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Diblock copolymer electrolytes based on a π-conjugated polyfluorene (PF) backbone were synthesized comprising nanodomains of a polymerized ionic liquid (PIL) and of a solid polymer electrolyte (SPE...

Conjugated polymers exhibit strong interactions with single-walled carbon nanotubes (SWNTs). These enable the selective dispersion of specific semiconducting SWNTs in organic solvents and polymer-m...

Funding: This research was funded by European commission under FP7 LAMP project “Laser Induced Synthesis of Polymeric Nanocomposite Materials and Development of Micro-patterned Hybrid Light Emitting Diodes (LED) and Transistors (LET)” (Grant No. 247928) 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. Publisher PDF

The availability of purely semiconducting single-walled carbon nanotube (s-SWCNT) dispersions has prompted their widespread application in solution-processed thin-film transistors with excellent device performance but has also raised the question of how their precise composition influences charge transport properties in random networks. Here, we compare hole and electron transport in three different polymer-sorted s-SWCNT networks from nearly monochiral (6,5) nanotubes (diameter 0.76 nm) to mixed networks of s-SWCNTs with medium (0.8–1.3 nm) and large (1.2–1.6 nm) diameters. Temperature-dependent field-effect mobilities are extracted from gated four-point probe measurements that exclude any contributions by contact resistance and indicate thermally activated transport. The mobility data can be fitted to the fluctuation-induced tunneling model, although with significant differences between the network compositions. The network with the broadest diameter and thus bandgap range results in the strongest tempe...

The aggregation-induced emission (AIE) phenomenon provides a new direction for the development of organic light-emitting devices. Here, we present a new class of emitters based on 4,4-difluoro-4-bora-3 a ,4 a -diaza- s -indacene (BODIPY), functionalized at different positions with tetraphenylethylene (TPE), which is one of the most famous AIE luminogens. Thanks to this modification, we were able to tune the photoluminescence of the BODIPY moiety from the green to the near-infrared (NIR) spectral range and achieve PL efficiencies of ~50% in the solid state. Remarkably, we observed an enhancement of the AIE and up to ~100% photoluminescence efficiencies by blending the TPE-substituted BODIPY fluorophores with a poly[(9,9-di- n -octylfluorene-2,7-diyl)- alt -(benzo[2,1,3]thiadiazol-4,7-diyl)] (F8BT) matrix. By incorporating these blends in organic light-emitting diodes (OLEDs), we obtained electroluminescence peaked in the range 650–700 nm with up to 1.8% external quantum efficiency and ~2 mW/cm2 radiance, a remarkable result for red/NIR emitting and solution-processed OLEDs.

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 (PC71BM). 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.

It is demonstrated that α,β-unsubstituted meso-positioning thienyl BODIPY is an electron deficient unit that leads to the development of ultra low optical band gap (Eoptg < 1 eV) π-conjugated D–A quarterthiophene polymers. Furthermore, it is revealed that the optoelectronic, electrochemical and charge transporting properties of the resulting α,β-unsubstituted meso-positioning thienyl BODIPY quaterthiophene-based polymers are alkyl side chain positioning dependent. Tail-to-tail (TT) positioning of the alkyl side chains at the two central thiophenes of the quaterthiophene segment results in lower Eoptg, higher energy levels and increased hole mobility as compared to head-to-head (HH) positioning. Finally, even though the synthesized polymers exhibit high electron affinity, higher even than that of the fullerene derivative [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), they present only p-type behaviour in field effect transistors (FETs) independent of the alkyl side chain positioning.

We employ photoluminescence and pumpprobe 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 thiopheneCNT sample.

