Advanced heterostructures for polymer organic semiconductor devices

Advanced Heterostructures for Polymer Organic Semiconductor Devices Rui Qi PNG In partial fulfillment of the requirements for the Degree of Doctor of Philosophy Department of Physics National University of Singapore 2011 To my parents To Raynaud Acknowledgements My years in Organic Nano Device Laboratory (ONDL) has been a very exciting and rewarding phase of my life This thesis would not have been possible without the assistance and support from many people First and most importantly, I owe my deepest gratitude to Professor Peter Ho and Dr Lay-Lay Chua for their guidance and continuous support throughout my PhD They have been truly inspiring and extremely encouraging I thank them for all the opportunities given to me I would also like to express my appreciation to Professor Sir Richard Friend for the excellent ideas and fruitful discussions I am also grateful to Dr Jeremy Burroughes and Dr Richard Wilson from Cambridge Display Technology Ltd I feel honoured to have the privilege to work on projects that are of immediate relevance and interest to CDT During the course of my PhD, I have had the pleasure to guide several students I would like to thank my team, especially Liu Bo, Dagmawi and Weiling I would also like to express my gratitude to all the members (and ex-members) of ONDL, especially Perq-Jon for starting me out in this exciting journey and Loke-Yuen, Lihong, Jing-Mei and Song Jie for their assistance and encouragement and all the wonderful times spent working together Lastly, my sincere gratitude goes to everyone else in ONDL and Department of Physics who have helped me in one way or another I II Abstract Modern electronic devices based on inorganic semiconductors such as silicon have reached a very high level of finesse However while functionality has increased tremendously, progress in large area integration has been more limited due to the unfavorable size–cost scaling in this industry The advent of solutionprocessable plastic electronics based on polymer organic semiconductors (OSCs) in the early 1990s provided a needed paradigm shift in both materials and processing This has opened the way to the development of low-cost large-area electronics that are manufacturable using materials- and energyefficient sustainable processes Sufficient advances have occurred in both materials and device performances to thus enable a fledgling industry to take off In the next phase of research activities, it will be important to increase the sophistication of heterostructures for these devices in order to make further progress This has been hampered previously by the dissolution of the underlayers unless orthogonal solvents are used Therefore work in this thesis has focused on heterostructures fabricated with a photocrosslinking step to circumvent the dissolution problem As a consequence, not only the traditional planar type heterostructures are possible, but also the novel nanotextured types due to the soft nature of these OSCs Performance gains across both polymer organic light-emitting diodes (OLEDs) and organic photovoltaic (OPV) solar cells have been measured Furthermore the use of chemical doping to create ultrahigh work function heterostructures and ohmic contacts has also been demonstrated Thus new opportunities for the design and control of plastic electronic devices have been opened In Chapter 2, an azide photocrosslinking methodology that is compatible with polymer OSC thin films is reported A new class of sterically-substituted bis(fluorophenyl azide)s (FPA) has been developed to enable deep-ultraviolet photocrosslinking of polymer films at will without significant degradation of their sensitive (opto)electronic semiconducting properties This crosslinking process has been monitored by gel III characteristic measurements, and Fourier-transform spectroscopy and the quality of the crosslinked films have been checked by photoluminescence yield measurements which are very sensitive to