One-Pot Synthesis of 4,8-Dibromobenzo[1,2-d;4,5-d′]bistriazole and Synthesis of its Derivatives as New Units for Conjugated Materials

The one-pot synthesis of 4,8-dibromobenzo[1,2-d;4,5-d′]bistriazole, followed by alkylation and Stille coupling to yield three different isomeric derivatives with markedly different optoelectronic properties, is reported. These derivatives show potential as new units in organic oligomers and polymers for electronics applications

Interfacial Charge Transfer Anisotropy in Polycrystalline Lead Iodide Perovskite Films

Solar cells based on organic–inorganic lead iodide perovskite (CH₃NH₃PbI₃) exhibit remarkably high power conversion efficiency (PCE). One of the key issues in solution-processed films is that often the polycrystalline domain orientation is not well-defined, which makes it difficult to predict energy alignment and charge transfer efficiency. Here we combine ab initio calculations and photoelectron spectroscopy to unravel the electronic structure and charge redistribution at the interface between different surfaces of CH₃NH₃PbI₃ and typical organic hole acceptor Spiro-OMeTAD and electron acceptor PCBM. We find that both hole and electron interfacial transfer depend strongly on the CH₃NH₃PbI₃ surface orientation: while the (001) and (110) surfaces tend to favor hole injection to Spiro-OMeTAD, the (100) surface facilitates electron transfer to PCBM due to surface delocalized charges and hole/electron accumulation at the CH₃NH₃PbI₃/organic interfaces. Molecular dynamic simulations indicate that this is due to strong orbital interactions under thermal fluctuations at room temperature, suggesting the possibility to further improve charge separation and extraction in perovskite-based solar cells by controlling perovskite film crystallization and surface orientation

Unique Reversible Crystal-to-Crystal Phase TransitionStructural and Functional Properties of Fused Ladder Thienoarenes

Donor–acceptor type molecules based on fused ladder thienoarenes, indacenodithiophene (IDT), and dithienocyclopenta–thienothiophene (DTCTT), coupled with benzothiadiazole, are prepared, and their solid-state structures are investigated. They display a rich variety of solid phases ranging from amorphous glass states to crystalline states, upon changes in the central aromatic core and side group structures. Most notably, the DTCTT-based derivatives showed reversible crystal-to-crystal phase transitions in heating and cooling cycles. Unlike what has been seen in π-conjugated molecules, variable temperature XRD revealed that structural change occurs continuously during the transition. A columnar self-assembled structure with slip-stacked π–π interaction is proposed to be involved in the solid state. This research provides the evidence of unique structural behavior of the DTCTT-based molecules through the detailed structural analysis. This unique structural transition paves the way for these materials to have self-healing of crystal defects, leading to improved optoelectronic properties

Multiphoton Harvesting in an Angular Carbazole-Containing Zn(II)-Coordinated Random Copolymer Mediated by Twisted Intramolecular Charge Transfer State

Multiphoton excited fluorescent probes with highly emissive, photostable, low cytotoxic properties are very important for photodynamic therapy, sensing, and bioimaging, etc., even though still challenging. Here, we report the synthesis and spectroscopic studies of two statistical Zn­(II)-coordinated copolymers containing different donor types and the same acceptor type (a dithienylbenzothiadiazole-based ditopic terpyridine ligand), aiming to achieving efficient multiphoton harvesting systems. Our results indicate that an angular carbazole-based ditopic terpyridine ligand donor shows a strong tendency to form a twisted intramolecular charge transfer (TICT) state. Taking advantage of the large multiphoton absorption coefficient in the donor and efficient Förster resonance energy transfer (FRET) mediated by TICT state, efficiently enhanced fluorescence from the acceptor under two- and even three-photon excitation is consequently achieved. In contrast, for a linear carbazole-based ditopic terpyridine ligand donor, the enhanced multiphoton excited fluorescence from the acceptor originates from reabsorption effect instead of FRET. For the first time, we have reported the multiphoton harvesting properties of metal–organic complexes, especially stressing the crucial role of TICT state in multiphoton excited FRET, which sheds light on how to design efficient multiphoton harvesting systems in general

