Latest News in Organic and Molecular Electronics for Spring 2006

05.26.06

Thiophenes for organic electronics: latest developments

(Minireview)

It would be no exaggeration to say that thiophenes occupied the top position among different types of conjugated materials for organic electronics. Superior chemical, physical, and electronic properties, synthetic availability, structural flexibility, and good environmental stability gave rise to extraordinary growth of interest to these compounds that resulted in boom of both academic and industrial research on electroconductive thiophenes.

Most recently, remarkable achievements in understanding and improving of charge carrier mobility of thiophene materials have been reported. Several scientific groups reported mobility values approaching or exceeding that of amorphous silicon for thin film transistor applications. Here we wish to summarize these events and track them down according to logical sequence of their appearance.

Morphology of thiophene oligomer and polymer films: improving charge carrier mobility

A large theoretical work published in JACS, 2005, 127, page 2339 by G. R. Hutchison, M. A. Ratner, and T. J. Marks from Northwestern University is dedicated to the mechanism of hole transport in conjugated heterocyclic oligomers and polymers. The article is entitled "Hopping transport in conductive heterocyclic oligomers: reorganization energies and substituent effects". The authors used computational methods to build a model for so-called hopping hole transport that occurs between molecules, chains, or grains of an organic semiconductor.

Greater impact of the hopping transport over intrachain (within one molecular chain) transport on overall mobility of an organic semiconductor should be mentioned. Hopping transport is likely to be a speed-determining step of overall transport process for most of types of organic semiconductors, because charge transfer within a molecular chain occurs much easier and faster than that between molecules or grains. It is known, that the slowest part of a process always determines speed of this process.
In support, the authors provide two equations: Marcus/Hush (1) for charge transfer rate (k_hole) and Einstein (2) for mobility m.

The equations show exponential dependence of mobility on molecular reorganization energy, l, a key parameter for organic conductors that may be practically modified. The reorganization energy is due to geometrical difference of an uncharged and charged state of a molecule, the energy necessary to overcome energy barrier of transformation from one state to another. Computed data suggest several important parameters affecting l, among them: oligomer length, inter-ring dihedral angles, nature of heteroatom, and presence of bulky substituents attached to heterocycle rings. Hence, variation of these parameters can be used in order to determine prospective ways of the carrier mobility improvement.

Thus, R. J. Kline, M. D. McGehee, and M. F. Toney from Stanford University recently reported very interesting observation on crystalline organization of low molecular weight poly-3-hexylthiophenes (P3HT) on silane self assembled monolayers (SAMs). The work "Highly oriented crystals at the buried interface in polythiophene thin-film transistors" is published in Nature materials, 2006, 5, page 222 followed by popularized article in the same journal by R. A. Street "The benefit of order", page 171.

The authors investigated mobility of thin films of both low and high molecular weight regioregular P3HT deposited on gate dielectric pretreated with either HMDS or OTS siane SAM. (For previous reports on use of these SAMs in organic electronics, please, see our article). They found reproducible increasing of mobility by 2-3 orders of magnitude when changing the SAM from HMDS to OTS. Thus, mobility values for polymer with molecular weight of ~3.5 kD were 1·10^-6 with HMDS SAM and 0.5·10^-2 with OTS SAM correspondingly. Somewhat smaller difference have been observed for polymer with molecular weight of ~9kD, and no or little difference for one of ~30kD.

Film morphology was investigated by grazing incidence X-ray scattering and the results were rationalized as follows. Directions of hole transport within polythiophene crystallite and between grains may be represented as (100), (010), and (001) (see scheme above). When holes travel in the direction (100), they meet 'insulation boundary', a double layer of alkyl chains, hence the transport in this direction is unfavorable. At the same time, transport in the p-stacked direction (010) and along the polymer backbones (001) should not be hampered.

A very thin (10 nm) layer of polythiophene near the surface of dielectric that is called 'buried interface' is responsible for conductance in TFTs. Study of morphology of this layer on HMDS and OTS SAMs suggest big difference in the grain's organization. The films deposited on the HMDS SAM were found to be disordered with many (100)-type inclusions. Deposition on OTS SAM revealed a high order of organization with only (010) and (001) types of grain stacking. The grains in the latter case are oriented with alkyl chains down (simplified scheme for (001)-type 'ideal' stacking see below) which might be expected due to lipophilic interactions between long alkyl chains of the OTS SAM and alkyl chains of P3HT. Noticeable, that this kind of grain orientation should be highly favorable for the transport in OTFT where holes travel in horizontal direction, but unfavorable for the transport in photovoltaics, where holes travel in vertical direction.

