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2012

Volume 2 (partial)

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Growth and characterization of InxGa1−xN alloys by metalorganic chemical vapor deposition for solar cell applications

Yong Huang, Andrew Melton, Balakrishnam Jampana, Muhammad Jamil, Jae-Hyun Ryou, Russell D. Dupuis, and Ian T. Ferguson

J. Photon. Energy 2, 028501 (Feb 23, 2012); http://dx.doi.org/10.1117/1.JPE.2.028501

Online Publication Date: Feb 23, 2012

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We report on the structural, morphological, and optical qualities of thick InxGa1−xN heteroepitaxial layers grown by metalorganic chemical vapor deposition with various growth conditions for applications in wide-bandgap solar cells. The indium incorporation depending on the growth temperature and indium precursor flow rate and the crystalline and optical qualities of InGaN layers depending on indium mole fraction were investigated. The InGaN layers with high structural and optical qualities were obtained for indium mole fractions, xIn<0.18, whereas significant degradation of material qualities was observed for xIn>0.18, which is associated with the change of growth mode induced by reduced growth temperature. Stokes shift and microscopic and macroscopic phase separations were also studied. Two types of additional phases besides InGaN matrix, i.e., indium-rich InGaN microstructures and macroscopic InGaN domains, were demonstrated to be suppressed by controlling surface adatom mobility and growth rates.
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Natural dye-sensitized solar cells using pigments extracted from Syzygium guineense

Sisay Tadesse, Atakilt Abebe, Yonas Chebude, Ignacio Villar Garcia, and Teketel Yohannes

J. Photon. Energy 2, 027001 (Mar 13, 2012); http://dx.doi.org/10.1117/1.JPE.2.027001

Online Publication Date: Mar 13, 2012

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A study on dye-sensitized solar cells (DSSCs) with extracts of Syzygium guineense as sensitizer is reported for the first time. DSSCs were assembled using natural dye extracted from Syzygium guineense as sensitizer. The photoelectrochemical performance of the quasi-solid state DSSCs based on the ethanol extract dye showed VOC of 0.506 V and JSC of 2.03  mAcm−2; and a power conversion efficiency of 0.51%. UV–vis spectroscopy studies of light absorption of the natural dye were done. Furthermore, the ethanol extract obtained from Syzygium guineense was further purified stepwise through solvent-solvent extraction. The photoelectrochemical performance for the extracts with different solvents indicated that the individual components have synergistic effect in the performance of the DSSC.
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Light-emitting diodes on curved ceramic substrate with primary optics for modification of luminous intensity

An-Chi Wei, Jyh-Rou Sze, and Jyh-Long Chern

J. Photon. Energy 2, 026501 (Apr 20, 2012); http://dx.doi.org/10.1117/1.JPE.2.026501

Online Publication Date: Apr 20, 2012

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Unlike the conventional light-emitting diode (LED) luminaire with a planar substrate and only the forward emission, the proposed LED luminaire with a curved ceramic substrate can perform both the forward and the backward emissions and inherits the merits of good heat-dissipation and low cost from the ceramic substrate. Assembled with the proper primary optics, an illustrated LED bulb has been designed, fabricated and measured. The measured luminous intensity of the LED bulb has shown the backward emission and designed distribution with the beam-angle of 133 deg. To broaden the application areas, such a LED bulb on a curved substrate has been modularized as a streetlight. The measured results of the proposed streetlight have shown that the beam angle of the luminous intensity and the luminaire efficiency are 132 deg and 86%, respectively. Meanwhile, its luminous characteristics also fit the Chinese standard for lighting design of urban roads.
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Photoelectrocatalysis and electrocatalysis on silicon electrodes decorated with cubane-like clusters

Yidong Hou, Billie L. Abrams, Peter C.K. Vesborg, Mårten E. Björketun, Konrad Herbst, Lone Bech, Brian Seger, Thomas Pedersen, Ole Hansen, Jan Rossmeisl, Søren Dahl, Jens K. Nørskov, and Ib Chorkendorff

