Christos L. Chochos

Electronic Properties and Photovoltaic Performances of a Series of Oligothiophene Copolymers Incorporating Both Thieno[3,2-b]thiophene and 2,1,3-Benzothiadiazole Moieties.

A series of donor-acceptor alternated conjugated copolymers, composed of thiophene, bithiophene, thieno[3,2-b]thiophene, and 2,1,3-benzothiadiazole units and differing from each other by the nature and the number of 3-alkylthiophene in the backbone, have been synthesized by Stille cross-coupling polymerization. The materials optical and electrochemical properties, in solution and in thin films, have been investigated using UV-Visible absorption and cyclic voltammetry. Bulk heterojunction solar cells using blends of the newly synthesized copolymers, as electron donor, and C60-PCBM or C70-PCBM, as electron transporting material, have been elaborated. A maximum power conversion efficiency of 1.8% is achieved with a 1:4 PPBzT(2) -C12:C70-PCBM weight ratio.

Novel Hybrid Materials Consisting of Regioregular Poly(3‐octylthiophene)s Covalently Attached to Single‐Wall Carbon Nanotubes

Facile routes for the synthesis of hybrid materials consisting of regioregular poly(3-octylthiophene)s covalently attached to single-wall carbon nanotubes are presented for the first time. These materials are easily processable using common organic solvents, and at the same time combine the properties of regioregular poly(3-alkylthiophene)s with those of single-wall carbon nanotubes. Moreover, studies of the properties of these materials have provided strong evidence for an electron transfer from the regioregular poly(3-octylthiophene) to the single-wall carbon nanotube.

Ultra low band gap α,β-unsubstituted BODIPY-based copolymer synthesized by palladium catalyzed cross-coupling polymerization for near infrared organic photovoltaics

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).

Theoretical study of phenyl-substituted indacenodithiophene copolymers for high performance organic photovoltaics

The theoretical estimation of energy levels and energy gaps of conjugated polymers for organic photovoltaics (OPVs) represents in principle a useful tool for the prescreening of new donor systems as a suitable pair for the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). In this study, ten tetraphenyl-substituted indacenodithiophene (IDT) copolymers (eight in the form of donor-acceptor), whose energy gaps vary in the range of 1.48-2.11 eV have been selected and their highest occupied molecular orbitals (HOMOs), lowest unoccupied molecular orbitals (LUMOs), and gap energies have been calculated by applying density functional theory (DFT) and/or time-dependent density functional theory (TD-DFT) methods. In spite of the examined molecular structure variety, nice correlations (theoretical models) between experimental and theoretical electronic parameters were found. It is shown that the theoretical band gap estimated by the TD-DFT using dimer model compounds and DFT using tetramer model compounds provide in good agreement the optical band gap of these polymers. Finally, the optimum theoretical limits of the LUMO offset between the fullerene and the IDT tetramer model compounds, for which high performance OPVs (efficiency > 6%) are obtained, is presented for the first time.

Highly Efficient Solid-State Near-infrared Organic Light-Emitting Diodes incorporating A-D-A Dyes based on α,β -unsubstituted “BODIPY” Moieties

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.

Porous organic polymers as emerging new materials for organic photovoltaic applications: current status and future challenges

Porous organic polymers are materials with covalently bonded (hydro)thermally stable backbones exhibiting high and accessible surface areas, and properties which are intriguing in the field of (opto)electronics. Especially in organic photovoltaics (OPVs), the electron rich backbone of this class of materials provides exceptional light absorption properties for improved charge formation/separation. Particularly for devices operating via bulk heterojunctions, the rigid open voids along the porous skeleton are rather interesting for improved phase separation and act as an ideal host to the prospective acceptors. Furthermore, their high dimensional geometry allows them to transport electrons/holes independent of orientation issues. However, the non-soluble nature of this class of materials limits their processability for forming uniform films, which is essential for device fabrication. In this review, a brief overview on the reported OPV devices fabricated by applying porous organic polymers in the active layer with the corresponding methods used for film formation will be presented, which will be followed by a discussion regarding possible improvements on the film formation methods and suggestions upon enhancement of the structural/electronic feasibility of the porous backbone.

Simple syntheses of cyclic polyamines using selectively N-tritylated polyamines and succinic anhydride

Abstract Treatment of selectively N -tritylated spermidine and spermine derivatives with succinic anhydride, followed by PyBrOP-mediated intramolecular amide bond formation and LiAlH 4 reduction, allows for an easy and general entry to cyclic polyamine derivatives.

The impact of thienothiophene isomeric structures on the optoelectronic properties and photovoltaic performance in quinoxaline based donor–acceptor copolymers

The influence of the monomers isomeric structure on the optical, electrochemical, charge transporting properties and photovoltaic performance of donor–acceptor (D–A) conjugated polymers has been demonstrated for the first time by studying two D–A copolymers consisting of bis(3-octyloxy)phenyl)quinoxaline as the electron deficient unit and the two isomeric structures of thienothiophene (thieno[3,2-b]thiophene and thieno[2,3-b]thiophene) as the electron rich units. The drastic effect of incorporating two different isomeric structures on the polymer backbone of these copolymers, manifests in changes observed in their optical, electrochemical and charge transporting properties. In contrast, the overall photovoltaic performance of the copolymers is similar, but distinct differences in the device photocurrents occur. These differences were attributed to morphology variations rather than the balanced mobility ratio. For further developments in the field, the isomeric structures of different functional monomers should be considered in the designing of new materials with even superior performance.

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.

4H-1,2,6-Thiadiazine-containing donor–acceptor conjugated polymers: synthesis, optoelectronic characterization and their use in organic solar cells

π-Conjugated donor–acceptor (D–A) polymers containing electron withdrawing non-S-oxidized 4H-1,2,6-thiadiazines and electron donating (het)aryl-substituted indacenodithiophenes (IDTs) were prepared and used in organic solar cell (OSC) devices with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the fullerene acceptor. The non-S-oxidized 4H-1,2,6-thiadiazine containing polymers were wide bandgap absorbers (Eoptg 1.80–1.95 eV) with low-lying highest occupied molecular orbital energy levels (EHOMO −5.5 to −6.1 eV) affording high open circuit voltages (Vocs 0.82–0.96 V) for polymer:fullerene bulk heterojunction (BHJ) OSCs. Owing to enhanced short circuit currents (Jscs) and fill factors (FFs), the 1,2,6-thiadiazin-4(H)-one-based polymers had significantly higher performances (up to 3.83%) vs. the N-(perfluorophenyl)-1,2,6-thiadiazin-4(H)-imine based-polymers (up to 1.37%).