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December 2009

Volume 2, Issue 12

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Low-voltage pentacene organic field-effect transistors with high-κ HfO2 gate dielectrics and high stability under bias stress

Xiao-Hong Zhang, Shree Prakash Tiwari, Sung-Jin Kim, and Bernard Kippelen

Appl. Phys. Lett. 95, 223302 (2009); http://dx.doi.org/10.1063/1.3269577 (3 pages)

Online Publication Date: 2 December 2009

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Low-voltage pentacene organic field-effect transistors are demonstrated (operating voltage of −3 V) with high-κ hafnium dioxide gate dielectrics grown by atomic layer deposition at 200 °C. A high hole mobility of 0.39 cm2/V s with low threshold voltage (<−0.5 V) and low subthreshold slope of 120 mV/dec is achieved with a HfO2 dielectric layer modified with a phosphonic acid based treatment. A high value of 94.8 nF/V s is obtained for the product of mobility and capacitance density. The devices show excellent bias stress stability with or without the phosphonic acid at the HfO2 gate dielectric surface.
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85.30.Tv Field effect devices
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
72.80.Le Polymers; organic compounds (including organic semiconductors)
81.05.Fb Organic semiconductors
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Low-voltage solution-processed n-channel organic field-effect transistors with high-k HfO2 gate dielectrics grown by atomic layer deposition

Shree Prakash Tiwari, Xiao-Hong Zhang, William J. Potscavage, Jr., and Bernard Kippelen

Appl. Phys. Lett. 95, 223303 (2009); http://dx.doi.org/10.1063/1.3269579 (3 pages)

Online Publication Date: 2 December 2009

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High performance solution-processed n-channel organic field-effect transistors based on [6,6]-phenyl C61 butyric acid methyl ester with low operating voltages (3 V) are demonstrated using a high-k hafnium dioxide gate dielectric grown by atomic layer deposition. Devices exhibit excellent n-channel performance with electron mobility values up to 0.14 cm2/V s, threshold voltages of ∼ 0.3 V, current on/off ratios >105, and very low values of subthreshold slope ( ∼ 140 mV/decade).
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85.30.Tv Field effect devices
73.40.Cg Contact resistance, contact potential
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
72.20.Fr Low-field transport and mobility; piezoresistance
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
77.55.D- High-permittivity gate dielectric films
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Rechargeable electronic textile battery

R. Bhattacharya, M. M. de Kok, and J. Zhou

Appl. Phys. Lett. 95, 223305 (2009); http://dx.doi.org/10.1063/1.3269907 (3 pages)

Online Publication Date: 3 December 2009

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We present a simple fabrication process that produces polymeric charge storage devices directly onto a textile surface. By using a coating of poly-(3,4-ethylenedioxythiophene):poly(styrene sulphonic acid) (PEDOT) as a solid electrolytic layer deposited between two woven silver coated polyamide yarns, we have created a simple polymeric textile battery. By studying the device’s surface morphology and chemical composition, we determined that in the presence of an electric field, silver ions from the anode yarn migrate to the cathode yarn through the PEDOT. Despite this migration, the device could be charged and discharged five times without suffering degradation in electrical performance.
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82.47.Cb Lead-acid, nickel-metal hydride and other batteries
82.45.Wx Polymers and organic materials in electrochemistry
82.45.Fk Electrodes
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Bias stress and condensation of mobile trap agents in printed organic transistors

Maki Hiraoka (平岡牧), Toshikazu Yamada (山田寿一), and Tatsuo Hasegawa (長谷川達生)

Appl. Phys. Lett. 95, 223304 (2009); http://dx.doi.org/10.1063/1.3269604 (3 pages)

Online Publication Date: 3 December 2009

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We report the relationship between the device instability and the presence of hydrophilic/hydrophobic boundary that is conventionally utilized to obtain well-defined film patterning in printed electronics devices. Pentacene thin-film transistors composed of inkjet-printed synthetic-metal electrodes exhibit notable bias stress effects whose appearance and disappearance depend critically on the positioning of the hydrophilic/hydrophobic boundary within the channel. The Kelvin probe force microscopy measurements revealed that the bias-stress effect is originated in the temporal evolution of trapped charge densities accumulated at the hydrophilic/hydrophobic boundary, in which the mobile nature of trap agents on the hydrophilic surface take crucial role.
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85.30.Tv Field effect devices
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
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Low-voltage organic transistors and inverters with ultrathin fluoropolymer gate dielectric

