Jaehan Jung

Low‐Cost Copper Zinc Tin Sulfide Counter Electrodes for High‐Efficiency Dye‐Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) are among the most promising photovoltaic devices for low-cost light-to-energy conversion with relatively high efficiency. A typical DSSC consists of three key components: a dye-sensitized semiconductor photoanode, an electrolyte with a redox couple (triiodide/iodide), and a counter electrode (CE). Upon photoexcitation, electrons generated from photoexcited dyes are injected into the conduction band of photoanode composed of TiO2 and the dyes are regenerated by redox reaction with the electrolyte. Oxidized ions (triiodide) in the electrolyte then diffuse to the CE and are finally reduced to iodide at the surface of the CE. An ideal CE should possess high electrocatalytic activity for the reduction of charge carriers in electrolyte as well as high conductivity. To date, the most commonly used CE is fluorine-doped tin oxide (FTO) glass coated with a thin layer of platinum. However, as a noble metal, the low abundance (0.0037 ppm) and high cost (US

1D nanocrystals with precisely controlled dimensions, compositions, and architectures

50/gram) prevent platinum from being used for largescale manufacturing. In this context, considerable efforts have been made to replace Pt with abundant low-cost alternatives, including carbon-based materials (for example, carbon nanotubes, carbon black, and graphite), conjugated polymers, and inorganic materials as CEs. In comparison to carbon materials and polymers, inorganic compounds carry many advantageous characteristics, such as simple preparation and a diversity of materials that can be used. In recent years, a variety of binary metal oxides, metal sulfides, metal nitrides, and metal carbides have been developed as CEs. To the best of our knowledge, the use of abundant ternary or quaternary materials as potential substitutes for Pt as low-cost CEs has not yet been explored. A quaternary chalcogenide semiconductor, copper zinc tin sulfide (hereafter referred to as CZTS), is most widely known as one of the most promising photovoltaic (PV) materials, and it is widely used in thin-film solar cells. Notably, CZTS is composed of naturally abundant elements in the Earth s crust and has very low toxicity: it is environmentally friendly compared to two high-efficiency thin-film solar cells with CdTe and Cu(In1 x,Gax)S2 (CIGS) that have toxic elements (Cd) and rare metals (indium and gallium). Recently, high-efficiency thin-film solar cells have been demonstrated based on the superior PV performance of CZTS as a p-type semiconductor owing to its direct band gap of 1.5 eV and a large absorption coefficient (> 10 cm ). However, no studies have centered on the electrocatalytic activity of CZTS for use in DSSCs. Herein, we present, for the first time, that CZTS can be exploited as an effective CE material to replace expensive Pt, yielding a low-cost, highefficiency DSSCs. It is noteworthy that a power conversion efficiency (PCE) of 7.37% was achieved by a simple process of spin-coating CZTS followed by selenization. This efficiency was highly comparable to the DSSC prepared by utilizing Pt (PCE= 7.04%) as the CE under the same device configuration. We employed a solution-base synthesis approach to prepare CZTS nanocrystals. Specifically, copper, zinc, and tin precursors dissolved in oleylamine (OLA) were purified at 130 8C and heated to 225 8C in argon. Subsequently, a sulfur solution was rapidly injected and stirred at 225 8C for 1 h. The product was centrifuged to yield CZTS nanoparticles (see the Experimental Section). Figure 1a,b shows scanning transmission electron microscope (STEM) images of CZTS nanoparticles. The nanoparticle diameter was approximately (15 6) nm and the lattice constant was 0.31 nm, corresponding to the (112) plane, which was consistent with the XRD result (Supporting Information, Figure S1). It is worth noting that compared to conventional costly and low-throughput highvacuum sputtering and vapor deposition of CZTS, the ability to produce a CZTS nanocrystal dispersion (that is, a nanocrystal “ink”) that can be sprayed and coated on surface and then thermally annealed into larger-grain thin film would substantially lower the manufacturing cost and allow highthroughput solar-cell production. The CZTS ink was then either spin-coated or drop-cast onto the clean FTO glass and sintered at 540 8C for 1 h in selenium vapor. The morphologies of resulting CZTS films after sintering in Se vapor are shown in Figure 1c,d. The thickness of the CZTS layer was approximately 180 nm for the spin-coated sample and 2.3 mm for the drop-cast sample, respectively. Cracks were clearly evident on the drop-cast sample sintered in Se vapor, which are due to the stress induced during the solvent evaporation (Supporting Information, Figure S2b). The compositions of CZTS nanocrystals before and after treatment with Se vapor (selenization to yield CZTSSe) were [*] X. Xin, W. Han, J. Jung, Prof. Z. Lin School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332 (USA) E-mail: zhiqun.lin@mse.gatech.edu

Graphene-based transparent flexible electrodes for polymer solar cells

The ability to synthesize a diverse spectrum of one-dimensional (1D) nanocrystals presents an enticing prospect for exploring nanoscale size- and shape-dependent properties. Here we report a general strategy to craft a variety of plain nanorods, core-shell nanorods, and nanotubes with precisely controlled dimensions and compositions by capitalizing on functional bottlebrush-like block copolymers with well-defined structures and narrow molecular weight distributions as nanoreactors. These cylindrical unimolecular nanoreactors enable a high degree of control over the size, shape, architecture, surface chemistry, and properties of 1D nanocrystals. We demonstrate the synthesis of metallic, ferroelectric, upconversion, semiconducting, and thermoelectric 1D nanocrystals, among others, as well as combinations thereof.

