Organic Thin Film Transistor - Research Paper Example

Organic Thin Film Transistors (Literature Review) Yousaf January Table of Contents Abbreviations and Variables 2 Introduction 3 Development, advantages and uses 3 2. Materials 4 3. Characteristics 7 4. Conclusion 10 List of Figures Figure 1: The basic structure of a p-type organic thin film transistor (top/bottom contact) 3 Figure 2: Improvements in the hole mobility of OTFTs over the years 4 Figure 3: Structure of a PDBT-co-TT based OTFT 5 Figure 4: Chemical structure of pentacene 5 Figure 5: Thin film transistor mobility for fabrication of C6FTTF and C12FTTF thin film transistors 6 Figure 6: Typical output and transfer characteristics of a pentacene based OTFT 7 Figure 7: Plots of drain current (ID) against drain voltage (VD) for different gate voltages 8 Figure 8: OTMS deposition by spin-casting 9 Figure 9: Illustration of a spray-deposition based system 10 Abbreviations and Variables Abbreviations used AMFPD Active Matrix Flat Panel Display FTTF Fluorene Bithiophene Fluorene HMTTF Hexamethylenetetrathiafulvalene LED Light Emitting Diode MOSFET Metal Oxide Silicon Field Effect Transistor OTFT Organic Thin Film Transistor OTMS Octadecyltrimethoxysilane OTS Octadecylsilanes RFID Radio Frequency Identification SAM Self-assembled Monolayer TES ADT Triethylsilylethynyl Anthradithiophene TFT Thin Film Transistor Variables mentioned ID Drain current I-V Current-Voltage Rs Series Resistance VD Drain Voltage VG Gate Source Voltage Veff Effective Gate Voltage Vth Threshold Voltage γ Prefactor μFE Field-effect Mobility (units: cm2/Vs) μ0 Zero-field Mobility. Introduction This paper reviews the literature on organic thin film transistors (OTFTs). It is divided into three sections to reflect the three important areas, namely (1) development, advantages and uses, (2) materials, and (3) characteristics. OTFTs, like their inorganic counterparts, can function as either p-type or n-type. The difference is that in the former, holes are the majority carriers whereas electrons are the major carriers in the n-type. Also, the p-type tends to have much better mobility. The focus herein is on the p-type. The basic structure of an OTFT is illustrated in the schematic diagrams below1. The top diagram (a) shows a top contact device in which the two (source and drain) electrodes are evaporated onto the semiconducting layer by applying a mask and the bottom diagram (b) shows a bottom contact device in which the semiconductor is deposited onto the gate insulator with prefabricated electrodes. 1. Development, advantages and uses Studies on organic semiconductors began in the 1940s and these components were mostly used in xerography due to their photoconductive properties. They have only gained more widespread attention in recent years due to impressive performance and efficiency improvements making it possible for a wider range of further applications. In particular, the considerable improvements in OTFTs have led to an enhanced understanding of their conduction mechanisms and performance characteristics, and fabrication technologies have been developed for optimising their morphology and structural order2. It should also be noted however, that despite the advancement in OTFTs and increasing uses for them, they are still not able to replace the more common inorganic based TFTs for certain applications in which faster switching speeds are required. This is due to their still relatively low mobility, despite considerable improvement in this regard, as shown in the chart below3. On the other hand, OTFTs are better suited where large area coverage is required and they offer compelling advantages of low cost, low temperature processing and structural flexibility. This makes them especially useful for switching in active matrix flat panel displays that consist of liquid crystal pixels or organic LEDs, for radio frequency identification tags (RFIDs), etc. 2. Materials As with inorganic semiconductors, organic semiconductors are also directly affected by such features as purity, crystallinity, molecular packing and growth mode4. The microstructure and morphology of all transistor devices are therefore important because they determine their electrical performance. Fine tuning these properties by selecting appropriate molecular side chain substituents for example, in order to devise better structures and optimising their processing conditions can improve the performance of OTFTs. Modification of the chemical structure of OTFTs at the molecular level affects their packing and their thin film morphology, both of which influence the mobility of the charge carrier. Morphology can be controlled by using appropriate surface chemistry and by dielectric modification. An important performance metric for comparing between OTFT materials and methods for fabrication is the field-effect mobility (μFE). This refers to the carrier speeds relationship with the applied electric field in a material, and in the case of organic semiconductors, it is also dependent on charge carrier concentration and its nature is non-linear rather than linear, as the electric fields are large5. Other metrics include the subthreshold swing, leakage currents and contact resistance6, threshold voltage (Vth), hysteresis and output conductance. The mobility of OTFTs with top contact structures is usually higher than for bottom contact OTFTs due to the presence of a greater contact area in the former type and a poorer morphology of the latter7. This highlights the difference that structure can make on the performance of OTFTs. As an example, a p-type 1,4-diketopyrrolo[3,4-c]pyrrole (DPP) and thieno[3,2-b]thiophene moieties, PDBT-co-TT based OTFT (see configuration and chemical structure below) was able to achieve a hole mobility as high as 0.94 cm2/Vs8. In contrast, previously, Fluorene