N-acyldithieno[3,2-b:2',3'-d]pyrrole-based low-band-gap conjugated polymer solar cells with amine-modified [6,6]-phenyl-C61-butyric acid ester cathode interlayers

Anthracene-Based Organic Small-Molecule Electron-Injecting Material for Inverted Organic Light-Emitting Diodes

A diphenylanthracene dimethylamine derivative (9-{3,5-di( N, N-dimethylaminoethoxy)phenyl}-10-phenyl-anthracene, DPAMA) was synthesized by the Suzuki-Miyaura cross-coupling reaction. Its ammonium salt, 9-{3,5-di(trimethylammonium ethoxy)phenyl}-10-phenyl-anthracene dichloride (DPAMA-Cl), was also synthesized as a reference material. DPAMA was characterized by UV-vis and fluorescence spectroscopy, cyclic voltammetry, photoelectron yield spectroscopy, and X-ray photoelectron spectroscopy to evaluate the work function-modifying ability of DPAMA on indium tin oxide (ITO) and ZnO. The work functions of ITO and ZnO changed from 4.4 and 4.0 eV (pristine) to 3.8 and 3.9 eV, respectively. Using this surface modification effect of DPAMA, inverted organic light-emitting diodes were fabricated with device structures of ITO/DPAMA/Alq₃/NPD/MoO₃/Al (Alq₃ = tris(8-hydroxyquinolinato)aluminum; NPD = N, N'-di-[(1-naphthyl)- N, N'-diphenyl]-1,1'-(biphenyl)-4,4'-diamine) and ITO/ZnO/DPAMA/Alq₃/NPD/MoO₃/Al. Both devices showed good performance at the range of current density, 1-300 mA/cm². The best inverted organic light-emitting diodes device showed luminance of 7720 cd/m², current efficiency of 4.51 cd/A, and external quantum efficiency of 1.45%. Also, poly(3-hexylthiophene):mixed phenyl-C₆₁ and C₇₁ butyric acid methyl ester-based organic solar cells using DPAMA and DPAMA-Cl as electron-transporting materials showed power conversion efficiencies of 3.3 and 3.4%, respectively.

N-Benzoyl dithieno[3,2-b:2',3'-d]pyrrole-based hyperbranched polymers by direct arylation polymerization

BACKGROUND

Although poly(N-acyl dithieno[3,2-b:2',3'-d]pyrrole)s have attracted great attention as a new class of conducting polymers with highly stabilized energy levels, hyperbranched polymers based on this monomer type have not yet been studied. Thus, this work aims at the synthesis of novel hyperbranched polymers containing N-benzoyl dithieno[3,23,2-b:2',3'-d]pyrrole acceptor unit and 3-hexylthiophene donor moiety via the direct arylation polymerization method. Their structures, molecular weights and thermal properties were characterized via 1H NMR and FTIR spectroscopies, GPC, TGA, DSC and XRD measurements, and the optical properties were investigated by UV-vis and fluorescence spectroscopies.

RESULTS

Hyperbranched conjugated polymers containing N-benzoyl dithieno[3,23,2-b:2',3'-d]pyrrole acceptor unit and 3-hexylthiophene donor moiety, linked with either triphenylamine or triphenylbenzene as branching unit, were obtained via direct arylation polymerization of the N-benzoyl dithieno[3,23,2-b:2',3'-d]pyrrole, 2,5-dibromo 3-hexylthiophene and tris(4-bromophenyl)amine (or 1,3,5-tris(4-bromophenyl)benzene) monomers. Organic solvent-soluble polymers with number-average molecular weights of around 18,000 g mol-1 were obtained in 80-92% yields. The DSC and XRD results suggested that the branching structure hindered the stacking of polymer chains, leading to crystalline domains with less ordered packing in comparison with the linear analogous polymers. The results revealed that the hyperbranched polymer with triphenylbenzene as the branching unit exhibited a strong red-shift of the maximum absorption wavelength, attributed to a higher polymer stacking order as a result of the planar structure of triphenylbenzene.

CONCLUSION

Both hyperbranched polymers with triphenylamine/triphenylbenzene as branching moieties exhibited high structural order in thin films, which can be promising for organic solar cell applications. The UV-vis absorption of the hyperbranched polymer containing triphenylbenzene as branching unit was red-shifted as compared with the triphenylamine-containing polymer, as a result of a higher chain packing degree.

