Organic Photovoltaic Devices Based on a New Class of Oligothienylenevinylene Derivatives as Donor Materials

A new class of donor–acceptor‐containing oligothienylenevinylenes with a triphenylamine donor and a dicyanovinyl group as acceptor has been synthesized and characterized. By extending the oligothiophene backbone, both the optical bandgaps and the charge‐transport properties can be tuned. These oligothienylenevinylene derivatives show intense charge‐transfer absorption bands that cover the entire visible spectrum, with low optical bandgaps of approximately 1.64 eV. In addition, electrochemical studies reveal that these compounds possess relatively large ionization potentials of approximately 5.5 eV. On the basis of these newly developed dicyanovinyl‐substituted chromophores as donor materials and C₆₀ as acceptor material, bilayer organic photovoltaic devices have been fabricated, with the best device showing a high power conversion efficiency (PCE) of 2.0 %, with an open‐circuit voltage of 0.68 V and a fill factor of 0.60 after thermal annealing. The obvious morphology change with the formation of small domains in thin films and the reduction of series resistance are believed to be responsible for the dramatic performance improvement upon thermal annealing.     

Synthesis and Photovoltaic Properties of Two‐Dimensional Low‐Bandgap Copolymers Based on New Benzothiadiazole Derivatives with Different Conjugated Arylvinylene Side Chains

A new series of 2,1,3‐benzothiadiazole (BT) acceptors with different conjugated aryl‐vinylene side chains have been designed and used to build efficient low‐bandgap (LBG) photovoltaic copolymers. Based on benzo[1,2‐b:3,4‐b′]dithiophene and the resulting new BT derivatives, three two‐dimensional (2D)‐like donor (D)–acceptor (A) conjugated copolymers have been synthesised by Stille coupling polymerisation. These copolymers were characterised by NMR spectroscopy, gel‐permeation chromatography, thermogravimetric analysis and differential scanning calorimetry. UV/Vis absorption and cyclic voltammetry measurements indicated that their optical and electrochemical properties can be facilely modified by changing the structures of the conjugated aryl‐vinylene side chains. The copolymer with phenyl‐vinylene side chains exhibited the best light harvesting and smallest bandgap of the three copolymers. The basic electronic structures of D–A model compounds of these copolymers were also studied by DFT calculations at the B3LYP/6‐31G* level of theory. Polymer solar cells (PSCs) with a typical structure of indium tin oxide (ITO)/poly(3,4‐ethylenedioxythiophene) (PEDOT):poly(styrenesulfonate) (PSS)/copolymer:[6,6]‐phenyl‐C₆₁(C₇₁)‐butyric acid‐methyl ester (PCBM)/calcium (Ca)/aluminum (Al) were fabricated and measured under the illumination of AM1.5G at 100 mW cm^?2. The results showed that the device based on the copolymer with phenyl‐vinylene side chains had the highest efficiency of 2.17 % with PC₇₁BM as acceptor. The results presented herein indicate that all the prepared copolymers are promising candidates for roll‐to‐roll manufacturing of efficient PSCs. Suitable electronic, optical and photovoltaic properties of BT‐based copolymers can also be achieved by fine‐tuning the structures of the aryl‐vinylene side chains for photovoltaic application.     

Synthetic Control of Spectroscopic and Photophysical Properties of Triarylborane Derivatives Having Peripheral Electron‐Donating Groups

The spectroscopic and photophysical properties of triarylborane derivatives were controlled by the nature of the triarylborane core (trixylyl‐ or trianthrylborane) and peripheral electron‐donating groups (N,N‐diphenylamino or 9H‐carbazolyl groups). The triarylborane derivatives with and without the electron‐donating groups showed intramolecular charge‐transfer absorption/fluorescence transitions between the π orbital of the aryl group (π(aryl)) and the vacant p orbital on the boron atom (p(B), π(aryl)–p(B) CT), and the fluorescence color was tunable from blue to red by the combination of peripheral electron‐donating groups and a triarylborane core. Detailed electrochemical, spectroscopic, and photophysical studies of the derivatives, including solvent dependences of the spectroscopic and photophysical properties, demonstrated that the HOMO and LUMO of each derivative were determined primarily by the nature of the peripheral electron‐donating group and the triarylborane core, respectively. The effects of solvent polarity on the fluorescence quantum yield and lifetime of the derivatives were also tunable by the choice of the triarylborane core.     

