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5 Mar 2012

Volume 100, Issue 10, Articles (10xxxx)

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Appl. Phys. Lett. 100, 101903 (2012); http://dx.doi.org/10.1063/1.3673327 (3 pages)

Michael Ian Lapsley, Anaram Shahravan, Qingzhen Hao, Bala Krishna Juluri, Stephen Giardinelli, Mengqian Lu, Yanhui Zhao, I-Kao Chiang, Themis Matsoukas, and Tony Jun Huang
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Direct probing of selective electron and hole accumulation processes along the channel of an ambipolar double-layer field-effect transistor by optical modulation spectroscopy

Le Zhang, Dai Taguchi, Takaaki Manaka, and Mitsumasa Iwamoto

Appl. Phys. Lett. 100, 103301 (2012); http://dx.doi.org/10.1063/1.3692581 (4 pages) | Cited 1 time

Online Publication Date: 6 March 2012

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By using optical modulation spectroscopy, the electronic state and selective carrier accumulation processes in the channel region of unipolar and ambipolar organic field effect transistors (OFETs) were directly probed. The modulated spectra of unipolar single-layer pentacene and C60 OFETs corresponding well with their I–V characteristics were understood in terms of the injected carrier induced effect. On the other hand, for the double-layer C60/pentacene OFETs, it is indicated that the modulated spectra under both hole and electron accumulation cases generated mainly from the pentacene layer but with different characteristics, implying that the injected carriers accumulated at spatially separated locations.
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85.30.Tv Field effect devices
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Electron depletion and accumulation regions in n-type copper-hexadecafluoro-phthalocyanine and their effects on electronic properties

Haibo Wang, Zengtao Liu, Ming Fai Lo, Tsz Wai Ng, Donghang Yan, and Chun-Sing Lee

Appl. Phys. Lett. 100, 103302 (2012); http://dx.doi.org/10.1063/1.3691257 (3 pages)

Online Publication Date: 6 March 2012

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We investigated interfacial electronic structures of organic/inorganic heterojunctions formed between copper-hexadecafluoro-phthalocyanine (F16CuPc) and molybdenum trioxide or lead monoxide by ultraviolet and x-ray photoemission spectroscopies. Obvious energy level bending in F16CuPc shows the formation of electron depletion and accumulation regions in its contact with molybdenum and lead oxides, respectively. Effects of these depletion/accumulation zones were studied via electronic characteristics of field-effect transistors constructed with these heterojunctions as active layers. Finally, a summary about the heterojunction including organic semiconductor (n-type and p-type) and metal oxide was given, which demonstrated that organic semiconductors can form various space charge regions like conventional inorganic semiconductors.
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73.21.Ac Multilayers
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
77.22.Jp Dielectric breakdown and space-charge effects
78.40.-q Absorption and reflection spectra: visible and ultraviolet
79.60.Bm Clean metal, semiconductor, and insulator surfaces
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Efficient polymer solar cells based on light-trapping transparent electrodes

Ziyang Hu, Jianjun Zhang, and Ying Zhao

Appl. Phys. Lett. 100, 103303 (2012); http://dx.doi.org/10.1063/1.3691906 (4 pages) | Cited 2 times

Online Publication Date: 6 March 2012

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Highly efficient and cost-effective polymer solar cells (PSCs) fabricated on textured fluorine doped transparent conductive (FTO) electrodes are achieved. Such electrodes with rough surface in combination with reflective back contacts provide efficient light-trapping by light scattering and multiple reflections to increase the path length of the light. Due to the increased light absorption in active layer, the resultant PSCs base on polymer:fullerene system show 10% and 8% improvement in short current density and efficiency, respectively, compared with the reference cell based on the flat electrodes.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
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Top-to-bottom optimization of the optical performance of the tandem organic solar cells with thin metal film as interlayer

Yongbing Long (龙拥兵), Liang Shen (沈亮), Shenping Ruan (阮圣平), Wenjuan Yu (于文娟), Yi Wang (王忆), Qingguang Zeng (曾庆光), and Jianyi Luo (罗坚义)

