Iseul Lim

High‐Performance Air‐Stable Single‐Crystal Organic Nanowires Based on a New Indolocarbazole Derivative for Field‐Effect Transistors

A new indolocabazole derivative possessing an extended aromatic core and solubilizing long aliphatic chains effectively self-assembles and crystallizes within the nanoscale channels to form single-crystal nanowires via a direct printing method from an ink solution. Single-crystal organic nanowire transistor arrays based on the π-extended indolocarbazole derivative exhibit an excellent hole mobility of 1.5 cm² V⁻¹ s⁻¹ and outstanding environmental stability.

Facile interfacial charge transfer across hole doped cobalt-based MOFs/TiO2 nano-hybrids making MOFs light harvesting active layers in solar cells

Efficient separation of charges and their mobility are key challenges in metal–organic-framework (MOF) based devices. In the present study, thin films of cobalt-based metal organic frameworks (MOFs) are synthesized using a layer-by-layer technique, and their electrical/optoelectronic properties are studied. The as-prepared MOF films show electrically insulating behavior, which after hole doping demonstrate p-type conduction behaviour. The measured HOMO–LUMO energy states of the MOF films are found to be well matched for sensitizing TiO2, and the photoluminescence quenching experiment demonstrates a facile photoelectron transfer path from the doped frameworks to TiO2. Consequently, the doped MOFs are employed successfully as light harvesting and charge transporting active layers in a fully devised TiO2-based solar cell. Two different organic ligands viz., benzene dicarboxylic acid and naphthalenedicarboxylic acid are used to synthesize two kinds of Co–MOFs having different geometrical dimensions of unit cells and pores, and their influence on hole doping and charge transportation is studied. Under optimized conditions, the Co–MOF based device demonstrates a solar-to-electric energy conversion efficiency of 1.12% with a short circuit current of 2.56 mA cm−2, showing promising future prospects of the application of Co–MOFs in photovoltaic devices. Further, the photovoltaic performance of the Co–MOF based device is comparatively studied with that of the previously reported Cu–MOF and Ru–MOF based similar devices, and the influence of different metal centers of MOFs on their light harvesting performance is discussed.

Indolocarbazole based small molecules: an efficient hole transporting material for perovskite solar cells

To date, Spiro-OMeTAD, which is an expensive organic compound, has been used as the benchmark hole transporting material (HTM) in perovskite based solid state solar cells. Development of an inexpensive HTM with competitive performance to Spiro-OMeTAD is therefore significantly important for the commercialization of perovskite cells. Herein, an indolocarbazole based small molecule derivative (C12-carbazole) has been introduced as an environmentally stable, cost effective and highly efficient HTM. In contrast to the power conversion efficiency of 9.62% exhibited by the Spiro-OMeTAD based solid state solar cell, the C12-carbazole based device under the same experimental conditions has demonstrated an enhanced power conversion efficiency of 11.26%. The improved photovoltaic performance of the C12-carbazole based device is attributed to reduced carrier recombination by a better hole extraction ability of the C12-carbazole, which has demonstrated remarkably higher hole mobility compared to Spiro-OMeTAD.

An ion exchange mediated shape-preserving strategy for constructing 1-D arrays of porous CoS1.0365 nanorods for electrocatalytic reduction of triiodide

Based on a coordination chemistry approach, the present work reports on the synthesis of thin films of various cobalt hydroxycarbonate nanostructures such as nanobeams, nanoneedles, and bending nanorods using three different cobalt precursors viz. Cl−, NO3− and CH3COO−. After pyrolysis in air, the hydroxycarbonate nanostructures are transferred into 1-D arrays of Co3O4 nanorods. The obtained 1-D Co3O4 nanostructures are then transformed into the corresponding analogous shaped 1-D arrays of porous cobalt sulfide (CoS1.0365) nanostructures using a wet chemical transformation method based on an ion exchange approach. The nanostructured films before and after the ion exchange reaction are characterized using field emission electron scanning microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy (TEM), and inductively coupled plasma mass spectroscopy (ICP-MS) measurements. As a proof-of-concept demonstration for the application, various shaped CoS1.0365 nanorod films synthesized are investigated as a Pt-free counter electrode in dye-sensitized-solar cells (DSSCs). The influence of three different counter anions of the cobalt precursors on the structural, textural, and morphological aspects, and thereby their influence on electronic and electrochemical properties, has been investigated. A correlation among electrical conductivity, charge transfer resistance and electrocatalytic performance of various CoS1.0365 nanorod films obtained from different cobalt precursors has been established. Among the various nanostructures, the thicker nanorod film synthesized using a chloride precursor has demonstrated the best electrocatalytic behavior toward triiodide reduction, which led to a short circuit current density of 18.04 mA cm−2 and energy conversion efficiency of 7.4% of the DSSC. This photovoltaic performance is highly competitive to a current density of 18.26 mA cm−2 and energy conversion efficiency of 7.7% exhibited by the standard Pt counter electrode.

