed. The leftmost panel shows the secondary shift of the sample as the P3HT was deposited. A tota l shift of 0.17 eV was detected between the first and last LIXPS measurement. The U PS measurement reduced the work function an additional 0.59 eV. The middle panel shows the UPS spectra measur ed after the final LIXPS measurement. The rightmost panel shows the HOMO region of the ITO sample after the final UPS exposure, pinning the HOM O cutoff at 0.81 eV (middle panel inset). Figure 22: ITO Work Function After Light Intensity X-ray Exposure 49

PAGE 58

Figure 23: LIXPS Measurements of ITO Sample After Exposure to Standard Intensity X-rays 50

PAGE 59

Figure 24: LIXPS and UPS Spectra for P3HT/ITO Interface P3HT/ITO Discussion Given the data on ITO work function altera tion from repeated x-ray exposure, it was determined that LIXPS measurements would not change the work function significantly. Over the course of 18 consecutive LIXPS measurements, the work function of the ITO decreased 0.18 eV. Since only ei ght LIXPS measurements were planned for the P3HT/ITO experiment, the work func tion change during the first eight LIXPS measurements was also recorded. As seen in Figure 21, the alteration in work function during the first 10 scans is much less than in the subsequent 14, suggesting that reliable measurements could be obtained if the num ber of LIXPS scans was kept below 10. 51

PAGE 60

A similar experiment to determine the re liability of standard XPS measurements found a shift of 0.53 eV after only six steps, an unacceptable deviation, and so standard XPS was ruled out for the measurement of the P3HT/ITO interface. A substantial deviation in ITO work function caused by a single UPS measurement also ruled out the use of UPS as a mechanism to further exam ine the interface, leaving LIXPS as the only viable option. Given the data available gathered fr om LIXPS measurements of the emerging P3HT overlayer, the orbital band lineup of the interface could be constructed. The valence band minimum (VBM) and conduc tion minimum (CBM) needed to be determined first. We calculated the VBM re lative to the Fermi level by using the In 3 d 5/2 peak located at 445.04 eV, since the VBM could not be determined by valence band spectra due to superimposed P3HT emissions. The known energy difference between the X peak and the VBM [15] is 441.8 eV, giving a VBM of 3.24 eV, which is in agreement with recent results [46]. Using an optical gap of 3.6 eV [15], the CBM was determined to be 0.36 eV. We used the previously measured ioni zation energy from the HOPG experiment of 4.85 eV for constructing the P3HT/ITO lineu p, because of the difficulty in analyzing the ionization energy from the ITO data. Using this and other data, the interfacial orbital lineup was constructed, shown in Figure 24. The work function of the sample decreases 0.17 eV from the bare ITO to the last P3HT deposition. This shift probably contains a LIXPS induced work function reduction of 0.05 eV corresponding to 8 LIXPS scans (Figure 21). This gives a to tal of 0.12 eV work function re duction, which is most likely due to a weak interface dipol e eD. Using the P3HT ioniza tion energy and aligning the 52

PAGE 61

ITO and P3HT vacuum levels (taking the di pole into account), we determined the HOMO cutoff of the P3HT layer to be 0.25 eV belo w the Fermi level. Subtracting the HOMO cutoff position from the ITO VBM (3.24 eV), a VBM to HOMO offs et of 2.99 eV was determined. The lowest occupied molecular orbital (LUMO) of the P3HT was calculated to be 1.45 eV above the Fermi level by using the P3HT optical gap of E g = 1.7 eV as determined by UV-vis absorption [43]. Th e ITO CBM to P3HT LUMO distance was calculated to be e = 1.09 eV, a lower estimate since optical band gap measurements include excitonic features, resulting in gene rally smaller band gaps than those actually encountered by charge carriers. The UPS measurement performed after the final LIXPS measurement further altered the configuratio n of the interface, as seen in Figure 24. UV exposure reduced the ITO work function 0.59 eV, for a total reduction of 3.96 eV. This implies a new interface dipole eD = 0.76 eV. Band bending is ruled out as an explana tion, as earlier investigations [34] showed no shifts in core levels during XPS core level measurements. The new HOMO level after UPS measuring wa s determined to be 0.81 eV (Figure 24, insert). This implied a new ionization en ergy of 4.77 eV for the P3HT layer, a small change from the earlier 4.85 eV [47]. This deviation possibl y results from weak charging artifacts, since small work function areas dominate secondary edge measurements, while primary peaks represent a true average over all areas of a sample. Figure 24 Shows the new orbital band lineup diagram after UV exposure. The larger interface dipole shifted the features of th e interface down to approximately 0.6 eV lower binding energies. 53

PAGE 62

Figure 25: Orbital Lineup of the P3HT /ITO Interface Before and After UPS Exposure 54