M. P. Walser, W. L. Kalb, T. Mathis, and B. Batlogg

Appl. Phys. Lett. 95, 233301 (2009); http://dx.doi.org/10.1063/1.3267055 (3 pages)

Online Publication Date: 7 December 2009

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We report on the simple fabrication of hysteresis-free and electrically stable organic field-effect transistors (OFETs) and inverters operating at voltages <1–2 V, enabled by the almost trap-free interface between the organic semiconductor and an ultrathin (<20 nm) and highly insulating single-layer fluoropolymer gate dielectric (Cytop). OFETs with PTCDI-C13 (N,N′-ditridecylperylene-3,4,9,10-tetracarboxylicdiimide) as semiconductor exhibit outstanding transistor characteristics; very low threshold voltage (0.2 V), onset at 0 V, steep subthreshold swing (0.1–0.2 V/decade), no hysteresis, and excellent stability against gate bias stress. It is gratifying to notice that such small OFET operating voltages can be achieved with the relatively simple processing techniques employed in this study.
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85.30.Tv Field effect devices
81.05.Fb Organic semiconductors
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Determination of gap defect states in organic bulk heterojunction solar cells from capacitance measurements

Pablo P. Boix, Germà Garcia-Belmonte, Udane Muñecas, Marios Neophytou, Christoph Waldauf, and Roberto Pacios

Appl. Phys. Lett. 95, 233302 (2009); http://dx.doi.org/10.1063/1.3270105 (3 pages)

Online Publication Date: 7 December 2009

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Energy distributions [density-of-states (DOS)] of defects in the effective band gap of organic bulk heterojunctions are determined by means of capacitance methods. The technique consists of calculating the junction capacitance derivative with respect to the angular frequency of the small voltage perturbation applied to thin film poly(3-hexylthiophene) (P3HT) [6,6]-phenyl C61-butyric acid methyl ester (PCBM) solar cells. The analysis, which was performed on blends of different composition, reveals the presence of defect bands exhibiting Gaussian shape located at E ≈ 0.38 eV above the highest occupied molecular orbital level of the P3HT. The disorder parameter σ, which accounts for the broadening of the Gaussian DOS, lies within the range of 49–66 meV. The total density of defects results of order 1016 cm−3.
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88.40.H- Solar cells (photovoltaics)
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Trapping-detrapping fluctuations in organic space-charge layers

Anna Carbone, Cecilia Pennetta, and Lino Reggiani

Appl. Phys. Lett. 95, 233303 (2009); http://dx.doi.org/10.1063/1.3271769 (3 pages)

Online Publication Date: 8 December 2009

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A trapping-detrapping model is proposed for explaining the current fluctuation behavior in organic semiconductors (polyacenes) operating under current-injection conditions. The fraction of ionized traps obtained from the current-voltage characteristics, is related to the relative current noise spectral density at the trap-filling transition. The agreement between theory and experiments validates the model and provides an estimate of the concentration and energy level of deep traps.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
71.55.Ht Other nonmetals
77.22.Jp Dielectric breakdown and space-charge effects
72.80.Le Polymers; organic compounds (including organic semiconductors)
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High-efficiency turquoise-blue electrophosphorescence from a Pt(II)-pyridyltriazolate complex in a phosphine oxide host

Unnat S. Bhansali, Evgueni Polikarpov, James S. Swensen, Wei-Hsuan Chen, Huiping Jia, Daniel J. Gaspar, Bruce E. Gnade, Asanga B. Padmaperuma, and Mohammad A. Omary

Appl. Phys. Lett. 95, 233304 (2009); http://dx.doi.org/10.1063/1.3268434 (3 pages)

Online Publication Date: 10 December 2009

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We demonstrate high-efficiency turquoise-blue electrophosphorescence from bis[3,5-bis(2-pyridyl)-1,2,4-triazolato]platinum(II) [Pt(ptp)2] doped in 4-(diphenylphosphoryl)-N,N-diphenylaniline(HM-A1). Organic light-emitting diodes (OLEDs) with 5% Pt(ptp)2:HM-A1 attain peak power efficiency of 61.2 lm/W, versus 40.8 lm/W for analogous devices employing the standard turquoise-blue phosphor bis[(4,6-difluorophenyl)-pyridinato-N,C2′](picolinato)iridium(III) (FIrpic). Devices with x% Pt(ptp)2:HM-A1 exhibit blue emission maxima (λmax ∼ 480 nm) with monotonic increase in excimer/monomer intensity ratio at higher doping levels within 1%–10%, causing color shift toward green and less charge balance. This work represents a significant step toward optimizing future white OLEDs from the same phosphor via combination of low-doped and higher-doped or neat films.
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85.60.Jb Light-emitting devices
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Organic vapor phase deposition for the growth of large area organic electronic devices