Influence of 4-<i>tert</i>-Butylpyridine in DSCs with Co^II/III Redox Mediator

The effect of TBP on Co­(II/III) redox electrolyte was examined. Photocurrent of the device can be limited in electrolyte with high TBP concentration due to its viscous nature. The higher amount of TBP in electrolyte increases the viscosity of electrolyte and consequently slows down the diffusion of Co^III species. This is a unique observation for Co­(II/III) electrolyte which is not observed in I^–/I₃^– electrolyte. An increase in TBP concentration from 0.1 to 0.5 M in a cobalt electrolyte containing 0.1 M Co­(bpy)₃(TFSI)₂ and 0.033 M Co­(bpy)₃(TFSI)₃ produced a 90 mV improvement in open-circuit potential (<i>V</i>_oc). Using electrochemical impedance spectroscopy (EIS), this enhancement could be attributed to the reduced interfacial recombination (33%) as well as a negative shift in the conduction band level of TiO₂ (67%). Although the influence of TBP in iodide/triiodide and cobalt-complex electrolytes is similar, the increase of viscosity at relatively high concentration of TBP in Co electrolyte should be taken into consideration in order to accomplish high efficiency DSCs based on Co-complex electrolytes

Naphthalene‑, Anthracene‑, and Pyrene-Substituted Fullerene Derivatives as Electron Acceptors in Polymer-based Solar Cells

A series of aryl-substituted fullerene derivatives were prepared in which the aromatic moiety of [6,6]-phenyl C₆₁-butyric acid methyl ester (PC₆₁BM) was modified by replacing the monocyclic phenyl ring with bicyclic naphthalene (NC₆₁BM), tricyclic anthracene (AC₆₁BM), and tetracyclic pyrene (PyC₆₁BM). The PC₆₁BM derivatives were synthesized from C₆₀ using tosylhydrazone and were tested as electron acceptors in poly­(3-hexylthiophene) (P3HT)-based organic photovoltaic cells (OPVs). The lowest unoccupied molecular orbital (LUMO) energy level of NC₆₁BM (−3.68 eV) was found to be slightly higher than those of PC₆₁BM (−3.70 eV), AC₆₁BM (−3.75 eV), and PyC₆₁BM (−3.72 eV). The electron mobility values obtained for the P3HT:PC₆₁BM, P3HT:NC₆₁BM, P3HT:AC₆₁BM, and P3HT:PyC₆₁BM blend films were 2.39 × 10^–4, 2.27 × 10^–4, 1.75 × 10^–4, and 2.13 × 10^–4 cm² V^–1 s^–1, respectively. P3HT-based bulk-heterojunction (BHJ) solar cells were fabricated using NC₆₁BM, AC₆₁BM, and PyC₆₁BM as electron acceptors, and their performances were compared with that of the device fabricated using PC₆₁BM. The highest power conversion efficiencies (PCEs) observed for devices fabricated with PC₆₁BM, NC₆₁BM, AC₆₁BM, and PyC₆₁BM were 3.80, 4.09, 1.14, and 1.95%, respectively, suggesting NC₆₁BM as a promising electron acceptor for OPVs

Synthesis and Characterization of a Series of Annelated Benzotriazole Based Polymers with Variable Bandgap

Here we report the synthesis and characterization of a series of annelated benzotriazole based polymers with variable bandgap. Benzobistriazole monomers reported by us previously were desymmetrized using partial reduction ring opening followed by ring closure to produce a wide range of annelated benzotriazole based monomers. These monomers were co-polymerized with a fluorene moiety to give polymers with bandgaps ranging from 1.16 to 2.41 eV

High Open Circuit Voltage Solution-Processed Tandem Organic Photovoltaic Cells Employing a Bottom Cell Using a New Medium Band Gap Semiconducting Polymer

Two donor–acceptor (D–A) copolymers, based on the donor unit TIPS substituted benzodithiophene (TIPSBDT) and the acceptor quinoxaline-based units with or without fluorine substitution (PTIPSBDT-DTQX and PTIPSBDT-DFDTQX), were designed and synthesized as a donor material for bulk-heterojunction (BHJ) photovoltaic cells. The introduction of F atoms with high electron affinity to be quinoxailine moieties is effective in further lowering both the HOMO and LUMO energy levels of PTIPSBDT-DFDTQX to attain higher open-circuit voltage (<i>V</i>_oc). Single junction photovoltaic cells were fabricated, and the polymers:PC₇₁BM active layer morphology was optimized by adding 1,8-diiodooctane (DIO) as an additive. In a single layer photovoltaic device, they showed power conversion efficiencies (PCEs) of 2–6%. The solution process inverted tandem photovoltaic cells, in which two photovoltaic cells with different absorption characteristics are linked to use a wider range of the solar spectrum, were fabricated with each layer processed from solution with the use of BHJ materials comprising semiconducting polymers and fullerene derivatives. We first report here on the design of PTIPSBDT-DFDTQX equivalent poly­(3-hexylthiophene), the current medium band gap polymer of choice, which thus is a viable candidate for use in the highly efficient bottom layer in inverted tandem cells