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May 2013

Volume 6, Issue 5

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In-situ monitoring of molecular vibrations of two organic semiconductors in photovoltaic blends and their impact on thin film morphology

Wing C. Tsoi, Weimin Zhang, Joseph Razzell Hollis, Minwon Suh, Martin Heeney, Iain McCulloch, and Ji-Seon Kim

Appl. Phys. Lett. 102, 173302 (2013); http://dx.doi.org/10.1063/1.4803912 (5 pages)

Online Publication Date: 1 May 2013

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We report in-situ simultaneous monitoring of molecular vibrations of two components in organic photovoltaic blends using resonant Raman spectroscopy. Blend films were composed of a low bandgap copolymer thieno[3,2-b]thiophene-diketopyrrolopyrrole (DPPTTT) and (6,6)-phenyl-C71-butyric acid ester (PC70BM). Changes in Raman spectra associated with crystallization processes of each component and their impact on thin film morphology were studied during thermal annealing and cooling processes. Transition temperatures to crystalline phases in blends were measured at ∼150 °C and ∼170 °C for DPPTTT and PC70BM, respectively. Such phase changes lead to modifications in local chemical composition reducing relative Raman peak intensities (IPC70BM/IDPPTTT) from ∼0.4 in PC70BM-rich domains to ∼0.15 in homogeneous areas.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
81.40.Gh Other heat and thermomechanical treatments
78.30.-j Infrared and Raman spectra
78.66.Qn Polymers; organic compounds
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Improved contact resistivity and intra-die variation in organic thin film transistors

S. Pankalla, D. Spiehl, H. M. Sauer, E. Dörsam, and M. Glesner

Appl. Phys. Lett. 102, 173303 (2013); http://dx.doi.org/10.1063/1.4804239 (5 pages)

Online Publication Date: 3 May 2013

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We studied the processing-related influence on contact resistivity of organic thin-film transistors in top gate architecture which are placed and oriented differently over flexible substrates. Appropriate plasma treatment reduces degradation of the source and drain contacts, increases effective contact surface for self-assembled monolayer treatment, and thus better injection. Increasing the semiconductor film thickness reduces the contact resistivity until a certain critical thickness. By these means, the contact resistivity has been reduced by two orders of magnitude. We did a mass characterisation of 366 solution-processed transistors on six samples that lead to a modified transfer line method in which we permutated the transistors to extract the contact resistivities. Thus, the intra-die dependency of the contact resistivity on the distance from the centre of the sample, the orientation of the transistor, its width, the pre-processing of the samples and on the semiconductor layer thickness has been analysed. These results serve as an evaluation of appropriate processes for printed organic transistors.
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85.30.Tv Field effect devices
84.32.Dd Connectors, relays, and switches
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Bimolecular recombination coefficient calculation by in situ potentiometry in a bulk heterojunction organic photovoltaic material

Eric Danielson, Zi-En Ooi, Christopher J. Lombardo, and Ananth Dodabalapur

Appl. Phys. Lett. 102, 173304 (2013); http://dx.doi.org/10.1063/1.4803512 (4 pages)

Online Publication Date: 3 May 2013

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Polymer fullerene bulk heterojunction (BHJ) systems are an important class of active materials; however, charge transport and recombination mechanisms in these materials are not yet completely understood. We use lateral bulk heterojunction devices to perform in situ potentiometry on the BHJ system P3HT:PCBM. From current vs. voltage measurements performed at different light intensities, we provide evidence that the recombination mechanism in this material is bimolecular. The potential profile of the device channel is also constructed from these measurements, which is then used to determine the mobility of both charge carriers and calculate the bimolecular recombination coefficient.
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88.40.jr Organic photovoltaics
85.65.+h Molecular electronic devices
88.40.hj Efficiency and performance of solar cells
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Optimized inverted polymer solar cells incorporating Cs2CO3-doped C60 as electron transport layer

A. Barbot, B. Lucas, C. Di Bin, B. Ratier, and M. Aldissi

Appl. Phys. Lett. 102, 193305 (2013); http://dx.doi.org/10.1063/1.4807388 (3 pages)

