Zhuoying Chen

Thieno[3,2-b]thiophene-Diketopyrrolopyrrole-Containing Polymers for High-Performance Organic Field-Effect Transistors and Organic Photovoltaic Devices

We report the synthesis and polymerization of a novel thieno[3,2-b]thiophene-diketopyrrolopyrrole-based monomer. Copolymerization with thiophene afforded a polymer with a maximum hole mobility of 1.95 cm(2) V(-1) s(-1), which is the highest mobility from a polymer-based OFET reported to date. Bulk-heterojunction solar cells comprising this polymer and PC(71)BM gave a power conversion efficiency of 5.4%.

High-performance ambipolar diketopyrrolopyrrole-thieno[3,2-b]thiophene copolymer field-effect transistors with balanced hole and electron mobilities.

Ambipolar OFETs with balanced hole and electron field-effect mobilities both exceeding 1 cm(2) V(-1) s(-1) are achieved based on a single-solution-processed conjugated polymer, DPPT-TT, upon careful optimization of the device architecture, charge injection, and polymer processing. Such high-performance OFETs are promising for applications in ambipolar devices and integrated circuits, as well as model systems for fundamental studies.

High Mobility Ambipolar Charge Transport in Polyselenophene Conjugated Polymers

Adv. Mater. 2010, 22, 2371–2375 2010 WILEY-VCH Verlag G Field-effect transistors (FETs) based on conjugated polymers and small molecules have been of extensive fundamental and practical interest for more than two decades. In terms of fundamental charge transport properties organic semiconductors have been recently shown to be intrinsically ambipolar, i.e., able to accumulate and transport both holes and electrons within the same material under suitable biasing conditions and device configurations. The discovery of the intrinsic ambipolar charge transport properties in common semiconducting polymers was made possible by the understanding of the crucial role played by electronegative trapping groups in the dielectric, such as hydroxyl groups on the surface of a SiO2 gate dielectric. [10] Ambipolar charge transport is not only of fundamental, but also of practical interest as it enables the realization of novel device architectures such as complementary-like voltage inverters with a single organic semiconductor as well as ambipolar light-emitting field-effect transistors (LFETs). Here we report the general observation of ambipolar charge transport characteristics in a series of regioregular polyselenophene-based polymers. Compared to the well-studied polythiophenes, which appear among the most promising solution processable organic semiconductors, polyselenophenes were recently developed as analogue systems providing several advantages over their predecessors. The highest occupied molecular orbital (HOMO) of polythiophenes has little contribution from the sulfur heteroatom, whereas the lowest unoccupied molecular orbital (LUMO) has significant electron density on the heteroatom. Polyselenophenes were initially developed as promising alternatives to polythiophenes for solar cell applications, mainly because of their reduced optical band gaps and their enhanced photostability due to the lower lying LUMO. For FET applications we expect the hole transport to be similar to that of polythiophenes, while the lower lying LUMO in polyselenophenes should result in improved electron transport due to enhanced electron injection from metal electrodes and lower susceptibility of electrons to trap states and oxidation. The regioregular polyselenophenes investigated in this work were: (1) poly(3,300-di-n-alkylterselenophene) (PSSS) of three different alkyl side-chains, namely PSSS-C10, PSSS-C8, and PSSS-C6; and (2) poly(3-octyl)selenophene (P3OS) (Fig. 1). We employed identical top-gate, bottom contact (TGBC) configurations with gold source-drain electrodes for all polymers. For ambipolar FETs the TGBC device configuration offers several advantages over a bottom-gate/bottom-contact (BGBC) configuration: (i) the freedom to select different gate dielectrics to minimize irreversible charge trapping at the semiconductordielectric interface and to act as encapsulation for the FET channel, and (ii) a lower contact resistance due to reduction of current-crowding effects. PSSS is the selenium analogue of the previously reported poly(3,300-dialkylterthiophene) (PTT) with a ‘‘spaced-out’’ distribution of the alkyl side-chains along the polymer backbone. 28] PTT was reported to readily self-assemble into a threedimensional lamellar p-stacking arrangement with an ‘‘edge-on’’

Barium titanate nanocrystals and nanocrystal thin films: Synthesis, ferroelectricity, and dielectric properties

Advanced applications for high k dielectric and ferroelectric materials in the electronics industry continues to demand an understanding of the underlying physics in decreasing dimensions into the nanoscale. We report the synthesis, processing, and electrical characterization of thin (<100nm thick) nanostructured thin films of barium titanate (BaTiO3) built from uniform nanoparticles (<20nm in diameter). We introduce a form of processing as a step toward the ability to prepare textured films based on assembly of nanoparticles. Essential to this approach is an understanding of the nanoparticle as a building block, combined with an ability to integrate them into thin films that have uniform and characteristic electrical properties. Our method offers a versatile means of preparing BaTiO3 nanocrystals, which can be used as a basis for micropatterned or continuous BaTiO3 nanocrystal thin films. We observe the BaTiO3 nanocrystals crystallize with evidence of tetragonality. We investigated the preparation of wel...

Structure direction of II-VI semiconductor quantum dot binary nanoparticle superlattices by tuning radius ratio.

We report a nanoparticle radius ratio dependent study of the formation of binary nanoparticle superlattices (BNSLs) of CdTe and CdSe quantum dots. While keeping all other parameters identical in the system, the effective nanoparticle radius ratio, gamma(eff), was tuned to allow the formation of five different BNSL structures, AlB(2), cub-NaZn(13), ico-NaZn(13), CaCu(5), and MgZn(2). For each structure, gamma(eff) is located close to a local maximum of its space-filling factor, based on a model for space filling principles. We demonstrate the ability to select specific BNSLs based solely on gamma(eff), highlighting the role of entropic forces as a driver for self-assembly.

