Photovoltaic materials encompass the technology of converting light into energy in the form of electricity. A photovoltaic device, also known as a solar cell, promises an ecologically benign source of energy by converting sunlight into electricity. The photovoltaic effect has been known since the French scientist Edmund Becquerel first described it in 1839. This research remained an interesting, but unused, phenomenon until the 1950s when solar cells were utilized by the U.S. space program. Considerable interest in photovoltaic devices has been generated in recent decades as scientist search for a clean alternative to fossil fuel energy generation. In contrast to photovoltaics, display technology is concerned with the conversion of energy into (visible) light. A variety of display technologies and devices are known.
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An organic electroluminescent (EL) device is a light-emitting device similar to light-emitting diodes (LEDs) made from semiconductors. Unlike LEDs, organic EL devices have large emitting areas and high brightness levels. Luminance exceeds 1000 cd/m2 below 10 V, with an external quantum efficiency of more than 1% photon/electron.1 Today, this type of device has a luminance of more than 100,000 cd/m2. It is about 10 times brighter than a common fluorescent lamp.2 In addition, various colors such as blue, green, red, and white can be obtained.3-8
Figure 1 shows various device structures. In these devices, organic emitter layers are sandwiched between two electrodes, and electric energy is transformed into light through the excitation of the organic molecules. Excitation mechanisms involve the recombination (reaction) of charge carriers such as electrons (radical anion) and holes (radical cation) that are injected into the organic layers from the electrodes. Hence, it is necessary for the organic component materials to be charge-transporting as well as fluorescent. This is shown in Figure 1 as a single-layer-type structure.
Figure 1. Typical device structures. The total thickness of the organic layers is ~ 1000 Å
Conjugated polymer systems have been used as a hole transport layer in double layer-type organic EL devices that have an electron transporting emitter layer. Such double-layer-type devices have a layered structure composed of the materials with different carrier transport properties: one transports holes, and the other transports electrons. Therefore, electrons and holes are injected into the organic layers from the electrodes through the corresponding carrier transport layer, and the recombination of the carriers occurs at or near the interface between the two organic layers. An EL device used for this purpose is a double-layer structure with a polymer layer as the hole transport layer and a tris(8-quinolinolato)aluminum(III) (Alq) complex layer as the emitting layer (the double-layer-type A in Figure 1). Alq is a luminescent metal complex with electron transport properties and has been used as an emitter layer.1,2 A polymer solution containing an appropriate amount of polymer is first dip coated or spin coated onto an indium-tin-oxide (ITO) coated glass substrate, except for the plasma polymerization system. The thickness of the polymer layer is usually 200-400 Å. Then, Alq is vacuum deposited at 3 x 10-5 Torr onto the polymer layer with a thickness of 500-700 Å. Finally, 2000 Å of magnesium and silver (10:1) is codeposited on the Alq layer surface as the top electrode at the same vacuum pressure. A small amount of Ag is necessary to stabilize the cathode. The deposition rate for Alq is 3 Å/s and for Mg:Ag is 11 Å/s. Charge transport polymers can also be used as an emitter layer in organic electroluminescent devices. For example, poly(vinylcarbazole) or PVK can be used as an emitter layer in multilayer-type devices in conjunction with appropriate electron transport layers. This device structure is depicted as the double layer-type B in Figure 1.
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Known more colloquially as plastics, semiconducting polymers are part of a broader class of organic materials that includes small molecules and oligomers. Semiconducting polymers in particular are generating excitement among scientists because of their potential commercial electronic applications.
To generate light with these materials, a thin film of semiconducting polymer is deposited on a glass or plastic substrate and sandwiched between two electrodes. Electrons and holes are then injected from the electrodes, and the recombination of these charge carriers results in luminescence. The bandgap (the energy difference between the valence and conduction bands of the polymer) determines the wavelength of the emitted light.13 By far, the most lucrative near-term application for light emitting polymers (LEPs) has been touted to be as small flat-panel displays.14
The Nobel Prize in Chemistry for 2000 was awarded to three scientists–Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa–the codiscoverers of electrically conductive polymers. Their work opened up polymer electronics, expected to be the electronics technology of the near future and likely finding wide applications in fields ranging from molecular electronics to foldable electronic newspapers.15
Electronic polymers are plastic materials with metallic and semiconductor characteristics, a combination of properties not exhibited by any other known material. The polymer possesses a conjugated p-electron system along its backbone, giving it the ability to support positive and negative charge carriers with high mobilities along the chain. The doping of semi-conducting, conjugated polymers such as polyaniline and polypyrrole leads to the presence of electronic states in the band gap (hopping states). At sufficient dopant concentrations, the band gap effectively disappears and the polymer acts as a metal with high conductivities; hence the term "synthetic metal".16,17
In addition to the change in electrical conductivity, the controlled addition of known, usually small (<10%) and non-stoichiometric quantities of chemical species can result in dramatic changes in the electrochemical, electronic, electrical, magnetic, optical, and structural properties and dimensions of the polymer. The resulting material lends itself to varied uses in sensors, MEMS-based actuators, batteries, corrosion inhibition, EMI shielding, and electrochromic camouflage coatings.18-21
The exploration of natural or pristine conjugated polymers for semiconductor device applications such as photovoltaic cells, field effect transistors, light-emitting diodes (LEDs), and Schottky diodes has become a major focal point of interest.22 Furthermore, the application of semiconducting conjugated polymers, that are based on polythiophene, PPV, and polyfluorene (PFO), among others in emerging display technologies of organic electroluminescence has generated immense interest. This is due to enormous advantages over classical semiconductors both in terms of ease of fabrication as well as design of new materials with different band gaps and electron affinities.23-27 The latter capability is crucial for constructing LEDs that are tunable over the entire visible spectrum with high luminescence quantum efficiency.