J. Photon. Energy 2, 026001 (Mar 19, 2012); http://dx.doi.org/10.1117/1.JPE.2.026001

Online Publication Date: Mar 19, 2012

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The influence of the cluster-core unit in cluster-decorated p-Si on photo-electrochemical (PEC) hydrogen evolution has been investigated using a homologous series of cubane-like heterobimetallic sulfide compounds. These compounds stem from the generic cluster structure A3S4 or A3B?startSend?4 (A = W, Mo; B = Co, Cu). We find that the Mo-based (A = Mo) cluster-decorated Si photoelectrodes show higher PEC performance than otherwise equivalent W-based (A = W) cluster-decorated ones. This is consistent with higher electrocatalytic activity of the Mo-based clusters supported on n-Si when measured in the dark. The result of stability tests is that photoelectrodes decorated with clusters without Co (B ≠ Co) can exhibit promising stability, whereas clusters of the structure A3CoS4  (A = W, Mo) yield photoelectrodes that are highly unstable upon illumination. X-ray photoelectron spectroscopy (XPS) results suggest that both oxidation and material loss play a role in deactivation of the A3CoS4 materials. Additionally, we observe that the photocurrent depends linearly on the light intensity in the limiting current region, and the corresponding incident photon to current efficiency (IPCE) may reach approximately 80%. Density functional theory (DFT) calculations of the clusters adsorbed on the hydrogen-terminated Si surface are used to estimate and compare cluster adsorption energies on the surface as well as the H-binding energies, which is a descriptor for electrocatalytic activity.
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OPEN ACCESS

Thermodynamically efficient solar concentrators

Roland Winston

J. Photon. Energy 2, 025501 (Apr 27, 2012); http://dx.doi.org/10.1117/1.JPE.2.025501

Online Publication Date: Apr 27, 2012

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Nonimaging Optics is the theory of thermodynamically efficient optics and as such depends more on thermodynamics than on optics. Hence, I propose a condition for the best design based on purely thermodynamic arguments, which I believe has profound consequences for design of thermal and even photovoltaic systems. This new way of looking at the problem of efficient concentration depends on probabilities, the ingredients of entropy, and information theory while optics in the conventional sense recedes into the background. The paper takes a pedagogical and retrospective approach, as to place the new approach in perspective, it helps to first appreciate what has succeeded in nonimaging optics, and turned some devices [e.g. the compound parabolic concentrator (CPC) cone] into a commodity. I will conclude with some speculative directions on where the new ideas might lead.
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Simulations of solar cell absorption enhancement using resonant modes of a nanosphere array

Jonathan Grandidier, Michael G. Deceglie, Dennis M. Callahan, and Harry A. Atwater

J. Photon. Energy 2, 024502 (May 16, 2012); http://dx.doi.org/10.1117/1.JPE.2.024502

Online Publication Date: May 16, 2012

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We propose an approach for enhancing the absorption of thin-film amorphous silicon solar cells using periodic arrangements of resonant dielectric nanospheres deposited as a continuous film on top of a thin planar cell. We numerically demonstrate this enhancement using three dimensional (3D) full field, finite difference time domain simulations and 3D finite element device physics simulations of a nanosphere array above a thin-film amorphous silicon solar cell structure featuring back reflector and anti-reflection coating. In addition, we use the full field finite difference time domain results as input to finite element device physics simulations to demonstrate that the enhanced absorption contributes to the current extracted from the device. We study the influence of a multi-sized array of spheres, compare spheres and domes, and propose an analytical model based on the temporal coupled mode theory.

Mobile display backlight light guide plates based on slanted grating arrays

Jyrki Kimmel and Tapani Levola

J. Photon. Energy 2, 024501 (Mar 12, 2012); http://dx.doi.org/10.1117/1.JPE.2.024501

Online Publication Date: Mar 12, 2012

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Modern mobile communication devices have user interfaces that are dominated by high-quality displays. Increased multimedia use imposes high demands on the design of display modules, as the content available for mobile use becomes visually richer. Especially the power dissipation of the display can limit the amount of time available for multimedia consumption and interaction. In the mobile liquid-crystal display (LCD), the energy efficiency is determined by the backlight design. State-of-the-art backlights direct white light through a display subpixel array, with high uniformity and up to 90% efficiency in white light output. Therefore, it is difficult to obtain system-level energy savings by improving the backlight design alone. Diffractive backlights have recently been proposed to reduce the power dissipation of the display module, and slanted grating arrays are among the enabling optical features that allow for reduction in power dissipation beyond what is available in the state of the art. By the use of diffractive grating arrays, the required primary color (red, green, or blue) is directed through the LCD subpixel array with geometrical registration, instead of flooding the whole LCD with white light and filtering the primary colors through the subpixel color filter array. We present a study on grating structures based on slanted grating arrays fabricated in high refractive index materials. The grating design principles and grating outcoupling results are provided, and an outline of a new embedded system design is given. Emphasis is on grating array design aspects for future energy-efficient display system design. The results show that savings in power consumption can be expected with advanced display system design based on embedded slanted grating array backlight light guide plates.
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Metastable electrical characteristics of polycrystalline thin-film photovoltaic modules upon exposure and stabilization

