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Analysis of improved photovoltaic properties of pentacene/C-60 organic solar cells: Effects of exciton blocking layer thickness and thermal annealing
Seunghyup Yoo, William J. Potscavage Jr., Benoit Domercq, Sung-Ho Han,
Tai-De Li, Simon C. Jones, Robert Szoszkiewicz, Dean Levi, Elisa Riedo,
Seth R. Marder, Bernard Kippelen; Sol. State Electron. 51, 1367–1375 (2007).
We report on the photovoltaic properties of organic solar cells based on pentacene and C60 thin films with a focus on their spectral responses and the effect of thermal annealing. Spectra of external quantum efficiency (EQE) are measured and analyzed with a onedimensional exciton diffusion model dependent upon the complex optical functions of pentacene films, which are measured by spectroscopic ellipsometry. An improvement in EQE is observed when the thickness of the bathocuproine (BCP) layer is decreased from 12 nm to 6 nm. Detailed analysis of the EQE spectra indicates that large exciton diffusion lengths in the pentacene films are responsible for the overall high EQE values near wavelengths of 668 nm. Analysis also shows that improvement in the EQE of devices with the thinner BCP layer can be attributed to a net gain in optical field distribution and improvement in carrier collection efficiency. An improvement in open-circuit voltage (VOC) is also achieved through a thermal annealing process, leading to a net increase in power conversion efficiency. Integration of the EQE spectrum with an AM1.5 G spectrum yields a predicted power conversion efficiency of 1.8 ± 0.2%. The increase in VOC is attributed to a significant reduction in the diode reverse saturation current upon annealing.
Ultrafast Optical image Processing Based on Third Hormonic Generation in Organic Thin Films
Canek Fuentes-Hernandez, Shuo-Yen Tseng, Daniel Owens, and Bernard Kippelen; Appl. Phys. Lett. 91, 131110-1 - 131110-1 (2007).

We report on the use of the noncollinear third-harmonic generation in an amorphous polymer film operating in the eye safe and elecommunication compatible near-infrared range to perform ultrafast
all-optical two-dimensional (2D) image processing at 1550 nm using 100 fs pulses. The background-free and nondegenerate outputs at 517 nm are easily spatially filtered and detected with low cost electronic components. We describe this Fourier transform based technique and
demonstrate its application to the classical problem of 2D image recognition.


High-performance and electrically stable C60 organic field-effect transistors
X.-H. Zhang, B. Domercq, and B. Kippelen; Appl. Phys. Lett. 91, 092114-1 - 092114-3 (2007).

The authors report on high-performance C60 organic field-effect transistors fabricated by physical vapor deposition. Electron mobility ranging from 2.7 to 5.0 cm2 /V s was achieved when treating the gate dielectric with divinyltetramethyldisiloxane bis(benzocyclobutene) and depositing C60 at room temperature. The transistors combine threshold voltages near zero, low subthreshold slopes (<0.7 V/decade), on/off current ratios larger than 106, excellent reproducibility, and good electrical stability under prolonged continuous dc bias stress.


