Redox-Active Molecular Wires Incorporating Ruthenium(II) σ-Arylacetylide Complexes for Molecular Electronics

The preparation and properties of novel ruthenium carbon-rich complexes for molecular electronics are reported. The synthetic procedure used in this work led to the first series of neutral redox-active conjugated molecular wires including mono-, bi-, and trimetallic bis(σ-arylacetylide) complexes (RunNC and CNRunNC, n = 1–3) having 1,4-diethynylbenzene spacers and one or two isocyanide terminal groups for surface binding. An analogous cationic σ-arylacetylide-allenylidene molecule (AllRuNC+) is also reported. These new structurally rigid complexes have lengths ranging from 1.8 to 4.5 nm and are excellent candidates for the building of alternative metal−molecule−metal junctions. Indeed, the molecules uniquely contain up to three metal-redox centers that are efficiently coupled by conjugated ligands to provide significant electronic communication along the molecular backbone, as indicated by the optical and electrochemical properties. Furthermore, the wires offer multiple low potential redox states that can lead to unusual current–voltage behavior and efficient charge conduction. Overall, these molecules will open a route to establish the structure–property relationships of conductive molecular wires and to gain valuable insights into the correlation between charge transport and molecular length

Measuring Relative Barrier Heights in Molecular Electronic Junctions with Transition Voltage Spectroscopy

Though molecular devices exhibiting potentially useful electrical behavior have been demonstrated, a deep understanding of the factors that influence charge transport in molecular electronic junctions has yet to be fully realized. Recent work has shown that a mechanistic transition occurs from direct tunneling to field emission in molecular electronic devices. The magnitude of the voltage required to enact this transition is molecule-specific, and thus measurement of the transition voltage constitutes a form of spectroscopy. Here we determine that the transition voltage for a series of alkanethiol molecules is invariant with molecular length, while the transition voltage of a conjugated molecule depends directly on the manner in which the conjugation pathway has been extended. Finally, by examining the transition voltage as a function of contact metal, we show that this technique can be used to determine the dominant charge carrier for a given molecular junction

Molecular Tunnel Junctions Based on π-Conjugated Oligoacene Thiols and Dithiols between Ag, Au, and Pt Contacts: Effect of Surface Linking Group and Metal Work Function

The tunneling resistance and electronic structure of metal–molecule–metal junctions based on oligoacene (benzene, naphthalene, anthracene, and tetracene) thiol and dithiol molecules were measured and correlated using conducting probe atomic force microscopy (CP-AFM) in conjunction with ultraviolet photoelectron spectroscopy (UPS). Nanoscopic tunnel junctions (∼10 nm2) were formed by contacting oligoacene self-assembled monolayers (SAMs) on flat Ag, Au, or Pt substrates with metalized AFM tips (Ag, Au, or Pt). The low bias (R) increased exponentially with molecular length (s), i.e., R = R0 exp(βs), where R0 is the contact resistance and β is the tunneling attenuation factor. The R0 values for oligoacene dithiols were 2 orders of magnitude less than those of oligoacene thiols. Likewise, the β value was 0.5 per ring (0.2 Å–1) for the dithiol series and 1.0 per ring (0.5 Å–1) for the monothiol series, demonstrating that β is not simply a characteristic of the molecular backbone but is strongly affected by the number of chemical (metal–S) contacts. R0 decreased strongly as the contact work function (Φ) increased for both monothiol and dithiol junctions, whereas β was independent of Φ within error. This divergent behavior was explained in terms of the metal–S bond dipoles and the electronic structure of the junction; namely, β is independent of contact type because of weak Fermi level pinning (UPS revealed EF – EHOMO varied only weakly with Φ), but R0 varies strongly with contact type because of the strong metal–S bond dipoles that are responsible for the Fermi level pinning. A previously published triple barrier model for molecular junctions was invoked to rationalize these results in which R0 is determined by the contact barriers, which are proportional to the size of the interfacial bond dipoles, and β is determined by the bridge barrier, EF – EHOMO. Current–voltage (I–V) characteristics obtained over a larger voltage range 0–1 V revealed a characteristic transition voltage Vtrans at which the current increased more sharply with voltage. Vtrans values were generally >0.5 V and were well correlated with the bridge barrier EF – EHOMO. Overall, the combination of electronic structure determination by UPS with length- and work function-dependent transport measurements provides a remarkably comprehensive picture of tunneling transport in molecular junctions based on oligoacenes

