Publications

Novel Triarylamine Polymers as Hole Transport Materials in OLEDs

Highly Efficient Solar Cells using TiO₂ Nanotube Arrays Sensitized with a Donor-Antenna Dye

Donor antenna dyes provide an exciting route to improving the efficiency of dye sensitized solar cells owing to their high molar extinction coefficients and the effective spatial separation of charges in the charge-separated state, which decelerates the recombination of photogenerated charges. Vertically oriented TiO(2) nanotube arrays provide an optimal material architecture for photoelectrochemical devices because of their large internal surface area, lower recombination losses, and vectorial charge transport along the nanotube axis. In this study, the results obtained by sensitizing TiO(2) nanotube arrays with the donor antenna dye Ru-TPA-NCS are presented. Solar cells fabricated using an antenna dye-sensitized array of 14.4 microm long TiO(2) nanotubes on Ti foil subjected to AM 1.5 one sun illumination in the backside geometry exhibited an overall conversion efficiency of 6.1%. An efficiency of 4.1% was obtained in the frontside illumination geometry using a 1 microm long array of transparent TiO(2) nanotubes subjected to a TiCl(4) treatment and then sensitized with the Ru-TPA-NCS dye. Open circuit voltage decay measurements give insight into the recombination behavior in antenna-dye sensitized nanotube photoelectrodes, demonstrating outstanding properties likely due to a reduction in the influence of the surface traps and reduced electron transfer from TiO(2) to ions in solution.

Influence of molecular weight on the solar cell performance of double-crystalline donor-acceptor block copolymers

We investigate the influence of the molecular weight of double-crystalline donor-acceptor block copolymers comprised of poly(3-hexylthiophene) as donor and poly(perylene bisimide acrylate) as acceptor segments on the device performance of polymer solar cells. Two block copolymers 1 and 2 exhibiting different molecular weights but the same composition are compared. Block copolymer 2 with the higher molecular weight shows an improvement in the hole carrier mobility μOFET of more than two orders of magnitude, and an improvement in the external quantum efficiency of one order of magnitude reaching 31%, which is the highest reported value for a block copolymer system.

A Competitive n‐Type OECT Material via Copolymerization of Electron Deficient Building Blocks

Abstract The classical acceptor motifs diketopyrrolopyrrole (DPP) and thienopyrrolodione (TPD) are copolymerized to yield the acceptor–acceptor polymer “Poly(DPP‐TPD).” The fundamental design idea is to maximize the electron affinity (EA), thus increasing the ambient stability of the reduced state against oxygen and water while ensuring high ion compatibility through the incorporation of hydrophilic oligoethylene glycol N‐substituents. Additionally, a highly planarized polymer structure is anticipated, due to the extended noncovalent interactions (conformational locking) between the carbonyl oxygen and the thiophene protons. Cyclic voltammetry, spectroelectrochemistry, ultraviolet photoelectron spectroscopy, ultraviolet‐visible absorption spectroscopy, and organic field effect transistor (OFET) characterization demonstrate the suitability for n‐type organic electrochemical transistor (OECT) devices. High EA, ionization potential, and good electron mobility (µ e(OFET) ) are shown, in addition to the electrochemical reduction of polymer films in aqueous electrolyte. In n‐type OECTs, poly(DPP‐TPD) demonstrates a moderate threshold voltage of V th = 0.58 V and an outstanding µC * value of 7.62 F cm −1 V −1 s −1 . Cycling studies consisting of pulsed on‐ and off‐switching of the device at gate voltages between V g = 0.6–0.8 V in the saturation regime reveal high stability for more than 2700 cycles with rapid switching kinetics.

