Věra Cimrová

Enhanced electroluminescence from light-emitting devices based on poly(9,9-dihexadecylfluorene-2,7-diyl) and polysilane blends

Photoluminescence and electroluminescence (EL) of poly(9,9-dihexadecylfluorene-2,7-diyl) (PFC16) and its blends with hole-transporting poly[methyl(phenyl)silanediyl] modified with pyrene were studied. Efficient blue light-emitting devices were fabricated. Blending of PFC16 with modified polysilane led to a significant improvement in the EL efficiency and stability compared with the devices fabricated from neat PFC16. An increase in the EL efficiency of up to two orders of magnitude was achieved. The increase in the EL efficiency was attributed to the modification of the charge transport and recombination in the blend.

Development of quinoxaline based polymers for photovoltaic applications

Polymer solar cells (PSCs) with a bulk heterojunction (BHJ) structure, i.e. a blend of a p-type conjugated polymer with an n-type semiconductor acceptor, have made rapid progress over the past decade. In comparison with inorganic semiconductor solar cells, PSCs have the advantages of low cost, light weight, solution processability and good mechanical flexibility. In the last few years, various classes of electron-donating polymers have been reported for PSCs. Among them, quinoxaline (Qx) and its derivatives have been widely used as building blocks for optoelectronic applications because they can be easily modified by varying the side chains, such as alkyl chains, conjugated aromatic rings, functional groups, etc. Recently, a power conversion efficiency (PCE) of over 11% was achieved for PSCs with Qx-based polymers. This PCE is among the best for PSCs, and it suggests that Qx-based polymers have great potential for highly efficient PSCs. In this article, we review the recent advances in the design and synthesis of such Qx-based conjugated polymers for photovoltaic applications. Particular attention is paid to the chemical structures of the polymers including flexible chains, conjugated side chains, functional groups, Qx derivatives and the effect of the molecular structure on device performance parameters. We believe that further development of Qx-based polymers will lead to a PCE >12% in the near future.

Free charge carrier formation in polymers under illumination

In addition to many extrinsic processes, free charge carriers in polymers can be photogenerated intrinsically via an intermediate stage of bound ion-pairs formed in consequence of band-to-band transitions or exciton autoionization. The excitation can be realized within the framework of (i) a chemical bond in the polymer backbone or (ii) a side group skeleton. An electron—hole pair with a short separation distance created by light on the same polymer chain or on the same side group usually recombines geminately very fast and does not contribute to the photocurrent. An inter- or intrachain charge transfer or transfer of the electron from the main chain to a side group is necessary to form a more stable ion-pair. Dissociation of the ion-pairs by Brownian motion subjected to a combination of Coulomb and applied electrical field, into free charge carriers, can be described in terms of the Onsager theory of geminate recombination. The photogeneration yield can be improved by an electron acceptor doping or by an electron accepting groups attached to side chains.

Charge carrier photogeneration in poly(methylphenylsilylene) and its derivatives with π-conjugated side groups

Abstract The photogeneration of charge carriers in thin films of poly(methylphenylsilylene) (PMPSi) and its derivatives with attached pendant π-conjugated chromophores was investigated as a function of the electric field and temperature by the photoinduced discharge current method. Six new substituted polysilylenes were used in this study. The electric field dependence of photogeneration efficiency suggests that the Onsager theory of geminate recombination is a suitable model for the description of the electron-hole pair dissociation during the photogeneration process in these polymers. The most effective separation distances of electron-hole pairs participating in the photogeneration process were found to be 1.8–3.5 nm and 0.9–1.75 nm for low and high electric fields, respectively.

Electrode-limited photoinjection at metal/polymer interface

The pulse photoconductivity in thin films of poly(methylphenylsilylene) with different electrodes was investigated as a function of electric field (F) and temperature (T) by the time-of-flight method. The electric field dependences of the apparent quantum efficiency (η) were analysed in the frame of electrode-limited photoinjection. For the samples irradiated through the ITO electrode, η followed the relation η = η0exp(-bFsol12), where η0 and b are constants. For irradiation through the gold electrode, the electrode-limited photoinjection was not so strong and at higher electric fields Schottky emission prevailed; the electric field dependence of the photogeneration efficiency was given by the relation η = η0exp(aF12-bF−12), where a and b are constants. The hole diffusion length was found to be in the range 1.8–6 nm, being longer for samples irradiated through gold electrodes.