the presence of exciton traps In Chapter 3, several practical heterostructures have been demonstrated for use in polymer OLEDs, solar cells, and organic field-effect transistors A separate-confinement-heterostructure LED based on transport energy mismatch has been found to help impose nearly perfect recombination of electrons and holes in the LED Two new contiguous polymer donor–acceptor heterostructures have also been demonstrated based on: (i) acceptor infiltration into a crosslinked polymer gel network, and (ii) backfilling of a crosslinked poriferous polymer layer These overcome the internal recombination losses in previous “bulk distributed heterostructures” in polymer solar cells, because they impose by design built-in carrier path continuity The further elaboration of these self-organised and hence potentially manufacturable heterostructures will be very interesting In Chapter 4, the existence of air-stable solution-processable p-doped polymer films with work function larger than 5.6 eV has been demonstrated, using high ionisation potential materials, such as the triphenylamine–fluorene and N,N,N’,N’-tetraphenylphenylenediamine–fluorene copolymers These materials can provide ohmic heterostructure contact for hole injection into deep ionisation-potential polymers, such as poly(9,9-dialkylfluorene) (F8), which are important host models for deep-blue emitters New findings suggest that the previous understanding is incomplete For example, the doped contacts can exhibit ohmic injection despite large apparent hole-injection barrier as deduced from low built-in potentials that have been measured This opens up new design considerations for ohmic contacts IV Table of Contents Acknowledgements I Abstract III List of Figures IX List of Tables XV Chapter Introduction 1.1 Organic semiconductors 1.2 Doping of organic semiconductors 1.3 Polymer light-emitting diodes 1.3.1 Structure and operation of a polymer light-emitting diode 1.3.2 Hole-injection layer: Poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid) 1.3.3 Challenges for higher efficiency polymer light-emitting diodes 1.4 Doped transport layers 12 1.5 References 15 Chapter Sterically-hindered bis(fluorophenyl azide)s photocrosslinking methodology 19 2.1 Introduction 20 2.1.1 The FPA photocrosslinking methodology 20 2.1.2 The FPA photocrosslinker development roadmap 23 2.2 Experimental methods 29 2.2.1 Synthesis of bis(fluorophenyl azide) photocrosslinker 29 2.3 Results and discussion 32 2.3.1 Bis(fluorophenyl azide) photocrosslinking methodology 32 2.3.2 Analysis of bis(fluorophenyl azide) photo-products in polymer matrices 38 2.3.3 Photocrosslinking efficiency: Gel curves 44 V 2.3.4 Suppression of interaction with OSC main chain with sterically hindered bis(fluorophenyl azide): FTIR analysis 47 2.3.5 Suppression of interaction with OSC main chain with sterically hindered bis(fluorophenyl azide): Photoluminescence efficiency 49 2.4 Summary 54 2.5 References 55 Chapter High-performance polymer semiconducting heterostructure devices 59 3.1 Introduction 60 3.2 Experimental methods 61 3.2.1 General materials and methods 61 3.2.2 Contiguous donor polymer network OC1C10-PPV: PCBM photovoltaic cells 63 3.2.3 Contiguous interpenetrating (columnar) PFB: F8BT photovoltaic cells 63 3.2.4 Separate confinement heterostructure light-emitting diodes 64 3.3 Results and discussion 64 3.3.1 Multilayer polymer stacks 64 3.3.2 Electron and hole currents in model OC1C10-PPV diodes 66 3.3.3 Contiguous donor polymer network PV cells 68 3.3.4 Contiguous interpenetrating heterostructure PV cells 72 3.3.5 Photo-crosslinked high-mobility polymer FETs and top-gate polymer FETs 76 3.3.6 Photopatterning of polymer LEDs 78 3.3.7 Separate-confinement-heterostructure polymer LEDs 79 3.4 Summary 81 3.5 References 82 Chapter Stable ultrahigh work function polymer hole-injection layers 85 4.1 Introduction 86 4.2 Experimental methods 91 VI 4.3.6 Ohmic injection from ultrahigh workfunction p-doped HIL into F8 Devices were made with some