Interestingly, bulky and long-chain-containing silane SAMs (HMDS and OTS) trigger deposition of organic semiconductors, whereas short-organic-chain dimethylsiloxane oligomers may efficiently protect surfaces from any depositing of organic semiconductors, at least in the case when dip coating method is utilized. This effect is probably due to partial hydrolysis of methoxy groups near the surface of the siloxane layer that results in some surface hydrophilicity. Dimethylsiloxanes may be easily patterned using dry microcontact printing, mCP, which was applied for the effective preparation of arrays of flexible OTFTs.

Thus, A. L. Briseno, Z. Bao and colleagues form California reported this novel method in J. Am. Chem. Soc, 2006, 128, page 3880; paper entitled "Patterning Organic Semiconductors Using "Dry" Poly(dimethysiloxane) Elastomeric Stamps for Thin Film Transistors". They succeeded to selectively deposit P3HT on patterned electrodes prepared from PEDOT (poly3,4-ethylenedioxythiophene) when the rest of the microchip was protected by dimethylsiloxane oligomers. Mobility of the devices, thus obtained, ranged from 0.01 to 0.03 cm²/Vs.

The other interesting work is dedicated to molecular organization of 'uniform' RR P3HTs of different molecular weights (and, correspondingly different lengths). The work by a group of scientists form Carnegie Mellon University and Cornell University is published in J. Am. Chem. Soc, 2006, 128, page 3480, and entitled "Nanostructure Dependence of Field-Effect Mobility in Regioregular Poly(3-hexylthuiophene) Thin Film Field Effect Transitors".

To check the theoretical predictions for the correlation: polymer length - reorganization energy - mobility (see above), the authors synthesized, separated, and investigated an impressive array of P3HTs with uniform molecular weights of 2.4, 4.8, 5.1, 7.0 ... up to 18.4 kDa. Overwhelming studies of drop-cast thin films of these polymers were carried out on complex equipment such as tapping mode atomic force microscopy (TMAFM), grazing incidence X-ray diffraction (GIXRD) and grazing incidence small-angle X-ray scattering (GISAXS).

These studies revealed organization of the polymers with weights from 2 to 7 kDa in nanofibrils of one molecular wide with molecules arranged perpendicular to the nanofibril axis (see scheme below). This fact itself might not be very striking, but furthermore, an exponential increase in mobility was observed when passed from shorter molecules to longer (thinner fibril to thicker). This tendency "leveled off", however, for polymers heavier than 10 kDa.

The knowledge found in this study may help to choose optimal polymer molecular weights for future uses in electronic devices.

The other intriguing work investigates J-aggregation of a-sexithiophene on silicon dioxide surfaces. The work by E. Da Como, M. A. Loi, M. Murgia, R. Zamboni, and M. Muccini from Italy "J-Aggregation in a-sexithiophene Submonolayer Films on Silicon Dioxide" is published in JACS, 2006, 128, page 4277.

The authors found, that very thin (submonolayer) films of a-6T can be obtained on SiO₂ substrate under strictly controlled deposition conditions. In these films, a-6T is largely arranged in a form of J-aggregate, where oligomer backbones 'rest' on the surface of silicon oxide (see scheme below). In thicker films, however, the molecules arrange in vertical positions to form H-aggregate as in the normal crystalline structure of the compound. J-aggregates of a-6T were found to possess outstanding photoluminescent properties due to exciton delocalization specific for J-aggregates. The PL yields were estimated between 0.6 and 1.

Novel thiophene oligomers and polymers for organic electronics

a-Sexithiophene and other thiophene oligomers possess both advantages and drawbacks in comparison to thiophene polymers. Major advantage is in higher organizational order and reproducibility of oligomer crystallites, whereas major drawback is in low solubility of oligomers. Good solubility is necessary for successful solution processing that is technologically simpler and cheaper than the vacuum deposition method. Even DH-a-6T with two hexyl groups in terminal positions possesses solubility insufficient for effective solution processing. Therefore, chemical modifications of oligomers that would provide high solubility along with retention of high crystalline order would be highly desirable.

A group of scientists from Germany reported in a recent JACS communication, 2006, 128, page 3914 a very interesting compound (1 on the scheme below) that possesses desired properties: both good solubility in chloroform and ordered crystalline structure. In addition, it gave good OTFT characteristics when wet-cast from chloroform solution.