Appl. Phys. Lett. 100, 103304 (2012); http://dx.doi.org/10.1063/1.3692597 (4 pages)

Online Publication Date: 7 March 2012

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Top-to-bottom optimization is developed to maximize the absorption for tandem organic solar cells with thin Ag interlayer connecting two subcells. By redshifting the cavity modes of the microcavity between the Ag interlayer and the top electrode, the absorption spectrum of the top cell can be shifted to the near-infrared wavelength range where the bottom cell has weak absorption. Correspondingly, subcells with highly complementary absorption spectrum are achieved, and there is an improvement of 17.8% in the total absorbed photons for the tandem device. Additionally, it is revealed that high transparency is not an essential property for the interlayer in tandem devices.
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88.40.jr Organic photovoltaics
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Simultaneous enhancement of charge transport and exciton diffusion in single-crystal-like organic semiconductors

Jianbing Yang, Feng Zhu, Bo Yu, Haibo Wang, and Donghang Yan

Appl. Phys. Lett. 100, 103305 (2012); http://dx.doi.org/10.1063/1.3692106 (4 pages) | Cited 2 times

Online Publication Date: 7 March 2012

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The charge transport and exciton diffusion have been simultaneously enhanced in the highly ordered and oriented zinc phthalocyanine (ZnPc) films which are fabricated by weak epitaxy growth method compared to the normal ZnPc films (charge mobility from 4 × 10−5 cm2/Vs to 1.1 × 10−4 cm2/Vs and exciton diffusion length from 9 nm to 16 nm). The temperature dependent current-voltage (I-V) characteristics are utilized to investigate the charge transport mechanism, which reveal the clear mechanism and the role of traps in different films. These results reveal the direct relationship between traps properties and transport behavior of both exciton and charge.
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71.35.-y Excitons and related phenomena
66.30.-h Diffusion in solids
68.55.-a Thin film structure and morphology
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Mode discretization in an organic microcavity including a perforated silver layer

C. Reinhardt, R. Brückner, J. Haase, M. Sudzius, S. I. Hintschich, H. Fröb, V. G. Lyssenko, and K. Leo

Appl. Phys. Lett. 100, 103306 (2012); http://dx.doi.org/10.1063/1.3690053 (4 pages)

Online Publication Date: 8 March 2012

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Two optical Tamm plasmons and a discretized microcavity state are observed simultaneously in an organic microcavity by angle-resolved photoluminescence spectroscopy. The Tamm plasmons form as a result of a 40 nm silver layer placed between the bottom distributed Bragg reflector and the λ/2 cavity layer. This silver layer is perforated by round holes of a few microns size, generating optical mesas from which discretized microcavity states are observed concurrently. The discretization and the intensity of the different states are studied as a function of angle and hole diameter and compared to analytical calculations.
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42.79.Bh Lenses, prisms and mirrors
42.82.-m Integrated optics
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
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High brightness alternating current electroluminescence with organic light emitting material

Ajay Perumal, Björn Lüssem, and Karl Leo

Appl. Phys. Lett. 100, 103307 (2012); http://dx.doi.org/10.1063/1.3692776 (4 pages) | Cited 3 times

Online Publication Date: 8 March 2012

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We report on an alternating current (AC) organic electroluminance (EL) device with high brightness of over 1500 cdm−2 and very low threshold voltage for the onset of AC luminance (10 V). The device consists of organic layers enclosed in a pair of either silicon dioxide (SiO2) or hafnium dioxide (HfO2) insulating dielectric layers. We observe internal charge carrier generation and luminance due to charge carrier recombination within the device under the AC drive without charge injection. We compare AC-EL demonstrated for the two devices and show that the threshold voltage for the onset of luminance can be significantly lowered using high dielectric constant (κ) HfO2 insulating layers. Further, we discuss the limitations of internal charge carrier generation and the device performance supported with the results from time resolved measurements.
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85.60.Jb Light-emitting devices
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