of these p-doped HILs to investigate their hole injecting characteristics into model deep work function OSC F8 with Ip of 5.8eV p-Doped mTFF (SbF6–) was made by solution-state doping as described in Section 4.2.2 A 20-nm-thick film was spin-cast on RCA-SC1-cleaned ITO/ glass substrates in a nitrogen glovebox The work function of this film was measured to be 5.7 eV F8 was then spin-cast from toluene solution (10–15 mg/ mL) over this p-doped mTFF layer to give a thickness of 85 nm Devices were then completed with the evaporation of 120-nm-thick Al as cathode Aluminium does not inject electrons significantly into F8 Reference devices with a 60-nm-thick 1:16 PEDT:PSSH film as the HIL were also prepared The work function of this PEDT:PSSH film was measured to be 5.2 eV Figure 4.20 shows the current density-voltage (JV) characteristics Four representative characteristics are shown for each type of diode There is clearly a three order of magnitude increase in current density over a wide voltage operation range when the PEDT:PSSH HIL is replaced by the p-doped mTFF film This improvement is also much larger than that observed with a ternary PEDT:PSSH:Nafion film (work function 5.5 eV) used in place of PEDT:PSSH as HIL (data not shown) 124 (b) 60-nm 1:16 PEDT:PSSH 20-nm p-doped mTFF 10-1 Current density (A cm-2) 10-3 10-4 10-2 10-3 10-5 10-6 10-7 100 Current density (A cm-2) (a) 10-2 diode1 diode2 diode3 diode4 Voltage (V) diode1 diode2 diode3 diode4 10-4 10-5 Voltage (V) Figure 4.20 JV characteristics of hole-only devices: (a) ITO/ PEDT:PSSH/ F8/ Al and (b) ITO/ p-mTFF/ F8/ Al Film thickness of the F8 is 85 nm We have also compared the diode JV characteristics to those obtained with p-doped F8 (SbF6–) films as the HIL in F8 diodes A 20-nm-thick film of F8 was prepared on RCA-SC1-cleaned ITO/ glass substrates in a nitrogen glovebox and doped with nitrosonium hexafluoroantimonate by thin-film contact doping as described in Section 4.2.2 F8 was then spin-cast from toluene solution (10–15 mg/ mL) over the p-doped polymer films to give various thicknesses Devices were then completed with the evaporation of 120-nmthick Al as cathode Reference devices with 60-nm-thick 1:16 PEDT:PSSH film as the HIL were also prepared within the same device batch 125 Figure 4.21 compares the electrical properties of the p-doped F8/ F8 contact and the PEDT/ F8 contact, for the devices with 75-nm-thick film of intrinsic F8 The device with PEDT:PSSH as HIL again clearly shows inferior JV characteristics, as described previously, in this case with a large hysteresis In contrast, the pdoped F8 layer gives a large improvement in the JV characteristics with no hysteresis The leakage current is also low 0.45 100 Current density (Acm-2) 10-1 10-2 Al cathode i-F8 (75nm) PEDT:PSSH or p-F8 ITO–glass p-F8 5.8 10-3 p-F8 10-4 3.35 5.65 F8 -6 3.35 5.2 10-6 10 Al PEDT 10-5 -7 Vbi=1.85 5.8 -4 -2 Voltage (V) Vbi=1.85 PEDT F8 Al Figure 4.21 JV characteristics of hole-only devices: ITO/ PEDT:PSSH/ F8/ Al (red curves) and ITO/ p-F8/ i-F8/ Al (orange curves) Inset: Energy level diagram based on electromodulated absorption spectroscopy determination of the built-in potential Vbi The Vbi was found to be 1.85 V in both cases 126 To determine the energy-level alignment, we measured the built-in potential Vbi by electromodulated absorption (EA) spectroscopy using a home-built rig.33,34 Figure 4.22 shows our EA setup In this method, an ac bias superimposed on a dc bias is applied to the device, and the reflection spectrum is collected, and demodulated as a function of wavelength to give the normalised modulated reflectance spectrum after normalisation by the dc spectrum (∆R/R) These spectra are plotted for different dc bias, and the Stark peak interpolated to the background to evaluate the dc bias at which the Stark signal disappears This gives the required applied dc bias for the zero internal field condition, i.e., flatband condition, in the diode, which directly gives the Vbi of the diode monochromator source device