The work is entitled "Solution Processable Organic Field-Effect transistors Utilizing an a,a'-Dihexylpentathiophene-Based Swivel Cruciform". Swivel cruciforms are compounds where "free rotation between the arms in addition to within the arms" can be realized. This property was expected to increase solubility of oligomeric thiophenes, which was confirmed after the synthesis of the compound 1 assigned as DHPT-SC. The authors stress folded p-stacked conformation of the compound in both solution and solid phase. Thin film transistors fabricated of the compound deposited from chloroform solution with P3HT source and drain electrodes and after thermal annealing gave next impressive characteristics: mobility 0.012 cm²/Vs, on/off ratio >10⁵, and very low turn-on voltages ~0V.

The other method that can be employed for the thin film preparation for OTFT applications are Langmuir-Blodgett (LB, vertical transfer) and Langmuir-Schafer (LS, horisontal transfer) techniques. These techniques are based on a film formation on the interface of water and air after evaporation of an organic solution containing a substance with amphiphilic properties. The method allows for better thickness control in comparison to the solution phase deposition method.

Thus, a group of scientists from Italy and France reported fabrication of an efficient OTFT from a new thiophene-phenylene copolymer POPT (2 on the scheme above) using Langmuir-Schafer technique. The work is published in Chemistry of Materials, 2006, 18, page 778 and entitled "Poly(alkoxyphenylene-thienylene) Langmuir-Schafer Thin Films for Advanced Performance Transistors".

The structure 2 was chosen to combine good electronic properties of thiophenes with good solubility properties and slight amphiphilic character of dialkoxy phenylenes. The authors succeeded to achieve field effect mobility of 5·10^-4 cm²/Vs for LS films of 2 that is highest reported to date for polyalkoxyphenylenes. The LS films of 2 also "strikingly outperform" solution cast ones.

Relatively high field-effect mobility of conjugated thiophenes is one of their most advantageous parameters over other types of conjugated conductive polymers. However, some other field-effect characteristics are nearly as important, for example on/off ratio. This parameter is a ratio of on current to off current and it determines efficiency and reliability of the device operation. Unfortunately, regioregular poly-3-alkylthiophenes generally do not show high on/off ratios, which tend to further decrease when the material exposed to the action of oxygen and moisture. This happens due to relatively low band gap of these compounds. Low band gap is otherwise useful property when high conductivity and mobility are needed, however organic materials with low band gap often exhibit reduced resistance to oxidation.

When a conjugated polymer reacts with oxygen of air, it can be reversibly or irreversibly p-doped (oxygen takes an electron from a polymer molecule and forms a hole). Hence, the off current becomes higher than it was before exposure to air, and on/off ratio decreases. Increasing of a band gap and, correspondingly, ionization potential (IP) of such materials along with no or little sacrifice in mobility would be highly desirable.

A group of scientists from Merck Chemicals, UK led by I. McCulloch succeeded to overcome this problem. This success reported in two sequential publications: "Influence of Molecular Design on the Field-Effect Transistor Characteristics of Terthiophene Polymers" Chem. Mater., 2005, 17, page 1381; and "Liquid-Crystalline Semiconducting Polymers with High Charge-Carrier Mobility" Nature Materials, 2006, 5, page 328. The results reported in the latter paper are so prominent that they were further popularized in a Science paper 2006, 311, page 1691 by R. F. Service "Plastics Break the speed Barrier" and in a S&EN article 2006, March 27, page 38: "Polymers Have Display Potential".

The first article is dedicated to modification of thiophene polymers in a way that would allow reduction of "conjugation density". This reduction of conjugation was expected to increase IP, stability, and on/off ratio of the polymers. Two approaches were chosen: 1. Arrangement of alkyl substituents in thiophene rings in the way that would force them twist out from the conjugation planes. 2. Introduction of a less conjugated unit "as a component for the polymer backbone". The latter approach appeared to be successful when naphtalenylene unit was introduced in the backbone. Thus, a polymer (3 on the scheme above) gave IP 5.4 eV, stable on/off ratio of 4·10⁴ even after several days of exposure to air, and mobility of 6·10^-4 cm²/V·s. RR P3HT (IP=4.8 eV) under identical conditions had on/off ratio of only 1·10¹ that was constantly dropping after several days exposure to air.

In the second work, the authors replaced naphtalenylene unit for thieno[3,2-b]thienylene unit (polymer 4 on the scheme above) and achieved striking increase of mobility in addition to good environmental characteristics of the former polymer. The new polymer 4 gave high saturated mobilities of 0.63-0.72 cm²/V·s The authors attribute improvement in mobility to special annealing technique when the polymers have been annealed in their liquid crystalline phase followed by controlled crystallization on cooling. This technique gave relatively large polymer grains that probably contributed strongly to the mobility. The mobility values for the new material are equal to those of amorphous silicon, which is currently used in commercial TFTs. In addition, the material showed high on/off ratio of 8·10⁷, stable in low humidity air, and low onset voltage of ~0 V.