Online Publication Date: 17 May 2013

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An efficient charge transfer between co-sublimed cesium carbonate (Cs2CO3) and fullerene C60 provides an n-type material exhibiting an electrical conductivity above 1 S/cm. This type of doped layers can be used in organic optoelectronic devices to reduce ohmic losses at organic-electrode interfaces. We report here an analysis of inverted polymer-based solar cells incorporating Cs2CO3 doped C60 as electron transport layer (ETL). The optimization of both dopant concentration and thickness resulted in a maximum efficiency of 3.79% compared to 3% for similar devices using undoped C60 as ETL and 2.13% for devices without any ETL.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
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Polymer–nanocrystal hybrid photodetectors with planar heterojunctions designed strategically to yield a high photoconductive gain

Tae Kyu An, Chan Eon Park, and Dae Sung Chung

Appl. Phys. Lett. 102, 193306 (2013); http://dx.doi.org/10.1063/1.4807422 (4 pages)

Online Publication Date: 17 May 2013

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We demonstrate an approach to enhancing the photoresponsivity of a polymer photodetector (PPD). Both conventional bulk heterojunction (BHJ) and planar heterojunction (PHJ) PPDs were fabricated considering that the interface between a CdSe nanocrystal and a polymer can create photoconductive gain. A systematic study of the illumination wavelength and light power dependence of the photocurrent gain, combined with the charge carrier transport analysis, suggested that the PHJ-PPD could yield a higher hole mobility than could be achieved in a BHJ-PPD without compromising on the selective electron trapping effects. The optimized PHJ-PPD led to a photoconductive detectivity of 1.3 × 1010 cm Hz1/2/W.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
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Topography-guided spreading and drying of 6,13-bis(triisopropylsilylethynyl)-pentacene solution on a polymer insulator for the field-effect mobility enhancement

Chang-Min Keum, Jin-Hyuk Bae, Min-Hoi Kim, Hea-Lim Park, Marcia M. Payne, John E. Anthony, and Sin-Doo Lee

Appl. Phys. Lett. 102, 193307 (2013); http://dx.doi.org/10.1063/1.4807461 (5 pages)

Online Publication Date: 17 May 2013

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We report on the enhancement of the field-effect mobility of solution-processed 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) by unidirectional topography (UT) of an inkjet-printed polymer insulator. The UT leads to anisotropic spreading and drying of the TIPS-pentacene droplet and enables to spontaneously develop the ordered structures during the solvent evaporation. The mobility of the UT-dictated TIPS-pentacene film (0.202 ± 0.012 cm2/Vs) is found to increase by more than a factor of two compared to that of the isotropic case (0.090 ± 0.032 cm2/Vs). The structural arrangement of the TIPS-pentacene molecules in relation to the mobility enhancement is described within an anisotropic wetting formalism. Our UT-based approach to the mobility enhancement is easily applicable to different classes of soluble organic field-effect transistors by adjusting the geometrical parameters such as the height, the width, and the periodicity of the UT of an inkjet-printed insulator.
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73.61.Ph Polymers; organic compounds
78.66.Qn Polymers; organic compounds
85.30.Tv Field effect devices
68.55.ag Semiconductors
68.03.Fg Evaporation and condensation of liquids
68.08.Bc Wetting
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Photoactive self-assembled monolayers for optically switchable organic thin-film transistors

Michael Salinas and Marcus Halik

Appl. Phys. Lett. 102, 203301 (2013); http://dx.doi.org/10.1063/1.4804595 (4 pages)

Online Publication Date: 20 May 2013

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We investigate the photoconductive and photovoltaic effects in organic thin-film transistors with thin hybrid dielectrics composed of aluminum oxide and self-assembled monolayers (SAMs). By using SAM molecules with an electro-optical functionality tuning of the photoinduced charge transfer at the interface of semiconductor and SAM upon illumination with laser light can be achieved. Control of the threshold voltage by the SAM composition enables the optical operation of the transistors without applying a gate voltage and affects the dynamics of photoinduced charge transfer.
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85.30.Tv Field effect devices
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Spectral dispersion of ultrafast optical limiting in Coumarin-120 by white-light continuum Z-scan