We acknowledge financial support from FP7 project “Laser Induced Synthesis of Polymeric Nanocomposite Materials and Development of Micro-Patterned Hybrid Light Emitting Diodes (LED) and Transistors (LET)”-LAMP (Project G.A.247928). A.K.B. and I.D.W.S. also acknowledge financial support from EPSRC Programme “Challenging the Limits of Photonics: Structured Light” Grant EP/J01771X/1. Quantum dots are of growing interest as emissive materials in light emitting devices. Here first we report the formation of highly luminescent organic capped colloidal cadmium sulfide (CdS) nanoparticles having the highest photoluminescence quantum yield of 69% in solutions and 34% in neat thin films in the near-infrared range. Second, we also show efficient electroluminescence in the near-infrared from solution processed hybrid light emitting diodes (LEDs) based on such colloidal CdS quantum dots embedded in an organic semiconductor matrix forming a nanocomposite active layer. We also discuss the device structure and role of the doped active layer in efficiency improvement. With optimized active layer thickness and concentration of QDs, the device exhibits an external electroluminescence quantum efficiency of 0.62% at a peak emission wavelength of 760 nm, providing a route to solution processable flexible light sources for biosensors and medicine. Publisher PDF

Printed random networks of polymer-wrapped multi-chiral semiconducting carbon nanotubes (s-SWCNTs) are an opportunity for mass-manufacturable, high-performance large-area electronics. To meet this goal, a deeper understanding of charge-transport mechanisms in such mixed networks is crucial. Here, charge transport in field-effect transistors based on inkjet-printed s-SWCNTs networks is investigated, obtaining direct evidence for the phases probed by charge in the accumulated channel, which is critical information to rationalize the different transport properties obtained for different printing conditions. In particular, when the fraction of nanotubes with smaller bandgaps is efficiently interconnected, the sparse network provides efficient charge percolation for band-like transport, with a charge mobility as high as 20.2 cm(2) V-1 s(-1). However, when the charges are forced by a less efficient morphology, to populate also higher bandgap nanotubes and and/or the wrapping polymer, thermally activated transport takes place and mobility drops. As a result, a trade-off between network density and charge transport properties is identified for device current optimization, in both p- and n-type regimes. These findings shed light on the fundamental aspects related to charge transport in printed s-SWCNT mixed networks and contribute to devise appropriate strategies for the formulation of inks and processes towards cost-effective mass production schemes of high-performance large-area electronics.

The authors acknowledge financial support from FP7 project “Laser Induced Synthesis of Polymeric Nanocomposite Materials and Development of Micro‐patterned Hybrid Light Emitting Diodes (LED) and Transistors (LET)” – LAMP project (G.A. 247928). M.T.S., A.R., and I.D.W.S. acknowledge support from the European Research Council (EXCITON grant 321305). I.D.W.S. acknowledges a Royal Society Wolfson Research Merit Award. The authors are grateful to EPSRC for an equipment grant (EP/L017008/1). Exciton diffusion plays an important role in functional materials used in organic optoelectronic devices, such as solar cells, organic light emitting diodes, and lasers. Here we explore how exciton diffusion can be controlled in highly fluorescent blue‐emitting polyfluorene materials by changing the length and type of side chains. We find that the exciton diffusion coefficient (D) decreases from 1.2 × 10−3 cm2 s−1 to 0.2×10−3 cm2 s−1 when the side chain length is increased from 8 to 12 carbon atoms. Other changes to the side chains led to enhancement of D up to 1.6 × 10−3 cm2 s−1. Our results show that small adjustments to the molecular structure can be helpful for the future development of high‐brightness organic light emitting devices. Postprint Postprint

Semiconducting single‐walled carbon nanotubes (s‐SWNTs) are used as a protective interlayer between the lead sulfide colloidal quantum dot (PbS CQD) active layer and the anode of the solar cells (SCs). The introduction of the carbon nanotubes leads to increased device stability, with 85% of the initial performance retained after 100 h exposure to simulated solar light in ambient condition. This is in sharp contrast with the behavior of the device without s‐SWNTs, for which the photoconversion efficiency, the open circuit voltage, the short‐circuit current, and the fill factor all experiencing a sharp decrease. Therefore, the inclusion of s‐SWNT as interlayer in CQD SCs, give rise to SCs of identical efficiency (above 8.5%) and prevents their performance degradation. ISSN:2196-7350