Polythiophenes are one of the more extensively studied classes of p-conjugated systems. Both the conducting and semiconducting forms are very stable and readily characterized. Applications of these materials in light-emitting devices, field effect transistors, as well as other molecular electronic devices have been fueled by the improved solubility and ease of processing of mono-, di-, and ring-substituted polythiophenes, while simultaneously allowing the electronic band gap to be tuned.28-32
The popularity of polyfluorenes (PFOs) as LEPs is due to their efficient blue photo- and electroluminescence coupled with their high chemical stability.26 The related poly(fluorenylene ethynylenes) (PFEs) have an intense solid-state fluorescence. Their emission spectra are reminiscent of PFOs.27
Since the discovery that conjugated polymers can be used as electroluminescent materials,23 PPV has been one of the most studied series of light-emitting polymers (LEPs) due to its excellent luminescent and mechanical properties. The PPV family of polymers serves as a prototypical conjugated polymer class for application as well as for fundamental understanding of the electronic processes in conjugated polymers. By a suitable modification of the chemical structure, the goal is to achieve electroluminescence that spans the visible and near-infrared regions.
Since unsubstituted PPV is insoluble and intractable, the p-xylylene derivative (Aldrich product 53,882-5) is polymerized to form the soluble polymeric precursor (Aldrich products 54,077-3 and 54,076-5), which is then cast into a film and thermally converted into a film of the intractable unsubstituted PPV on a suitable substrate.33-35
In order to improve the processability of PPV, flexible side chains are introduced on the polymer back-bone resulting in PPV derivatives such as MEH-PPV and others. A phenyl-substituted PPV, BEHP-PPV (Aldrich product 54,661-5), is more stable than MEH-PPV and has a higher solubility.36 Recently, a semiconjugated organic polymer, PmPV, (Aldrich product 55,516-9, and others) was used to purify carbon nanotubes as well as in an LED device.37,38
Other potential device applications of PPVs include photodiodes, photodetectors, and photovoltaic cells. Large-area photodiodes based on C60-doped MEH-PPV showed excellent visible-UV sensitivity. Optical recording devices based on PPVs, current-controlled electrical switching from a combination of PPV and ferroelectric KH2PO4, and metal-insulator-semiconductor field-effect transistors based on DMPPV are among the novel device applications of PPVs.
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Light-Emitting Polymer (LEP) Displays
LEP based display technology is widely accepted as the most likely replacement for the cathode-ray tube and liquid-crystal displays (LCD). It offers several significant advantages over both technologies such as enhanced clarity, unlimited viewing angles, faster image-refresh rates, thinner profile, lighter weight, and the availability of all colors of the visible spectrum. In addition, LEPs can achieve high brightness at low drive voltages and current densities, which results in lower power consumption than other technologies. Moreover, polymer materials can be processed into large-area thin films using simple and inexpensive technology, unlike inorganic LEDs, which require a highly doped semiconductor layer for ohmic contact.9 Hence, large-area pixellated displays made from a single sheet are possible. Current research focuses on the use of polymer materials to make electroluminescent displays with both passive- and active-matrix technologies.10 Figure 2 shows a cross section of three addressable subpixels (red, green, and blue) in a polymer light-emitting display. Each pixel of the display consists of the glass substrate, indium tin oxide (ITO) anode, a hole conduction layer, the light-emitting polymer layer, and the top cathode (a low-work function metal). In an active-matrix display, the array is divided into a series of row and column lines, with each pixel formed at the intersection of a row and column line, just as in a passive-matrix display. Each pixel now consists of an organic light-emitting diode (OLED) in series with a thin-film transistor (TFT). The TFT is a switch that can control the amount of current flowing through the OLED. In an active-matrix OLED display (AMOLED), information is sent to the transistor in each pixel, dictating the brightness of the pixel. The TFT then stores this information and continuously controls the current flowing through the OLED. In this manner the OLED is operating continuously, avoiding the need for the very high currents necessary in a passive-matrix display. By contrast, to illuminate any particular pixel line a passive-matrix display, electrical signals are applied to the row line and column line. The more current pumped through each pixel diode, the brighter the pixel.11 The technology used in an active-matrix screen tends to be a more expensive but better quality than a passive-matrix display.