Importance of 4-<i>tert</i>-Butylpyridine in Electrolyte for Dye-Sensitized Solar Cells Employing SnO₂ Electrode

The photovoltaic performance of dye-sensitized solar cells (DSSCs) employing SnO2 electrodes was investigated while increasing the content of 4-tert-butylpyridine (TBP) in the conventional liquid-type electrolyte. As the added TBP content increased, the open circuit voltage (Voc) and conversion efficiency were highly enhanced while the short circuit current (Jsc) was not much affected. With the electrolyte of 2.0 M TBP, the Voc and conversion efficiency were increased by 26 and 33%, respectively, compared with the conventional electrolyte (0.5 M TBP). The electrochemical impedance spectra revealed that the enhancement of Voc resulted from the negative shift of the SnO2 conduction band potential and the increase in resistance of electron recombination by 1 order of magnitude. It is noteworthy that the optimized concentration of TBP for the SnO2 electrode is greatly larger than that for the TiO2 electrode. This may be due to the much faster electron recombination rate and more positive conduction band potential of the SnO2 electrode. The SnO2 electrode modified with TiO2 shell showed only slightly enhanced performance due to the similar effects of shell layer and those of the TBP. In contrast to the SnO2, TiO2 electrodes did not show performance enhancement with the electrolyte of TBP concentration larger than 0.5 M. The impedance spectra of symmetric dummy cells employing Pt counter electrodes indicated that the catalytic effect of Pt was deteriorated, and the resistance of electrolyte diffusion was increased by the higher concentration of TBP. This brings up the need for development of a counter electrode that TBP is not easily adsorbed on, and alternative additives to TBP which are not highly viscous

Positional Effect of the 2‑Ethylhexyl Carboxylate Side Chain on the Thiophene π‑Bridge of Nonfullerene Acceptors for Efficient Organic Solar Cells

Three nonfullerene acceptors (NFAs) with an acceptor−π–donor−π–acceptor (A−π–D−π–A) structure, i.e., IDT-3EsT, IDT-3EsT-2IC2F, and IDT-4EsT-2IC2F, were designed and synthesized by inserting a 2-ethylhexyl carboxylate-substituted thiophene π-bridge between the indacenodithiophene (IDT) core and acceptor end group [2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)­malononitrile (IC) or 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)­malononitrile (IC2F)]. These acceptors were systematically modified through variation of the end group and the position of 2-ethylhexyl carboxylate on the thiophene π-bridge. The effect of the 2-ethylhexyl carboxylate group on absorption and energy levels, as well as photovoltaic performance, was investigated. The wide-bandgap PBDB-T polymer was selected as the donor material to fabricate the conventional type of organic solar cells (OSCs) based on the PBDB-T:NFAs blend. Among three NFA-based OSCs, those fabricated using the PBDB-T:IDT-3EsT-2IC2F blend film exhibited an optimized power conversion efficiency (PCE) of 7.54% with a short-circuit current density (JSC) of 14.68 mA cm–2, an open-circuit voltage (VOC) of 0.88 V, and a fill factor (FF) of 58%. OSCs fabricated using PBDB-T:IDT-3EsT showed a promising PCE of 7.43% with a significantly improved VOC of 0.98 V. PBDB-T:IDT-4EsT-2IC2F-based OSCs showed the lowest device performance with a PCE of 5.45% due to the formation of larger domains in the PBDB-T:IDT-4EsT-2IC2F blend film, which had adverse effects on exciton diffusion, dissociation, and charge transport properties. These results demonstrated that modifying the position of substituents on the thiophene π-bridge of NFAs is critical in determining the photovoltaic performance of OSCs

Immobilization of Conjugated Polymer Domains for Highly Stable Non-Fullerene-Based Organic Solar Cells

To commercialize organic solar cells (OSCs), changes in the optimized morphology of the photoactive layer caused by external stimuli that cause degradation must be addressed. This work improves OSC stability by utilizing the cross-linking additive 1,8-dibromooctane (DBO) and a sequential deposition process (XSqD) to fabricate the photoactive layer. The cross-linking additive in the donor polymer (PTB7-Th) improves polymer crystallinity and immobilizes the crystalline morphology by partial photo-cross-linking. Ellipsometry experiments confirm the increase in the glass transition temperature of cross-linked PTB7-Th. The polymer crystallinity is further improved after removal of non-cross-linked polymer and residual additive by chlorobenzene. The cross-linked polymer layer forms an efficient and stable heterojunction with a nonfullerene acceptor (IEICO-4F) layer via an XSqD process. The OSC based on the immobilized PTB7-Th exhibits excellent stability against light soaking and thermal aging