Anodically fabricated self-organized nanoporous tin oxide film as a supercapacitor electrode material

Self-organized nanoporous tin oxide films were fabricated by anodizing a tin substrate in an aqueous electrolyte containing oxalic or phosphoric acid. The films were characterized using FE-SEM, XRD, XPS, and TGA. In addition, the supercapacitive properties of the porous oxide films were measured using cyclic voltammetry and galvanostatic charge/discharge technique. The film demonstrated a maximum specific capacitance of 274 F g−1 with long life in electrochemical charge/discharge cycles.

Interfacially treated dye-sensitized solar cell with in situ photopolymerized iodine doped polythiophene.

A thin film of iodine doped polythiophene was grown photoelectrochemically around the dye-sensitized TiO(2) nanoparticles in a Grätzel cell, and the effect of iodine doping level on the cell performance was investigated using X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and photovoltage decay. At an optimum doping level, the cell demonstrated the enhanced energy conversion efficiency by 27.52% compared to the cell without polythiophene.

Interfacial Engineering for Enhanced Light Absorption and Charge Transfer of a Solution-Processed Bulk Heterojunction Based on Heptazole as a Small Molecule Type of Donor.

In the present study, a solution-processed organic semiconductor based on indolocarbazole derivative (heptazole) is introduced as a p-type donor material for a bulk-heterojunction photovoltaic device. The heptazole has an optical band gap of 2.97 eV, and its highest occupied molecular orbital-lowest unoccupied molecular orbital energy levels are compactable with the PC60BM to construct a donor-acceptor heterojuction for energy harvesting and transfer. When the bulk-heterojunction photovoltaic devices consisting of ITO/PEDOT:PSS/heptazole:PC60BM/Al with different blending ratio of heptazole:PC60BM were constructed, the cell with 1:1 blending ratio exhibited the best power conversion efficiency. Further, when an indoline organic dye (D149) was introduced as an interfacial modifier to the above donor/acceptor bulk heterojunction, the device demonstrated an enhanced overall power conversion efficiency from 1.26% to 2.51% hence demonstrating enhancement by the factor of 100%. The device was further characterized using electronic absorption spectroscopy, photoluminescence spectroscopy, electrochemical impedance spectroscopy, and the photovoltage decay kinetics. These studies reveal that the enhanced power conversion efficiency of the device is due to the enhanced charge transfer with the complementary light absorption feature of the interfacial D149 dye molecules.

Photoelectrochemical cells by design: 3D nanoporous CdO–CdSe architectures on ITO

We report on the synthesis of CdO–CdSe nanoporous architectures from porous CdO architectures by a controlled solution anion exchange method. Monolithic CdCO3 micron sized crystals synthesized in the first step were decomposed by calcination to evolve CO2 to form porous CdO crystals, having a similar outer shape but with a tailored internal nanostructure. Controlled Se2− ion exchange with CdO crystals yielded CdO–CdSe architectures with tunable composition and optoelectronic properties. Pearsons acid–base concept is used as a guiding principle for controlling the morphology and composition of the formed architectures. The photoelectrochemical properties of the system comprising CdO–CdSe architectures were investigated at various stages of ion exchange intervals. CdO–CdSe architectures on indium tin oxide (ITO) is a promising photoelectrode with excellent photovoltaic properties exhibiting a short-circuit current density of 7.36 mA cm−2 under 1 Sun illumination.

Interfacial Engineering of CdO–CdSe 3D Microarchitectures with in situ Photopolymerized PEDOT for an Enhanced Photovoltaic Performance

In the present work, porous 3D CdO‐microstructured electrode obtained by pyrolysis of 3D CdCO3 microstructures is self‐sensitized with CdSe using an ion exchange reaction. After sensitization, an interfacial treatment of the CdO–CdSe interface is performed by depositing a thin film of PEDOT using a photoinduce polymerization route. The microstructured electrode before and after interfacial treatment is characterized using field‐emission scanning microscope, energy dispersive X‐ray analyzer, contact angle measurement, UV–Visible absorption spectrophotometer and X‐ray photoelectron spectrometer. After constructing a liquid junction solar cell with a Pt counter electrode, the photovoltaic performance and interfacial charge transfer kinetics across the CdO–CdSe interface before and after PEDOT treatment are investigated. The results exhibit an improved interfacial charge‐transfer resistance after the PEDOT treatment, which leads to enhance the short‐circuit current by 15.81% and the power conversion efficiency by 19.82%.

A facile approach for carburization of anodically grown titania nanotubes: towards metallization of nanotubes

The present work demonstrates a facile, low cost and environmentally friendly technique for carburization of TiO2 nanotubes. XRD and XPS investigations suggest that the anodically grown self-organized TiO2 nanotubes when annealed in an argon filled steel nut–bolt cavity working as an autogenic pressure reactor undergo carburization at 650 °C, which converts TiO2 into TiOyCz. TEM-SAED suggests that the carburized nanotubes are polycrystalline, and also contain some reduced oxides of titanium. The conductivity measurement of the carburized nanotubes shows their conductivity to be close to metals. The electrochemical investigation of the carburized nanotubes demonstrates that the material can be used as a conductive electrode material for electrochemical reactions.