Noncovalent functionalization of single-walled carbon nanotubes (SWNTs) using π-conjugated polymers has become one of the most effective techniques to select semiconducting SWNTs (s-SWNTs). Several conjugated polymers are used, but their ability to sort metallic and semiconducting species, as well as the dispersions yields, varies as a function of their chemical structure. Here, three polymers are compared, namely, poly[2,6-(4,4-bis-(2-dodecyl)-4H-cyclopenta[2,1-b;3,4b′]dithiophene)-alt-4,7(2,1,3-ben-zothiadiazole)] (P12CPDTBT), poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PF12), and poly(3-dodecylthiophene-2,5-diyl) (P3DDT) in their ability to select two types of carbon nanotubes comprising small (≈1 nm) and large (≈1.5 nm) diameters. P12CPDTBT is a better dispersant than PF12 for small diameter nanotubes, while both polymers are good dispersants of large diameter nanotubes. However, these dispersions contain metallic species. P3DDT, instead presents the best overall performance regarding the selectivity toward semiconducting species, with a dispersion yield for s-SWNTs of 15% for small and 21% for large diameter nanotubes. These results are rationalized in terms of electronic and chemical structure showing that: (i) the binding energy is stronger when more alkyl lateral chains adsorb on the nanotube surface; (ii) the binding energy is stronger when the polymer backbone is more flex-ible; (iii) the purity of the dispersions seems to depend on a strong polymer– nanotube interaction.

The authors acknowledge financial support from the UK EPSRC (Grants EP/E065066/1, EP/E062636/1, EP/J009318/1 and EP/J009019/1), from the EPSRC Scottish Centre for Doctoral training in Condensed Matter Physics and from the European Union Seventh Framework Programme under Grant Agreement 321305. Electronic energy transfer (EET) in organic materials is a key mechanism that controls the efficiency of many processes, including light harvesting antennas in natural and artificial photosynthesis, organic solar cells, and biological systems. In this paper we have examined EET in solid-state thin-films of polyfluorene, a prototypical conjugated polymer, with ultrafast photoluminescence experiments and theoretical modeling. We observe EET occurring on a 680 ± 300 fs time scale by looking at the depolarisation of photoluminescence. An independent, predictive microscopic theoretical model is built by defining 125 000 chromophores containing both spatial and energetic disorder appropriate for a spin-coated thin film. The model predicts time-dependent exciton dynamics, without any fitting parameters, using the incoherent Förster-type hopping model. Electronic coupling between the chromophores is calculated by an improved version of the usual line-dipole model for resonant energy transfer. Without the need for higher level interactions, we find that the model is in general agreement with the experimentally observed 680 ± 300 fs depolarisation caused by EET. This leads us to conclude that femtosecond EET in polyfluorene can be described well by conventional resonant energy transfer, as long as the relevant microscopic parameters are well captured. The implications of this finding are that dipole-dipole resonant energy transfer can in some circumstances be fully adequate to describe ultrafast EET without needing to invoke strong or intermediate coupling mechanisms. Postprint

A new ultra low band gap (LBG) α,β-unsubstituted BODIPY-based conjugated polymer has been synthesized by conventional cross coupling polymerization techniques (Stille cross coupling) for the first time. The polymer exhibits a panchromatic absorption spectrum ranging from 300 nm to 1100 nm and an optical band gap (Eoptg) of 1.15 eV, suitable for near infrared (NIR) organic photovoltaic applications as electron donor. Preliminary power conversion efficiency (PCE) of 1.1% in polymer : [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) 1 : 3 weight ratio bulk heterojunction (BHJ) solar cells has been achieved, demonstrating very interesting and promising photovoltaic characteristics, such as good fill factor (FF) and open circuit voltage (Voc). These results showing that by the proper chemical design, new α,β-unsubstituted BODIPY-based NIR copolymers can be developed in the future with suitable energy levels matching those of PC71BM towards more efficient NIR organic photovoltaics (OPVs).

