Dictionary of Terms. Specific Terms: P.
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Photopolymers is a general term for a large group of organic polymeric/oligomeric materials that may undergo photochemical reactions of polymerization or vice-versa, depolymerization, in the spots subjected to light-irradiation. Polymerization gives rise to higher molecular weight or cross-linked polymers that make the irradiated spots firmer, resistant to the action of solvents, whereas the rest of the non-irradiated, 'pristine' material can be easily removed with a solvent. In contrast, the photochemical depolymerization brakes polymer linkages and makes the material softer and solvent-soluble. In this case, the material from the irradiated spots can be easier removed. Both types of materials are known as positive and negative photoresists correspondingly, which are used for the patterning of integrated circuits in microelectronics in various lithographical methods. Irradiation can also result in changing of optical properties, such as optical density or luminescent responce of a photopolymer that is widely used in optoelectronics and optical data storage. 'Burning' of a photopolymer in CD, DVD discs by means of laser is patterning of arrows of irradiated dots, whose optical response on 'reading' light is different from that of surrounding 'pristine' material. Conventional optical discs store information on a two-dimensional surface. Use of holographic techniques could allow storage of information in a three-dimensional volume that would dramatically increase data storage density. For recent progress in 3-dimensional optical data storage please see one of our news articles.
Photovoltaics are semiconductor devices that generate electric current under the action of any light. Solar cells are semiconductor devices that are used for the direct transformation of the solar light to electricity.
Monolayered inorganic solar cells are composed of an inorganic semiconductor (usually silicon) sandwiched between two metallic electrodes with different electrochemical potentials (Fermi levels), where one of electrodes is semitransparent or grid-like. They can generate voltage when light penetrates into the semiconductor. A photon breaks a covalent bond between silicon atoms and 'kicks' one electron out. The atom that loses the electron gets a positive charge and becomes a hole. Further electrons are transported to the anode and holes to the cathode driven by a difference in electrochemical potentials of the electrodes. Monolayered devices usually possess rather low power conversion efficiency. Bilayered devices (scheme below) are composed of two types of semiconductor: p-type and n-type. In this case, the charge separation occurs near the border between p-type and n-type semiconductors and it is much more efficient, because the charges are 'captured' and held strong by 'host' semiconductors. Triple-layered and multi-layered devices are also known. Inorganic solar cells are usually chemically and thermally stable devices. Power conversion efficiencies of modern inorganic solar cells reach 30% barrier. This is a major advantage of inorganic solar cells over organic devices. However, inorganic solar cells also possess their own drawbacks. Their production is costly and energy consuming because it requires thorough purification procedures. Therefore, inorganic cells still cannot provide cost-efficient alternative to other 'green' energy sources, such as hydropower.
Operation of organic solar cells is mechanistically more complex. First, a molecule of an organic compound absorbs a photon and forms an excited state (exciton). Further, the exciton diffuses to a junction border between n- and p-types of semiconductor where it dissociates to form free charge carriers. Organic p- and n-transporters are also known as donors and acceptors correspondingly. If there is no junction border nearby, the exciton may recombine (decay) via photoluminescence, or thermally, back into the ground state of the molecule. This is the main reason, why mono- and bilayered (scheme above) organic solar cells were poorly performing devices until a new concept of bulk heterojunction has been introduced. Bulk heterojunction is a tight blend of a p-type conductor (donor), and n-type conductor (acceptor) in the photoactive layer of a device, where the concentration of each component often gradually increases when approaching to the corresponding electrode. This affords vast expansion of p-n-junction's total surface and strongly facilitates the exciton's dissociation. The implication of this concept in practice allowed increase of power conversion efficiencies of up to 5% for all-organic solar cells.
