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March 2013

Volume 6, Issue 3

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Disorder-tuned charge transport in organic semiconductors

Feng Xu, Dong Qiu, and Dadong Yan

Appl. Phys. Lett. 102, 083304 (2013); http://dx.doi.org/10.1063/1.4793399 (4 pages)

Online Publication Date: 1 March 2013

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We propose that the polaron transport in organic semiconductors is remarkably tuned by the fluctuation of polarization energy. The tuning effect of energetic fluctuation not only causes a continuous transition from non-Arrhenius to Arrhenius temperature activated charge transport with increasing moderate disorder strengths but also results in a band-like conduction in the low disorder regime which benefits from the enhanced mobilities in shallow trap states. As a result, a unified description of polaron transport is obtained for a set of typical organic semiconductors.
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72.20.Ee Mobility edges; hopping transport
72.20.Fr Low-field transport and mobility; piezoresistance
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.80.Le Polymers; organic compounds (including organic semiconductors)
81.05.Fb Organic semiconductors
71.38.-k Polarons and electron-phonon interactions
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Morphological characterization of plastic solar cells using polarized neutron reflectivity

B. W. Guralnick, B. J. Kirby, C. F. Majkrzak, and M. E. Mackay

Appl. Phys. Lett. 102, 083305 (2013); http://dx.doi.org/10.1063/1.4793423 (4 pages)

Online Publication Date: 1 March 2013

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Thermal annealing is known to significantly improve the photovoltaic performance of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction solar cells. We have used polarized neutron reflectometry to compare the PCBM depth profiles of the same P3HT:PCBM film before and after annealing at conditions know to improve the photovoltaic efficiencies of similarly prepared devices. Contrary to previous findings, annealing is shown to have little effect on the vertical PCBM concentration profile. We attribute the improved efficiency from thermal annealing to increased P3HT crystallinity and/or change in crystal orientation, which inhibits exciton recombination.
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88.40.jr Organic photovoltaics
81.40.Gh Other heat and thermomechanical treatments
82.45.Wx Polymers and organic materials in electrochemistry
88.40.H- Solar cells (photovoltaics)
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Charge modulation infrared spectroscopy of rubrene single-crystal field-effect transistors

R. Uchida, H. Yada, M. Makino, Y. Matsui, K. Miwa, T. Uemura, J. Takeya, and H. Okamoto

Appl. Phys. Lett. 102, 093301 (2013); http://dx.doi.org/10.1063/1.4794055 (5 pages)

Online Publication Date: 4 March 2013

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Polarized absorption spectra of hole carriers in rubrene single crystal field-effect transistors were measured in the infrared region (725–8000 cm−1) by charge modulation spectroscopy. The absorptions, including the superimposed oscillatory components due to multiple reflections within thin crystals, monotonically increased with decreasing frequency. The spectra and their polarization dependences were well reproduced by the analysis based on the Drude model, in which the absorptions due to holes in rubrene and electrons in the gate electrodes (silicon), and multiple reflections were fully considered. The results support the band transport of hole carriers in rubrene.
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85.30.Tv Field effect devices
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Interfacial design for reducing charge recombination in photovoltaics

Shinnosuke Hattori, Weiwei Mou, Pankaj Rajak, Fuyuki Shimojo, and Aiichiro Nakano

Appl. Phys. Lett. 102, 093302 (2013); http://dx.doi.org/10.1063/1.4794983 (5 pages)

Online Publication Date: 8 March 2013

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Key to high power conversion efficiency of organic solar cells is to minimize charge recombination (CR) at electron donor/acceptor interfaces. Here, nonadiabatic quantum molecular dynamics simulation shows how the interfacial structure can be controlled by molecular design at acene/C60 interfaces to suppress CR. Orders-of-magnitude reduction of the CR rate is achieved through drastic modification of interfacial structure by attaching phenyl groups to tetracene. This finding confirms a molecular design principle for efficient organic photovoltaics underlying a recent experimental study.
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88.40.H- Solar cells (photovoltaics)
71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations
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Organic transistor and inverter based on assembly of organic nanowires achieved by optimizing surface morphology

