Transparent conducting film
Transparent conducting films (TCFs) are thin films of optically
TCFs for
Transparent conducting films are typically used as
Transparent conducting oxides
Overview
Transparent conductive oxides (TCO) are doped metal oxides used in optoelectronic devices such as flat panel displays and photovoltaics (including inorganic devices, organic devices, and
To date, the industry standard in TCOs is ITO, or indium tin oxide. This material boasts a low resistivity of ~10−4 Ω·cm and a transmittance of greater than 80%.[clarification needed][15] ITO has the drawback of being expensive. Indium, the film's primary metal, is rare (6000 metric tons worldwide in 2006), and its price fluctuates due to market demand (over $800 per kg in 2006).[16] For this reason, doped binary compounds such as aluminum-doped zinc oxide (AZO) and indium-doped cadmium oxide have been proposed as alternative materials. AZO is composed of aluminum and zinc, two common and inexpensive materials, while indium-doped cadmium oxide only uses indium in low concentrations. Several transition metal dopants in indium oxide, particularly molybdenum, give much higher electron mobility and conductivity than obtained with tin[17] and Ta is a promising alternative dopant for tin oxide.[18] Other novel transparent conducting oxides include barium stannate and the correlated metal oxides strontium vanadate and calcium vanadate.
Binary compounds of metal oxides without any intentional impurity doping have also been developed for use as TCOs. These systems are typically n-type with a carrier concentration on the order of 1020 cm−3, provided by interstitial metal ions and oxygen vacancies which both act as donors. However, these simple TCOs have not found practical use due to the high dependence of their electrical properties on temperature and oxygen partial pressure.[13]
In current research, labs are looking to optimize the electrical and optical characteristics of certain TCOs. Researchers deposit TCO onto the sample by using a sputtering machine. The targets have been changed and researchers are looking at materials such as IZO (Indium Zinc Oxide), ITO (Indium Tin Oxide) and AZO (Aluminum Zinc Oxide), and they are optimizing these materials by changing parameters within the sputtering deposition machine. When researchers vary parameters such as concentration of the gases within the sputtering, the pressure within the sputtering machine, power of the sputtering, and pressure, they are able to achieve different carrier concentrations and sheet resistivities within the machine. Carrier concentrations affect the short circuit current of the sample, and a change in sheet resistivity affects the fill factor of the sample. Researchers have varied parameters enough and found combinations that will optimize the short circuit current as well as the fill factor for TCOs such as indium tin oxide.[citation needed]
Fabrication
Doped metal oxides for use as transparent conducting layers in photovoltaic devices are typically grown on a
For AZO thin film deposition, the coating method of reactive
Theory
Charge carriers in these n-type oxides arise from three fundamental sources: interstitial metal ion impurities, oxygen vacancies, and doping ions. The first two sources always act as electron donors; indeed, some TCOs are fabricated solely using these two intrinsic sources as carrier generators. When an oxygen vacancy is present in the lattice it acts as a doubly charged electron donor. In ITO, for example, each oxygen vacancy causes the neighboring In3+ ion 5s orbitals to be stabilized from the 5s conduction band by the missing bonds to the oxygen ion, while two electrons are trapped at the site due to charge neutrality effects. This stabilization of the 5s orbitals causes a formation of a donor level for the oxygen ion, determined to be 0.03 eV below the conduction band.[21] Thus these defects act as shallow donors to the bulk crystal. Common notation for this doping is Kröger–Vink notation and is written as:
Here "O" in the subscripts indicates that both the initially bonded oxygen and the vacancy that is produced lie on an oxygen lattice site, while the superscripts on the oxygen and vacancy indicate charge. Thus to enhance their electrical properties, ITO films and other transparent conducting oxides are grown in reducing environments, which encourage oxygen vacancy formation.
Dopant ionization within the oxide occurs in the same way as in other semiconductor crystals. Shallow donors near the conduction band (n-type) allow electrons to be thermally excited into the conduction band, while acceptors near the valence band (p-type) allow electrons to jump from the valence band to the acceptor level, populating the valence band with holes. It is important to note that carrier scattering in these oxides arises primarily from ionized impurity scattering at high dopant levels (>1 at%). Charged impurity ions and point defects have scattering cross-sections that are much greater than their neutral counterparts. Increasing the scattering decreases the mean-free path of the carriers in the oxide, which leads to low electron mobility and a high resistivity. These materials can be modeled reasonably well by the
where aH* is the mean ground state Bohr radius. For ITO, this value requires a minimum doping concentration of roughly 1019 cm−3. Above this level, the conduction type in the material switches from semiconductor to metallic.[21]
Transparent conducting polymers
Applications
Transparent conductive polymers are used as electrodes on light emitting diodes and photovoltaic devices.[26] They have conductivity below that of transparent conducting oxides but have low absorption of the visible spectrum allowing them to act as a transparent conductor on these devices. However, because transparent conductive polymers do absorb some of the visible spectrum and significant amounts of the mid to near IR, they lower the efficiency of photovoltaic devices.[citation needed]
The transparent conductive polymers can be made into flexible films making them desirable despite their lower conductivity. This makes them useful in the development of flexible electronics where traditional transparent conductors will fail.