New thiophenes for optoelectronics

Interesting thiophene-containing photochromic molecule has been reported by a group of scientists from China in a Chemistry of Materials paper 2006, 18, page 235, entitled "Highly fluorescent contrast for rewritable optical storage based on photochromic bisthienylethene-bridged naphatlimide dimer". The compound BTE-NA, 5 on the scheme below "allows the read, write, and erase functions to be controlled by light". The authors succeeded to obtain this high-performance system by combining a known efficient photochromic bisthienylethene unit (in the center of the molecule) with two efficient naphtalimide chromophores (side groups of the molecule).

The naphtalimide chromophores emit photoluminescent light under near UV illumination (442 nm) when the compound is in the 'open' state 5 of the central bisthienylethene. Irradiation with higher energy UV light (365 nm) gives rise to ring closure in the central bisthienylethene unit (6 on the scheme below): writing of the information. After the ring is closed, the naphtalimide chromophores do not luminescent any more under the 442 nm illumination: reading of the information. Efficient quenching of luminescence is attributed to efficient intramolecular energy transfer from excited chromophores to the central bisthienylethene. Erasing of the information occurs, when the 'closed' form 6 is irradiated with visible light of 510 nm, and the molecule transforms into the initial state 5.

The authors stress next advantages of their rewritable optical storage device prototype, which was fabricated by dopping of 5 in polymethylmethacrylate: high contrast (on/off ratio = 85:1) and high stability of the recording.

The other work by a group of scientists from Korea and USA, entitled "Novel dendritic chromophores for electro-optics: influence of binding mode and attachment flexibility on electro-optic behavior", is dedicated to electro-optic properties of dendrimer-tethered thiophene-containing chromophores. The work is published in Chem. Mater. paper 2006, 18, page 344.

It is known, that thiophene derivatives containing both strong electron-donating substituent in 2-position and electron-withdrawing substituent in 5-position (example-7) possess nonlinear optical properties and may exhibit large electro-optic (EO) response (r₃₃). Recently, an application of chromophore-tethered dendrimers have been introduced to improve EO-responce through control of electrostatic interactions between chromophore sites. Attachment of chromophores to a dendrimer assures 'random' orientation of chromophore sites that reduces dipole-dipole interactions and improves EO response.

The authors of aforementioned research used binding of chromophore 7 to a variety of dendritic systems to evaluate EO-effects. It was found that binding through thiophene ring ('side-on'-tethering 7a) using short and rigid binding moieties (succinate diesters) affords strong (3-fold) enhancement in EO coefficient. Both 'side-on' (7a) and 'end-on' (7b) tethering affords also significantly higher thermal stability of the EO response.

New thiophene compounds with high potential for use in organic electronics

Ethylenedioxythiophene (EDOT) is a monomer, the synthetic precursor of polymer PEDOT, which has found numerous applications as an organic electronic material possessing outstanding physical and electronic properties. These unique properties include low band gap, rigid conjugation due to sulfur-oxygen interactions, good conductivity and charge carrier mobility in combination with optical transparency and environmental stability.

P. Leriche, P. Balnchard, J. Roncali and coworkers from Universite d'Angers, France reported synthesis of unsaturated analog of EDOT which they named as VDOT (8 on the scheme below). The work entitled "3,4-Vinylenedioxythiophene (VDOT): a new building block for thiophene-based p-conjugated systems" is published in Chemical Communications 2006, page 275, followed by popularized article in C&EN's "concetrates" 2006, Jan. 16, page 26.

The authors determined band gap of VDOT that appeared to be little lower than that of EDOT (5.10 and 5.76 eV respectively). Surprisingly, the attempts to electropolymerize VDOT were unsuccessful, which the authors attributed to low density of unpaired electron at the linking position of intermediate cation-radical. Lithiation followed by oxidative dimerization of VDOT gave it's dimer BVDOT (9 on the scheme below) that possesses lower band gap of 4.09 eV and does undergo electropolymerization. The polymer, pBVDOT has higher ionization potential than PEDOT and, according to common tendency (see also this review above), is expected to possess higher environmental stability.

A thiophene derivative that may form stable bication-radical "bipolaron" (10 on the scheme above) has been recently reported by a group of scientists from USA and Russia. The work, entitled "Isolation and crystal structures of two singlet bis(triarylamine) dications with nonquinoidal geometries", is published in J. Am. Chem. Soc, 2006, 128, page 1812.