In spite of the impressive results achieved with the realization of the bulk-heterojunction concept, the organic cells and materials still need to be strongly improved in order to find commercial application. Advantages of organic solar cells are: lightweight, environmentally friendly, no requirements for rare metals and minerals, no high temperatures and purity demand on the production stage, potentially inexpensive, virtually unlimited room for further material modification and improvement. Hybrid (blended organic-inorganic) solar cells Hybrid organic-inorganic solar cells combine advantages of both 'all organic' (no strict requirements to high material purity, similar to 'bulk heterojunction' structure, potential for flexibility and large area applications) and 'all inorganic' (good environmental and thermal stability) cells. Two major types of hybrid cells are known that are somewhat different in structure and mechanism of operation: micro- or nanocrystalline hybrid cells and dye-sensitized cells
Micro- or nanocrystal-polymer blend hybrid solar cells: Inorganic materials were chosen to play a role of n-transporters in hybrid cells because they usually posses much higher electron mobility, stability, and overall efficiency over organic n-conductors. Recent studies demonstrate, that hybrid cells often exhibit power conversion efficiencies approaching 5% barrier that is close to that of the best devices containing fullerenes as n-conductors and superior over those containing other organic n-transporters. However, a disadvantage of inorganic crystalline hybrid cells should be mentioned. Inorganic crystals often possess low miscibility with organic compounds that reduces overall efficiency and hampers fabrication of devices, and may result in subsequent phase separation and further reduction of the cell efficiency over a time. Dye-sensitized solar cells: Mechanism of operation of dye-sensitized solar cells includes absorption of light by a molecule of a dye, which forms highly energetic exciton. The latter surrenders the excited electron to a particle of an inorganic semiconductor where it is further transferred to the anode. The former organic exciton becomes a positively charged cation that is reduced back to the initial dye molecule by an electron coming from a special reducing medium - redox electrolyte. The oxidized molecules of the redox electrolyte are reduced in turn by electrons from the cathode, when the circuit is connected. Initially, the solutions of iodine-iodide mixtures in volatile solvents, usually acetonitrile, have been used as redox-electrolytes: Electrolyte: J2 + J- ↔ J3- Recently, solvent-free redox electrolytes, prepared from ionic liquids (liquid ionic organic compounds) or from ionically-conducting polymer-nanocrystal blends were found to be very efficient. Although photoelectrochemical cells may operate without of an organic dye, the efficiency of such cells is very low due to low light-harvesting ability of inorganic n-conductors that normally absorb light of only high-energy ultraviolet region of the solar spectrum. The introduction of an organic dye allows for vast increase of absorption ability of the cells that expands through almost entire region of the solar spectrum.
Dye-sensitized solar cells showed power conversion efficiencies of up to 12%, that are much superior over those of 'all organic' and crystal-polymer blend hybrid devices. At the same time, DSCs were found to perform with high incident photon to current conversion efficiencies (IPCE), >80% over a large region of the solar spectrum, which is significantly wider than that of 'all inorganic' devices. Therefore, DSCs are already considered possessing high potential for commercial applications. The most important characteristics of photovoltaics are: Organic materials for PVs, general overview: Summary of conducting oligomers for small molecule PVs Latest developments in organic materials for photovoltaics: Efficient planar-heterojunction solar cells based on a-8T and GaAs Detailed and comprehensive scientific information on organic photovoltaics may be found in books. Interesting scientific information on the balk-heterojunction all-organic solar cells is summarized also in a comprehensive review: Ref.. Summary of the most important classes of conducting polymers: Polyacetylene was first obtained in a form of nonconductive powder by Natta and coworkers, and Luttinger in early 1960s. Shirakawa and Ikeda were the first scientists who succeeded to obtain conductive films of the polymer (1978). Preparation: Polyacetylene possesses the simplest structure among fully conjugated polymers, it is synthesized by polymerization of acetylene. For the comprehensive scientific reviews on the synthesis of polyacetylenes, please, see ref., page 197.
Properties and applications: Polyacetylenes posses a very low band gap (~1.6 eV), and metallic-type conductivity in a doped state. The polymers are very sensitive to the action of air, moisture, or light and conductivity may be strongly altered with time. They are also insoluble and infusible. Polyacetylenes found limited use as conductive powders and additives, however they are not suitable for semiconductor or optoelectronic applications. Conductivity of polyaniline salts was reported for the first time by Green and Woodhead as long ago as 1910. Later it was rediscovered by Epstein and MacDiarmid, who studied the material and its properties in detail. Triarylamine oligomers are 'shorter' analogs of polyanilines for use in 'small-molecule' electronics. Preparation: Polyanilines are usually synthesized by chemical or electrochemical polymerization of aniline.