International audience; The influence of annealing temperature on the structural and electrical properties of conjugated poly(dodecyl-quaterthiophene) (PQT-12) polymer films is exploited. The temperature induced changes of structural parameters are monitored by in situ grazing incident X-ray diffraction (GIXD) and the conductivity. They are complemented by studies of the dielectric properties using variable angle spectroscopic ellipsometry (VASE). An increase of the scattered intensity, the size of the crystalline domains, and the current response is observed for a first thermal cycle with stepwise heating up to 90 degrees C, which revealed two polymorphs with different degrees of interdigitation in PQT-12. Irreversible changes are observed for the second cycle with a higher thermal budget up to 140 degrees C and are connected with a transition from the highly ordered to powder-like disordered phase for the main PQT-12 form whereas the second polymorph with stronger interdigitation completely vanished. In agreement with these observations high-temperature VASE studies demonstrated a blue shift of the transitions with a reduction in the conjugation length caused by an increase in the twist and torsion of the backbone. Combined GIRD, VASE, and electrical characterizations show that PQT-12 exhibits a complex interplay between two polymorphs with a strong influence on the charge carrier transport depending on the thermal budget employed.

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 cm(2) 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.

We take advantage of a recent breakthrough in the synthesis of α,β-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.

One of the key aspects to achieve high efficiency in ternary bulk-hetorojunction solar cells is the physical and chemical compatibility between the donor materials. Here, we report the synthesis of a novel conjugated polymer (P1) containing alternating pyridyl[2,1,3] thiadiazole between two different donor fragments, dithienosilole and indacenodithienothiophene (IDTT), used as a sensitizer in a host system of indacenodithieno[3,2-b]thiophene, 2,3-bis(3-(octyloxy)phenyl)quinoxaline (PIDTTQ) and [6,6]-phenyl C-70 butyric acid methyl ester (PC71BM). We found that the use of the same IDTT unit in the host and guest materials does not lead to significant changes in the morphology of the ternary blend compared to the host binary. With the complementary use of optoelectronic characterizations, we found that the ternary cells suffer from a lower mobility-lifetime (mu tau) product, adversely impacting the fill factor. However, the significant light harvesting in the near infrared region improvement, compensating the transport losses, results in an overall power conversion efficiency enhancement of similar to 7% for ternary blends as compared to the PIDTTQ:PC71BM devices.

The authors acknowledge financial support from FP7 project “Laser Induced Synthesis of Polymeric Nanocomposite Materials and Development of Micro-patterned Hybrid Light Emitting Diodes (LED) and Transistors (LET)” – LAMP project (G.A. 247928). AKB and IDWS also acknowledge financial support from EPSRC Programme grant “Challenging the limits of photonics: Structured light” EP/J01771X/1. Conjugated polymer-semiconductor quantum dot (QD) composites are attracting increasing attention due to the complementary properties of the two classes of materials. We report a convenient method for in situ formation of QDs, and explore the conditions required for light emission of nanocomposite blends. In particular we explore the properties of nanocomposites of the blue emitting polymer poly[9,9-bis(3,5-di-tert-butylphenyl)-9H-fluorene] together with cadmium sulphide (CdS) and cadmium selenide (CdSe) precursors. We show the formation of emissive quantum dots of CdSe from thermally decomposed precursor. The dots are formed inside the polymer matrix and have a photoluminescence quantum yield of 7.5%. Our results show the importance of appropriate energy level alignment, and are relevant to the application of organic-inorganic systems in optoelectronic devices. Publisher PDF