Octupolar Derivatives Functionalized with Superacceptor Peripheral Groups: Synthesis and Evaluation of the Electron‐Withdrawing Ability of Potent Unusual Groups

Novel tripodal derivatives with a triphenylamine core and that bear “superacidifiers” (i.e., fluorinated sulfoximinyl blocks) or novel sulfiliminyl moieties as peripheral groups were synthesized. These new chromophores show strong absorption in the near‐UV region and emission in the visible region. The fluorinated sulfoximinyl moieties were found to behave as potent auxochromic and electron‐withdrawing (EW) groups, thus leading to redshifted absorption and emission. These moieties promote a core‐to‐periphery intramolecular charge transfer (ctp‐ICT) transition, the energy of which was found to be correlated to their EW strength. In this study, we provide evidence of a linear correlation between the Hammett constant (σ_p) values and the electronic gap between the ground and first excited state of the three‐branched derivatives. This in turn was used to derive σ_p values of fluorinated sulfoximinyl moieties. These EWGs show unprecedentedly high σ_p values, up to 1.45 relative to 0.8 for NO₂. Also, by using this method, the sulfiliminyl moiety was shown to exhibit similar EW strength as NO₂, while promoting improved transparency and solubility. Finally, the superior EW strength of the fluorinated sulfoximine peripheral moieties was shown to induce significant enhancement of the two‐photon absorption responses in the red near‐IR region of the three‐branched derivatives relative to similar octupoles that bear more usual strong EW groups. These characteristics (improved nonlinear responses or transparency) open new routes for the design of nonlinear optical (NLO) chromophores for optical limiting or electro‐optical modulation. Such building blocks could also be of interest for optoelectronic applications, including the development of solar cells.     

Adjusting the Proportion of Electron‐Withdrawing Groups in a Graft Functional Polymer for Multilevel Memory Performance

A polymer containing aldehyde active groups (PVB) was synthesized by atom transfer radical polymerization (ATRP), acting as a polymer precursor to graft a functional moiety via nucleophilic addition reaction. DHI (2‐(1,5‐dimethyl‐hexyl)‐6‐hydrazino‐benzo[de]isoquinoline‐1,3‐dione) and NPH (nitrophenyl hydrazine) groups, which contain naphthalimides that act as narrow traps and nitro groups that act as deep traps, were anchored onto the PVB at different ratios. A series of graft polymers were obtained and named PVB‐DHI, PVB‐DHI₄‐NPH, PVB‐DHI‐NPH₄, and PVB‐NPH. The chemical composition of the polymers was analyzed by ¹H‐NMR spectroscopy and X‐ray photoelectron spectroscopy (XPS). Memory devices were prepared from the polymers, and I–V characteristics were measured to determine the performance. By adjusting the ratio of different electron acceptors (DHI and NPH) to 4:1, ternary memory behavior was achieved. The relationship between memory behavior of PVB‐DHI_xNPH_y and acceptor groups as well as their conduction mechanism were studied in detail.     

New Low Bandgap Dithienylbenzothiadiazole Vinylene Based Copolymers: Synthesis and Photovoltaic Properties

Two new low‐bandgap block copolymers derived from dithienylbenzothiadiazole (DTBT) and different electron‐rich functional groups (dioctoxyl benzene and N‐octyl‐diphenylamine), poly(1,4‐dioctoxyl‐2,5‐divinylbenzene‐co‐4,7‐dithiophene‐2′‐yl‐2,1,3‐benzothiadiazole) (PPV‐DTBT), poly(3,8‐divinyl‐N‐octyl‐diphenylamine‐co‐4,7‐dithiophene‐2′‐yl‐2,1,3‐benzothiadiazole) (PDPAV‐DTBT), were synthesized by Heck cross‐coupling polymerization. PPV‐DTBT and PDPAV‐DTBT are easily soluble in common organic solvents such as o‐dichlorobenzene and chloroform. DSC and TGA results indicate that these copolymers possess good thermal stabilities. PPV‐DTBT and PDPAV‐DTBT films exhibit broad absorption bands at 300–765 nm (with an optical bandgap of 1.62 eV) and 300–733 nm (with an optical bandgap of 1.69 eV), respectively. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PPV‐DTBT were estimated by cyclic voltammetry to be −5.43 and −3.74 eV, respectively, and the HOMO and LUMO of PDPAV‐DTBT were −5.37 and −3.7 eV, respectively. Preliminary photovoltaic cells based on the composite structure of ITO/PEDOT: PSS/PPV‐DTBT:PCBM (1: 2, w/w)/Al showed an open‐circuit voltage of 0.75 V, a power conversion efficiency of 0.6%, and a short circuit current of 1.7 mA · cm⁻².

     

Enhanced Electron‐Transfer Properties of Cofacial Porphyrin Dimers through π–π Interactions

π–π assisted : Photoinduced electron transfer from cofacial porphyrin dimers to electron acceptors is prominently accelerated, whereas the back electron transfer is decelerated, relative to the corresponding porphyrin monomer (see figure).