Nanoscale Morphology from Donor–Acceptor Block Copolymers: Formation and Functions

Macroscopic Vertical Alignment of Nanodomains in Thin Films of Semiconductor Amphiphilic Block Copolymers

Though several techniques have been reported on the alignment of conventional block copolymers, the macroscopic vertical orientation of semiconductor block copolymer microdomains in thin films has still not been accomplished. Here, we report the control on the alignment of nanostructures in a semiconductor amphiphilic block copolymer comprising an amorphous triphenyldiamine hole conductor block and a hydrophilic poly(styrene sulfonate) segment. Three different compositions with a hole conductor content of 57, 72, and 79 wt % were synthesized using a combination of controlled reversible addition/fragmentation transfer polymerization and "click" chemistry. All polymers feature a narrow molecular weight distribution. Cryo-TEM reveals the formation of micelles in DMF solutions of the amphiphilic copolymer having nanoscopic dimensions. The micelle size correlates well with the X-ray analysis of dried bulk samples. Atomic force microscopy (AFM) confirms the micellar structure in the as-cast films. Thermal annealing causes an aggregation of micelles but did not lead to morphologies known for conventional block copolymers. However, annealing in saturated DMF vapor induces a morphology transition and a vertical orientation of the microdomains which was determined by grazing incidence small-angle X-ray scattering and AFM. The morphology varies from lamella to cylinders with increasing content of the hole-conductor block. The orientation arises from the controlled evaporation of the solvent, a mechanism that is similar to that observed for conventional block copolymers. Our approach demonstrates the macroscopic vertical alignment of nanodomains in semiconductor block copolymers which is a key requirement for applications in hybrid devices.

Microphase‐Separated Donor–Acceptor Diblock Copolymers: Influence of HOMO Energy Levels and Morphology on Polymer Solar Cells

Abstract The synthesis of novel semiconducting donor–acceptor (D–A) diblock copolymers by means of nitroxide‐mediated polymerization (NMP) is reported. The copolymers contain functional moieties for hole transport, electron transport, and light absorption. The first block, representing the donor, is made up of either substituted triphenylamines (poly(bis(4‐methoxyphenyl)‐4′‐vinylphenylamine), PvDMTPA) or substituted tetraphenylbenzidines (poly( N , N ′‐bis(4‐methoxyphenyl)‐ N ‐phenyl‐ N ′‐4‐vinylphenyl‐(1,1′‐biphenyl)‐4,4′‐diamine), PvDMTPD). The second block consists of perylene diimide side groups attached to a polyacrylate backbone (PPerAcr) via a flexible spacer. This block is responsible for absorption in the visible range and for electron‐transport properties. The electrochemical properties of these fully functionalized diblock copolymers, PvDMTPA‐ b ‐PPerAcr and PvDMTPD‐ b ‐PPerAcr, are investigated by cyclic voltammetry (CV), and their morphology is investigated by transmission electron microscopy (TEM). All diblock copolymers exhibit microphase‐separated domains in the form of either wire‐ or wormlike structures made of perylene diimide embedded in a hole‐conductor matrix. In single‐active‐layer organic solar cells, PvDMTPD‐ b ‐PPerAcr reveals a fourfold improvement in power conversion efficiency (η = 0.26 %, short‐circuit current ( I SC ) 1.21 mA cm –2 ), and PvDMTPA‐ b ‐PPerAcr a fivefold increased efficiency (η = 0.32 %, I SC = 1.14 mA cm –2 ) compared with its unsubstituted analogue PvTPA‐ b ‐PPerAcr (η = 0.065 %, I SC = 0.23 mA cm –2 ).

Correlation of charge transport with structural order in highly ordered melt‐crystallized poly(3‐hexylthiophene) thin films

ABSTRACT A series of well‐defined poly(3‐hexylthiophene)s (P3HT) of different molecular weight (MW) and high regioregularity was investigated for charge transport properties in as‐cast and melt‐crystallized films. The semicrystalline structure of the P3HT was characterized by X‐ray scattering and Atomic force microscopy. Crystallization by cooling from the melt led to a substantial increase in crystallinity and a stronger alignment of the crystals in comparison to as‐cast films. The increase in crystallinity went along with an increase in hole mobility of up to an order of magnitude as measured by the space charge limited current method. Additionally, the hole mobility depended on the long period of P3HT lamellae and consequently on the MW. In compliance with the long period, the charge carrier mobility first increased with the MW before decreasing again at the onset of chain folding. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51 , 943–951