LIA cryostat voltage DMM modulated drive voltage: Vac Vdc time V = Vdc + Vac sin(ωt) Figure 4.22 Schematic of the ONDL electromodulated absorption spectroscopy setup 127 The Vbi for the device with the PEDT:PSSH as HIL is 1.85 V, which is exactly what is expected for vacuumlevel alignment at the PEDT/ F8 interface (see Figure 4.21) This gives rise to a thermodynamic barrier to hole injection of 0.6 eV The surprising result is that the device with the p-doped F8 as HIL also gives the same Vbi of 1.85 V This means that there is also an apparent 0.6 eV hole-injection barrier, arising from a 0.45-eV vacuum level offset This suggests there is hole transfer across the doped contact Nevertheless this experiment provides the first indication that a large apparent barrier at the injection contact is still compatible with good injection if the contact is doped In other words, the apparent hole-injection barrier, and the related Vbi are not adequate predictors of the quality of contact as previously assumed It appears that here even though the apparent hole injection barrier is large, the presence of a heavily δ-doped F8 layer, which sets up the interface dipole that makes the apparent hole-injection barrier large in the first place, is sufficient to drive that contact to ohmic behavior.33 This result is robust, as it has appeared in other doped contacts that I have measured Figure 4.23 shows the log–log plot of current density vs forward bias above flatband, i.e., V – Vbi, for devices with the p-doped F8/ i-F8 hole-injection contact and different thicknesses of the i-F8 layer (40, 130 and 170 nm), and a non-injecting cathode (Al) This is a classic test for space-charge-limited-currents (SCLC) The HIL is kept the same, 20nm of p-doped F8 The JV curves reveal the ideal power-law behaviour with the correct scaling as the inverse of the cube of film thickness This demonstrates that SCLC has been attained, and the hole-injection contact is indeed ohmic, even for a 40-nm-thick i-F8 film Repeat scans for the current–voltage characteristics did not suggest evidence of severe counter-ion migration The diodes can be repeatedly measured over weeks and cycled between 30 and 298 K, so the 128 doped profiles have sufficient stability for further investigations Thus the simple p-doped HILs here provide a good model system to study the physics of ultrahigh work-function contacts 101 100 Current density (Acm-2) 10-1 10-2 10-3 10-4 Al cathode i-F8 (x nm) p-F8 (20 nm) ITO–glass 40nm 130nm 10-5 10-6 0.1 170nm V-Vbi (V) 10 Figure 4.23 Log–log JV characteristics of ITO/ p-F8/ i-F8/ Al diodes plotted against the forward bias voltage above flat band, i.e., V–Vbi Following the ideas described here, we expect that a reliable doping procedure can also be developed for an electron injection layer (EIL) at the cathode contact If so, this can significantly improve the efficiency of electron injection With the continued development of the bis(fluorophenyl azide) photocrosslinking methodology, we anticipate that both electron blocking layer and hole blocking layer can eventually be introduced between the HIL and EIL and LEP interfaces respectively This should provide for more 129 sophisticated exciton confinement and charge carrier confinement that will further improve luminance efficiency of polymer OLED devices 4.4 Summary We have demonstrated ohmic injection to a polymer organic semiconductor with deep work function HIL by simply providing a high work-function contact that is now available through several routes via doping of the HIL Stable p-doped HILs with deep work functions up to eV are now available New aspects of the semiconductor device physics of doping and of energy-level alignment at the doped contacts are suggested by the results described here, which will be pursued in further work In particular we have evidence now that doping does not always lead to shifting of states into the gap with respect to vacuum level, so it is possible to achieve work function that is close to or even