Fluorene‐Based Copolymers for Red‐Light‐Emitting Diodes

The syntheses of new fluorene‐based π‐conjugated copolymers; namely, poly((5,5″‐(3′,4′‐dihexyl‐2,2′;5′,2″‐terthiophene 1′,1′‐dioxide))‐alt‐2,7‐(9,9‐dihexylfluorene)) (PFTORT), poly((5,5″″‐(3″,4″‐dihexyl‐2,2′:5′,2′:5″,2‴:5‴,2″″‐quinquethiophene 1″,1″‐dioxide))‐alt‐2,7‐(9,9‐dihexylfluorene)) (PFTTORTT), and poly((5,5‐E‐α‐(2‐thienyl)methylene)‐2‐thiopheneacetonitrile)‐alt‐2,7‐(9,9‐dihexylfluorene)) (PFTCNVT), are reported. In the solid state, PFTORT and PFTCNVT present red–orange emission (with a maximum at 610 nm) while PFTTORTT shows a red emission with a maximum at 666 nm. In all cases, electrochemical measurements have revealed p‐ and n‐dopable copolymers. All these copolymers have been successfully tested in simple light‐emitting diodes and show promising results for orange‐ and red‐light‐emitting devices.     

Organic Thin‐Film Transistors Based on Tolyl‐Substituted Oligothiophenes

Thin films based on the tolyl‐substituted oligothiophenes 5,5′′‐bis(4‐methylphenyl)‐2,2′:5′,2′′‐terthiophene ( 1 ), 5,5′′′‐bis(4‐methylphenyl)‐2,2′:5′,2′′:5′′,2′′′‐quaterthiophene ( 2 ) and 5,5′′′′‐bis(4‐methylphenyl)‐2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′‐quinqethiophene ( 3 ) exhibit hole‐transport behavior in a thin‐film transistor (TFT) configuration, with reasonable mobilities and high current on/off (I_on/I_off) ratios. Powder X‐ray diffraction (PXRD) reveals that these films, grown by vacuum deposition onto the thermally grown silicon oxide surface of a TFT, are highly crystalline, a characteristic that can be attributed to the general tendency of phenyl groups to promote crystallinity. Atomic force microscopy (AFM) reveals that the films grow layer by layer to form large domains, with some basal domain areas approaching 1000 μm². The PXRD and AFM data are consistent with an “end‐on” orientation of the molecules on the oxide substrate. Variable‐temperature current–voltage (I–V) measurements identified the activation regime for hole transport and revealed shallow level traps in thin films of 1 and 2 , and both shallow and deep level traps in thin films of 3 . The activation energies for thin films of 1 , 2 , and 3 were similar, with values of E_a = 121, 100, and 109 meV, respectively. The corresponding trap densities were N_trap/N_v = 0.012, 0.023, and 0.094, where N_trap is the number of trap states and N_v is the number of conduction states. The hole mobilities for the three compounds were similar (μ ? 0.03 cm² V^–1 s^–1), and the I_on/I_off ratios were comparable with the highest values reported for organic TFTs, with films of 2 approaching I_on/I_off = 10⁹ at room temperature.     

Size‐Specific Interactions Between Single‐ and Double‐Stranded Oligonucleotides and Cationic Water‐Soluble Oligofluorenes

An improved synthetic approach was developed for the synthesis of 1,4‐bis[9′,9′‐bis(6″‐(N,N,N‐trimethylammonium)‐hexyl)‐fluoren‐2′‐yl]benzene tetrabromide ( 1a ), 1,4‐bis[9′,9′;9″,9″‐tetra(6″′‐(N,N,N‐trimethylammonium)‐hexyl)‐7′,2″‐bisfluoren‐2′‐yl] benzene octabromide ( 1b ) and 1,4‐bis[9′,9′;9″,9″;9″′,9″′‐hexakis(6″″‐(N,N,N‐trimethylammonium)‐hexyl)‐7′,2″,7″,2″′‐trifluoren‐2′‐yl] benzene dodecabromide ( 1c ). These molecules provide a size‐specific series of water‐soluble oligofluorene molecules with increasing numbers of repeat units to model the interactions between cationic conjugated polymers and DNA. Fluorescence quenching and energy‐transfer measurements were performed with 1a – c and single‐stranded (ss) DNA and double‐stranded (ds) DNA, with and without fluorescein (Fl). These studies show that, on a per‐negative‐charge basis, ssDNA quenches the emission of 1a – c more effectively than dsDNA. Furthermore, we show that the energy‐transfer ratios dsDNA–Fl/ssDNA–Fl are dependent on the number of repeat units in 1a – c .     