PAGE 63

Conclusion P3HT/Au In this experiment P3HT was deposited on to a clean Au surface in several steps, with x-ray and ultraviolet photoemission spect roscopy performed between each step. The measurements were performed in situ in an ultra high vacuum environment. Once a suitable layer of P3HT was grow n, the data were analyzed usi ng Igor Pro software and an orbital band lineup diagram was created. Seve ral of the properties of the interface were measured, including the HOMO, charge injectio n barriers, and the presence of absence of interface dipoles. The P3HT interface was found to have slightly modified electronic properties from similar oligomeric interfaces. P3HT/HOPG In this experiment P3HT was deposited onto a cleaved HOPG surface in several steps, with x-ray and ultr aviolet photoemission spectroscopy performed between each step. The measurements were performed in si tu in an ultra high vacuum environment. Once a suitable layer of P3HT was grown, the data were analyzed using Igor Pro software and an orbital band lineup diagram was created. Several of the properties of the interface were measured, including the HOMO, charge injection barriers, and the presence of absence of interface dipoles. 55

PAGE 64

P3HT/ITO In this experiment bare ITO was exposed to light and standard intensity x-rays to determine their effect on the ITOs work function. P3HT was deposited onto a second ITO substrate in several steps, with light intensity x-ray spectroscopy performed between each step. The measurements were performed in situ in an ultra high vacuum environment. Once a suitable layer of P3HT was grown, the data were analyzed using Igor Pro software and an or bital band lineup diagram was created. Several of the properties of the interface were measured, including the HOMO, charge injection barriers, and the presence of absence of interface dipoles. 56

PAGE 65

References 1. Huynh, W.U., J.J. Dittmer, and A.P. Alivisatos, Hybrid Nanorod-Polymer Solar Cells 2002. p. 2425-2427. 2. Hoppe, H. and N.S. Sariciftci, Organic solar cells: An overview. Journal of Materials Research, 2004. 19(7): p. 1924-1945. 3. Stutzmann, N., R.H. Friend, and H. Sirringhaus, Self-Aligned, Vertical-Channel, Polymer Field-Effect Transistors. 2003. p. 1881-1884. 4. Tanase, C., et al., Unification of the Hole Transpor t in Polymeric Field-Effect Transistors and Light-Emitting Diodes. Physical Review Letters, 2003. 91(21): p. 216601. 5. Waldrop, J.R. and R.W. Grant, Semiconductor Heterojunction Interfaces: Nontransitivity of Energy-band Discontiuities. Physical Review Letters, 1979. 43(22): p. 1686. 6. Salaneck, W.R., et al., The electronic structure of polymer-metal interfaces studied by ultraviolet ph otoelectron spectroscopy. Materials Science & Engineering R-Reports, 2001. 34(3): p. 121-146. 7. Atreya, M., et al., In situ x-ray photoelectron spectr oscopy studies of interactions of evaporated metals with Poly(p-pheny lene vinylene) and its ring-substituted derivatives 1999, AVS. p. 853-861. 8. Chandekar, A. and J.E. Whitten, Ultraviolet photoemission and electron loss spectroscopy of oligothiophene films. Synthetic Metals, 2005. 150(3): p. 259-264. 9. Fujimoto, H., et al., Ultraviolet photoemission study of oligothiophenes: pi-band evolution and geometries 1990, AIP. p. 4077-4092. 10. Cascio, A.J., et al., Investigation of a polythiophene interface using photoemission spectroscopy in combination with electrospray thin-film deposition 2006, AIP. p. 062104. 11. Lyon, J.E., et al., Photoemission study of the poly (3-hexylthiophene)/Au interface 2006, AIP. p. 222109. 57

PAGE 66

12. Yi, Y., et al., Characterization of indium tin oxid e surfaces and interfaces using low intensity x-ray photoemission spectroscopy 2006, AIP. p. 093719. 13. Kasap, S.O., Principles of Electronic Materials and Devices. 2002. 14. Ishii, H., et al., Energy level alignment and interfac ial electronic structures at organic/metal and organic/organic interfaces (vol 11, pg 605, 1999). Advanced Materials, 1999. 11(12): p. 972-972. 15. Gassenbauer, Y. and A. Klein, Electronic surface prope rties of rf-magnetron sputtered In2O3 : Sn. Solid State Ionics, 2004. 173(1-4): p. 141-145. 16. Roncali, J., Conjugated Poly(Thiophenes) Sy nthesis, Functionalization, and Applications. Chemical Reviews, 1992. 92(4): p. 711-738. 17. Bokman, F., Analytical Aspects of Atmosphe ric Pressure Ionization in Mass Spectrometry 2002. 18. Gilbert, W., De Magnete 1600, Londini. 19. Bose, G.M., Recherches Sur La Cause Et Sur La Veritable Theorie De L'Electricite 1745, Wittembergae. 20. Nollet, J.A., Recherches Sur Les Causes Particulieres Des Phenomenes Electrique. 1753, Paris. 21. Taylor, G.I., Proceedings of the Royal Society in London A A280. 22. Whitehouse, C.M., et al., Electrospray Interface for Liquid Chromatographs and Mass Spectrometers. Analytical Chemistry, 1985. 57(3): p. 675-679. 23. Yamashita, M. and J.B. Fenn, Electrospray Ion-Source Another Variation on the Free-Jet Theme. Journal of Physical Ch emistry, 1984. 88(20): p. 4451-4459. 24. Alexandrov, M.L., et al., Ion Extraction from Solutions at Atmospheric-Pressure a Method of Mass-Spectrometric An alysis of Bioorganic Substances. Doklady Akademii Nauk Sssr, 1984. 277(2): p. 379-383. 25. Bruins, A.P., T.R. Covey, and J.D. Henion, Ion Spray Interface for Combined Liquid Chromatography/Atmospheric Pre ssure Ionization Mass-Spectrometry. Analytical Chemistry, 1987. 59(22): p. 2642-2646. 26. Wong, S.F., C.K. Meng, and J.B. Fenn, Multiple Charging in Electrospray Ionization of Poly(Ethylene Glycols). Journal of Physical Chemistry, 1988. 92(2): p. 546-550. 58

