Zhiqun Lin

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

High Efficiency Dye-Sensitized Solar Cells Based on Hierarchically Structured Nanotubes

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

p-n Heterojunction photoelectrodes composed of Cu2O-loaded TiO2 nanotube arrays with enhanced photoelectrochemical and photoelectrocatalytic activities

Dye-sensitized solar cells (DSSCs) based on hierarchically structured TiO(2) nanotubes prepared by a facile combination of two-step electrochemical anodization with a hydrothermal process exhibited remarkable performance. Vertically oriented, smooth TiO(2) nanotube arrays fabricated by a two-step anodic oxidation were subjected to hydrothermal treatment, thereby creating advantageous roughness on the TiO(2) nanotube surface (i.e., forming hierarchically structured nanotube arrays-nanoscopic tubes composed of a large number of nanoparticles on the surface) that led to an increased dye loading. Subsequently, these nanotubes were exploited to produce DSSCs in a backside illumination mode, yielding a significantly high power conversion efficiency, of 7.12%, which was further increased to 7.75% upon exposure to O(2) plasma.

Inorganic-modified semiconductor TiO2 nanotube arrays for photocatalysis

Cu2O/TiO2 p–n heterojunction photoelectrodes were prepared by depositing different amounts of p-type Cu2O nanoparticles on n-type TiO2 nanotube arrays (i.e., forming Cu2O/TiO2 composite nanotubes) via an ultrasonication-assisted sequential chemical bath deposition. The success of deposition of Cu2O nanoparticles was corroborated by structural and composition characterizations. The enhanced absorption in the visible light region was observed in Cu2O/TiO2 composite nanotubes. The largely improved separation of photogenerated electrons and holes was revealed by photocurrent measurements. Consequently, Cu2O/TiO2 heterojunction photoelectrodes exhibited a more effective photoconversion capability than TiO2 nanotubes alone in photoelectrochemical measurements. Furthermore, Cu2O/TiO2 composite photoelectrodes also possessed superior photoelectrocatalytic activity and stability in the degradation of Rhodamine B. Intriguingly, by selecting an appropriate bias potential, a synergistic effect between electricity and visible light irradiation can be achieved.

Learning from “Coffee Rings”: Ordered Structures Enabled by Controlled Evaporative Self‐Assembly

Semiconductor photocatalysis is a promising physicochemical process for the photodegradation of organic contaminants and bacterial detoxification. Among various oxide semiconductor photocatalysts, TiO2 has garnered considerable attention because of its outstanding properties including strong oxidizing activity, chemical and mechanical stability, corrosion resistance, and nontoxicity. This Review briefly introduces the key mechanisms of photocatalysis, highlights the recent developments pertaining to pure TiO2 nanotube arrays and TiO2 nanotube arrays modified by non-metals, metals and semiconductors, and their applications in the photocatalytic degradation of organic dyes. The improved photocatalytic efficiencies of modified TiO2 nanotube arrays are compared with unmodified counterparts. Current challenges and prospective areas of interest in this rich field are also presented.

Towards high-performance polymer-based thermoelectric materials

Research into the evaporation of solutions is not only aimed at a better understanding the physics of evaporation, but increasingly at capitalizing on the extremely simple method it offers to assemble diverse nonvolatile solutes into complex ordered structures on the submicron and longer length scales. This Review highlights recent advances in evaporative assembly of confined solutions, focusing especially on recently developed approaches that provide structures with unprecedented regularity composed of polymers, nanoparticles, and biomaterials, by controlled evaporation-driven, flow-aided self-assembly. A broad range of variables that can control the deposition are explored and the future directions of this rich field are presented.