Ambipolar Organic Field‐Effect Transistors from Cross‐Conjugated Aromatic Quaterthiophenes; Comparisons with Quinoidal Parent Materials

This contribution presents an electrochemical, Raman spectroscopic, and theoretical study probing the differences in molecular and electronic structure of two quinoidal oligothiophenes (3′,4′‐dibutyl‐5,5″‐bis(dicyanomethylene)‐5,5″‐dihydro‐2,2′:5′,2″‐terthiophene and 5,5′‐bis(dicyanomethylene)‐3‐hexyl‐2,5‐dihydro‐4,4′‐dihexyl‐2,2′,5,5′‐tetrahydro‐tetrathiophene) with terminal tetracyanomethylene functionalization and aromatic oligothiophenes where acceptor moieties are positioned at lateral positions along the conjugated chain (6,6′‐dibutylsulfanyl‐[2,2′‐bi‐[4‐dicyanovinylene‐4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene]). In this way, the consequences of linear and cross conjugation are compared and contrasted. From this analysis, it is apparent that organic field‐effect transistors fabricated with cross‐conjugated tetrathiophene semiconductors should combine the benefits of an electron‐donor aromatic chain with strongly electron‐accepting tetracyanomethylene substituents. The corresponding organic field‐effect transistors exhibit ambipolar transport with rather similar hole and electron mobilities. Moreover, n‐channel conduction is enhanced to yield one of the highest electron mobilities found to date for this type of material.     

Complementary Absorbing Star‐Shaped Small Molecules for the Preparation of Ternary Cascade Energy Structures in Organic Photovoltaic Cells

Two anthracene‐based star‐shaped conjugated small molecules, 5′,5″‐(9,10‐bis((4‐hexylphenyl)ethynyl)anthracene‐2,6‐diyl)bis(5‐hexyl‐2,2′‐bithiophene), HBantHBT, and 5′,5″‐(9,10‐bis(phenylethynyl)anthracene‐2,6‐diyl)bis(5‐hexyl‐2,2′‐bithiophene), BantHBT, are used as electron‐cascade donor materials by incorporating them into organic photovoltaic cells prepared using a poly((5,5‐E‐alpha‐((2‐thienyl)methylene)‐2‐thiopheneacetonitrile)‐alt‐2,6‐[(1,5‐didecyloxy)naphthalene])) (PBTADN):[6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM) blend. The small molecules penetrate the PBTADN:PC₇₁BM blend layer to yield complementary absorption spectra through appropriate energy level alignment and optimal domain sizes for charge carrier transfer. A high short‐circuit current (J_SC) and fill factor (FF) are obtained using solar cells prepared with the ternary blend. The highest photovoltaic performance of the PBTADN: BantHBT :PC₇₁BM blend solar cells is characterized by a J_SC of 11.0 mA cm^?2, an open circuit voltage (V_OC) of 0.91 V, a FF of 56.4%, and a power conversion efficiency (PCE) of 5.6% under AM1.5G illumination (with a high intensity of 100 mW^?2). The effects of the small molecules on the ternary blend are investigated by comparison with the traditional poly(3‐hexylthiophene) (P3HT):[6,6]‐phenyl‐C61‐butyric acid methyl ester (PC₆₁BM) system.     

Electroactive Polymeric Networks Created by Oxidation of Oligothiophene‐Functionalized Perylene Bisimides

