Cooperative Plasmonic Effect of Ag and Au Nanoparticles on Enhancing Performance of Polymer Solar Cells

This article describes a cooperative plasmonic effect on improving the performance of polymer bulk heterojunction solar cells. When mixed Ag and Au nanoparticles are incorporated into the anode buffer layer, dual nanoparticles show superior behavior on enhancing light absorption in comparison with single nanoparticles, which led to the realization of a polymer solar cell with a power conversion efficiency of 8.67%, accounting for a 20% enhancement. The cooperative plasmonic effect aroused from dual resonance enhancement of two different nanoparticles. The idea was further unraveled by comparing Au nanorods with Au nanoparticles for solar cell application. Detailed studies shed light into the influence of plasmonic nanostructures on exciton generation, dissociation, and charge recombination and transport inside thin film devices

Match the Interfacial Energy Levels between Hole Transport Layer and Donor Polymer To Achieve High Solar Cell Performance

The interfacial energy level alignment is shown to play an important role in determining solar cell performance. Replacing hole transport layer poly­(3,4-ethylene dioxythiophene)–(polystyrene sulfonic acid) (PEDOT:PSS) with vanadium pentoxide (V₂O₅) leads to a simultaneous improvement in short-circuit current density (<i>J</i>_sc), open-circuit voltage (<i>V</i>_oc) and fill factor (FF) for two donor polymers with deep HOMO energy levels, resulting in a power conversion efficiency (PCE) of 7.03% and 4.14%. This is 18% and 106% increase in PCE over the 5.97% and 2.01% achieved with PEDOT:PSS. V₂O₅ is shown to increase <i>J</i>_sc and FF by enhancing hole mobility, reducing bimolecular recombination, and facilitating charge collection and to maximize <i>V</i>_oc by providing a better ohmic contact. We also demonstrate that PEDOT:PSS still works better for donor polymers with a HOMO energy level around 5.1 eV, such as PTB7

Donor–Acceptor Porous Conjugated Polymers for Photocatalytic Hydrogen Production: The Importance of Acceptor Comonomer

Porous conjugated polymer (PCP) is a new kind of photocatalyst for photocatalytic hydrogen production (PHP). Here, we report the importance of the electronic properties of acceptor comonomer in determining the reactivity of 4,8-di­(thiophen-2-yl)­benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene (DBD)-based PCP photocatalyst for PHP application. It was found that the incorporation of nitrogen-containing ligand acceptor monomers into PCP network is an effective strategy to enhance the PHP activity. These moderately electron-deficient comonomers enhanced the dipole polarization effect. These PCPs exhibit appropriate solid-state morphology for charge transport. Powder X-ray diffraction (XRD) studies demonstrate that these PCP materials are semicrystalline materials. A strong correlation between the crystalline property and PHP activity is observed. The replacement of nitrogen-containing ligand acceptors with ligand-free strong acceptors is proved to be detrimental to the PHP process, indicating the proper choice in the electronic properties of monomer pair is important for achieving high photoactivity

Transport Properties of a Single-Molecule Diode

Charge transport through single diblock dipyrimidinyl diphenyl molecules consisting of a donor and acceptor moiety was measured in the low-bias regime and as a function of bias at different temperatures using the mechanically controllable break-junction technique. Conductance histograms acquired at 10 mV reveal two distinct peaks, separated by a factor of 1.5, representing the two orientations of the single molecule with respect to the applied bias. The current–voltage characteristics exhibit a temperature-independent rectification of up to a factor of 10 in the temperature range between 300 and 50 K with single-molecule currents of 45–70 nA at ±1.5 V. The current–voltage characteristics are discussed using a semiempirical model assuming a variable coupling of the molecular energy levels as well as a nonsymmetric voltage drop across the molecular junction, thus shifting the energy levels accordingly. The excellent agreement of the data with the proposed model suggests that the rectification originates from an asymmetric Coulomb blockade in combination with an electric-field-induced level shifting

Molecular Rectification Tuned by Through-Space Gating Effect

Inspired by transistors and electron transfer in proteins, we designed a group of pyridinoparacyclophane based diodes to study the through-space electronic gating effect on molecular rectification. It was shown that an edge-on gate effectively tunes the rectification ratio of a diode via through-space interaction. Higher rectification ratio was obtained for more electron-rich gating groups. The transition voltage spectroscopy showed that the forward transition voltage is correlated to the Hammett parameter of the gating group. Combining theoretical calculation and experimental data, we proposed that the change in rectification was induced by a shift in HOMO level both spatially and energetically. This design principle based on through-space edge-on gate is demonstrated on molecular wires, switches, and now diodes, showing the potential of molecular design in increasing the complexity of single-molecule electronic devices

Effect of Acceptor Strength on Optical and Electronic Properties in Conjugated Polymers for Solar Applications