Richard R. Lunt, Brian E. Lassiter, Jay B. Benziger, and Stephen R. Forrest

Appl. Phys. Lett. 95, 233305 (2009); http://dx.doi.org/10.1063/1.3271797 (3 pages)

Online Publication Date: 11 December 2009

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We demonstrate that material utilization efficiencies of 50% and deposition nonuniformities ≤ 2.5% are achievable over substrate diameters of 200 mm using a simplified, organic vapor phase deposition (OVPD) system. The OVPD system is used to demonstrate doped electrophosphorescent organic light emitting diodes whose performance is comparable to those grown by vacuum thermal evaporation. Through continuum modeling, we demonstrate that analogous systems whose chamber dimensions are comparable to the substrate width are scalable to substrate sizes of at least 1500×1800 mm2 with deposition nonuniformities between 1.5% and 2.5%. These results indicate that OVPD is useful in the large area deposition of displays, lighting, and other organic electronic devices.
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85.60.Jb Light-emitting devices
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
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Using copper substrate to enhance the thermal conductivity of top-emission organic light-emitting diodes for improving the luminance efficiency and lifetime

Yu-Sheng Tsai, Shun-Hsi Wang, Chuan-Hung Chen, Chien-Lung Cheng, and Teh-Chao Liao

Appl. Phys. Lett. 95, 233306 (2009); http://dx.doi.org/10.1063/1.3272110 (3 pages)

Online Publication Date: 11 December 2009

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The influence of heat dissipation on the performances of organic light-emitting diode (OLED) is investigated by measuring junction temperature and by calculating the rate of heat flow. The calculated rate of heat flow reveals that the key factors include the thermal conductivity, the substrate thickness, and the UV glue. Moreover, the use of copper substrate can effectively dissipate the joule heat, which then reduces the temperature gradient. Finally, it is shown that the use of a high thermal conductivity thinner substrate can enhance the thermal conductivity of OLED and the luminance efficiency as well.
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85.60.Jb Light-emitting devices
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Compact polarization-insensitive biopolymer multimode interference waveguide splitter

J. Zhou, Z. Y. Wang, and E. Y. B. Pun

Appl. Phys. Lett. 95, 243301 (2009); http://dx.doi.org/10.1063/1.3271680 (3 pages)

Online Publication Date: 14 December 2009

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Compact 1×4 multimode interference splitter with tapered sections has been designed and fabricated based on deoxyribonucleic acid-cetyltrimethylammonium biopolymer material. A multistep processing and reactive ion etching technique have been developed to fabricate the device. There is excellent agreement between experimental and theoretical results and the splitter exhibits low excess loss, negligible polarization dependent loss, and well output uniformity for both TE and TM modes at 1550 nm wavelength.
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42.70.Jk Polymers and organics
42.79.Fm Reflectors, beam splitters, and deflectors
42.25.Hz Interference
87.14.gk DNA
87.50.wf Biophysical mechanisms of interaction
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Low-voltage high-performance organic thin film transistors with a thermally annealed polystyrene/hafnium oxide dielectric

Ying Wang, Orb Acton, Guy Ting, Tobias Weidner, Hong Ma, David G. Castner, and Alex K.-Y. Jen

Appl. Phys. Lett. 95, 243302 (2009); http://dx.doi.org/10.1063/1.3268455 (3 pages)

Online Publication Date: 16 December 2009

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Low-voltage pentacene-based organic thin film transistors (OTFTs) are demonstrated with polystyrene (PS)/hafnium oxide (HfOx) hybrid dielectrics. Thermal annealing of PS films on HfOx at 120 °C (PS-120) induces a flatter orientation of the phenyl groups (tilt angle 65°) at the surface compared to PS films without annealing (PS-RT) (tilt angle 31°). The flatter phenyl group orientation leads to better matching of surface energy between pentacene and PS. Pentacene deposited on PS-120 display higher quality thin films with larger grain sizes and higher crystallinity. Pentacene OTFTs with PS-120/HfOx hybrid dielectrics can operate at low-voltage (<3 V) with high field-effect mobilities (1 cm2/V s), high on/off current ratios (106), and low subthreshold slopes (100 mV/dec).
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85.30.Tv Field effect devices
81.05.Fb Organic semiconductors
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Enhancement of electron transport by horizontal molecular orientation of oxadiazole planar molecules in organic amorphous films