Synthesis and Photophysical and Electroluminescent Properties of Poly(1,4-phenylene−ethynylene)-alt-poly(1,4-phenylene−vinylene)s with Various Dissymmetric Substitution of Alkoxy Side Chains

The synthesis and characterization of a set of conjugated polymers, poly(1,4-phenylene–ethynylene)-alt-poly(1,4-phenylene–vinylene)s (PPE–PPVs), with a dissymmetrical configuration (partial or total) of alkoxy side chains is reported. Five new polymers bearing octyloxy and/or octadecyloxy side chains at the phenylene–ethynylene and phenylene–vinylene segments, respectively, were obtained. Two symmetrical substituted polymers were used for comparison. Polymers with weight-average molecular weight, Mw, up to 430 000 g/mol and degree of polymerization between 17 and 322 were obtained by a Horner–Wadsworth–Emmons olefination polycondensation reaction of the respective luminophoric dialdehydes and bisphosphonates. As expected, identical conjugated backbones in all polymers results in very similar photophysical response in dilute solution, with high fluorescence quantum yields between 50% and 80%. In contrast, the thin film properties are dependent on the combinatorial effects of side chain configuration, molecular weight, and film thickness parameters, which are the basis of the resulting comparison and discussion.

Optimizing the conjugated side chains of quinoxaline based polymers for nonfullerene solar cells with 10.5% efficiency

Quinoxaline (Qx) has an easily modifiable structure, which allows for fine-tuning its properties through optimizing the length of side chains and the kinds of aromatic rings in conjugated side chains. Qx based polymer PBDTT-ffQx with alkoxy substituted fluorobenzene side chains shows a power conversion efficiency (PCE) of 8.47% in nonfullerene solar cells. In this work, we change the alkoxy substituted fluorobenzene side chains to alkyl substituted fluorothiophene in the Qx unit and employ the benzodithiophene (BDT) unit with different lengths of alkyl thiophene side chains to obtain two new Qx based polymers, named TTFQx-T1 and TTFQx-T2, respectively. The TTFQx-T1:ITIC blend film has clear nanoscale phase separation and a suitable domain size, and the device with the TTFQx-T1:ITIC blend film shows lower geminate recombination and higher charge mobility than those with TTFQx-T2:ITIC. Therefore, the TTFQx-T1:ITIC based device exhibits a higher PCE of 10.52% than that based on the TTFQx-T2:ITIC blend with a moderate PCE of 7.22%. The inverted devices from TTFQx-T1:ITIC blends with a larger active area of 16 mm2 than the conventional device (4.5 mm2) show a good PCE of 9.21%. The results highlight that the alkyl lengths and the kinds of aromatic rings in side chains are significant for the development of high performance photovoltaic polymers. Optimizing the conjugated side chains of Qx based polymers is an efficient way to improve photovoltaic properties, and Qx based polymers are potential candidates for fabricating highly efficient polymer solar cells.

Semiconducting Conjugated Copolymer Series for Organic Photonics and Electronics

Donor–acceptor copolymer series containing 4,6-di (thien-2′-yl) thieno [3,4-c][1,2,5] thiadiazole or its derivatives serving as electron-acceptor units and various electron-donor units such as 9,9-bis (alkyl) fluorene, benzene, bithiophene or carbazole derivatives is reported. These copolymers possess narrow optical band gap in the range of 1.0 - 1.5 eV depending on the character of the donor units. They exhibit relatively high electron affinity. Absorption of copolymer thin films covers the whole visible spectral region extended up to NIR for some copolymers. The influence of side chain nature and molecular weight on their photophysical properties is shown. Selected copolymers are used in the blends with fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester ([60]PCBM) as active layers in bulk heterojunction photovoltaic devices. The results are discussed in relation to the copolymer structure, side chain nature and molecular weight.

New two-step synthesis of N-(2-ethylhexyl)-2,7-diiodocarbazole as a monomer for conjugated polymers

A reasonably efficient two-step synthesis of N-(2-ethylhexyl)-2,7-diiodocarbazole is presented. In the first step, the 4,4′-diiodobiphenyl was nitrated to a mixture of 4,4′-diiodo-2-nitrobiphenyl and 4-iodo-4′-nitrobiphenyl. In the second step, the mixture of these compounds was converted by simultaneous carbazole ring closure and N-alkylation to the N-(2-ethylhexyl)-2,7-diiodocarbazole by means of tris(2-ethylhexyl) phosphite. The synthesis represents a shorter alternative to the known more tedious procedures. The prepared compound is a suitable monomer for synthesis of conjugated polymers for optoelectronic applications.

Novel and Simple Synthesis of Brominated 1,10-Phenanthrolines

A novel, simple, and reasonably efficient synthesis of 3,8-dibromo-1,10-phenanthroline, 3,6-dibromo-1,10-phenanthroline, 3,5,8-tribromo-1,10-phenanthroline, and 3,5,6,8-tetrabromo-1,10-phenanthroline is presented herein. The crucial role of a new catalyst (sulfur dichloride – SCl2) for the bromination of 1,10-phenanthroline is reported. The bromination of 1,10-phenanthroline monohydrate in the presence of SCl2 and pyridine yielded the brominated compounds, previously only possible through the complicated multi-step and tedious Skraup synthesis method. The application of the bromination catalyst SCl2 as a medium-strength Lewis acid is demonstrated for the first time, and the results are compared with the behaviours of known weak (sulfur chloride – S2Cl2) and strong (thionyl chloride – SOCl2) bromination catalysts. A reaction mechanism was proposed.