Four new low-bandgap electron-accepting polymerspoly­(4,10-bis­(2-butyl­octyl)-2-(2-(2-ethylhexyl)-1,1-dioxido-3-oxo-2,3-dihydrothieno­[3,4-<i>d</i>]­isothiazol-4-yl)­thieno­[2′,3′:5,6]­pyrido­[3,4-<i>g</i>]­thieno­[3,2-<i>c</i>]­isoquinoline-5,11­(4<i>H</i>,10<i>H</i>-dione) (PNSW); poly­(4,10-bis­(2-butyl­octyl)-2-(5-(2-ethylhexyl)-4,6-dioxo-5,6-dihydro-4<i>H</i>-thieno­[3,4-<i>c</i>]­pyrrol-1-yl)­thieno­[2′,3′:5,6]­pyrido­[3,4-<i>g</i>]­thieno­[3,2-<i>c</i>]­isoquinoline-5,11­(4<i>H</i>,10<i>H</i>)-dione) (PNTPD); poly­(5-(4,10-bis­(2-butyl­octyl)-5,11-dioxo-4,5,10,11-tetrahydrothieno­[2′,3′:5,6]­pyrido­[3,4-<i>g</i>]­thieno­[3,2-<i>c</i>]­isoquinolin-2-yl)-2,9-bis­(2-decyldodecyl)­anthra­[2,1,9-<i>def</i>:6,5,10-<i>d′e′f′</i>]­diisoquinoline-1,3,8,10­(2<i>H</i>,9<i>H</i>)-tetraone) (PNPDI); and poly­(9,9-bis­(2-butyl­octyl)-9<i>H</i>-fluorene-bis­((1,10:5,6)­2-(5,6-dihydro-4<i>H</i>-cyclopenta­[<i>b</i>]­thiophene-4-ylidene)­malonitrile)-2-(2,3-dihydrothieno­[3,4-<i>b</i>]­[1,4]­dioxine)) (PECN)containing thieno­[2′,3′:5′,6′]­pyrido­[3,4-<i>g</i>]­thieno­[3,2-<i>c</i>]­isoquinoline-5,11­(4<i>H</i>,10<i>H</i>)-dione and fluorenedicyclopentathiophene dimalononitrile, were investigated to probe their structure–function relationships for solar cell applications. PTB7 was also investigated for comparison with the new low-bandgap polymers. The steady-state, ultrafast dynamics and nonlinear optical properties of all the organic polymers were probed. All the polymers showed broad absorption in the visible region, with the absorption of PNPDI and PECN extending into the near-IR region. The polymers had HOMO levels ranging from −5.73 to −5.15 eV and low bandgaps of 1.47–2.45 eV. Fluorescence upconversion studies on the polymers showed long lifetimes of 1.6 and 2.4 ns for PNSW and PNTPD, respectively, while PNPDI and PECN showed very fast decays within 353 and 110 fs. PECN exhibited a very high two-photon absorption cross section. The electronic structure calculations of the repeating units of the polymers indicated the localization of the molecular orbitals in different co-monomers. As the difference between the electron affinities of the co-monomers in the repeating units decreases, the highest occupied and lowest unoccupied molecular orbitals become more distributed. All the measurements suggest that a large difference in the electron affinities of the co-monomers of the polymers contributes to the improvement of the photophysical properties necessary for highly efficient solar cell performance. PECN exhibited excellent photophysical properties, which makes it to be a good candidate for solar cell device applications

Covalently Bound Clusters of Alpha-Substituted PDIRival Electron Acceptors to Fullerene for Organic Solar Cells

A cluster type of electron acceptor, TPB, bearing four α-perylenediimides (PDIs), was developed, in which the four PDIs form a cross-like molecular conformation while still partially conjugated with the BDT-Th core. The blend TPB:PTB7-Th films show favorable morphology and efficient charge dissociation. The inverted solar cells exhibited the highest PCE of 8.47% with the extraordinarily high <i>J</i>_sc values (>18 mA/cm²), comparable with those of the corresponding PC₇₁BM/PTB7-Th-based solar cells

Solution Phase Exciton Diffusion Dynamics of a Charge-Transfer Copolymer <b>PTB7</b> and a Homopolymer <b>P3HT</b>

Using ultrafast polarization-controlled transient absorption (TA) measurements, dynamics of the initial exciton states were investigated on the time scale of tens of femtoseconds to about 80 ps in two different types of conjugated polymers extensively used in active layers of organic photovoltaic devices. These polymers are poly­(3-fluorothienothiophenebenzodithiophene) (<b>PTB7</b>) and poly-3-hexylthiophene (<b>P3HT</b>), which are charge-transfer polymers and homopolymers, respectively. In <b>PTB7</b>, the initial excitons with excess vibrational energy display two observable ultrafast time constants, corresponding to coherent exciton diffusion before the vibrational relaxation, and followed by incoherent exciton diffusion processes to a neighboring local state after the vibrational relaxation. In contrast, <b>P3HT</b> shows only one exciton diffusion or conformational motion time constant of 34 ps, even though its exciton decay kinetics are multiexponential. Based on the experimental results, an exciton dynamics mechanism is conceived taking into account the excitation energy and structural dependence in coherent and incoherent exciton diffusion processes, as well as other possible deactivation processes including the formation of the pseudo-charge-transfer and charge separate states, as well as interchain exciton hopping or coherent diffusion

Edge-on Gating Effect in Molecular Wires

This work demonstrates edge-on chemical gating effect in molecular wires utilizing the pyridinoparacyclophane (PC) moiety as the gate. Different substituents with varied electronic demands are attached to the gate to simulate the effect of varying gating voltages similar to that in field-effect transistor (FET). It was observed that the orbital energy level and charge carrier’s tunneling barriers can be tuned by changing the gating group from strong electron acceptors to strong electron donors. The single molecule conductance and current–voltage characteristics of this molecular system are truly similar to those expected for an actual single molecular transistor

Nanoporous Porphyrin Polymers for Gas Storage and Separation

This article describes the synthesis of four porous polymers containing Ni–porphyrin units with Brunauer–Emmet–Teller (BET) specific surface areas up to 1711 m²/g achieved. The isotherm gas adsorptions of hydrogen, methane and carbon dioxide over these polymers were measured. The adsorption selectivity for methane and carbon dioxide over nitrogen were also investigated. While the initial isosteric heat of adsorption (Δ<i><i>H</i></i>_<i>ads</i>) was around 8–9 kJ/mol for hydrogen, it reached 23 kJ/mol for methane and 29 kJ/mol for carbon dioxide. CO₂/N₂ selectivity as high as 19 (calculated from single gas adsorption isotherms) was also achieved with one of these four polymers