Michael F. Durstock Air Force Research Laboratory
Anvar Zakhidov University of Texas-Dallas
Michael Graetzel Ecole Polytechnique Federale de Lausanne
Lynne A. Samuelson U. S. Army RDECOM
Z1: Polymer-Based Devices I
Tuesday PM, April 10, 2007
Room 2009 (Moscone West)
9:30 AM - **Z1.1
Challenges in Flexible Organic Phovoltaics.
Christoph Brabec 1 , J. Hauch 1 , S. Choulis 1 , P. Schilinsky 1 , E. Zeira 1 , I. Sokolik 1 Show Abstract
1 , Konarka Technologies Inc., Lowell, Massachusetts, United States
Polymer solar cell materials in the laboratory are steadily improving their light conversion efficiencies. Efficiencies of 5% can be achieved, which is regarded as the gate performance for market entry. Besides efficiency, costs and lifetime are as important and equally challenging, especially for a technology on flexible substrates. The presentation gives an outlook on the challenges to overcome transferring the organic photovoltaic technology to flexible substrates. The water and oxygen sensitivity is one issue, further issues are the development of a set of electrodes compatible to the flex requirements. The differences in performance for glass based and flex based solar cells will be analyzed in detail.
10:00 AM - Z1.2
P3HT:PCBM Bulk Heterojunction Solar Cells: Morphological And Electrical Characterization And Performance Optimization.
Tom Aernouts 1 , Peter Vanlaeke 1 2 3 , Ilse Haeldermans 2 3 , Jan D'Haen 2 3 , Jean Manca 2 3 , Paul Heremans 1 , Jef Poortmans 1 Show Abstract
1 MCP, IMEC, Leuven Belgium, 2 IMOMEC, IMEC, Diepenbeek Belgium, 3 Institute for Materials Research, Hasselt University, Diepenbeek Belgium
The performance of organic solar cells based on the blend of regioregular poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) is strongly influenced by the morphology of the active layer on the nanoscale level. X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) measurements show that ordering of P3HT plays a key role in optimizing the photovoltaic performance. It is demonstrated that the natural tendency of regioregular P3HT to crystallize is disturbed by the addition of PCBM. The crystallinity of the photo-active blend is typically restored by an annealing procedure resulting in improved device performance, characterized by a spectral broadening of the optical absorption.The morphological changes upon annealing of the P3HT:PCBM blends are accompanied by electrical changes as shown in charge carrier mobility measurements. Space-charge limited current measurements have been performed in hole-only devices with various P3HT:PCBM blend ratios. The mobility before and after annealing is compared and from temperature dependent measurements the width of the density of states distribution (DOS) is determined. The hole mobility in pristine P3HT remains practically unaffected by the annealing treatment. The as-produced P3HT:PCBM blends on the other hand, with a more disordered P3HT phase, have a much lower hole mobility. Annealing is capable of increasing the P3HT ordering with as a result an orders of magnitude larger hole mobility, approaching the value found in pristine P3HT. The DOS bandwidths are affected similarly. In the as-produced blend films a value of 100 meV is found, larger than in the annealed films, there reaching a value around 70 meV similar as in pristine P3HT.Variation of the processing solvent demonstrated however that an optimized morphology and charge transport situation can also be obtained without an additional annealing step. It is shown that in that case the as-produced active layer has already a favorable crystalline morphology. We argue that the high boiling point of the solvent plays an important role in this by influencing the evaporation speed during deposition of the photo-active blend. Further proof is delivered that indeed slowing down the evaporation speed can beneficially influence the solar cell performance. Power conversion efficiency over 4% has been achieved in this way.
10:15 AM - Z1.3
Self Assembly of a Donor/Acceptor Block Copolymer and Its Effect to Photovoltaic Property
Sam-Shajing Sun 1 , Cheng Zhang 1 , Soobum Choi 1 , Kang Seo 1 , Carl Bonner 1 Show Abstract
1 Center for Materials Research, Norfolk State University, Norfolk, Virginia, United States
Future polymeric or ‘plastic’ photovoltaic materials and devices are attractive for solar energy conversion applications where large area, low cost, lightweight, and flexible shape are desired. However, the photoelectric power conversion efficiencies of currently reported organic/polymeric photovoltaic materials are still relatively low (typically less then 5%) compared to inorganic crystalline photovoltaic materials (typically over 15%). This low efficiency photoelectric conversion in organics can be attributed mainly to the three major losses including the “photon loss”, the “exciton loss” and “carrier loss” due to improper materials frontier orbital levels and poor spatial morphologies. In this presentation, the optimization approaches of polymeric solar cells in spatial/morphology regime will be discussed. For instance, a ‘tertiary’ block copolymer supra-molecular nano structure has been designed, and a series of –DBAB- type of block copolymer, where D is a conjugated donor block, A is a conjugated acceptor block, and B is a non-conjugated and flexible bridge unit, have been synthesized, characterized, and preliminarily examined for photovoltaic applications. In comparison to simple donor/acceptor (D/A) blend system having the same energy levels, -DBAB- block copolymer exhibited much better photoluminescence (PL) quenching, and better photovoltaic properties (Jsc of 0.058 mA/cm2 vs. 0.017 mA/cm2, and Voc of 1.10 volt vs. 0.14 volt under identical conditions in a not yet optimized device). Morphological studies revealed molecular self assembly ordered pattern in the block copolymer thin film under simple thermal annealing. We attributed the improved photovoltaic property to an improvement in spatial regime that results in improved charge carrier generation and transportation.
10:30 AM - Z1.4
Optimizing Materials for Bulk-Heterojunction Polymer:Fullerene Photovoltaics.
Kevin Sivula 1 , Barry Thompson 1 , Scott Backer 1 , David Kavulak 1 , Bumjoon Kim 1 , Jean Frechet 1 Show Abstract
1 Departments of Chemistry and Chemical Engineering, University of California, Berkeley, Berkeley, California, United States
10:45 AM - Z1.5
Development of Conjugated Polymers for use in Polymer-Polymer Bulk-Heterojunction Photovoltaic Devices.
Barry Thompson 1 , Kevin Sivula 2 , David Kavulak 1 , Jean Frechet 1 2 Show Abstract
1 Chemistry, University of California, Berkeley, Berkeley, California, United States, 2 Chemical Engineering, University of California, Berkeley, Berkeley, California, United States
In an effort to optimize the performance of conjugated polymer based solar cells we have focused on the structural design of polymers for use in all-polymer photovoltaic devices with bulk-heterojunction architectures. This general approach is attractive as the utilization of complementary electron donor and electron acceptor polymers offers significant advantages over the polymer-fullerene platform that has become ubiquitous in the field. These advantages include the ability to design electron-donating polymers with electronic characteristics free of the constraints of a complementary fullerene acceptor. Further, the specific design of complementary electron accepting and transporting polymers allows optimization of device open circuit voltage, balanced charge transport, and the absorption of the maximum amount of light over the greatest breadth of the solar spectrum. Toward these ends, we present here the synthesis of a family of novel, soluble donor and acceptor polymers and the fundamental characterization of the electronic properties of these materials. Electron donating and hole transporting materials based on polythiophenes and poly(phenylene vinylene)s are used in combination with electron accepting and transporting polymers based on either cyanovinylene linkages or the use of electron deficient aromatic heterocycles. Within this context, donor and acceptor polymers that exhibit fully conjugated backbones, discrete conjugation length, and fully nonconjugated backbones are discussed relative to application as complementary donor-acceptor pairs in photovoltaic devices.
11:30 AM - **Z1.6
Spatial Fourier Transform Analysis of the Morphology of Bulk Heterojunction Materials Used in ``Plastic" Solar Cells.
Alan Heeger 1 , Wanli Ma 1 Show Abstract
1 , UCSB, Santa Barbara, California, United States
12:00 PM - Z1.7
Chemically Fixed Polymer Homojunction Photovoltaic Cells.
Janelle Leger 1 , Glenn Bartholomew 1 Show Abstract
1 Chemistry, University of Washington, Seattle, Washington, United States
12:15 PM - Z1.8
Degradation Kinetics of Polymer/fullerene Bulk Heterojunction Solar Cells.
Sabine Bertho 1 , Wouter Moons 1 , Ilse Haeldermans 1 , Ann Swinnen 1 , Laurence Lutsen 2 , Jean Manca 1 2 , Dirk Vanderzande 1 2 Show Abstract
1 Institute for Materials Research, Hasselt University, Diepenbeek Belgium, 2 IMOMEC, IMECvzw, Diepenbeek Belgium
12:30 PM - Z1.9
Air-stable Polymer Solar Cells
Kwanghee Lee 1 , Jin Young Kim 1 , A. Heeger 1 Show Abstract
1 Center for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, California, United States
Despite promising expectations of technological impact, electronic devices based on semiconducting and metallic polymers are not yet utilized in large scale commercial applications. The thin film form factor of polymer devices (thicknesses of approximately 100 nm) inevitably leads to vulnerability to the diffusion of oxygen and water vapor into the active layers. Moreover, most semiconducting polymer materials degrade when exposed to humidity and/or oxygen, and photo-oxidation can be a serious problem. Here, we report an innovative approach that significantly extends the lifetime of polymer-based solar cells. By introducing a solution-based titanium oxide (TiOx) layer between the active layer and the aluminum cathode in polymer solar cells, we have demonstrated devices with excellent air stability and with enhanced performance. The TiOx layer acts as a shielding and scavenging layer which prevents the intrusion of oxygen and humidity into the electronically active polymers, thereby improving the lifetime of unpackaged devices exposed to air by nearly two orders of magnitude.
12:45 PM - Z1.10
Microwave Annealing Processes in Polymer Photovoltaic Devices.
Fang-Chung Chen 1 , Chu-Jung Ko 2 , Yi-Kai Lin 1 Show Abstract
1 Department of Photonics and Display Institute, National Chiao Tung University, Hsinchu Taiwan, 2 Department of Photonics & Institute of Electro-optical Engineering, National Chiao Tung University, Hsinchu Taiwan
Z2: Polymer-Based Devices II
Tuesday PM, April 10, 2007
Room 2009 (Moscone West)
2:30 PM - **Z2.1
A Library of Fullerene Derivatives for Application in Bulk Heterojunctions.
Jan Hummelen 1 Show Abstract
1 Molecular Electronics, Materials Science Centre Plus, University of Groningen, Groningen Netherlands
Since the introduction of the bulk heterojunction principle in 1995, the combination of fullerene derivatives with conjugated polymers has been extremely fruitful in materials science research and applications. The bulk heterojunction is a nanostructured material in which an electron donor and an acceptor component are conceptually present in such a way that the morphology resembles that of two bi-continuous interpenetrating networks. The desired domain size of each component may be limited by the application (for example, in a solar cell). Most efficient donor-acceptor combinations have been realized using conjugated polymers (as donors) in combination with fullerene derivatives (as the acceptors). In order to obtain an optimal morphology and optimal (photo)electrical behavior of the blend with a specific conjugated polymer, the structure of the fullerene derivative can be varied substantially. Here, we describe the range of fullerene derivatives that we have made available for combining with donor materials. Solar cells, in which a thienyl analog of PCBM is used in combination with regioregular P3HT, are presented. These cells are more efficient than the previous PCBM-based ones by 0.7% absolute power conversion efficiency.
3:00 PM - Z2.2
Integrated Organic Photovoltaic Modules with a Scalable Voltage Output.
William Potscavage 1 , Seunghyup Yoo 1 , Benoit Domercq 1 , Jungbae Kim 1 , Joe Holt 1 , Bernard Kippelen 1 Show Abstract
1 School of Electrical and Computer Engineering and Center for Organic Photonics and Electronics (COPE), Georgia Institute of Technology, Atlanta, Georgia, United States
Solar cells based on organic small molecules and conjugated polymers are being developed at a rapid pace in hope of thin, lightweight, and low-cost alternatives to conventional inorganic solar cells. In order to demonstrate their full commercial potential, however, scalable organic photovoltaic module technologies must be developed at the same time so that one can accommodate various needs of practical applications. Here, we will report that efficient organic solar cells can be fabricated based on a mixture of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C71 butyric acid methyl ester (PCBM-70), which was relatively rarely used when compared to the more common electron acceptor [6,6]-phenyl C61 butyric acid methyl ester (PCBM-60). Using the same mixture, we will report on the fabrication and characterization of integrated organic photovoltaic modules with a scalable voltage output of VOC = N × 0.621 V with a maximum VOC of 2.48 V for N = 4 without short-circuit current reduction, under simulated solar illumination (AM1.5 G, 85 mW/cm2).
3:15 PM - Z2.3
Photovoltaic Performance of Solution Processed Oligoacene:fullerene Bulk Heterojunctions.
Matthew Lloyd 1 , Alex Mayer 1 3 , John Anthony 2 , George Malliaras 1 Show Abstract
1 Department of Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 3 Department of Materials Science and Engineering, Stanford Univeristy, Palo Alto, California, United States, 2 Department of Chemistry, University of Kentucky, Lexington, Kentucky, United States
Organic photovoltaics comprised of small molecules have demonstrated power conversion efficiencies as high as 4.2% in a layered device structure. In order to overcome low exciton diffusion lengths, interpenetrating networks of polymeric donors and small molecule acceptor materials are used to make the highest performance organic photovoltaic cells to date. We have exploited the high hole-mobility (1.7 cm^2/Vs) of oligoacene small molecules for use as a donor material in extended heterojunctions with soluble fullerene derivates as the acceptor. Two realizations were key to obtain 100% solution processable devices with power conversion efficiencies of 0.8%. First, oligoacene appears to form an energy transfer system with C60. Consequently, for both materials to contribute to the external quantum efficiency, the bandgap of the oligoacene must be tuned to permit energy transfer from donor to accepter prior to exciton dissociation. Secondly, we have observed that when cast from a single solvent, the resulting homogeneous donor-acceptor blend yields inefficient charge separation and charge extraction. The nanometer to submicron scale phase-separation target, however, can be achieved in the oligoacene:fullerene system by the introduction of a second low boiling-point solvent. X-ray diffraction scans and polarized light microscopy show extensive crystallization of the oligoacene phase. From such measurements, we find the degree of crystallinity for a given device to be directly proportional to the short circuit current. To quantify the scale of the phase-separation, we have exploited the high fluorescence yield of the oligoacene component to measure micron-sized features with optical fluorescence microscopy and submicron domains with scanning near-field optical microscopy.