Daisuke Yokoyama, Akio Sakaguchi, Michio Suzuki, and Chihaya Adachi

Appl. Phys. Lett. 95, 243303 (2009); http://dx.doi.org/10.1063/1.3274135 (3 pages)

Online Publication Date: 16 December 2009

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To demonstrate the effect of molecular orientation on charge transport characteristics in vacuum-deposited organic amorphous films, the electrical and optical properties of films of two oxadiazole derivatives were investigated. One of them has bulky tert-butyl terminals and forms an isotropic film, whereas the other has planar bipyridyl terminals and forms a highly anisotropic film. The very large optical anisotropy of the latter means that the planar molecules stack horizontally, leading to large overlaps of π-orbitals and more than 30 times higher electron mobility though the geometric and electronic structures of the main conformers of the two derivatives are quite similar.
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73.61.Ph Polymers; organic compounds
68.55.-a Thin film structure and morphology
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
73.50.Dn Low-field transport and mobility; piezoresistance
71.20.Rv Polymers and organic compounds
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Molecular structure and carrier distributions at semiconductor/dielectric interfaces in organic field-effect transistors studied with sum frequency generation microscopy

Ikuyo F. Nakai, Masaaki Tachioka, Akito Ugawa, Tadashi Ueda, Kazuya Watanabe, and Yoshiyasu Matsumoto

Appl. Phys. Lett. 95, 243304 (2009); http://dx.doi.org/10.1063/1.3275805 (3 pages)

Online Publication Date: 17 December 2009

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Infrared-visible sum frequency generation (SFG) microscopy was applied to the observation of semiconductor/dielectric interfaces in organic field-effect transistors fabricated with pentacene films and polyvinyl phenol dielectric layers. SFG intensity at the interface was greatly increased by carrier injection. The large enhancement in SFG intensity enables us to observe clearly the vibrational spectra of molecules and the spatial distributions of charge density at the interface.
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85.30.Tv Field effect devices
FREE

Flexible pentacene organic thin film transistor circuits fabricated directly onto elastic silicone membranes

Ingrid M. Graz and Stéphanie P. Lacour

Appl. Phys. Lett. 95, 243305 (2009); http://dx.doi.org/10.1063/1.3265737 (3 pages)

Online Publication Date: 18 December 2009

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We have fabricated flexible pentacene thin film transistors (OTFTs) and active load inverter circuits directly onto polydimethylsiloxane (PDMS) membranes. Gold electrodes, parylene C gate dielectric, and pentacene films are deposited using a room temperature and all-vapor phase process on spin-coated or cast PDMS. The channel stack on PDMS is wrinkled but crack-free. The devices have good electrical properties with saturation mobilities up to 0.2 cm2/V s, and on/off current ratio of 5×104. The OTFT circuits on PDMS withstand modest repeated bending without electrical failure.
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85.30.Tv Field effect devices
85.40.-e Microelectronics: LSI, VLSI, ULSI; integrated circuit fabrication technology
FREE

Bistable electrical switching and electronic memory effect in a solution-processable graphene oxide-donor polymer complex

Gang Liu, Xiaodong Zhuang, Yu Chen, Bin Zhang, Jinhui Zhu, Chun-Xiang Zhu, Koon-Gee Neoh, and En-Tang Kang

Appl. Phys. Lett. 95, 253301 (2009); http://dx.doi.org/10.1063/1.3276556 (3 pages)

Online Publication Date: 21 December 2009

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A solution-processable and electroactive complex of poly(N-vinylcarbazole)-derivatized graphene oxide (GO-PVK) was prepared via amidation of end-functionalized PVK, from reversible addition fragmentation chain transfer polymerization, with tolylene-2,5-diisocyanate-functionalized graphene oxide. The Al/GO-PVK/ITO device exhibits bistable electrical conductivity switching and nonvolatile rewritable memory effects. Both the OFF and ON states of the memory device are stable under a constant voltage stress of −1 V for up to 3 h, or under a pulse voltage stress of −1 V for up to 108 read cycles, with an ON/OFF state current ratio in excess of 103.
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61.41.+e Polymers, elastomers, and plastics
72.80.Le Polymers; organic compounds (including organic semiconductors)
82.35.Jk Copolymers, phase transitions, structure
81.05.ue Graphene
84.30.Sk Pulse and digital circuits
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Chemically modified ink-jet printed silver electrodes for organic field-effect transistors

