Nanometal conductive layers and organic heterostructures for polymer semiconductor devices

NANOMETAL CONDUCTIVE LAYERS AND ORGANIC HETEROSTRUCTURES FOR POLYMER SEMICONDUCTOR DEVICES SANKARAN SIVARAMAKRISHNAN A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2009 To my parents and satish Acknowledgements I am grateful to Dr Peter HO for including me as a member of the Organic Nano Device Laboratory (ONDL) at which the work described in this thesis is performed. I thank Peter for his constant guidance and for his ideas during this period. I am happy that I was part of the first few members of ONDL and helped build this lab. It was a privilege working at ONDL. I would also like to thank Lay-Lay CHUA, Loke Yuen WONG, Rui Qi PNG, Bibin Thomas ANTO, Roland GOH, Jingmei ZHUO and Lihong ZHAO and all the members of the ONDL for their support. I would like to thank Perq Jon CHIA and Mi ZHOU for their contributions to this work. I would like to thank Huijuan CHE and Jiecong TANG for the constant supply of anhydrous solvents. I am grateful to Choon-Wah TAN and his team at the Physics Workshop for their prompt help and suggestions, and in general the Department of Physics for hosting and support of this work. I would like to thank my friends for the constant encouragement and in particular Leo and Ravi whose constant enquiries into the state of my thesis hastened its completion. Finally I would like to thank the National University of Singapore for the scholarship provided by them. Abstract The further development of organic polymer electronics crucially depends on continued advances in printable materials systems possessing high quality insulator, metal or semiconductor properties and in high quality multilayered semiconductor heterostructures. Previous efforts to address these have focused mostly on the development of new polymer insulator, metal or semiconductor systems. Here we demonstrate new approaches based on control of the final morphology of the materials. In the first part of this work, we modified the Brust–Schiffrin process by mixed ωfunctionalised carboxy- and hydroxyl-alkylthiol monolayers to protect gold nanoparticles and found these become extremely water dispersible, and their thin films can be annealed to the high conductivity state at temperatures below 250ºC. This makes it possible to deposit these materials directly on organic underlayers without damaging these layers. Furthermore the critical challenges of coalescence-induced cracking and poor adhesion of gold nanoparticle films can be overcome by the formation of nanocomposites with compatibilising polymer matrices. These composite materials are potentially useful as source, drain and gate electrodes in field effect transistors, and as current carrying interconnects in light emitting diodes and photovoltaic cells. In the second part of this work we demonstrate that sequentially solution-deposited semiconducting polymer films can be doped to give p–i–n structures by contact with dopant solutions or dry dopant films. While small molecules have been routinely doped by co-evaporation with dopant molecules, this has been a particular challenge with solution- processed polymer semiconductors due to the re-dissolution of the underlayers when the next layer is deposited. Here we overcome this using a newly developed photocrosslinking methodology and demonstrate efficient p-i-n light emitting diodes based on a pdoped, intrinsic and n-doped layers of poly(9,9-dioctylfluorene-alt-benzothiadiazole). In chapter 1, we give a background to the work described in this thesis. In chapter 2, we describe the synthesis of highly water and alcohol soluble gold nanoparticles in the particle size range 1-5 nm and the incorporation of these nanoparticles into a conducting matrix like poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDT) or an insulating polymer matrix like poly(4-hydroxystyrene) (PHOST) to form printable nanocomposites. These nanocomposites undergo an insulator to metal transformation at 195–250ºC. The final conductivity is controlled by the mean Au particle size, volume fraction and anneal temperature and can be tuned between 10–4 and a few 105 S cm–1. The polymer matrix wets the Au and forms an ultra thin film at the surface after annealing which is advantageous as PEDT is a better hole injector than Au. The transformation can also be induced electrically thus allowing for memory applications. In chapter 3, we show that the spectral shape of the plasmon excitation at 500–600 nm in thin films of gold colloids and their nanocomposites