Chris Deline, Joseph del Cueto, David S. Albin, and Steve Rummel

J. Photon. Energy 2, 022001 (Mar 02, 2012); http://dx.doi.org/10.1117/1.JPE.2.022001

Online Publication Date: Mar 02, 2012

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The significant features of a series of stabilization experiments conducted at the National Renewable Energy Laboratory (NREL) between May 2009 and the present are reported. These experiments evaluated a procedure to stabilize the measured performance of thin-film polycrystalline cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) thin-film photovoltaic (PV) modules. The current-voltage (I-V) characteristics of CdTe and CIGS thin-film PV devices and modules exhibit transitory changes in electrical performance after thermal exposure in the dark and/or bias and light exposures. We present the results of our case studies of module performance versus exposure: light soaked at 65°C; exposed in the dark under forward bias at 65°C; and, finally, longer-term outdoor exposure. We find that stabilization can be achieved to varying degrees using either light-soaking or dark-bias methods and that the existing IEC 61646 light-soaking interval may be appropriate for CdTe and CIGS modules with one caveat: it is likely that at least three exposure intervals are required for stabilization.
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Spectrally stable deep blue-emitting polyfluorenes containing dibenzothiophene-S,S-dioxide moiety

Lei Ying, Yunhua Xu, Na Li, Jing’ai Yan, Yuanyuan Li, Wei Yang, and Junbiao Peng

J. Photon. Energy 2, 021212 (May 10, 2012); http://dx.doi.org/10.1117/1.JPE.2.021212

Online Publication Date: May 10, 2012

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Efficient deep blue-emitting poly[9,9-bis(2-ethylhexyl)fluorene] derivatives were synthesized via incorporating 2,8-diyl-dibenzothiophene-S,S-dioxide (FSO) moiety into polyfluorene backbone. The photoluminescence (PL) spectra exhibit excellent thermal stability and PL quantum efficiencies distinctly improved upon thermal annealing at various temperatures. Spectrally stable light-emitting diodes were obtained based on resulting copolymers at various driving voltages and operation times. The best device performances were achieved based on copolymer PF-FSO10, which shows a maximum external quantum efficiency of 1.78%, and 2.27% upon thermal annealing in the air, and as high as 3.52% by inserting an additional electron injection layer poly(9,9-bis(6′-(bromilamino)hexyl))fluorene (PFNBr) between the polymer-cathode interface. The stable electroluminescence spectra with CIE coordinates of (0.16, 0.08) and superior device performances indicates that the copolymers could be a promising candidate of deep blue emitters in full-color displays.

Integration of high brightness and low operating voltage green organic light-emitting diodes on complementary metal-oxide semiconductor backplane

Vipul Gohri, Julien Boizot, Henri Doyeux, and Gunther Haas

J. Photon. Energy 2, 021211 (Apr 26, 2012); http://dx.doi.org/10.1117/1.JPE.2.021211

Online Publication Date: Apr 26, 2012

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We report high brightness and low operating voltage efficient green organic light-emitting diodes (OLEDs) based on silicon complementary metal-oxide semiconductor (CMOS) backplane which can be used in applications such as microdisplays. The small molecule top-emitting OLEDs are based on a fluorescent green emitter accompanied by blocking, doped charge transport layers, and an anode fabricated with standard CMOS processes of a 200 mm integrated circuit (IC) fab. The devices are designed to maximize the efficiency under low operative bias so as to fit the limited voltage budget of the IC. This was done by making optical simulations of the device structure, optimizing the organic layer thicknesses and charge injection in the n and p transport layers. The devices reach a current efficacy of 21.6  cd/A at a luminance of 20,000  cd/m2. The devices exhibit a voltage swing as low as 2.95 V for a contrast ratio of 1000. The optimized devices have a high lifetime of 6000 and 8800 h at 5000  cd/m2. Furthermore, aging inside the emission layer is investigated.

Organic light-emitting diodes with roll-up character

Jwo-Huei Jou, Pin-Chu Chen, Ming-Chun Tang, Yi-Shan Wang, Chun-Hao Lin, Szu-Hao Chen, Chien-Chih Chen, Ching-Chiun Wang, and Chien-Tien Chen

J. Photon. Energy 2, 021208 (Mar 19, 2012); http://dx.doi.org/10.1117/1.JPE.2.021208

Online Publication Date: Mar 19, 2012

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Highly efficient organic light-emitting diodes (OLEDs) are strongly demanded for both display and illumination applications. High efficiency would also help prolong the device lifespan. However, many OLEDs encounter significant roll-off problems, leading to undesired low device efficiency at high luminance, which is unfavorable to their commercial realization for lighting. Hence, OLED devices with mild or even little roll-off are highly expected. We have, nevertheless, observed some OLEDs that exhibit roll-up phenomenon, i.e., that their external quantum efficiency (EQE) or current efficiency increases as the applied voltage or brightness is increased. By taking such advantage of device architecture design, OLEDs with an approaching or even above the theoretical limit EQE are obtained at high luminance. In this report, we present how this works for a yellow OLED that exhibits a record-high power efficiency among reported fluorescent yellow OLEDs, a very-low color temperature OLED with a record-breaking efficacy based on the same color-temperature, and a green OLED. Notably, the yellow OLED exhibits an EQE that increases from 5.4 to 6.2% and current efficiency from 16.4 to 18.7  cd/A as the luminance increases from 1000 to 10,000  cd/m2. Plausible mechanisms regarding why roll-up occurs in these devices are discussed.

