Understanding the Effects of a High Surface Area Nanostructured Indium Tin Oxide Electrode on Organic Solar Cell Performance.

Organic solar cells (OSCs) are a complex assembly of disparate materials, each with a precise function within the device. Typically, the electrodes are flat, and the device is fabricated through a layering approach of the interfacial layers and photoactive materials. This work explores the integration of high surface area transparent electrodes to investigate the possible role(s) a three-dimensional electrode could take within an OSC, with a BHJ composed of a donor-acceptor combination with a high degree of electron and hole mobility mismatch. Nanotree indium tin oxide (ITO) electrodes were prepared via glancing angle deposition, structures that were previously demonstrated to be single-crystalline. A thin layer of zinc oxide was deposited on the ITO nanotrees via atomic layer deposition, followed by a self-assembled monolayer of C60-based molecules that was bound to the zinc oxide surface through a carboxylic acid group. Infiltration of these functionalized ITO nanotrees with the photoactive layer, the bulk heterojunction comprising PC71BM and a high hole mobility low band gap polymer (PDPPTT-T-TT), led to families of devices that were analyzed for the effect of nanotree height. When the height was varied from 0 to 50, 75, 100, and 120 nm, statistically significant differences in device performance were noted with the maximum device efficiencies observed with a nanotree height of 75 nm. From analysis of these results, it was found that the intrinsic mobility mismatch between the donor and acceptor phases could be compensated for when the electron collection length was reduced relative to the hole collection length, resulting in more balanced charge extraction and reduced recombination, leading to improved efficiencies. However, as the ITO nanotrees increased in height and branching, the decrease in electron collection length was offset by an increase in hole collection length and potential deleterious electric field redistribution effects, resulting in decreased efficiency.

Phosphorus Pentachloride Initiated Functionalization of Silicon Nanocrystals.

Phosphorus pentachloride (PCl5) has long been used to chlorinate hydrocarbons. It has also been applied in silicon surface chemistry to facilitate alkylation via a two-step halogenation/Grignard route. Here we report a study of the reaction of PCl5 with hydride-terminated silicon nanocrystals (H-SiNCs). An examination of the reaction mechanism has allowed us to establish a functionalization protocol that uses PCl5 as a surface radical initiator to introduce alkyl and alkenyl moieties to the surface of H-SiNCs. The reaction proceeds quickly in a single step, at room temperature and the functionalized silicon nanocrystals retained their morphology and crystallinity. The resulting materials exhibited size-dependent photoluminescence that was approximately 3× as bright as that observed for thermally hydrosilylated SiNCs. Furthermore, the absolute PL quantum yield (AQY) was more than double. The high AQY is expected to enable SiNCs to compete with chalcogenide-based quantum dots in various applications.

Instantaneous Functionalization of Chemically Etched Silicon Nanocrystal Surfaces.

Remarkable advances in surface hydrosilylation reactions of C=C and C=O bonds on hydride-terminated silicon have revolutionized silicon surface functionalization. However, existing methods for functionalizing hydride-terminated Si nanocrystals (H-SiNCs) require long reaction times and elevated temperatures. Herein, we report a room-temperature method for functionalizing H-SiNC surfaces within seconds by stripping outermost atoms on H-SiNC surfaces with xenon difluoride (XeF2 ). Detailed analysis of the reaction byproducts by in situ NMR spectroscopy and GC-MS provided unprecedented insight into NC surface composition and reactivity as well as the complex reaction mechanism of XeF2 activated hydrosilylation.

Alkoxy-Terminated Si Surfaces: A New Reactive Platform for the Functionalization and Derivatization of Silicon Quantum Dots.

Alkoxy-terminated silicon quantum dots (SiQDs) were synthesized via hydrosilylation of aliphatic ketones on hydride-terminated SiQD (H-SiQD) surfaces under microwave-irradiation. Aromatic ketones undergo hydrosilylation on H-SiQD surfaces at room temperature without requiring any catalyst. The alkoxy-terminated SiQDs are soluble in organic solvents, colloidally stable, and show bright and size dependent photoluminescence (PL). The alkoxy-functionalized silicon surfaces were used as reactive platform for further functionalization via unprecedented ligand exchange of the alkoxy-surface groups with alkyl or alkenyl-surface groups in the presence of BH3·THF. Proton nuclear magnetic resonance ((1)H NMR), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) spectroscopy confirmed alkoxy-terminated surfaces and their ligand exchange reactions in the presence of various alkenes and alkynes.

Grafting Poly(3-hexylthiophene) from Silicon Nanocrystal Surfaces: Synthesis and Properties of a Functional Hybrid Material with Direct Interfacial Contact.

