Nanostructuring with Surfactants: The Self-Assembly of a New Poly(thiophene-phenylene) Conjugated Polymer Bearing Azacrown Ether Pendant Groups.

We report the synthesis of a new conjugated polymer bearing crown ether moieties, poly[(N(1-aza-[18]crown-6)carbamido)thiophene-2,5-diyl-alt-1,4-phenylene] (BG2). In water, BG2 forms a dispersion with a slightly cloudy appearance. We have studied the effect of adding surfactants, with different polar head groups, on these polymer-polymer aggregates. Special attention is given to the system with the anionic surfactant, sodium dodecyl sulfate (SDS). The combination of photophysical techniques with electrical conductivity, NMR (1H, 13C, and 27Na), DFT calculations, molecular dynamics simulations, and small-angle neutron scattering (SANS) provides a detailed picture on the behavior of the SDS/BG2 system in aqueous solution and in thin films. NMR, electric conductivity, and DFT results suggest that hydrophilic interactions occur between the polar headgroup of the surfactant (OSO3- Na+) and the aza-[18]-crown-6 moiety. DFT calculations confirmed the capability of BG2 to form stable complexes with the Na+ cations, where the cation can be either inside the azacrown cavity or sandwiched between the cavity and the polymer chain, which seem to determine the position of the surfactant hydrocarbon chain and, therefore, be responsible for the disruption of the BG2 aggregates and subsequent increase in the photoluminescence quantum yields. SANS measurements, made with hydrogenated and deuterated SDS in D2O, clearly show how micron-sized aggregates of BG2 are broken down by SDS and then how BG2 becomes preferentially incorporated within joint colloidal particles of BG2 and SDS with increasing [SDS]/[BG2] molar ratio.

New n-Type Solution Processable All Conjugated Polymer Network: Synthesis, Optoelectronic Characterization, and Application in Organic Solar Cells.

The efficient synthesis of a new solution-processable n-type conjugated polymer network (PNT1) is reported through palladium-catalyzed Stille cross-coupling reaction conditions following the A3 + B2 synthetic approach. A benzo[1,2-b:3,4-b':5,6-b″]trithiophene derivative is used as the A3 knot and an alkyl functionalized naphthalenediimide is utilized as the B2 linker. The thermal, optical, and electrochemical properties are examined in detail, showing high thermal stability, absorbance in the visible part of the solar spectrum, and reversible reduction characteristics similar to those of the fullerene derivative [6,6]-phenyl-C71 -butyric acid methyl ester (PC71 BM). PNT1 is employed as the electron acceptor in solution-processed bulk heterojunction organic solar cells, demonstrating the potential of this new type of materials for optoelectronic applications.

Beyond Donor-Acceptor (D-A) Approach: Structure-Optoelectronic Properties-Organic Photovoltaic Performance Correlation in New D-A1 -D-A2 Low-Bandgap Conjugated Polymers.

Low-bandgap near-infrared polymers are usually synthesized using the common donor-acceptor (D-A) approach. However, recently polymer chemists are introducing more complex chemical concepts for better fine tuning of their optoelectronic properties. Usually these studies are limited to one or two polymer examples in each case study so far, though. In this study, the dependence of optoelectronic and macroscopic (device performance) properties in a series of six new D-A1 -D-A2 low bandgap semiconducting polymers is reported for the first time. Correlation between the chemical structure of single-component polymer films and their optoelectronic properties has been achieved in terms of absorption maxima, optical bandgap, ionization potential, and electron affinity. Preliminary organic photovoltaic results based on blends of the D-A1 -D-A2 polymers as the electron donor mixed with the fullerene derivative [6,6]-phenyl-C71 -butyric acid methyl ester demonstrate power conversion efficiencies close to 4% with short-circuit current densities (J sc ) of around 11 mA cm-2 , high fill factors up to 0.70, and high open-circuit voltages (V oc s) of 0.70 V. All the devices are fabricated in an inverted architecture with the photoactive layer processed in air with doctor blade technique, showing the compatibility with roll-to-roll large-scale manufacturing processes.

Very High Solid State Photoluminescence Quantum Yields of Poly(tetraphenylethylene) Derivatives.