A general and robust strategy for the synthesis of nearly monodisperse colloidal nanocrystals

Thermoelectric materials have garnered considerable attention due to their unique ability to directly convert heat to electricity and vice versa. Polymers carry many intrinsic advantages such as low thermal conductivity, solution processability, and roll-to-roll production for fabricating high-performance, light-weight, and flexible thermoelectric modules. In this review, we highlight recent advances in the preparation, modification and optimization of polymer thermoelectric materials, focusing especially on the current state-of-the-art strategies to minimize the thermal conductivity and maximize the power factor, and finally provide an outlook on the future development of this field.

Thermopower enhancement in conducting polymer nanocomposites via carrier energy scattering at the organic–inorganic semiconductor interface

Colloidal nanocrystals exhibit a wide range of size- and shape-dependent properties and have found application in myriad fields, incuding optics, electronics, mechanics, drug delivery and catalysis, to name but a few. Synthetic protocols that enable the simple and convenient production of colloidal nanocrystals with controlled size, shape and composition are therefore of key general importance. Current strategies include organic solution-phase synthesis, thermolysis of organometallic precursors, sol-gel processes, hydrothermal reactions and biomimetic and dendrimer templating. Often, however, these procedures require stringent experimental conditions, are difficult to generalize, or necessitate tedious multistep reactions and purification. Recently, linear amphiphilic block co-polymer micelles have been used as templates to synthesize functional nanocrystals, but the thermodynamic instability of these micelles limits the scope of this approach. Here, we report a general strategy for crafting a large variety of functional nanocrystals with precisely controlled dimensions, compositions and architectures by using star-like block co-polymers as nanoreactors. This new class of co-polymers forms unimolecular micelles that are structurally stable, therefore overcoming the intrinsic instability of linear block co-polymer micelles. Our approach enables the facile synthesis of organic solvent- and water-soluble nearly monodisperse nanocrystals with desired composition and architecture, including core-shell and hollow nanostructures. We demonstrate the generality of our approach by describing, as examples, the synthesis of various sizes and architectures of metallic, ferroelectric, magnetic, semiconductor and luminescent colloidal nanocrystals.

High efficiency perovskite solar cells: from complex nanostructure to planar heterojunction

The energy-filtering effect was successfully employed at the organic–inorganic semiconductor interface of poly(3-hexylthiophene) (P3HT) nanocomposites with the addition of Bi2Te3 nanowires, where low-energy carriers were strongly scattered by the appropriately engineered potential barrier of the P3HT–Bi2Te3 interface. The resulting P3HT–Bi2Te3 nanocomposites exhibited a high power factor of 13.6 μW K−2 m−1 compared to that of 3.9 μW K−2 m−1 in P3HT. The transport characteristics of nanocomposites, including the carrier concentration, mobility, and energy-dependent scattering parameter, were revealed by the experimental measurements of electrical conductivity, Seebeck coefficient, and Hall coefficient to quantitatively elucidate the carrier energy scattering at the P3HT–Bi2Te3 interface. The ability to rationally engineer the organic–inorganic semiconductor interfaces of polymer nanocomposites to achieve an improved Seebeck coefficient and power factor provides a potential route to high-performance, large-area, and flexible polymer thermoelectric materials.

Noble metal–metal oxide nanohybrids with tailored nanostructures for efficient solar energy conversion, photocatalysis and environmental remediation

Perovskite solar cells have garnered great attention in recent years as promising high performance next-generation solar cells with long-term stability at low cost. Since the seminal work of Miyasaka and others in 2009, the power conversion efficiency (PCE) of perovskite-based dye-sensitized solar cells (DSSCs) has rapidly increased from 3.8% to 15% over the past four years, exceeding the highest efficiency of conventional organic dye-sensitized DSSCs. Recently, the perovskite has been demonstrated to act successfully as an active layer in simple planar-heterojunction solar cells with no need of complex nanostructured DSSC architectures, leading to an attractively high PCE of 15.4% at a competitive low manufacturing cost. In this Feature Article, we aim to review the recent impressive development in perovskite solar cells, and discuss the prognosis for future progress in exploiting perovskite materials for high efficiency solar cells.