Sarita Yadav, Akanksha Sharma, and Subhasis Ghosh

Appl. Phys. Lett. 102, 093303 (2013); http://dx.doi.org/10.1063/1.4795150 (5 pages)

Online Publication Date: 8 March 2013

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We have demonstrated that assemblies of organic nanowires can be grown on Si and SiO2 substrates by controlling growth parameters. At higher growth temperatures, anisotropic growth dominates over isotropic growth, resulting in surface morphologies consisting of nanowire-like elongated grains. These elongated grains provide better π-π stacking, leading to higher carrier mobility and better performance of organic transistors. Using this approach, we have demonstrated organic inverter using complementary semiconducting materials, p-type copper phthalocyanine and n-type copper hexadecafluoro phthalocyanine. These results indicate that small organic molecule-based nanowires are promising candidates for future organic based microelectronics.
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85.30.Tv Field effect devices
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Electronic transport in sub-micron square area organic field-effect transistors

F. Golmar, P. Stoliar, M. Gobbi, F. Casanova, and L. E. Hueso

Appl. Phys. Lett. 102, 103301 (2013); http://dx.doi.org/10.1063/1.4795014 (5 pages)

Online Publication Date: 12 March 2013

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Scaling down organic field effect transistors to channel areas well below the micron square could improve positively its speed and integration capabilities. Here, we report a careful study of the electronic carrier transport for such nanoscale devices. In particular, we explore the validity of standard analysis for parameters extraction in this size regime. We also study the effect of the large longitudinal electric field and fringe currents, especially their influence on the ON/OFF ratio.
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85.30.Tv Field effect devices
85.35.-p Nanoelectronic devices
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Ligand chemistry of titania precursor affects transient photovoltaic behavior in inverted organic solar cells

Jong Bok Kim, Seokhoon Ahn, Seok Ju Kang, Colin Nuckolls, and Yueh-Lin Loo

Appl. Phys. Lett. 102, 103302 (2013); http://dx.doi.org/10.1063/1.4795287 (4 pages)

Online Publication Date: 14 March 2013

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The chemistry of the precursor from which charge transport layers are formed can significantly affect the device performance of organic solar cells. Here, we compare two common precursors that are used to generate titania electron transport layers and elucidate their effects on the transient characteristics of inverted bulk-heterojunction polymer solar cells comprising poly(3-hexyl hiophene) and [6,6]-phenyl-C61-butyric acid methyl ester. Substituting the isopropyl ligands of titanium isopropoxide with 2-methoxyethanol leads to electron transport layers that require a shorter illumination time to fill shallow electron traps. Furthermore, organic solar cells with titania electron transport layers prepared with such pre-modified titania precursor exhibit higher power-conversion efficiencies stemming from lower trap densities.
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88.40.H- Solar cells (photovoltaics)
88.40.jr Organic photovoltaics
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The impact of substrate temperature on the size and aspect ratio of inkjet-dissolved via holes in thin poly(4-vinyl phenol) dielectric layers

Y. Zhang (张岩), C. Liu, and D. C. Whalley

Appl. Phys. Lett. 102, 103303 (2013); http://dx.doi.org/10.1063/1.4795447 (4 pages)

Online Publication Date: 14 March 2013

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The authors demonstrate the effect of substrate temperature on the relationship between the inkjet-etched via hole size and the number of drops of etchant dispensed. A mechanism for the different via hole size evolution versus the number of drops is proposed. An explanation for the interrelationship between the solvent evaporation rate and polymer re-deposition is presented. The aspect ratio of via holes produced is found to increase with the substrate temperature. Therefore, higher temperatures can be used to reduce the size and increase the aspect ratio of via holes fabricated by inkjet etching.
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85.40.Ls Metallization, contacts, interconnects; device isolation
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Identifying the roles of the excited states on the magnetoconductance in tris-(8-hydroxyquinolinato) aluminum

Y. L. Lei, Q. M. Zhang, L. J. Chen, X. H. Yang, and Z. H. Xiong

Appl. Phys. Lett. 102, 113301 (2013); http://dx.doi.org/10.1063/1.4795937 (4 pages)