3:30 PM - **Z2.4
Organic Phototransistors vs. Organic Photodetectors
Birendra Singh 1 , Nenad Marjanovic 1 , Niyazi Sariciftci 1 Show Abstract
1 Physical Chemistry, Johanes Kepler University Linz, Linz Institute of Organic Solar Cells(LIOS), Linz, Linz, Austria
Photo-induced phenomena were investigated in photoresposive organic field-effect transistors (photOFETs) based on solution-processed conjugated polymer/fullerene as well evaporated small molecules such as phthalocyanine/fullerene solid-state mixtures as active semiconductor layer and divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) as gate dielectrics. The devices were characterized both in under dark showing n-type transistor behaviour with linear and saturated mobility of 1.7 x 10-3 cm2/Vs and 2.7 x 10-2 cm2/Vs respectively, and under white light illumination condition, where large shifts in the threshold voltage in the transfer characteristics were obtained. photOFETs show a high responsivities under AM 1 illumination. A typical phototransistor behaviour in a wide range of illumination intensities are observed in these devices. The photocurrent amplification mechanisms in this device were verified from experimental results as photovoltaic (turn-on) and photocurrent effect (turn-off). We will compare the performance of organic phototransistors and that of organic photodetectors realised from these materials.
4:30 PM - **Z2.5
Charge-Separation and Exciton-Migration Processes in Organic Semiconductors for Solar-Cell Applications
Jean-Luc Bredas 1 Show Abstract
1 Chemistry and Biochemistry, Georgia Tech, Atlanta, Georgia, United States
Conjugated organic oligomer and polymer materials are being increasingly considered for incorporation as the active semiconductor elements in devices such as photo-voltaic cells, light-emitting diodes, or field-effects transistors. In the operation of these devices, electron-transfer and energy-transfer processes play a key role, for instance in the form of charge transport, energy transport, charge separation, or charge recombination. Here, we focus on a theoretical description of charge-separation phenomena based on electron-transfer theory, which allows us to provide a molecular, chemically-oriented understanding.
5:00 PM - Z2.6
Tuning the Photophysical Properties of Polyfluorene by Conformational Engineering of the Single Chain.
Enrico Da Como 1 , Klaus Becker 1 , Manfred Walter 2 , John Lupton 2 , Jochen Feldmann 1 Show Abstract
1 Department of Physics, Ludwig-Maximilians-Universität Munich, Munich Germany, 2 Department of Physics, University of Utah, Salt Lake City, Utah, United States
The development of photovoltaic devices based on fluorene polymers or copolymers depends on the control of their photophysical properties and the ability to fabricate nanostructures with appropriate dimensions inside thin films. In particular, the order at the molecular level can strongly influence the exciton diffusion length and consequently the exciton migration to heterojunctions, where the electron-hole dissociation takes place. Within this context, we correlate the photophysics and the chain conformation of a model fluorene polymer, poly-(9,9-dioctylfluorene) (PFO), at the single molecule level.PFO shows different phases that can be identified by photoluminescence spectroscopy. Well resolved and red-shifted emission bands are observed for the planarized beta phase while a less structured blue emission is characteristic of the glassy one. The identification of these spectroscopic features at the single molecule level, with a remarkable photo stability of the beta phase , has raised the question of how to control intramolecular chain conformation and consequently photophysics. Here we use a vapour swelling procedure  to increase the relative percentage of beta phase chains with respect to glassy ones in a single molecule sample. By low temperature single molecule polarization anisotropy we probe the conformation of isolated chains in both phases. The results demonstrate that beta phase molecules are characterized by higher polarization values than the glassy ones, reflecting an elongated character of the chain. Contrary to that, the random values of the dihedral angle between repeat units in the glassy phase lead to a decrease in the overall chain stiffness and reduce the anisotropy. Additionally, we observe single chains with both types of emission, indicating only a partial conversion of the initial glassy segments to planarized beta segments. Our results show a strong correlation between the zero phonon line width and the anisotropy in the emission of beta phase molecules. Extremely narrow emission lines (0,6 meV) are observed for completely polarized emission, indicating that beta phase PFO in effect constitutes a 1D crystalline polymer. The truly 1D nature of this material promises outstanding exciton and charge carrier mobility previously only found in polydiacetylenes . Moreover, elongated beta phase chains are suitable in order to obtain a highly blended pi-conjugated polymer in an electron transporting material, thus controlling phase segregation phenomena in polymer-blend based solar cells. K. Becker, J. M. Lupton, J. Am. Chem. Soc., 127, 7306 (2005) M. Grell et al., Macromolecules, 32, 5810, (1999) F. Dubin et al., Nat. Phys., 2, 32 (2006)
5:15 PM - Z2.7
Transparent Polymer Solar Cells with Invert Structure
Gang Li 1 , Chi-Wei Chu 1 , Vishal Shrotriya 1 , Jinsong Huang 1 , Yang Yang 1 Show Abstract
1 MSE, UCLA, Los Angeles, California, United States
We investigate the effect of interfacial buffer layers – vanadium oxide (V2O5) and cesium carbonate (Cs2CO3) on the performance of polymer solar cells based on regioregular poly-(3-hexylthiophene) (RR-P3HT) and [6,6]-phenyl C60 butyric acid methyl ester (PCBM) blend. The polarity of solar cells can be controlled by the relative positions of these two interfacial layers. Efficient inverted polymer solar cells were fabricated with the structure of indium tin oxide (ITO)/Cs2CO3/polymer blend/ vanadium oxide (V2O5)/aluminum (Al). Short-circuit current of 8.42 mA/cm2, open-circuit voltage of 0.56 V, and power conversion efficiency of 2.25% under AM1.5G 130 mW/cm2 condition were achieved. The interfacial layers were also used to fabricate polymer solar cells using ITO and thin gold (Au) layer as the transparent electrodes. The thickness of V2O5 layer (10 nm) makes it an effective protective layer for the active layer so that ITO can be used for both the electrodes, enabling highly efficient transparent polymer solar cells (i.e., polymer solar cells with transparent electrodes). Application of this structure for multiple-stacking polymer solar cells will also be discussed.
5:30 PM - Z2.8
Optical Enhancement in Semitransparent Organic Photovoltaic Cells.
Ging Meng Ng 1 , Elizabeth Lekha Paul 1 , Thomas Kietzke 1 , Li Wei Tan 1 , Pooi Kwan Liew 1 , Furong Zhu 1 Show Abstract
1 , Institute of Materials Research and Engineering, Singapore Singapore
This work will report the efforts to study the optical enhancement in semitransparent organic photovoltaic (OPV) cells. We discuss possible designs to improve the conversion efficiency as well as the optical transparency of OPV cells. An optical admittance analysis is used to optimize the structure of OPV cells involving ITO as both anode and cathode. With the help of the optical admittance analyses, a P3HT:PCBM based semitransparent organic solar cell was optimized and demonstrated. The results of this work have yielded a promising mean external quantum efficiency of ~ 45% and a power conversion efficiency of 1.32% for a semitransparent OPV cell having a 75 nm thick polymer blend of P3HT:PCBM.
5:45 PM - Z2.9
Design and Optimization of Organic Photovoltaics with Oxadiazole Containing Poly(p-phenylenevinylene)s [OXA-PPVs].
Changhee Ko 1 , Olga Zolotarskaya 1 , Yashpal Bhandari 1 , Mary Galvin 2 Show Abstract
1 Materials Science and Engineering, University of Delaware, Newark, Delaware, United States, 2 , Air Products and Chemicals, Inc., Allentown, Pennsylvania, United States
Michael F. Durstock Air Force Research Laboratory
Anvar Zakhidov University of Texas-Dallas
Michael Graetzel Ecole Polytechnique Federale de Lausanne
Lynne A. Samuelson U. S. Army RDECOM
Z3: Variable Bandgap Materials and Nanostructured Devices I
Wednesday AM, April 11, 2007
Room 2009 (Moscone West)
9:30 AM - **Z3.1
Low Band Gap and Multi-junction Polymer Solar Cells.
Rene Janssen 1 , Martijn Wienk 1 , Arjan Zoombelt 1 , Jan Gilot 1 , Mathieu Turbiez 1 Show Abstract
1 Molecular Materials and Nanosystems, Eindhoven University of Technology, Eindhoven Netherlands
10:00 AM - **Z3.2
Functional and Variable Gap Conjugated Polymers in Hybrid Photovoltaic Devices
John Reynolds 1 , Kirk Schanze 1 Show Abstract
1 Chemistry, University of Florida, Gainesville, Florida, United States
By controlling the energy levels of the HOMO and LUMO states, along with the magnitude of the electronic band gap, the properties of light absorbing and electron donating conjugated polymers have been optimized for electron transfer to PCBM. This will be discussed for polymers where the HOMO is set for air stability, the LUMO for excited state donation to the PCBM acceptor, and the band gap for broad absorption of the solar spectrum. Our use of 3,4-alkylenedioxythiophene (XDOT) donors, along with cyanovinylene and pyridopyrazine acceptors has led to the development of a representative class of soluble narrow gap (Eg = 1.5 to 1.8 eV) polymers; nicely absorbing across the solar spectrum. Varying the side chains between linear and branched alkyl and alkoxy substituents provides polymers with enhanced morphologies when blended with PCBM. Understanding and optimizing the detailed mechanisms of exciton lifetime and diffusion lengths, charge carrier creation efficiencies, back electron transfer to annihilate the charge separated state, and complete charge separation to allow high photocurrents has been the focus of an effort utilizing a new class of thienylene linked platinum-acetylide (p-PtTh) organometallic polymers which effectively form long-lived triplet states. In this work, we have been able to demonstrate that photovoltaic cells that contain the p-PtTh and PCBM operate efficiently and that by broadening the spectral absorption using donor-acceptor chromophores these solar efficiencies can be further enhanced.
10:30 AM - Z3.3
A Red-absorbing Polyfluorene as Donor and Acceptor in Hybrid Photovoltaic Devices.
Henry Mo Pun Wong 1 , Peng Wang 1 , Agnese Abrusci 1 , Mattias Svensson 2 , Mats Andersson 2 , Neil Greenham 1 Show Abstract
1 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom, 2 Materials and Surface Chemistry, Chalmers University of Technology, Göteborg Sweden
10:45 AM - Z3.4
Optimizing Absorption and Transport in APFO3:PCBM Polymer Solar Cells.
Fengling Zhang 1 , Mats Andersson 2 , Olle Inganas 1 Show Abstract
1 , Linköping University, Linköping Sweden, 2 , Chalmers University of Technology, Göteborg Sweden
11:45 AM - Z3.6
Photocurrent Enhancement by 3-D Transparent Carbon Nanotube Charge Collectors in Polymeric Solar Cells.
Anvar Zakhidov 1 , Kamil Mielczarek 1 , Sergey Lee 1 , Mei Zhang 1 , Shaoli Fang 1 , Ray Baughman 1 Show Abstract
1 Physics, University of Texas at Dallas, Richardson, Texas, United States
Strong transparent multiwall carbon nanotube sheets (t-CNT) can be used as anodes in organic photovoltaic cells (OPV). We demonstrate here that t-CNT play an essential role as three-dimensional hole collecting network with enhanced interface in OPV of bulk heterojunction type (BH). Although the sheet resistance of as-prepared t-CNT films is higher than of typical ITO, we prove here that combining 3-D t-CNT network with planar ITO can lead to twice increased photocurrent due to combined effect of enhanced hole collection by 3-D t-CNT and improved transport via ITO part. So even in unoptimized OPV, a short circuit current of 11 mA/cm2 can be easily obtained, which is significant increase, compared to only ~ 5.5 mA/cm2 of photocurrent in sole ITO device, or 5 mA/cm2 in sole t-CNT case, increasing the overall efficiency to > 3%,. The role of thin PEDOT-PSS coating on t-CNT is discussed, both for planarizing CNT network and avoiding shorts as well as for blocking electrons from recombination on t-CNTs. Flexible OPVs on plastic substrates also show enhanced performance with hybrid t-CNT-ITO anodes.
12:00 PM - Z3.7
Ordered Structures for Photovoltaic Devices
Michael Durstock 1 , Adam Smith 2 1 , Britt Minch 2 1 , Tae-Sik Kang 1 , Barney Taylor 3 1 Show Abstract
1 , The Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio, United States, 2 , UES, Inc., Beavercreek, Ohio, United States, 3 , Universal Technology Corp., Dayton, Ohio, United States
The development of low-cost, lightweight, and flexible solar cells are an enabling technology for many different types of applications. The fabrication of highly efficient devices, however, has yet to be achieved due to a variety of contributing factors. Poor charge transport in organic and organic-hybrid devices is one of these factors and is a result of low charge carrier mobilities and relatively random thin film morphologies. Our efforts to address this situation include developing materials and fabrication methodologies that result in ordered structures to permit enhanced charge transport. A templating approach using nanoporous alumina as a framework in which to grow titania nanotubes and rods has been developed and will be discussed. Using this methodology, highly ordered structures can be fabricated in which the titania tubes are close-packed and perpendicular to the substrate and have dimensions (length and wall thickness) which can be tuned. An alternative approach uses holographic interference lithography to produce 1-D, 2-D, or 3-D patterns in a photo-polymerizable matrix. By tailoring the synthesis of electroactive materials, a variety of bi-continuous networks can be fabricated for use in photovoltaic devices. Finally, discotic liquid crystalline materials are of much interest for use in these devices due to their ability to organize into columnar domains with high charge carrier mobilities. By using a facile synthetic methodology, solution-processable versions of phthalocyanines have been fabricated by attaching solubilizing side-chains. Recent development in these approaches will be highlighted as a step towards more efficient photovoltaic devices.