The role of colloidal plasmonic nanostructures in organic solar cells

Plasmonic particles can contribute via multiple processes to the light absorption process in solar cells. These particles are commonly introduced into organic solar cells via deposition techniques such as spin-coating or dip-coating. However, such techniques are inherently challenging to achieve homogenous surface coatings as they lack control of inter-particle spacing and particle density on larger areas. Here we introduce interface assisted colloidal self-assembly as a concept for the fabrication of well-defined macroscopic 2-dimensional monolayers of hydrogel encapsulated plasmonic gold nanoparticles. The monolayers showed a pronounced extinction in the visible wavelength range due to localized surface plasmon resonance with excellent optical homogeneity. Moreover this strategy allowed for the investigation of the potential of plasmonic monolayers at different interfaces of P3HT:PCBM based inverted organic solar cells. In general, for monolayers located anywhere underneath the active layer, the solar cell performance decreased due to parasitic absorption. However with thick active layers, where low hole mobility limited the charge transport to the top electrode, the plasmonic monolayer near that electrode spatially redistributed the light and charge generation close to the electrode led to an improved performance. This work systematically highlights the trade-offs that need to be critically considered for designing an efficient plasmonically enhanced organic solar cell.

Supermolecular Control of Charge Transfer in Dye‐Sensitized Nanocrystalline TiO₂ Films: Towards a Quantitative Structure–Function Relationship

Trennung auf Zeit: In einem Farbstoff-sensibilisierten nanokristallinen TiO2-Film (siehe Schema) kann ein bemerkenswert langlebiger (4 s) ladungsgetrennter Zustand erzeugt werden. Als Schlüsselparameter für die Steuerung des Ladungstransfers an der Farbstoff/TiO2-Grenzfläche erwies sich der Abstand zwischen dem Kationenzentrum des Farbstoffs und der Elektrodenoberfläche. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2001/2005/z500363_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Influence of Poly(Diketopyrrolopyrrole) Chain Length and Chemical Structure on Photocatalytic Hydrogen Evolution in Composites With TiO₂

For this study, we have synthesized two different poly(diketopyrrolopyrrole) copolymers with different chain lengths. The diketopyrrolopyrrole (DPP) core is substituted with oligoethylene glycol side chains to increase its compatibility with water and copolymerized with either fluorene or carbazole moieties. The polymers form a composite photocatalyst with anatase TiO2. Detailed characterization such as NMR spectroscopy, UV?Vis, DRIFT, and UPS is used to analyze the structural and optical properties as well as the frontier orbital energy levels of the components and the composite materials. The optical properties of the polymers are tunable with respect to the copolymer used, opening up the possibility of optimizing the photocatalytic activity. These composite materials (without the addition of a co-catalyst) provide up to an eightfold enhancement of the hydrogen evolution reaction (HER) compared to pristine TiO2. The polymers also exhibit stability in the reaction medium as shown by solid-state NMR, DRIFT, and UV?Vis spectroscopy. A significant influence of the chain length of the polymers on HER is found as well. As the chain length increases, the activity toward hydrogen evolution increases. We show a correlation between hydrogen evolution and PDPP chain length whereby the active site of the photocatalytic process remains the inorganic semiconductor.

The effect of fluorination on chain transfer reactions in the radical polymerization of oligo ethylene glycol ethenesulfonate monomers

The polymerization details of new ethenesulfonate monomers with pendant fluorinated oligo ethylene glycol side-chains are reported.

Charge Separation at Self‐Assembled Nanostructured Bulk Interface in Block Copolymers

Proper percolation of charges in solar cells is possible with the self-assembled nanostructure of a block-copolymer film (see the TEM cross section of a solar cell). A photovoltaic device made from such a block copolymer exhibits a ten-times higher short circuit current and a considerably larger photovoltage than its polymer-blend analogue.

Optical gating of perylene bisimide fluorescence using dithienylcyclopentene photochromic switches

The emission of millions of fluorescence photons from a chromophore is controlled by the absorption of a few tens of photons in a photochromic molecule. The parameters that determine the efficiency of this process are investigated, providing insights for the development of an all-optical gate.