Benoy Anand, Nivya Roy, S. Siva Sankara Sai, and Reji Philip

Appl. Phys. Lett. 102, 203302 (2013); http://dx.doi.org/10.1063/1.4807151 (5 pages)

Online Publication Date: 20 May 2013

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Measurement of the wavelength dispersion of optical limiting in materials can provide invaluable information useful for laser safety device applications. However, it can be a tedious task when conventional tunable laser sources like the optical parametric amplifier are employed for excitation. Here we report the spectral dispersion of ultrafast optical limiting in the laser dye Coumarin-120 in the wavelength region 630 to 900 nm, measured in a single Z-scan, using the white-light continuum as the excitation source. Optical limiting is found to arise from two-photon absorption, and its spectrum agrees very well in shape with the linear absorption spectrum.
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42.55.Mv Dye lasers
42.50.Md Optical transient phenomena: quantum beats, photon echo, free-induction decay, dephasings and revivals, optical nutation, and self-induced transparency
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
07.60.Rd Visible and ultraviolet spectrometers
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Silver nanoparticle facilitated charge generation in tandem organic light-emitting devices

Fei Yan, Rui Chen, H. D. Sun, and Xiao Wei Sun

Appl. Phys. Lett. 102, 203303 (2013); http://dx.doi.org/10.1063/1.4807664 (4 pages)

Online Publication Date: 22 May 2013

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By inserting an ultrathin silver nanoparticles (Ag NPs) layer between n- and p-type layer of charge generation connector, the performance of the tandem organic light emitting device was improved drastically; the driving voltage was lowered and the luminous efficiency was increased significantly. As the common electrode for two emissive units, Ag NP layer sits at the PN heterojunction interface responsible for charge generation and offers more charge carriers under a lower driving voltage. Additionally, the surface plasmonic resonance of Ag NPs also plays an important role on device improvement.
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85.60.Jb Light-emitting devices
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Energy transfer process between exciton and surface plasmon: Complete transition from Forster to surface energy transfer

Arunandan Kumar, Priyanka Tyagi, Ritu Srivastava, D. S. Mehta, and M. N. Kamalasanan

Appl. Phys. Lett. 102, 203304 (2013); http://dx.doi.org/10.1063/1.4806979 (5 pages)

Online Publication Date: 23 May 2013

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The energy transfer process between surface plasmons and excitons was studied by varying the filling fraction of gold (Au) nano-clusters (NCs) and by placing a spacer of different thickness between Au NC and organic semiconductor layer. The intensity enhancement has occurred for 10%-50% filling fractions and 4-14 nm spacer thicknesses. Energy transfer mechanism was found to switch from Forster type to surface type by increase in filling fraction. Transverse electric field for Au NCs was simulated and we observed that for filling fraction <30%, Au NCs behave like 1-dimensional dipole and for >60%, they behave like 2-dimensional dipoles.
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71.35.-y Excitons and related phenomena
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
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Improving light-emitting electrochemical cells with ionic additives

Yulong Shen, Daniel D. Kuddes, Clint A. Naquin, Travis W. Hesterberg, Caroline Kusmierz, Bradley J. Holliday, and Jason D. Slinker

Appl. Phys. Lett. 102, 203305 (2013); http://dx.doi.org/10.1063/1.4807674 (5 pages)

Online Publication Date: 23 May 2013

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Light-emitting electrochemical cells with ionic transition metal complexes (iTMCs) comprising the active layer have great potential for lighting applications, but to date high luminance and fast turn on times have been challenging to attain without significant loss of lifetime. Here, we demonstrate iridium iTMC devices with high luminances of 3000–5000 cd/m2, reduced turn on times, and long lifetimes through incorporation of ionic additives. Furthermore, we show additional reduction of turn on time to seconds through heat processing. We rationalize these results as the enhancement of carrier injection through improved double-layer formation at the electrodes.
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85.60.Jb Light-emitting devices
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