12:15 PM - Z3.8
Optimization of ZnO Nanorod Array Morphology and Chemistry for Polymer Hybrid Photovoltaic Devices
Yun-Ju Lee 1 , Dana Olson 1 , David Scrymgeour 1 , Darren Dunphy 1 , James Voigt 1 , Julia Hsu 1 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Aligned ZnO nanorod arrays (NRAs) grown from aqueous precursors with subsequent infiltration of poly(3-hexylthiophene) (P3HT) represent a promising method to fabricate inexpensive, high surface area hybrid photovoltaic (PV) devices. Control of the ZnO NRA morphology and surface defects may improve exciton transport within the polymer, charge separation at the polymer-oxide interface, and carrier transport through both polymer and oxide, which in turn would enhance the efficiency of such nanostructured photovoltaic devices. Here, we systematically vary NRA alignment, nanorod diameter, and spacing between nanorods by controlling seeding and growth conditions. The resulting NRAs are characterized using scanning electron microscopy, x-ray diffraction, atomic force microscopy, surface area measurement, and UV-vis spectroscopy. After P3HT is infiltrated in the NRAs, the response and efficiency of ZnO-P3HT PV devices are correlated with the NRA characteristics. By increasing the thickness of a sol-gel seed layer, the degree of alignment of the resulting ZnO NRA changes from disordered to ordered, as demonstrated by XRD and SEM. The degree of NRA alignment affects both the infiltration and crystallinity of the P3HT, leading to competing effects on the performance of the PV devices. We adjust the inter-rod spacing in highly aligned NRA by changing the pH of the growth solution, and find a strong relationship between the amount of polymer infiltrated between rods and the performance of the PV device. Finally, we examine the effect of Mg doping on the conduction band position in the oxide and, consequently, the open circuit voltage of the PV device. By adding a magnesium precursor in the growth solution, highly aligned Zn1-xMgxO arrays are synthesized. The amount of Mg doping is measured by UV-vis spectroscopy and x-ray diffraction, and the effect of Mg doping on NRA work function is determined by Kelvin probe measurement. Changes in the PV device performance as a function of Mg doping will be presented. Possible strategy for further improvements in the PV device efficiency will also be discussed.
12:30 PM - Z3.9
Holographically Patterned Organic Photovoltaic Devices
Adam Smith 2 1 , Eric Beckel 3 1 , Augustine Urbas 1 , Vincent Tondiglia 4 1 , Barney Taylor 5 1 , Timothy Bunning 1 , Michael Durstock 1 Show Abstract
2 , UES, Inc., Beavercreek, Ohio, United States, 1 , Wright Patterson Air Force Base, Wright Patterson AFB, Ohio, United States, 3 , General Dynamics Information Technology, Dayton, Ohio, United States, 4 , SAIC, Dayton, Ohio, United States, 5 , Universal Technology Corporation, Dayton, Ohio, United States
The power conversion efficiencies of bulk-heterojunction organic solar cells are limited, in large part, due to poor charge transport. One reason for this is the random morphology of the photoactive layer in these devices. Control over the thin-film morphology through the fabrication of a regular microstructure may dramatically enhance charge transport and, ultimately, device efficiency. Current thrusts designed to meet these charge transport challenges include the use of templating strategies and the incorporation of self-assembling electroactive components. An alternative strategy that has been extensively utilized in the formation of optical diffraction gratings involves the use of laser interference to holographically write regular, repeating patterns via a photopolymerization mechanism. This technology seems quite promising since numerous 1D, 2D, and 3D morphologies can be holographically written by simple changes in experimental design. Of particular interest for photovoltaic contexts may be lamellae or cylindrical architectures aligned normal to the film surface comprised of a bicontinuous donor:acceptor network. Such micropatterns may enable efficient charge transport when incorporated in the photoactive layer of bulk-heterojunction solar cells. Our efforts exploring the use of a thiol-ene photopolymerization mechanism with electroactive components for patterning the active layer of organic photovoltaic devices will be presented.
12:45 PM - Z3.10
Engineering Vertical Composition Variations in Spin-Coated Thin Films of Low-Bandgap Polyfluorene Copolymers and PCBM for Solar Cell Applications.
Cecilia Bjorstrom 1 , Jakub Rysz 2 , Adrzej Bernasik 3 , Andrzej Budkowski 2 , Ellen Moons 1 Show Abstract
1 Department of Physics, Karlstad University, Karlstad Sweden, 2 Institute of Physics, Jagiellonian University, Kraków Poland, 3 Faculty of Physics and Applied Computer Science , AGH- University of Science and Technology, Kraków Poland
Thin films of conjugated polymers blended with fullerene derivatives are frequently used as the active material in photovoltaic cells, where the blends are spin-coated from solution directly onto the bottom electrode. For such devices, so-called bulk-heterojunction solar cells, the blend film morphology has been shown to have a strong effect on the performance of the solar cell. The film formation during spin-coating is characterised by rapid solvent quenching and phase separation, and results in a non-equilibrium film morphology. There are several parameters that may affect the final film morphology. One important factor is the difference in surface energy between the blend components. This difference may give rise to surface-directed spinodal decomposition if one of the blend components is attracted to one or both of the external surfaces, i.e. the free surface with air or the interface with the substrate. This, in turn, will lead to the initial formation of vertically stratified phases that can either be frozen in or break up by interfacial instabilities and yield a lateral domain structure.Here we present results from morphology studies on spin-coated thin films of low-bandgap alternating polyfluorene copolymers (APFO’s) and fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). We show that the compositional depth profile of the films can be determined with a resolution of about 10 nm by means of dynamic secondary ion mass spectrometry (SIMS). The polymer component was identified by its sulphur and the CN signatures and for detection of PCBM we used deuterium labelling of the phenyl-group (d5-PCBM).We analysed blend films and bilayers. The blend films were spincoated from chloroform and chlorobenzene solutions, and we found that the morphology can be controlled from a vertical multilayer structure to a homogeneous distribution, by adjusting the volume ratio of the solvents. Rapid evaporation causes quenching into the two-phase region, which results in vertical variations in composition, expressed as multilayers of polymer-rich and PCBM-rich phases. In this spontaneously formed structure the surface of the film is strongly enriched with polymer, the low surface energy component. Despite this structure with the hole-transporting material near the low-workfunction metal electrode being electronically non-ideal, the solar cells produced from this active layer have shown the highest energy conversion efficiency among all low-bandgap polyfluorene:PCBM blend devices. To understand this better, we have fabricated bilayers with reversed structures as compared to the spontaneously formed multilayers. Their compositional depth profiles determined by SIMS are a clear proof for the bilayer structure. Depending on the preparation parameters, the bilayer structure can be formed with a sharp or more graded composition at the polymer/PCBM interface.
Z4: Small Molecule-Based Devices
Wednesday PM, April 11, 2007
Room 2009 (Moscone West)
2:30 PM - **Z4.1
Solution Processed Photovoltaic Cells from Small Molecules.
George Malliaras 1 , Matt Lloyd 1 , John Anthony 2 Show Abstract
1 Materials Science, Cornell University, Ithaca, New York, United States, 2 Department of Chemistry, University of Kentucky, Lexington, Kentucky, United States
Organic photovoltaic cells are being developed as low-cost alternatives to inorganic ones. The two main approaches to efficient organic photovoltaic cells are polymer blends and small molecule heterojunctions. We discuss the development of efficient photovoltaic cells from solution-deposited small molecule blends. This materials system combines the advantages of the two approaches above. Namely, small molecules can be easily synthesized and their structure tuned to control their electronic properties, as well as their packing in solid state. They can be easily purified, and their chemical structure can be modified to improve stability towards undesirable (photo-)reactions, which has the potential to improve device stability. Finally, they can be processed from solution, which makes them ideal candidates for large-area, inexpensive photovoltaic cells. The discussion will focus on blends of oligoacenes with fullerenes and the relationship between structure and morphology of the blend and its performance in photovoltaic cells.
3:00 PM - Z4.2
Synthesis and Photovoltaic Applications of Discotic Liquid Crystalline Phthalocyanines.
Britt Minch 1 2 , Adam Smith 1 2 , Barney Taylor 1 3 , James Deneault 1 3 , Michael Durstock 1 Show Abstract
1 MLBP, AFRL, Wright-Patterson AFB, Ohio, United States, 2 , UES, Dayton, Ohio, United States, 3 , UTC, Beavercreek, Ohio, United States
Organic dye-based photovoltaics are attractive for the tunability of the absorbances of the organic dyes and because they allow for the production of flexible devices. Although organic dyes typically have poorer charge transport properties than amorphous silicon, fabrication of thinner organic films can be used to obtain higher photovoltaic efficiencies. Therefore, organic dyes that can absorb light strongly are of great interest. Phthalocyanines are an important class of organic dyes that are of particular interest for photovoltaics due to their strong absorption in the UV-Vis region. The addition of side chains can lead to the formation of discotic liquid crystalline mesophases, which can be oriented. Discotic mesophases are capable of uni-axial charge transport, but the synthesis and purification of these discotic liquid crystals can be very tedious. We report an improved synthetic method that uses extremely mild conditions to obtain good yields of metal free phthalocyanine. The materials synthesized have been characterized with respect to their thermal and optical properties. The phthalocyanines have been incorporated into first generation devices using PCBM and perylene the electron acceptors. The device performance of these first generation photovoltaics will be presented.
3:15 PM - Z4.3
Enhancing the Absorption of Organic Solar Cells by Photon Harvesting Antenna Systems
Robert Koeppe 1 , Olivia Bossart 2 , Gion Calzaferri 2 , Niyazi Sariciftci 1 Show Abstract
1 LIOS, JKU Linz, Linz Austria, 2 Department of Chemistry and Biochemistry, University of Berne, Bern Switzerland
We present small molecule organic solar cells fabricated by vacuum evaporation of zinc-phthalocyanine and C60 reaching external quantum efficiencies of close to 50% between 600 and 720nm. Low absorption of the used materials below 580nm limits the performance of the solar cells under white light illumination. We propose the use of antenna systems absorbing incoming light of lower wavelengths and donating the energy to the solar cell. Two promising approaches are identified: 1. Concentrating the illumination spatially as well as spectrally using a luminescent concentrator waveguide. Experiments show an efficient funneling of the illumination collected in the concentrator towards the attached solar cell. An external quantum efficiency of >20% in the wavelength range from 400 to 600nm was realized by using a 1.5x1.5x0.3cm^3 concentrator with a concentration factor of 5. Around 550nm, the quantum efficiency of the solar cell is increased by a factor of 2 in comparison with direct illumination.2. Absorbing the illumination in an energy transfer cascading structure such as dye-loaded zeolite L crystals or layers of semiconductor nanocrystals with consecutively increasing size. An efficient resonant energy transfer from the last stage of the energy transfer cascade onto a solar cell material will improve the photon harvesting. Fluorescence studies show an efficient energy transfer from CdSe/ZnS nanocrystals as well as dye loaded zeolite L crystals to zinc-phthalocyanine.G. Calzaferri et al, Angew. Chem. Int. Ed. 42: 3732-3758 (2003)T. Franzl et. al, Nano Lett. 4: 1599 (2004)
3:30 PM - Z4.4
Hydrogen-bonded Phthalocyanine Molecular Assemblies Demonstrating Homeotropic Alignment: Studies of Molecular Order and Electrical Properties
Niranjani Kumaran 1 , Alexander Veneman 1 , Britt Minch 2 , Wei Xia 3 , Neal Armstrong 1 Show Abstract
1 Department of Chemistry, University of Arizona, Tucson, Arizona, United States, 2 , Air Force Research Laboratories, Wright-Patterson AFB, Ohio, United States, 3 , Veeco Metrology, Inc, Santa Barbara, California, United States
Discotic molecular assemblies are of interest for a variety of organic electronic device applications such as light-emitting diodes, photovoltaic cells and thin-film transistors. The orientation of the molecular assembly, at the nanometer and micron length scales, is important since the highest charge-carrier mobilities are expected to occur along the long axis of these rod-like assemblies. Aggregates whose rod axis is parallel to the substrate plane are, of course, optimum for thin-film transistors, but organic solar cells require that the rod axis be perpendicular to the substrate plane, which has heretofore been difficult to achieve simply by deposition from solution. This presentation will focus on our efforts to construct solution-deposited thin films of phthalocyanine molecular aggregates, with the rod-axis constrained to be perpendicular to the substrate plane. Our first target molecule is a derivative of previously reported octa-(thioether)-benzyloxy Copper Phthalocyanine (CuPc), in which a H-bonding amide group has been incorporated into the eight side chains. Films deposited from concentrated solutions of this Pc consist of both monoclinic and orthorhombic phases of cofacially stacked Pcs, with the rod-axis mainly parallel to the substrate plane. Films adsorbed from extremely dilute solutions, however, on ultra-smooth substrates modified with self-assembled monolayers terminating in H-bonding amide groups, form large sheet-like deposits, exhibiting layer-by-layer growth over 100-500 nm length scales, with a separation between the sheets (0.35 nm) consistent with the expected Pc-Pc separation distance in homeotropically aligned aggregates. Confirmation of the layered structures of these thin films is provided by Polarized Optical Microscopy (POM), X-ray diffractometry (XRD), visible wavelength Attenuated Total Reflection Spectroscopy (ATR), Reflection Absorption Infrared Spectroscopy (RAIRS), and Atomic Force Microscopy (AFM). Preliminary characterization of the electrical properties of these layered materials using conducting tip AFM (C-AFM) will also be presented.
3:45 PM - Z4.5
One-Dimensional Morphology and Controlled Orientation of Donor-Acceptor Blends Made of Self-Assembled Phthalocyanine-Based Molecular Fibers.
Volodimyr Duzhko 1 , Kenneth Singer 1 Show Abstract
1 Physics, Case Western Reserve University, Cleveland, Ohio, United States
We report on the controlled self-assembly of phthalocyanine-based discotic molecules into molecular fibers in organic solvents and on the fabrication of one-dimensional donor-acceptor blends. The internal structure, morphology and alignment of the fibers in external fields were studied by spectroscopic techniques, optical and scanning electron microscopies. We demonstrate that alkoxy-substituted phthalocyanine molecules self-assemble into sub-micron diameter fibers with discrete diameter. The assembly can be controlled by various processing conditions, e.g. temperature and solvent. The internal structure of fibers resembles those found in condensed liquid crystalline mesophases, which have been shown to exhibit efficient carrier transport. Homeotropic alignment of the fibers in a dc electric field allows for the fabrication of quasi-one-dimensional donor-acceptor architectures with orientation of the transport channels perpendicular to the conducting substrate planes. We discuss the relation between the morphology and donor-acceptor interface charge transfer in blends of phthalocyanine and perylene diimide derivatives as well as electronic transport in columnar liquid crystal mesophases as related to photovoltaic applications.
4:30 PM - **Z4.6
Fullerene-Based Organic Photovoltaics Using Various Hole Transporters.