Five different poly(arylene-diarylvinylene)s have been synthesized by reductive polyolefination starting from the corresponding bis(α,α-dichlorobenzyl)-substituted monomers and dicobaltoctacarbonyl as reducing agent. The resulting polymers all contain main chain tetraphenylethylene units. Thanks to the aggregation-induced emission effect, the corresponding polymer films show remarkably high photoluminescence quantum yields (PLQYs) of 32%-73%. The polymer with the highest PLQY is tested as solid state sensing material for the PL-quenching-based detection of nitroaromatic analytes (1,3,5-trinitrobenzene as prototypical analyte).

Scope and limitations of a direct arylation polycondensation scheme in the synthesis of PCPDTBT-type copolymers.

Direct arylation polycondensation represents a promising alternative to the currently used aryl-aryl coupling schemes for conjugated polymer synthesis that is characterized by a potentially lower impact on the environment and reduced costs. However, scope and limitations of this novel protocol are not fully understood until now. Two main aspects are, hereby, i) the chemical nature of side reactions that occur during coupling of nonactivated and dihalogenated aromatic monomers, and ii) the influence of steric and electronic factors on the reactivity of the monomers. Within this communication, the 4,4-bis(2-ethylhexyl)-cyclopenta[1,2-b:5,4-b']dithiophene (CPDT)/2,1,3-benzothiadiazole (BT) monomer couple for direct arylation synthesis of the alternating copolymer poly(4,4-(2-ethylhexyl)-cyclopenta[2,1-b:3,4-b']dithiophene-alt-2,1,3-benzothiadiazole) (PCPDTBT) is studied and homocoupling identified as the dominating side reaction. The study demonstrates that homocoupling can be almost completely suppressed through a clever choice of the reaction conditions. Finally, the findings implicate that mainly electronic factors control the reactivity of both monomers.

Charge photogeneration in donor-acceptor conjugated materials: influence of excess excitation energy and chain length.

We investigate the role of excess excitation energy on the nature of photoexcitations in donor-acceptor π-conjugated materials. We compare the polymer poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b;3,4-b']dithiophene)-4,7-benzo[2,1,3]thiadiazole) (PCPDTBT) and a short oligomer with identical constituents at different excitation wavelengths, from the near-infrared up to the ultraviolet spectral region. Ultrafast spectroscopic measurements clearly show an increased polaron pair yield for higher excess energies directly after photoexcitation when compared to the exciton population. This effect, already observable in the polymer, is even more pronounced for the shorter oligomer. Supported by quantum chemical simulations, we show that excitation in high-energy states generates electron and hole wave functions with reduced overlap, which likely act as precursors for the polaron pairs. Interestingly, in the oligomer we observe a lifetime of polaron pairs which is one order of magnitude longer. We suggest that this behavior results from the intermolecular nature of polaron pairs in oligomers. The study excludes the presence of carrier multiplication in these materials and highlights new aspects in the photophysics of donor-acceptor small molecules when compared to polymers. The former are identified as promising materials for efficient organic photovoltaics.

Ternary photovoltaic blends incorporating an all-conjugated donor-acceptor diblock copolymer.

We present a new fully conjugated diblock copolymer, P3HT-b-PFTBTT, containing donor and acceptor blocks with suitably positioned energy levels for use in a solar cell. This is the first block copolymer to be based on an existing high-performance polymer:polymer blend. We observe phase separation of the blocks and self-assembly behavior. In ternary blends with the respective homopolymers the diblock copolymer introduces lateral nanostructure without restricting P3HT crystallization in the charge transport direction, resulting in standing lamellae. By adding the diblock to the homopolymer blend as a compatibilizer, we prevent phase separation at elevated temperatures and benefit from a dramatic increase in P3HT ordering, allowing us to demonstrate polymer blend photovoltaics where the nanostructure is thermodynamically, rather than kinetically, controlled.

Enhancing Electrochemical Transistors Based on Polymer-Wrapped (6,5) Carbon Nanotube Networks with Ethylene Glycol Side Chains.

