Showing papers by "Alex K.-Y. Jen published in 2013"

High-Performance Perovskite-Polymer Hybrid Solar Cells via Electronic Coupling with Fullerene Monolayers

Abstract: A plethora of solution-processed materials have been developed for solar cell applications. Hybrid solar cells based on light absorbing semiconducting polymers infiltrated into mesoporous TiO2 are an interesting concept, but generating charge at the polymer–metal oxide heterojunction is challenging. Metal–organic perovskite absorbers have recently shown remarkable efficiencies but currently lack the range of color tunability of organics. Here, we have combined a fullerene self-assembled monolayer (C60SAM) functionalized mesoporous titania, a perovskite absorber (CH3NH3PbI3–xClx), and a light absorbing polymer hole-conductor, P3HT, to realize a 6.7% efficient hybrid solar cell. We find that photoexcitations in both the perovskite and the polymer undergo very efficient electron transfer to the C60SAM. The C60SAM acts as an electron acceptor but inhibits further electron transfer into the TiO2 mesostructure due to energy level misalignment and poor electronic coupling. Thermalized electrons from the C60SAM a...

The role of spin in the kinetic control of recombination in organic photovoltaics

Abstract: In biological complexes, cascade structures promote the spatial separation of photogenerated electrons and holes, preventing their recombination. In contrast, the photogenerated excitons in organic photovoltaic cells are dissociated at a single donor-acceptor heterojunction formed within a de-mixed blend of the donor and acceptor semiconductors. The nanoscale morphology and high charge densities give a high rate of electron-hole encounters, which should in principle result in the formation of spin-triplet excitons, as in organic light-emitting diodes. Although organic photovoltaic cells would have poor quantum efficiencies if every encounter led to recombination, state-of-the-art examples nevertheless demonstrate near-unity quantum efficiency. Here we show that this suppression of recombination arises through the interplay between spin, energetics and delocalization of electronic excitations in organic semiconductors. We use time-resolved spectroscopy to study a series of model high-efficiency polymer-fullerene systems in which the lowest-energy molecular triplet exciton (T1) for the polymer is lower in energy than the intermolecular charge transfer state. We observe the formation of T1 states following bimolecular recombination, indicating that encounters of spin-uncorrelated electrons and holes generate charge transfer states with both spin-singlet ((1)CT) and spin-triplet ((3)CT) characters. We show that the formation of triplet excitons can be the main loss mechanism in organic photovoltaic cells. But we also find that, even when energetically favoured, the relaxation of (3)CT states to T1 states can be strongly suppressed by wavefunction delocalization, allowing for the dissociation of (3)CT states back to free charges, thereby reducing recombination and enhancing device performance. Our results point towards new design rules both for photoconversion systems, enabling the suppression of electron-hole recombination, and for organic light-emitting diodes, avoiding the formation of triplet excitons and enhancing fluorescence efficiency.

Rational Design of Advanced Thermoelectric Materials

Abstract: Advanced thermoelectric technologies can drastically improve energy efficiencies of industrial infrastructures, solar cells, automobiles, aircrafts, etc. When a thermoelectric device is used as a solid-state heat pump and/or as a power generator, its efficiency depends pivotally on three fundamental transport properties of materials, namely, the thermal conductivity, electrical conductivity, and thermopower. The development of advanced thermoelectric materials is very challenging because these transport properties are interrelated. This paper reviews the physical mechanisms that have led to recent material advances. Progresses in both inorganic and organic materials are summarized. While the majority of the contemporary effort has been focused on lowering the lattice thermal conductivity, the latest development in nanocomposites suggests that properly engineered interfaces are crucial for realizing the energy filtering effect and improving the power factor. We expect that the nanocomposite approach could be the focus of future materials breakthroughs.