In the past years, high-quality semiconducting single-walled carbon nanotube (s-SWCNT) inks obtained by conjugated polymer wrapping using toluene as solvent have been used for the fabrication of high-performance field-effect transistors. Charge-carrier mobilities up to 50 cm(2) V-1 s(-1) and on/off ratios above 10(8) have been reported for devices based on networks of s-SWCNT. However, devices fabricated from inks that are only a few weeks old generally show a marked decrease in performance, indicating the limited shelf life of toluene-based inks. The use of o-xylene as applicator solvent to obtain high quality and very stable s-SWCNT inks is reported. The charge carrier mobility of field-effect transistors fabricated with this new ink show a twofold increase in magnitude compared to devices prepared from a toluene solution. More importantly, the device-to-device performance shows improved reproducibility, which is ascribed to the higher degree of homogeneity of the s-SWCNT network deposited from o-xylene with respect to the one from toluene. Finally, the o-xylene inks maintain their initial properties for longer than one year. This very long shelf life is an important pre-condition for the industrial use of s-SWCNT inks.

Future application of conjugated (co)polymers, e.g. in electronic devices, requires the availability of up-scalable synthetic procedures. “Conventional” (hetero)aryl-(hetero)aryl coupling schemes often produce toxic or environmentally risky by-products. Here, so-called direct arylation schemes without use of organometallic reagents or related anion equivalents came in the focus of attention as simplified alternative to standard coupling procedures. Now, a couple of structurally defined conjugated (co)polymers can be generated in high yield and molecular weight via direct arylation polycondensation. The promising application potential of these materials as active component of organic solar cells has been demonstrated.

A detailed investigation of the open circuit voltage (V-OC) of organic bulk heterojunction solar cells comprising three different donor polymers and two different fullerene-based acceptors is presented. Bias amplified charge extraction (BACE) is combined with Kelvin Probe measurements to derive information on the relevant energetics in the blend. On the example of P3HT:PC70BM the influence of composition and preparation conditions on the relevant transport levels will be shown. Moderate upward shifts of the P3HT HOMO depending on crystallinity are observed, but contrarily to common believe, the dependence of V-OC on blend composition and thermal history is found to be largely determined by the change in the PCBM LUMO energy. Following this approach, we quantified the energetic contribution to the V-OC in blends with fluorinated polymers or higher adduct fullerenes.

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.

The recent development of donor–acceptor copolymers has led to an enormous improvement in the performance of organic solar cells and organic field-effect transistors. Here we describe the synthesis, detailed characterisation, and application of a series of structurally modified copolymers to investigate fundamental structure–property relationships in this class of conjugated polymers. The interplay between chemical structure and optoelectronic properties is investigated. These are further correlated to the charge transport and solar cell performance, which allows us to link their chemical structure to the observed physical properties.

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).

Poly(4,4-dialkyl-cyclopenta[2,1-b:3,4-b′]dithiophene-alt-2,1,3-benzothiadiazole) (PCPDTBT), a potentially interesting low bandgap donor copolymer for bulk heterojunction-type organic solar cells with a power conversion efficiency >5.5%, can be now synthesized in a direct arylation scheme starting from 4,4-dialkyl-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) and 4,7-dibromo-2,1,3-benzothiadiazole (4,7-dibromo-BT) as monomers. The direct arylation procedure leads to PCPDTBT with an Mn of up to 40 000 and circumvents the use of costly diboronic acid/ester or distannyl monomers.

The application of new polymer materials for organic solar cells requires a detailed understanding of the orientation and the electronic properties of the system because these have a dramatic effect on the device performance and efficiency. The films were prepared by doctor blade casting, a simple preparation route from solution. We study the orientation of the low band gap polymers poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) and poly-[2,6-(4,4-bis(2-ethyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene)-alt-5,8-(2,3-dioctylquinoxaline)] (PCPDTQ) as well as of the state of the art polymer poly(3-hexyl-thiophene) (P3HT) using near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Clear angular dependence in the NEXAFS spectra is observed for P3HT and PCPDTBT pointing to well-orientated films.