Improvement of device efficiency in PIN-OLEDs by controlling the charge carrier balance and intrinsic outcoupling methods

Falk Löser, Tilmann Romainczyk, Carsten Rothe, Domagoj Pavicic, Andreas Haldi, Michael Hofmann, Sven Murano, Tobias Canzler, and Jan Birnstock

J. Photon. Energy 2, 021207 (Mar 12, 2012); http://dx.doi.org/10.1117/1.JPE.2.021207

Online Publication Date: Mar 12, 2012

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Organic light-emitting diodes (OLEDs) with high power efficiency are desirable for lighting applications. A prerequisite for high power efficiency is the achievement of low driving voltages, which can be done via charge carrier doping of the transport layers in PIN organic light-emitting diodes (PIN- OLEDs). We have looked at how to combine low voltage with high current efficiency and long lifetime and have therefore investigated different ways of changing the charge carrier balance in PIN-OLEDs. The carrier supply to the emitting layer was adjusted by electron and hole-blocking layers to allow for an undisturbed exciton formation and radiative decay while keeping the total voltage low. In further investigations, we developed highly efficient and stable white PIN bottom emission OLED devices using novel evaporation processable outcoupling enhancement materials inside the PIN OLED stack. In white bottom emission OLEDs the use of this outcoupling enhancement material in combination with a standard micro-lens arrays (MLA) outcoupling film can yield an efficiency enhancement of up to 1.8.
By combining the well-balanced stack with the internal outcoupling approach we achieved a power efficiency of 51.9  lm/W. With additional flat external outcoupling, the power efficiency has been further increased to 60  lm/W. The color coordinates are 0.470/0.429 with color rendering index (CRI) of 87. The 50% lifetime of the OLED could be estimated to 90,000 h.

On the way to wafer-level quantum dot light-emitting diodes

Hélène Bourvon, Stéphanie Le Calvez, Hani Kanaan, Sylvia Meunier Della Gatta, Cécile Philippot, and Peter Reiss

J. Photon. Energy 2, 021206 (Apr 11, 2012); http://dx.doi.org/10.1117/1.JPE.2.021206

Online Publication Date: Apr 11, 2012

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Solution-processed quantum dot light-emitting diodes (QDLEDs) have recently paved the way for low-cost and color-saturated displays. Indeed, quantum dots (QDs) have the appealing property of emitting at a tunable wavelength determined by their chemical composition and diameter. Therefore, QDLED electroluminescent spectra exhibit a narrow bandwidth (FWHM at approximately 30 nm) and QD-based displays offer high color purity and saturation. Moreover, QDs may be deposited by spin coating, inkjet printing, and stamping, methods already implemented for polymer LEDs at the wafer level. We report the optimization steps and performances of our recent developed devices emitting in the visible range. We present pros and cons relative to each method of QD deposition and develop a strategy for realizing wafer-level QDLEDs.

Phosphorescent organic light-emitting diodes for high-efficacy long-lifetime solid-state lighting

Peter A. Levermore, Alexey B. Dyatkin, Zeinab Elshenawy, Huiqing Pang, Jeffrey Silvernail, Emory Krall, Raymond C. Kwong, Ruiqing Ma, Michael S. Weaver, Julie J. Brown, Xiangfei Qi, and Stephen R. Forrest

J. Photon. Energy 2, 021205 (Mar 26, 2012); http://dx.doi.org/10.1117/1.JPE.2.021205

Online Publication Date: Mar 26, 2012

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We report data for a pair of singlestack all-phosphorescent 15×15  cm2 organic light emitting-diode (OLED) light panels with high efficacy, long lifetime, and very low operating temperature: Panel 1 has 62  lm/W efficacy, CRI = 81 and lifetime to LT70 = 18,000  h at 1000  cd/m2, while Panel 2 has 58  lm/W efficacy, CRI = 82 and lifetime to LT70 = 30,000  h at 1000  cd/m2. Operating at a higher luminance of 3000  cd/m2, Panel 2 has 49  lm/W efficacy with lifetime to LT70 = 4000  h. Excellent panel lifetime is enabled by a stable light blue phosphorescent materials system. Panel temperatures are within 10°C of ambient temperature at 3000  cd/m2. Panel 2 was further used as a building block to demonstrate an all-phosphorescent OLED luminaire for under-cabinet lighting applications. Operating at approximately 3000  cd/m2, the luminaire delivers 570 lm with 52  lm/W total system efficacy, CRI = 86 and CCT = 2940  K.