can be quantitatively modelled in a surprisingly simple way by treating the nanoparticle quantum-size effect, the core-shell nanostructure effect and the thin film optical effect, in an overall transfer matrix formalism. The results show there is an initial nanocore relaxation followed by a progressive desorption of the ligand shell leading to formation of percolated paths. Partial percolation is already sufficient to attain the desired conductivity. The optical transformation due to plasmon coupling between the Au cores precedes the electrical transformation which requires the development of macroscopic percolation. In chapter 4, we describe solution-based p- and n- doping of conjugated polymers and devices. The doping has been followed by ultraviolet–visible and Fourier transform infrared spectroscopies. By a combination of solution contact p doping and solid state contact n doping, p–i–n LEDs based on poly(9,9-dioctylfluorene-alt-benzothiadiazole) with a p-doped layer (over ITO) as hole-transport and injection layer, and n-doped layer as electron-transport and injection layer (capped by Al) were fabricated. The devices exhibit rectifying current–voltage characteristics, high built-in potential (2.2 V) and good electroluminescence efficiency (1.2% ph/el) indicating balanced carrier injection. This shows that stable electrodes can be used with doped polymer semiconductor layers to achieve efficient injection into the intrinsic semiconductor. CONTENTS Chapter 1.1 Organic electronics 10 1.2 Electrically conductive materials system 1.2.1 Requirements and available materials systems 11 1.2.2 Nanoparticle inks as conductors 15 1.3 Metal organic semiconductor junctions 17 1.4 Organic semiconductors 19 1.5 Fundamentals of doping in organic semiconductors 21 1.6 References 27 Chapter 2.1 Introduction 34 2.2 Synthesis of Au polymer nanocomposites 2.2.1 Synthesis, purification and ion-exchange of nano-Au dispersions having high solubility in water and alcohols 2.2.2 Au nanoparticles dispersed in polymer matrix 37 41 2.3 Insulator-metal transformation in nanoparticles and nanocomposites 2.3.1 Experimental details 2.3.1.1 Conductivity measurements as a function of heat treatment 43 2.3.1.2 Optical micrographs and AFM 44 2.3.1.3 X-ray photoelectron spectroscopy 44 2.3.2 Conductivity measurements : Results and discussions 46 2.4 Film morphology and treatment : the role of the polymer matrix 2.3.3.1 Morphology change with annealing 51 2.3.3.2 XPS characterization : Wetting of Au nanoparticles 53 by the polymer matrix 2.5 Applications 2.5.1 “All-printed” transistor 56 2.5.2 Electrically induced insulator-metal transformation – memory devices 60 2.6 Summary 63 Appendix 2.1 Estimation of intercluster spacing 64 2.7 References 67 Chapter 3.1 Introduction 75 3.2 Experimental details 3.2.1 Ultraviolet-Visible absorption spectroscopy 76 3.2.2 FTIR spectroscopy 76 3.3 Results and discussions 3.3.1 Ultraviolet-Visible absorption spectra: Experimental and calculated spectra 3.3.1.1 Spectral features 77 3.3.1.2 Optical model : Features and Comparison 78 3.3.1.3 Optical transformation and electrical transformation 88 3.4.1 FTIR spectra 3.5 Summary 93 96 3.6 References 97 Chapter 4.1 Introduction 4.1.1 Doping of polymers 102 103 4.2 Experimental details 4.2.1 Ultraviolet-visible absorption and FTIR spectra – p and n doping 109 4.2.2 Fabrication of diodes and IVL measurements 110 4.2.3 Modulated photocurrent measurements 113 4.3 Results and Discussions 4.3.1 Ultraviolet –visible and Fourier transform infrared spectroscopy 114 4.3.2 The built in potential: Modulated photocurrent measurements 123 4.3.3 Diode IVL characteristics 126 4.4 Summary 131 4.5 References 132 Outlook 139 Appendix A Publications arising from the work described in this thesis 141 Appendix B Publications arising from work carried out during the period but not described in this thesis Appendix C Conference presentations 142 143 Chapter Introduction 1.1 Organic electronics Organic electronics has its origin in the discovery of doping of polyacetylene[1]. However this discovery didn’t really spur on the immense growth we see now. Organic electronics can be said to have truly arrived with the discovery and understanding of electroluminescence in conjugated polymers [2, 3]. This discovery was an important step in the growth of organic electronics and led to the fabrication of other devices and increased understanding of the device physics[4, 5]. However organic electronics is still at a nascent stage. Our understanding of organic electronic devices[6] has increased