Gregory Lewis Whiting and Ana Claudia Arias

Appl. Phys. Lett. 95, 253302 (2009); http://dx.doi.org/10.1063/1.3276913 (3 pages)

Online Publication Date: 21 December 2009

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Modification of ink-jet printed silver source and drain electrodes for organic field-effect transistors (FETs) with the electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) was investigated. Solution-based deposition of F4TCNQ onto ink-jet printed silver electrodes formed using either a nanoparticle-based or a metal organic decomposition ink, lead to a greater than tenfold improvement in FET mobility. Using these modified electrodes with the organic semiconductor 6,13-bis(triisopropylsilylethynyl) pentacene yields devices with a charge carrier mobility up to 0.9 cm2 V−1 s−1 and a current on/off ratio of 106.
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85.30.Tv Field effect devices
72.20.Fr Low-field transport and mobility; piezoresistance
82.45.Fk Electrodes
FREE

Hybrid p-n junction light-emitting diodes based on sputtered ZnO and organic semiconductors

Jong H. Na, M. Kitamura, M. Arita, and Y. Arakawa

Appl. Phys. Lett. 95, 253303 (2009); http://dx.doi.org/10.1063/1.3275802 (3 pages)

Online Publication Date: 21 December 2009

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We fabricated light-emitting hybrid p-n junction devices using low temperature deposited ZnO and organic films, in which the ZnO and the organic films served as the n- and p-type component, respectively. The devices have a rectification factor as high as ∼ 103 and a current density greater than 2 A/cm2. Electroluminescence of the hybrid device shows the mixture of the emission bands arising from radiative charge recombination in organic and ZnO. The substantial device properties could provide various opportunities for low cost and large area multicolor light-emitting sources.
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85.30.Kk Junction diodes
85.60.Jb Light-emitting devices
81.15.Cd Deposition by sputtering
73.40.Ei Rectification
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
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Improved performance of blue phosphorescent organic light-emitting diodes with a mixed host system

Jonghee Lee, Jeong-Ik Lee, Jun Yeob Lee, and Hye Yong Chu

Appl. Phys. Lett. 95, 253304 (2009); http://dx.doi.org/10.1063/1.3276075 (3 pages)

Online Publication Date: 21 December 2009

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We have investigated a relationship between device performances and host properties in the emissive layer (EML) of iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate doped blue phosphorescent organic light-emitting diodes (PHOLEDs). We found that the recombination zone as well as charge carrier injection properties of the EML could be systematically engineered through a mixed host system. Therefore, light-emitting performances of blue PHOLEDs were greatly improved with external quantum and power efficiencies of 20.5% and 43.5 lm/W at a luminance of 56 cd/m2, respectively.
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85.60.Jb Light-emitting devices
FREE

Proton migration mechanism for the instability of organic field-effect transistors

A. Sharma, S. G. J. Mathijssen, M. Kemerink, D. M. de Leeuw, and P. A. Bobbert

Appl. Phys. Lett. 95, 253305 (2009); http://dx.doi.org/10.1063/1.3275807 (3 pages)

Online Publication Date: 22 December 2009

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During prolonged application of a gate bias, organic field-effect transistors show an instability involving a gradual shift of the threshold voltage toward the applied gate bias voltage. We propose a model for this instability in p-type transistors with a silicon-dioxide gate dielectric, based on hole-assisted production of protons in the accumulation layer and their subsequent migration into the gate dielectric. This model explains the much debated role of water and several other hitherto unexplained aspects of the instability of these transistors.
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85.30.Tv Field effect devices
FREE

Low voltage active pressure sensor based on polymer space-charge-limited transistor

Yu-Chiang Chao, Wei-Jen Lai, Chun-Yu Chen, Hsin-Fei Meng, Hsiao-Wen Zan, and Sheng-Fu Horng

Appl. Phys. Lett. 95, 253306 (2009); http://dx.doi.org/10.1063/1.3266847 (3 pages)