Organic electrochemical transistors (ECTs) are an important building block for bioelectronics. To promote the required ion transport through the active layer, state-of-the-art semiconducting polymers feature hydrophilic ethylene glycol side chains that increase the volumetric capacitance and transconductance of the devices. Here, we apply this concept to polymer-wrapped single-walled carbon nanotubes (SWCNTs) as a high-mobility semiconducting material. We replace the polyfluorene copolymer (PFO-BPy), which is used for selectively dispersing semiconducting (6,5) SWCNTs and contains octyl side chains, by an equivalent polymer with tetraethylene glycol side chains. Aerosol-jet printed networks of these SWCNTs are applied as the active layer in water-gated ECTs. These show high hole mobilities (3-15 cm2·V-1·s-1), significantly improved volumetric capacitances and larger transconductances. Thin networks of SWCNTs reach (219 ± 16) F·cm-1·V-1·s-1 as the product of mobility and volumetric capacitance. In situ photoluminescence measurements show more efficient quenching of the near-infrared fluorescence for nanotube networks with hydrophilic glycol side chains compared to those with hydrophobic alkyl side chains, thus corroborating more complete charging under bias. Overall, networks of semiconducting SWCNTs with such tailored wrapping polymers provide excellent device performance. Combined with their inherent mechanical flexibility and durability, they constitute a competitive material for bioelectronics.

Reduced charge transfer exciton recombination in organic semiconductor heterojunctions by molecular doping.

We investigate the effect of molecular doping on the recombination of electrons and holes localized at conjugated-polymer-fullerene interfaces. We demonstrate that a low concentration of p-type dopant molecules (<4% weight) reduces the interfacial recombination via charge transfer excitons and results in a favored formation of separated carriers. This is observed by the ultrafast quenching of photoluminescence from charge transfer excitons and the increase in photoinduced polaron density by ~70%. The results are consistent with a reduced formation of emissive charge transfer excitons, induced by state filling of tail states.

Development of Quantum Dot (QD) Based Color Converters for Multicolor Display.

Many displays involve the use of color conversion layers. QDs are attractive candidates as color converters because of their easy processability, tuneable optical properties, high photoluminescence quantum yield, and good stability. Here, we show that emissive QDs with narrow emission range can be made in-situ in a polymer matrix, with properties useful for color conversion. This was achieved by blending the blue-emitting pyridine based polymer with a cadmium selenide precursor and baking their films at different temperatures. To achieve efficient color conversion, blend ratio and baking temperature/time were varied. We found that thermal decomposition of the precursor leads to highly emissive QDs whose final size and emission can be controlled using baking temperature/time. The formation of the QDs inside the polymer matrix was confirmed through morphological studies using atomic force microscopy (AFM) and transmission electron microscopy (TEM). Hence, our approach provides a cost-effective route to making highly emissive color converters for multi-color displays.

Electronic structure and interface properties of a model molecule for organic solar cells.

We study the electronic structure of 4,7-bis(5-methylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (MTBT) and its interface properties with gold using X-ray photoemission spectroscopy (XPS), valence-band ultraviolet photoemission spectroscopy (UPS), X-ray absorption spectroscopy (XAS), as well as resonant photoemission (ResPES). MTBT can be regarded as a model molecule for PCPDTBT, a promising candidate for efficient bulk heterojunction solar cells. Almost no contribution of sulfur and only a weak contribution of nitrogen to the HOMO level is found. At the interface with gold, a strong chemical interaction between the sulfur of the benzothiadiazole and gold occurs, which may have consequences for interface properties in devices.

Organic semiconductors for solution-processable field-effect transistors (OFETs).

The cost-effective production of flexible electronic components will profit considerably from the development of solution-processable, organic semiconductor materials. Particular attention is focused on soluble semiconductors for organic field-effect transistors (OFETs). The hitherto differentiation between "small molecules" and polymeric materials no longer plays a role, rather more the ability to process materials from solution to homogeneous semiconducting films with optimal electronic properties (high charge-carrier mobility, low threshold voltage, high on/off ratio) is pivotal. Key classes of materials for this purpose are soluble oligoacenes, soluble oligo- and polythiophenes and their respective copolymers, and oligo- and polytriarylamines. In this context, micro- or nanocrystalline materials have the general advantage of somewhat higher charge-carrier mobilities, which, however, could be offset in the case of amorphous, glassy materials by simpler and more reproducible processing.

