Research Interests and Projects
The following projects are tied to the general interests of my group in organic electronics materials and devices. Particular examples are organic light-emitting diodes (OLEDs), solar cells and organic thin film transistors (OTFTs). Among these devices, OLEDs are finding worldwide recognition as the leading contender for ultra-thin flat panel displays. The central scientific issue to be studied will be the conductivities of organic electronic materials that are relevant to organic electronic devices. Most of the projects are designed to measure conductivities and advance the understanding the mechanisms of conductivity in organic electronic materials. Besides conductivities, we are also interested in forming efficient charge injection contacts to organics.
General References
S.C. Tse, C.H. Cheung, and S.K. So, "Organic electronics: materials, processing, devices and applications", F. So Ed., Chap. 3, CRC Press, Francis and Taylor (2010).
K.K. Tsung and S.K. So, "Carrier trapping and scattering in amorphous organic hole transporter", Appl. Phys. Lett. 92, 103315 (2008).
[Abstract]
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Abstract: The effects of dopants on the hole transporting properties of N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine (NPB) have been studied by time of flight. Five dopants: copper phthalocyanine (CuPc), 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyrle)-4 H-pyran (DCM1), 4-dicyanomethylene-2-methyl-6-[2-(2,3,6,7-tetra-hydro-1H,5H-benzo[ij] quinolizin-8-yl)vinyl]-4H-pyran [DCM2], 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (tBu-PBD), and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) are used in this study. The dopant molecules behave like hole traps or scatterers. Their detailed behaviors are determined by their highest occupied molecular orbital relative to that of NPB. Generally, traps are found to induce significant reduction in hole mobility while there is a slight reduction for scattering. Two different underlying charge transport mechanisms are proposed.
Copyright Notice: Copyright (2008) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
Link: http://link.aip.org/link/?APL/92/103315
S.K. So, S.C. Tse, and K.L. Tong, "Charge transport and injection to Phenylamine-based hole transporters for OLEDs applications", Journal of Display Technology 3, 225-232 (2007).
1. Organic solar cells
Solar cells absorb photons in the visible range and convert them into from free electrons and holes. Traditionally, solar cells are constructed from semiconducting silicon. This project will investigate the possibility of using organic electronic materials for solar cells fabrication. The project is quite challenging as it comprises of sample fabrication, optical and electrical characterization, and some device modeling.
Reference:
S. Gunes, H. Neugebauer, and N.S Sariciftci, "Conjugated Polymer-Based Organic Solar Cells", Chemical Reviews 107, 1324-1338 (2007).
2. Dark injection space-charge-limited-current
Organic electronic devices are now widely used in optoelectronics. Examples are organic light-emitting diodes (OLEDs) and organic thin film transistors (OTFTs). The technology has matured sufficiently, and commercial products such as flat panel displays are beginning to be available. In these devices, they typically have a structure of anode/organic materials/cathode. Proper operations of devices require the injection of external charge carriers from the contact electrodes into the active organic electronic material. Thus the ability to control and to quantify carrier injection is of critical importance for improving performances of organic electronic devices. This project uses a very useful technique, known as dark injection space-charge-limited-current (DI-SCLC), to study charge injection phenomena in organic electronic devices. The principles and merits of this technique of will be investigated. The outcome of this project will allow us to quantify the quality of a charge injection contact to an organic electronic material.
References:
C.H. Cheung, W.J. Song, and S.K. So,"Role of air exposure in the improvement of injection efficiency of transition metal oxide/organic contact", Organic Electronics 11,89-94 (2010).
S.C. Tse, S.W. Tsang, and S.K. So, "Polymeric conducting anode for small organic transporting molecules in dark injection experiments", J. Appl. Phys. 100, Art. No. 063708 (2006).