The compound 10 forms stable dication as a crystalline salt when oxidized with antimony pentachloride. Unusual stability of the salt is attributed to charges delocalization due to electron donating properties of the thiophene ring and conjugated system of the molecule. The authors stress first time isolation and crystal structure elucidation of this type of dications. Crystallographic and NMR data of the salt suggest the structure to be partly quinoidal, but to exist mostly in a form of a singlet biradical. The results of this work may be useful for better understanding of charge carrier transport in related conjugated structures that are widely used in organic electronics.

A funny thiophene macrocycle C[12]T (11 on the scheme below) was synthesized by E. Mena-Osteritz and P. Bauerle from University of Ulm, Germany. The work is published in Advanced Materials, 2006, 18, page 447, and entitled: "Complexation of C₆₀ on a cyclothiophene monolayer template".

The authors observed "highly organized monolayers" of the material in a form of two-dimensional (2D) crystals when self-assembled on graphite. The monolayers "uniformly cover large areas of the surface" to form a layer of p-donor semicondutor. Furthermore, molecules of fullerene - C₆₀ were found to settle down nicely on the top of the macrocycles as balls in sockets to form a layer of p-acceptor semicondutor.

The authors analyzed organization and dynamics of C[12]T-C₆₀ complexes by means of scanning tunneling microscopy (STM) and spectroscopy (STS) and observed formation of "true host-guest" complexes. I-V characteristics of the complex were also measured, however no obvious conclusions on electroconductive properties were provided, although promised in forthcoming publications. To the personal opinion of the author of this review, there must be some properties of this material, that would be highly useful for photovoltaic or other electronic applications.

04.12.06

Latest Developments in Organic Materials for Photovoltaics

(Minireview)

Solar cells are utmost environmentally friendly, the most promising alternate energy source for the future fossil fuels replacement. Organic photovotaics are particularly attractive for the next generation large scale solar cell commercialization due to possibility of inexpensive, thin film, lightweight, large area applications, and some other reasons we also mentioned before.

For example, one can imagine a photovoltaic paint. Imagine, we go to a nearest supermarket and buy four cans of paint. We paint roof and walls of our house with paint (likely black) from the first can: this will be an anode for our future electricity-generating house. The second layer from the second can may be variably colored, likely darkish; it will be a light-harvesting active layer for our electricity-generating house. Then, we use the third can of paint, transparent, the cathode for our house's solar cell. Finally, we put a transparent finish to protect our freshly painted solar cell from excessive action of oxygen and moisture.

The same way we can paint our garage, fence, and even driveway. Would you imagine all huge surface of our highways generating electricity? Of course, every surface would be pretreated with a special network of metallic contacts that guide the electricity generated to a set of batteries.

In addition, we can decorate our windows with photovoltaic curtains or blinds. The window glass itself may be covered or filled with a light-sensitive obscuring organic photovoltaic layer that would become darker and generate electricity under the bright sunlight.

After a some while (say 10 years), the productivity of our electricity-generating yard would decline, but nothing to worry about. We would go to the supermarket again and buy a special solvent that may wash the old solar cell away. Then we buy four more cans of a fresh paint, cheaper and more efficient after 10 years of constant improvements...

However, to achieve that scale of application, many existing parameters of organic photovoltaic materials have to be strongly improved. First of all it is power conversion efficiency (PCE) the most important parameter that determines relative amount of electricity generated from a certain surface area under certain illumination conditions. Currently this parameter reaches 5% for 'all organic' solar cells and 12% for dye sensitized solar cells. Other highly important parameters are environmental and thermal stability. For these parameters, dye-sensitized cells are also superior at present time over 'all organic' devices.

All organic solar cells:

Search for highly efficient and stable n-conductors (electron acceptors) for bulk heterojunction active layers is one of possibilities to improve PCE of 'all organic' photovoltaics. Known organic n-transporters are sparse and not quite efficient. Derivatives of buckminsterfullerene (1) were found to be one of the most efficient acceptor types. The fullerene (1) itself is rather inorganic material than organic and it possesses high order of symmetry and low affinity to organic compounds resulting in tendency for aggregation and phase separation. Therefore, chemically modified fullerenes such as PCBM (2) are currently employed as electron acceptors to afford in some cases the highest PCE achieved to date near 5%.

Organic side chain in PCBM prevents it from excessive aggregation when the compound is blended with a donor component, an organic p-type conductor. A significant choice of p-type conductors exists, among them derivatives of polyphenylenevinylene (PPV) and polythiophenes have been most intensively studied. Thus, different processes and conditions for polymerization of 3-hexylthiophene may give rise to either regioregular (RR) (3 on the scheme below) or regiorandom (5) forms of poly-3-hexylthiophene (P3HT). P3HTs were found to match well with fullerenes in band gap energy levels to afford the most efficient cells.