Poly(3,4-ethylenedioxythiophene) (PEDOT)
Poly(3,4-ethylenedioxythiophene) (PEDOT) has conductivity of up to around 1,000 S/cm.[27] Thin oxidized PEDOT films have approx. 10% or less absorption in the visible spectrum and excellent stability.[28] However, PEDOT is insoluble in water making processing more difficult and costly.
The bandgap of PEDOT can be varied between 1.4 and 2.5 eV by varying the degree of π-overlap along the backbone.[28] This can be done by adding substituents along the chain, which result in steric interactions preventing π-overlap. Substituents can also be electron-accepting or donating which will modify the electronic character and thus modify the bandgap. This allows for the formation of a wide bandgap conductor which is transparent to the visible spectrum.
PEDOT is prepared by mixing EDT monomer with an oxidizing agent such as FeCl3. The oxidizing agent acts as an initiator for polymerization. Research has shown that increasing the ratio of [FeCl3]/[monomer] decreases the solubility of the PEDOT.[28] This is thought to be a result of increased crosslinking in the polymer making it more difficult to dissolve in a solvent.
Poly(3,4-ethylenedioxythiophene) PEDOT: poly(styrene sulfonate) PSS
Doping PEDOT with poly(styrene sulfonate) can improve the properties over the unmodified PEDOT. This PEDOT:PSS compound has become the industry leader in transparent conductive polymers. PEDOT:PSS is water-soluble, making processing easier.[29] PEDOT:PSS has a conductivity ranging from 400 to 600 S/cm while still transmitting ~80% of visible light.[30] Treatment in air at 100 °C for over 1000 hours will result in a minimal change in conductivity.[31] Recently, it was reported that the conductivity of PEDOT:PSS can be improved to be more than 4600 S/cm.[32]
PEDOT:PSS is prepared by polymerizing EDT monomer in an aqueous solution of PSS using Na2S2O8 as the oxidizing agent. This aqueous solution is then spin coated and dried to make a film.[31]
Poly(4,4-dioctyl cyclopentadithiophene)
Poly(4,4-dioctyl cyclopentadithiophene) can be doped with
DDQ itself has a conductivity of 1.1 S/cm. However, DDQ-doped poly(4,4-dioctyl cyclopentadithiophene) also tends to decrease its conductivity in air. DDQ-doped polymer has better stability than the iodine-doped polymer, but the stability is still below that of PEDOT. In summary, poly(4,4-dioctyl cyclopentadithiophene) has inferior properties relative to PEDOT and PEDOT:PSS, which need to be improved for realistic applications.
Poly(4,4-dioctyl cyclopentadithiophene) is solution polymerized by combining monomer with iron(III) chloride. Once the polymerization is complete the doping is done by exposing the polymer to iodine vapor or DDQ solution.[33]
Carbon nanotubes
Advantages
Transparent conductors are fragile and tend to break down due to fatigue. The most commonly used TCO is Indium-Tin-Oxide (ITO) because of its good electrical properties and ease of fabrication. However, these thin films are usually fragile and such problems as lattice mismatch and stress-strain constraints lead to restrictions in possible uses for TCFs. ITO has been shown to degrade with time when subject to mechanical stresses. Recent increases in cost are also forcing many to look to carbon nanotube films as a potential alternative.
Preparation of CNT thin films
The preparation of CNT
In order to separate the grown tubes, the CNTs are mixed with surfactant and water and sonicated until satisfactory separation occurs. This solution is then sprayed onto the desired substrate in order to create a CNT thin film. The film is then rinsed in water in order to get rid of excess surfactant.
One method of spray deposition used for CNT film creation is an ultrasonic nozzle to atomize CNTs in solution to form PEDOT layers.[36][37]
By optimizing spray parameters, including surfactant, drop size (dictated by the ultrasonic nozzle frequency) and solution flow rate, sheet resistance characteristics can be tuned. Due to the ultrasonic vibration of the nozzle itself, this method also provides an additional level of sonification during the spray process for added separation of agglomerated CNTs.
Comparing CNTs to TCOs
CNTs can also be used in addition to transparent conducting oxides (TCOs) in
As stated previously, nanotube chirality is important in helping determine its potential aid to these devices. Before mass production can occur, more research is needed in exploring the significance of tube diameter and chirality for transparent conducting films in photovoltaic applications. It is expected that the conductivity of the SWNT thin films will increase with an increase in CNT length and purity. As stated previously, the CNT films are made using randomly oriented bundles of CNTs. Ordering these tubes should also increase conductivity, as it will minimise scattering losses and improve contact between the nanotubes.
Conducting nanowire networks and metal mesh as flexible transparent electrodes
Randomly conducting networks of wires or metal meshes obtained from templates are new generation transparent electrodes. In these electrodes, nanowire or metal mesh network is charge collector, while the voids between them are transparent to light.[39] These are obtained from the deposition of silver or copper nanowires, or by depositing metals in templates such as hierarchical patterns of random cracks, leaves venation and grain boundaries etc. These metal networks can be made on flexible substrates and can act as flexible transparent electrodes.[40] For better performance of these conducting network based electrodes, optimised density of nanowires has to be used as excess density, leads to shadowing losses in solar cells, while the lower density of the wires, leads to higher sheet resistance and more recombination losses of charge carriers generated in solar cells.[41][42]
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