Online Publication Date: 23 December 2009

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Low voltage active pressure sensor is realized by vertically stacking a pressure sensitive rubber on a polymer space-charge-limited transistor. The sensor can be turned on and off by modulating the metal-grid base voltage within the range of 3 V. The output current is irrelevant to the pressure as the sensor is off. As the sensor is turned on, the output current values can be used to monitor the pressure. Reversible pressure sensing characteristics is observed below the pressure of 7.11 psi. The response time of the sensor to the pressure is as short as 22 ms.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
85.30.Fg Bulk semiconductor and conductivity oscillation devices (including Hall effect devices, space-charge-limited devices, and Gunn effect devices)
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High-resolution transparent carbon electrodes for organic field-effect transistors patterned by laser sintering

Hiroshi Wada and Takehiko Mori

Appl. Phys. Lett. 95, 253307 (2009); http://dx.doi.org/10.1063/1.3276771 (3 pages)

Online Publication Date: 23 December 2009

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Organic transistors are fabricated by selective laser sintering of carbon films formed from carbon solution. This method achieves low-cost and high-resolution carbon electrodes with the channel length down to 2 μm without using vacuum process and lithography. The resulting bottom-contact pentacene transistor realizes high performance owing to the low carbon/organic interfacial potential. Since the carbon film is as thin as 60 nm, the film is practically transparent.
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85.30.Tv Field effect devices
42.62.-b Laser applications
81.16.Nd Micro- and nanolithography
82.45.Fk Electrodes
81.05.Fb Organic semiconductors
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Double wavelength ultraviolet light sensitive organic photodetector

Fei Yan, Huihui Liu, Wenlian Li, Bei Chu, Zisheng Su, Guang Zhang, Yiren Chen, Jianzhuo Zhu, Dongfang Yang, and Junbo Wang

Appl. Phys. Lett. 95, 253308 (2009); http://dx.doi.org/10.1063/1.3266861 (3 pages)

Online Publication Date: 23 December 2009

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The authors demonstrate an organic ultraviolet (UV) photodetector (PD) device in which 1,3,5-tris(3-methylphenyl-phenylamino)-triphenyamine and 1,3,5-tris(N-phenylbenzimidazol-2-yl)-benzene were used as the electron donor and acceptor, respectively. The PD diode offers responses of 75.2 and 22.5 mA/W as the 365 and 330 nm UV light with 1.0 mW/cm2 intensities illuminate the PD diode through anode and cathode sides, respectively. It is interesting that only the planar heterojunction structure diode can provide the special response feature while bulk-heterojunction device could not do it. The working mechanism of the PD diode was also discussed in detail.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
71.55.-i Impurity and defect levels
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Hierarchical assembly of light-emitting polymer nanofibers in helical morphologies

Stefano Pagliara, Andrea Camposeo, Roberto Cingolani, and Dario Pisignano

Appl. Phys. Lett. 95, 263301 (2009); http://dx.doi.org/10.1063/1.3275727 (3 pages)

Online Publication Date: 28 December 2009

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Single electrospun nanofibers of light-emitting conjugated polymers hierarchically assemble at nano- to macroscopic lengthscales in various helical morphologies. At nanoscopic lengthscales, molecular chains follow the microscopic assembly, prevalently aligning along the fiber dynamic axis, as demonstrated by polarized photoluminescence spectroscopy. The role of molecular weight in the resulting assembling and optical properties is highlighted and discussed. Nanofibers based on the heaviest polymer exhibit the most stretched helical geometries and the highest suppression of the excitonic energy migration, resulting in the most blue-shifted photoluminescence with respect to thin films.
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61.41.+e Polymers, elastomers, and plastics
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
78.55.Kz Solid organic materials
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A near infrared organic photodiode with gain at low bias voltage

I. H. Campbell and B. K. Crone

Appl. Phys. Lett. 95, 263302 (2009); http://dx.doi.org/10.1063/1.3279133 (3 pages)

Online Publication Date: 28 December 2009

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We demonstrate an organic photodiode with near infrared optical response out to about 1100 nm with a gain of ∼ 10 at 1000 nm under 5 V reverse bias. The diodes employ a soluble naphthalocyanine with a peak absorption coefficient of ∼ 105 cm−1 at 1000 nm. In contrast to most organic photodiodes, no exciton dissociating material is used. At zero bias, the diodes are inefficient with an external quantum efficiency of ∼ 10−2. In reverse bias, large gain occurs and is linear with bias voltage above 4 V. The observed gain is consistent with a photoconductive gain mechanism.
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85.60.Dw Photodiodes; phototransistors; photoresistors
81.05.Fb Organic semiconductors
72.40.+w Photoconduction and photovoltaic effects
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