Hybrid functional materials (HFMs) comprised of semiconductor nanoparticles and conjugated polymers offer the potential of synergetic photophysical properties. We have developed HFMs based upon silicon nanocrystals (SiNCs) and the conductive polymer poly(3-hexylthiophene) (SiNC@P3HT) by applying surface-initiated Kumada catalyst transfer polycondensation (SI-KCTP). One unique characteristic of the developed SiNC@P3HT is the formation of a direct covalent bonding between SiNCs and P3HT. The presented method for obtaining direct interfacial attachment, which is not accessible using other methods, may allow for the development of materials with efficient electronic communication at the donor-acceptor interfaces. Systematic characterization provides evidence of a core-shell structure, enhanced interfacial electron and/or energy transfer between the P3HT and SiNC components, as well as formation of a type-II heterostructure.

Solution-processed zinc phosphide (α-Zn3P2) colloidal semiconducting nanocrystals for thin film photovoltaic applications.

Zinc phosphide (Zn3P2) is a promising earth-abundant material for thin film photovoltaic applications, due to strong optical absorption and near ideal band gap. In this work, crystalline zinc phosphide nanoparticles are synthesized using dimethylzinc and tri-n-octylphosphine as precursors. Transmission electron microscopy and X-ray diffraction data show that these nanoparticles have an average diameter of ∼8 nm and adopt the crystalline structure of tetragonal α-Zn3P2. The optical band gap is found to increase by 0.5 eV relative to bulk Zn3P2, while there is an asymmetric shift in the conduction and valence band levels. Utilizing layer-by-layer deposition of Zn3P2 nanoparticle films, heterojunction devices consisting of ITO/ZnO/Zn3P2/MoO3/Ag are fabricated and tested for photovoltaic performance. The devices are found to exhibit excellent rectification behavior (rectification ratio of 600) and strong photosensitivity (on/off ratio of ∼10(2)). X-ray photoelectron spectroscopy and ultraviolet photoemission spectroscopy analyses reveal the presence of a thin 1.5 nm phosphorus shell passivating the surface of the Zn3P2 nanoparticles. This shell is believed to form during the nanoparticle synthesis.

Mixed bis(µ-silylene) and (µ-silylene)/(µ-germylene) complexes involving the Rh/Ir metal combination: nature of the Si···Si interactions in the bis(µ-silylene) species.

A series of mixed bis(µ-silylene) complexes of rhodium and iridium [RhIr(CO)(2)(µ-SiHR)(µ-SiR(1)R(2))(dppm)(2)] (R = R(1) = R(2) = Ph (4); R = R(1) = Ph, R(2) = Cl (5); R = R(1) = Ph, R(2) = Me (6); R = 3,5-C(6)H(3)F(2), R(1) = Ph, R(2) = Me (7); R = 3,5-C(6)H(3)F(2), R(1) = 2,4,6-C(6)H(2)Me(3), R(2) = H (8)) have been synthesized by the reaction of the silylene-bridged dihydride complexes, [RhIr(H)(2)(CO)(2)(µ-SiHR)(dppm)(2)] (1, R = Ph; 2, R = C(6)H(3)F(2)), with a number of secondary or primary silanes (Ph(2)SiH(2), PhClSiH(2), PhMeSiH(2), C(6)H(2)Me(3)SiH(3)). The influence of substituents and π-stacking interactions on the Si···Si distance (determined by X-ray crystallography) in this series and the implications regarding the nature of the Si···Si interactions are discussed. A series of novel (µ-silylene)/(µ-germylene) complexes, [RhIr(CO)(2)(µ-SiHPh)(µ-GePh(2))(dppm)(2)] (9) and [RhIr(CO)(2)(µ-SiR(1)R(2))(µ-GeHPh)(dppm)(2)] (R(1) = Ph, R(2) = H (11); R(1) = R(2) = Ph (12); R(1) = Ph, R(2) = Me (13)), have also been synthesized by reaction of the silylene-bridged dihydride complex, [RhIr(H)(2)(CO)(2)(µ-SiHPh)(dppm)(2)] (1), with 1 equiv of diphenylgermane and by reaction of the germylene-bridged dihydride complex, [RhIr(H)(2)(CO)(2)(µ-GeHPh)(dppm)(2)] (3), with 1 equiv of the respective silanes. These complexes have been characterized by multinuclear NMR spectroscopy and X-ray crystallography.

Germyl- and germylene-bridged complexes of Rh/Ir and subsequent chemistry of a bridging germylene group.