A series of fourfold oligothienyl‐functionalized perylene bisimides, N,N′‐bis(2,6‐diisopropylphenyl)‐1,6,7,12‐tetra(4‐(7‐[2,2′]bithien‐5‐yl)‐heptanoyloxyphenoxy)perylene‐3,4:9,10‐tetracarboxylic acid bisimide ( 7a ), N,N′‐bis(2,6‐diisopropylphenyl)‐1,6,7,12‐tetra(4‐(7‐[2,2′;5′,2′′]terthien‐5‐yl)‐heptanoyloxyphenoxy)perylene‐3,4:9,10‐tetracarboxylic acid bisimide ( 7b ), and N,N′‐bis(2,6‐diisopropylphenyl)‐1,6,7,12‐tetra(4‐(7‐(5″‐Methyl‐[2,2′;5′,2″]terthien‐5‐yl))‐heptanoyloxyphenoxy)perylene‐3,4:9,10‐tetracarboxylic acid bisimide ( 7c ), have been synthesized. Oligothienyl and perylene bisimide chromophores in these dyads display their characteristic optical UV/vis absorption properties. Upon excitation of the oligothiophene subunits, fluorescence resonance energy transfer (FRET) occurs to the perylene bisimide core. Cyclic voltammetric studies revealed that the reduction of the perylene bisimide moiety is not affected by the presence of oligothiophenes, showing two waves at around ‐0.7 and ‐1.0 V versus Ag/AgCl, respectively. On the other hand, the oxidation of the oligothienyl moieties leads to oxidative coupling for 7a and 7b , providing electroactive sexithiophene‐ and quaterthiophene‐perylene bisimide networks, respectively. Electrochemical deposition of compounds 7a , b was performed and the films were characterized using cyclic voltammetry and in situ conductance, which reveal remarkable p‐type conductivity. Significantly, two separate regimes of electrical conductance have been observed for the films generated from 7b .     

Plastic Solar Cells Based on Fluorenone‐Containing Oligomers and Regioregular Alternate Copolymers

Oligomers and regioregular copolymers based on fluorenone subunits are synthesized and used in bulk‐heterojunction photovoltaic cells. These are 2,7‐bis(5‐[(E)‐1,2‐bis(3‐octylthien‐2‐yl)ethylene])‐fluoren‐9‐one (TVF), the product of its oxidative polymerization, that is, (poly[(5,5′‐(bis‐(E)‐1,2‐bis(3‐octylthien‐2‐yl)ethylene]‐alt‐(2,7‐fluoren‐9‐one)]) (PTVF), and an alternate copolymer of fluoren‐9‐one and di‐n‐alkylbithiophene, namely poly[(5,5′‐(3,3′‐di‐n‐octyl‐2,2′‐bithiophene))‐alt‐(2,7‐fluoren‐9‐one)] (PDOBTF). The interpenetrating networks of active layers consisting of these new compounds as electron donors and of methanofullerene [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) as an acceptor exhibit an extended absorption band in the visible part of the spectrum with an absorption edge close to 700 nm. The external power conversion efficiencies (EPCEs) and the external quantum efficiency of the various TVF‐, PTVF‐, and PDOBTF‐based photovoltaic cells have been determined. EPCE values of up to 1 % have been achieved, which demonstrate the potential of fluorenone‐based materials in solar cells. It has also been demonstrated that fluorenone subunits are efficient photon absorbers for the conversion. Interestingly, some cell parameters such as, for example, the fill factor, have been improved as compared to photovoltaic cells with a “classical” poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylenevinylene]/PCBM active layer, fabricated and studied under the same experimental conditions.     

Diketopyrrolopyrrole‐Based Conjugated Polymers Synthesized via Direct Arylation Polycondensation for High Mobility Pure n‐Channel Organic Field‐Effect Transistors

High‐performance unipolar n‐type conjugated polymers (CPs) are critical for the development of organic electronics. In the current paper, four “weak donor–strong acceptor” n‐type CPs based on pyridine flanked diketopyrrolopyrrole (PyDPP), namely PPyDPP1‐4FBT, PPyDPP2‐4FBT, PPyDPP1‐4FTVT, and PPyDPP2‐4FTVT, are synthesized via direct arylation polycondensation by using 3,3′,4,4′‐tetrafluoro‐2,2′‐bithiophene (4FBT) or (E)‐1,2‐bis(3,4‐difluorothien‐2‐yl)ethene (4FTVT) as weak donor unit. All four polymers exhibit low‐lying highest occupied molecular orbital (≈ ?5.90 eV) and lowest unoccupied molecular orbital energy levels (≈ ?3.70 eV). Top‐gate/bottom‐contact organic field‐effect transistors based on all four polymers display unipolar n‐channel characteristics with electron mobility (µ_e) above 1 cm² V^?1 s^?1 in air, and presented linear |I_SD|^1/2 ?V_GS plots and weak dependence of the extracted moblity on gate voltage (V_GS), indicative of the reliability of the extracted mobility values. Importantly, the devices based on PPyDPP1‐4FBT and PPyDPP2‐4FBT show a pure unipolar n‐channel transistor behavior as revealed by the typical unipolar n‐channel output characteristics and clear off‐regimes in transfer characteristics. Attributed to its high crystallinity and favorable thin film morphology, PPyDPP2‐4FBT shows the highest µ_e of 2.45 cm² V^?1 s^?1, which is among the highest for unipolar n‐type CPs reported to date. This is also the first report for DPP based pure n‐type CPs with µ_e greater than 1 cm² V^?1 s^?1.     