Zakya Kafafi 1 Show Abstract
1 , Naval Research Laboratory, Washington , District of Columbia, United States
Fullerene-based organic photovoltaics (OPVs) using various hole transporters such as acenes and metallophthalocyanines were fabricated and characterized in terms of their electrical and spectroscopic properties. The present study focuses on understanding the role played by the electron donor/hole transporter on the efficiency of exciton formation, transport and dissociation in OPVs. For instance, creating a large built-in chemical potential at the donor/acceptor interfaces requires maximizing the separation between the energy levels of the highest occupied molecular orbital (HOMO) of the electron donor and the lowest unoccupied molecular orbital (LUMO) of the electron acceptor, which should result in efficient charge separation and a large open circuit voltage. A close look at the relationship between the open circuit voltage and the HOMO-LUMO offset reveals close to a linear dependence for the various hole transporters used in this study. In addition, the dynamics of exciton formation, diffusion and dissociation including possibly Förster resonance energy transfer from donor to acceptor molecules in the charge photogeneration process(es) are investigated using transient spectroscopic measurements.
5:00 PM - Z4.7
Synthesis and Characterization of Electropolymerized Porphyrin Nanofibers
Carl Wamser 1 , Michael Walter 1 Show Abstract
1 Chemistry, Portland State University, Portland, Oregon, United States
Electrochemical oxidation of tetrakis-5,10,15,20-(4-aminophenyl)porphyrin (TAPP) on transparent fluorine-doped tin oxide (FTO) electrodes leads to a conductive polymeric film (poly-TAPP) with a nanostructured fibrous morphology. Film growth and film features were monitored by cyclic voltammetry, uv-vis spectroscopy, scanning electron microscopy, atomic force microscopy, and an electrochemical quartz crystal microbalance. The nanofibrous film morphology and growth rate are enhanced by the presence of pyridine in the dichloromethane solvent and by the use of a constant potential delay at the oxidizing end of the cyclic voltammetry sweeps (-0.3 to +0.7 V vs Ag/AgNO3). Typical film thicknesses are about 80 nm after 3 CV cycles and up to 300 nm after 15 cycles. Typical fiber diameters are in the range 40-100 nm, highly interconnected in a network with pores in the range of 50-500 nm. Electropolymerization in the absence of pyridine leads to poly-TAPP with a spectrum indicative of protonated porphyrin units, a more highly bundled nanofibrous structure with larger pores, and much higher electrical conductivity. Preliminary testing of the poly-TAPP electrodes as photovoltaic cells indicated modest photoactivity. Using iodide/triiodide electrolyte solution, short-circuit current densities of 3 μA/cm2 and open-circuit potential values of 120 mV were measured under AM 1.5 simulated solar illumination. A bilayer organic solar cell was created by incorporating PCBM (a fullerene derivative) into the porous poly-TAPP film followed by deposition of an Al/LiF counter electrode. This cell exhibited short-circuit current densities of 140 μA/cm2 and open-circuit potential values of 500 mV under AM 1.5 illumination.
5:15 PM - Z4.8
Role of the Metal Center in Determining the Electrical Characteristics of Metallophthalocyanine-based Organic Photovoltaics.
Gary Kushto 1 , Mason Wolak 1 , Zakya Kafafi 1 Show Abstract
1 , U.S. Naval Research Laboratory, Washington, District of Columbia, United States
5:30 PM - Z4.9
Electrical and Photoelectrical Properties of Nanostructured ZnO Thin Films for Photovoltaic Applications.
Stefan Antohe 1 , Lucian Ion 1 , Cezar Tazlaoanu 1 , Gabriel Socol 2 , Larisa Magherusan 1 , Ion Mihailescu 2 , Vlad-Andrei Antohe 1 Show Abstract
1 Faculty of Physics, University of Bucharest, Bucharest-Magurele Romania, 2 , National Institute for Plasma Physics, Bucharest-Magurele Romania
Michael F. Durstock Air Force Research Laboratory
Anvar Zakhidov University of Texas-Dallas
Michael Graetzel Ecole Polytechnique Federale de Lausanne
Lynne A. Samuelson U. S. Army RDECOM
Z5: Nanostructured Devices II and Dye-Sensitized Cells I
Thursday AM, April 12, 2007
Room 2009 (Moscone West)
9:15 AM - *Z5.1
Nanostructured Polymer-titania Solar Cells.
Michael McGehee 1 Show Abstract
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
10:00 AM - Z5.2
The Importance of Relating Charge Transport and Recombination Rates to Charge Carrier Density in Nano-material Based Photovoltaics (and Other Electronics).
Brian O'Regan 1 , Chris Shuttle 1 , James Durrant 1 Show Abstract
1 , Imperial College London, London United Kingdom
The application of nano-materials to large area applications such as photovoltaics requires high volume low cost production methods. The trade off between cost and purity, as well as the high surface area, will insure the existence of inhomogeneity and defects in large area nano-material applications. Some of these defects will in turn act as charge carrier traps (and possibly points of instability.) Despite conventional wisdom, these trap sites do not necessarily impair device function. In all cases however, important characteristics of nano-electronic devices will depend on the extent of trap filling. Most obviously this includes charge transport, but also charge separation, charge recombination, and possibly morphological stability in some cases. To provide accurate models of the function and stability of nano-scale photovoltaic cells (e.g. dye sensitized, polymer) the dependence of the electronic characteristics on trap filling will have to be quantified. In the field of dye sensitized cells, it has been realized for some time that various parameters are dependent on light intensity and/or voltage, however only recently has the underlying parameter, trap filling been examined. The importance of trap filling leads to the question, how can trap filling be correctly quantified under various conditions? Optical solutions have been examined (e.g. photoinduced absorption, transient absorption, electro-absorption), however in these cases separate measurement of the charge carrier absorption coefficient is also necessary and non-trivial. Charge extraction measurements, where charges existing in the system are removed as current and integrated, have been proposed and are proving useful for both dye sensitized and polymer based cells. Charge extraction measurements are only accurate when there is little or no loss of carriers during the extraction process. This presumption is always violated to some degree. Especially when studying treatments that presumably increase or decrease overall quantum efficiency (e.g. annealing or degradation), charge extraction measurements alone are not useful. We will show how a combination of charge extraction and optical measurements can help determine total charge, and the charges lost to recombination, thus providing convincing answers to questions regarding fabrication and degradation of nano-material electronics. Results from the application of the method to dye cells and P3HT/PCBM cells will be given.
10:15 AM - Z5.3
Dye Ssensitized Solar Cells using Well- Aligned ZnO Nanotips Arrays on Transparent Conducting Substrates.
Aurelien Du Pasquier 1 , Hanhong Chen 2 , Yicheng Lu 2 Show Abstract
1 MSE, Rutgers, The State University of New Jersey, Piscataway, New Jersey, United States, 2 ECE, Rutgers, The State University of New Jersey, Piscataway, New Jersey, United States
10:30 AM - Z5.4
Nanostructured Hybrid Solar Cells.
Vignesh Gowrishankar 1 , Albert Chan 1 , Shawn Scully 1 , Michael McGehee 1 , Qi Wang 2 , Howard Branz 2 Show Abstract
1 Materials Science, Stanford University, Stanford, California, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Hybrid solar cells seek to increase efficiencies by combining the advantages of semiconducting polymers and inorganic semiconductors. We report on the fabrication and performance of two such bilayer, hybrid systems, hydrogenated amorphous silicon (a-Si:H) / poly-3-hexylthiophene (P3HT) and a-Si:H / poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV). These cells are compared with bilayer titania / P3HT and titania / MEH-PPV cells. The a-Si:H / P3HT system is found to be at least as efficient as the titania / P3HT system, despite the fact that the external quantum efficiency of the former is predicted to be more than 3 times lower than that of the latter, due to differences in the refractive indices, absorption coefficients and thicknesses of the titania and a-Si:H layers. By comparing the four systems of P3HT and MEH-PPV on a-Si:H and titania, we highlight the need for careful choice of inorganic and organic materials to make efficient hybrid solar cells, especially with regard to the bandgaps, the LUMO levels and the HOMO levels. In these four systems, upon illumination, we demonstrate that exciton-diffusion followed by forward-electron-transfer may be competing with energy-transfer followed by backward-hole-transfer for splitting excitons. We observe that in systems where energy transfer cannot occur (e.g. titania / polymer), the LUMO offset is critical, while in systems where strong energy transfer can occur (e.g. a-Si:H / MEH-PPV), the HOMO offset may become critical. The most efficient of these four hybrid systems is found to be a-Si:H / P3HT, with a power-conversion efficiency of about 0.16% (under simulated 1000 W/m2 AM 1.5 illumination). To achieve higher efficiencies it is necessary to increase the interface area between the two semiconductors so that more excitons can be split. To effect this we use a nanostructure, in the a-Si:H, of pillars / ridges separated by trenches into which the P3HT is infiltrated. About 150 – 200 nm deep trenches would contain enough P3HT to absorb all the light. Trenches that are less than two exciton diffusion lengths wide would conceivably allow for maximized exciton splitting. Furthermore, the vertical walls of the trenches would allow for superior chain packing and thus higher charge carrier mobilities in the P3HT. We use techniques such as Nanosphere Lithography and Block Copolymer Lithography to create the nanopattern template on the a-Si:H. Liftoff followed by Reactive Ion Etching yields the required nanostructured a-Si:H. After melt-infiltration of the P3HT into the nanostructure, we investigate the extent of efficiency enhancement as well as potential problems present in these nanostructured hybrid solar cells.
10:45 AM - Z5.5
A New Strategy to Fabricate a Colloidal Array Templated TiO2 Photoelectrode for Dye-sensitized Solar Cells.
Su-Chul Yang 1 , Eun-sik Kwak 1 , Il-doo Kim 1 , Junkyung Kim 1 , Hyunjung Lee 1 Show Abstract
1 , KIST, Seoul Korea (the Republic of)
11:30 AM - **Z5.6
Solid-state and Fully Electronic ``liquid" Hole-transporter-based Dye-sensitized Solar Cells.
Henry Snaith 1 2 , Michael Graetzel 1 Show Abstract
1 Institute of Chemical Science and Engineering, Ecole Polytechnique Federale de Lausanne, Lausanne, Vaud, Switzerland, 2 Department of Physics, University of Cambridge, Cambridge United Kingdom
12:15 PM - Z5.8
Solid State Dye-Sensitized Solar Cells using Polydiacetylene for Hole Conduction.
Yanping Wang 1 , Lian Li 1 , Fadong Yan 1 , Ke Yang 1 , Lynne Samuelson 2 , Jayant Kumar 1 Show Abstract
1 Center for Advanced Materials, University of Massachusetts Lowell, Lowell, Massachusetts, United States, 2 Nanomaterials Science Team, U.S. Army Natick Soldier, RDECOM, Natick, Massachusetts, United States
Solid state dye-sensitized TiO2 solar cells were fabricated using in situ topochemically polymerized 10,12-pentacosadiynoic acid as the hole conductor. FT-Raman spectroscopy was used to characterize the polymerization of the diacetylene monomer within the nanoporous TiO2 film. The morphology of the polydiacetylene/TiO2 nanocomposite was investigated via scanning electron microscopy. Under AM 1.5 condition (100 mWcm-2), with N719 as the sensitizer, an open-circuit voltage over 600 mV and a short-circuit current density up to 4.91 mAcm-2 were measured, yielding an overall power conversion efficiency of 1.21%.
12:30 PM - Z5.9
Towards ''Green" Solid-state Polythiophene-sensitized Solar Cells using a Fully Water Based Preparation Method.
Ilse Haeldermans 1 2 , Koen Vandewal 1 , Wouter Moons 1 , Ine Truijen 2 , Marlies Van Bael 2 3 , Jan D’Haen 1 , Jean Manca 1 3 , Jules Mullens 2 Show Abstract
1 Institute for Materials Research, Hasselt University, Diepenbeek Belgium, 2 Inorganic and Physical Chemistry, Hasselt University, Diepenbeek Belgium, 3 IMECvzw, division IMOMEC, Hasselt University, Diepenbeek Belgium
12:45 PM - Z5.10
Improvement of Efficiency in DSCs by using the Combination of N719 and P3HT as Sensitizers.
Osamu Yoshikawa 1 , Supachai Ngamsinlapasathian 1 , Takashi Sagawa 1 , Susumu Yoshikawa 1 Show Abstract
1 Institute of Advanced Energy, Kyoto Univeristy, Kyoto Japan
Z6: Dye-Sensitized Cells II and Nanoparticle Hybrid Cells I
Thursday PM, April 12, 2007
Room 2009 (Moscone West)
2:30 PM - Z6.1
Electrolyte Effects on the Thermodynamics and Kinetics of Interfacial Electron Transfer in TiO2 Dye-Sensitized Solar Cells.
Jordan Katz 1 , Amanda Smeigh 2 , Bruce Brunschwig 1 , James McCusker 2 , Nathan Lewis 1 Show Abstract
1 Chemistry, California Institute of Technology, Pasadena, California, United States, 2 Chemistry, Michigan State University, East Lansing, Michigan, United States
The effect of small cations, Li+ and H+, in the electrolyte of working TiO2 dye-sensitized solar cells has been characterized by both steady state and time-resolved techniques. In the dark, current-voltage curves were shifted up to 300 mV to more negative bias upon reducing the concentration of small cations in the electrolyte solution. Similarly, under illumination large increases in open-circuit voltage (Voc) were obtained, although photo- currents were reduced by as much as a factor of five. Under monochromatic illumination, the quantum yield dropped monotonically across the spectrum, consistent with the overall loss of photocurrent under AM1.5 illumination. The increase in Voc is attributed to a cation-dependent shift in the conduction band edge, as was confirmed by a unique spectro-chronocoulometric technique. Band edge shifts were determined by correlating the amount of charge injected, monitored simultaneously by both chronocoulometry and time-resolved UV/Vis absorption spectroscopy, to the applied bias in a series of potential step experiments. To account for the loss of photocurrent at low cation concentrations, ultra-fast electron injection dynamics from the Ru dye N3 to TiO2 were studied by femtosecond transient absorption (TA) spectroscopy. In the absence of redox couple electron injection rates were similar at all concentrations of small cations. In contrast, in working cells with the I-/I3- couple in solution, the presence of small cations increased electron injection rates. Nanosecond TA spectroscopy revealed that the regeneration of the oxidized dye by I- was also significantly accelerated by the presence of small cations, which can be explained by a either a shift in the electrochemical ground-state potential of the dye or a TiO2 surface charging effect due to cation intercalation, creating a local electric field. The kinetics of the reduction of the oxidized dye by a series of neutral ferrocene derivatives with ground-state potentials varying by nearly 0.5 V has been used to distinguish these mechanisms.
