Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session L16: Focus Session: Polymers and Energy: Photovoltaics II |
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Sponsoring Units: DPOLY GERA DMP Chair: Tom Russell, University of Massuchusetts at Amherst Room: B115 |
Tuesday, March 16, 2010 2:30PM - 2:42PM |
L16.00001: Photo-physics of P3HT blended with highly enriched metallic and semiconducting single-walled carbon nanotubes Josh Holt, Kevin Mistry, Andrew Ferguson, Jeff Blackburn, Nikos Kopidakis, Garry Rumbles Single-walled carbon nanotubes (SWNTs) possess unique properties that may potentially benefit photovoltaic (PV) devices, including high carrier mobilities, convenient work functions, and tunable optical transitions that span most of the solar spectrum. However, significant polydispersity in both diameter and electronic structure have hindered the realization of efficient PV cells incorporating SWNTs. In this presentation, we report the use of advanced techniques to separate single-walled carbon nanotubes (SWNTs) created by laser vaporization into highly enriched semiconducting and metallic species. The enriched SWNTs are then blended with regioregular poly(3-hexylthiophene) (P3HT) to serve as a model electron donor/acceptor system, analogous to systems typically used in organic PV devices. We investigate the photo-physical properties of charge generation and transfer using primarily time-resolved microwave conductivity (TRMC) and photoluminescence excitation spectroscopy and discuss the disparities between metallic vs semiconducting SWNT acceptors. [Preview Abstract] |
Tuesday, March 16, 2010 2:42PM - 2:54PM |
L16.00002: Modeling organic bulk-heterojunction solar cells: Parameter stability and photocurrent transients Roger Hausermann, Evelyne Knapp, Michael Moos, Nils Reinke, Thomas Flatz, Beat Ruhstaller An opto-electronic device model for organic bulk-heterojunction solar cells is presented (setfos by fluxim). First, the optical in-coupling into a multilayer stack is calculated. From the photon absorption profile a charge-transfer (CT) exciton profile is derived. These CT-excitons are then dissociated according to the Onsager-Braun model. The resulting motion of electrons and holes is modeled considering both drift and diffusion. We analyze measurements on P3HT:PCBM based solar cells and derive a set of parameter values, including values for CT-exciton dissociation. The experiments are well described and the stability of the parameters under various conditions is tested. This includes the simulation of current-voltage curves, the dependence of the short-circuit current on the layer thickness, and transient photo-currents. It is shown that simulating the transient photo-current is particularly helpful in determining the values of electron and hole mobility. This highlights the need to measure transient photo-currents to extract device parameters such as mobilities and CT-exciton dissociation constants.\\[4pt] J. Appl. Phys. \textbf{106}, 104507 (2009) [Preview Abstract] |
Tuesday, March 16, 2010 2:54PM - 3:06PM |
L16.00003: Effect of recombination on the open circuit voltage in polymer-fullerene solar cells Sarah Cowan, Anshuman Roy, Alan Heeger Polymer-fullerene solar cell response measurements, including intensity-dependent current-voltage and bias-dependent internal quantum efficiency and photoconductivity measurements, probe the density of trap states\footnote{R. Street, M. Schoendorf, A. Roy, J.H. Lee. ``Recombination in organic solar cells.'' Phys. Rev.B. (submitted for publication).} for three polymers (poly(3-hexylthiophene) (P3HT), poly[N-9$\prime $-heptadecanyl-2,7-carbazole-alt-5,5-(4$\prime $,7$\prime $-di-2-thienyl-2$\prime $,1$\prime $,3$\prime $-benzothiadiazole)] (PCDTBT), and poly{\{}[4,4'-bis(2-ethylhexyl)dithieno(3,2-b;2',3'-d)silole]-2,6-diyl-alt-(2,1,3-benzothidiazole)-4,7-diyl{\}} (PSBTBT). The short circuit current shows linear dependence and the open circuit voltage shows logarithmic dependence on intensity over four orders of magnitude in intensity. When compared with silicon counterparts, polymer solar cells maintain high open circuit voltage at low light intensity, making them ideal solar cells for low light conditions. [Preview Abstract] |
Tuesday, March 16, 2010 3:06PM - 3:18PM |