Synthesis of 2,7‐Carbazolenevinylene‐Based Copolymers and Characterization of Their Photovoltaic Properties

New electroactive and photoactive conjugated copolymers consisting of alternating 2,7‐carbazole and oligothiophene moieties linked by vinylene groups have been developed. Different oligothiophene units have been introduced to study the relationship between the polymer structure and the electronic properties. The resulting copolymers are characterized by UV‐vis spectroscopy, size‐exclusion chromatography, and thermal and electrochemical analyses. Bulk heterojunction photovoltaic cells from different copolymers and a soluble fullerene derivative, [6,6]‐phenyl‐C₆₁ butyric acid methyl ester, have been fabricated, and promising preliminary results are obtained. For instance, non‐optimized devices using poly(N‐(4‐octyloxyphenyl)‐2,7‐carbazolenevinylene‐alt‐3″,4″‐dihexyl‐2,2′;5′,2″;5″,2″′;5″′,2″″‐quinquethiophenevinylene 1″,1″‐dioxide) as an absorbing and hole‐carrier semiconductor exhibit power conversion efficiency up to 0.8 % under air mass (AM) 1.5 illumination. These features make 2,7‐carbazolenevinylene‐based and related polymers attractive candidates for solar‐cell applications.     

An Electron Acceptor with Broad Visible–NIR Absorption and Unique Solid State Packing for As‐Cast High Performance Binary Organic Solar Cells

A novel acceptor–donor–acceptor (A–D–A) type electron acceptor 6TIC‐4F with terthieno[3,2‐b]thiophene (6T) as the core unit is rationally designed and synthesized, which exhibits an extraordinarily narrow bandgap (≈1.24 eV) and strong absorption between 650 and 1000 nm. X‐ray crystallographic analysis reveals that it has unique intermolecular π–π stacking. The solar cells based on the as‐cast poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione))]) (PBDB‐T): 6TIC‐4F binary blends exhibit an excellent power conversion efficiency (PCE) of 11.14% with a high J_SC of 23.00 mA cm^?2, and a high fill factor of 0.67, which represents one of the best PCE values for low bandgap (E_g < 1.3 eV)–based organic solar cells.     

2,5‐Functionalized Spiro‐Bisiloles as Highly Efficient Yellow‐Light Emitters in Electroluminescent Devices

New spiro‐bisilole molecules functionalized with nitrogen‐containing heterocyclic groups including 7‐azaindolyl, indolyl, and 2,2′‐dipyridylamino have been synthesized. These molecules are found to display good chemical and thermal stability. They are luminescent in solution and in the solid state with an emission color ranging from blue–green to yellow, depending on the functional group. In the solid state, they display high photoluminescence quantum efficiency (32–40 %). The electroluminescence properties for one of the new molecules, 2,3,3′,4,4′,5‐hexaphenyl‐2′,5′‐bis(p‐2,2′‐dipyridylaminophenyl)spiro‐bisilole, have been investigated by fabricating single‐layer and double‐layer electroluminescent devices. The double‐layer device, in which N,N′‐bis(1‐naphthyl)‐N,N′‐diphenylbenzidine acts as the hole‐transport layer and the functionalized spiro‐bisilole functions as the emitter (emission wavelength = 566 nm) and the electron‐transport layer, displays a brightness of 8440 cd m^–2 at 9 V with a current efficiency of 1.71 cd A^–1. No evidence of exiplex emission is observed.     

Alkylsilyl Functionalized Copolymer Donor for Annealing‐Free High Performance Solar Cells with over 11% Efficiency: Crystallinity Induced Small Driving Force