Metal Acetylacetonates as General Precursors for the Synthesis of Early Transition Metal Oxide Nanomaterials

A versatile, convenient, and nontoxic solvothermal method for the synthesis of nanocrystalline iron, chromium, and manganese oxides is described. This method employs the reactions of metal acetylacetonate precursors and oxygen-containing solvents in a reaction to prepare metal oxide nanoparticles. Characterization of these nanocrystalline materials was carried out employing transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD), and elemental analysis.

Charge trapping dynamics in PbS colloidal quantum dot photovoltaic devices.

The efficiency of solution-processed colloidal quantum dot (QD) based solar cells is limited by poor charge transport in the active layer of the device, which originates from multiple trapping sites provided by QD surface defects. We apply a recently developed ultrafast electro-optical technique, pump-push photocurrent spectroscopy, to elucidate the charge trapping dynamics in PbS colloidal-QD photovoltaic devices at working conditions. We show that IR photoinduced absorption of QD in the 0.2-0.5 eV region is partly associated with immobile charges, which can be optically detrapped in our experiment. Using this absorption as a probe, we observe that the early trapping dynamics strongly depend on the nature of the ligands used for QD passivation, while it depends only slightly on the nature of the electron-accepting layer. We find that weakly bound states, with a photon-activation energy of 0.2 eV, are populated instantaneously upon photoexcitation. This indicates that the photogenerated states show an intrinsically bound-state character, arguably similar to charge-transfer states formation in organic photovoltaic materials. Sequential population of deeper traps (activation energy 0.3-0.5 eV) is observed on the ~0.1-10 ns time scales, indicating that most of carrier trapping occurs only after substantial charge relaxation/transport. The reported study disentangles fundamentally different contributions to charge trapping dynamics in the nanocrystal-based optoelectronic devices and can serve as a useful tool for QD solar cell development.

Organic Cation Rotation and Immobilization in Pure and Mixed Methylammonium Lead-Halide Perovskites

Three-dimensional lead-halide perovskites have attracted a lot of attention due to their ability to combine solution processing with outstanding optoelectronic properties. Despite their soft ionic nature these materials demonstrate a surprisingly low level of electronic disorder resulting in sharp band edges and narrow distributions of the electronic energies. Understanding how structural and dynamic disorder impacts the optoelectronic properties of these perovskites is important for many applications. Here we combine ultrafast two-dimensional vibrational spectroscopy and molecular dynamics simulations to study the dynamics of the organic methylammonium (MA) cation orientation in a range of pure and mixed trihalide perovskite materials. For pure MAPbX3 (X = I, Br, Cl) perovskite films, we observe that the cation dynamics accelerate with decreasing size of the halide atom. This acceleration is surprising given the expected strengthening of the hydrogen bonds between the MA and the smaller halide anions, but can be explained by the increase in the polarizability with the size of halide. Much slower dynamics, up to partial immobilization of the organic cation, are observed in the mixed MAPb(ClxBr1-x)3 and MAPb(BrxI1-x)3 alloys, which we associate with symmetry breaking within the perovskite unit cell. The observed dynamics are essential for understanding the effects of structural and dynamical disorder in perovskite-based optoelectronic systems.

Reduced Carrier Recombination in PbS - CuInS2 Quantum Dot Solar Cells

Energy loss due to carrier recombination is among the major factors limiting the performance of TiO2/PbS colloidal quantum dot (QD) heterojunction solar cells. In this work, enhanced photocurrent is achieved by incorporating another type of hole-transporting QDs, Zn-doped CuInS2 (Zn-CIS) QDs into the PbS QD matrix. Binary QD solar cells exhibit a reduced charge recombination associated with the spatial charge separation between these two types of QDs. A ~30% increase in short-circuit current density and a ~20% increase in power conversion efficiency are observed in binary QD solar cells compared to cells built from PbS QDs only. In agreement with the charge transfer process identified through ultrafast pump/probe spectroscopy between these two QD components, transient photovoltage characteristics of single-component and binary QDs solar cells reveal longer carrier recombination time constants associated with the incorporation of Zn-CIS QDs. This work presents a straightforward, solution-processed method based on the incorporation of another QDs in the PbS QD matrix to control the carrier dynamics in colloidal QD materials and enhance solar cell performance.

Optical, structural, and electrical properties of PEDOT:PSS thin films doped with silver nanoprisms

Conductive thin films of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) are common buffer layers widely applied in organic solar cells. In order to explore the implications of the localized surface plasmon resonance of noble metal nanoparticles in such applications, we prepared a series of hybrid PEDOT:PSS thin films doped with different proportions of colloidal Ag nanoprisms. Various characterization techniques, including transmission electron microscopy, regular and conductive atomic force microscopy, optical absorption, goniophotometry, and four-point probe resistance measurements, were applied to study the effects of Ag nanoprisms on the optical, structural, and electrical properties of the hybrid films. Through analyzing the Bidirectional Reflectance Distribution Functions (BRDF) of different hybrid films, we compared among different hybrid films the proportions of light being scattered and absorbed over various reflected angles. In terms of optical properties, with higher Ag nanoprism concentration, increased light scattering was found in the hybrid films which can potentially improve the light harvest in organic solar cells. In terms of structural and electrical properties, the surface roughness and the global sheet resistance of hybrid films were found to increase as the concentration of Ag nanoprism increases. The magnitude of the sheet resistance in these hybrid films was reduced to a level comparable to or smaller than those observed in pristine PEDOT:PSS films by increasing the extent of post-synthesis nanoprism purification and by applying organic solvent additives.