Figure 2. Cross-section of three addressable subpixels in a flat-panel display less than 100 mm thick, with a thin film of semiconducting polymer sandwiched between electrodes. (Image courtesy of Cambridge Display Technologies)
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Fabrication methods depend on the materials used. For example, in molecular systems, based on photoluminescent dyes, organic thin layers can be formed by vacuum deposition. However, the crystallization or aggregation of the vacuum-deposited molecules sometimes causes the destruction of the layered structure and the degradation of the device.12 In contrast, polymeric materials have stronger mechanical strength and are less crystalline than low molecular weight materials. Also, thin polymer films can be formed by various coating techniques such as spin coating and dip coating. It is therefore reasonable to use macromolecular systems.
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Organic Photonic Materials
There has been extensive research over the past several years on active organic materials, both polymeric and small molecule. Applications that have emerged include light-emitting devices (OLEDs, PLEDs), solar cells, thin-film transistors, and organic semiconductor lasers,39,40 all of which require the growth of organic thin films. Of the numerous techniques that can be used to deposit organic thin films (spin casting, vacuum deposition, self assembly, etc.), small molecule organics are particularly amenable to vacuum deposition enabling precise thickness control, extremely high purity, and a controlled environment for device fabrication.39 Aldrich offers a comprehensive selection of such materials for multilayer organic electroluminescent and photovoltaic devices.
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More recently, there has been a surge of new device applications for these luminescent materials as highlighted below.
As Emissive Dopants in Organic Electroluminescent (EL) Devices
- Voltage-tunable multicolor emission with enhanced luminance (~1000 cd/m2) was observed using varying amounts of DCM dye (Aldrich product 41,049-7) in a polymer light-emitting diode (PLED).41
- When used as the emissive dopant in an Alq host layer, DMQA Aldrich product 55,758-7) provided improved operational stability.42
- Several coumarin derivatives have been employed as dopants in PVK based multilayered EL devices.43,44
As Dye Sensitizers
- Fluorescent dye-sensitized photo-electrochemical cells showed an enhanced photocurrent with remarkable stability.45
- Cyano-coumarin derivatives have been used as photosensitizers to generate high-speed photopolymer coating layers.46
Other Device Applications
- 3D optical storage media.
- Laser active media.
- Optical, thin-film, polymeric sensors of aqueous halide ions based on fluorescence quenching.47
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Charge Transport Agents
In addition to long lifetime, high quantum efficiency of the device is a critical issue in the commercialization of PLEDs. An effective method for improving device efficiency would be to balance injected electrons and holes by adding an electron transport layer (ETL), which is commonly used in small-molecule-based organic LEDs.48
Charge transport polymers have been shown to be useful as a component material in organic electroluminescent devices. A hole transport layer and an emitter layer can be made from polymers. These polymer-based devices show different characteristics from the devices based on vacuum-deposited molecular materials. Mechanical strength and processability of the polymer materials are the major advantages over molecular materials.
Such electron transporters, hole transporters, and other charge-transport/photosensitizing materials are available from Aldrich. The hole transfer rate constant of poly(1-vinylnaphthalene), given below, is twice as large as that of poly(2-vinylnaphthalene) (Aldrich products 19,193-0: Avg. Mw ca. 100,000; 46,194-6: Avg. Mw ca.175,000).49 Polyaniline has been used not only as the anode in flexible PPV-LEDs, but also as an efficiency-enhancing interfacial layer between MEH-PPV and ITO.50-51
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Inorganic LED Materials
Red light emitting diodes (LEDs) based on gallium arsenic phosphide (GaAsP) dates back to the early 1960s. The mid 1970s brought about green LEDs and blue LEDs were identified in the 1980s. These higher wavelength LEDs were dim and had short lifetimes. Not until group 13-15 compounds (such as indium gallium aluminum phosphide) were synthesized in the 1990’s did bright-blue LEDs become available. Efficient, modern LEDs have lifetimes of over 100,00 hours or more than a decade of continuous use.
With the recent development of bright blue LEDs and the inherent, fast switching times of LEDs (in the order of 300-400 nanoseconds), light emitting diode clusters can now be used for large scale displays. In addition to their brightness, LED displays also exhibit considerable reliability and ruggedness as well as low power requirements making them excellent materials for open-air displays such as those seen in your local sports arena.
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Indium tin oxide (ITO) is electrically conductive and transparent in the near-IR to UV range (300 nm to >2600 nm). Due to its optical transparency (85-90% transmission in the visible range), ITO-coated glass slides are ideal electrodes for the hole-injecting contact in optical light-emitting diodes (OLEDs).52-54 The sensitivity of visible spectroelectrochemical instruments can be increased 200 times or more using ITO slides in an integrated optic waveguide.55 ITO slides are useful deposition substrates for optoelectric materials56 and liquid crystal displays.57 Phosphor-coated indium tin oxide plates are also used in flat panel displays.58 Aldrich now offers indium tin oxide coated float glass, aluminosilicate glass, and PET coated slides.
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