Thermal study of the photonic band gap effect on a resonance energy transfer process

Luis González-Urbina, Javier Pérez-Moreno, Branko Kolaric, and Koen Clays

J. Photon. Energy 2, 021204 (Mar 26, 2012); http://dx.doi.org/10.1117/1.JPE.2.021204

Online Publication Date: Mar 26, 2012

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Optical confinement can induce enhancement of the resonance energy transfer between fluorescent molecules by influencing the interaction between the different available energy levels. We study the energy transfer between a pair of molecules, tris(2-phenylpyridine) iridium and bis(2-methyl-8-quinolinato)-4-phenylphenolate aluminum, which are extensively used in organic light-emitting diode technologies. These molecules have previously shown Förster energy transfer. We present the result of the dipolar coupling of these two molecules embedded in a poly(N-vinylcarbazole) film and inserted in a colloidal photonic crystal. Due to the presence of the photonic band gap, the efficiency of the energy transfer has been improved. A thermal study of the emission under the effect of the photonic band gap has been performed.

Highly efficient white organic light-emitting device using a single emitter

Nathan Bakken, Zixing Wang, and Jian Li

J. Photon. Energy 2, 021203 (Mar 02, 2012); http://dx.doi.org/10.1117/1.JPE.2.021203

Online Publication Date: Mar 02, 2012

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White organic light-emitting devices (WOLEDs) can be fabricated using a simple, low-cost device structure with a single uniformly doped emissive layer. The Pt-17 emitter used in these devices obtains excellent color rendering (CRI = 80) as well as bright white electrophosphoresence (CIE x = 0.37, y = 0.40) by combining efficient monomer and efficient excimer emission as demonstrated by excellent external quantum efficiency (ηext = 15.9%). The Pt-17 based WOLED is also compatible with state-of-the-art charge injection and blocking materials as well as high out-coupling device structures. Application of these existing technologies is expected to extend luminance efficiencies of Pt-17 devices to world-class values (46  lm/?startWend? and 100  lm/?startWend? respectively). In addition to avoiding the difficulty and cost of fabricating more complex device structures, the color of a single-doped device also is uniquely independent of voltage, current density, and age. Molecules like fluorine-free Pt-17 are uniquely positioned to utilize excimer emissions in order to reduce manufacturing costs and provide solutions to satisfy many of the requirements for the next generation of organic solid-state lighting.

High-performance phosphorescent white-stacked organic light-emitting devices for solid-state lighting

Vadim I. Adamovich, Peter A. Levermore, Xin Xu, Alexey B. Dyatkin, Zeinab Elshenawy, Michael S. Weaver, and Julie J. Brown

J. Photon. Energy 2, 021202 (Mar 12, 2012); http://dx.doi.org/10.1117/1.JPE.2.021202

Online Publication Date: Mar 12, 2012

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In this work we report exceptional efficacy, lifetime and color stability for all-phosphorescent white stacked organic light-emitting devices (SOLED®s). We report data for all-phosphorescent white SOLED pixels with two emissive units connected in series by a charge generation layer (CGL). At 3,000  cd/m2, efficacy = 54 to 56  lm/W and lifetime to 70% of initial luminance LT70 ≈ 20,000  h, with color rendering index (CRI) = 82 to 83 and chromaticity meeting Energy Star criteria. We further report data for a 15  cm×15  cm white SOLED panel that operates at 3,000  cd/m2 with 48  lm/W efficacy, CRI = 86 and chromaticity meeting Energy Star criteria. The panel has extremely low operating temperature that is only 6.4°C above ambient, and exceptional lifetime of LT70 ≈ 13,000  h when operated at 3,000  cd/m2.

Theoretical design for carrier-transporting molecules in view of vibronic couplings

Tohru Sato, Katsuyuki Shizu, Kazuyoshi Tanaka, and Hironori Kaji

J. Photon. Energy 2, 021201 (Mar 12, 2012); http://dx.doi.org/10.1117/1.JPE.2.021201

Online Publication Date: Mar 12, 2012

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Vibronic coupling constants (VCC) in aromatic diboranes are calculated. For a carrier-transporting molecule, vibronic couplings should be small. Vibronic couplings, or reorganization energy, can be controlled by applying the vibronic coupling density (VCD) analysis. To suppress vibronic couplings, electron density difference should be localized not on bonds but on atoms. Aromatic diboranes as electron-transporting molecules are designed based on this design principle. Introducing the protecting groups that prevent the boron atoms in the diborane from being attacked suppresses the vibronic couplings. Based on the nonequilibrium Green’s function (NEGF) theory, energy dissipations through a single molecule are also calculated.
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Flexible transparent conductive oxide-less flat and cylinder dye-sensitized solar cells