Toward Perfect Control of End Groups and Polydispersity in Poly(3-hexylthiophene) via Catalyst Transfer Polymerization

We report the influence of the active Grignard monomer formation on the end groups and polydispersity of poly(3-hexylthiophene) (P3HT) for the catalyst transfer polymerization. The rate of the active Grignard monomer formation of 2,5-dibromo-3-hexylthiophene using t-BuMgCl was studied using 1H NMR. Only in the absence of unreacted/excess t-BuMgCl, polymers with 100% H/Br end groups were formed. If the active Grignard monomer formation was incomplete and thus unreacted t-BuMgCl remained, the end groups depended on the polymerization time; the ratio of H/Br to H/H end groups decreased with increasing time. LiCl was shown to accelerate the active Grignard monomer formation but negatively affects the regioregularity to a small extent. It also increases the molecular weight of P3HT when used in combination with Ni(dppp)Cl2 as a catalyst. Further, MeOH as a quenching reagent was identified to cause chain−chain coupling and hence an increase in the polydispersity. Thus, important parameters influencing the kinetics of the catalyst transfer polymerization were studied, and a series of P3HTs with 100% H/Br end groups and low polydispersities were synthesized by an optimized procedure. These findings are very relevant for extending this polymerization method to new monomers and for the realization of well-defined block copolymers.

Subsurface Mapping of Amorphous Surface Layers on Poly(3-hexylthiophene)

ADVERTISEMENT RETURN TO ISSUEPREVCommunication to the...Communication to the EditorNEXTSubsurface Mapping of Amorphous Surface Layers on Poly(3-hexylthiophene)M. Zerson*†, E.-C. Spitzner†, C. Riesch†, R. Lohwasser‡, M. Thelakkat‡, and R. Magerle*†View Author Information† Fakultät für Naturwissenschaften, Technische Universität Chemnitz, Chemnitz, Germany‡ Angewandte Funktionspolymere, Makromolekulare Chemie I, Universität Bayreuth, Bayreuth, GermanyE-mail: [email protected] (M.Z.); [email protected] (R.M.).Cite this: Macromolecules 2011, 44, 15, 5874–5877Publication Date (Web):July 8, 2011Publication History Received17 May 2011Revised23 June 2011Published online8 July 2011Published inissue 9 August 2011https://pubs.acs.org/doi/10.1021/ma2011359https://doi.org/10.1021/ma2011359rapid-communicationACS PublicationsCopyright © 2011 American Chemical SocietyRequest reuse permissionsArticle Views729Altmetric-Citations12LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose SUBJECTS:Annealing (metallurgy),Layers,Materials,Organic polymers,Thin films Get e-Alerts

Thickness dependence of device parameters in solid state dye sensitized solar cells

The Journal of the National Science Foundation of Sri Lanka publishes the results of research in all aspects of Science and Technology. The journal also has a website at http://www.nsf.gov.lk/. 2021 Impact Factor: 0.682The JNSF provides immediate open access to its content on the principle that making research freely available to the public supports a greater global exchange of knowledge.

Synthesis and Application of Dimeric 1,3,5-Triazine Ethers as Hole-Blocking Materials in Electroluminescent Devices

A series of low molecular weight dimeric 1,3,5-triazine ethers with high glass transition temperatures is synthesized from 2-(4-fluorophenyl)-4,6-diphenyl-1,3,5-triazine and various bisphenols. The thermal and electrochemical properties of these materials are examined using differential scanning calorimetry and cyclic voltammetry, respectively. The glass transition temperatures (Tg) are in the range of 106−144 °C, and all of them have similar thermal stability and show no weight loss up to 380 °C in thermogravimetric measurements. Some of these dimers form stable glasses which do not recrystallize on annealing at or above the Tg. Cyclic voltammetry studies reveal that these compounds undergo reversible reduction between −2.09 and −2.27 V vs ferrocene/ferrocenium as internal standard which correspond to LUMO values between −2.5 and −2.7 eV, respectively. The application of one of these dimeric ethers as a hole-blocking/electron-transport material in organic light-emitting devices is demonstrated.