Doping of Fullerenes via Anion‐Induced Electron Transfer and Its Implication for Surfactant Facilitated High Performance Polymer Solar Cells

Abstract: Simple and solution-processible tetrabutyl-ammonium salts (TBAX) can dope fullerene and its derivatives to achieve conductive thin films (σ as high as 0.56 S/m). The electron transfer between the anions of TBAXs and n-type semiconductors induces doping without encountering any harsh activation. These provide valid support for the surfactant interfacial doping of fullerene in polymer solar cells for enhanced device performance.

Non-halogenated solvents for environmentally friendly processing of high-performance bulk-heterojunction polymer solar cells

Abstract: High-performance bulk-heterojunction (BHJ) polymer solar cells (PSCs) processed from non-halogenated solvents with power conversion efficiency values over 7% are demonstrated for the first time. The effects of these solvents on the morphology, charge mobility, and PSC device performance have been systematically studied. The general applicability of these non-halogenated solvents to diverse polymer/fullerene systems has also been demonstrated.

Flexible and twistable non-volatile memory cell array with all-organic one diode–one resistor architecture

Abstract: Flexible organic memory devices are one of the integral components for future flexible organic electronics. However, high-density all-organic memory cell arrays on malleable substrates without cross-talk have not been demonstrated because of difficulties in their fabrication and relatively poor performances to date. Here we demonstrate the first flexible all-organic 64-bit memory cell array possessing one diode-one resistor architectures. Our all-organic one diode-one resistor cell exhibits excellent rewritable switching characteristics, even during and after harsh physical stresses. The write-read-erase-read output sequence of the cells perfectly correspond to the external pulse signal regardless of substrate deformation. The one diode-one resistor cell array is clearly addressed at the specified cells and encoded letters based on the standard ASCII character code. Our study on integrated organic memory cell arrays suggests that the all-organic one diode-one resistor cell architecture is suitable for high-density flexible organic memory applications in the future.

Molecular Weight Effect on the Absorption, Charge Carrier Mobility, and Photovoltaic Performance of an Indacenodiselenophene-Based Ladder-Type Polymer

Abstract: Selenium substitution on a ladder-type indacenodithiophene-based polymer (PIDT-DFBT) is investigated in order to reduce band gap, improve charge mobilities, and enhance the photovoltaic performance of the material. The new indacenodiselenophene-based polymer (PIDSe-DFBT) possessed improved absorption over its sulfur analogue in films, as well as substantially higher charge mobilities (0.15 and 0.064 cm2/(V s) hole and electron mobility, respectively, compared to 0.002 and 0.008 cm2/(V s) for PIDT-DFBT). The enhanced material properties led to an improved power conversion efficiency of 6.8% in photovoltaic cells, a 13% improvement over PIDT-DFBT-based devices. Furthermore, we examined the effect of molecular weight on the properties of PIDSe-DFBT and found not only a strong molecular weight dependence on mobilities, but also on the absorptivity of polymer films, with each 15 000 g/mol increase in weight, leading to a 25% increase in the absorptivity of the material. The molecular weight dependence of the m...

Solution‐Processible Highly Conducting Fullerenes

Abstract: n-Doping of solution-processible organic semiconductors: highly conductive fullerenes are demonstrated through solution-processed fulleropyrrolidinium iodide (FPI) and FPI-doped PCBM to reach a high conductivity (3.2 S/m). The n-doping proceeds via anion-induced electron transfer between the iodide on FPI and the fullerene in the solid state.