Thieno[3,2‑<i>b</i>]thiophene-Substituted Benzo[1,2‑<i>b</i>:4,5‑<i>b</i>′]dithiophene as a Promising Building Block for Low Bandgap Semiconducting Polymers for High-Performance Single and Tandem Organic Photovoltaic Cells

We designed and synthetized a new poly­{4,8-bis­((2-ethyl­hexyl)­thieno­[3,2-<i>b</i>]­thiophene)-benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithio­phene-<i>alt</i>-2-ethyl­hexyl-4,6-dibromo-3-fluoro­thieno­[3,4-<i>b</i>]­thio­phene-2-carboxylate} (PTTBDT-FTT) comprising bis­(2-ethyl­hexyl­thieno­[3,2-<i>b</i>]­thio­phenyl­benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithio­phene (TTBDT) and 2-ethylhexyl 3-fluorothieno­[3,4-<i>b</i>]­thiophene-2-carboxylate (FTT). The optical bandgap of PTTBDT-FTT was 1.55 eV. The energy levels of the highest occupied and lowest unoccupied molecular orbitals of PTTBDT-FTT were −5.31 and −3.73 eV, respectively. Two-dimensional grazing-incidence X-ray scattering measurements showed that the film’s PTTBDT-FTT chains are predominantly arranged with a face-on orientation with respect to the substrate, with strong π–π stacking. An organic thin-film transistor fabricated using PTTBDT-FTT as the active semiconductor showed high hole mobility of 2.1 × 10^–2 cm²/(V·s). Single-junction bulk heterojunction photovoltaic cells with the configuration ITO/PEDOT:PSS/PTTBDT-FTT:PC₇₁BM/Ca/Al were fabricated, which showed a maximum power conversion efficiency (PCE) of 7.44%. Inverted photovoltaic cells with the structure ITO/PEIE/PTTBDT-FTT:PC₇₁BM/MoO₃/Ag were also fabricated, with a maximum PCE of 7.71%. A tandem photovoltaic device comprising the inverted PTTBDT-FTT:PC₇₁BM cell and a P3HT:ICBA-based cell as the top and bottom cell components, respectively, showed a maximum PCE of 8.66%. This work demonstrated that the newly developed PTTBDT-FTT polymer was very promising for applications in both single and tandem solar cells. Furthermore, this work highlighted the fact that an extended π-system in the electron-donor moiety in low bandgap polymers is crucial for improving polymer solar cells

Transition from Tunneling to Hopping Transport in Long, Conjugated Oligo-imine Wires Connected to Metals

We report the electrical transport characteristics of conjugated oligonaphthalenefluoreneimine (ONI) wires having systematically varied lengths up to 10 nm. Using aryl imine addition chemistry, ONI wires were built from gold substrates by extending the conjugation length through imine linkages between highly conjugated building blocks of alternating naphthalenes and fluorenes. The resistance and current−voltage characteristics of ONI wires were measured as a function of molecular length, temperature, and electric field using conducting probe atomic force microscopy (CP-AFM). We have observed a transition in direct current (DC) transport from tunneling to hopping near 4 nm as previously established for oligophenyleneimine (OPI) wires. Furthermore, we have found that long ONI wires are less resistive than OPI wires. The single-wire conductivity of ONI wires is ∼1.8 ± 0.1 × 10−4 S/cm, a factor of ∼2 greater than that of OPI wires, and consistent with the lower transport activation energy (∼0.58 eV versus 0.65 eV or 13 versus 15 kcal/mol). Quantum chemical calculations reveal that charge is preferentially localized on the fluorene subunits and that the molecules are substantially twisted. Overall, this work confirms that imine addition chemistry can be used to build molecular wires long enough to probe the hopping transport regime. The versatility of this chemistry, in combination with CP-AFM, opens up substantial opportunities to probe the physical organic chemistry of hopping conduction in long conjugated molecules