«GRAPHENE MODIFIED INDIUM TIN OXIDE ELECTRODES FOR ORGANIC SOLAR CELLS CHANG CI’EN SHARON (B. Sc.(Hons.), NATIONAL UNIVERSITY OF SINGAPORE) A THESIS ...»
GRAPHENE MODIFIED INDIUM TIN OXIDE
ELECTRODES FOR ORGANIC SOLAR CELLS
CHANG CI’EN SHARON
(B. Sc.(Hons.), NATIONAL UNIVERSITY OF SINGAPORE)
A THESIS SUBMITTED
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
NUS GRADUATE SCHOOL FOR INTEGRATIVE
SCIENCES AND ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE&
DEPARTMENT OF MATERIALS
IMPERIAL COLLEGE LONDONDeclaration I hereby declare that this thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis.
This thesis has also not been submitted for any degree in any university previously.
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Chang Ci’En Sharon 28 July 2014 i Dedicated to my loving family and my partner ii Acknowledgement Praise God from whom all blessings flow These four years of PhD could not have been made possible without the help of many people who rendered assistance at work, gave a listening year, or blessed me with their friendship and prayers. To begin, I would like to express my gratitude towards my supervisors, Prof Andrew Wee and Sandrine Heutz, for their invaluable support, advice and guidance that goes beyond the sciences; for knowing when to push me to achieve more, and when to encourage a break to refresh and take stock. Chen Wei and David McPhail, as part of my thesis advisory committee, also provided timely feedback and useful discussions.
I would like to thank my colleagues past and present in the Surface Science Lab NUS, in Heutz’s group in the ICL, and in the LCN office. Their friendship and aid, shared joys and grieves, encouragement and small talks, helped me through the bleak parts of my PhD (and the bleak London winter). I would like to specially thank Hendrik Glowatzki, Cao Liang, Wei Da Cheng, Wang Rui, Luke Fleet and James Gilchrist who invested so much time and energy in imparting experimental techniques and safety considerations; patiently discussed and analysed experimental data; and even advising on the finer details such as the presentation of data. Their integrity and rigor towards proper scientific methodology and data handling have left a lasting impression on me. I am grateful to the assistance of Kendra Kam and Dr. Chua Lay Lay for the provision of some of the samples for synchrotron measurements, to James for his partnership with all the solar cell device work, and to Sarah Fearn for her expertise with all the TOF5SIMS measurements.
These people beyond the scope of work have also played an integral part in this process: Fish, Nadia, Clarence, Jiahui, Cedric, Boredin, Valerie, Peggy, Ivy, Aunt Sau Har and the many others
throughout all these years who encouraged and prayed for me; who selflessly sacrificed their time, energy, sleep and resources to walk this journey with me. Thank you for being my pillar of support throughout all the years of my life, for being my best friends, and for making this dream of graduate studies a reality. Finally, to my dearest Taffy, who embarked on life’s journey together with me over the last five and a half years, thank you for loving, blessing, and waiting for me.
Gilchrist, J. B., Basey5Fisher, T. H., Chang, S. C’ E.*, Scheltens, F., McComb, D. W. & Heutz, S.
Uncovering the Buried Interface in Molecular Photovoltaics. Adv. Funct. Mater. 24, 647356483 (2014).
Chang, S. C’ E.*, Fearn, S., McPhail, D., Wee, A. T. S. & Heutz, S. TOF5SIMS Investigation of F45TCNQ Diffusion Through CuPc Molecules. In preparation (2014).
Chang, S. C’ E.*, Liang, C., Gilchrist, J. B., Wei, C., Heutz, S. & Wee, A. T. S. Molecular Modification of Graphene to Control the Structural and Electronic Properties of CuPc in Organic Solar Cells. In preparation (2014).
Chang, S. C’ E.*, Liang, C., Wei, C., Heutz, S. & Wee, A. T. S. Thin Film Properties of F45 TCNQ as an Interface Dopant on ITO and Graphene Modified ITO. In preparation (2014).