L16.00004: Comparison of different models of transport in organic photovoltaics Paul Haney, Behrang Hamadani, Nikolai Zhitenev Organic photovoltaics are relatively low-cost, easy-to-fabricate materials that can harness solar energy, and have been the subject of intense study in recent years. There is a rich array of physics which underlie the operation of these materials, and various models have been proposed to capture the most relevant mechanisms and help explain experimental results. Understanding and modeling transport in these structures is particularly difficult as they are highly disordered. We perform a critical analysis of various models currently used in the field, and in particular, consider the relevance of different models of charge transport (i.e., drift-diffusion versus hopping) to overall model behavior. We also consider the effect of a series resistance on measured current-voltage characteristics. Comparisons to experiment reveal that under high illumination, series resistance should be included in any microscopic or mesoscopic model of an organic solar cell. [Preview Abstract] |
Tuesday, March 16, 2010 3:18PM - 3:30PM |
L16.00005: Effect of doping spin 1/2 radical impurities on the performance of polymer/fullerene bulk heterojunctions solar cell devices Ye Zhang, Golda Hukic-Markosian, Debra Mascaro, Z. Valy Vardeny We use a variety of regio-regular P3HT and PCBM blends to fabricate bulk heterojunction organic photovoltaic devices (solar cells) doped with spin 1/2 galvinoxyl radical impurities. We show that the device performance can be significantly improved by doping the device active layer by these radicals. Compared to pristine photovoltaic devices, the doped devices show improved short-circuit current density, fill factor, and consequently also the power conversion efficiency in enhanced. We explore the origin of this effect by varying the polymer/fullerene composition, as well as comparing the radical-doped devices with devices doped with small molecules that are electron donors (CuPc). We also conducted light-induced electron spin resonance measurements for investigating the spin 1/2 radical activities upon light excitation. Our results show that the enhanced device performance is attributed to improved charge separation and carrier transport in the fullerene phase of the active layer. [Preview Abstract] |
Tuesday, March 16, 2010 3:30PM - 3:42PM |
L16.00006: Three-dimensional nanomorphology in blended organic solar cells explored by photocurrent microscopy Behrang Hamadani, Suyong Jung, Paul Haney, Nikolai Zhitenev The morphology of the active layer in blended bulk heterojunction solar cells strongly influences charge transport of photogenerated carriers to the contacts. Here, we present our study of nanoscale morphology and phase segregation in blended organic solar cells based on P3HT and PCBM by use of a photoconductive atomic force microscope (PCAFM). The film/air interface material blending is investigated by direct PCAFM scanning of the top film, whereas the bulk blending morphology is explored by combining the PCAFM with the use of a shallow angle microtomy technique designed at removing the film top layers by creating wedge structures along certain cutting directions. Our results show that the top layer in these photovoltaic systems is enriched with a polymer skin layer with small concentration of PCBM nanocrystals reaching the top film and creating lateral spatial segregation on the scale of 100-500 nm. In the bulk, however, much finer material blending is observed, with sub-100 nm length scales for photogenerated charge transport. [Preview Abstract] |
Tuesday, March 16, 2010 3:42PM - 3:54PM |
L16.00007: Photonic Crystal Geometry for Organic Solar Cells Edward Samulski, Rene Lopez, Doo-Hyun Ko, John Tumbleston Efficient absorption of light calls for thicker PV active layers whereas carrier transport always benefits from thinner ones, and this dichotomy is at the heart of an efficiency/cost conundrum that has kept solar energy expensive relative to fossil fuels. We report a 2-D, photonic crystal morphology that enhances the efficiency of organic photovoltaic cells relative to conventional planar cells.[1] The morphology is developed by patterning an organic photoactive bulk heterojunction blend using PRINT a process that lends itself to large area fabrication of nanostructures.[2] The photonic crystal cell morphology increases photocurrents generally, and particularly through the excitation of resonant modes near the band edge of the organic PV material. [1] Ko, D.-H.; Tumbleston, J. R.; Zhang, L.; Williams, S.; DeSimone, J. M.; Rene, L.; Samulski, E. T. \textit{Nano Lett.} \textbf{2009}, $9$, 2742--2746. [2] Hampton et al. \textit{Adv. Mater.} \textbf{2008}, $20$, 2667. [Preview Abstract] |
Tuesday, March 16, 2010 3:54PM - 4:06PM |
