Chalmers 2006-05-17 Examiner: Thorvald Andersson

Miniproject in FMI040 Semiconductor Materials Physics

Organic devices and electronics -for non-display application

Tutor: Måns Andreasson Group members: Peishan Chien Jose Juan Zacarias Haiping Lai Zengwei Guo Qiguang Li

Content 1. Introduction.............................................................. - 2 -

2. Applications ............................................................. - 2 -

2.1. Organic Diodes.................................................. - 2 -

2.2. Organic Transistors ........................................... - 5 -

2.3. Organic Lasers................................................... - 6 -

2.4. Gas Sensors ....................................................... - 8 -

2.5. Solar Cells ....................................................... - 10 -

3. Conclusions ............................................................ - 12 -

4. References .............................................................. - 12 -

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1. Introduction

The large majority of actual devices based on semiconductors are using inorganic crystalline materials, with single-crystalline silicon dominating when compared to other materials like GaAs. Despite the advantages of single-crystalline inorganic semiconductors like high mobility and high stability, these materials are less suitable if applications require low cost and large area. As an alternative to inorganic semiconductors, organic materials have recently gained much interest and attention. Actually some research studies have developed properties such as mobility in single crystals about a few cm2/Vs at room temperature and higher values at low temperature. For practical applications organic semiconductors with disordered structures are prevailing. In photoconductors for copiers and laser printers organic semiconductors are already widely applied. In this report we will mention some of the principles and applications of organic devices such as diodes, transistors, lasers, gas sensors and solar cells. Organic semiconductors have unique physical properties, which offer many advantages to inorganic semiconductors, these are: (i) The extremely high absorption coefficients in the visible range of some dyes offer the possibility to prepare very thin photodetectors and photovoltaic cells. Due to the small thickness of the layers, the requirements on chemical and structural perfection are reduced since the excitation energy does not have to travel long ways. (ii) Many fluorescent dyes emit strongly red shifted to their absorption. Thus, there are almost no reabsorption losses in organic light emitting diodes (OLEDs). (iii) Since organic semiconductors consist of molecular structures with saturated electron systems, the number of intrinsic defects in disordered systems is much lower than in inorganic amorphous semiconductors, where a large number of dangling bonds exist. (iv) There are no limitations in the number of chemical compounds available, and it is possible to tailor materials. [1]

2. Applications

2.1. Organic Diodes 1) History

Organic light-emitting diodes (OLEDs) are nowadays one of the most attractive devices based on organic semiconductors due to their various merits for flat-panel display applications which work on the principle of electroluminescence. Organic electroluminescence was first observed in thick organic crystals sandwiched between electrodes in the 1960s. In 1987, Tang and VanSlyke reported the first practical organic light-emitting diodes (OLEDs); later in 1989, Tang, Van Slyke, and Chen further boosted the device efficiency by two to three times with the emissive-doping technique; in 1990 the group of Cambridge University demonstrated polymer LEDs (PLEDs) with conjugated polymers. Over the past decade, the OLED display technology has made

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rapid progress, and various types of OLED displays have been demonstrated or commercialized. In 1997, Pioneer Corporation was a monochrome (green) passive-matrix OLED display with a moderate resolution. With further advance of OLED display technologies toward even higher image quality, higher efficiency, and larger areas, the passive-matrix architecture can no longer meet the requirements and the active-matrix architecture, which integrates OLEDs with transistor memory/driving circuits in each display pixel, has become the technology of choice. [2]

2) Structure Actually, an organic light-emitting diode is a thin-film light-emitting diode which the emissive layer is an organic compound. [3] The basic structure of OLEDs consists of multilayers of organic materials sandwiched between two electrodes (Figure 1). The total thickness of organic materials is usually of the order of 100 nm and is comparable to the emission wavelength. Therefore emission properties of devices not only depend on intrinsic properties of emitting materials but also often are modified significantly by the optical structures of OLEDs. For instance, a typical OLED, which usually has one reflective metal cathode and one transparent indium tin oxide (ITO) anode on glass substrates (Figure 1), behaves like a weak microcavity. The device exhibits wide-angle interference between directly emitting and reflected radiation, as represented in Figure 2. On the other hand, in some OLED structures, such as microcavity OLEDs and top-emitting OLEDs, etc., two electrodes of strong reflection are implemented into the devices, and one obtains a strong one-dimensional microcavity. The optical characteristics of such strong microcavity structure usually are significantly tailored not only by wide-angle interference but also by multiple-beam interference. [2]

Figure 1. Device structure of a conventional bottom-emitting OLED.