The most recent results demonstrate that morphology of fullerene-containing bulk heterojunction solar cells is critical for the cell performance. These results are summarized in a comprehensive review published by H. Hoppe and N. S. Sariciftci from Linz Institute for Organic Solar Cells (LIOS), Austria in J. Mater. Chem., 2006, 16, page 45. The article entitled "Morphology of polymer/fullerene bulk heterojunction solar cells" contains 'in depths' analysis of all aspects of the structure-morphology-PCE correlations of the fullerene-based devices.

The other article by W. Ma, C. Yang, X. Gong, K. Lee, and A. J. Heeger from University of California at Santa Barbara published in Adv. Funct. Mater., 2005, 15, page 1617 reports improvement in morphology, thermal stability, and efficiency of PCBM-P3HT solar cells after special thermal treatment that is called annealing. The results suggest also that the PCE improvement may be attributed to formation of chemical bonds between organic layers and metallic anode during controlled heating of the device.

One of the most recent reports demonstrates importance of the level of regioregularity (RR) of P3HT in performance of PCBM-P3HT devices. The article by a group of scientists from United Kingdom and South Korea published in Nature Materials, 2006, 5, page 197 is entitled "A strong regioregularity effect in self-organizing conjugated polymer films and high-efficiency polythiophene:fullerene solar cells". It is known that hole mobility of P3HT strongly increases when regiorandom polymer (5) (scheme above) is replaced with regioregular (3).

However 100% RR P3HT (3) is very difficult to prepare. Therefore the authors investigated available 90.7% RR (near 4 shown on the scheme above) and 95.2% RR P3HTs and found a dramatic difference in such important parameter of the device performance as short circuit current density: J_SC. Thus, increase in RR only by 4.5% gave rise to increase of J_SC by more than 100%! The researchers achieved also 4.4% of PCE with 95.2% RR polythiophene without optimization of electrodes.

Though 'all organic' solar cells based on fullerene - n-transporters are the best performing devices fabricated to date, there are two major drawbacks for the practical use of these materials: cost and tendency of fullerenes to undergo aggregation and phase separation that may give rise to strong decline in the efficiency with time. Therefore, it would be very attractive to replace fullerenes with inexpensive and stable organic n-transporters possessing adequate performance. There are only few types of effective organic n-transporters and one of them are derivatives perylene teracarboxylic acid diimide (PDI).

We have already reported the use of one such derivative in a couple with RR P3HT (3) for the solar cell prototype. Here, we wish to summarize some of the latest advances. Thus the researches W. S. Shin, S.-H. Jin and coworkers from universities of Korea investigated a series of differently substituted PDIs in a couple with RR P3HT and identical other device parameters with a purpose to identify influence of the substituents on the device performance. The article is published in J. Mater. Chem., 2006, 16, page 384 and entitled: "Effects of functional groups at perylene diimide derivatives on organic photovoltaic device application".

The study ended up with determination of a positive effect of thermal annealing in most of the cases. The highest efficiency (PCE) of 0.18% and current density J_SC of 1.32 mA/cm^-2 has been obtained for PDI-C9 (7 on the scheme below) that contains long "swallow-tailed" alkyl chains. Alkyl chains increase solubility and prevent from crystallization and phase separation in the active layer. At the same time 5-PDI (8 on the scheme below) possessing strong electron donating groups at the perylene core was found to absorb efficiently in red region of visible light spectra which is rare and very valuable property. In addition, the device with 8 gave highest open circuit voltage V_OCof 0.66V, though overall PCE was only 0.04%.

The other research by J. Li, F. Dierschke, J. Wu, A. C. Grimsdale, and K. Mullen from Max-Planck Institute for Polymer Research, Germany, demonstrates higher performance of polycarbazole (Pcz-9 on the scheme above) as a donor and p-transporter than polythiophene P3HT (3) when coupled with PDI-C4 (6) for solar cell applications. The work "Poly(2,7-carbazole) and perylene tetracarboxydiimide: a promising donor/acceptor pair for polymer solar cells" is published in J. Mater. Chem., 2006, 16, page 96.

The effect of higher efficiency of the blend of polycarbazole and PDI is attributed to better matching of their band gaps energy levels than that of P3HT and PDI. Thus a device fabricated of a blend of Pcz-9 : PDI-C4 (6) = 1 : 4 gave power conversion efficiency (PCE) of 0.63% whereas a device with P3HT (3) : PDI-C4 (6) = 1 : 4 gave PCE of only 0.19% under identical illumination conditions.