List of Tables
List of Figures
List of Abbreviations
Chapter 1 : Introduction
1.1 Organic Photovoltaics Devices
1.1.1 Basic Properties of OPV Devices
1.1.2 Structural Templating in OPV Devices
1.1.3 Energy Level Alignment in OPV Devices
1.2 Structural Properties of CuPc
1.3 Thesis Overview
Chapter 2 : Experimental Methodology
2.1 The OMBD Growth System
2.2 Characterization Techniques
2.2.1 Working Principle of PES Measurements
2.2.2 NEXAFS Measurements
2.2.3 Time5of5Flight Secondary Ion Mass Spectrometry Working Principles
2.2.4 X5ray Diffraction
2.2.5 Atomic Force Microscopy
2.2.7 Ultraviolet5Visible Spectroscopy
2.2.8 Current5Voltage Characterization
2.3 Sample Preparation
2.3.1 Sample Cleaning
2.3.2 Transfer of Graphene to ITO
188.8.131.52 Characterization of Graphene Films
2.3.3 Thin Film Deposition
Chapter 3 : Controlling the Molecular Orientation of CuPc Using Graphene Interlayer on ITO.. 52
3.2 Energetic Properties of CuPc on ITO and G/ITO
3.3 Molecular Orientation of CuPc on ITO and G/ITO
3.4 OPV Device Characterization using ITO and G/ITO as Anode Layer
3.5 Conclusion and Future Work
Chapter 4 : F45TCNQ Thin Film Properties
4.2 Calibration of F45TCNQ Film Thickness
4.3 Electronic Structure of F45TCNQ on ITO and G/ITO
4.4 Structural Analysis of F45TCNQ on ITO and G/ITO
Chapter 5 : Modification of ITO and G/ITO Anodes with F45TCNQ
5.2 Structural Properties of CuPc
5.2.1 CuPc Deposited on F45TCNQ Pre5covered G/Cu and Cu
5.2.2 CuPc Deposited on F45TCNQ Pre5covered Si & G/Si, and ITO & G/ITO................ 108
5.3 Optical Absorption of CuPc on F45TCNQ Pre5Covered ITO and G/ITO
5.4 Interfacial Energetics of CuPc on F45TCNQ Pre5Covered ITO and G/ITO
5.6 Device Characterization of OPV
5.7 Conclusion and Outlook
Chapter 6 : Diffusion of F45TCNQ Molecules
6.2 Diffusion of Interface F45TCNQ into Bulk CuPc Film Deposited on ITO, G/ITO and G/Cu
6.2.1 Influence of CuPc Molecular Packing on F45TCNQ Diffusion Dynamics................ 143 6.2.2 Effect of Interfacial Interaction on F45TCNQ Diffusion
6.2.3 Diffusion of F45TCNQ through CuPc Deposited on ITO versus G/ITO
6.3 Co5deposition of F45TCNQ and CuPc as a Method to Estimate Dopant Diffusion.......... 152 6.3.1 Preparation of Co5deposited Films
6.3.2 F5 Profiles for Co5Deposited Samples
6.4 Conclusion and Outlook
Chapter 7 : Thesis Summary
7.1 Thesis Summary
7.2 Future Work
Appendix A – Characterization of G/Si
Appendix B – Solar Cell Data
Appendix D – Depth Resolution for TOF5SIMS
Appendix E – TOF5SIMS Depth Profile of ITO
Appendix F – TOF5SIMS Depth Profile of 6.5mol% F45TCNQ Co5deposited with CuPc......... 171
In this thesis, we explore the use of graphene incorporated onto indium tin oxide (G/ITO) as a structural template to modify the orientation of copper phthalocyanine (CuPc) molecules for organic photovoltaic (OPV) device applications. We also investigate the effectiveness of 2,3,5,65 tetrafluoro57,7,8,85tetracyanoquinodimethane (F45TCNQ) as a work function modifier for G/ITO without compromising the templating properties of graphene. Photoemission spectroscopy (PES) is employed to assess the electronic properties at the anode5CuPc interface, while X5ray diffraction (XRD) and near5edge X5ray absorption fine structure (NEXAFS) are used to determine the molecular orientation of CuPc. OPV devices are fabricated to attempt to correlate the observations at the microscopic level with the macroscopic device performance.
First, we investigate the electronic properties of CuPc deposited on G/ITO and ITO using PES.