Online Publication Date: 18 March 2013

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Magnetoconductance response (MC) in illuminated tris-(8-hydroxyquinolinato) aluminum (Alq3) single layer has been studied to clarify the role of excitons and electron-hole (e-h) pairs on the MC generation. By fitting the MC curves at different bias conditions, the contribution of e-h pairs and excitons to the MC can be differentiated in the same material. The fitting results indicated that hyperfine mixing between the singlet and triplet e-h pairs results in a Lorentzian type MC within hyperfine field and charge reaction of triplet excitons induces a high-field MC following a non-Lorentzian shape, respectively. Moreover, the characteristic field width for the high-field MC is very different at forward and reverse bias regime, implying that the rate strength for the hole/electron-exciton reactions in Alq3 should be treated separately.
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81.05.Fb Organic semiconductors
72.20.My Galvanomagnetic and other magnetotransport effects
71.70.Jp Nuclear states and interactions
71.35.-y Excitons and related phenomena
72.80.Le Polymers; organic compounds (including organic semiconductors)
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Electronic structure disorder, vibronic coupling, and charge transfer excitons in poly(fluorene-alt-bithiophene):fullerene films

I. Riisness and M. J. Gordon

Appl. Phys. Lett. 102, 113302 (2013); http://dx.doi.org/10.1063/1.4796118 (5 pages)

Online Publication Date: 19 March 2013

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Charge transfer processes in conjugated polymer:fullerene blends play an important role in the operation of organic solar cells and organic light emitting diodes. Herein, near-infrared emission from poly-(9,9-dioctylfluorene-alt-bithiophene) (F8T2) and [6,6]-phenyl-C61-butyric acid methyl ester blends was studied and attributed to charge transfer exciton (CTX) recombination. Polymer and CTX emission were monitored via low-temperature/transient photoluminescence and absorbance to elucidate the effects of annealing and composition on donor-acceptor morphology. CTX emission decreased and F8T2 vibronic structure was partially restored due to lower fullerene dispersion and polymer realignment upon annealing. Differences in the temperature-dependent emissions of the polymer singlet vs. CTX were attributed to exciton diffusion in the polymer phase vs. enhanced quenching at the donor-acceptor interface, respectively.
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71.23.-k Electronic structure of disordered solids
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
78.55.-m Photoluminescence, properties and materials
71.35.Pq Charged excitons (trions)
78.66.Sq Composite materials
81.40.Gh Other heat and thermomechanical treatments
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Semitransparent organic solar cells with hybrid monolayer graphene/metal grid as top electrodes

Peng Lin, Wallace C. H. Choy, Di Zhang, Fengxian Xie, Jianzhuo Xin, and C. W. Leung

Appl. Phys. Lett. 102, 113303 (2013); http://dx.doi.org/10.1063/1.4798254 (4 pages)

Online Publication Date: 20 March 2013

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Hybrid transparent monolayer graphene/metal grid is proposed as top electrode of semitransparent organic solar cells. The hybrid electrode using gold grid on flexible polyethylene terephthalate substrate shows very low sheet resistance of 22 ± 3 Ω/ and high optical transmittance of 81.4%, which is comparable to conventional indium tin oxide/glass electrode. Using lamination process, the layer of poly(3,4-ethylenedioythiophene):poly(styrenesulfonate) doped with D-sorbitol plays an important role in the electrical performance of the laminated devices. In addition, the devices show best power convention efficiency of 3.1% and fill factor of 55.0%, which are much better than those of similar graphene-based semitransparent organic solar cells.
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88.40.H- Solar cells (photovoltaics)
88.40.jr Organic photovoltaics
82.45.Fk Electrodes
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Bias stress effect in polyelectrolyte-gated organic field-effect transistors

H. Sinno, S. Fabiano, X. Crispin, M. Berggren, and I. Engquist

Appl. Phys. Lett. 102, 113306 (2013); http://dx.doi.org/10.1063/1.4798512 (5 pages)