by leaps and bounds but it is only the proverbial tip of the iceberg. Lot of work is required to be carried out on the various aspects from materials development to novel device architectures to new fabrication techniques. The demand for organic electronics lies in the easy processibility leading to reduced costs and the possibility of niche applications which are beyond the purview of inorganic electronics like flexible display devices[7], wearable electronics etc. Especially solution processible materials are becoming more prevalent because of the advantages of ease of device fabrication, large area applications, compatibility with light weight and mechanically flexible base materials, and control of electrical, optical and magnetic properties. Various printing techniques are being explored for printing of organic electronic circuits[8, 9]. Screen printing and inkjet printing are two practical printing techniques that have shown great promise.[10, 11] 10 Fig 4.12 shows the energy level diagram of the undoped and doped device at flat band conditions. It is generally agreed that the electrode work function should be matched with the ionization potential or electron affinity for efficient hole or electron injection. For ohmic contacts, this is not always valid. On doping, sub gap states are created which result in the decrease of the injection barrier and subsequent increase in the current density. The electrode Fermi level is pinned inside the gap and creates a large apparent gap to the transport level. The gap states furnished by the doping allows for charge injection into them resulting in an ohmic contact [54]. The effect of doping here is to create doped interfaces and differs from isolated bulk chemical doping due to madelung stabilization [55]. This is reflected in the increase in the built in potential, for the p-i device to 1.5V and for the p–i–n doped device to 2.2 V. The leakage current is also suppressed in the p–i–n device as compared to the intrinsic device by more than two orders. This is probably due to the planarization of the polymer film due to doping thereby eliminating the local shorts that cause the high leakage current in the intrinsic device. 128 Flat band Flat band EV EV LUMO e- LUMO Ef h+ e- HOMO ITO (anode) i-F8BT EV Δe Vbi Δh h+ EV Al (cathode) h+ e- Δe Vbi Δh HOMO ITO p-F8BT i-F8BT n-F8BT Al (cathode) (anode) Fig 4.12 Energy band diagrams of the intrinsic and doped device at flat band conditions showing the increase in built in potential and decrease in barrier heights. On doping, sub-gap density of states are built up which provide electronic states at the Fermi level into which charge injection occurs. This is reflected in the increase in the built in potential. 129 10-1 Temperature: 350 K Current (A) 10-3 300 K 240 K 10-5 180 K 150 K 10-7 100 K 10-9 10-11 30 K -6 -4 -2 Voltage (V) Fig 4.13 I-V data for the p–i–n LED at different temperatures after cooling down to 30 K and then heating up to 350 K. The diode characteristics were reproducible during the heating and cooling cycles indicative of its stability The p–i–n diode was subjected to low temperature measurements up to 30K and then heated to 350K. Fig 4.13 shows the I-V characteristics of the p–i–n device when cooled down to 30K in a Janis cryostat under vacuum and then subsequently heated to 350 K. The diode characteristics were reproducible during the heating and cooling cycles indicative of its stability. 130 4.4 Summary A p–i–n polymer light-emitting diode based on doped layers of F8BT has been demonstrated. Hole injection can occur efficiently from the ITO/ p-doped F8BT into the intrinsic layer, while electron injection can also occur efficiently from the Al/ n-doped F8BT contact. The p-doping was implemented by solution contact-doping with a NO2+ SbF6– solution, while the n-doping was by solid-state contact-doping with a thin film of Na+ Np– on PDMS stamp. 