Figure 2. Illustration of optical phenomena in OLEDs---wide-angle interference in noncavity (weak-microcavity) OLED.

3) Mechanism

Electroluminescence in OLEDs is mainly governed by the fluorescence from excited singlet states, which have large transition probabilities providing the major radiative pathway. The key to the operation of an OLED is an organic luminophore. An exciton, which consists of a bound, excited electron and hole pair, is generated inside the emissive layer. To create the excitons, a thin film of the luminophore is sandwiched between electrodes of differing work functions. Electrons are injected into one side from a metal cathode, while holes are injected in the other from an anode. The electron and the hole move into the emissive layer and when the exciton's electron and hole combine, a photon can be emitted. An exciton can be in one of two states, singlet or triplet. Only one in four excitons is a singlet because the singlet–triplet exciton formation statistics is usually given by 1:3 partitions due to the quantum constrains. The materials currently employed in the emissive layer are typically fluorophors, which can only emit light when a singlet

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exciton forms, which reduces the OLED's efficiency. Luckily, the control of the spin statistics could improve the OLED efficiency and help the switching from singlet to triplet device operation. By incorporating transition metals into a small-molecule OLED, the triplet and singlet states can be mixed by spin-orbit coupling, which leads to emission from the triplet state; that is, the ‘‘forbidden’’ triplet state emission can be activated by increasing spin–orbit coupling via dye doping. However, this emission is always red-shifted, making blue light more difficult to achieve from a triplet excited state. It is pointed out that triplet emitters can be four times more efficient than OLED technology. A major challenge in OLED manufacture is tuning the device such that an equal number of holes and electrons meet in the emissive layer. This is difficult because, in an organic compound, the mobility of an electron is much lower than that of a hole. Besides, the analytical Fabry–Perot formulations and rigorous electromagnetic modelling are the basis in discussing optical characteristics and structures of various OLEDs. The concept of the Fabry–Perot cavity gives clearer physical insights but is limited to treatment of characteristics along the forward direction of devices. The Fabry-Perot formulation treats only the optical characteristics along the forward direction of an OLED, in which there is no difference for or polarizations. However, when considering detailed emission characteristics along different viewing angles of an OLED, the dependence on both wavelength and polarization must be considered. Furthermore, in the more accurate treatment of optical characteristics of OLEDs, the influence of cavity on the transition rate of molecular excited states must be included. All these can be considered by adopting the rigorous and full-vectorial electromagnetic modeling of OLEDs. [4]

4) Application Commercial Uses OLED technology is being used in commercial applications such as small screens for mobile phones and portable digital music players (mp3 players), car radios and digital cameras and also in high resolution microdisplays for head-mounted displays. [3] Also, prototypes have been made of flexible and rollable displays which take advantage of OLEDs unique characteristics. OLEDs could also be used as solid state light sources. As by now the OLED efficacies and lifetime go already beyond those of tungsten bulbs, white OLEDs are under worldwide investigation as source for general illumination (e.g. the EU OLLA project). Advanced OLEDs Various advanced organic light-emitting devices (OLEDs) for enhancing performances of OLED displays, particularly active-matrix OLED displays. These include top-emitting OLEDs, inverted OLEDs, high-contrast OLEDs, microcavity OLEDs, and their combinations. Top-emitting OLEDs have been used in AMOLEDs for increasing aperture ratios of pixels. Inverted OLEDs render feasible implementation of AMOLEDs using higher-performance n-type transistors in pixel circuits. High-contrast OLED displays may be realized using high-contrast OLEDs to reduce impact on cost and fabrication. Finally, incorporation of microcavity structures in any type of OLEDs could be used to enhance colour purity, brightness, and efficiency of OLED displays.