Online Publication Date: 21 March 2013

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A main factor contributing to bias stress instability in organic transistors is charge trapping of mobile carriers near the gate insulator-semiconductor interface into localized electronic states. In this paper, we study the bias stress behavior in low-voltage (p-type) polyelectrolyte-gated organic field effect transistors (EGOFETs) at various temperatures. Stressing and recovery in these EGOFETs are found to occur six orders of magntiude faster than typical bias stress/recovery reported for dielectric-gated OFETs. The mechanism proposed for EGOFETs involves an electron transfer reaction between water and the charged semiconductor channel that promotes the creation of extra protons diffusing into the polyelectrolyte.
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85.30.Tv Field effect devices
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Dual roles of MoO3-doped pentacene thin films as hole-extraction and multicharge-separation functions in pentacene/C60 heterojunction organic solar cells

Yan-Hui Lou, Mei-Feng Xu, Zhao-Kui Wang, Shigeki Naka, Hiroyuki Okada, and Liang-Sheng Liao

Appl. Phys. Lett. 102, 113305 (2013); http://dx.doi.org/10.1063/1.4798281 (4 pages)

Online Publication Date: 21 March 2013

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The authors demonstrate a pentacene/C60 heterojunction organic solar cell utilizing MoO3-doped pentacene thin films as an interfacial layer at anode and a multicharge-separation layer between pentacene and C60, respectively. The short-circuit current density and the open-circuit voltage were improved simultaneously compared with the reference device, resulting in an improvement in power conversion efficiency from 0.97% to 2.29%. Absorption spectra measurement, surface morphology analysis, and interfacial evaluation at anode side in the hole-dominant devices were carried out to reveal the functions of MoO3-doped pentacene films in OSCs.
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88.40.jp Multijunction solar cells
82.45.Fk Electrodes
61.72.up Other materials
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Influence of exciton lifetime on charge carrier dynamics in an organic heterostructure

Kanika L. Agrawal, Matthew E. Sykes, Kwang Hyup An, Bradley Frieberg, P. F. Green, and Max Shtein

Appl. Phys. Lett. 102, 113304 (2013); http://dx.doi.org/10.1063/1.4795523 (4 pages)

Online Publication Date: 21 March 2013

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Interactions between charge carriers and excitons, as well as between excitons and optical cavity modes in organic optoelectronic devices are fundamental to their operational limits and chief in preventing the realization of certain phenomena, such as electrically pumped organic lasing. We uncovered a previously unreported phenomenon, wherein optical cavity-modulated exciton decay rate leads to a concomitant modulation in the electrical current of an archetypal NPD/Alq3 organic light emitting device operated in forward bias. The magnitude of this variation is sensitive to the local dielectric environment of the device and is found to be as large as 15%.
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85.60.Jb Light-emitting devices
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The role of Ag nanoparticles in inverted polymer solar cells: Surface plasmon resonance and backscattering centers

Peng Xu, Liang Shen, Fanxu Meng, Jiaqi Zhang, Wenfa Xie, Wenjuan Yu, Wenbin Guo, Xu Jia, and Shengping Ruan

Appl. Phys. Lett. 102, 123301 (2013); http://dx.doi.org/10.1063/1.4798553 (4 pages)

Online Publication Date: 26 March 2013

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Here, we demonstrate silver (Ag) nanoparticles (NPs) existing in molybdenum trioxide (MoO3) buffer layers can improve the photocurrent by surface plasmon resonance (SPR) and backscattering enhancement. The device structure is glass/indium tin oxides/titanium dioxide (TiO2)/regioregular poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester/MoO3/Ag NPs/MoO3/Ag. Compared to the device without Ag NPs, the short current density (Jsc) is improved from 7.76 ± 0.14 mA/cm2 to 8.89 ± 0.12 mA/cm2, and the power conversion efficiency is also enhanced from 2.70% ± 0.11% to 3.35% ± 0.08%. The transmittance spectra show that the device with Ag NPs has weaker transmittance than the device without, which could be attributed to the photons absorption of Ag NPs and light scattering by Ag NPs. The absorption profile of the devices with or without Ag NPs is simulated using finite-difference time-domain methods. It is approved that the Ag NPs result in the absorption improvement by SPR and backscattering enhancement.
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88.40.jr Organic photovoltaics
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
88.40.hj Efficiency and performance of solar cells
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