C–H activation: making diketopyrrolopyrrole derivatives easily accessible

Abstract: Diketopyrrolopyrrole (DPP) derivatives are an important class of high-performance pigment used in inks, paints, plastics, and organic electronics. Until now, DPP derivatives containing sophisticated aryl units at the DPP core have usually been obtained via Suzuki, Stille, or Negishi cross-coupling reactions, which require organometallic precursors. In this work, a series of DPP-based π-conjugated molecules bearing diverse aryl substituents on the thiophene- or benzene-DPPs were facilely synthesized in moderate to excellent yields through the Pd-catalyzed direct arylation of C–H bonds. The synthetic procedures feature advantages over traditional C–C cross-coupling reactions such as: (1) avoidance of the use of organometallic reagents in the starting materials leading to simpler byproducts and higher atom economy, (2) fewer synthetic steps, (3) higher yields, (4) better compatibility with chemically sensitive functional groups, and (5) simpler catalytic systems free of phosphine ligands. These advantages make the present protocol an ideal and versatile strategy for the synthesis of DPP derivatives, especially for structurally complicated DPPs that may possess chemically sensitive functionalities. The optical and electrochemical properties of the synthesized DPPs (17 compounds) were systematically investigated using UV-vis spectroscopy, steady-state fluorescence spectroscopy, and cyclic voltammetry (CV).

High-Efficiency Polymer Solar Cells Achieved by Doping Plasmonic Metallic Nanoparticles into Dual Charge Selecting Interfacial Layers to Enhance Light Trapping

Abstract: Significantly increased power conversion efficiency (PCE) of polymer solar cells (PSCs) is achieved by applying a plasmonic enhanced light trapping strategy to a low bandgap conjugated polymer, poly(indacenodithiophene- co-phananthrene-quinoxaline) (PIDT-PhanQ) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) based bulk-heterojunction (BHJ) system. By doping both the rear and front charge-selecting interfacial layers of the device with different sizes of Au NPs, the PCE of the devices is improved from 6.65% to 7.50% (13% enhancement). A detailed study of processing, characterization, microscopy, and device fabrication is conducted to understand the underlying mechanism for the enhanced device performance. The success of this work provides a simple and generally applicable approach to enhance light harnessing of low bandgap polymers in PSCs.

Wide optical spectrum range, subvolt, compact modulator based on an electro-optic polymer refilled silicon slot photonic crystal waveguide.

Abstract: We design and demonstrate a compact and low-power, band-engineered, electro-optic (EO) polymer refilled silicon slot photonic crystal waveguide (PCW) modulator. The EO polymer is engineered for large EO activity and near-infrared transparency. A PCW step coupler is used for optimum coupling to the slow-light mode of the band-engineered PCW. The half-wave switching voltage is measured to be Vπ=0.97±0.02 V over an optical spectrum range of 8 nm, corresponding to the effective in-device r33 of 1190 pm/V and Vπ×L=0.291±0.006 V×mm in a push–pull configuration. Excluding the slow-light effect, we estimate that the EO polymer is poled with an efficiency of 89 pm/V in the slot.

A versatile fluoro-containing low-bandgap polymer for efficient semitransparent and tandem polymer solar cells

Abstract: The versatility of a fl uoro-containing low band-gap polymer, poly[2,6-(4,4bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b’]dithiophene)-alt-4,7-(5-fl uoro2,1,3-benzothia-diazole)] (PCPDTFBT) in organic photovoltaics (OPVs) applications is demonstrated. High boiling point 1,3,5-trichlorobenzene (TCB) is used as a solvent to manipulate PCPDTFBT:[6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) active layer morphology to obtain high-performance single-junction devices. It promotes the crystallization of PCPDTFBT polymer, thus improving the charge-transport properties of the active layer. By combining the morphological manipulation with interfacial optimization and device engineering, the single-junction device exhibits both good air stability and high power-conversion effi ciency (PCE, of 6.6%). This represents one of the highest PCE values for cyclopenta[2,1-b;3,4-b’]dithiophene (CPDT)-based OPVs. This polymer is also utilized for constructing semitransparent solar cells and double-junction tandem solar cells to demonstrate high PCEs of 5.0% and 8.2%, respectively.