The contradiction between enlarging the offset between energy levels of donor/acceptor and the required driving force for exciton split leads to a trade‐off between open circuit voltage (V_OC) and short circuit current density (J_SC), which is a big challenge for development of high performance polymer solar cells (PSCs). Some advanced works reported the PSCs with low photon energy loss (E_loss) and small driving force, but the correlation of molecular structures of light‐harvesting system and driving force is still unclear. In this work, a new alkylsilyl functionalized copolymer donor PBDS‐T (PBDST: poly[(2,6trialkylsilyl thiophen2yl)benzo[1,2b:4,5b′]dithiophene))alt(5,5(1′,3′di2thienyl5′,7′bis(2ethylhexyl)benzo[1′,2′c:4′,5′c′]dithiophene4,8dione))]) with low‐lying energy levels was designed for efficient PSCs. By monitoring the Photoluminescence quenching of the bulk and bilayer heterojunctions, small driving forces, ?E_HOMO of 0.15 eV and ?E_LUMO of 0.22 eV were founded to allow for efficient charge transfer, which were observed to correlate with the crystalline PBDS‐T and the optimal morphology in PBDS‐T:ITIC (ITIC: 3,9bis(2methylene(3(1,1dicyanomethylene)indanone))5,5,11,11tetrakis(4hexylphenyl)dithieno[2,3d:2′,3′d′]sindaceno[1,2b:5,6b′]dithiophene). Simultaneously improved V_OC, J_SC and small Eloss boosted the PCE over 11%, which is one of the highest values for annealing‐free device. These results shield a light on precise design of a light‐harvesting system with small driving force to simultaneously improve the V_OC and J_SC for highly efficient PSCs.     

Very Low Degree of Energetic Disorder as the Origin of High Mobility in an n‐channel Polymer Semiconductor

Charge transport is investigated in high‐mobility n‐channel organic field‐effect transistors (OFETs) based on poly{[N,N′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} (P(NDI2OD‐T2), Polyera ActivInk? N2200) with variable‐temperature electrical measurements and charge‐modulation spectroscopy. Results indicate an unusually uniform energetic landscape of sites for charge‐carrier transport along the channel of the transistor as the main reason for the observed high‐electron mobility. Consistent with a lateral field‐independent transport at temperatures down to 10 K, the reorganization energy is proposed to play an important role in determining the activation energy for the mobility. Quantum chemical calculations, which show an efficient electronic coupling between adjacent units and a reorganization energy of a few hundred meV, are consistent with these findings.     

Bulk Heterojunction Solar Cells: Impact of Minor Structural Modifications to the Polymer Backbone on the Polymer–Fullerene Mixing and Packing and on the Fullerene–Fullerene Connecting Network

The morphology of the active layer of a bulk heterojunction solar cell, made of a blend of an electron‐donating polymer and an electron‐accepting fullerene derivative, is known to play a determining role in device performance. Here, a combination of molecular dynamics simulations and long‐range corrected density functional theory calculations is used to elucidate the molecular‐scale effects that even minor structural changes to the polymer backbone can have on the “local” morphology; this study focuses on the extent of polymer–fullerene mixing, on their packing, and on the characteristics of the fullerene–fullerene connecting network in the mixed regions, aspects that are difficult to access experimentally. Three representative polymer donors are investigated: (i) poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3′″‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2′″‐quaterthiophen‐5,5′″‐diyl)] (PffBT4T‐2OD); (ii) poly[(2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3′″‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2′″‐quaterthiophen‐5,5′″‐diyl)] (PBT4T‐2OD), where the fluorine atoms in the benzothiadiazole moieties of PffBT4T‐2OD are replaced with hydrogen atoms; and (iii) poly[(2,2′‐bithiophene)‐alt‐(4,7‐bis((2‐decyltetradecyl)thiophen‐2‐yl)‐5,6‐difluoro‐2‐propyl‐2H‐benzo[d][1,2,3]triazole)] (PT2‐FTAZ), where the sulfur atoms in the benzothiadiazole moieties of PffBT4T‐2OD are replaced with nitrogen atoms carrying a linear C₃H₇ side‐chain; these polymers are mixed with the phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) acceptor. This study also discusses the nature of the charge‐transfer electronic states appearing at the donor–acceptor interfaces, the electronic couplings relevant for the charge‐recombination process, and the electron‐transfer features between neighboring PC₇₁BM molecules.     