Encapsulation of pentacene/C60 organic solar cells with Al2O3 deposited by atomic layer deposition
W. J. Potscavage, S. Yoo, B. Domercq, and B. Kippelen; Appl. Phys. Lett. 90, 253511-1 - 253511-3 (2007).
Organic solar cells based on pentacene/C60 heterojunctions were encapsulated using a 200-nm-thick film of Al2O3 deposited by atomic layer deposition (ALD). Encapsulated devices maintained power conversion efficiency after exposure to ambient atmosphere for over 6000 h, while devices with no encapsulation degraded rapidly after only 10 h of air exposure. In addition, thermal annealing associated with the ALD deposition is shown to improve the open-circuit voltage and power
conversion efficiency of the solar cells.
Variable splitting ratio 2x2 MMI couplers using multimode waveguide holograms
Shuo-Yen Tseng, Canek Fuentes-Hernandez, Daniel Owens, and Bernard Kippelen; Optics Express, 15, 9015-9021 (2007).
Variable power splitting ratio 2×2 MMI couplers using multimode waveguide holograms are analyzed. Theoretical analysis shows that variable splitting ratios can be obtained with surface relief holograms on
MMI couplers with fixed dimensions. Devices with paired-imaging lengths
are designed on a silicon-on-insulator (SOI) platform. Beam propagation
simulations are used to verify a matrix theory analysis and to investigate proposed device performance. Fabrication tolerance of the proposed device is also analyzed.
Integrated organic photovoltaic modules with a scalable voltage output
S. Yoo, W. J. Potscavage, B. Domercq, J. Kim, J. Holt and B. Kippelen; Appl. Phys. Lett. 90, 253511-1 - 253511-3 (2006).
Efficient organic photovoltaic modules and cells have been fabricated based on a blend of poly(3-hexylthiophene) (P3HT) and a soluble C70 derivative, [6,6]-phenyl C71 butyric acid methyl ester (PCBM-70). Power conversion efficiency of P3HT/PCBM-70 cells is estimated at 4.1% for the AM1.5 G condition from the spectra of external quantum efficiency. Integrated photovoltaic modules, in which N- individual P3HT/PCBM-70 cells (N=1–4) are connected in series, exhibit a short-circuit current ISC of 1.4±0.1 mA and an open-circuit voltage VOC of Nx0.621 V with a maximum VOC of 2.48 V for N=4 under simulated solar illumination (AM1.5 G, 85 mW/cm2).
Large-aperture switchable thin diffractive lens with interleaved electrode patterns
Guoqiang Li, Pouria Valley, M. S. Giridhar, David L. Mathine, Gerald Meredith,
Joshua N. Haddock, Bernard Kippelen, and N. Peyghambarian, Appl. Phys. Lett. 89, 141121-141123 (2006).
The authors report on a high-performance large-aperture switchable diffractive lens using nematic liquid crystal that can be used as an adaptive eyewear. The odd- and even-numbered ring electrodes are separated in two layers, avoiding the gaps between the neighboring electrodes and allowing high diffraction efficiency. It is easier to avoid shorts between neighboring conductive electrodes and fabricate lenses with larger aperture and smaller feature size. With a four-level phase modulation, a 15 mm aperture, 2 dpt lens with small aberrations and diffraction efficiency of above 75% could be demonstrated with low operating voltages. The thickness of the liquid crystal is only 5 micrometer. The lens switching time is about 180 ms. The on and off states of the electrically controlled lens allows near and distance vision, respectively. The focusing power of the lens can be adjusted to be either positive or negative. This structure can be extended to higher-level phase modulation with even higher efficiencies.
Exciton Diffusion in Multilayer Thin-Film Pentacene/C60 Photovoltaic Cells: Modeling of Spectral Response
S. Yoo, W. J. Potscavage, B. Domercq, and B. Kippelen; Nonlinear Optics & Quantum Optics, 30, 27-38 (2006).
A model describing exciton diffusion is used to describe the spectral
response of multilayer organic photovoltaic cells based on Pentacene/C60 thin film heterojunctions. The electric field distribution inside a cell is calculated from the complex optical constants of the layers that comprise the cell. Exciton diffusion lengths are estimated in pentacene and C60 by using this model to fit the spectrum of the external quantum efficiency of charge generation. A large exciton diffusion length of ca. 50–70 nm is found for pentacene.
A comprehensive study of short channel effects in organic field-effect transistors