Ferroelectric self-assembled molecular materials showing both rectifying and switchable conductivity

Advanced molecular materials that combine two or more physical properties are typically constructed by combining different molecules, each being responsible for one of the properties required. Ideally, single molecules could take care of this combined functionality, provided they are self-assembled correctly and endowed with different functional subunits whose strong electronic coupling may lead to the emergence of unprecedented and exciting properties. We present a class of disc-like semiconducting organic molecules that are functionalized with strong dipolar side groups. Supramolecular organization of these materials provides long-range polar order that supports collective ferroelectric behavior of the side groups as well as charge transport through the stacked semiconducting cores. The ferroelectric polarization in these supramolecular polymers is found to couple to the charge transport and leads to a bulk conductivity that is both switchable and rectifying. An intuitive model is developed and found to quantitatively reproduce the experimental observations. In a larger perspective, these results highlight the possibility of modulating material properties using the large electric fields associated with ferroelectric polarization.

A hybrid organic–inorganic three-dimensional cathode interfacial material for organic solar cells

An alcohol soluble hybrid organic–inorganic three-dimensional material POSS-FN has been synthesized and assessed as a cathode interlayer within organic solar cells consisting of a PBDT-BT:PC₆₁BM bulk heterojunction.

[6,6]-phenyl-C₆₁-butyric acid 2-((2-(dimethylamino)ethyl)(methyl)amino)-ethyl ester as an acceptor and cathode interfacial material in polymer solar cells

An amine-based, alcohol-soluble fullerene [6,6]-phenyl-C61-butyric acid 2-((2-(dimethylamino)ethyl)(methyl)amino)-ethyl ester (PCBDAN) with 4-fold electron mobility of 6,6-phenyl-C61-butyric acid methyl ester (PCBM) is applied successfully as an acceptor and cathode interfacial material in polymer solar cells ITO/P3HT:PCBDAN/MoO3/Ag, where indium tin oxide (ITO) alone is used as the cathode and poly(3-hexylthiophene) (P3HT) is used as a donor. The X-ray photoelectron spectroscopy (XPS) depth profile confirming a favorable vertical phase separation is formed where P3HT is rich at the air/active blend interface and PCBDAN is rich at the buried interface with ITO and, thus, reduces the work function of ITO for use as the cathode. A moderate power conversion efficiency (PCE) of 3.1% is achieved. The slightly low PCE could be due to unoptimized morphology and low structure ordering of P3HT in the blends. However, this result demonstrates that the amine-based fullerene could be used as the acceptor and cathode interfacial material, which eliminated the multilayer device fabrication process. Because PCBDAN has high electron mobility, it would have potential applications in nano-structured organic solar cells. In the near future, alcohol-processable, high-efficient organic/polymer solar cells can be anticipated.

Self-organization of amine-based cathode interfacial materials in inverted polymer solar cells

We present a strategy to fabricate polymer solar cells in inverted geometry by self-organization of alcohol soluble cathode interfacial materials in donor-acceptor bulk heterojunction blends. An amine-based fullerene [6,6]-phenyl-C61-butyric acid 2-((2-(dimethylamino)-ethyl)(methyl)amino)ethyl ester (PCBDAN) is used as an additive in poly(3-hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM) blend to give a power conversion efficiency of 3.7% based on devices ITO/P3HT:PCBM:PCBDAN/MoO3/Ag where the ITO alone is used as the cathode. A vertical phase separation in favor of the inverted device architecture is formed: PCBDAN is rich on buried ITO surface reducing its work function, while P3HT is rich on air interface with the hole-collecting electrode. The driving force of the vertical phase separation is ascribed to the surface energy and its components of the blend compositions and the substrates. Similar results are also found with another typical alcohol soluble cathode interfacial materials, poly[(9,9-bis(3'-(N, N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN), implying that self-organization may be a general phenomenon in ternary blends. This self-organization procedure could eliminate the fabrication of printing thin film of interlayers or printing on such thin interlayers and would have potential application for roll-to-roll processing of polymer solar cells.