Properties and applications: Polyaniline bases are insulators, soluble in many organic solvents, whereas polyaniline salts are conductors with metallic-type conductivity, insoluble and infusible. The salts are used as highly conductive powders and additives in xerocopying, corrosion inhibition and other useful applications. Some types of salts were also found to be optically transparent and suitable for the replacing of ITO in treatment of glass substrates. Recently, truly metallic conductivity of a highly ordered emeraldine salt has also been discovered.
Polypyrroles may possess metallic-type conductivity in a doped state along with higher stability and variability of properties than polyacetylenes. Preparation: Polypyrolles are usually synthesized by chemical-oxidative, or electrochemical polymerization of pyrroles. For the comprehensive scientific reviews on the synthesis of polypyrroles, please, see ref., page 259.
Properties and applications: Inexpensive simplest polypyrrole (1) is insoluble and infusible, possesses metallic-type conductivity in a doped state. It has been used in a form of highly conductive powder for various applications. Soluble 3-alkylpyrroles (3) are still too expensive to find significant commercial application, whereas variably N-functionalized derivatives (2) have been tested for chemical and biological sensors, artificial muscles, and other interesting and prospective uses.
Polythiophenes are one of the most valuable types of conducting polymers that may be easily modified to afford a variety of useful electrical and physical properties such as solubility, electrical conductivity, mobility and others. Oligothiophenes are 'shorter' analogs of polythiophenes for small-molecule electronic applications. Preparation: Polythiophenes are usually synthesized by chemical-oxidative, or electrochemical polymerization of thiophenes. For the synthesis of highly regioregular or structurally ordered polymers, some other methods are also used, such as the ones involving directed reductive couplings of selectively-halogenated precursors, or template polymerization methods. For the comprehensive scientific reviews on the synthesis of thiophene polymers, please, see ref. 1, pages 225, 259, 277, 311; ref. 3; ref. 1; ref. 2; ref. 3 Structure: The simplest member of the family: polythiophene (1) forms linear chains in the ideal case. Depending on a method of the preparation, the polymer may contain variable percentage of defects with 2,4, or 3,4-type ring connections, the same as in the case of polypyrrole but at lesser extent. 3-Alkyl-substituted polythiophenes (2-3) usually free of this type of defects. In addition, they possess lower band gap and better electronic properties. They are widely varied in a level of regularity of the relative position of alkyl substituents: from regiorandom (about 50 to 50% of 'head to head' and 'head to tail': 3) to 100% regioregular (RR) form (100% 'head to tail' 2). In 3,4-disubstituted polymers, like PEDOT 4, the monomeric units may be 'twisted' out of the conjugation plane due to steric interactions between substituents. The 'twisting' of the thiophene rings usually reduces conjugation and, therefore, conductivity. However, it may give rise also to some very useful properties such as increased ionization potential and stability.
Properties and applications: Polythiophenes usually do not possess metallic type conductivity even in a doped state. Therefore, they are much more commonly used as organic semiconductors. Many of them possess also good luminescent, nonlinear-optical, and other useful optoelectronic properties. The simplest polythiophene (1) is insoluble and infusible, therefore inconvenient for semiconductor processing. Hence, it is not widely used even though it is very cheap. 3-Alkylpolythiophenes, especially regioregular ones (RR) (2) found wide application as the main components of p-transporting and light-absorbing or emitting layers almost in every type of organic electronic devices, including OLEDs, photovoltaics, and OTFTs. The most widely used: poly-3-hexylthiophene (P3HT), R = C6H13, poly-3-octylthiophene (P3OT), R = C8H17, poly-3-dodecylthiophene (P3DT), R = C12H25, and poly-3-methylthiophene (P3MT), R = CH3. The latter is inexpensive but also hardly processible as well as 1. For the latest advances in improving of mobility of P3ATs, please, see also our review. Polyethylenedioxythiophene PEDOT (4 - upper part), a patented product of H. C. Starck, Inc., Bayer group, found a variety of applications as a good conductor and hole-transport material possessing also optical transparency. PEDOT-PSS, copolymer of PEDOT and polystyrene sulfonic acid (4 - lower part) is known under commercial name BAYTRON®. It possesses high conductivity and solubility in water, hence, it may be processed from water solutions or dispersions. It found numerous commercial applications from antistatic treatment to complex OLED technology.