Side-Chain Effect on Cyclopentadithiophene/Fluorobenzothiadiazole-Based Low Band Gap Polymers and Their Applications for Polymer Solar Cells

Abstract: A series of cyclopentadithiophene-based low band gap conjugated polymers with varied side-chain patterns and F-substituents were synthesized. By replacing the shorter 2-ethylhexyl (EH) side-chain with the longer 3,7-dimethyloctyl (DMO) side-chain, it resulted in significant changes to the optical, electrochemical, and morphological properties of the polymers, as well as the subsequent performance of devices made from these materials. Solar cells fabricated from polymer with 2-ethylhexyl (EH) side-chain and monofluoro substituent exhibits increased open circuit voltage, short circuit current and fill factor, resulting in the highest power conversion efficiency (5.5%) in this series of polymers. This finding provides valuable insight for making more efficient low band gap polymers.

High color purity ZnSe/ZnS core/shell quantum dot based blue light emitting diodes with an inverted device structure

Abstract: Deep-blue, high color purity electroluminescence (EL) is demonstrated in an inverted light-emitting device using nontoxic ZnSe/ZnS core/shell quantum dots (QDs) as the emitter. The device exhibits moderate turn-on voltage (4.0 V) and color-saturated deep blue emission with a narrow full width at half maximum of ∼15 nm and emission peak at 441 nm. Their maximum luminance and current efficiency reach 1170 cd/m2 and 0.51 cd/A, respectively. The high performances are achieved through a ZnO nanoparticle based electron-transporting layer due to efficient electron injection into the ZnSe/ZnS QDs. Energy transfer processes between the ZnSe/ZnS QDs and hole-transporting materials are studied by time-resolved photoluminescence spectroscopy to understand the EL mechanism of the devices. These results provide a new guide for the fabrication of efficient deep-blue quantum dot light-emitting diodes and the realization of QD-based lighting sources and full-color panel displays.

Effects of self-assembled monolayer structural order, surface homogeneity and surface energy on pentacene morphology and thin film transistor device performance

Abstract: A systematic study of six phosphonic acid (PA) self-assembled monolayers (SAMs) with tailored molecular structures is performed to evaluate their effectiveness as dielectric modifying layers in organic field-effect transistors (OFETs) and determine the relationship between SAM structural order, surface homogeneity, and surface energy in dictating device performance. SAM structures and surface properties are examined by near edge X-ray absorption fine structure (NEXAFS) spectroscopy, contact angle goniometry, and atomic force microscopy (AFM). Top-contact pentacene OFET devices are fabricated on SAM modified Si with a thermally grown oxide layer as a dielectric. For less ordered methyl- and phenyl-terminated alkyl ∼(CH2)12 PA SAMs of varying surface energies, pentacene OFETs show high charge carrier mobilities up to 4.1 cm2 V−1 s−1. It is hypothesized that for these SAMs, mitigation of molecular scale roughness and subsequent control of surface homogeneity allow for large pentacene grain growth leading to high performance pentacene OFET devices. PA SAMs that contain bulky terminal groups or are highly crystalline in nature do not allow for a homogenous surface at a molecular level and result in charge carrier mobilities of 1.3 cm2 V−1 s−1 or less. For all molecules used in this study, no causal relationship between SAM surface energy and charge carrier mobility in pentacene FET devices is observed.

Highly Efficient Organic Electrooptic Materials and Their Hybrid Systems for Advanced Photonic Devices

Abstract: This review summarizes recent development of highly efficient organic electricoptic (OEO) materials and their applications in hybrid photonic devices. New generation of highly efficient EO polymers possessing large Pockels coefficients of 200-250 pm/V at 1.3 μm and excellent thermal and photochemical stability have been developed for advanced photonic devices. In addition, new pyroelectric poling process provides an efficient and reliable high field poling for EO polymers in multilayered thin films and nanophotonic waveguides. Using OEO materials as a key enabling element, significant progress has been made in the development of innovative hybrid EO devices, including EO polymer hybrid nanophotonic waveguides with high-index semiconductors, all polymer- and polymer/sol-gel-based EO modulators using low-refractive-index cladding layers, and EO polymer-based electric field sensors. At the end, this review also provides an outlook of future development of OEO materials and their hybrid systems for advanced photonic technologies.