Enhanced‐Light‐Harvesting Amphiphilic Ruthenium Dye for Efficient Solid‐State Dye‐Sensitized Solar Cells

A ruthenium sensitizer (coded C101, NaRu (4,4′‐bis(5‐hexylthiophen‐2‐yl)‐2,2′‐bipyridine) (4‐carboxylic acid‐4′‐caboxylate‐2,2′‐bipyridine) (NCS)₂) containing a hexylthiophene‐conjugated bipyridyl group as an ancillary ligand is presented for use in solid‐state dye‐sensitized solar cells (SSDSCs). The high molar‐extinction coefficient of this dye is advantageous compared to the widely used Z907 dye, (NaRu (4‐carboxylic acid‐4′‐carboxylate) (4,4′‐dinonyl‐2,2′‐bipyridine) (NCS)₂). In combination with an organic hole‐transporting material (spiro‐MeOTAD, 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine) 9, 9′‐spirobifluorene), the C101 sensitizer exhibits an excellent power‐conversion efficiency of 4.5% under AM 1.5 solar (100 mW cm^?2) irradiation in a SSDSC. From electronic‐absorption, transient‐photovoltage‐decay, and impedance measurements it is inferred that extending the π‐conjugation of spectator ligands induces an enhanced light harvesting and retards the charge recombination, thus favoring the photovoltaic performance of a SSDSC.     

Fluorenone‐Based Molecules for Bulk‐Heterojunction Solar Cells: Synthesis,Characterization, and Photovoltaic Properties

A series of four conjugated molecules consisting of a fluorenone central unit symmetrically coupled to different oligothiophene segments are conceptually designed and synthesized to provide new electroactive materials for application in photovoltaic devices. The combination of electron‐donating oligothiophene building blocks with an electron‐accepting fluorenone unit results in the emergence of a new band assigned to an intramolecular charge transfer transition that gives rise to the extension of the absorption spectral range of the resulting molecules. Detailed spectroscopic and voltammetric investigations show that all studied molecules have highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) level positions, which make them good candidates for the application as electron‐donors in bulk‐heterojunction photovoltaic cells, with (6,6)‐phenyl‐C61‐butyric acid methyl ester (PCBM)‐C₆₀ as electron acceptor component. Moderate device performances, with power conversion efficiencies (PCEs) comprised between 0.3 and 0.6%, were obtained with rigid molecules, containing either the bridging units between the thiophene rings, i.e., (2,7‐bis(4,4′‐dioctyl‐cyclopenta[2,1‐b:3,4‐b′]dithiophen‐2‐yl)‐fluoren‐9‐one (SCPTF) and 2,7‐bis(4‐(dioctylmethylene)‐cyclopenta[2,1‐b:3,4‐b′]dithiophen‐5‐yl)‐fluoren‐9‐one (MCPTF) or a vinylene unit 2,7‐bis(5‐[(E)‐1,2‐bis(3‐octylthien‐2‐yl)ethylene])‐fluoren‐9‐one (TVF), whereas with (2,7‐bis‐(3,3?‐dioctyl‐[2,2′;5′,2″;5″,2?]quaterthiophen‐5‐yl)‐fluoren‐9‐one (QTF) PCE up to 1.2% (under AM 1.5 illumination, 100 mW cm^?2, active area 0.28 cm²) was obtained. The strong π‐stacking interactions in the solid state for this oligomer leading to improved morphology could explain the good performances of QTF‐based devices, which rank among the highest recorded for non‐polymeric materials. Consequently, fluorenone‐based non‐polymeric molecules constitute highly attractive materials for solution‐processable solar cell applications.     

High‐Mobility n‐Type Organic Transistors Based on a Crystallized Diketopyrrolopyrrole Derivative

A new high‐performing small molecule n‐channel semiconductor based on diketopyrrolopyrrole (DPP), 2,2′‐(5,5′‐(2,5‐bis(2‐ethylhexyl)‐3,6‐dioxo‐2,3,5,6‐tetrahydropyrrolo[3,4‐c]pyrrole‐1,4‐diyl)bis(thiophene‐5,2‐diyl))bis(methan‐1‐yl‐1‐ylidene)dimalononitrile (DPP‐T‐DCV), is successfully synthesized. The frontier molecular orbitals in this designed structure are elaborately tuned by introducing a strong electron‐accepting functionality (dicyanovinyl). The well‐defined lamellar structures of the crystals display a uniform terrace step height corresponding to a molecular monolayer in the solid‐state. As a result of this tuning and the remarkable crystallinity derived from the conformational planarity, organic field‐effect transistors (OFETs) based on dense‐packed solution‐processed single‐crystals of DPP‐T‐DCV exhibit an electron mobility (μ_e) up to 0.96 cm² V^?1 s^?1, one of the highest values yet obtained for DPP derivative‐based n‐channel OFETs. Polycrystalline OFETs show promise (with an μ_e up to 0.64 cm² V^?1 s^?1) for practical utility in organic device applications.     