An Unconventional Route to Monodisperse and Intimately Contacted Semiconducting Organic–Inorganic Nanocomposites

Conjugated polymer-based bulk heterojunction (BHJ) solar cells are widely recognized as a promising alternative to their inorganic counterparts for achieving low-cost, roll-to-roll production of large-area flexible lightweight photovoltaic devices. Current research in designing new polymers and optimizing device architectures has been devoted to improving the film morphology, photovoltaic performance and stability of polymer BHJ solar cells. Conjugated block copolymers (BCPs), including rod–coil and rod–rod BCPs, exhibit excellent flexibility for tuning the bandgap of semiconductor polymers, regulating the molecular organization of donor (and/or acceptor) units, templating the film morphology of active layers, and achieving well-defined BHJ architectures. In this Feature Article, we summarize the recent developments over the past five years in the synthesis, self-assembly, and utilization of conjugated rod–coil and all-conjugated rod–rod BCPs for solar energy conversion; highlight the correlation between the microphase-separated morphology and photovoltaic properties in conjugated BCPs; and finally provide an outlook on the future of BCP-based photovoltaic devices.

A general route to nanocrystal kebabs periodically assembled on stretched flexible polymer shish

We developed an unconventional route to produce uniform and intimately contacted semiconducting organic-inorganic nanocomposites for potential applications in thermoelectrics. By utilizing amphiphilic star-like PAA-b-PEDOT diblock copolymer as template, monodisperse PEDOT-functionalized lead telluride (PbTe) nanoparticles were crafted via the strong coordination interaction between PAA blocks of star-like PAA-b-PEDOT and the metal moieties of precursors (i.e., forming PEDOT-PbTe nanocomposites). As the inner PAA blocks are covalently connected to the outer PEDOT blocks, the PEDOT chains are intimately and permanently tethered on the PbTe nanoparticle surface, thereby affording a well-defined PEDOT/PbTe interface, which prevents the PbTe nanoparticles from aggregation, and more importantly promotes the long-term stability of PEDOT-PbTe nanocomposites. We envision that the template strategy is general and robust, and offers easy access to other conjugated polymer-inorganic semiconductor nanocomposites for use in a variety of applications.

Semiconducting Conjugated Polymer–Inorganic Tetrapod Nanocomposites

Organic-inorganic 1D periodic necklace-like nanostructures are fabricated using confined synthesis of inorganic nanocrystals. Assembling nanoparticles into one-dimensional (1D) nanostructures with precisely controlled size and shape renders the exploration of new properties and construction of 1D miniaturized devices possible. The physical properties of such nanostructures depend heavily on the size, chemical composition, and surface chemistry of nanoparticle constituents, as well as the close proximity of adjacent nanoparticles within the 1D nanostructure. Chemical synthesis provides an intriguing alternative means of creating 1D nanostructures composed of self-assembled nanoparticles in terms of material diversity, size controllability, shape regularity, and low-cost production. However, this is an area where progress has been slower. We report an unconventional yet general strategy to craft an exciting variety of 1D nanonecklace-like nanostructures comprising uniform functional nanodiscs periodically assembled along a stretched flexible polymer chain by capitalizing on judiciously designed amphiphilic worm-like diblock copolymers as nanoreactors. These nanostructures can be regarded as organic-inorganic shish-kebabs, in which nanodisc kebabs are periodically situated on a stretched polymer shish. Simulations based on self-consistent field theory reveal that the formation of organic-inorganic shish-kebabs is guided by the self-assembled elongated star-like diblock copolymer constituents constrained on the highly stretched polymer chain.

Hairy Uniform Permanently Ligated Hollow Nanoparticles with Precise Dimension Control and Tunable Optical Properties

Cadmium telluride (CdTe) tetrapods were synthesized via multiple injections of the Te precursor by utilizing bifunctional ligands. Subsequently, tetrapod-shaped semiconducting inorganic-organic nanocomposites (i.e., P3HT-CdTe tetrapod nanocomposites) were produced by directly grafting conjugated polymer ethynyl-terminated poly(3-hexylthiophene) (i.e., P3HT-≡) onto azide-functionalized CdTe tetrapods (i.e., CdTe-N3) via a catalyst-free click chemistry. The intimate contact between P3HT and CdTe tetrapod rendered the effective dispersion of CdTe tetrapods in nanocomposites and facilitated their efficient electronic interaction. The success of coupling reaction was confirmed by Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy. The grafting density of P3HT chains on the CdTe tetrapods was estimated by thermogravimetric analysis. The photophysical properties of P3HT-CdTe tetrapod nanocomposites were studied using UV-vis and photoluminescence spectroscopies. These intimate semiconducting conjugated polymer-tetrapod nanocomposites may offer a maximized interface between conjugated polymers and tetrapods for efficient charge separation and enhanced charge transport regardless of their orientation for potential application in hybrid solar cells with improved power conversion efficiency.