Joshua N. Haddock, Xiaohong Zhang, Shijun Zheng, Qing Zhang, Seth R. Marder, and Bernard Kippelen; Organic Electronics, 7, 45-54 (2006).
We present a comprehensive study of short channel effects in organic field-effect transistors by measuring the electrical characteristics of devices with fixed channel width and varying channel length. Our studies are conducted on a hole transport organic semiconductor, E,E-2,5-bis-{40-bis-(400-methoxy-phenyl)amino-styryl}-3,4-ethylenedioxy-thiophene, that is spin-coated from solution to form bottom contact organic field-effect transistors. Drain–source currents from transistors with a channel length of 50 lm show excellent agreement with the square law equations derived for crystalline Si MOSFETs in both the linear and saturation regimes. As the channel length is incrementally reduced to 1 lm, device characteristics such as saturation regime channel conductance, sub-threshold current and threshold voltage, behave in a manner similar to Si MOSFETs of decreasing channel length. Results of these studies indicate the presence of non-destructive current
punch-through and in addition, behavior similar to channel-length modulation and threshold-voltage roll-off, neither
of which have previously been reported in OFETs.
Switchable electro-optic diffractive lens with high efficiency for ophthalmic applications
Guoqiang Li, David L. Mathine, Pouria Valley, PekkaAyras , Joshua N. Haddock, M. S. Giridhar, Gregory Williby, Jim Schwiegerling, Gerald R. Meredith, Bernard Kippelen, Seppo Honkanen, and Nasser Peyghambarian; PNAS, 103, 182-187 (2006).
Presbyopia is an age-related loss of accommodation of the human
eye that manifests itself as inability to shift focus from distant to near objects. Assuming no refractive error, presbyopes have clear vision of distant objects; they require reading glasses for viewing near objects. Area-divided bifocal lenses are one example of a treatment for this problem. However, the field of view is limited in such eyeglasses, requiring the user to gaze down to accomplish near-vision tasks and in some cases causing dizziness and discomfort. Here, we report on previously undescribed switchable, flat, liquid-crystal diffractive lenses that can adaptively change their focusing power. The operation of these spectacle lenses is based on electrical control of the refractive index of a 5-micrometer-thick layer of nematic liquid crystal using a circular array of photolithographically defined transparent electrodes. It operates with high transmission, low voltage (<2 Vrms), fast response (<1 sec), diffraction efficiency > 90%, small aberrations, and a power-failure-safe configuration. These results represent significant advance in stateof- the-art liquid-crystal diffractive lenses for vision care and other applications. They have the potential of revolutionizing the field of presbyopia correction when combined with automatic adjustable focusing power.
Fullerene based n-type organic thin film transistors
N.J. Haddock, Xiaohong Zhang, B. Domercq and B. Kippelen, Organic Electronics 6, 182-187 (2005).
Significant progress has been made in the area of p-type organic field effect transistors while progress in developing n-type materials and devices has been comparatively lacking, a limiting factor in the pursuit to develop complementary organic electronic circuits. Given the need for n-type organic semiconductors we have carried out studies using two different fullerene molecules, C60 and C70. Here, we report mobilities for C60 ranging from 0.02 cm2/V s up to 0.65 cm2/V s (depending on channel length), and mobilities from 0.003 cm2/V s up to 0.066 cm2/V s for C70. All devices were fabricated with organic films deposited under high vacuum but tested at ambient pressures under nitrogen.
High Charge-Carrier Mobility in an Amorphous Hexaazatrinaphthylene Derivative
Bilal R. Kaafarani, Takeshi Kondo, Junsheng Yu, Qing Zhang, Davide Dattilo, Chad Risko,
Simon C. Jones, Stephen Barlow, Benoit Domercq, Fabrice Amy, Antoine Kahn, Jean-Luc Bre´das, Bernard Kippelen, and Seth R. Marder, J. Am. Chem. Soc. 127, 16358-16359 (2005).
Here we report on a new tris(pentafluorobenzyl ester) derivative, both morphology and the effective charge-carrier mobility of thin films depend on whether pure isomer mixture is used, but we have measured effective mobilities as high as 0.07 cm2 V-1 s-1.