Jun Usagawa, Takeshi Kogo, Kengo Sadamasu, Shyam S. Pandey, Yuhei Ogomi, and Shuzi Hayase

J. Photon. Energy 2, 021011 (May 17, 2012); http://dx.doi.org/10.1117/1.JPE.2.021011

Online Publication Date: May 17, 2012

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Fabrication and photovoltaic performances for flexible transparent conductive oxide-less (TCO-less) flat and cylinder dye-sensitized solar cells (DSCs) are reported. The cylinder solar cell consists of a porous silicone tube, a protected stainless steel metal mesh (protected SUS mesh) working as a counter electrode, a gel electrolyte sheet, a dye/porous titania layer fabricated on a protected SUS mesh working as a anode, and a thermally shrinkable plastic tube, from the inside to the outside. The thermally shrinkable tube was used to reduce the gap between a cathode and an anode. In addition, a porous silicone tube was used for injecting electrolytes smoothly into the gel electrolyte layer. 5.08% efficiency (FF: 0.68; Voc: 0.68 V; Jsc: 11.07  mA/cm2) was observed. A flexible TCO-less flat DSCs with 6.1% efficiency which was improved by narrowing a gap between two electrodes is also reported.

Efficient organic solar cells based on spray-patterned single wall carbon nanotube electrodes

Sungsoo Kim, Xuhua Wang, Jong Hyuk Yim, Wing Chung Tsoi, Ji-seon Kim, Soonil Lee, and John C. deMello

J. Photon. Energy 2, 021010 (Apr 26, 2012); http://dx.doi.org/10.1117/1.JPE.2.021010

Online Publication Date: Apr 26, 2012

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We report efficient organic bulk heterojunction solar cells, utilizing spray-patterned films of single-wall carbon nanotubes for the transparent electrode. High power conversion efficiencies of up to 3.6% were obtained using a blend of poly(3-hexylthiophene) and phenyl-C61 butyric acid methyl ester as the active layer, comparable to conventional devices utilizing indium tin oxide as the transparent electrode.

Enhancement of poly(3-hexylthiophene)-based solar cell with thermal-evaporated [6,6]-phenyl-C60 butyric acid methyl ester layers

Wei-Hsuan Tseng, Jeng-Yu Wang, Mei-Hsin Chen, Chung-Yen Wang, Hung Lo, and Chih-I Wu

J. Photon. Energy 2, 021009 (Apr 27, 2012); http://dx.doi.org/10.1117/1.JPE.2.021009

Online Publication Date: Apr 27, 2012

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We provide a more compact fabrication process than the prior work using [6,6]-phenyl C61-butyric acid methyl ester (PCBM) buffer layer to enhance the performance of organic solar cells. The device with an extra PCBM layer exhibits an improved power conversion efficiency (PCE) of 20% as compared with the devices with conventional structures. Ultraviolet and x-ray photoemission spectroscopy indicate that thermal annealing treatments result in better mixture structures of PCBM layers with poly(3-hexylthiophene) at the blended surfaces. Therefore, the device with inserted PCBM provides more ideal hetero-structures and vertically graded PCBM distribution to optimize the charge extraction efficiency.

Implementing nanometer-scale confinement in organic semiconductor bulk heterojunction solar cells

Jonathan E. Allen, Kevin G. Yager, Htay Hlaing, Chang-Yong Nam, Benjamin M. Ocko, and Charles T. Black

J. Photon. Energy 2, 021008 (Mar 14, 2012); http://dx.doi.org/10.1117/1.JPE.2.021008

Online Publication Date: Mar 14, 2012

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We discuss fabrication processes for implementing nanometer scale confinement in an organic bulk heterojunction device architecture, including formation and integration of the confining self-assembled template. Such confinement has a beneficial influence on the electrical properties of blended poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester organic solar cell active layers. Crystallization of the blend upon annealing is inhibited by the confining template, which we understand through analysis of x-ray scattering measurements.

Lateral mobility measurements in organic bulk heterojunctions: comparison of field-effect and space charge mobilities

Christopher Lombardo, Eric Danielson, Zi En Ooi, and Ananth Dodabalapur

J. Photon. Energy 2, 021007 (Apr 05, 2012); http://dx.doi.org/10.1117/1.JPE.2.021007

Online Publication Date: Apr 05, 2012

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Measurement of charge transport and recombination parameters in bulk heterojunction (BHJ) materials is of great interest to better understand the science underlying organic solar cells. We discuss the use of space charge limited current measurements under solar illumination to measure the mobility of lateral structures in which charge transport is parallel to the plane of the substrate. We compare these mobilities with mobilities calculated from field-effect measurements of the same structures. Changes in mobility as a function of device length and electric field are consistent with numerical simulations of these lateral structures. Lateral organic photovoltaic structures are potentially very useful in evaluating a number of basic material properties of BHJs including three-dimensional transport.