A Silicon-Polymer Hybrid Modulator—Design, Simulation and Proof of Principle

Abstract: Optimal dimensions are found for the silicon waveguide in an electro-optic (EO) polymer cladding-based silicon waveguide modulator. The confinement factor as well as the effective index of the mode are taken into account. The influence of the coplanar electrode spacing and electrode height on performance are examined and a figure of merit formula for choosing the optimal device geometry is shown. The design space for both 1550 nm and 1310 nm wavelengths is explored. With the optimal 275 nm waveguide width and 4 μm electrode spacing, a Vπ of a few volts can be achieved even with moderate r33 EO polymers. Experimental results on a fabricated modulator are shown and compared with the predicted performance.

Enhanced performance of polymer solar cells using solution-processed tetra-n-alkyl ammonium bromides as electron extraction layers

Abstract: We investigate solution-processed tetra-n-alkyl ammonium bromides (TAABs) as electron extraction layers (EELs) in bulk heterojunction (BHJ) solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). The EELs of TAABs lead to simultaneous increases in open-circuit voltage (VOC), short-circuit current density, and fill factor, enhancing the power conversion efficiency of BHJ solar cells from 2.38% to 4.02–4.19%. It is interesting that the same increase in VOC of about 0.14 V is obtained for devices with Al, Ag, and Au cathodes by using the EEL of TOAB. The self-assembly of TOAB into the lamellar structure stacked upright atop P3HT:PCBM is corroborated by synchrotron X-ray diffraction. The surface morphologies of TAABs atop P3HT:PCBM, explored by atomic force microscopy, show that there are ultrathin layers of TAABs in the regions beside large island clusters. The underlying mechanism is inferred that the TAAB molecules introduce anisotropic dipoles toward P3HT:PCBM which significantly elevate the vacuum level of the metal cathode for the efficient electron extraction. Our results provide a simple method to fabricate high performance BHJ solar cells by solution processing.

The effect of thieno[3,2-b]thiophene on the absorption, charge mobility and photovoltaic performance of diketopyrrolopyrrole-based low bandgap conjugated polymers

Abstract: A series of two-dimensional diketopyrrolopyrrole-based low band gap conjugated polymers were synthesized. Replacing thiophene with thieno[3,2-b]thiophene in the side chain and bridge resulted in significant changes to the optical, electrochemical, and morphological properties of the polymers, as well as the subsequent performance of devices made from these materials. The polymer with thieno[3,2-b]thiophene as a bridge instead of a side chain exhibited an increased absorption coefficient and hole mobility, and resulted in the highest power conversion efficiency (5.34%) in this series of polymers. This finding provides valuable insight for the development of more efficient low band-gap polymers.

Reducing cross-sensitivity of TiO2-(B) nanowires to humidity using ultraviolet illumination for trace explosive detection

Abstract: Environmental humidity is an important factor that can influence the sensing performance of a metal oxide. TiO2-(B) in the form of nanowires has been demonstrated to be a promising material for the detection of explosive gases such as 2,4,6-trinitrotoluene (TNT). However, the elimination of cross-sensitivity of the explosive detectors based on TiO2-(B) toward environmental humidity is still a major challenge. It was found that the cross-sensitivity could be effectively modulated when the thin film of TiO2-(B) nanowires was exposed to ultraviolet (UV) light during the detection of explosives under operating conditions. Such a modulation of sensing responses of TiO2-(B) nanowires to explosives by UV light was attributed to a photocatalytic effect, with which the water adsorbed on the TiO2-(B) nanowire surface was split and therefore the sensor response performance was less affected. It was revealed that the cross-sensitivity could be suppressed up to 51% when exposed to UV light of 365 nm wavelength with an intensity of 40 mW cm−2. This finding proves that the reduction of cross-sensitivity to humidity through UV irradiation is an effective approach that can improve the performance of a sensor based on TiO2-(B) nanowires for the detection of explosive gas.