Bithiophene Fluorene (FTTF) based thin films had demonstrated 0.3 cm2/Vs charge carrier mobility (for p-type)9. Highly efficient pathways were able to be established for facilitating charge carrier transport as a result of interconnected polymer networks forming an ordered lamellar structure. Charge carrier transport refers to the movement or flow of electrons and holes (in the case of p-type) through the semiconductor material from the source to the drain10 Generally, this movement is strongly influence by molecular orientation and by crystallinity in the semiconductor layer whereas the overall charge mobility is determined by the grain and crystal grain boundaries11. In a semiconductor, this transport of the charge carriers mostly occurs in the first few monolayers close to the interface between the semiconductor and the dielectric. Whereas previous research examined such parameters as the dielectric surface energy, chemical functionality and roughness in terms of their impact on performance, it is now recognised that a surface modification layer can make a big difference for optimising OTFTs12. Thus, a dense octadecylsilane (OTS) crystalline layer was used to facilitate 2D semiconductor growth and this led to consistent higher mobility as compared to disordered OTS. The mobilities were 5.3cm2/Vs for C60 and 2.3cm2/Vs for pentacene (C22H14). Pentacene is a polycyclic aromatic hydrocarbon that consists of five linearly fused benzene rings as shown below. It is a popular organic semiconductor used in OTFTs due to the nature of its crystals and thin films that make it behave as a p-type organic semiconductor. The possibility of a high mobility of OTFTs has also been shown to be linked to side-by-side intermolecular interactions that occur parallel to the surfaces13. This led to the fabrication of a hexamethylenetetrathiafulvalene (HMTTF) high performance OTFT. For the active layer of OTFTs, asymmetrically substituted derivatives of FTTF have shown great potential by exhibiting similar and even greater hole transport mobility compared to symmetrical arrangements14. The charge mobility is directly affected by the formation of 2D and 3D polycrystallite from the change in the molecular aspect ratio. The degree of crystallinity can vary for different substrate temperatures allowing for higher quality crystallites. In general, for OTS substrates, a higher substrate temperature for growing the films leads to improved crystallinity. This increases the likelihood of a lower trap density and therefore results in higher charge carrier mobility. The figure below illustrates the better mobility for C12FTTF than C6FTTF and for OTS treated surface compared to a plain substrate. Also, having well-packed asymmetric derivatives places the conjugated FTTF core in direct contact with the surface of the substrate making the single alkyl chain pointed away from the semiconductor interface. Shorter alkyl chains could improve the properties of charge carrier transport but performance also depends on such factors as trap densities and grain size. Besides, a longer length can improve the molecular aspect ratio, which helps to support layer-by-layer large grain growth15. Alkyl chain substitutions are known to be flexible and this makes them stable even during bending of the substrate16. The stability can scale according to the chain length and it can even surpass the stability of pentacene devices. Material properties such as semiconductor thickness do not affect device simulation17. Rather, an increase in thickness tends to increase contact resistance and decrease channel mobility, not only for p-type but also for n-type OTFTs18. In contrast, thin film morphology is an important parameter for optimising OTFT structures because it has been shown that even high vacuum evaporated thin films can be made to behave as p-type semiconducting layers due to the active layers19. This was demonstrated in a comparison of the structural and electrical properties of pentacene with three such oligomers functioning as active layers in OTFTs. The high vacuum evaporated thin films were found to possess two important attributes of OTFTs useful for low cost applications, namely high air stable mobility at a low substrate temperature and a reduced bias stress effect. Hence, it was established that the molecular structure relating to the active layer and thin film morphology are both important parameters for optimising OTFT structures. Moreover, the evaporation of the semiconductor onto the substrate surface helps in the formation of thin films, and thereby also its performance. This means that other important parameters are the method used for the deposition, as this can affect grain shape, size, orientation, molecular packing, etc. and the rate of evaporation20. In practice however, OTFT pose manufacturing challenges relating to depositing the materials and layers and in patterning them. These can be overcome though, for example, by using organic etch masks and plasma etching for the depositing and by using predefined features for transfer printing or direct dry printing. A soluble acene derivative has also been used for fabricating OTFTs for high performance using a rapid film-forming technique called spin-coating21. Such OTFTs exhibit high field-effect mobility, which makes them suitable for large area electronics developed at low cost and at novel room temperature. Crystalline growth was able to be induced on source/drain contacts by modifying their surface properties in order to improve the OTFT electrical characteristics. 