The authors investigated organic tandem solar cells with solution processed recombination contacts made from a TiO2 sol in combination with PEDOT:PSS. Tandem cells were prepared either with two P3HT:PCBM-based subcells or by combining PCDTBT:PC[70]BM and PCPDTBT:PC[70]BM. Optical modeling is used to predict experimental short circuit currents of the tandem solar cells as a function of layer thickness. Both types of tandem cells yield energy conversion efficiencies of ca. 3.3%. We propose that a significant improvement of the performance of the PCDTBT:PC[70]BM - PCPDTBT:PC[70]BM tandem is possible by optimization of the recombination contact and layer thicknesses of both subcells.

Conjugated, alternating copolymers composed of electron-rich (so-called “donor”) and electron-poor (so-called “acceptor”) building blocks are in the focus of very intense research activities both on the academic and industrial scale. Main driving force for this research is their application as hole-generation/hole-transport material (donor material) in bulk heterojunction-type organic solar cells (BHJ-OSCs). [There is some confusion in the use of the term “donor”. “Donor” is both used for the electron-rich building blocks of the alternating copolymers and the copolymer material itself in its function as hole-generating/hole-transport component of the active polymer/fullerene blend in bulk heterojunction-type organic solar cells (BHJ-OSCs).] The number of published copolymer structures is currently rapidly increasing. A couple of examples allow, in combination with soluble fullerene acceptors (PC61BM, PC71BM), for power conversion efficiencies (PCEs) of >5.5%. Unfortunately, reliable design rules in the selection of the electron-rich and electron-poor building blocks of the alternating copolymers are not available until now. Nevertheless, a few donor and acceptor building blocks have proved to be effective toward maximum PCEs including N-alkylcarbazole-2,7-diyl and 4,4-dialkyl-cyclopenta[2,1-b:3,4-b0]dithiophene-2,6-diyl (CPDT) as “donor” and benzo-2,1,3-thiadiazole-4, 7-diyl, 4,7-dithienyl-2,1,3-benzothiadiazole-50,50 0-diyl,9,10 and thiophene-3,4-dicarbimide-2,5-diyl as “acceptor” units. The combination of 4,4-dialkyl-cyclopenta[2,1-b:3,4-b0]dithiophene-2,6-diyl and benzo-2,1,3-thiadiazole-4,7-diyl building blocks leads to the well-known poly(4,4-dialkyl-cyclopenta[2,1-b:3,4-b0]dithiophene2,6-diyl-co-benzo-2,1,3-thiadiazole-4,7-diyl) PCPDTBT copolymer (introduced by Konarka) for which a PCE of 5.5% in alternating copolymer/fullerene BHJ-OSCs has been reported after a careful optimization of blend composition and processing conditions by Peet et al. One shortcoming in the application of PCPDTBT and related copolymers in donor-acceptor couples with soluble fullerene acceptors (PC61BM, PC71BM) is the relatively low open circuit voltage of the resulting BHJ-OSCs with a Voc of ca. 0.6-0.7 V. An increase of the Voc value in related alternating copolymers can be expected for a decreased HOMO energy level of the alternating copolymer. Hereby, the energetic position of the copolymer HOMO level is mainly determined by the structure of the electronrich (“donor”) building block of the alternating copolymer. The favorable downshift of the HOMO energy level can be accomplished by a successive replacement of aromatic (sp) carbons of the thiophene unit by sp nitrogens. Following this idea we have focused our synthetic efforts on 7,7-dialkyl-cyclopenta[1,2-d:4,3d0]dithiazole-2,5-diyl-type monomers as target for further condensation with suitable electron-poor “acceptor” moieties. Results and Discussion

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.

Two swivel cruciform oligothiophenes are investigated for the occurrence of oxidative ring closure reactions. Surprisingly, instead of intramolecular cyclizations, a regioselective side chain oxidation of two of the four alpha-methylene groups next to the terminal thiophene rings is detected for one of the oligomers. Such side chain oxidations may be one unintended degradation pathway in oligothiophene-based organic semiconductors used as the active layer of organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), and organic solar cells (OSCs).