A series of neutral and cationic germylene-bridged complexes and a neutral germyl(germylene) complex have been synthesized and characterized by NMR spectroscopy and X-ray crystallography. Reaction of 1 equiv of primary germanes, RGeH(3) (R = Ph, (t)Bu), with [RhIr(CO)(3)(dppm)(2)] (1) at low-temperature yields [RhIr(GeH(2)R)(H)(CO)(3)(dppm)(2)] (R = Ph (3) or (t)Bu (4)), the products of single Ge-H bond activation, which upon warming transform to the germylene-bridged dihydrides, [RhIr(H)(2)(CO)(2)(µ-GeHR)(dppm)(2)] (R = Ph (5) or (t)Bu (6)) by activation of a second Ge-H bond accompanied by CO loss. Both classes of compounds have the diphosphines folded back in a "cradle-shaped" geometry. Although compound 5 reacts with additional phenylgermane at -40 °C to give a germylene-bridged/germyl product, [RhIr(GeH(2)Ph)(H)(2)(CO)(2)(κ(1)-dppm)(µ-GeHPh)(µ-H)(dppm)] (7), warming results in decomposition. However, reaction of 5 with 1 equiv of diphenylgermane at ambient temperature results in a novel mixed bis(µ-germylene) complex, [RhIr(CO)(2)(µ-GeHPh)(µ-GePh(2))(dppm)(2)] (8), containing both mono- and disubstituted germylene fragments. Reaction of 1 equiv of diphenylgermane with complex 1 produces a similar monogermylene-bridged product, [RhIr(H)(2)(CO)(2)(µ-GePh(2))(dppm)(2)] (9), while reaction of 1 with 2 equiv of diphenylgermane yields the germyl/germylene product [RhIr(H)(GeHPh(2))(CO)(3)(κ(1)-dppm)(µ-GePh(2))(dppm)] (10). The above reactions, incorporating first one and then a second equivalent of primary and secondary germanes, were studied by low-temperature multinuclear NMR spectroscopy, revealing details about the stepwise activations of multiple Ge-H bonds. Reaction of diphenylgermane with the cationic complex [RhIr(CH(3))(CO)(2)(dppm)(2)][CF(3)SO(3)] (2) leads to a cationic A-frame-type germylene- and hydride-bridged product, [RhIr(CO)(2)(µ-H)(µ-GePh(2))(dppm)(2)][CF(3)SO(3)] (3), which reversibly activates H(2), yielding a germyl-bridged dihydride and reacts stoichiometrically with water, methanol, and HCl to yield the respective germanol, germamethoxy, and germylchloride products.

Multiple silicon-hydrogen bond activations at adjacent rhodium and iridium centers.

The reaction of 1 equiv of primary silanes, SiH(3)R (R = Ph, Mes), with [RhIr(CO)(3)(dppm)(2)] yields mono(silylene)-bridged complexes of the type [RhIr(H)(2)(CO)(2)(µ-SiHR)(dppm)(2)] (R = Ph or Mes), while for R = Ph the addition of 2 equiv yields the bis(silylene)-bridged complexes, [RhIr(CO)(2)(µ-SiHPh)(2)(dppm)(2)]. The kinetic isomer of this bis(silylene)-bridged product has the phenyl substituent axial on one silylene unit and equatorial on the other, and in the presence of excess silane this rearranges to the thermodynamically preferred "axial-axial" isomer, in which the phenyl substituents on each bridging silylene unit are axial and parallel to one another. The reaction of 1 equiv of diphenylsilane with [RhIr(CO)(3)(dppm)(2)] produces the mono(silylene)-bridged product, [RhIr(H)(2)(CO)(2)(µ-SiPh(2))(dppm)(2)], and the subsequent addition of silane in the presence of CO yields the silyl/silylene product [RhIr(H)(SiPh(2)H)(CO)(3)(κ(1)-dppm)(µ-SiPh(2))(dppm)]. The reaction of [RhIr(CO)(3)(dppm)(2)] with 2 equiv of SiH(2)Me(2) yields the analogous product [RhIr(H)(SiMe(2)H)(CO)(3)(κ(1)-dppm)(µ-SiMe(2))(dppm)]. Low-temperature NMR spectroscopic observation of some key intermediates, such as [RhIr(H)(SiH(2)Ph)(CO)(2)(µ-CO)(dppm)(2)], formed during the formation of the mono(silylene)-bridged species provides evidence for a mechanism involving initial Si-H bond activation at Rh, followed by the subsequent Si-H bond activation at Ir. The Si-H bond activation of a second equivalent of silane seems to be initiated by dissociation of the Rh-bound end of one diphosphine. The reaction of diphenylsilane with the cationic complex [RhIr(CH(3))(CO)(2)(dppm)(2)][CF(3)SO(3)] gives rise to a different reactivity pattern in which Si-H bond activation is initiated at Ir. In this case, the cationic silyl-bridged species, [RhIr(CH(3))(CO)(2)(κ(1):η(2)-SiHPh(2))(dppm)(2)][CF(3)SO(3)], contains an agostic Si-H interaction with Rh. In solution, at ambient temperature, this complex converts to two species, [RhIr(H)(COCH(3))(CO)(µ-H)(µ-SiPh(2))(dppm)(2)][CF(3)SO(3)] and [RhIr(CO)(2)(µ-H)(µ-SiPh(2))(dppm)(2)] [CF(3)SO(3)], formed by the competing methyl migration to CO and reductive elimination of methane, respectively. In the diphenylsilylene dihydride product, a weak interaction between the bridging silicon and the terminal Ir-bound hydride is proposed on the basis of NMR evidence.