3. Characteristics Two important electrical measures that represent the device characteristics of OTFTs are transfer and output curves. The transfer characteristics show the drain source current (ID) for values of the gate source voltage (VG) and the output characteristics show the drain source current (ID) for values of the drain source voltage (VD). A curve is then fitted over these values marked on a graph to provide a model for the OTFTs characteristics. The fitting of a transfer curve allows such values to be determined as the threshold voltage (Vth), series resistance (Rs), zero-field mobility (μ0) and prefactor (γ). The diagram below shows typical transfer (on main chart) and output (in the inset) characteristics of a pentacene based OTFT22. The symbols indicate the measured values and the lines are modelled. The typical I-V (output) curves for a p-type OTFT (top contact) are shown below for a wider range of gate voltages, with the semiconductor based on polycrystalline, a vapour deposited pentacene film, and the gate insulator based on SiO2 modified by a 1-diethoxy-1-silacyclopent-3-ene SAM23. In this particular case, the linear regime mobility is 0.8 cm2/Vs at VD=-10V and the saturation regime mobility is 1.03cm2/Vs at VD=-100V. These I-V output characteristics in general resemble the characteristics for metal oxide silicon field effect transistors (MOSFET)24. The distribution of states in the energy gap has also been modelled for OTFTs based on pentacene to describe the three regimes of below threshold, linear and saturation25. The model devised accounts for the entire range of operating conditions and is useful in analogue circuit simulations. A modification of the physical/chemical properties of surfaces, such as by using certain alkyl derivatives as mentioned earlier, can lead to the formation of self-assembled monolayers (SAMs) through absorption or covalent bonding. The use of octadecylsilanes (OTS) for example, have been shown to be able to provide a very high charge carrier mobility for various semiconductors, and they have also proven to be useful for bottom-gated OFETs due to their growth behaviour being greatly dependent on the dielectric surfaces quality and composition26. The performance improvement was due to the 2D growth on the crystalline OTS, which resulted in well-connected and highly conductive films. Smooth crystalline OTS can be deposited using a spin-coating technique, as illustrated below for octadecyltrimethoxysilane (OTMS). Several recent studies have shown that the electrical characteristics of OTFTs can be affected significantly by depositing SAMs at their semiconductor-dielectric interface27. In one study, this ability of SAM to significantly change the transfer characteristics was investigated using two-dimensional drift-diffusion simulations. The impact was modelled by means of a permanent space charge layer resulting from chemical reactions with the active material and by using a dipole layer as an array of ordered molecules. In addition, the threshold voltage was also affected. The effective gate voltage (Veff) is modified such that the threshold voltage (Vth) shifts rigidly. The magnitude of this shift is dependent on the role of SAM. It can be from a few volts to larger if the SAM results in charges at the interface. Other techniques have also been discovered recently to improve the characteristics of OTFTs. For example, as an alternative to spin-coating, a method of fabrication using spray-deposition for coating organic devices has proven to be not only comparable performance-wise, but it also offers more advantages28. It was shown to consume 20 times less organic semiconductor and to make rapid coating of large-areas easier. Yet it remains a simple method, ensures good homogeneity, it is inexpensive, it permits processing at room temperature, and it can be used conveniently for large-area electronics. Spray-deposited triethylsilylethynyl anthradithiophene (TES ADT) transistors yielded mobilities of 0.2cm2/Vs. The spray-deposition based system is illustrated below albeit for a field-effect transistor. At small voltage values, the feature of non-linearity suggests some poor subthreshold behaviour such as the possibility of parasitic contact effects causing roughness. Parasitic contact resistance can be made around two orders of magnitude lower than present in conventional devices by using a solid electrolyte for the gate dielectric 29. Greater linearity in the output characteristics of OTFTs besides effective mobilities, can be achieved by using arrays of carbon nanotube electrodes due to the more efficient charge injection compared to using noble metal electrodes30. High purity semiconducting carbon nanotube-based transistors are currently showing signs of promise for developing circuit applications including highly linear RF electronics31. 4. Conclusion A selection of recent studies was surveyed on the development, advantages, uses, materials and electrical characteristics of p-type organic thin film transistors. These OTFTs benefit from structural flexibility, low temperature processing requirements and can be produced at low cost. A study of their microstructure and morphology is important because they affect the mobility of the charge carriers. Thus, molecular orientation, crystallinity, enhanced contact area, molecular packing, the presence of a surface modification layer, etc. are all important considerations for improving their performance. The practice of evaporation of the semiconductor onto the substrate surface in forming the thin films and spin-coating and other similar techniques are also useful. Important performance metrics of OTFTs include field-effect mobility and threshold voltage that are determined from their transfer (ID versus VG) and output (ID versus VD) characteristics. Latest research points for example, to the potential of depositing self-assembled monolayers at the semiconductor-dielectric interface, spray-deposition methods for coating, using a solid electrolyte as the gate dielectric, and using carbon nanotube electrodes. References Read More