A Highly Stable,New Electrochromic Polymer: Poly(1,4‐bis(2‐(3′,4′‐ethylenedioxy) thienyl)‐2‐methoxy‐5‐2″‐ethylhexyloxybenzene)

A highly stable new electrochromic polymer, poly(1,4‐bis(2‐(3′,4′‐ethylenedioxy)thienyl)‐2‐methoxy‐5‐2″‐ethylhexyloxybenzene) (P(BEDOT‐MEHB)) was synthesized and its electrochemical and electrochromic properties are reported. P(BEDOT‐MEHB) showed a very well defined electrochemistry with a relatively low oxidation potential of the monomer at + 0.44 V versus Ag/Ag⁺, E_1/2 at – 0.35 V versus Ag/Ag⁺ and stability to long‐term switching up to 5000 cycles. A high level of stability to over‐oxidation has also been observed as this material shows limited degradation of its electroactivity at potentials 1.4 V above its half‐wave potential. Spectroelectrochemistry showed that the absorbance of the π–π* transition in the neutral state is blue‐shifted compared to PEDOT, displaying a maximum at 538 nm (onset at 640 nm), thus giving an almost colorless, highly transparent oxidized polymer with a bandgap of 1.95 eV. Different colors observed at different oxidation levels and strong absorption in the near‐IR make this polymer a good candidate for several applications.     

Molecular Engineering of Highly Efficient Small Molecule Nonfullerene Acceptor for Organic Solar Cells

A new molecularly engineered nonfullerene acceptor, 2,2′‐(5,5′‐(9,9‐didecyl‐9H‐fluorene‐2,7‐diyl)bis (benzo[c][1,2,5]thiadiazole‐7,4‐diyl)bis (methanylylidene))bis (3‐hexyl‐1,4‐oxothiazolidine‐5,2‐diylidene))dimalononitrile ( BAF‐4CN ), with fluorene as the core and arms of dicyano‐n‐hexylrhodanine terminated benzothiadiazole is synthesized and used as an electron acceptor in bulk heterojunction organic solar cells. BAF‐4CN shows a stronger and broader absorption with a high molar extinction coefficient of 7.8 × 10⁴m ^?1 cm^?1 at the peak position (498 nm). In the thin film, the molecule shows a redshift around 17 nm. The photoluminescence experiments confirm the excellent electron accepting nature of BAF‐4CN with a Stern–Volmer coefficient (K _sv) of 1.1 × 10⁵m ^?1. From the electrochemical studies, the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of BAF‐4CN are estimated to be ?5.71 and ?3.55 eV, respectively, which is in good synchronization with low bandgap polymer donors. Using BAF‐4CN as an electron acceptor in a poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3″′‐di(2‐octyldodecyl) 2,2′;5′,2″;5″,2″′‐quaterthiophen‐5,5″′‐diyl)] based bulk‐heterojunction solar cell, a maximum power conversion efficiency of 8.4% with short‐circuit current values of 15.52 mA cm^?2, a fill factor of 70.7%, and external quantum efficiency of about 84% covering a broad range of wavelength is achieved.     

Semiconducting Polymers via Microwave‐Assisted Suzuki and Stille Cross‐Coupling Reactions

A fully conjugated para‐phenylene ladder polymer ( P1 ) and the alternating copolymers {2,7‐[9,9‐bis(2‐ethylhexyl)fluorene]‐5,5′‐(2,2′‐bithiophene)} ( P3 ) and {2,7‐[9,9‐dioctylfluorene]‐5,5′‐(2,2′‐bithiophene)} ( P4 ) have been prepared via metal‐mediated cross‐coupling reactions, using microwaves as a heat source. The procedure, which yields polymeric material in ca. ten minutes, has no adverse effects on the quality of the polymers and displays a high degree of reproducibility. Transfer of the optimized conditions to the synthesis of a new naphthalene‐based polyarylene‐ketone ( P2 ) and a (1,5‐dioctoxynaphthylene‐2,6‐diyl‐alt‐2,2′‐bithiophene‐5,5′‐diyl) copolymer ( P5 ) confirmed the versatility of the procedure and the dramatic reduction in reaction times compared with conventional heating. In the case of the Stille‐type coupling reaction of the electron‐rich, less reactive dibromo monomer 1,5‐dioctoxy‐2,6‐dibromo‐naphthalene, the microwave‐assisted protocol results in a marked increase in both yield and molecular weight.