Thin air-plasma-treated alkali fluoride layers for improved hole extraction in copper phthalocyanine/C70-based solar cells

Teng Xiao, Weipan Cui, Min Cai, Rui Liu, James W. Anderegg, Joseph Shinar, and Ruth Shinar

J. Photon. Energy 2, 021006 (Mar 12, 2012); http://dx.doi.org/10.1117/1.JPE.2.021006

Online Publication Date: Mar 12, 2012

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Alkali fluorides, mostly LiF and CsF, are well-known to improve electron injection/extraction in organic light-emitting diodes (OLEDs) and organic solar cells (OSCs). They are also utilized, though to a lesser extent, for hole injection in OLEDs. Here we demonstrate a new role for such fluorides in enhancing OSCs’ hole extraction. We show that an ultrathin air-plasma-treated alkali fluoride layer between the indium tin oxide (ITO) anode and the active layer in copper phthalocyanine (CuPc)/C70-based OSCs increases the short circuit current by up to ∼ 17% for cells with LiF and ∼ 7% for cells with NaF or CsF. The effects of the fluoride layer thickness and treatment duration were evaluated, as were OSCs with oxidized and plasma-treated Li and UV-ozone treated LiF. Measurements included current voltage, absorption, external quantum efficiency (EQE), atomic force microscopy, and x-ray photoelectron spectroscopy, which showed the presence of alkali atoms F and O at the treated ITO/fluoride surface. The EQE of optimized devices with LiF increased at wavelengths >560  nm, exceeding the absorption increase. Overall, the results indicate that the improved performance is due largely to enhanced hole extraction, possibly related to improved energy-level alignment at the fluorinated ITO/CuPc interface, reduced OSC series resistance, and in the case of LiF, improved absorption.

Optimization of transparent electrode processing conditions for bulk heterojunction solar cells

Yechuan Sun, Annie Ng, Man Kin Fung, Alan Man Ching Ng, Aleksandra B. Djurišić, and Wai Kin Chan

J. Photon. Energy 2, 021005 (Mar 02, 2012); http://dx.doi.org/10.1117/1.JPE.2.021005

Online Publication Date: Mar 02, 2012

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In this work, semi-transparent inverted polymer solar cells with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate; PEDOT:PSS) top electrodes were fabricated by spin-coating process. Poly(3-hexylthiophene; P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) was used as a model material combination for a bulk heterojunction solar cell, because this material combination has been frequently studied, and its properties and performance have been well established. For enhancing the wetting of P3HT:PCBM blend film, different plasma etching conditions were tried. In addition, different high boiling point organic additives were tried to enhance the conductivity of PEDOT:PSS. The performance of solar cells with different fabrication conditions for the top electrode was compared. The best performance was obtained for Ar plasma etching to improve wetting of PEDOT:PSS and the addition of ethylene glycol to improve conductivity.

Dependence of alkyl-substituent length for bulk heterojunction solar cells utilizing 1,4,8,11,15,18,22,25-octaalkylphthalocyanine

Tetsuro Hori, Yasuo Miyake, Tetsuya Masuda, Takeshi Hayashi, Kaoru Fukumura, Hiroyuki Yoshida, Akihiko Fujii, Yo Shimizu, and Masanori Ozaki

J. Photon. Energy 2, 021004 (Mar 02, 2012); http://dx.doi.org/10.1117/1.JPE.2.021004

Online Publication Date: Mar 02, 2012

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Bulk heterojunction organic thin-film solar cells utilizing soluble phthalocyanine derivatives, 1,4,8,11,15,18,22,25-octaalkylphthalocyanine (CnPcH2, n = 6, 7, 9, 10), were investigated. Two broad peaks existing in the external quantum efficiency spectra almost correspond to the Q-band and B-band of CnPcH2. The solar cell utilizing C6PcH2 had the best photovoltaic properties as evidenced by open-circuit voltage, short-circuit current density, fill factor, and energy conversion efficiency. Almost the same photovoltaic properties were observed in the solar cells utilizing C9PcH2 and C10PcH2. We discuss the photovoltaic properties by taking into consideration the crystal structure and electronic state of CnPcH2 from the results of the absorbance spectra, X-ray diffraction measurement, and polarization microscope observation.