From Broadband to Electrochromic Notch Filters with Printed Monochiral Carbon Nanotubes.

Dense layers of semiconducting single-walled carbon nanotubes (SWNTs) serve as electrochromic (EC) materials in the near-infrared with high optical density and high conductivity. EC cells with tunable notch filter properties instead of broadband absorption are created via highly selective dispersion of specific semiconducting SWNTs through polymer-wrapping followed by deposition of thick films by aerosol-jet printing. A simple planar geometry with spray-coated mixed SWNTs as the counter electrode renders transparent metal oxides redundant and facilitates complete bleaching within a few seconds through iongel electrolytes with high ionic conductivities. Monochiral (6,5) SWNT films as working electrodes exhibit a narrow absorption band at 997 nm (full width at half-maximum of 55-73 nm) with voltage-dependent optical densities between 0.2 and 4.5 and a modulation depth of up to 43 dB. These (6,5) SWNT notch filters can retain more than 95% of maximum bleaching for several hours under open-circuit conditions. In addition, different levels of transmission can be set by applying constant low voltage (1.5 V) pulses with modulated width or by a given number of fixed short pulses.

Highly Efficient Solid-State Near-infrared Organic Light-Emitting Diodes incorporating A-D-A Dyes based on α,ß-unsubstituted "BODIPY" Moieties.

We take advantage of a recent breakthrough in the synthesis of α,ß-unfunctionalised 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) moieties, which we symmetrically conjugate with oligothienyls in an unexpectedly stable form, and produce a "metal-free" A-D-A (acceptor-donor-acceptor) oligomer emitting in the near-infrared (NIR) thanks to delocalisation of the BODIPY low-lying lowest unoccupied molecular orbital (LUMO) over the oligothienyl moieties, as confirmed by density functional theory (DFT). We are able to retain a PL efficiency of 20% in the solid state (vs. 30% in dilute solutions) by incorporating such a dye in a wider gap polyfluorene matrix and demonstrate organic light-emitting diodes (OLEDs) emitting at 720 nm. We achieve external quantum efficiencies (EQEs) up to 1.1%, the highest value achieved so far by a "metal-free" NIR-OLED not intentionally benefitting from triplet-triplet annihilation. Our work demonstrates for the first time the promise of A-D-A type dyes for NIR OLEDs applications thereby paving the way for further optimisation.

Excited-State Interaction of Semiconducting Single-Walled Carbon Nanotubes with Their Wrapping Polymers.

We employ photoluminescence and pump-probe spectroscopy on films of semiconducting single-walled carbon nanotubes (CNTs) of different chirality wrapped with either a wide band gap polyfluorene derivative (PF12) or a polythiophene with narrower gap (P3DDT) to elucidate the excited states' interplay between the two materials. Excitation above the polymer band gap gives way to an ultrafast electron transfer from both polymers toward the CNTs. By monitoring the hole polaron on the polymer via its mid infrared signature, we show that also illumination below the polymer band gap leads to the formation of this fingerprint and infer that holes are also transferred toward the polymer. As this contradicts the standard way of discussing the involved energy levels, we propose that polymer-wrapped CNTs should be considered as a single hybrid system, exhibiting states shared between the two components. This proposition is validated through quantum chemical calculations that show hybridization of the first excited states, especially for the thiophene-CNT sample.

On-Chip Chemical Self-Assembly of Semiconducting Single-Walled Carbon Nanotubes (SWNTs): Toward Robust and Scale Invariant SWNTs Transistors.

In this paper, the fabrication of carbon nanotubes field effect transistors by chemical self-assembly of semiconducting single walled carbon nanotubes (s-SWNTs) on prepatterned substrates is demonstrated. Polyfluorenes derivatives have been demonstrated to be effective in selecting s-SWNTs from raw mixtures. In this work the authors functionalized the polymer with side chains containing thiols, to obtain chemical self-assembly of the selected s-SWNTs on substrates with prepatterned gold electrodes. The authors show that the full side functionalization of the conjugated polymer with thiol groups partially disrupts the s-SWNTs selection, with the presence of metallic tubes in the dispersion. However, the authors determine that the selectivity can be recovered either by tuning the number of thiol groups in the polymer, or by modulating the polymer/SWNTs proportions. As demonstrated by optical and electrical measurements, the polymer containing 2.5% of thiol groups gives the best s-SWNT purity. Field-effect transistors with various channel lengths, using networks of SWNTs and individual tubes, are fabricated by direct chemical self-assembly of the SWNTs/thiolated-polyfluorenes on substrates with lithographically defined electrodes. The network devices show superior performance (mobility up to 24 cm2 V-1 s-1 ), while SWNTs devices based on individual tubes show an unprecedented (100%) yield for working devices. Importantly, the SWNTs assembled by mean of the thiol groups are stably anchored to the substrate and are resistant to external perturbation as sonication in organic solvents.