     

Ferrocene‐Substituted Naphthalenediimide with Broad Absorption and Electron‐Transport Properties in the Segregated‐Stack Structure

A new naphthalenediimide (NDI) molecule, where two ferrocene (Fc) units were directly attached to both imide nitrogens ( Fc‐NDI‐Fc ), was synthesized. The Fc units provide high crystallinity to Fc‐NDI‐Fc with good solubility to conventional organic solvents. The Fc units also work as electron‐donating substituents, in contrast to the electron‐deficient NDI unit, resulting in broad charge‐transfer absorption of Fc‐NDI‐Fc from the UV region to 1500 nm in the solid state. The crystal structure analysis revealed that Fc‐NDI‐Fc formed a segregated‐stack structure. The DFT calculation based on the crystal structure showed that the NDI π‐orbitals extended over two axes. The extended π‐network of the NDI units led to the electron‐transport properties of Fc‐NDI‐Fc , which was confirmed using a flash‐photolysis time‐resolved microwave conductivity technique.     

Synthesis and Plasmon‐Induced Charge‐Transfer Properties of Monodisperse Gold‐Doped Titania Microspheres

Functional spheres : Monodisperse gold‐doped titania spheres with tunable sizes under high concentration of titanium precursor have been synthesized by introducing trace amounts of chloroauric acid into the reaction system. Surface photovoltage, surface photocurrent, and transient photovoltage measurements (see figure) of annealed samples reveal that gold nanodots can act as both electron acceptors and donors under the illumination of different wavelengths of light.

     

Different Steric‐Twist‐Induced Ternary Memory Characteristics in Nonconjugated Copolymers with Pendant Naphthalene and 1,8‐Naphthalimide Moieties

Herein, novel random copolymers PMNN and PMNB were designed and synthesized, and the memory devices Al/ PMNN and PMNB /ITO both exhibited ternary memory performance. The switching voltages of the OFF–ON1 and ON1–ON2 transitions for both memory devices are around −2.0 and −3.5 V, respectively, and the ON1/OFF, ON2/ON1 current ratios are both up to 10³. The observed tristable electrical conductivity switching could be attributed to field‐induced conformational ordering of the naphthalene rings in the side chains, and subsequent charge trapping by 1,8‐naphthalimide moieties. More interestingly, by adjusting the connection sites of 1,8‐naphthalimide moieties to tune the steric‐twist effect, different memory properties were achieved ( PMNN showed nonvolatile write once, read many (WORM) memory behavior, whereas PMNB showed volatile static RAM (SRAM) memory behavior). This result will offer a guideline for the design of different high‐performance multilevel memory devices by tuning the steric effects of the chemical moieties.     

Effects of Block Length in Copolymers Based on Regioregular Oligothiophenes Linked With Electron‐Accepting Units

Copolymers with an alternating structure of regioregular oligo(3‐hexylthiophene) (O3HT) with different lengths and 2,5‐dibutyl‐3,6‐di(thiophen‐2‐yl)pyrrolo[3,4‐c]pyrrole‐1,4(2H,5H)‐dione (DPP) were synthesized through Stille coupling reaction. The light absorption of the copolymers can be rationally tuned to have a broad spectrum across the visible region by adjusting the length of O3HT. Organic solar cells fabricated with the copolymers and PCBM showed a broad photoresponse and a comparable efficiency to that of poly(3‐hexylthiophene) (P3HT):PCBM cells. The external quantum efficiency and fluorescence spectra suggested that the intrachain energy transfer from the O3HT block to the vicinity of the DPP unit could limit the photovoltaic performance of the copolymers.     

Intermolecular Interactions in Weak Donor–Acceptor Complexes from Symmetry‐Adapted Perturbation and Coupled‐Cluster Theory: Tetracyanoethylene–Benzene and Tetracyanoethylene–p‐Xylene

The interactions in the complexes of tetracyanothylene (TCNE) with benzene and p‐xylene, often classified as weak electron donor–acceptor (EDA) complexes, are investigated by a range of quantum chemical methods including intermolecular perturbation theory at the DFT‐SAPT (symmetry‐adapted perturbation theory combined with density functional theory) level and explicitly correlated coupled‐cluster theory at the CCSD(T)‐F12 level. The DFT‐SAPT interaction energies for TCNE–benzene and TCNE–p‐xylene are estimated to be ?35.7 and ?44.9 kJ mol^?1, respectively, at the complete basis set limit. The best estimates for the CCSD(T) interaction energy are ?37.5 and ?46.0 kJ mol^?1, respectively. It is shown that the second‐order dispersion term provides the most important attractive contribution to the interaction energy, followed by the first‐order electrostatic term. The sum of second‐ and higher‐order induction and exchange–induction energies is found to provide nearly 40 % of the total interaction energy. After addition of vibrational, rigid‐rotor, and translational contributions, the computed internal energy changes on complex formation approach results from gas‐phase spectrophotometry at elevated temperatures within experimental uncertainties, while the corresponding entropy changes differ substantially.     