[Abstract]
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Abstract: Poly(3,4-ethylenedioxythiophene) doped with polystrenesulphonic acid (PEDOT:PSS) is used as a hole-injecting anode for small organic hole transporters in current-voltage (JV) and dark injection space-charge-limited current (DI-SCLC) experiments. The hole transporters under investigation are phenylamine-based 4,4',4"-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (MTDATA), N,N'-diphenyl-N,N'-bis(1-naphthyl) (1,1'-biphenyl)-4,4'diamine (NPB), and N,N'-diphenyl-N,N'-bis(3-methylphenyl)(1,1'-biphenyl)-4,4'diamine (TPD). Clear DI-SCLC transient peaks were observed over a wide range of electric fields in all cases. For MTDATA and NPB, hole mobilities evaluated by DI experiments are in excellent agreement with mobilties deduced from independent time-of-flight technique. It can be concluded that, for the purpose of JV and DI experiments, PEDOT:PSS forms an Ohmic contact with MTDATA and a quasi-Ohmic contact with NPB despite the relatively low-lying highest occupied molecular orbital of the latter. In the case of TPD, hole injection from PEDOT:PSS deviates substantially from Ohmic injection, leading to a lower than expected DI-extracted hole mobility. The performances of other hole-injecting anodes for DI experiments were also examined.
Copyright Notice: Copyright (2006) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
Link: http://link.aip.org/link/?JAP/100/063708
3. Time-of-flight measurements
The conductivity of a non-crystalline solid is determined by the mobility of charges inside the solid. The mobility is a measure of how fast charge carriers can move inside the solid. In general, the mobility is affected by temperature and external applied electric field. An effective means of measuring charge mobility in a solid is by the time-of-flight (TOF) technique. In TOF, a pulsed laser is used to create mobile carriers in the material. The time required for charges to migrate through a fixed distance is measured from which the carrier mobility is derived. The objective of this project is to use TOF to measure the mobilities of amorphous thin films and to understand their conduction mechanisms.
References:
K.K. Tsung and S.K. So, "Carrier trapping and scattering in amorphous organic hole transporter", Appl. Phys. Lett. 92, 103315 (2008).
[Abstract]
[Free PDF File,
Copyright Notice]
Abstract: The effects of dopants on the hole transporting properties of N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine (NPB) have been studied by time of flight. Five dopants: copper phthalocyanine (CuPc), 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyrle)-4 H-pyran (DCM1), 4-dicyanomethylene-2-methyl-6-[2-(2,3,6,7-tetra-hydro-1H,5H-benzo[ij] quinolizin-8-yl)vinyl]-4H-pyran [DCM2], 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (tBu-PBD), and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) are used in this study. The dopant molecules behave like hole traps or scatterers. Their detailed behaviors are determined by their highest occupied molecular orbital relative to that of NPB. Generally, traps are found to induce significant reduction in hole mobility while there is a slight reduction for scattering. Two different underlying charge transport mechanisms are proposed.
Copyright Notice: Copyright (2008) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
Link: http://link.aip.org/link/?APL/92/103315
S.C. Tse, K.C. Kwok, and S.K. So, "Electron transport in naphthylamine-based organic compounds", Appl. Phys. Lett. 89, Art. No. 262102 (2006).
[Abstract]
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Abstract: Two naphthylamine-based hole transporters, namely, N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine (NPB) and 4,4',4''-tris(n-(2-naphthyl)-n-phenyl-amino)-triphenylamine (2TNATA), were found to possess electron transporting (ET) abilities. From time-of-flight measurements, values of electron mobilities for NPB and 2TNATA are (6-9)x10-4 and (1-3)x10-4 cm2/V s, respectively, under an applied electric field range of 0.04-0.8 MV/cm at 290 K. An organic light-emitting diode that employed NPB as the ET material was demonstrated. The electron conducting mechanism of NPB and 2TNATA in relation to the Marcus theory [Rev. Mod. Phys. 65, 599 (1993)] from quantum chemistry will be discussed.
