Seokhoon Ahn

Using self-organization to control morphology in molecular photovoltaics

This work explores the formation of well-defined molecular p-n junctions in solution-processed self-assembled heterojunction solar cells using dodecyloxy-substituted contorted hexabenzocoronene (12-c-HBC) as a donor material and phenyl-C(70)-butyric acid methyl ester (PC(70)BM) as an acceptor. We find that the contorted 12-c-HBC molecules effectively assemble in solution to form a nested structure with the ball-shaped PC(70)BM. The result is a self-assembled molecular-scale p-n junction. When this well-defined p-n junction is embedded in active films, we can make efficient self-assembled solar cells with minimal amounts of donor material relative to the acceptor. The power conversion efficiency is drastically enhanced by the mode of donor and acceptor assembly within the film.

Six different assemblies from one building block: two-dimensional crystallization of an amide amphiphile.

A C(s)-symmetric amide amphiphile containing a C(18) alkyl chain exists in at least six crystalline phases at the liquid/solid interface; several of these phases display regularly arranged nanoscopic voids. Structural analysis of each phase reveals that highly symmetric and/or complex patterns arise through adopting various aggregates via noncovalent interactions, several of which are mediated by the unique hydrogen-bonding properties of the primary amide. The formation of each phase is interpreted in the context of the kinetic and thermodynamic behaviors, with some phases showing concentration-dependent stabilities, while others are purely kinetic in origin. This investigation contributes to understanding the factors that give rise to solid form diversity in two- and three-dimensional crystallization.

Importance of Direct Metal−π Coupling in Electronic Transport Through Conjugated Single-Molecule Junctions

We study the effects of molecular structure on the electronic transport and mechanical stability of single-molecule junctions formed with Au point contacts. Two types of linear conjugated molecular wires are compared: those functionalized with methylsulfide or amine aurophilic groups at (1) both or (2) only one of its phenyl termini. Using scanning tunneling and atomic force microscope break-junction techniques, the conductance of mono- and difunctionalized molecular wires and its dependence on junction elongation and rupture forces were studied. Charge transport through monofunctionalized wires is observed when the molecular bridge is coupled through a S-Au donor-acceptor bond on one end and a relatively weak Au-π interaction on the other end. For monofunctionalized molecular wires, junctions can be mechanically stabilized by installing a second aurophilic group at the meta position that, however, does not in itself contribute to a new conduction pathway. These results reveal the important interplay between electronic coupling through metal-π interactions and quantum mechanical effects introduced by chemical substitution on the conjugated system. This study affords a strategy to deterministically tune the electrical and mechanical properties through molecular wires.

Highly symmetric 2D rhombic nanoporous networks arising from low symmetry amphiphiles.

Highly symmetric 2D nanoporous molecular networks containing rhombic voids are demonstrated to be accessible from low symmetry amphiphilic molecules. The amide amphiphiles overcome the barrier to symmetry generation in the two-dimensional crystal through forming an aggregate as a building block. This aggregate consists of three inequivalent amphiphiles that assemble to create 3- and 6-fold rotation axes through hydrogen bonding. In the 6-fold rotation axis, an unusual hydrogen bonding network, supported by high resolution scanning tunneling microscopy (STM) images and computation, is observed. This network formed by amide groups significantly contributes to constructing the rhombic nanoporous network, whereas carboxylic acid amphiphiles do not adopt this nanoporous network due to a geometric difference of hydrogen bonding. This investigation demonstrates that a high symmetry pattern is achievable without correlation with molecular symmetry through the proper combination of noncovalent interactions of simple amphiphilic molecules.

Additive Perturbed Molecular Assembly in Two-Dimensional Crystals: Differentiating Kinetic and Thermodynamic Pathways

During attempts to produce novel two-dimensional cocrystals by coadsorbing components in a binary mixture, the formation of a metastable form was observed in analogy to the phenomenon of additive-induced polymorph formation reported in three-dimensional crystallization. Mechanistic insights into this phenomenon were gained through the use of scanning tunneling microscopy and several adsorbate/additive combinations. One additive plays a critical role in forming a disordered assembly through a process that is primarily kinetic whereas another additive thermodynamically stabilized an intermediate form, resulting in interrupting a phase transformation to a more stable form. These additive effects elucidate one of the potential pathways to kinetically isolate a metastable polymorph formed during cocrystallization in three-dimensional crystallization.