Dye-sensitized solar cells:

One of the best performing dyes for dye sensitized solar cells was found to be ruthenium bipyridine complex coded as N3 (10 on the scheme below). This complex contains four carboxylic groups (shown in red) that strongly bind to the surface of inorganic n-transporter, titanium dioxide. N3 was found to afford up to 12% power conversion efficiency that is the highest value achieved to date for solar cells containing any organic component. However, N3 has been applied with a volatile solvent-containing redox electrolyte, which is impractical due to technological difficulties and problems associated with heat-induced electrolyte evaporation.

New modified analogs of N3 that may be effectively used with nonvolatile binary ionic liquids and in addition possess higher molar extinction coefficients have been recently reported by D. Kuang, S. M. Zakeeruddin, M. Gratzel and coworkers from Switzerland. The paper entitled "High molar extinction coefficient heteroleptic ruthenium complexes for thin film dye-sensitized solar cells" is published in J. Am. Chem. Soc., 2006, 128, page 4146.

The authors synthesized complexes 11 and 12 on the scheme below which they coded as K-19 and K-73 respectively. These complexes possess elongated (p-alkyloxystyryl) conjugated chains that increase both the dye extinction coefficient and hydrophobic properties of new dyes in comparison to N3. The dyes exhibited "strikingly" efficient performance and high stability in the presence of nonvolatile ionic liquid electrolytes with PCEs of 7.0% (K-19) and 9.0% (K-73).

Though ruthenium complexes possess outstanding efficiency as sensitizing dyes, they unlikely to find large scale industrial application since they contain rare and expensive noble metal - ruthenium. Even low concentrations of an expensive metal can make the costs unbearable on metric tons quantities. Therefore, search for suitable substitute for the ruthenium complexes is currently underway.

Thus, scientists S. Ushiroda, N. Ruzycki, M. T. Spitler, and B. A. Parkinson form Colorado State University investigated a series of thiacyanine dyes (13 and analogs). These dyes are sulfur-containing analogs of cyanine dyes, and they were functionalized with carboxylic groups to bind to anatase surface (JACS, 2006, 128, page 5158). The authors used just single crystal anatase (natural form of crystalline titanium dioxide) electrodes to identify dye surface binding properties, and some photovoltaic parameters, such as incident photon to current efficiency (IPCE) and absorbed photon to current efficiency (APCE). It was found that this type of dyes may bind to the surface in either monomeric or dimeric form depending on absorption conditions. Both forms may give somewhat differing parameters. The most efficient dye was found to be a compound coded as G-12 (13) on the scheme above that afforded IPCE of 0.70 for monomeric and 0.76 for dimeric form respectively.

Mimicry from nature:

Since the discovery of chlorophylls, active light harvesting 'antennas' of plants (one of them, 'chlorophyll a' 14 on the scheme below), scientists have been trying to reproduce photosynthetic reactions in the laboratory conditions. Photosynthesis is formation of a sugar-type organic compound and oxygen from carbon dioxide of air and water. This process requires an electron - a powerful reducing agent that reduces almost everything, even stable molecules of carbon dioxide to form carbohydrates: cellulose, starch, sugars etc.

Where plants take electron to reduce carbon dioxide? Yes, they take it from the same organic photovoltaic process: charge separation in a neutral organic compound under action of light. The difference between photovoltaic and photosynthetic processes is in subsequent stages: Separated charges, electrons and holes gather on different poles of photovoltaic cell to afford voltage. And separated charges trigger chemical photosynthetic reactions in plants: electrons reduce carbon dioxide to sugars, holes oxidize water to oxygen.

The chlorophylls are responsible for the charge separation in plants. A critical part of the chlorophyll molecule is porphyrin core (shown in green, 14 on the scheme above). Therefore scientists use modified porphyrin-containing compounds to achieve light-induced charge separation on the laboratory scale.

Thus, a group of scientists from the Netherlands and United Kingdom have reported extremely fast and reversible charge separation in a porphyrin containing complex (15 on the scheme above) that also possesses an acceptor counterpart rhenium complex (shown in purple). Both parts are connected through a chain of unsaturated conjugated bonds to facilitate electron transfer. The work is published in J. Am. Chem. Soc., 2006, 128, page 4253.

However, the porphyrins are known to have low absorption in red region of the solar light spectrum. Phtalocyanine core (purple in 16 and green in 17 on the scheme below) may be represented as p-extended porphyrin core, and it is known that extension of conjugation shifts the maximums of light absorption and emission to the red region.