How intermolecular geometrical disorder affects the molecular doping of donor-acceptor copolymers.

Molecular doping of conjugated polymers represents an important strategy for improving organic electronic devices. However, the widely reported low efficiency of doping remains a crucial limitation to obtain high performance. Here we investigate how charge transfer between dopant and donor-acceptor copolymers is affected by the spatial arrangement of the dopant molecule with respect to the copolymer repeat unit. We p-dope a donor-acceptor copolymer and probe its charge-sensitive molecular vibrations in films by infrared spectroscopy. We find that, compared with a related homopolymer, a four times higher dopant/polymer molar ratio is needed to observe signatures of charges. By DFT methods, we simulate the vibrational spectra, moving the dopant along the copolymer backbone and finding that efficient charge transfer occurs only when the dopant is close to the donor moiety. Our results show that the donor-acceptor structure poses an obstacle to efficient doping, with the acceptor moiety being inactive for p-type doping.

In situ formation and photo patterning of emissive quantum dots in small organic molecules.

Nanostructured composites of inorganic and organic materials are attracting extensive interest for electronic and optoelectronic device applications. Here we report a novel method for the fabrication and patterning of metal selenide nanoparticles in organic semiconductor films that is compatible with solution processable large area device manufacturing. Our approach is based upon the controlled in situ decomposition of a cadmium selenide precursor complex in a film of the electron transporting material 1,3,5-tris(N-phenyl-benzimidazol-2-yl)-benzene (TPBI) by thermal and optical methods. In particular, we show that the photoluminescence quantum yield (PLQY) of the thermally converted CdSe quantum dots (QDs) in the TPBI film is up to 15%. We also show that laser illumination can form the QDs from the precursor. This is an important result as it enables direct laser patterning (DLP) of the QDs. DLP was performed on these nanocomposites using a picosecond laser. Confocal microscopy shows the formation of emissive QDs after laser irradiation. The optical and structural properties of the QDs were also analysed by means of UV-Vis, PL spectroscopy and transmission electron microscopy (TEM). The results show that the QDs are well distributed across the film and their emission can be tuned over a wide range by varying the temperature or irradiated laser power on the blend films. Our findings provide a route to the low cost patterning of hybrid electroluminescent devices.

Quantifying Charge Extraction in Organic Solar Cells: The Case of Fluorinated PCPDTBT.

We introduce a new and simple method to quantify the effective extraction mobility in organic solar cells at low electric fields and charge carrier densities comparable to operation conditions under one sun illumination. By comparing steady-state carrier densities at constant illumination intensity and under open-circuit conditions, the gradient of the quasi-Fermi potential driving the current is estimated as a function of external bias and charge density. These properties are then related to the respective steady-state current to determine the effective extraction mobility. The new technique is applied to different derivatives of the well-known low-band-gap polymer PCPDTBT blended with PC70BM. We show that the slower average extraction due to lower mobility accounts for the moderate fill factor when solar cells are fabricated with mono- or difluorinated PCPDTBT. This lower extraction competes with improved generation and reduced nongeminate recombination, rendering the monofluorinated derivative the most efficient donor polymer.

Carbon nanotube network ambipolar field-effect transistors with 10(8) on/off ratio.

Polymer wrapping is a highly effective method of selecting semiconducting carbon nanotubes and dispersing them in solution. Semi-aligned semiconducting carbon nanotube networks are obtained by blade coating, an effective and scalable process. The field-effect transistor (FET) performance can be tuned by the choice of wrapping polymer, and the polymer concentration modifies the FET transport characteristics, leading to a record on/off ratio of 10(8) .