2:45 PM - Z6.2
A Novel Efficient, Iodide-free Redox Mediator for Dye-sensitized Solar Cells.
Zhipan Zhang 1 , Peter. Chao-Yu Chen 1 , Shaik Mohammed Zakeeruddin 1 , Jacques-Edouard Moser 1 , Michael Graetzel 1 Show Abstract
1 Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne, Lausanne Switzerland
3:00 PM - Z6.3
Stepwise Synthesis of a Multifunctional Strong π-electron Acceptor for Solar Cell Applications
Dinesh Patel 1 , Janelle Leger 1 , Nick Bastianon 1 , Glenn Bartholomew 1 Show Abstract
1 Chemistry, University of Washington, Seattle, Washington, United States
All-organic dyes have good potential for use in dye-sensitized solar cells because of their tuneable properties and high molar extinction coefficients. Most all-organic dyes employed utilize a carboxylic acid for binding to titania via an ester linkage. Because charge recombination is a problem that limits solar cell efficiency, a large number of dyes rely on the electron withdrawing cyanoacetic acid moiety to aid in charge separation and injection into the titania conduction band. We are currently examining analogues of a nonlinear optical dye that uses the 2-dicyanomethylene-2,5-dihydro-4,5,5-trimethylfuran-3-carbonitrile (TCF) acceptor, one of the strongest acceptors known, which we believe will aid in charge separation and injection. TCF has no means to effectively bind to titania, and accordingly we have exchanged one cyano group for a carboxylic acid functionality and synthesized a series of dyes containing an electron donating group and the modified TCF acceptor. These dyes, which are of varying conjugation length and donor strength, absorb light strongly in the red region of the solar spectrum. Nanocrystalline titania solar cells will be assembled and the performance of the dyes will be compared to that of the N3 dye typically used in solar cells. Cyclic voltammetry and UV-Vis spectroscopy will be used to rationalize device performance. Furthermore, a high yield synthesis of 2,5-dihydro-2-imino-4,5,5-trimethylfuran-3-carbonitrile is described. This critical precursor to the carboxylic acid modified TCF is isolated in high purity after a multi-step synthesis. A focus of the work is the unusual conditions made necessary by working with highly electron deficient intermediates.
3:15 PM - Z6.4
Origin of Enhanced Light Harvesting in Photonic-Crystal-Based Dye-Sensitised Solar Cells.
Hernan Miguez 1 , Agustin Mihi 1 , Francisco Javier Lopez-Alcaraz 1 , Manuel Ocana 1 , Silvia Colodrero 1 Show Abstract
1 Institute of Materials Science of Seville, Spanish Research Council, Sevilla Spain
3:30 PM - Z6.5
Porous Nanocrystalline TiO2 Thin Films for Dye-sensitized Solar Cells.
Xiaojuan Fan 1 , Claudia Swanson 1 , David Rogow 1 , Akhilesh Tripathi 2 , Scott Oliver 1 Show Abstract
1 Chemistry & Biochemistry, UC at Santa Cruz, Santa Cruz, California, United States, 2 , Rigaku Americas Corporation, The Woodlands, Texas, United States
3:45 PM - Z6.6
A Potential of Large Scale of Dye Sensitized Solar Cells using Metallic Titanium Sheet as the Substrate for Photoelectrode.
Kinji Onoda 1 , Supachai Ngamsinlapasathian 1 , Takuya Fujieda 1 , Susumu Yoshikawa 1 Show Abstract
1 , Kyoto University, Uji Japan
4:30 PM - **Z6.7
Hybrid-Nanorod Polymer Solar Cells.
Paul Alivisatos 1 Show Abstract
1 Chemistry, U.C. Berkeley, Berkeley, California, United States
This talk will describe our recent work related to the fabrication of solar cells, based upon blends of inorganic semiconductor nanorods with conjugated polymers such as P3HT. Such blends offer the possibility of paint-on type cells that combine high electron and hole mobilities, but require the development of methods to reliably control the dispersion of the two materials on the nanometer scale. Several approaches will be described, including the incorporation of so-called tetrapods, branched four-armed inorganic nanocrystals that spontaneously align on a substrate and hyper-branched rods with tree-like features that provide pre-formed percolation pathways for photogenerated electrons.
5:00 PM - Z6.8
Photoelectrochemical and Photovoltaic Cells Based on CdSe Nanoparticles: Electron-Rich Thiophene Capping Agents which Provide for Incorporation Into Textured Photoelectrodes and Blended Heterojunctions.
R. Shallcross 1 , Gemma D'Ambruoso 1 , Jeffrey Pyun 1 , Neal Armstrong 1 Show Abstract
1 Chemistry, University of Arizona, Tucson, Arizona, United States
Semiconductor nanoparticles are potentially useful components of hybrid photovoltaic cells, and as the principal light-absorbing component in solar hydrogen producing photoelectrochemical cells. Both device types rely upon formation of a rectifying condensed phase or solution phase heterojunction, and when optimized, should produce sizeable photovoltages and photocurrents. This presentation will focus on our recent development of extremely monodisperse CdSe semiconductor nanoparticles (SC-NP), and electron-rich thiophene capping ligands, which provide for electrochemical cross-linking into a poly-(thiophene) (PEDOT or PRODOT) matrix. Under illumination the poly-(thiophene) matrix serves to stabilize the SC-NP through electron injection, and the SC-NP injects electrons into solution redox couples (solar hydrogen production) or an electron transport layer (e.g. C60 or its derivatives) for photovoltaic applications. Once the SC-NPs are electrochemically “wired” to the substrate electrode (ITO), one may monitor the photoluminescence as a function of applied potential in order to interrogate the efficiency of charge transfer in the system. The efficiency of these charge transfer events appears to correlated with the “band-edge offsets” in these hybrid materials, as revealed both by UV-photoelectron spectroscopic (UPS) studies of monolayers of the SC-NP (using He(II) excitation), and electrochemical characterization of both the SC-NP and the poly-(thiophene) matrix.
5:15 PM - Z6.9
Quantum Dot-Sensitized Nanowires for Photovoltaic Devices.
Kurtis Leschkies 1 , Divakar Ramachandran 1 , Joysurya Basu 1 , C. Carter 1 , Uwe Kortshagen 2 , David Norris 1 , Eray Aydil 1 Show Abstract
1 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States, 2 Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States
CdSe semiconductor nanocrystals (or quantum dots) were combined with single-crystal ZnO nanowires to demonstrate a new type of quantum dot-sensitized solar cell. An array of ZnO nanowires (lengths and diameters between 2–12 µm and 75–125 nm, respectively) were grown vertically from a fluorine-doped tin oxide conducting substrate seeded with 3–5 nm ZnO nanoparticles from an aqueous solution of Zn(NO3)2 and methenamine. Separately, CdSe quantum dots (3–4 nm in diameter) were prepared and capped with 3-mercaptopropionic acid. The carboxyl end-groups of the quantum dots were used as an anchoring group to graft the quantum dots onto the surface of the nanowires. The optical absorption properties of the quantum dots were preserved when the quantum dots were attached to the ZnO nanowires. Intimate quantum dot-nanowire contact creates a driving force to aid in charge separation at the interface of the two materials. When illuminated with visible light, the excited CdSe quantum dots injected electrons across the quantum dot-nanowire interface and our devices exhibited the photovoltaic effect. The topology of the nanowires guarantees that the photoinjected electrons have a direct connection to the collection electrode. The positively charged quantum dots can be neutralized by either hole injection into a hole conductor or through an electrochemical reaction with a redox couple in a liquid electrolyte. Using a liquid electrolyte as the hole transport medium, quantum dot-sensitized nanowire solar cells exhibited short-circuit currents ranging from 0.1–0.2 mA/cm2 and open-circuit voltages of ~0.5 V under simulated AM1.5 illumination.
5:30 PM - Z6.10
Nanostructured Heterojunction Photovoltaic Devices Fabricated in Porous Aluminum Oxide (PAO) Templates.
Martin Schierhorn 1 , Shannon Boettcher 1 , Nicholas Strandwitz 2 , Emily Norvell 3 , Galen Stucky 1 2 , Martin Moskovits 1 Show Abstract
1 Department of Chemistry and Biochemistry, UCSB, Santa Barbara, California, United States, 2 Department of Materials, UCSB, Santa Barbara, California, United States, 3 Department of Materials Engineering, California Polytechnic State University, San Louis Obispo, California, United States
Z7: Poster Session
Friday AM, April 13, 2007
Salon Level (Marriott)
9:00 PM - Z7.1
Organic-Inorganic Nanohybrids in Complex Media.
Zhiqun Lin 1 , Jun Xu 1 Show Abstract
1 Materials Science and Engineering, Iowa State University, Ames, Iowa, United States
Conjugated polymer-quantum dot (CP-QD) nanohybrids, in which CPs chemically anchor to the QD surface, are synthesized. Such CP-QD nanohybrids possess a well-defined interface that significantly promotes the charge or energy transfer between these two components. Subsequently, the nanohybrids are confined in nanoscopic geometries. The photophysical properties of nanohybrids depend on the molecular weight of CP and the dimension of cylindrical nanopores. The versatility of these hybrid nanomaterials is further demonstrated by constructing and photovoltaic devices and measuring their performance.
9:00 PM - Z7.10
Effect of Surface Modification by Solvent Exchange on Hybrid Bulk Heterojunction Solar Cell Performance.
Matthew Monroe 1 , Young Wook Kim 2 , Truong Nguyen Nguyen 2 , Bou-hye Kim 2 , Jaeseung Seol 3 , Timothy Anderson 1 , Chinho Park 2 Show Abstract
1 Chemical Engineering Department, University of Florida, Gainesville, Florida, United States, 2 School of Display and Chemical Engineering, Yeungnam University, Kyeongsan, Kyeongsangbukdo, Korea (the Republic of), 3 , LGMicron Co, Ltd, Gumi Korea (the Republic of)
Bulk heterojunction solar cells are promising candidates for high-efficiency low-cost photovoltaic energy conversion. In the hybrid bulk heterojunction solar cell, inorganic semiconductor nanocrystals are dispersed in a semiconducting conjugated polymer to create the cell’s active layer. For efficient device performance, percolation must be achieved in both the nanocrystal and the polymer phases, and the nanocrystals must be well-dispersed to provide efficient interfaces for exciton dissociation. This is accomplished through a uniform dispersion of nanocrystals in the film. A fundamental challenge limiting the performance of hybrid bulk heterojunction cells is controlling the morphology of the active layer film. In this study the microstructure of cadmium selenide nanocrystal and poly-3-hexylthiophene (P3HT) blends was investigated using Atomic Force Microscopy (AFM). The CdSe nanocrystals were synthesized using a solution-based process resulting in 5 nm diameter nanocrystals capped with tri-n-octylphosphine oxide (TOPO) surfactant. When the surface of the nanocrystals is treated so as to replace TOPO with pyridine, it was found that a blue-shift occurs in the crystal’s photoluminescence spectra. The morphology of composite films deposited from a binary solvent mixture of chloroform and pyridine was compared for TOPO and pyridine nanocrystal surfactants. Additionally, attempts were made to identify an optimal single solvent to for mixtures of P3HT and pyridine-coated CdSe nanocrystals.Investigation of the surface roughness of bulk heterojunction films has shown that for TOPO-coated CdSe nanocrystals in P3HT surface roughness of less than 10 nm can be achieved using a 1:1 solvent mixture of chloroform and pyridine. Using pyridine-coated CdSe nanocrystals, sub-10 nm roughness is achieved using a pyridine concentration of less than 10% in the solvent. In addition to being compatible with low pyridine content solvents, the pyridine-coated nanocrystals provide charge transfer pathways to allow efficient transport between the nanocrystal and polymer as evidenced through photoluminescence data. In testing alternate solvents for these mixtures, it was found that P3HT is insoluble in highly polar solvents such as dimethyl formamide (DMF) and methyl ethyl ketone (MEK). Low surface roughness was obtained for composite films of pyridine-coated CdSe and P3HT deposited from pure solvents of chloroform, chlorobenzene, and toluene. Hybrid bulk heterojunction solar cells were fabricated from these films, and the device performance was compared to that of the binary solvent mixture.
9:00 PM - Z7.11
Materials Aspects of Laminated Flexible Dye Sensitized Solar Cells.
Aurelien Du Pasquier 1 Show Abstract
1 , Rutgers, The State University of New Jersey, North Brunswick, New Jersey, United States
9:00 PM - Z7.12
Size-Controlled Synthesis of ZnS-Nanoparticles in an Organic Solvent and their Application in Photovoltaic Devices.
Thomas Rath 1 , Monika Piber 1 , Thomas Griesser 1 , Robert Saf 1 , Dieter Meissner 2 , Gregor Trimmel 1 , Franz Stelzer 1 Show Abstract
1 Graz University of Technology, Institute for Chemistry and Technology of Organic Materials, Graz Austria, 2 Upper Austrian University of Applied Sciences, co: Energy Engineering, Wels Austria
9:00 PM - Z7.13
Growth and Characterisation of α-Fe2O3 Nanowires for Photovoltaic Applications.
Pritesh Hiralal 1 , U. Wijayantha 3 , H. Emrah Unalan 1 , J. Driscoll 2 , G. Amaratunga 1 Show Abstract
1 Engineering, University of Cambridge, Cambridge United Kingdom, 3 Department of Chemistry, University of Loughborough, Loughborough United Kingdom, 2 Materials Science, University of Cambridge, Cambridge United Kingdom
9:00 PM - Z7.14
Controlled Growth of Novel Platinum Nanostructures on Silicon and their Application to Photoelectrochemical Catalysis.
Arnold Forman 1 , Galen Stucky 1 2 , Eric McFarland 3 Show Abstract
1 Department of Chemisty, University of California, Santa Barbara, Santa Barbara, California, United States, 2 Materials Department, University of California, Santa Barbara, Santa Barbara, California, United States, 3 Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, California, United States
9:00 PM - Z7.15
Doped Polyaniline/Crystalline Silicon pn Heterostructure Solar Cells.