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2.2. Organic Transistors Field effect transistors (FETs) using organic materials have low speed, low power, and relatively high operational voltage due to their low mobility and high resistivity. On the other hand the static induction transistor (SIT) is a promising device because it has high speed and high power of operation. Its excellent characteristics arise from the vertical structure with a very short distance between the source, drain, and gate electrodes. Organic light emitting transistors (OLET) combined with the organic SIT and OLED have several advantages, such as low-voltage, high speed operation, high luminous efficiency, and simple fabrication process. During the fabrication of the OLET, the controllability of the grid-type gate electrode is important. The thicker part of the grid gate blocks the current flow from the source to the drain electrode, due to the formation of double Schottky barriers, and the wider gap region of the gate electrode also does not control the current flow effectively. OLETs with a narrow W have low response time because the operational speed is strongly related to the edge feature of the gate electrode. Future developments of organic SITs operating with high power and high speed are expected by optimizing the fabrication process of the gate electrode and by choosing high mobility organic semiconductors.

Figure 3. Cross-sectional structures of: a) OLED and b) OLET. The latter showing a structure similar to

the OLED. [5] The following figure shows an organic field-effect transistor incorporating the electron transport material C60 as the active semiconductor. Maximum field-effect mobility was 0.65 cm2/Vs. [6]

Figure 4. OFET device geometry where source, drain and gate were electrically deposited.

1) Applications Organic thin-film transistors (OTFTs) are useful in a many electronic applications, including active-matrix liquid crystal displays, chemical sensors, electronic paper, and low cost microelectronics due to their low temperature processing. [7]

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Organic semiconductors have been used to fabricate field-effect transistors for application in polymer integrated circuits. Applications have been demonstrated in research environment, like pixel switches, drivers for displays, and integrated circuits. Other new application area of organic transistors is for sensors. Polymeric semiconductors can be made to optimize their response to chemical agents and improve chemical sensing. Organic semiconductors are also suited for printing techniques like ink-jet printing, micro-contact printing, offset and gravure printing. In this way it will be possible to fabricate low cost, low weight and very large surface area sensors.

2) Examples Alcohol sensors OTFTs employing alkoxysubstituted polytertiophene thin films can work as alcohol sensors with sensitivities as good as 0.7 ng/ppm. Using transistor configuration represents superior performance with respect to simple resistor. FETs can provide more measuring parameters, such as threshold voltage changes, mobility variations, as well as a change in the device current. Explosive vapour sensors For detecting the explosive vapours, the high reduction potential of TNT molecules are used. The interaction between the strong acceptor and the p-type polymer creates electronic states, which changes the electrical properties of a field effect transistor. Figure 5 shows a typical organic TFT device. A heavily n-doped silicon wafer is used as a substrate, and the gate dielectric is a 200 nm thick (thermally grown) SiO2 insulating layer. [8]

Figure 5. Schematic of an organic thin film transistor structure.

The chemical structure of the organic semiconductor used in this experiment is shown in Figure 6. [8]

Figure 6

2.3. Organic Lasers Semiconductor lasers are widely used in modern science and technology. Compared with conventional inorganic semiconductors, organic semiconductor offers more

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potential advantages. Lasers based on organic semiconductors will be easier and cheaper to manufacture than existing devices that rely on conventional inorganic semiconductors. The first electrically powered organic laser has been demonstrated by physicists at Bell Laboratories in the US (J H Schön et al). The new device uses a tetracene crystal, which consists of four benzene rings linked together, and conducts electricity well. Laser action has previously been observed in many semiconducting polymers and single crystals, but the emitted light was stimulated by other lasers. [9] And now laser action in organic materials structures is studied in a variety of optically pumped structures that demonstrate the feasibility of organic thin-film materials as active laser medial. The new systems represent that the first electrically pumped organic laser is desirable, because they are more compact and can be integrated into complex electronic circuitry.