larger than the ionisation potential of the neutral material This is related to the energy of the SOMO and its stabilisation or destabilisation by the Madelung potential which can be tuned This opens new approaches to the design of ultrahigh work function materials Secondly, doped contacts can exhibit apparently large thermodynamic barrier to hole injection, yet can still provide ohmic injection, because of the participation of the δ-doped carrier density at the interface This opens intriguing possibilities for energy tuning of interfaces in devices 130 4.5 References Pfeiffer, M., Forrest, S R., Zhou, X & Leo, K A low drive voltage, transparent, metal-free n-i-p electrophosphorescent light emitting diode Org ELectron 4, 21-26 (2003) Harada, K et al Organic homojunction diodes with a high built-in potential: Interpretation of the current-voltage characteristics by a generalized Einstein relation Phys Rev Lett 94, 036601 (2005) Matsushima, T & Adachi, C Extremely low voltage organic light-emitting diodes with p-doped alpha-sexithiophene hole transport and n-doped phenyldipyrenylphosphine oxide electron transport layers Appl Phys Lett 89, 253506 (2006) Walzer, K., Maennig, B., Pfeiffer, M & Leo, K Highly efficient organic devices based on electrically doped transport layers Chem Rev 107, 1233-1271 (2007) Parthasarathy, G., Shen, C., Kahn, A & Forrest, S R Lithium doping of semiconducting organic charge transport materials J Appl Phys 89, 4986 (2001) Werner, A et al n-Type doping of organic thin films using cationic dyes Adv Funct Mater 14, 255-260 (2004) Chan, C K., Kahn, A., Zhang, Q., Barlow, S & Marder, S R Incorporation of cobaltocene as an ndopant in organic molecular films J Appl Phys 102, 014906 (2007) Pfeiffer, M et al Doped organic semiconductors: Physics and application in light emitting diodes Org ELectron 4, 89-103 (2003) Chiang, C K et al Polyacetylene, (CH)x: n-type and p-type doping and compensation Appl Phys Lett 33, 18 (1978) 10 Lögdlund, M., Lazzaroni, R., Stafström, S & Salaneck, W R Direct observation of charge-induced pi-electronic structural changes in a conjugated polymer Phys Rev Lett 63, 1841-1844 (1989) 11 Fahlman, M et al Experimental and theoretical studies of the electronic structure of Na-doped poly(p-phenylenevinylene) Chem Phys Lett 214, 327-332 (1993) 131 12 Hwang, J & Kahn, A Electrical doping of poly(9,9-dioctylfluorenyl-2,7-diyl) with tetrafluorotetracyanoquinodimethane by solution method J Appl Phys 97, 103705 (2005) 13 Yim, K H et al Controlling electrical properties of conjugated polymers via a solution-based p-type doping Adv Mater 20, 3319-3324 (2008) 14 Ho, P K H., Thomas, S., Friend, R H & Tessler, N All-polymer optoelectronic devices Science 285, 233-236 (1999) 15 Ho, P K H et al Molecular-scale interface engineering for polymer light-emitting diodes Nature 404, 481-484 (2000) 16 Groenendaal, L B., Jonas, F., Freitag, D., Pielartzik, H & Reynolds, J R Poly(3,4ethylenedioxythiophene) and Its derivatives: past, present, and future Adv Mater 12, 481-494 (2000) 17 Kirchmeyer, S & Reuter, K Scientific importance, properties and growing applications of poly(3,4ethylenedioxythiophene) J Mater Chem 15 (2005) 18 Kim, J S., Friend, R H., Grizzi, I & Burroughes, J H Spin-cast thin semiconducting polymer interlayer for improving device efficiency of polymer light-emitting diodes Appl Phys Lett 87, 023506-023501-023503 (2005) 19 Mauritz, K A & Moore, R B State of understanding of Nafion Chem Rev 104, 4535-4585 (2004) 20 Lee, T W et al Hole-injecting conducting-polymer compostitions for highly efficient and stable organic light-emtting diodes Appl Phys Letts 87, 231106 (2005) 21 Lee, T W., Chung, Y., Kwon, O & Park, J J Self-organized gradient hole injection to improve the performance of polymer electroluminescent devices Adv Funct Mater 17, 390-396 (2007) 22 Chia, P J et al Injection-induced de-doping in a conducting polymer during device operation: asymmetry in thoe hole injection and extraction rates Adv Mater 19, 4202-4207 (2007) 23 Png, R Q et al Electromigration of the conducting polymer in organic semiconductor devices and its stabilization by crosslinking Appl Phys Lett 91, 013511 (2007) 132 24 Guk, Y V., Ilyushin, M A., Golod, E L & Gidaspov, B V Nitronium salts in organic chemistry Russ Chem Rev 52, 287-297 (1983) 25 Borodkin, G I & Shubin, V G Nitrosonium complexes of organic compounds Structure and reactivity Russ Chem Rev 70, 211-230 (2001) 26 Kim, E K & Kochi, J K Oxidative aromatic nitration with charge-transfer complexes of arenes and nitrosonium salts J Org Chem 54, 1692-1702 (1988) 27 Sivaramakrishnan, S et al Solution-processed conjugated polymer organic p-i-n light-emitting diodes with high built-in potential by solution- and solid-state doping Appl Phys Lett 95, 213303213301-213303 (2009) 28 Connelly, N G & Geiger, W E Chemical redox agents for organometallic chemistry Chem Rev 96, 877-910 (1996) 29 Meyer, J et al Highly efficient simplified organic light emitting diodes Appl Phys Lett 91, 113506 (2007) 30 Zhang, M., Irfan, Ding, H., Gao, Y & Tang, C W Organic schottky barrier photovoltaic cells based on MoOx / C60 Appl Phys Lett 96, 183301 (2010) 31 Chia, P J et al Direct evidence for the role of the Madelung potential in determining the work function of doped organic semiconductors Phys Rev Lett 102, 096602-096601-096604 (2009) 32 Lögdlund, M., Lazzaroni, R., Stafström, S., Salaneck, W R & Brédas, J L Direct observation of charge-induced π-electronic structural changes in a conjugated polymer Phys Rev Lett 63, 1841-1844 (1989) 33 Zhou, M et al The role of delta-doped interfaces for Ohmic contacts to organic semiconductors Phys Rev Lett 103, 036601 (2009) 34 Zhou, M et al Determination of the interface d-hole density in a blue-emitting organic semiconductor diode by electromodulated absorption spectroscopy Appl Phys Lett 97, 113505 (2010) 133 134 Appendix (A) Publications related to work done in this thesis R.Q Png, P.J Chia, J.C Tang, B Liu, S Sivaramakrishnan, M Zhou, S.H Khong, H.S.O Chan, J.H Burroughes, L.L Chua, R.H Friend, P.K.H Ho, “High-performance Polymer Semiconducting Heterostructure Devices by Nitrene-mediated Photocrosslinking of Alkyl Side Chains”, Nature Materials, 9, 152 (2010) (B) Publications (up till 2011) for work not described in this thesis B Liu, R Q Png, L.-H Zhao, R.H Friend, L.-L Chua, P.K.-H Ho, “Polymer network solar cells with enhanced power conversion efficiency: Control of donor-acceptor morphology by molecular acceptor infiltration into crosslinked polymer donor network”, Manuscript in preparation L.-H Zhao, R Q Png, J.-M Zhuo, J.-C Tang, L Y Wong, R H Friend, L.-L Chua, P.K.-H Ho, “The ringtwist transition in semicrystalline π-conjugated organic semiconductors: Inducing polaron delocalization through suppression of intrachain relaxation ” , Manuscript submitted L.-H Zhao, R Q Png, J.-M Zhuo, L Y Wong, J.-C Tang, Y.-S Su, L.-L Chua, “A General Method to Induce Macroscopically Well-Oriented Lamellar Order π-Stackable Polymer Films Using Borderline−Poor Solvents”, Manuscript submitted L.-H Zhao, R Q Png, J.-C Tang, J.-M Zhuo, L.-L Chua, “Moelcular-weight dependence of the liquidcrystalline transitions of poly(bithiophene–alt–thienothiophene): evidence for the role of π-stacking interactions and a new nematic phase”, Manuscript submitted 135 M Zhou, R.Q Png, S Sivaramakrishnan, P.J Chia, C.K Yong, L.L Chua, P.K.H Ho, “Determination of the Interface δ−hole Density in a Blue-Emitting Organic Semiconductor Diode by Electromodulated Absorption Spectroscopy”, Applied Physics Letters, 97, 113505 (2010) L.Y Wong, R.Q Png, F.B Shanjeera Silva, L.L Chua, D.V Maheswar Repaka, S Chen, X.Y Gao, L Ke, S.J Chua, A.T.S Wee, P.K.H Ho, “Interplay of Processing, Morphological Order, and Chargecarrier Mobility in Polythiophene Thin Films Deposited by Different Methods: Comparison of Spin-cast, Drop-cast, and Inkjet-printed Films”, Langmuir, 26, 15494 (2010) B.T Anto, L.Y Wong, R.Q Png, S Sivaramakrishnan, L.L Chua, P.K.H Ho, “Printable Metal