Layers made from soluble low molecular weight polythiophene PQT-12 with low polydispersity exhibit a highly ordered structure and charge-carrier mobilities of the order of 10−3 cm2/(V s), which we attribute to its proximity to monodispersity. We propose that polydispersity is a decisive factor with regard to structure formation and transport properties of soluble low molecular weight polythiophenes.

Polythiophenes are being intensively studied for use in polymer/fullerene bulk-heterojunction photovoltaic (PV) devices because they absorb light throughout the entire visible spectrum and show high hole mobility, which are the two most important parameters for a successful donor in these devices. A large body of anecdotal information exists about what morphological features lead to a high efficiency bulk-heterojunction PV device, but few design parameters for improved polymers exist. We compare two polythiophene isomers, poly(3-hexylthiophene) (P3HT) and poly(3,3′′′-didodecylquaterthiophene) (PQT-12) to determine which parameters make these two polymers more and less suitable for use in bulk-heterojunction solar cells. We find that although they have nearly identical absorption spectra and hole mobilities, the PV devices made using P3HT are far superior to those made using PQT-12 because the PQT-12 forms crystalline nanoparticles in solution that do not form electrical connections in the thin films. The ...

This work is part of the TIRAMISU project, funded by the European Commission’s Seventh Framework Programme (FP7/2007-2013) under grant agreement n◦284747 and the Engineering and Physical Sciences Research Council (EPSRC) grants EP/J009016/1 and EP/F059922/1. I.D.W.S. and P.J.S. are Royal Society Wolfson Research Merit Award holders. The density of organic semiconductor films is an important quantity because it is related to intermolecular spacing which in turn determines the electronic and photophysical properties. We report thin film density and refractive index measurements of polyfluorenes and star-shaped oligofluorene truxene molecules. An ellipsometer and a procedure using a spectrophotometer were used to determine film thickness and mass of spin-coated films, respectively. We present a study of the effect of alkyl side-chain length on the volumetric mass density and refractive index of the materials studied. The density measured for poly(9,9-di-n-octylfluorene) (PF8) was 0.88 ± 0.04 g/cm3 and decreased with longer alkyl side chains. For the truxene molecule with butyl side chains (T3 butyl), we measured a density of 0.90 ± 0.04 g/cm3, which also decreased with increasing side-chain length. Postprint

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.

We report on the investigation of the temperature effect on the selective dispersion of single-walled carbon nanotubes (SWNTs) by Poly(3-dodecylthiophene-2,5-diy1) wrapping. The interaction mechanism between polymer chains and SWNTs is studied by controlling the polymer aggregation via variation of the processing temperature. Optical absorption and photoluminescence measurements including time resolved photoluminescence spectroscopy are employed to study the degree of interaction between the polymer in different aggregation states and the carbon nanotubes. At low processing temperatures, results are consistent with the planarization of the polymer chains and with SWNTs working as seeds for polymer aggregation. The formation of small clusters due to the inter-digitation of alkyl tails between neighboring polymer-wrapped SWNTs allows the formation of the SWNT bundles, as experimentally evidenced and investigated by molecular dynamics simulations. The interaction between the tubes within the bundles, which is reflected in the variation of the photoluminescence dynamics of the polymer, can be suppressed by warming up the sample. (C) 2014 Elsevier Ltd. All rights reserved.

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. This journal is © The Royal Society of Chemistry.

We report our investigations of the photophysical properties of a so-called “donor-acceptor” (D-A) electroluminescent polymer incorporating N-alkyl-4,7-di(thien-2-yl)-2,1,3-benzothiadiazole-5,6-dicarboxylic imide (DI) as the electron acceptor and benzo[1,2-b:4,5-b′]dithiophene (BDT) acceptor units. Polymer light-emitting diodes (PLEDs) based on this copolymer give the best efficiency (external quantum efficiency EQE∼0.04%) and show a pure near-infrared (NIR) electroluminescence (EL) with a peak at 870 nm.