1) The working principle of laser Laser is the device that the light is amplified by the stimulated emission of radiation. The customized opinion of laser is generally associated with the strict criteria: 1. A spectral narrow line emission coinciding with resonant cavity modes 2. A spatial coherence 3. Strongly polarized output 4. A lasing threshold 5. A power characteristic of the order of three to seven Generally, a laser device is constructed by the combination of the laser medium and an optical feedback structure. Figure 7 (a) shows the high gain medium that serves as the active material is energetically pumped by either optical excitation (optically pumped laser device) or electrical excitation(electrical injection laser, laser diode) . By virtue of the feedback structure the build-up of the laser oscillation is restricted to a few resonant modes. [10] Figure 7 (b) is the device structure of the organic semiconductor laser. The resonator is based on a periodic modulation in the surface of the substrate, onto which the organic film (blue in the figure) is deposited. Light emission is perpendicular to the sample surface. [11]

(a) (b)

Figure 7 (a) Principal laser device scheme with optical feedback. Figure 7 (b) Device structure of the organic semiconductor laser.

2) Organic laser materials

With the significant improvements of laser instrumentation, there is an ongoing search for advanced laser materials with the aim to improve laser performance and extend

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wavelength range. In particular, recently there is a lot of research for novel solid-state lasing media which open up new possibilities for applications and technologies. The steady progress of chemical synthesis widely opens the spectrum for new organic materials, and new materials can be constructed and designed with specific properties. Furthermore, it is of important interest to develop new designs of compact laser sources attractive to the widespread field of photo-physical and photo-chemical applications. Vacuum deposition of small organic molecules (dyes) provides the advantage of defined film growth, whereas an important perspective in favour of the conjugated polymers is the possibility of directly linking the luminescence characteristics with the electronic transport properties .For the organic approach to solid-state lasers, one can either utilize dilute systems of organic dyes embedded in host matrices or alternatively employ films of highly luminescent conjugated polymers. Both methods have successfully demonstrated optically pumped laser devices. [10]

3) The development of the organic laser Laser source based on the low-cost organic semiconductors which are less expensive than the inorganic semiconductor materials used in today's lasers, so it may be possible to decrease production costs of individual lasers. Alternatively, it is an attractive alternative in the market due to their appealing electronic and optical properties combined with the ease of processing. [12] Organic laser materials, particularly the recently improved materials, will continue to be useful in researching the properties of resonators. There is currently much interest in lasers based on the photonic band-gap concept. [13] Organic gain media have a number of properties that are particularly suitable for these geometries. For example, they can be quite easily made to fill the sub wavelength-sized holes and crevices that are characteristic features of photonic band-gap structures. The accelerated pace of research will make the future of organic solid-state lasers at least as interesting as the past. Looking at the impressive way in which organic solid-state materials have become established as new laser media within the last three years. However previous organic lasers had only been powered by light sources, such as other lasers, which limit their applications. It is not surprising that main efforts now focus on the development of electrically pumped organic lasers. With respect to easy processing, lower cost, and flexibility, electrically pumped organic lasers, which integrated with complex electronic circuitry, are preferred. [12] Hence, the electrically driven lasers based on organic semiconductors might find a wide range of applications in the near future.

2.4. Gas Sensors A real sensor era has started in 1970s during which semiconductor combustible gas sensors, solid electrolyte oxygen sensors and humidity sensors were commercialized for non-professional uses. Since then, extensive efforts have been compiled not only for advancing these sensors but also for developing various new gas sensors, which have been in great demand to make sure safety, health, amenity, environmental reservation, energy saving and so on. In the last few years, there has been an intense search for better-performing organic semiconductors for thin-film-transistors TFT’s, and a range of materials, with quite different chemical and physical properties, are now available. Organic materials such as conducting polymers have also been employed in gas sensors, achieved by assembling many sensors into arrays commonly called electronic noses.

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The present trend to improve pattern recognition is to increase the number of sensors forming the array.

1) Working Principle A Scheme of a typical OTFT structure is presented in Figure 8. An OTFT comprises a highly conductive Si wafer with a dielectric layer on top. The silicon substrate with a gold contact functions as the gate, and the top layer as the gate dielectric. Gold source and drain contacts are photolithographically defined on SiO2 such that W=250 μm is the channel width and L=4μm is the channel length. An organic layer is then deposited on top of the pre-patterned dielectric by vacuum sublimation, spin coating, or solution casting. In the present case, a 50-nm-thick NTCDA film is thermally evaporated.