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Lett., 2009 102, 096602 138 Outlook In this work we have shown the synthesis of highly water soluble nanocomposites which can be used as conductors in organic electronic circuits. The transformation temperature in these nanocomposites has been further reduced to temperatures compatible with plastic substrates by using suitable combinations of ligands. They are already increasingly used in simple circuits like RFIDs and in some cases in transistors. The challenge now is to produce them in large quantities retaining the high solubilities to be commercially viable. The nanocomposites can be then printed on to flexible plastic substrates. There are still many issues concerning the integration of the various layers. Printing of the semiconductor and gate dielectric haven’t been demonstrated satisfactorily. Novel surface treatment techniques are required to print the semiconducting polymer which is usually soluble in organic solvents. Gate dielectrics like BCB are too viscous to be printed using inkjet printing. A combination of various printing techniques like screen printing and inkjet printing might be needed to be used in the future. All printed electronic circuits are a possibility but several challenges have to be surmounted. We have also seen that the conductivities of conjugated polymers can be raised by doping. Due to low mobilities, the conductivity of these materials after doping is still much lower than their inorganic counterparts but are sufficient enough for a variety of devices that don’t require too high current densities like OLEDs. Controlled doping of polymers is still a mighty challenge. In this work we have shown the successful p and importantly n doping of some polymers and also demonstrated a vertical p–i–n polymer heterojunctions. However, the level of doping and precise control over the doping still remains a challenge. The immediate challenge to confront is to acquire control over the doping process and stabilise the doped polymers. The stability of the doped 139 devices is dependent on the ability of the dopant counterion to not get involved in migration or diffusion reactions. While p-type doping has been available for some time, more progress needs to take place for n-type doping. This is not trivial as it requires transfer of electrons to rather high-lying orbitals. From a materials perspective, the progress on the synthesis of new conjugated polymers is quite rapid. This should lead to an increase in the research activities on new stable dopants and facile doping methods involving solution based techniques. 140 Appendix A. Publications arising from the work described in this thesis 1. S. Sivaramakrishnan, P.-J. Chia, Y.-C. Yeo, L.-L. Chua and P.K-H. Ho, "Controlled insulator-to-metal transformation in printable polymer composites with nanometal clusters", Nature Mater., (2007) 149 2. S. Sivaramakrishnan, Bibin T. Anto, P. K-H. Ho “Optical modeling of the plasmon band of monolayer-protected nanometal clusters in pure and in polymer matrix thin films as a function of heat treatment”, Appl. Phys. Lett. 94 (2009) 091909 3. S. Sivaramakrishnan, Zhou Mi, Aravind kumar, Chen Zhili, Png Ri Qi, Chua Lay-lay, P. K-H. Ho “Solution-processed conjugated polymer organic p-i-n light-emitting diodes with high builtin potential by solution- and solid-state doping”, Appl. Phys. Lett. 95 (2009) 213303 141 B. Publications arising from work carried out during the period but not described in this thesis 1. P.J. Chia, L.L. Chua, S. Sivaramakrishnan, J.M. Zhuo, L.H. Zhao, W.S. Sim , Y.C. Yeo, P.K.H. Ho, " Injection-induced de-doping in a conducting polymer during device operation: asymmetry in the hole injection and extraction rates", Advanced Materials 19 (2007) 4202 2. R.Q. Png, P.J. Chia, S. Sivaramakrishnan, L.Y. Wong, M. Zhou, L.L. Chua and P.K.H. Ho, "Electromigration of the conducting polymer in organic semiconductor devices and its stabilization by cross-linking", Applied Physics Letters 91 (2007) 013511 3. S.-H. Khong, S. Sivaramakrishnan, R.-Q. Png, L.-Y. Wong, P.-J. Chia, L.-L. Chua and P.K.H. Ho, "General photo-patterning of polyelectrolyte thin films via efficient ionic bis(Fluorinated phenyl azide) photo-crosslinkers and their post-deposition modification", Advanced Functional Materials 17 (2007) 2490 4. H.-J. Che, P.-J. Chia, L.-L. Chua, S. Sivaramakrishnan, J.-C. Tang, A.T.S. Wee, H.S.O. Chan, P.K.H. Ho, "Robust reproducible large-area molecular rectifier junctions", Applied Physics Letters 92 (2008) 253503 5. S. Wang, P.-J. Chia, L.-L. Chua, L.-H. Zhao, R.-Q. Png, S. Sivaramakrishnan, M. Zhou, R.G.-S. Goh, R.H. Friend, A.T.-S. Wee, P.K.