Polyparaphenylenes may be awarded 'champion' status among polymeric semiconductors in chemical and environmental stability, which is highly valuable property. Oligo-para-phenylenes are 'shorter' analogs of PPPs for small-molecule electronic applications. Preparation: In contrast to all conducting polymers described above, poly-para-phenylenes can not be synthesized by oxidative polymerization of a monomer precursor. Instead, PPPs are synthesized by polymerization involving reductive coupling reactions of appropriately-substituted halogenated benzenes, such as intermolecular Suzuki polymerization. The other efficient approach, so-called "precursor" approach employs the synthesis of a nonaromatic precursor of PPP followed by aromatization on the final step. For a comprehensive review on the synthetis of PPPs see also ref. 1, page 209. Structure:The simplest member of the family, 'parent' poly-para-phenylene (1) is completely insoluble and intractable, therefore, the well-processible alkyl-substituted derivatives (2) are most commonly used. To achieve higher degree of planarization and conjugation, a variety of synthetically available polyfluorenes (3) have been introduced. Polyfluorenes can also be easily modified by alkylation with alkyl halides, including 'swallow-tailed' ones for better solubility and film-forming properties. 'Ladder-type' PPPs (4) are rigid and planar that may be useful for a variety of applications.
Properties and applications: PPPs do not exhibit metallic-type conductivity, even in a doped state, therefore they are typical organic semiconductors, p-transporters. Attractive features of PPPs are high ionization potential, high thermal, and environmental stability, and structural flexibility. PPPs also possess excellent film-forming and luminescent properties (many of them luminescent in a blue region of visible light spectrum), therefore they are highly attractive for use in OLED technology.
Poly-para-phenylene-vinylenes (PPV): PPVs may be awarded 'champion' status among polymeric semiconductors in solubility and film-forming properties. Preparation: PPVs as well as PPPs, are synthesized utilizing two main approaches: the 'direct' route and the 'precursor' route. Although both approaches use polycondensation reactions, e.g. intermolecular condensations of appropriately-functionalized benzenes, the direct route gives rise to PPVs, whereas the 'precursor' route affords the polymers with saturated bonds instead of the double bonds between phenyl rings. The latter polymers, called 'precursors', are much better soluble and may 'gain' much higher molecular weight than those obtained via the 'direct' route.
The 'precursor' polymers are subsequently converted to PPVs through the reactions of elimination to afford the products of higher order, molecular weight, and quality than that obtained through the 'direct' route. For the comprehensive review on the synthesis of PPVs see also ref. 1, page 343.
Structure: The structure of PPVs my be seen as a 'mix' of polyacetylene and polyphenylene. In contrast to all classes of conducting polymers described above, all PPVs, even the simplest member (1) possess essential solubility, processibility, and film forming properties. This valuable feature of PPVs in combination with excellent luminescent and optoelectronic properties made these polymers the most popular materials for OLED and photovoltaic applications. MEH-PPV (2) is one of the most widely used PPVs, commercially available, highly luminescent p-transporter that possesses electron donating p-conjugated system, and excellent film-forming properties due to 'swallow-tailed' aliphatic side-chains. MEH-CN-PPV (3) is a usual n-transporting counterpart to MEH-PPV. Due to conjugation with electron-withdrawing cyano-groups, MEH-CN-PPV (3) possesses electron-deficient conjugated system with essential n-transporting properties and increased ionization potential (IP).
Properties and applications: PPVs are typical organic semiconductors. Carrier-transporting properties of PPVs may be easily 'tuned' and altered by manipulation with substituents attached to either benzene rings or double bonds. PPVs are one of the most common types of conducting polymers used in OLED and organic photovoltaic technology.
In photovoltaics: For the comparison of solar cells, following standards are used: Ac = 1 cm2, Standard illumination conditions: Pin = 100 mW/cm2.
Where:
Maximum power point: is a point on I/V curve, where I x V represents a maximum value (maximum power). Standard illumination conditions: (one sun) is the power of solar radiation per square centimeter with "air mass 1.5-spectrum" (AM 1.5) at solar noon on a clear equinox day at the equator (~100 mW/cm2).
In light emitting diodes:
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