Figure 8. Scheme of a typical OTFT structure. [14] The reason why an organic TFT (OTFT) can be used as a novel sensor is that four parameters can be measured when the device, held at room temperature, is exposed to chemical species. These parameters are: i. The bulk conductivity of the organic thin film ii. The two-dimensional field-induced conductivity iii. The transistor threshold voltage VT iv. The field effect mobility m The first one is typical of the material employed as sensing layer; the other three are device parameters that can be easily extracted from the experimental data. So measurements of these parameters may allow for recognition of molecular species. [15]

2) Application Gas sensor has the ability to detect and monitor various species of the chemical gas, so it is very important for many applications. The most common one is environmental monitoring. The atmospheric air we live in contains numerous kinds of chemical species, natural and artificial, some of which are vital to our life while many others are harmful more or less. The vital gases like O2 and humidity should be kept at adequate levels in living atmospheres, while hazardous gases should be controlled to be under the designated levels. So we can detect the presence and concentration of the toxic gas which come from leaks. Another application is quality control and industrial monitoring, especially in the industries like food processing, perfume, beverage and some other chemical products.

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2.5. Solar Cells A solar cell is a semiconductor device that converts photons from the sun (solar light) into electricity. [16] The general term for a solar cell including both solar and non-solar sources of light (such as photons from incandescent bulbs) is termed a photovoltaic cell. Fundamentally, the device needs to fulfill only two functions: photogeneration of charge carriers (electrons and holes) in a light-absorbing material, and separation of the charge carriers to a conductive contact that will transmit the electricity. This conversion is called the photovoltaic effect, and the field of research related to solar cells is known as photovoltaics. The first cell was invented in 1813 by Peter Scholey who was experimenting with black powder and a mirror. Until now, photovoltaics (PV) have been dominated by solid-state junction devices, often made of silicon (Grätzel, 2001). [17] Current PV technology, mainly based on silicon wafers, is more expensive than conventional power generation and, despite rapid growth, still only competes in niche and subsidized application. As a result, research is progressing on a wide range of alternative materials in an effort to improve performance and reduce costs. Recent development is focusing on organic devices (usually polymer-based as we know) which can perform as substitution to silicon based devices. Organic devices are born to be attractive because of their advantages. People from not only academic institutes, but also many other such as commercial companies, military organizations, are attracted by the promising future of these materials. High efficiency but cheap costs make them the first choice in this field. Some other properties, such as flexible, easy process, give organic solar cell a wider application area. To understand the electrical behavior, it is useful to create a model which is electrically equivalent, and is based on discrete component which behavior is well-known. An ideal solar cell can be treated as a current source in parallel with a diode. In practice, no solar cell is ideal, so a shunt resistance and a series of resistance should be added to the model. A structural model of a crystalline solar cell can be derived from the traditional design of silicon-based cells. Doped positive silicon layer (p-type) and negative layer (n-type) are combined together, which is called p-n-junction at the boundary. This model is shown in Figure 9. [18]

Figure 9. Model of a crystalline solar cell

Organic solar cells have a similar structure with a crystalline one, by replacing the substrates and molecules contained between substrates with organic devices and molecules. Organic solar cells and Polymer solar cells are built from thin films

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(typically 100 nm) of organic semiconductors such as polymers and small-molecule compounds like polyphenylene vinylene, copper phthalocyanine and carbon fullerenes. Energy conversion efficiencies achieved to date using conductive polymers are very low. [16] The structural model is shown in Figure 10. [19]

Figure 10. Model of an organic solar cell

The application of polymer materials as an alternative to silicon-based semiconductor materials is based on the fact that conducting polymers are capable of photoinduced charge transfer (B.J. Landi, et al, 2005). [20] The polymers which have been used most commonly for this application are poly(3-hexylthiophene)-(P3HT), poly(3-octylthiophene)-(P3OT), and poly(para-phenylenevinylene)-(PPV). Conducting polymers like these have the ability to generate excitons (bound electron–hole pairs) upon optical absorption. Therefore, organic compounds that have similar structure can be tried to apply for this purpose.