Copyright Notice: Copyright (2006) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
Link: http://link.aip.org/link/?APL/89/262102
H.H. Fong, K.C. Lun, and S.K. So, "Hole transports in molecularly doped triphenylamine derivative", Chem. Phys. Lett. 353, 407-413 (2002).
4. Impedance spectroscopy
Impedance spectroscopy (IS) is a very useful technique for characterizing the electronic properties of materials. In IS, the material under investigation is subject to a small ac voltage and the complex impedance is measured. From the complex impedance, one can deduce dielectric properties and conductivities of the material. The impedance of the sample will also be monitored as a function of frequency and dc biased voltage. The application of IS to organic electronic materials (e.g. guest-host systems) will be examined in this project. By fitting the impedance data to equivalent circuit models, we can deduce the capacitance and the conductivity of the organic electronic materials.
References:
K.K. Tsung and S.K. So, "Advantages of admittance spectroscopy over time-of-flight technique for studying dispersive charge transport in an organic semiconductor", J. Appl.Phys. 106, 083710 (2009).
[Abstract]
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Abstract: We show that admittance spectroscopy (AS) is a better technique than time of flight (TOF) to study the charge transport properties in dispersive materials. The hole transport properties of N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'-diamine (NPB) doped with different traps were evaluated by AS and TOF techniques. It was found that both techniques can show clear signals for measuring the mobility of NPB doped with shallow traps. When NPB was doped with deep traps, the AS signals were still clear for mobility extraction. In sharp contrast, the TOF transients become featureless and the carrier transit time cannot be determined. The validity of AS in mobility determination was demonstrated by comparing the extracted AS to TOF mobilities. Generally, the hole mobilities extracted by these two techniques were in excellent agreement. In addition, we will demonstrate that AS can be employed to measure carrier dispersion.
Copyright Notice: Copyright (2009) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
Link: http://link.aip.org/link/?JAP/106/083710
S.W. Tsang, S.K. So, and J.B. Xu, "Application of admittance spectroscopy to evaluate carrier mobility in organic charge transport materials", J. Appl. Phys. 99, Art. No. 013706 (2006).
[Abstract]
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Abstract: We examine the feasibility of admittance spectroscopy (AS) and susceptance analysis in the determination of the charge-carrier mobility in an organic material. The complex admittance of the material is analyzed as a function of frequency in AS. We found that the susceptance, which is the imaginary part of the complex admittance, is related to the carrier transport properties of the materials. A plot of the computer-simulated negative differential susceptance versus frequency yields a maximum at a frequency taur-1. The position of the maximum taur-1 is related to the average carrier transit time taudc by taudc=0.56 taur. Thus, knowledge of taur can be used to determine the carrier mobility in the material. Devices with the structure ITO/4,4',4'-tris[N,-(3-methylphenyl)-N-phenylamino] triphenylamine/Ag have been designed to investigate the validity of the susceptance analysis in the hole mobility determination. The hole mobilities were measured both as functions of the electric field and the temperature. The hole mobility data extracted by susceptance analysis were in excellent agreement with those independently obtained from time-of-flight (TOF) measurements. Using the temperature dependence results, we further analyzed the mobility data by the Gaussian disorder model (GDM). The GDM disorder parameters are also in good agreement with those determined from TOF.
Copyright Notice: Copyright (2006) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
Link: http://link.aip.org/link/?JAP/99/013706
K.L. Tong, S.W. Tsang, K.K. Tsung, S.C. Tse, and S.K. So, "Hole transport in molecularly doped naphthyl diamine", J. Appl. Phys. 102, 093705 (2007).
[Abstract]
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Abstract: The effects of dopants on the hole-transporting properties of NPB, i.e., (N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4' diamine), were studied by time-of-flight technique and admittance spectroscopy. Three dopants were chosen in this study. They were 4-dicyanmethylene-2-methyl-6-4H-pyran (DCM1), rubrene (RB), and tris-(8-hydroxyquinoline) aluminum (Alq3). It can be shown that DCM1 behaves as hole traps whereas Alq3 behaves as hole scatterers in NPB. Generally, both trapping and scattering lower hole mobilities in NPB. The hole mobilities decrease when DCM1 and Alq3 are introduced into NPB whereas the hole mobility remains nearly unchanged when RB is doped into NPB. The effect of doping on carrier dispersion is also studied.