Colourless Molecular Dopants for Low Operating Voltage Nematic Liquid Crystals
J.N. Haddock, B. Kippelen, D. L. Mathine, J. Schmidtke, S. Kumaraswamy, S. R. Marder, O. Kwon, E. Jojer, Jean-Luc Bre´das; Mol. Cryst. Liq. Cryst. 428, 17-32 (2005).

In this paper we present a study of two similar, colorless molecular dopants designed to decrease the operating voltages of nematic liquid crystals by increasing the dielectric anisotropy. Experimental analysis shows that these materials reduced the threshold voltage (voltage associated with the Freedericksz transition) by up to 20%, but via a reduction in the liquid crystal order parameter. To gain insight into the decrease of the order parameter, we used density-functional and single-point MP2 theory to investigate the conformational preferences of our dopants; the calculations suggest that, because of steric interactions, the conformations adopted are rather rigid.

High mobility C60 organic field-effect transistors
J.N. Haddock, B. Domercq, and B. Kippelen; Electron. Lett. 41 (Mar. 31st) 444-446 (2005).

Organic field effect transistors incorporating the electron transport material C 60 as the active semiconductor have been fabricated under high vacuum conditions and tested in an inert nitrogen atmosphere at ambient pressure. The maximum field effect mobility was found to be 0.65 cm 2/Vs, comparable to the state of the art value measured under ultra-high vacuum conditions.


Intensity-dependent equivalent circuit parameters of organic solar cells based on pentacene and C60
S. Yoo, B. Domercq, and B. Kippelen; J. Appl. Phys. 97 (May 15) 103706-1 - 103706-9 (2005).

We present studies of the current-voltage characteristics of organic solar cells based on heterojunctions of pentacene and C 60 as a function of illumination intensity. The photovoltaic response at a given illumination level is parameterized and modeled using the equivalent-circuit model developed for inorganic pn-junction solar cells. Reduction in shunt resistance and increase in diode reverse saturation current density are observed upon increase of the light intensity.


Efficient thin-film organic solar cells based on pentacene/C60 heterojunctions
S. Yoo, B. Domercq, and B. Kippelen; Appl. Phys. Lett. 85 (Nov. 29) 5427-5429 (2004).

We have fabricated an efficient organic photovoltaic cell based on a heterojunction of pentacene and C60. Photocurrent action spectra exhibit broad light-harvesting throughout the visible spectrum with a peak external quantum efficiency (EQE) of 58% at short-circuit condition. Modeling studies indicate that high EQE can be partly attributed to the large exciton diffusion length in the pentacene film as well as efficient dissociation of excitons at the pentecen/C60 heterojunction


Direct imaging through scattering media by use of efficient third-harmonic generation in organic materials
G. Ramos-Ortiz, M. Cha, B. Kippelen, G.A. Walker, S. Barlow, and S.R. Marder; Opt. Lett. 29 (Nov. 1) 2515-2517 (2004).

We report on real-time, time-gated, direct imaging through scattering media with an attenuation of 14 mean-free paths by use of third-harmonic generation in the eye-safe and telecommunication compatible near-IR spectral region (1550 nm).


Photorefractive polymer composite operating at the optical communication wavelength of 1550 nm
S. Tay, J. Thomas, M. Eralp, G. Li, B. Kippelen, S.R. Marder, G. Meredith, A. Schulzgen, and N. Peyghambarian; Appl. Phys. Lett. 85 (Nov. 15) 4561-4563 (2004).

A photorefractive polymer composite sensitized at 1550 nm through direct two-photon absorption has been developed. We show an external diffraction efficiency of 3% in four-wave mixing experiments and perform hologrpahic reconstruction of distorted images utilizing thin-film devices made of this polymer composite.



Ultrafast-pulse diagnostic using third-order frequency-resolved optical gating in organic films
G. Ramos-Ortiz, M. Cha, S. Thayumanavan, J. Mendez, S.R. Marder, and B. Kippelen; Appl. Phys. Lett. 85 (Oct. 18) 3348-3350 (2004).

We report on the diagnostic of ultrafast pulses by frequency-resolved optical gating (FROG) based on strong third harmonic generation (THG) in amorphous organic thin films. The high THG conversion effficiency of these films allows for the characterization of sub-nanojoule short pulses emitting at the telecommunication wavelengths using a loe cost portable fiber spectrometer.


Video-rate compatible photorefractive polymers with stable dynamic properties under continuous operation
C. Fuentes-Hernandez, J. Thomas, R. Termine, G. Meredith, N. Peyghambarian, B. Kippelen, S. Barlow, G.A. Walker, S.R. Marder, M. Yamamoto, K. Cammack, and K. Matsumoto; Appl. Phys. Lett. 85 (Sep. 13) 1877-1879 (2004).

We report on photorefractive polymer composites that exhibit stable dynamic properties under continuous operation. These materials are based on a bis-arylamine side-chain polymer matrix with a low ionization potential. The evolution of the response time for exposures up to 4 kJ/cm2 was studied and compared with that obtained in PVK composites. In the composites, operational stability is combined with video-rate compatible dynamics, large dynamic range at moderate fields, and long shelf lifetimes.