Great efforts have been made to develop conductive polymeric solar cells, which may be expected to substitute crystalline technology in the future. The invention of those materials gives the potential reduction in processing cost, improved scalability, and opportunity for lightweight, flexible devices. However, all organic solar cells made to date suffer from degradation upon exposure to UV light, and hence have lifetimes which are far too short to be viable. [16] The conjugated double bond systems in the polymers, which carry the charge, are always susceptible to breaking up when radiated with shorter wavelengths. This is due to the highly bipolar nature of the polymers. Additionally, conductive polymers are highly sensitive to air and moisture, making commercial applications difficult.

Despite of some disadvantages are still in existing, the steps of people in searching more effective organic materials for solar cell devices will never stop. Polymeric solar cells have been emerged as credible alternative to conventional devices, and a promising future can be expected.

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3. Conclusions

In this paper, we evaluated several organic devices for non-display purpose. Their basic principles, advantages and disadvantages, which are proposed as evaluation of the future development, are accessed. As described above, organic semiconductor devices are promising and have great potential in many areas; this without considering the already existing technology for displays. There are good expectations for a major choice in semiconductor areas; these can take up most of the research energy and marketing proportion in the following decade.

4. References

[1] Pfeiffer, M., Doped organic semiconductors: Physics and application in light emitting diodes. Organic Electronics 4 (2003) 89–103. [2] Chung-Chih Wu, et al., Advanced Organic Light-Emitting Devices for Enhancing Display Performances, Journal of Display Technology, Vol. 01, No. 2, December 2005. [3] Internet: http://en.wikipedia.org/wiki/OLED[4] Bergenti, I., et al., Transparent manganite films as hole injectors for organic light emitting diodes, Journal of Luminescence 110 (2004) 384–388. [5] Kazuhiro Kudo, Organic light emitting transistors. Current Applied Physics 5 (2005) 337–340. [6] Haddock, N. J., High mobility C60 organic field-effect transistors. Electronics Letters, 31st March 2005 Vol. 41 No. 7. [7] Zhang, Jian, Organic thin-film transistors in sandwich configuration. Applied Physics Letters, Volume 84, Number 1 5, January 2004. [8] Bentes, E., Detection of explosive vapors using organic thin-film transistors. Faculty of Sciences and Technology, University of Algarve, Faro, Portugal. IEEE, 2004. [9] Internet: http://physicsweb.org/articles/news/4/7/11[10] Kranzelbinder, G., Leising, G., Organic solid state laser. Rep.Prog.Phys, 63 (2000). [11] Internet: http://www.kompetenznetze.de/vdi/generator/navi/en/root,did=99272.html[12] Internet: http://www.lucent.com/press/0700/000728.bla.html[13] Joannopoulos, D. J., et al., Solid State Commun 102, 165 (1997). [14] L. Torsi, A. Dodabalapur. Multi-parameter gas sensors based on organic thin-film transistors, Sensors and Actuators B 67 (2000) 312–316. [15] Noboru Yamazoe. Toward innovations of gas sensor technology, Sensors and Actuators B 108 (2005) 2–14. [16] Internet: http://en.wikipedia.org/wiki/Solar_cell[17] Grätzel, Michael., Photoelectrochemical cells. Nature 414, 338-344 (2001) [18] Internet: http://www.solarserver.de/wissen/photovoltaik-e.html[19] Internet: http://www.org.kemi.uu.se/Research/HelenaG/hgsolcell2a.shtm[20] Landi, B.J., Castro, S.L., Ruf, H.J., et al., Cd Se quantum dot-single wall carbon nanotube complexes for polymeric solar cells., Solar Energy Materials & Solar Cells 87, 733-746 (2005). [21] Internet: http://www.howstuffworks.com/solar-cell.htm[22] Internet: http://www.livescience.com/technology/041224_solar_panels.html

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