Oligothiophene monomers with a fixed length varying from 3 to 5 thiophene rings were prepared by Stille coupling. They were functionalised with one or two methacrylate groups to allow polymerization and cross-linking. These monomers can be patterned with the help of soft silicon stamps (e.g., with the micro-injection moulding in capillaries process) on substrates like glass or flexible polymer foils. If a photoinitiator has been added, they can then be hardened by exposure to UV-light through the transparent stamp. Afterwards the stamp can be removed. This allows the preparation of several centimeters long oligothiophene lines of a width varying between 50 and 0.5 μm. These lines of semiconducting organic material can be used e.g., as an orientation layer for liquid crystals. The resulting line patterns from embossing oligomer 17, line width 500 nm.

Poly(3-alkylthiophenes) are one of the most frequently used conjugated polymer classes in organic photovoltaics. Here, a generalized packing model for the polythiophene main chains in the crystalline form I of high molecular weight regioregular poly(3-alkylthiophenes) with extended side chains (pentyl through octyl) is reported. The model is based on structural constraints from solid-state NMR: short internuclear distances of less than 4.0 A of neighboring thiophene protons parallel to the stacking direction and the isotropic chemical shift for the thiophene protons is high-field shifted by 0.9 ± 0.1 ppm. Nucleus-independent chemical shift calculations show that only the most recent structure for P3HT (space group P21/c) is compatible with these structural constraints. On this basis, a generalized packing model is developed, showing that slipping parallel to the stacked polymer chains of up to 1.5 A is allowed, while out-of-plane tilts perpendicular to the stacked chains are only tolerated up to 20°.

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.

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.

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) .

In this work we discuss an elegant, alternative strategy to extend the spectral sensitivity of wide bandgap polymers in the near IR region. We discuss the microstructure of different functional ternary systems and compare them from different perspectives.

We report on improved power conversion efficiencies (PCEs) approaching 3.5% for bulk-heterojunction (BHJ) hybrid solar cells based on CdSe nanorods and the low band gap polymer poly[2,6-(4,4-bis-(2-ethylhexyl)- 4H -cyclopenta[2,1- b ;3,4- b ′]-dithiophene)- alt -4,7-(2,1,3-benzothiadiazole)] (PCPDTBT). Different post-synthetic surface treatments of CdSe nanorods are applied and the performances of the resulting hybrid solar cells are compared. The improved PCE values are attributed to the advanced post-synthetic surface treatment of the CdSe nanorods, leading to an effective reduction of synthesis ligands from the nanorod surfaces prior to ligand exchange with pyridine. The sequential decrease of the ligands is confirmed by a combination of thermogravimetric analysis-mass spectrometry (TGA-MS).

We have applied time-delayed collection field (TDCF) and charge extraction by linearly increasing voltage (CELIV) to investigate the photogeneration, transport, and recombination of charge carriers in blends composed of PCPDTBT/PC70BM processed with and without the solvent additive diiodooctane. The results suggest that the solvent additive has severe impacts on the elementary processes involved in the photon to collected electron conversion in these blends. First, a pronounced field dependence of the free carrier generation is found for both blends, where the field dependence is stronger without the additive. Second, the fate of charge carriers in both blends can be described with a rather high bimolecular recombination coefficients, which increase with decreasing internal field. Third, the mobility is three to four times higher with the additive. Both blends show a negative field dependence of mobility, which we suggest to cause bias-dependent recombination coefficients.

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.

We have synthesised and characterised a new low-gap conjugated polymer, with a broad absorption profile. In blends with a C(70) derivative we demonstrate power conversion efficiencies of 0.76%. We show electroluminescence from the polymer peaking at 956 nm, and quantum efficiency of 0.02% in a blend.