Quantum Soldering of Individual Quantum Dots

Making contact to a quantum dot: Single quantum-dot electronic circuits are fabricated by wiring atomically precise metal chalcogenide clusters with conjugated molecular connectors. These wired clusters can couple electronically to nanoscale electrodes and be tuned to control the charge-transfer characteristics (see picture).

Two-dimensional crystals from reduced symmetry analogues of trimesic acid

The two-dimensional assembly of multicarboxylated arenes is explored at the liquid-graphite interface using scanning tunneling microscopy. Symmetry variations were introduced via phenylene spacer addition and the influence of these perturbations on the formation of hydrogen-bonded motifs from an alkanoic acid solvent is observed. This work demonstrates the importance of symmetry in 2D crystal formation and draws possible links of this behavior to prediction of coordination modes in three-dimensional coordination polymers.

All-solid-state flexible asymmetric micro supercapacitors based on cobalt hydroxide and reduced graphene oxide electrodes

In-plane micro supercapacitors (micro-SC) have attracted interest due to their high areal and volumetric capacitance that is dependent upon their electrode structure. This study proposes simply-fabricated micro-SCs based on cobalt hydroxide and electrochemically-reduced graphene oxide (erGO). The Au electron collector was prepared via photolithography, and Co(OH)2 and erGO were deposited on the Au surface via electrodeposition. Using facile two-step fabrication method and cost-effective materials, the prepared micro-SCs exhibit good electrochemical performance in PVA–KOH–KI solid electrolyte. The in-plane interdigitated electrode maximizes the areal capacitance by increasing the facing area between the anode and cathode. The micro-SC presents good power density (100.38 μW cm−2) and a wide potential window due to the electric double-layer properties of erGO and pseudocapacitive performance of Co(OH)2. These fabricated micro-SCs are flexible and can be used in various wearable and small-scale energy storage devices.

Ligand chemistry of titania precursor affects transient photovoltaic behavior in inverted organic solar cells

The chemistry of the precursor from which charge transport layers are formed can significantly affect the device performance of organic solar cells. Here, we compare two common precursors that are used to generate titania electron transport layers and elucidate their effects on the transient characteristics of inverted bulk-heterojunction polymer solar cells comprising poly(3-hexyl hiophene) and [6,6]-phenyl-C61-butyric acid methyl ester. Substituting the isopropyl ligands of titanium isopropoxide with 2-methoxyethanol leads to electron transport layers that require a shorter illumination time to fill shallow electron traps. Furthermore, organic solar cells with titania electron transport layers prepared with such pre-modified titania precursor exhibit higher power-conversion efficiencies stemming from lower trap densities.

Electronic transport and mechanical stability of carboxyl linked single-molecule junctions

We characterize electron transport across Au-molecule-Au junctions of heterogeneous carboxyl and methyl sulfide terminated saturated and conjugated molecules. Low-bias conductance measurements are performed using the scanning tunneling microscopy based break-junction technique in the presence of solvents and at room temperature. For a series of alkanes with 1-4 carbon atoms in the hydrocarbon chain, our results show an exponential decrease in conductance with increasing molecule length characterized by a decay constant of 0.9 ± 0.1 per methylene group. Control measurements in pH 11 solutions and with COOMe terminations suggest that the carboxylic acid group binds through the formation of a COO(-)-Au bond. Simultaneous measurements of conductance and force across these junctions yield a rupture force of 0.6 ± 0.1 nN, comparable to that required to rupture a Au-SMe bond. By establishing reliable, in situ junction formation, these experiments provide a new approach to probe electronic properties of carboxyl groups at the single molecule level.