Nanoparticle inks”, Handbook of Nanophysics, Functional Nanomaterials, Chapter 2, ISBN: 978-14200-7552-6 S Sivaramakrishnan, M Zhou, A.C Kumar, Z Chen, R.Q Png L.L Chua, P.K.H Ho, “Solutionprocessed Conjugated Polymer Organic p-i-n Light-emitting Diodes with High Built-in Potential by Solution- and Solid-state Doping”, Applied Physics Letters, 95, 213303 (2009) J.M Zhuo, L.H Zhao, R.Q Png, L.Y Wong, P.J Chia, J.C Tang, S Sivaramakrishnan, M Zhou, E.C.W Ou, S.J Chua, W.S Sim, L.L Chua and P.K.H Ho, “Direct Spectroscopic Evidence for a Photo-Doping Mechanism in Polythiophene and Poly(bithiophene-alt-thienothiophene) Organic Semiconductor Thin Films Involving Oxygen and Sorbed Moisture”, Advanced Materials, 21, 4747 (2009) 10 M Zhou, L.L Chua, R.Q Png, C.K Yong, S Sivaramakrishnan, P.J Chia, A.T.S Wee, R.H Friend and P.K.H Ho, “Role of delta-Hole-Doped Interfaces at Ohmic Contacts to Organic Semiconductors”, Physical Review Letters, 103, 036601 (2009) 136 11 P.J Chia, S Sivaramakrishnan, M Zhou, R.Q Png, L.L Chua, R.H Friend and P.K.H Ho, “Direct Evidence for the Role of the Madelung Potential in Determining the Work Function of Doped Organic Semiconductors”, Physical Review Letters, 102, 096602 (2009) 12 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 and P.K.H Ho, “Band-like Transport in Surface-Functionalized Highly SolutionProcessable Graphene Nanosheets”, Advanced Materials, 20, 3440 (2008) 13 S Wang, J.C Tang, L.H Zhao, R.Q Png, L.Y Wong, P.J Chia, H.S.O Chan, P.K.H Ho and L.L Chua, “Solvent effects and multiple aggregate states in high-mobility organic field-effect transistors based on poly(bithiophene-alt-thienothiophene)”, Applied Physics Letters, 93, 162103 (2008) 14 L Ke, S Bin Dolmanan, L Shen, C Vijila, S.J Chua, R.Q Png, P.J Chia, L.L Chua, and P.K.H Ho “Impact of self-assembled monolayer on low frequency noise of organic thin film transistors” Applied Physics Letters, 93, 153507 (2008) 15 L Ke, S Bin Dolmanan, L Shen, C Vijila, S.J Chua, R.Q Png, P.J Chia, L.L Chua, and P.K.H Ho “Low frequency noise analysis on organic thin film transistors” Journal of Applied Physics, 104, 124502 (2008) 16 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) PhotoCrosslinkers and their Post-Deposition Modification”, Advanced Functional Materials, 17, 2490 (2007) 17 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, 013511 (2007) 137 (C) Conference presentations (presenting author underlined): R.Q Png, J.H Burroughes, L.L Chua, R.H Friend, P.K.H Ho, “Recent Insights in Making and Characterising Ohmic Contacts to Organic Semiconductors”, E-MRS Spring 2011, Nice (France) Talk R.Q Png, P.J.Chia, J.C Tang, B Liu, S Sivaramakrishnan, M Zhou, S.H Khong, H.S.O Chan, J.H Burroughes, L.L Chua, R.H Friend, P.K.H Ho, “High Performance Heterostructure Polymer Devices by Nitrene-Mediated Photocrosslinking of Alkyl Side Chains”, MRS Spring 2010, San Francisco Talk R.Q Png, P.J Chia, L.L Chua, J.C Tang, B Liu, S Sivaramakrishnan, M Zhou, S.H Khong, H.S.O Chan, J.H Burroughes, R.H Friend, P.K.H Ho, “New Opportunities in Heterostructure Polymer Organic Photovoltaics, Transistors and Light-Emitting Diodes”, ICMAT 2009, Singapore Poster R.Q Png, L.H Zhao, L.Y Wong, L.L Chua, P.K.H Ho, “Free-Form Contact Resistance Extraction: New Insights into the Nature of Mobility and Contact Resistance in OFETs”, E-MRS Spring 2008, France Poster 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”, ICMAT 2007, Singapore Poster 138 ... report stands for weight ratio of crosslinker to polymer) .11 Thus while bis(phenyl azide)s may be fine for photoresist applications, they are not suitable for organic semiconductor devices because... layers Controlled and stable doping is therefore desirable for the realisation of this and the improved efficiency of these organic devices Doping of organic semiconductors to make p–i–n structures... 1.1 Organic semiconductors 1.2 Doping of organic semiconductors 1.3 Polymer light-emitting diodes 1.3.1 Structure and operation of a polymer light-emitting