Crafting Core/Graded Shell-Shell Quantum Dots with Suppressed Re-absorption and Tunable Stokes Shift as High Optical Gain Materials.

The ability to tailor the size and shape of nanoparticles (NPs) enables the investigation into the correlation between these parameters and optical, optoelectronic, electrical, magnetic, and catalytic properties. Despite several effective approaches available to synthesize NPs with a hollow interior, it remains challenging to have a general strategy for creating a wide diversity of high-quality hollow NPs with different dimensions and compositions on demand. Herein, we report on a general and robust strategy to in situ crafting of monodisperse hairy hollow noble metal NPs by capitalizing on rationally designed amphiphilic star-like triblock copolymers as nanoreactors. The intermediate blocks of star-like triblock copolymers can associate with metal precursors via strong interaction (i.e., direct coordination or electrostatic interaction), followed by reduction to yield hollow noble metal NPs. Notably, the outer blocks of star-like triblock copolymers function as ligands that intimately and permanently passivate the surface of hollow noble metal NPs (i.e., forming hairy permanently ligated hollow NPs with superior solubility in nonpolar solvents). More importantly, the diameter of the hollow interior and the thickness of the shell of NPs can be readily controlled. As such, the dimension-dependent optical properties of hollow NPs are scrutinized by combining experimental studies and theoretical modeling. The dye encapsulation/release studies indicated that hollow NPs may be utilized as attractive guest molecule nanocarriers. As the diversity of precursors are amenable to this star-like triblock copolymer nanoreactor strategy, it can conceptually be extended to produce a rich variety of hairy hollow NPs with different dimensions and functionalities for applications in catalysis, water purification, optical devices, lightweight fillers, and energy conversion and storage.

Light-enabled reversible self-assembly and tunable optical properties of stable hairy nanoparticles

The key to utilizing quantum dots (QDs) as lasing media is to effectively reduce non-radiative processes, such as Auger recombination and surface trapping. A robust strategy to craft a set of CdSe/Cd(1-x)Zn(x)Se(1-y)S(y)/ZnS core/graded shell-shell QDs with suppressed re-absorption, reduced Auger recombination rate, and tunable Stokes shift is presented. In sharp contrast to conventional CdSe/ZnS QDs, which have a large energy level mismatch between CdSe and ZnS and thus show strong re-absorption and a constrained Stokes shift, the as-synthesized CdSe/Cd(1-x)Zn(x)Se(1-y)S(y)/ZnS QDs exhibited the suppressed re-absorption of CdSe core and tunable Stokes shift as a direct consequence of the delocalization of the electron wavefunction over the entire QD. Such Stokes shift-engineered QDs with suppressed re-absorption may represent an important class of building blocks for use in lasers, light emitting diodes, solar concentrators, and parity-time symmetry materials and devices.

Enhancement of optical gain characteristics of quantum dot films by optimization of organic ligands

Significance This work reports a versatile and robust strategy for creating monodisperse plasmonic nanoparticles (NPs) intimately and permanently capped with photoresponsive polymers via capitalizing on amphiphilic star-like diblock copolymer nanoreactors. The reversibly assembled nanostructures comprising photoresponsive NPs may exhibit a broad range of new attributes, functions, and applications as a direct consequence of size-dependent physical property from individual NP and the collective property originated from the NP interaction due to their close proximity within nanostructure. The ability to dynamically organize functional nanoparticles (NPs) via the use of environmental triggers (temperature, pH, light, or solvent polarity) opens up important perspectives for rapid and convenient construction of a rich variety of complex assemblies and materials with new structures and functionalities. Here, we report an unconventional strategy for crafting stable hairy NPs with light-enabled reversible and reliable self-assembly and tunable optical properties. Central to our strategy is to judiciously design amphiphilic star-like diblock copolymers comprising inner hydrophilic blocks and outer hydrophobic photoresponsive blocks as nanoreactors to direct the synthesis of monodisperse plasmonic NPs intimately and permanently capped with photoresponsive polymers. The size and shape of hairy NPs can be precisely tailored by modulating the length of inner hydrophilic block of star-like diblock copolymers. The perpetual anchoring of photoresponsive polymers on the NP surface renders the attractive feature of self-assembly and disassembly of NPs on demand using light of different wavelengths, as revealed by tunable surface plasmon resonance absorption of NPs and the reversible transformation of NPs between their dispersed and aggregated states. The dye encapsulation/release studies manifested that such photoresponsive NPs may be exploited as smart guest molecule nanocarriers. By extension, the star-like block copolymer strategy enables the crafting of a family of stable stimuli-responsive NPs (e.g., temperature- or pH-sensitive polymer-capped magnetic, ferroelectric, upconversion, or semiconducting NPs) and their assemblies for fundamental research in self-assembly and crystallization kinetics of NPs as well as potential applications in optics, optoelectronics, magnetic technologies, sensory materials and devices, catalysis, nanotechnology, and biotechnology.