Copyright Notice: Copyright (2007) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
Link: http://link.aip.org/link/?JAP/102/093705
5. Fabrication and characterization of organic thin film transistors
Field effect transistors (FETs) are critical ingredients in modern microelectronics. They are being used in amplifiers and nearly for all digital combinational logic circuits because of their versatility and low power consumption. Thin film transistors (TFTs) are special FETs in which the active, semiconducting material (usually silicon) is grown as a very thin film on an insulating substrate. They are used in electronic applications that require a large area, e.g., liquid crystal display monitors. Organic thin film transistors (OTFTs) make use of semiconducting organic molecules or polymers as the active materials to accomplish functions analogous to inorganic TFTs. They have clear novelty in comparison to Si-based TFT because OTFT can be grown on flexible substrates --- hence a realization of plastic/flexible electronic devices. Their fabrication process is less complex. The goal of this project is to use OTFTs to evaluate the carrier mobility (and hence the conductivity) and investigate the physics of charge transport in organic semiconductors. Particular emphasis is on spin-cast semi-conducting polymer. Extension to other emerging and novel organic charge conductors will be considered.
References:
W.H. Choi, C.H. Cheung, and S.K. So, "Can an organic phosphorescent dye act as a charge transporter?", Organic Electronics 11, 872-875 (2010).
C.H. Cheung, K.K. Tsung, K.C. Kwok, and S.K. So, "Using thin film transistors to quantify carrier transport properties of amorphous organic semiconductors", Appl. Phys. Lett. 93, 083307 (2008).
[Abstract]
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Abstract: The hole transport properties of two phenylamine-based compounds were evaluated by thin film transistor (TFT) measurement and time-of-flight (TOF) technique. The compounds were N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'biphenyl)-4,4'diamine (NPB) and 4,4',4"-tris[n-(2-naphthyl)-n-phenyl-amino] triphenylamine (2TNATA). With tungsten oxide/gold as the charge injecting electrode, the field effect mobility of NPB was found to be 2.4x10-5 cm2/V s at room temperature, which was about one order of magnitude smaller than that obtained from independent TOF experiments (3x10-4 cm2/V s). Similar observations were found for 2TNATA. Temperature dependent measurements were carried out to study the energetic disorder of the materials. It was found that the energetic disorder was increased in the neighborhood of a gate dielectric layer.
Copyright Notice: Copyright (2008) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
Link: http://link.aip.org/link/?APL/93/083307
C.H. Cheung, K.C. Kwok, S.C. Tse, and S.K. So, "Determination of carrier mobility in phenylamine by time-of-flight, dark-injection, and thin film transistor techniques", J. Appl. Phys. 103, 093705 (2008).
[Abstract]
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Abstract: The hole transport property of a phenylamine-based compound, 4,4',4"-tris(n-(2-naphthyl)-n-phenyl-amino)-triphenylamine, was independently studied by time-of-flight (TOF), dark-injection space-charged-limited-current (DI-SCLC), and thin film transistor (TFT) techniques. With UV-ozone treated gold as the injecting anode, clear DI-SCLC transient peaks were observed over a wide range of electric fields. The hole mobilities evaluated by DI-SCLC experiment were in excellent agreement with the mobilities obtained from the TOF technique. The injection contact was demonstrated to be Ohmic by an independent current-voltage (J-V) experiment. However, with the same injecting electrode, the mobility deduced from the TFT method was found to be 9.8x10-7cm2/Vs, which was about one order of magnitude smaller than the TOF mobility (~1.2x10-5cm2/Vs). The origin of the discrepancy is discussed.
Copyright Notice: Copyright (2008) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
Link: http://link.aip.org/link/?JAP/103/093705
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