Charge separation dynamics at inorganic/organic nanostructured hybrid photovoltaic interfaces

Rainer Eichberger, Christian Strothkämper, Inara Thomas, Thomas Hannappel, Klaus Schwarzburg, Carlo Fasting, Andreas Bartelt, and Robert Schütz

J. Photon. Energy 2, 021003 (Mar 12, 2012); http://dx.doi.org/10.1117/1.JPE.2.021003

Online Publication Date: Mar 12, 2012

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A new Förster resonance energy transfer (FRET) concept for a multichromophoric organic sensitizer in a dye-sensitized solar cell is presented based on a phenyl base body that accommodates the separately linked donor and acceptor moieties. The whole assembly is attached to the surface of a ZnO nanorod electrode via a carboxylic anchor group. FRET activity was demonstrated with UV-VIS measurements, and the charge separation dynamics at the inorganic/organic interface were analyzed with fs transient absorption and terahertz pump/probe for the precursors and fully assembled FRET units.

Solar cells based on semi-random P3HT analogues containing dithienopyrrole: influence of incorporating a strong donor

Beate Burkhart, Petr P. Khlyabich, and Barry C. Thompson

J. Photon. Energy 2, 021002 (Mar 05, 2012); http://dx.doi.org/10.1117/1.JPE.2.021002

Online Publication Date: Mar 05, 2012

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Novel semi-random poly(3-hexylthiophene) (P3HT) based polymers P3HTT-DTP, P3HTT-BTD-DTP, P3HTT-TP-DTP and P3HTT-DPP-DTP containing the strong donor dithienopyrrole (DTP) as well as different acceptors (benzothiadiazole (BTD), thienopyrazine (TP) or diketopyrrolopyrrole (DPP)) were synthesized by Stille copolymerization and their optical, electrochemical, charge transport, and photovoltaic properties were investigated. All polymers (except for the all donor polymer P3HTT-DTP) show considerably broadened absorption compared to P3HT due to the donor–acceptor effect and their multichromophoric nature. The introduction of the strong donor DTP leads to increased HOMO energies and thus decreased open-circuit voltage (Voc) (compared to previously published semi-random polymers) as well as an amorphous character of P3HTT-DTP, P3HTT-BTD-DTP and P3HTT-TP-BTD resulting in low hole mobilities and moderate solar cell efficiencies (0.18% to 0.36%). The exception is P3HTT-DPP-DTP, which is semi-crystalline and has a high hole mobility of 1.4×10−4  cm2 ?startVend?−1 s−1 comparable to P3HT, as well as increased photocurrent (10.7  mA/cm2) due to broad and uniform photoresponse up to 850 nm leading to a promising non-optimized device efficiency of 2.83%.

Charge photogeneration in donor/acceptor organic solar cells

Safa Shoaee, Tracey M. Clarke, Mattias P. Eng, Chun Huang, Stephen Barlow, Eva Espíldora, Juan Luis Delgado, Bert Campo, Seth R. Marder, Martin Heeney, Iain McCulloch, Nazario Martín, Dirk Vanderzande, and James R. Durrant

J. Photon. Energy 2, 021001 (Mar 12, 2012); http://dx.doi.org/10.1117/1.JPE.2.021001

Online Publication Date: Mar 12, 2012

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We focus upon the role of interfacial energetics and morphology in influencing the separation of CT states into dissociated charge carriers. In particular, we undertake transient optical studies of films comprising regioregular poly(3-hexylthiophene) (P3HT) blended with a series of perylene-3,4:9,10-tetracarboxydiimide (PDI) fullerene electron acceptors. For the PDI film series, we observe a close correlation between the PDI electron affinity and the efficiency of charge separation. This correlation is discussed in the context of studies of charge photogeneration for other organic donor/acceptor blend films, including other polymers, blend compositions, and the widely used electron phenyl-C61-butyric acid methyl ester(PCBM). Furthermore, we compare the charge recombination dynamics observed in films comprising P3HT blended with three fullerene derivatives: PCBM and two alternative pyrazolinofullerenes. Transient absorption data indicate that replacement of PCBM with either of the pyrazolinofullerene derivatives results in a transition from nongeminate to monomolecular (geminate) recombination dynamics. We show that this transition cannot be explained by a difference in interfacial energetics. However, this transition does correlate with nanomorphology data that indicate that both pyrazolinofullerenes yield a much finer phase segregation with correspondingly smaller domain sizes than observed with PCBM. Our results therefore provide clear evidence of the role of nanomorphology in determining the nature of recombination dynamics in such donor/acceptor blends.
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Publisher’s Note: Growth and characterization of InxGa1−xN alloys by metalorganic chemical vapor deposition for solar cell applications

Yong Huang, Andrew Melton, Balakrishnam Jampana, Muhammad Jamil, Jae-Hyun Ryou, Russell D. Dupuis, and Ian T. Ferguson

J. Photon. Energy 2, 020101 (Apr 06, 2012); http://dx.doi.org/10.1117/1.JPE.2.020101

Online Publication Date: Apr 06, 2012

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