Pd‐Catalysed Direct Arylation Polymerisation for Synthesis of Low‐Bandgap Conjugated Polymers and Photovoltaic Performance

Low‐bandgap conjugated copolymers based on a donor–acceptor structure have been synthesised via palladium‐complex catalysed direct arylation polymerisation. Initially, we report the optimisation of the synthesis of poly(cyclopentadithiophene‐alt‐benzothiadiazole) (PCPDTBT) formed between cyclopentadithiophene and dibromobenzothiadiazole units. The polymerisation condition has been optimised, which affords high‐molecular‐weight polymers of up to M _n = 70 k using N‐methylpyrrolidone as a solvent. The polymers are used to fabricate organic photovoltaic devices and the best performing PCPDTBT device exhibits a moderate improvement over devices fabricated using the related polymer via Suzuki coupling. Similar polymerisation conditions have also been applied for other monomer units.     

Self‐Assembly of Pyridinium‐Functionalized Anthracenes: Molecular‐Skeleton‐Directed Formation of Microsheets and Microtubes

Two amphiphilic regioisomers, 9‐AP (1‐[11‐(9‐anthracenylmethoxy)‐11‐oxoundecyl]pyridinium bromide), and 2‐AP (1‐[11‐(2‐anthracenyl methoxy)‐11‐oxoundecyl]pyridinium bromide), were synthesized and their assembly behaviors were studied. Due to the anisotropic features of the anthracene structure, different substituted positions on the anthracene ring lead 9‐AP and 2‐AP to adapt “shaver” and “spatula”‐like molecular shapes, respectively, which consequently dictate the structure of their final assemblies. While “shaver”‐shaped 9‐AP assembled into microsheets, driven by π–π interactions, “spatula”‐shaped 2‐AP assembled into microtubular structures, promoted primarily by charge‐transfer interactions.     

Competition Between π–π and CH/π Interactions: A Comparison of the Structural and Electronic Properties of Alkoxy‐Substituted 1,8‐Bis((propyloxyphenyl)ethynyl)naphthalenes

The structural and electronic consequences of π–π and C?H/π interactions in two alkoxy‐substituted 1,8‐bis‐ ((propyloxyphenyl)ethynyl)naphthalenes are explored by using X‐ray crystallography and electronic structure computations. The crystal structure of analogue 4 , bearing an alkoxy side chain in the 4‐position of each of the phenyl rings, adopts a π‐stacked geometry, whereas analogue 8 , bearing alkoxy groups at both the 2‐ and the 5‐positions of each ring, has a geometry in which the rings are splayed away from a π‐stacked arrangement. Symmetry‐adapted perturbation theory analysis was performed on the two analogues to evaluate the interactions between the phenylethynyl arms in each molecule in terms of electrostatic, steric, polarization, and London dispersion components. The computations support the expectation that the π‐stacked geometry of the alkoxyphenyl units in 4 is simply a consequence of maximizing π–π interactions. However, the splayed geometry of 8 results from a more subtle competition between different noncovalent interactions: this geometry provides a favorable anti‐alignment of C?O bond dipoles, and two C?H/π interactions in which hydrogen atoms of the alkyl side chains interact favorably with the π electrons of the other phenyl ring. These favorable interactions overcome competing π–π interactions to give rise to a geometry in which the phenylethynyl substituents are in an offset, unstacked arrangement.     

Interaction of Aromatic Derivatives with Single‐Walled Carbon Nanotubes

Fluorescence of semiconducting single‐walled carbon nanotubes (SWNTs) normally exhibits diameter‐dependent oxidative quenching behaviour. This behaviour can be changed substantially to become an almost diameter‐independent quenching phenomenon in the presence of electron‐withdrawing nitroaromatic compounds, including o‐nitrotoluene, 2,4‐dinitrotoluene, and nitrobenzene. This change is observed for SWNTs suspended either in sodium dodecyl sulfate or in Nafion upon titration with hydrogen peroxide. Benzene, toluene, phenol, and nitromethane do not show such change. These findings suggest the possibility of forming an electron donor–acceptor complex between SWNTs and nitroaromatic compounds, resulting in leveling the redox potential of different SWNT species. The observation appears to provide a new method for modifying the electrochemical potentials of SWNTs through donor–acceptor complex formation.