Scientists from Spain and Germany: A. de la Escosura, M. V. Martinez-Diaz, D. M. Guldi, and T. Torres, JACS, 2006, 128, page 4112 used a tandem of electron-deficient, palladium-coordinated phtalocyanine complex 16 and electron-donating, zinc-coordinated phtalocyanine complex 17 to achieve complete and relatively stable (hundreds of nanoseconds) light-induced charge separation. The effect is observed even under red and close infrared illumination. Electron donating counterpart 17 possesses a powerful electron trap buckminsterfullerene (see above) that contributes to stabilization. This kind of the charge direction is also known as 'energy funneling' in photosynthesis terminology.

03.10.06

Organic Compounds for Red OLEDs

Recently, we have summarized some of new developments in organic materials for blue emitting OLEDs. A main difficulty in the selection of organic compounds for blue emitters is due to high energy transformations that are necessary to overcome high energy barrier to transform the molecule in its excited state. These high energy transformations are damaging to organic materials, which results in short operational lifetimes. In contrast to the blue emitters, red emitting materials have to possess narrow (low) band gap, energy difference between Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO). To further simplify, an electron on HOMO in a basic state of a molecule should receive relatively little energy in order to jump on LUMO and form an exciton, therefore no high energy (eventually voltage applied to a device) is necessary.

However, there is another problem that arises. Electrons on HOMO of molecules with low band gap are usually highly energetic. In other words, these electrons may not only easily jump on LUMO, form excitons, they also may react easily with everything around, with oxygen of air, for example. These compounds are usually strong reducing agents and can be easily oxidized or photochemically decomposed. Therefore, selection of materials for red emitters is also a challenge.

There are well recognized methods for modifying of organic molecules to reduce band gap. Both introduction of electron donating substituents and increase of conjugation length may give the desired result. For example, while anthracene derivatives built of three fused benzene rings are usually blue emitters, addition of two more fused benzene rings forms pentacene derivatives and strongly increases conjugation. Pentacene derivatives already 'prefer' emitting red.

Thus, B.-B. Jang, S. H. Lee, and Z. H. Kafafi, USA, synthesized pentacene derivative DMPDPP (1 on the scheme below), which exhibited near saturated red emission with Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of (x, y) = (0.67, 0.32) when used as a 'guest' material in Alq₃ (2) host emitting layer of a fabricated smOLED. The work entitled "Asymmetric Pentacene Derivatives for Organic Light-Emitting Diodes" is published in Chem. Mater., 2006, 18, page 449. The authors mention that the chromaticity results they obtained are very close to the National Television Standards Committee (NTSC) red specification (x, y) = (0.67, 0.33).

A serious advancement in efficiency of phosphorescent polymeric OLED materials has been reported by international team of scientists from USA, Taiwan, and Canada in Chem. Mater., 2005, 17, page 3532. In an introduction part of the article entitled "Highly Efficient Electrophosphorescent Devices with Saturated Red Emission from a Neutral Osmium Complex" the authors emphasize importance of phosphorescent dopants for increase of quantum efficiency of OLEDs through "the harvest of both singlet and triplet excitons".

However, a major drawback of the phosphorescent materials is in decline of the phosphorescent efficiency at higher current density due to triplet-triplet annihilation. The authors succeeded to reduce this problem by use of a phosphorescent dopant with a short phosphorescence lifetime. Osmium complexes are known to have very short phosphorescent lifetimes of less than 1 ms, and the authors have chosen an osmium complex 3 (on the scheme below) as a phosphorescent dopant. By doping this complex in a scary looking host polymer PF-TPA-OXD (4), great parameters for the fabricated red PHOLED have been obtained such as external quantum efficiency of 12.8% and brightness >19000 cd/m² with CIE coordinates (x, y) = (0.67, 0.33). The polymer 4 structure was specially designed to serve as both hole (left, triarylamine part of the scheme) and electron (right, oxadiazole part of the scheme) transporter at the same time. A drawback for the practical use of these complex materials should be mentioned: they are costly.

The other work by Chinese scientists: T.-S. Yen, T.-J. Chow, C.-X. Zhao and coworkers, Chem. Mater., 2006, 18, page 832, is interesting also because of natural origin of the dye derivatives used. Thus, structural analogs of bisindolylmaleimide (5 on the scheme below) found in slime molds (Myxomycetes). These compounds preserve red emission in the solid state due to ability to form amorphous glasses. The structural feature of 5 ensures twisting and nonplanar geometry of the molecule because of mutual repel of two methyl groups in 2' positions of indole moieties. That in turn prohibits crystallization of compound in thin films and quenching of the fluorescence. An OLED fabricated of 5 as emitting layer and TPBI (6) as electron transport layer gave CIE coordinates (x, y) = (0.63, 0.36).