-H. Ho, “Band-like transport in surfacefunctionalized highly solution processible grapheme nanosheets”, Adv.Mater 20 (2008) 3440 6. P.J. Chia, S. Sivaramakrishnan, M. Zhou, R.Q. Png, Lay-Lay Chua, R.H. Friend, P.K.H. Ho, “Direct Evidence for the Role of the Madelung Potential in Determining the Work Function of Doped Organic Semiconductors”, Phys.Rev.Lett. 102 (2009) 096602 142 C. Conference presentations (presenting author underlined) 1. S. Sivaramakrishnan, L-L. Chua and P.K.H. Ho “Gold Nanoparticle–Conducting Polymer Nanocomposites as Printable Conductors in Flexible Electronics”, ICMAT 2005 (oral presentation) 2. S. Sivaramakrishnan, P-J. Chia, R-Q. Png, L.Y. Wong, L-L. Chua and P.K.H. Ho, “Controlled insulator-metal transformation in printable Au polymer nanocomposites”, MRS 2007, San Francisco (oral presentation) 3. P-J. Chia, R-Q. Png, S. Sivaramakrishnan, Yeo, L-L. Chua, P.K.H. Ho, “Injection-induced dedoping of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) in solid state devices”, ICMAT 2007, Singapore. (oral presentation) 4. M. Zhou, L-L. Chua, S. Sivaramakrishnan, P.K.H. Ho, “ Built-in potential measurement in organic light-emitting diodes”, ICMAT 2007, Singapore. (oral presentation) 143 [...]... the metal and semiconductor The understanding of the interfacial electronic structure[41] forms the basis for understanding and improving the performance of organic electronic devices A proper choice of materials can provide a low resistance ohmic contact However for a lot of organic semiconductors there is no appropriate metal available Whenever a metal and a semiconductor are in intimate contact,... introducing SAM layers on to the metal electrodes[42] Another way is by doping the polymer to increase the charge carrier density thereby leading to improved device performance Before taking a closer look into doping of organic semiconductors, let us take a brief look at organic semiconductors 1.4 Organic Semiconductors Organic semiconductors can be divided broadly into two main groups: i) conjugated polymers,... poortmans, and R Mertens, injection- and space charge limited-currents in doped conducting organic materials J Appl Phys., 2001 89: p 3804-3810 51 Pron, A and P Rannou, Processible conjugated polymers: from organic semiconductors to organic metals and superconductors Prog Polym Sci., 2002 1: p 135-190 52 de Leeuw, D.M., M.M.J Simenon, A.R Brown, and R.E.F Einerhand, Stability of n-type doped conducting polymers... conducting polymers and consequences for polymer microelectronic devices Synth Met., 1997 87: p 53-59 53 Walzer, K., B Maenning, M Pfeiffer, and K Leo, Highly efficient organic devices based on electrically doped transport layers Chem Rev., 2007 107: p 1233-1271 32 Chapter 2 Printable gold polymer nanocomposites as conductive materials system and the controlled insulator to metal transformation in these... 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Burns, and A.B Holmes, Light-emitting diodes based on conjugated polymers Nature, 1997 347: p 539-541 4 Sirringhaus, H., Device physics of solution-processed organic field-effect transistors Adv Mater., 2005 17: p 2411 5 Burroughes, J.H., C.A Jones, and R.H Friend, New semiconductor devices physics in polymer diodes and transistors Nature, 1988 335: p 137 6 Koch, N., Organic electronic devices and their... contacts to wire up and deliver the appropriate power and signals to the various circuit components, but also at the 11 component level as electrode contacts to the devices (for example, as cathode and anode of light emitting diodes and of photodiodes, and as the source, drain and gate electrodes of field-effect transistors, and of tunnel-dielectric-based electrically programmable memory devices) In some... Printable organic and polymeric semiconducting materials and devices J Mater Chem., 1999 9: p 1895-1904 11 Sirringhaus, H., T Kawase, R.H Friend, T Shimoda, M Inbasekaran, W Wu, and E.P Woo, High-resolution inkjet printing of all -polymer transistor circuits Science, 2000 290: p 2123-2126 12 Huang, J., M Pfeiffer, A Werner, J Blochwitz, K Leob, and S Liu, Low-voltage organic electroluminescent devices. .. Thiem, and U Scherf, Organic semiconductors for solution-processable field-effect transistors (OFETs) Angew Chem Int Ed., 2008 47: p 4070-4098 16 Ling, Q.-D., D.-J Liaw, C.-X Zhu, D.S.-H Chan, E.-T Kang, and K.-G Neoh, Polymer electronic memories: Materials, devices and mechanisms Prog Polym Sci., 2008 33: p 917-978 17 http://www.periodictable.com/ 18 Granqvist, C.G and A Hultaker, Transparent and conducting . NANOMETAL CONDUCTIVE LAYERS AND ORGANIC HETEROSTRUCTURES FOR POLYMER SEMICONDUCTOR DEVICES SANKARAN SIVARAMAKRISHNAN A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR. of the metal and semiconductor. The understanding of the interfacial electronic structure[41] forms the basis for understanding and improving the performance of organic electronic devices. A. taking a closer look into doping of organic semiconductors, let us take a brief look at organic semiconductors. 1.4 Organic Semiconductors Organic semiconductors can be divided broadly