Weining Wang 1 , Eric Schiff 1 Show Abstract
1 Physics, Syracuse University, Syracuse, New York, United States
9:00 PM - Z7.16
Network Structure Polymer/PCBM Organic Photovoltaic Devices
So Yeon Kim 2 , Kwang Hee Lee 2 , Byung Doo Chin 1 , Jai Kyeong Kim 1 , Jae-Woong Yu 1 Show Abstract
2 Deparment of Polymer Science & Engineering, Inha University, Incheon Korea (the Republic of), 1 Optoelectronic Materials Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
The performance of thin-film organic photovoltaic cells based on polymer blended with [6,6]-phenyl-C60-butyric acid methyl ester (PCBM) has been strongly related to morphology of active layer. Major factors influencing morphology of polymer/PCBM blend are the composition ratio and conditions of thermal treatment. Best polymer solar cell performance about 5% made with polymer/PCBM blend film has been obtained with an interpenetrating network structure at a fixed composition ratio and thermal treatment condition. However, at the solar cell operation temperature near 80°C, PCBM can diffuse into host polymer causing further phase separation and leading decrease in power conversion efficiency. In this study, [6,6]-phenyl-C60-butyric acid 3-ethylthiophene ester has been electrochemically copolymerized with 3-otcylthiophene so that PCBM unit is connected to the conducting polymer network. In this structure, PCBM can not move around even at the elevated temperature. The effect of anchoring PCBM unit in the conducting polymer matrix will be discussed.
9:00 PM - Z7.17
Optical and Electrical Properties of Composite Mesoporous Titania-P3HT films on Anodic Alumina Template
Hyun Jung Her 1 , Jung-Min Kim 1 , Chi-Jung Kang 1 , Yung-Jin Choi 1 , Yong-Sang Kim 1 2 Show Abstract
1 Nano Science and Engineering, Myong-Ji Univ., Yong-In Korea (the Republic of), 2 Electrical Engineering, Myong-Ji Univ., Yong-In Korea (the Republic of)
The nano-structured semiconductor materials has become popular in microfabrication since these materials can form very unique structures with high aspect ratios and possess superior electrical properties. Particularly in photovoltaic processes, titania (TiO2) is widely used as electron recipient medium. Photovoltaic cells which are consisted of conjugated or conducting polymer as the photoactive material, light absorption results to the creation of bound electron-hole pairs called excitons. The excitons have to be separated at the interface between an electron donor and acceptor material in order to create mobile charge carriers. The efficiency of photovoltaic cell depends on the number of excitons created during light emission and on the reaction of materials. Moreover, the average interfacial distance between electron donor and acceptor greatly affects photovoltaic efficiency. Therefore by maximizing the reaction area and by assuring that the interfacial distance between the donor and acceptor surfaces is within the exciton diffusion length, improved photovoltaic cell efficiency is expected. The surface to volume ratio of titania surface can be increased by creating mesoporous structure. This is accomplished by producing highly uniform nanoporous thin films of the dense array of titania (TiO2) pores with 70~80 nm diameter by nanoimprinting technique. The mesoporous titania was synthesized by coating the sol-gel mixture of titanium ethoxide (IV), HCl and 2-propanol on an indium tin oxide (ITO) surface. The coated surface was then embossed with an array of PMMA nanopoles which was produced using a nanoporous alumina (Al2O3) template. The potential of the fabricated composite mesoporous titania-poly(3-hexylthiophene) (P3HT) structure on photovoltaic cell synthesis is currently under investigation. Comparative studies between the conventional smooth titania films will be conducted hereafter.
9:00 PM - Z7.18
Spray Coated Dye Solar Cell with CNT Counter Electrode.
Won Jae Lee 1 , Easwaramoorthi Ramasamy 1 2 , Dong Yoon Lee 1 , Jae Sung Song 1 Show Abstract
1 Electric and Magnetic Devices Research Group, Korea Electrotechnology Research Institute, Changwon, Gyeongnam-do, Korea (the Republic of), 2 , University of Science & Technology, Daejeon Korea (the Republic of)
9:00 PM - Z7.21
Investigation of Device Optimization of Organic Bi-layer Solar Cells
Young Wook Kim 1 , Matthew Monroe 2 , Jiyoun Seol 1 , Jaeseung Seol 3 , Do-Hoon Kim 1 , Timothy Anderson 2 , Chinho Park 1 Show Abstract
1 School of Display and Chemical Engineering, Yeungnam University, Kyeongsan, Kyeongsangbukdo, Korea (the Republic of), 2 Department of Chemical Engineering, University of Florida, Gainesville, Florida, United States, 3 , LGMicron, Co, Ltd, Gumi Korea (the Republic of)
Organic solar cells offer the possibility of inexpensive and efficient energy conversion, and one design currently being investigated is the organic bi-layer solar cell. The bi-layer cell design is composed of p- and n-type thin films sandwiched together with charge transport enhancement layers between two electrodes. Recently, bi-layer organic solar cell efficiency has approached 5%. One reason for the low efficiency of organic devices is their low electron mobility compared to inorganic semiconductors. To improve device efficiency, this investigation attempts to develop a viable surface treatment technique for the ITO surface and optimize the thickness of the active layer materials. To quantify the effect of these tests, organic bi-layer solar cells with the structure ITO/HTL/CuPc/C60/BCP/Al were fabricated and the device performance measured.The effects of N2 and O2 plasma and electron beam treatment were compared using XPS, AFM, contact angle measurement, and solar cell performance. The efficiency of solar cells built on N2 plasma-treated ITO substrates was nearly double that of cells built on untreated substrates and was higher than that of other treatment techniques. This improvement is attributed to an increased hydrophilic nature of the surface, decreased surface roughness, and a slight decease in the In/Sn ratio in the ITO films. Parametric studies have been performed to identify fabrication conditions for optimum PV performance. A HTL (Hole Transport Layer) of PEDOT:PSS or m-MTDATA was deposited between ITO and the active layer. This layer provides a smooth surface for active layer deposition and reduces the energy barrier for hole collection at the ITO electrode. CuPc serves as the active layer where exictons are generated through photon absorption. C60 is the n-type material in this organic solar cell which transports electrons to the back electrode and provides a p-n junction to separate excitons. The BCP layer is an EBL (Exciton Blocking Layer) which helps to prevent exciton recombination at the Al electrode. We have optimized the organic solar cell structure through sequential optimization of each layer using fabricated cell efficiency as our primary performance measure.
9:00 PM - Z7.22
Two Dimensional X-shaped Oligothiophenes for Optoelectronic Applications.
Olga Zolotarskaya 1 , Yashpal Bhandari 1 , Mary Galvin 2 Show Abstract
1 Materials Science and Engineering, University of Delaware, Newark, Delaware, United States, 2 , Air Products and Chemicals, Allentown, Pennsylvania, United States
9:00 PM - Z7.24
Electric-Field-Assisted Aerosol Deposition of Metal Nanoparticles for Metal Nanostructure-Enhanced Organic Photovoltaic Cells
Shigeo Fujimori 1 2 , Rostam Dinyari 1 , Peter Peumans 1 Show Abstract
1 Electrical Engineering, Stanford University, Stanford, California, United States, 2 Electronic & Imaging Materials Research Labs., Toray Industries, Inc., Otsu, Shiga, Japan
Metal nanostructures can confine and guide electromagnetic energy on a nanometer scale over a wide spectral range that covers the solar spectrum. Metal core/insulator shell-nanoparticles are one class of metal nanostructures that can be incorporated into organic photovoltaic cells to bring about large gains in efficiency and to tune the spectral response of the cell. Such metal core-insulator shell structures lead to locally enhanced absorption but do not quench excitons by charge-transfer and can hence be used in the device active region, unlike prior demonstrations based on bare metal nanoparticles where the metal has to be spatially separated from the active region. However, the fabrication of multilayer devices with embedded solution-synthesized metal nanostructures cannot be realized via vacuum deposition because the metal nanoparticles cannot be sublimed. Solution processing is not an option because of solvent orthogonality requirements and because exposure to solvents degrades the performance of small molecular weight organic materials. To address this challenge, we have developed an aerosol deposition technique to introduce metal core/insulator shell nanoparticles into organic thin-film devices in a vapor-phase process. In our method, droplets of an organic solvent containing the nanoparticles and/or organic material are transported in an inert carrier gas to a substrate after removal of the solvent by evaporation. To transport the nanoparticles to the substrate effectively, the droplets are charged by corona discharge and an electric field is used to assist delivery of the nanoparticles to the substrate. Monolayer films of surfactant-coated Au-nanoparticles of varying density were deposited on glass substrates by this electric-field-assisted aerosol deposition. We will discuss how the aerosol deposition method was optimized to achieve deposition without clustering of the metal nanoparticles. We will also present our results on organic donor-acceptor solar cells with surfactant-coated Au nanoparticles embedded at the donor-acceptor interface.
9:00 PM - Z7.25
Direct Deposition of PbSe Nanoparticles in a Polymer Matrix by an Integrated Microwave Plasma-spin Coating Process.
Sarath Witanachchi 1 , Gayan Dedigamuwa 1 , Marek Merlak 1 , Pritish Mukherjee 1 , Xiaomei Jiang 1 Show Abstract
1 Department of Physics, University of South Florida, Tampa, Florida, United States
9:00 PM - Z7.26
Swollen Polymers for the Synthesis of Photovoltaic Devices
Claudia Swanson 1 , David Rogow 1 , Xiaojuan Fan 1 , Scott Oliver 1 Show Abstract
1 Chemistry and Biochemistry, University of California, Santa Cruz, Santa Cruz, California, United States
We are studying swollen block copolymers as a medium in which to grow macroporous zirconium oxide for photovoltaic devices. Polystyrene/polymethylmethacrylate and polystyrene/polybutadiene block copolymers form a highly viscous mixture with zirconium n-propoxide. The solution is air-dried to give a hybrid inorganic-organic polymer material, and is then pyrolyzed to yield a three-dimensional ZrO2 framework. The material has been characterized by optical microscopy, SEM, TEM and powder X-ray diffraction. The porous material will be dip-coated with a photosensitizer to determine both its electrical conductivity and feasibility for photovoltaic applications.
9:00 PM - Z7.27
Synthesis and Self-assembled Nanostructure of Polythiophenes Grafted Copolymers.
Xiwen Chen 1 , Xu Han 1 , George Vamvounis 1 , Steven Holdcroft 1 Show Abstract
1 Chemistry, SFU , Burnaby, British Columbia, Canada
9:00 PM - Z7.28
Enhanced Open Circuit Voltage in SWCNTs/P3HT Nanocomposite Photovoltaics
Tingying Zeng 1 , Jianxin Geng 1 Show Abstract
1 Chemistry, Western Kentucky University, Bowling Green, Kentucky, United States
Carbon nanotube/semiconductor polymer nanocomposite films play an important role in the development of organic photovoltaics (OPVs). The designed OPV performance is directly depend on the formed nanoarchitectures. In this paper, our experiments demonstrated a new phenomenon that purified single wall carbon nanotubes (SWCNTs) enhanced the crystallinity of poly(3-hexylthiophene) (P3HT), which led to a significant increase of open circuit voltage of the SWCNTs/P3HT nanocomposite photovoltaic cells. The possible reason might result from the interaction of sulfur atoms in thiophene ring on polymer chain with the pentagon defects on the wall of SWCNTs due to the electron pairs on the sulfur atoms stacking with the Pi-Pi orbitals of carbon nanotubes, which may cause the SWCNT work function changed. (Accepted by JACS). ACKNOWLEDGMENT This work is partially supported by the Junior Faculty Scholarship at Western Kentucky University through the Office of Sponsored Programs, by the U.S. National Science Foundation under contract #0520789, and by KY NSF EPSCoR REG Program under contract #260501, respectively.
9:00 PM - Z7.29
The role of DNA thin film in PCBM/P3HT Bulk Heterojunction Solar cells.
Vidyalakshmi Kolachure 1 , Michael Jin 1 Show Abstract
1 Materials Science and Engineering, University of Texas at Arlington, Arlington, Texas, United States
We present the use of DNA-CTMA (Deoxyribonucleic acid-Cetyl trimethyl ammonium chloride) thin film as a hole transport layer replacing PEDOT-PSS in conventional bulk heterojunction solar cells. Recent work has demonstrated that DNA-CTMA can be used as a hole transport layer in organic light-emitting diodes (OLEDs) showing electrical conduction behavior similar to the organic semiconducting polymers . With a LUMO (Lowest Unoccupied Molecular Orbit) level of 0.9 eV, DNA-CTMA is also an efficient electron blocking layer improving the efficiency of the OLEDs . DNA-CTMA provides another benefit because it is soluble in organic solvents preventing any inconvenient wetting problem often faced in the spin coating process of the PEDOT-PSS on top of transparent conducting oxides and typical PCBM/P3HT can be also easily spin-coated on top of DNA-CTMA layer without dissolving it.We have fabricated bulk heterojunction solar cells using regioregular poly(3-hexylthiophene) (P3HT) as an electron donor material and the fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) as an electron acceptor material. Thin film DNA-CTMA is spin-coated from its butanol solution on top of the bottom electrode prior to the bulk heterojunction formation. With a proper choice of solvents, both DNA-CTMA and PCBM/P3HT solutions provide the excellent processibility with uniform layers. Cyclic Voltammetry (CV) measurements of the DNA-CTMA are carried out to find the effect of thin film process parameters on the oxidation and the reduction potential of the complex. Solar cell characteristics will be further discussed.  Kunio Hirata, Takahito Oyamada, Toshiro Imai, Hiroyuki Sasabe, Chihaya Adachi and Tamami Koyama, Appl. Phys. Lett. 85, 1627 (2004).  J. A. Hagen, W. Li, A. J. Steckl, and J. G. Grote, Appl. Phys. Lett. 88, 171109 (2006).
9:00 PM - Z7.3
Effects of C70 Derivative in Low band Gap Polymer Photovoltaic Devices: Spectral Complementation and Morphology Optimization.