While the interaction between CuPc molecules and ITO and G/ITO is similar, the hole injection barrier (HIB) is ~0.9 eV for CuPc/G/ITO as compared to 0.5 eV for CuPc/ITO. Therefore, further modification of G/ITO to reduce the HIB is required. The XRD spectrum of CuPc molecules deposited onto graphene grown on copper foil (G/Cu) verifies that graphene is an effective structural template, causing CuPc molecules to ‘lie’ on the substrate. NEXAFS data shows that the orientation of CuPc molecules changes from ‘standing’ on ITO to ‘tilted’ on G/ITO.
Next, the effectiveness of F45TCNQ deposited on ITO and G/ITO as a work function modifier is assessed. A thin layer of F45TCNQ is able to increase the substrate work function to ~5 eV, which is close to the ionization potential of CuPc molecules. This suggests that barrierless extraction of holes from CuPc into F45TCNQ modified ITO or G/ITO may be possible. F45TCNQ molecules are found to be predominantly tilted on G/ITO, suggesting that the templating property of graphene may be propagated through F45TCNQ molecules. CuPc molecules deposited onto F45 TCNQ/G/ITO attain a ‘lying’ configuration, confirming that the templating property of graphene
eV for CuPc/F45TCNQ/G/ITO, and ~0.1 eV for CuPc/F45TCNQ/ITO. Optical absorption of templated CuPc molecules over the visible range is enhanced by over 40% as compared to the non5templated molecules. Therefore, the structure of F45TCNQ/G/ITO appears to be a potential anode design to improve OPV device performance. Our test cells however do not show an improvement in OPV parameters due to the poor quality of transferred graphene, and the high series resistance in our unoptimized OPV device.
Finally, the diffusion of F45TCNQ through a CuPc film is studied using time5of5flight secondary ion mass spectrometry (TOF5SIMS). The F5 depth profiles establish that a higher quantity of F45 TCNQ molecules diffuse into CuPc on the G/ITO sample. This is attributed to the weaker interfacial adhesion between F45TCNQ and graphene, and the crystallinity of the templated CuPc film. The quantity of diffused F45TCNQ in the G/ITO sample is only about 0.2 mol%. At this dopant concentration, the conductivity of the film should increase; thus doping of the whole organic film may be favourable for OPV devices.
xi List of Tables Table 2-1 Growth rates and deposition temperatures of the various materials used in this thesis.
The growth rate of Al was 0.2 Å/s for the first 20 nm deposited directly on the organic materials to minimize sample damage by the hot Al molecules, but increased to 0.5 Å/s for the next 80 nm.
Table 4-1 Summary of the peak intensities of the (2 1 1) and (0 2 0) in the F45TCNQ/ITO and F45 TCNQ /G/ITO films, as well as F45TCNQ powder diffraction.
Figure 1-2 Schematic drawings showing the effect of structural templating on planar molecules.
Individual molecules are shown and the direction of the stacking axis is indicated by the dashed arrow. (a) ‘Standing’ orientation of the molecules before templating, and (b) the ‘lying’ orientation with the inclusion of a template layer.
Figure 1-3 Schematic drawing showing the energy levels in an OPV. Eex and Eb refer to the Coulombically bound exciton energy and exciton binding energy respectively. The HOMO and LUMO positions of the donor (subscript D) and acceptor (subscript A) materials, and the HIB are also shown. The dashed arrows in the donor and acceptor bands indicate the directions of the hole and electron diffusion respectively.
Figure 1-4 (a) Chemical structure of a CuPc molecule. The structure consists of carbon atoms (grey), nitrogen atoms (blue) and a central Cu (red). (b) Geometric illustration of a CuPc column, and (c) a brick5stack arrangement of the one5dimensional CuPc column as proposed by Hoshino.86 The grey shaded area in (c) represents a 25dimensional unit cell.
Figure 1-5 Flow chart showing the key systems and experimental investigations in each chapter, and cohesion between the chapters
Figure 2-1 (a) Experimental setup of an OMBD system with a glove box. The positions of the sample holder and organic sources within the OMBD chamber are shown by the dashed boxes marked (b) and (c) respectively. Images of (b) the sample holder and (c) several sources for organic materials. A QCM is highlighted in (c). Figures (b) and (c) are obtained from the Kurt J.
Figure 2-2 (a) Energy level alignment between the sample and analyser when they are in good electrical contact. (b) A typical UPS spectrum of organic molecular thin films.