Solvent-Dispersed Benzothiadiazole-Tetrathiafulvalene Single-Crystal Nanowires and Their Application in Field-Effect Transistors

Abstract: A new organic semiconductor (BT-TTF) based on molecular moieties of benzothiadiazole and tetrathiafulvalene was designed and synthesized, and its structure, molecular packing and charge-transporting properties were determined. Thermal properties, electrochemical behaviors, and optical absorption of this molecule were studied by using differential scanning calorimetry/thermal gravimetric analysis, cyclic voltammetry, and ultraviolet–visible spectroscopy, respectively. Its bulk and nanowire single crystals were prepared and characterized by X-ray crystallography, scanning electron microscopy, transmission electron microscopy, and field-effect transistors. It is found that short intermolecular S···S (3.41 A), S···C (3.49 A), and S···N (3.05 A) contacts define the solid-state structure of BT-TTF single crystals which π-stack along the [100] with interplanar distances of 3.49 A. Solvent-cast single-crystal nanowire transistors showed mobilities as large as 0.36 cm2/(V s) with current on/off ratios of 1 × 106. ...

Optical and electrical effects of plasmonic nanoparticles in high-efficiency hybrid solar cells

Abstract: Plasmonics have been proven to be an effective way to harness more incident light to achieve high efficiency in photovoltaic devices. Herein, we explore the possibility that plasmonics can be utilized to enhance light trapping and power conversion efficiency (PCE) for polymer–quantum dot (QD) hybrid solar cells (HSCs). Based on a low band-gap polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and a CdSe QD bulk-heterojunction (BHJ) system, gold nanoparticles were doped at different locations of the devices. Successfully, an improved PCE of 3.20 ± 0.22% and 3.16 ± 0.15% was achieved by doping the hole transporting layer and the active layer, respectively, which are among the highest values reported for CdSe QD based HSCs. A detailed study of processing, characterization, microscopy, and device fabrication is conducted to understand the underlying mechanism for the enhanced device performance. The success of this work provides a simple and generally applicable approach to enhance light harnessing of polymer–QD hybrid solar cells.

Demonstration of effective in-device r 33 over 1000 pm/V in electro-optic polymer refilled silicon slot photonic crystal waveguide modulator

Abstract: We demonstrate a band engineered slot photonic crystal waveguide refilled with electro-optic (EO) polymer. The combined effects of slow-light and high performance EO polymer makes possible effective in-device r 33 of 1012pm/V and V π ×L of 0.345Vmm.

Morphology evolution by controlling solvent-solute interactions using a binary solvent in bulk heterojunction solar cells

Abstract: Improved power conversion efficiency of poly(indacenodithiophene-co-phananthrene-quinoxaline) (PIDT-PhanQ)/[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) based bulk- heterojunction (BHJ) solar cells was achieved upon adding tetrahydrofuran (THF) as a co-solvent to 1,2-dichlorobenzene (DCB). This reasonably large enhancement is achieved due to THF changes the morphology in the active layer by reducing the solvent-solute interaction. The Flory-Huggins interaction parameter and cohesive energy densities for PIDT-PhanQ and PD71BM with different solvents were further studied to understand the underlying phase separation mechanism in BHJ films.

Cascading retro-Diels-Alder cycloreversion and sydnone-maleimide based double 1,3-dipolar cycloaddition for quantitative thermal cross-linking of an amorphous polymer solid

Abstract: A new approach to polymer cross-linking is investigated using a cascading cycloreversion of a maleimide-furan adduct and a double 1,3-dipolar cycloaddition between a sydnone and maleimide. The cross-linking proceeds quantitatively above 63 °C, despite the polymer possessing no observable glass transition temperature. The resulting polymer network possesses a high thermal stability (>300 °C) due to the irreversibility of the sydnone-maleimide cycloaddition, which releases CO2 during the cross-linking, driving the reaction. The rigid three-dimensional structure of the bis-maleimide-sydnone cycloadduct produced local free volumes in films, decreasing the dielectric constant of the material.