Yan Yao 1 , Gang Li 1 , Qibing Pei 1 , Yang Yang 1 Show Abstract
1 Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California, United States
9:00 PM - Z7.30
Colloidal III-V Quantum Dots for Polymer Photovoltaics
Annick Anctil 1 , Brian Landi 1 , James Worman 1 , Ryne Raffaelle 1 Show Abstract
1 Physics, Rochester Institute of Technology, Rochester, New York, United States
Semiconducting quantum dots (QDs) are being investigated as multifunctional additives in polymer photovoltaic devices. Such nanomaterials are capable of size-dependent optical absorption, exciton dissociation centers in conjugated polymers, and a means for carrier transport. Generally, polymer photovoltaic devices rely solely on photon absorption by the conducting polymer and as such are bandgap (Eg)-limited (typically > 2 eV) in regard to the solar spectrum. Therefore, QDs with optical bandgaps below the conducting polymer Eg can allow composite devices to absorb a larger portion of the solar spectrum. Therefore, we have focused on the synthesis of III-V semiconducting QDs like GaAs, InAs, GaSb, and InSb; since they are ideal candidates for this role based on the quantum mechanical tuning of their optical bandgaps by size, composition, and surface chemistry. We will review the colloidal synthesis schemes involving the use of Tris(trimethylsilyl)arsine and Tris(trimethylsilyl)antimony as precursors in the growth of these QD structures. Optical characterization involving absorption and fluorescence will demonstrate the size-dependent bandgaps over the desired near-infrared range. In addition, spectral response and air mass (1.5) current-voltage measurements will be shown for QDs dispersed into the conventional ITO/PEDOT/poly(3-hexylthiophene)/Al device structure.
9:00 PM - Z7.31
Hybrid Monolithic Tandems of CIGS and DSSC with Transparent Carbon Nanotube Recombination Layer
Anvar Zakhidov 1 , Hasan Shodiev 1 , William Shafarman 2 Show Abstract
1 Physics, University of Texas at Dallas, Richardson, Texas, United States, 2 Institute of Energy Conversion, University of Delaware, Newark, Delaware, United States
Multi-junction solar cells enable harvesting of wider regions of the solar radiation spectrum leading thereby to increased overall efficiencies. Graetzel’s group at EPFL has developed a double junction dye sensitized solar cells (DSC), where the top and bottom cells harvest respectively the visible and near IR regions of the solar spectrum. Recently this group has also demonstrated a greater than 15% total efficiency tandem array involving an inorganic thin film CIGS (CuInGaSe2) bottom cell and DSC as a top cell. These demonstrated tandem devices with DSCs have been composed of stacked individual cells, each built on a substrate using a separate set of electrodes, and electrically connected in series. A natural next step is attempting to create monolithic multi-junction cells with DSCs and CIGS, which, in principle, could lead to smaller losses and more efficient and economic structures. We present here a first study of a hybrid monolithic structure composed of DSCs with DSCs and OPVs or DSCs with thin film inorganic CIGS. We have created several architectures of monolithic multi-junction cells and address fundamental connectivity issues by using sheets of strong, transparent carbon nanotubes (T-CNTs) recently produced at UTD  as a uniform interlayer platform.. Such T-CNTs are dry-spun as a free standing 3-D aerogel sheet from a forest of multiwall CNTs and have high inherent electrical and thermal conductivities. The UTD team has demonstrated advantages of flexible and chemically stable T-CNTs, densified from aerogels into 50-100 nm films as anodes in OLEDs  as well as in an OPV with unoptimized h = 2.4 % and in a DSC . Free-standing T-CNT networks can be applied (like a scotch tape) onto any surface and their usefulness as transparent interlayers in monolithic tandems is shown here for a tandem in which a non-finished CIGS (the top ITO layer is absent) is coated by T-CNTs. Such CIGS with T-CNT shows Voc=0.6 V and Isc ~ 10 mA/cm2. It has been combined with DSC as a photoactive counter electrode, in a conventional DSC with iodine based electrolyte and well known Ru-dye o TiO2 mesoscopic electrode (both from Dyesol). The tandem demonstrated Voc+ 0.82 V, which is higher than Voc of our sole DSC and Isc= 1mA/cm2, which is smaller than photocurrent of DSC (typical Isc=8 mA/cm2) due to unbalanced current and shading of light by unoptimized structure.
9:00 PM - Z7.32
Solar Cells from Colloidal Type-1 Quantum Dots and Type-2 Quantum Dots.
Jiwon Bang 1 Show Abstract
1 , Postech, Pohang Korea (the Republic of)
9:00 PM - Z7.33
Hybrid Solar Cells from Polymers and Silicon Nanocrystals
Chin-Yi Liu 1 , Uwe Kortshagen 1 Show Abstract
1 Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States
9:00 PM - Z7.5
Template-Assisted Large Scale Vertical Implantation of Crystalline Titania Nanotube Arrays.
Svetlana Khvan 1 , Junkyung Kim 1 , Sang-Soo Lee 1 Show Abstract
1 Polymer Hybrid Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
The ability to control the nanostructure of titania (TiO2) promises to make a great impact on various fields of science and technology. For this reason, recent research has focused on developing unique nanostructures by manipulating their architecture at the nanoscale to adapt these materials to specific applications. Among the many applications of nanosized titania, low cost, high efficiency solar cells are of particular interest to anyone engaged in harnessing solar energy. Dye-sensitized solar cells (DSSC) consisting of a nanoparticular TiO2 layer have been extensively investigated for the last few decades. It is believed that one of the options for further improving the overall conversion efficiency of DSSC would be a vertical alignment (perpendicular to a substrate) of well-ordered titania nanostructures. Replacement of TiO2 nanoparticles by high-aspect-ratio nanotubes is one of the most promising approaches because by using an ordered one-dimensional nanostructure, the intercrystalline contacts can be substantially reduced, thus minimizing inefficient electron percolation pathways and facilitating electron transport through the titania layer to the current collecting electrode. Moreover, higher surface area will result in higher dye adsorption on the titania surface.This work presents a template-assisted method to fabricate the TiO2 nanotube arrays implanted on a transparent electro-conducting glass substrate. TiO2 nanostructures were synthesized by a sol-gel process using an anodic alumina (AAO) membrane as a template. A procedure for the vertical implantation of the nanotube arrays on the substrate was developed. The titania nanotubes well aligned within the template were adhered orthogonally to the conductive glass substrate with the assistance of a thin particulate titania layer. A length of the titania nanotube array was shortened to approximately 20-30 μm by polishing or etching the membrane surface with rf power. By controlling the fabrication procedure such as pre-etching, sintering, thickness of the AAO template and etching conditions, and by manipulating the filter layer of the AAO membrane, diverse morphology of the final titania nanostructure layer was attained. As observed from SEM images, the assisting nanoparticulate titania layer provided implantation of the high-aspect-ratio titania nanotube arrays perpendicularly to the substrate. It is likely that the thickness of the assisting titania layer could be further reduced to a minimum. Compositional analysis performed with XPS and EDS together with TEM observation confirmed that the alumina template was dissolved by chemical etching.The method discussed here enables the designing of an adaptable architecture for nanostructured titania film applicable to diverse applications. Exploration for an optimal design of the nanostructured titania film to achieve a high performance of the electrode in DSSC is the challenge of our on-going work.
9:00 PM - Z7.6
Silicon Nanoparticles for Photovoltaic Applications.
Cedrik Meier 1 , Nadine v.d. Schoot 1 , Hartmut Wiggers 1 , Andre Ebbers 2 , Axel Lorke 1 Show Abstract
1 Physics and Engineering Departments, University of Duisburg-Essen, Duisburg Germany, 2 , Degussa AG, Duisburg Germany
9:00 PM - Z7.8
Modeling-Based Optimization of the Ordered Quantum Dot Superlattices for the Photovoltaic Efficiency Enhancement.
Qinghui Shao 1 , Alexander Balandin 1 , A. Fedoseyev 2 , M. Turowski 2 , S. Bailey 3 Show Abstract
1 Nano-Device Laboratory, Department of Electrical Engineering, University of California, Riverside, Riverside, California, United States, 2 , CFD Research Corporation, Huntsville, Alabama, United States, 3 John H. Glenn Research Center , National Aeronautics and Space Administration, Cleveland, Ohio, United States
Quantum dots can form three-dimensional (3D) ordered superlattices with the inter-dot spacing sufficiently small for the strong electronic coupling and formation of mini-bands . The mini-band formation can enhance the photovoltaic efficiency via several possible mechanisms. It creates an intermediate band, which allows one the harvesting of a much larger portion of the available solar spectrum; improves the long-range electron transport and collection efficiency; slows the carrier cooling and permits the transport and collection of the hot carriers to produce a higher photovoltaic potential. It was also suggested that the quantum dot superlattices (QDS) offer improved radiation hardness . Any practical application of QDS in solar cells would require accurate tuning of the QDS parameters such as quantum dot size, shape, inter-dot separation, dot regimentation, material properties and interface quality. Modeling of the carrier mini-bands, absorption processes, and charge transport in realistic quantum dots and surrounding materials is essential for the development of QDS photovoltaic elements. In this presentation we will discuss the modeling-based optimization of the QDS for the radiation-hardened solar cells applications. Specifically, we utilized the theoretical formalism previously developed by some of us [1, 3] to simulate the mini-bands and carrier transport in InAs/GaAs QDS (with the dot sizes varying from 2 to 11 nm). We show that the mini-band formation and corresponding changes in the electron (hole) density of states result in the strong modification of the electrical current through the QDS. The onset of non-Ohmic behavior in QDS has been observed at the lower electric field intensity than that in corresponding bulk material. Strong dependence of the electrical conductivity tensor on the parameters of QDS creates an exciting opportunity for re-engineering the photovoltaic properties of these structures through modification of their electronic states. This work has been supported by the NASA project on the Novel Solar Cell Nanotechnology for Improved Efficiency and Radiation Hardness.  O.L. Lazarenkova and A. Balandin, J. Appl. Phys., 89, 5509 (2001).  A. I. Fedoseyev , M. Turowski, Q. Shao, A.A. Balandin , “Novel Solar Cell Nanotechnology for Improved Efficiency and Radiation Hardness” (invited), in Photonics for Space Environments XI, Proceedings of SPIE (ISBN: 0-8194-6387-6), Vol. 6308, Edited by E.W. Taylor (2006).  O.L. Lazarenkova and A.A. Balandin, Phys. Rev. B, 66: 245319 (2002).
9:00 PM - Z7.9
Tuning the Morphology of Organic Bulk-Hetero Junction Photovoltaic Devices for a more Balanced Charge Transport.
Lacramioara Popescu 1 , Harry Jonkman 1 , Jan Hummelen 1 Show Abstract
1 Materials Science Center, University of Groningen, Groningen Netherlands
The charge carrier mobility is often the limiting factor in the overall efficiency of bulk hetero-junction photovoltaic devices. Low mobility combined with an unbalance between the electron and hole transport channels lead often to severe space charge limited currents in these devices. The carrier mobility turns out to be very sensitive for the overall morphology and in special to the degree of crystallinity of the photo-active layer. We did study the effect of the morphology of devices consisting of a blend of regioregular poly(3-hexylthiophene) (P3HT) as the electron donor and 1-(3-methoxycarbonyl)propyl-1-thienyl-[6,6]-methanofullerene (ThCBM) as the new electron acceptor. It turns out that slow growing conditions, i.e. active layers are spin cast from high boiling point solvents, lead to more crystalline films. This enhances the interchain interaction in the P3HT phase and scanning probe microscopy shows the formation of a superstructure which can be indentified as a whisker like structure. This overall morphology improves the hole transport in the blend by an order of magnitude relative to annealed fast grown active layers. This leads to a more balanced charge transport in the blend .Although the open circuit voltage tends to be lowered (with approximately 40 mV) in devices with active layers with a higher degree of molecular organization, it is the dramatic increase in fill factor (up to 70 %) which gives much higher power conversion efficiencies.
Michael F. Durstock Air Force Research Laboratory
Anvar Zakhidov University of Texas-Dallas
Michael Graetzel Ecole Polytechnique Federale de Lausanne
Lynne A. Samuelson U. S. Army RDECOM
Z8: Nanoparticle Hybrid Cells II and Surfaces & Interfaces
Friday AM, April 13, 2007
Room 2009 (Moscone West)
9:30 AM - **Z8.1
Carrier-Multiplication and Field-Enhancement Effects From the Perspective of Solar Energy Conversion.
Victor Klimov 1 Show Abstract
1 Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Solar cells represent an important clean source of energy. However, in order to make them competitive with traditional energy sources, the cost-to-efficiency ratio of photovoltaics must be reduced appreciably. Cost considerations have been a strong driver for the development of non-Si devices that are instead based on, e.g., polymers (plastic cells) and dye-sensitized porous metal oxides (Graetzel cells). Increases in efficiency have typically relied on iterative improvements of material quality (for both Si and non-Si systems) and/or device engineering aspects including, e.g., the use of tandem architectures. There exist, however, approaches that can potentially lead to a leap in photovoltaic performance through the use of new principles for harvesting solar energy and converting it into charge carriers. Two such principles that will be discussed in this presentation involve the use of carrier multiplication in semiconductor nanocrystals [1-5] for increasing the conversion efficiency of solar photons to electrical charges and hybrid semiconductor/metal “nano-antenna” structures  for enhancing the light-harvesting efficiency of photovoltaic devices. These new ideas can be implemented using inexpensive wet-chemistry techniques, which should allow one to simultaneously address both the efficiency and cost issues. 1.Schaller, R.D. and V.I. Klimov, High efficiency carrier multiplication in PbSe nanocrystlas: Implications for solar-energy conversion. Phys. Rev. Lett., 2004. 92: p. 186601-1-4.2.Schaller, R.D., M. Sykora, J.M. Pietryga, and V.I. Klimov, Seven excitons at a cost of one: Redefining the limits for conversion efficiency of photons into charge carriers. Nano Lett., 2006. 6: p. 424-429.3.Schaller, R.D. and V.I. Klimov, Non-Poissonian exciton populations in semiconductor nanocrystals via carrier multiplication. Phys. Rev. Lett., 2006. 96: p. 097402-1-4.4.Klimov, V.I., Mechanisms for photogeneration and recombination of multiexcitons in semiconductor nanocrystals: Implications for lasing and solar energy conversion. J. Phys. Chem. B, 2006. 110(34): p. 16827-16845.5.Klimov, V.I., Detailed-balance power conversion limits of nanocrystal-quantum-dot solar cells in the presence of carrier multiplication. Appl. Phys. Lett., 2006. 89: p. 123118-1-3.6.Liu, N., B.S. Prall, and V.I. Klimov, Hybrid gold/silica/nanocrysta-quantum-dot superstructures: Synthesis and analysis of semiconductor-metal interactions. J. Am. Chem. Soc., 2006.