PAGE 67

27. Wilm, M. and M. Mann, Analytical properties of th e nanoelectrospray ion source. Analytical Chemistry, 1996. 68(1): p. 1-8. 28. Smith, D.P.H., The Electrohydrodynamic Atomization of Liquids. Ieee Transactions on Industry Applications, 1986. 22(3): p. 527-535. 29. Dole, M., L.L. Mack, and R.L. Hines, Molecular Beams of Macroions. Journal of Chemical Physics, 1968. 49(5): p. 2240-&. 30. Thomson, B.A. and J.V. Iribarne, Field-Induced Ion Evaporation from Liquid Surfaces at Atmospheric-Pressure. Journal of Chemical Physics, 1979. 71(11): p. 4451-4463. 31. Einstein, A., Generation and conversion of light with regard to a heuristic point of view. Annalen Der Physik, 1905. 17(6): p. 132-148. 32. Hufner, S. 2003. 33. Gargagliano, Characterization of L-cysteine Thin Films Via Photoemission Spectroscopy 2005: Tampa. 34. Beerbom, M.M., et al., Direct comparison of photo emission spectroscopy and in situ Kelvin probe work function meas urements on indium tin oxide films. Journal of Electron Spectroscopy and Rela ted Phenomena, 2006. 152(1-2): p. 12-17. 35. Kojima, I. and M. Kurahashi, Application of Asymmetr ical Gaussian Lorentzian Mixed-Function for X-Ray Photoelectron Curve Synthesis. Journal of Electron Spectroscopy and Related Phenomena, 1987. 42(2): p. 177-181. 36. Schlaf, R., H. Murata, and Z.H. Kafafi, Work function measurements on indium tin oxide films. Journal of Electron Spectrosc opy and Related Phenomena, 2001. 120(1-3): p. 149-154. 37. Dannetun, P., et al., The Chemical and Electronic-Structure of the Interface between Aluminum and Polythiophene Semiconductors. Journal of Chemical Physics, 1993. 99(1): p. 664-672. 38. Salaneck, W.R., et al., Thermochromism in Poly(3-Hexylthiophene) in the SolidState a Spectroscopic Study of Temper ature-Dependent Conf ormational Defects. Journal of Chemical Physics, 1988. 89(8): p. 4613-4619. 39. Knupfer, M. and G. Paasch, Origin of the interface dipole at interfaces between undoped organic semiconductors and metals. Journal of Vacuum Science & Technology A, 2005. 23(4): p. 1072-1077. 59

PAGE 68

40. Nambu, A., et al., Film growth and X-ray induced chemical reactions of thiophene adsorbed on Au(111). Surface Science, 2003. 530(1-2): p. 101-110. 41. Burgi, L., et al., Close look at charge carrier in jection in polymer field-effect transistors. Journal of Applied P hysics, 2003. 94(9): p. 6129-6137. 42. Wan, A., et al., Impact of electrode contamina tion on the alpha-NPD/Au hole injection barrier. Organic Electronics, 2005. 6(1): p. 47-54. 43. Chen, T.A., X.M. Wu, and R.D. Rieke, Regiocontrolled Synthesis of Poly(3Alkylthiophenes) Mediated by Rieke Zinc Their Characterization and Solid-State Properties. Journal of the American Chemical Society, 1995. 117(1): p. 233-244. 44. Fujimoto, H., et al., Ultraviolet Photoemission-Study of Oligothiophenes PiBand Evolution and Geometries. Journal of Chemical Physics, 1990. 92(7): p. 4077-4092. 45. Schwieger, T., et al., Electronic properties of interfaces between different sexithiophenes and gold. Journal of Applied Physics, 2005. 97(12). 46. Lagel, B., et al., Investigation of the poly[2-met hoxy-5-(2 '-ethyl-hexyloxy)-1,4phenylene vinylene]/indium tin oxide in terface using photoemission spectroscopy. Journal of Applied Physics, 2005. 98(2). 47. Lyon, J.E., et al., Photoemission study of the poly (3-hexylthiophene)/Au interface. Applied Physics Letters, 2006. 88(22). 60