10:00 AM - Z8.2
Germanium Nanocrystals for Thin-Film Photovoltaic Devices
Zak Holman 1 , Ryan Gresback 1 , Uwe Kortshagen 1 Show Abstract
1 Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States
Semiconductor nanocrystals have received much attention recently due to their size-tunable optical and electronic properties. However, their application in economical, environmentally-benign electronic devices (e.g.—solar cells) has been limited thus far by costly or toxic constituent materials (e.g.—cadmium), and processing difficulties. We report on the synthesis and applications of non-toxic germanium (Ge) nanocrystals, which hold promise for thin-film nanocrystal electronic devices. Germanium tetrachloride is dissociated in a non-thermal plasma reactor to form spherical, crystalline Ge nanoparticles. TEM studies have verified that this method, unlike other techniques used to synthesize Ge nanocrystals, produces crystals that are free-standing and nearly monodisperse (10-15% mean particle size standard deviation), with diameters which may be tuned from 4-20 nm by varying the plasma parameters. Experiments have been done in which crystals are deposited directly from the gas phase onto glass substrates and subsequently coated with a polystyrene matrix, yielding networks of Ge nanocrystal agglomerates encapsulated in polymer, as seen with SEM. Room-temperature current-voltage measurements were then performed both in the dark and under AM1.5 solar-simulating light across aluminum electrodes patterned on the substrates prior to nanocrystal deposition. These measurements reveal that the samples exhibit conductivities which are field-dependent, photo-activated, and surprisingly large (~10-6 S/cm at 104 V/cm in the light, ~3 × 10-7 S/cm in the dark). Nanocrystals synthesized in the plasma may also be transferred to the liquid phase, where we have utilized colloidal chemistry techniques to attach ligands to the Ge crystals’ surfaces which passivate dangling bonds, inhibit surface oxidation, and prevent agglomeration. Solubility tests in non-polar solvents indicate that 1-dodecene, octadecene, styrene, and hexylamine molecules have all been successfully attached to the Ge nanocrystals’ surfaces. In the case of 1-dodecene, FTIR measurements have also been performed and clearly indicate that the carbon double bond in 1-dodecene is broken and a germanium-carbon single bond is formed during the reaction. Through TEM studies of 1-dodecene- and hexlyamine-terminated crystals drop-cast onto TEM grids, we have observed the self-assembly of Ge nanocrystals into glassy thin films. The spacing between adjacent crystals in the films is roughly twice the length of the ligands, suggesting that the intercrystal spacing—and therefore important film properties which depend on the superlattice structure—can be tailored by adjusting the terminating ligands.This work was supported by NSF under NIRT grant CBET-0506672 and in part under MRSEC grant DMR-0212302, and by IREE grant LG-C5-2005.
10:15 AM - Z8.3
Surface Plasmons as a Route to Light Concentration: The Case of Quantum Dots in Self-Assembled Hybrid Nanostructures.
Manuel Romero 1 , Jao van de Lagemaat 1 , Garry Rumbles 1 , Mowafak Al-Jassim 1 Show Abstract
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Surface plasmons (SPs)–quantized collective oscillations of free electrons with a photon-electron dual nature–can be confined and concentrated in nano-structured metals and, therefore, represent a very attractive alternative to concentration in solar cells. We are investigating how quantum dots (QDs) respond to local SP excitation by scanning tunneling microscopy (STM). Dots are self assembled on gold substrates covered by a monolayer of 1,6-hexanedithiol. We have produced QD sub-monolayers, single monolayers, multilayers and multidot structures.In STM, confinement of SPs to the optical cavity formed under the tip stimulates the emission of light. We use this plasmon nanosource as the central component of a scanning optical microscopy. The combination of superb resolution in STM and energy resolution in scanning tunneling spectroscopy is applied to investigate the above-mentioned self-assembled QD structures. Resonant extinction of the SP emission is observed when a QD is found in the tunneling gap during STM measurements. Such resonances follow the optical fundamental transition of the QD. The strong coupling of plasmons with the energy levels of the QD can lead to massive enhancements in the absorption of incoming photons.In this contribution we address the following questions: How does the frequency of the SP affects the resonant extinction by QDs? Is the behavior of QDs influenced by any near-field effects? How does the local environment of the dots influence the energy transfer? At the end, the applicability of these concepts to photovoltaics will be discussed.
10:30 AM - Z8.4
Hybrid Gold/Silica/Nanocrystal-Quantum-Dot Superstructures: Synthesis and Analysis of Semiconductor-Metal Interactions.
Bradley Prall 1 , Nanguo Liu 1 , Victor Klimov 1 Show Abstract
1 , Los Alamos National Lab, Los Alamos, New Mexico, United States
We report on the synthesis and spectroscopic characterization of well-defined hybrid structures that consist of a gold core overcoated with a silica shell, followed by a dense monolayer of CdSe nanocrystal quantum dots (QDs). The dielectric silica spacer of a controlled thickness provides a simple means for tuning interactions between the QD emitters and the metal core. To illustrate this tunability, we demonstrate switching between QD emission quenching and enhancement by varying the silica shell thickness. Synthetic procedures developed here employ a final step of self-assembly of QDs onto the silica shell performed via simple titration of the QD solution with pre-fabricated core/shell Au/SiO2 particles. This approach allows us to perform an accurate quantitative analysis of the effect of the metal on the QD emission intensity. One important result of this analysis is that nonuniformity of nonradiative rates across the QD ensemble has a significant effect on both the magnitude and the shell-thickness dependence of the emission enhancement/quenching factors.
10:45 AM - Z8.5
Photocatalytic Assemblies Based on CdSe Nanocrystal Quantum Dots and Surface Adsorbed Ru-polypyridine complexes.
Milan Sykora 1 , Melissa Petruska 1 , James Alstrum-Acevedo 2 , Ilya Bezel 1 , Jason Leonard 1 , Thomas Meyer 2 , Victor Klimov 1 Show Abstract
1 C-PCS, LANL, Los Alamos, New Mexico, United States, 2 Chemistry, University of North Carolina, Chapel Hill, North Carolina, United States
Ru-polypyridine complexes have been studied extensively with one point of interest being their possible inclusion in solar energy conversion schemes as molecular catalysts. Utilization of Ru-complexes as components of photocatalytic systems requires that they are activated by an appropriate sensitizer. In the presented work, we demonstrate a new approach to sensitization of Ru-complexes by using CdSe nanocrystals (NCs) as light sensitizers. Here, we provide a summary of interfacial energy transfer (ET) and charge transfer (CT) studies in assemblies consisting of high quality CdSe nanocrystals (NCs) (size dispersion ~5-8%) and surface adsorbed Ru-polypyridine complexes. We show that in NC/Ru-complex assemblies the NQD photoluminescence (PL) is quenched with efficiencies up to 100%. The mechanism of quenching is either ET or CT, depending on NC size. In the case of small NCs (PLmax = 520 nm), the dominant interaction mechanism is dipole-dipole type ET due to the large overlap of the NC PL and the absorption spectrum of the complex. The energy transfer dynamics are sufficiently fast to compete with non-radiative carrier recombination in the CdSe NCs. In the case of large NCs (PLmax = 610 nm), the overlap of the NC PL and the absorption spectrum of the complex is small and the dominant interaction mechanism is CT. Studies of CT dynamics via combination of ultrafast transient absorbance and upconversion PL methods reveals that the hole transfer occurs with the time constant of ~5ps. This constant is shorter than the characteristic Auger recombination times in NCs, suggesting that the NC/Ru-complex assemblies can be utilized for extraction of multiple redox equivalents produced in the NC via the carrier multiplication process [1, 2]. This opens interesting opportunities for designing new types of photocatalytic materials for solar energy conversion applications. Schaller, R. D.; Klimov, V. I. Phys. Rev. Lett. 2004, 92, 186601-1–4.  Schaller, R. D.; Sykora, M.; Pietryga, J. M.; Klimov, V. I. Nano Lett. 2006, 6, 424-429.
11:30 AM - Z8.6
Interfacial Studies of Polymer/ZnO Hybrid Solar Cells
Dana Olson 1 , Yun-Ju Lee 1 , Erik Spoerke 1 , Matthew White 2 3 , Sean Shaheen 2 , David Ginley 2 , Terry Guilinger 1 , William Steen 1 , James Voigt 1 , Julia Hsu 1 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 Physics, University of Colorado, Boulder, Colorado, United States
Recently, much research has been conducted on the synthesis, characterization, and optimization of hybrid conjugated polymer/metal oxide photovoltaic (PV) devices. Such devices may offer a low-cost alternative to current inorganic PV technologies through the use of solution grown nanostructured organic and inorganic composite materials. Upon photoexcitation, the poly(3-hexylthiophene) (P3HT) transfers an electron to the zinc oxide (ZnO) at the heterojunction interface. Here, we study the effect of the interface and interfacial modifications on the optical and morphological properties of the P3HT, and the hybrid device performance.In order to understand the effect of interfacial interactions, we use a model system of P3HT/ZnO bilayer films synthesized through solution-based processes. The composite films were characterized using optical absorption spectroscopy before and after subsequent annealing to enhance crystallinity of the P3HT as well as for infiltration of P3HT for ZnO nanorod-based devices. We have found that the characteristics of the final P3HT layer depend on the substrate on which the P3HT was deposited, the solvent of the P3HT solution, the solution concentration, and subsequent annealing and cooling processes. P3HT films less than 15 nm thick exhibit a blue-shift (> 55 nm) when deposited on ZnO-coated substrates, which is further enhanced after annealing. This is believed to indicate a disruption in the conjugation length in the polymer chains, which leads to an increased band gap and degrades the transport properties of P3HT. Functionalizing the ZnO surface with organic modifiers, such as catechol, various dopamine and functionalized fullerene molecules, or inorganic coatings can eliminate the changes in P3HT in these composite films. The effect of the interface modifiers were studied using optical absorption, photoluminescence quenching, Kelvin probe, and through the PV performance of bilayer devices. Such interfacial modifications will then be incorporated into ZnO nanorods in order to enhance the performance of nanostructured devices. Additionally, devices based on spin coated polymer films are compared to those with polymers formed through electrochemical polymerization.The authors would like to thank the IC Post Doctoral Fellowship for partial funding this research. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
11:45 AM - Z8.7
Understanding Blended Solar Cell Performance with Electrical Scanning Probes
David Coffey 1 , Obadiah Reid 1 , David Ginger 1 Show Abstract
1 Chemistry, University of Washington, Seattle, Washington, United States
Organic composites comprising blends of conjugated polymers with fullerenes, nanocrystals, or other polymers make promising materials for low cost photovoltaic applications. Different processing conditions are known to impact the efficiency of these blended solar cells by creating a variety of nanostructured film morphologies. However, the relationship between local film structure and device efficiency is not fully understood. We apply time-resolved Electrostatic Force Microscopy (trEFM) and photocurrent conducting Atomic Force Microscopy (pcAFM) to study photoinduced charge generation and transport with resolutions better than 50 nm in several model organic semiconductor device structures under photoexcitation. We show that our trEFM measurements and pcAFM can correlate local variations with overall external quantum efficiencies for several blend systems, providing a direct link between local morphology, local optoelectronic properties, and device performance. We show that in some systems domain interfaces visible in topographic AFM lead to reduced photocurrent, while in other blends, such features lead to elevated photocurrents. We explain these differences in terms of local compositional and structural variations, and discuss how this improved understanding of microscopic charge generation, recombination and transport can be used to improve the design of blended organic solar cells.
12:00 PM - Z8.8
Scanning Probe Microscopy on Operational Organic Solar Cells.
Klara Maturova 1 , Martijn Kemerink 1 , Rene Janssen 1 Show Abstract
1 Molecular Materials and Nanosystems, Eindhoven University of Technology, Eindhoven Netherlands
12:15 PM - Z8.9
ITO Electrical Non-Uniformity and the Diode Quality Factor: A Conducting-Tip Atomic Force Microscope Study
Alexander Veneman 1 , Michael Brumbach 1 , Thomas Schulmeyer 1 , Neal Armstrong 1 Show Abstract
1 Chemistry, University of Arizona, Tucson, Arizona, United States
Organic photovoltaics (OPVs) are typically deposited on transparent conducting oxides, such as Indium-Tin Oxide (ITO). These are complex oxides with surfaces known to be heterogeneous both in terms of composition and electrical properties on a nanometer length scale. We have recently determined that this compositional and electrical heterogeneity can seriously affect the device parameters and performance of OPVs which are scaled up to larger device areas. This presentation focuses on the use of Conducting Tip Atomic Force Microscopy (C-AFM) to explore the relation between electrical heterogeneity of the ITO surface and the performance of OPV devices. C-AFM with Pt tips, which form ohmic contacts with most of our organic layers, has been used to obtain current-voltage curves on ca. 20nm2 areas across ITO/organic thin films. In this study one of three surface treatments (detergent cleaning, oxygen plasma treatment or an aggressive acid etch) was applied to ITO, followed by vacuum deposition of ca. 8 nm of copper phthalocyanine, the first organic layer of our OPV devices (ITO/Copper Phthalocyanine/C60/Aluminum Quinolate/Aluminum). In general the current density varies by several orders of magnitude (from ca. 104 A/cm2 to 107 A/cm2 at 1V bias) for different points on a given ITO/Pc film. Prior aggressive etching of the ITO film with strong halo acids leads to Pc/ITO junctions with I/V profiles exhibiting current densities that are constant within a factor of ca. 10 (106 to 107 A/cm2 at 1V bias). OPVs made on acid-etched ITO reproducibly exhibit a diode quality factor of 2.09 +/- 0.05 as predicted for an interface recombination-limited diode, whereas OPVs made on oxygen plasma etched ITO and detergent cleaned ITO show average diode quality factors of 3 or 4 respectively. These higher diode quality factors are a consequence of greater spatial variation in the rate of hole injection at the ITO/PC interface, resulting in non-uniform carrier concentrations at the donor-acceptor interface. This leads to lower fill factors and poor repeatability device to device. This result shows that the operating parameters of a macroscopic OPV can be drastically affected by chemical heterogeneity on a nanometer length scale and gives an explanation for unphysically large diode quality factors sometimes seen in OPV devices.
12:30 PM - Z8.10
Surface Functionalization and Passivation of GaAs Nanocrystals.
Matthew Traub 1 , Bruce Brunschwig 2 , Nathan Lewis 1
1 Chemistry, California Institute of Technology, Pasadena, California, United States, 2 Molecular Materials Research Center, California Institute of Technology, Pasadena, Californ