L16.00008: High Performance Phototransistor based on Nanostructured Regioregular Poly (3-hexylthiophene) Tanusri Pal, M. Arif, Saiful I. Khondaker We have demonstrated high performance phototransistors based on regioregular poly 3-hexylthiophene (rr-P3HT) by tuning the nanomorphology of the P3HT thin film. The morphology of the solution processing polymer has been controlled by the selection of organic solvents (p-xylene, dichlorobenzene and chloroform). Under illumination of light drain current increased significantly and threshold voltage shifted towards positive direction whereas mobility remains unaffected. Change in threshold voltage corresponds to change in carrier density due to illumination. Conversely the field-effect mobility is relatively unaffected, indicates that the electronic structure of the polymer is not affected by the illumination. Devices made from p-xylene and dichlorobenzene solution show responsivity (Photocurrent/Optical power) of 16 A/W and 21A/W respectively at V$_{G}$=0V. The responsivity further increased up to one order of magnitude high by tuning the gate bias. While devices made from chloroform solvent show maximum responsivity of up to 2A/W. The maximum photosensitivity (Photocurrent/ Dark current) of our device is 3.8X10$^{3}$. [Preview Abstract] |
Tuesday, March 16, 2010 4:06PM - 4:18PM |
L16.00009: Patterned nanocontacts on blended photovoltaic films for probing of local photoresponse Nikolai Zhitenev, Suyong Jung, Paul Haney, Behrang Hamadani Probing of nanoscale photocurrent by photoconductive atomic force microscopy (PCAFM) provides spatially resolved information on the nature of inhomogeneity related to material blending in organic bulk heterojunction solar cells. However, interpretation of the data is often complicated due to the nontrivial nature of tip/sample contact. Here, we pattern the active layer of the P3HT:PCBM solar cells by arrays of silver nanodots with sub-micron spatial resolution, and probe the photoresponse from each metal dot by a PCAFM system under illumination. The rigid geometry of the nanodot, the well-defined contact and the low work function of silver allows for better characterization of photoresponse from the film and a trend that allows us to generalize their response to macroscopic devices. We also show modeling results based on equivalent circuit elements to better understand the current-voltage characteristics of these photovoltaic nanodevices. [Preview Abstract] |
Tuesday, March 16, 2010 4:18PM - 4:30PM |
L16.00010: Utilizing Scanning Probe Microscopy to Study Organic Photovoltaic Materials Ashley Kibel, Shreya Bhattacharyya, Paul Liddell, Devens Gust, Stuart Lindsay Organic photovoltaics have the potential to provide cheaper alternatives to traditional silicon solar cells due to flexibility in design and engineering. Understanding how charge is transported in these materials is important for the future design and fabrication of efficient organic solar cells. We utilize scanning probe microscopy techniques to study the electrical properties of biomimetic organic molecules that have photovoltaic potential. We present results from conducting atomic force microscopy measurements performed on bare substrates commonly utilized in organic photovoltaic applications as well as measurements on organic thin films self assembled on these substrates. Furthermore, we present the results of single molecule conductivity experiments performed using a scanning tunneling microscope on novel donor-acceptor molecules. We discuss benefits, as well as challenges, to using scanning probe techniques on organic photovoltaic systems. [Preview Abstract] |
Tuesday, March 16, 2010 4:30PM - 4:42PM |
L16.00011: STM Study of Donor-Bridge-Acceptor Molecules Having Different Bridge Structure Chenggang Tao, Jibin Sun, Xiaowei Zhang, Ryan Yamachika, Daniel Wegner, Yasaman Bahri, Georgy Samsonidze, Marvin L. Cohen, Steven G. Louie, T. Don Tilley, Rachel A. Segalman, Michael F. Crommie Composite molecular solar cells form an interesting alternative to traditional silicon or gallium arsenide solar cells. In order to increase the efficiency these molecular systems, a deeper understanding of the microscopic mechanisms at work in organic solar cells is needed. Using scanning tunneling microscopy and spectroscopy we have investigated donor-bridge-acceptor molecules having different bridge structures covalently bonding similar donor and acceptor elements. Structural and electronic properties of individual molecules and self-assembled molecular chains will be presented. This study is aimed toward controlling energy conversion pathways within molecular solar cells and for developing higher efficiency solar cell materials. [Preview Abstract] |
Tuesday, March 16, 2010 4:42PM - 4:54PM |
L16.00012: Nanoscale manipulation and conduction anisotropy in oligothiophene monolayers, a CS-AFM study Florent Martin, Bas Hendriksen, Allard Katan, Miquel Salmeron, Nenad Vukmirovic, Ling-Wang Wang, Clayton Mauldin, Jean Frechet We used Langmuir-Blodgett organic monolayers on SiO2/p+Si as a model system to study the relationship between molecular order and conductivity in organic materials. Local manipulation of the polycrystalline monolayer by the AFM tip provided clear proof of the correlation between in plane conduction and crystalline order in the monolayer. Lattice resolution AFM imaging was used to show how scanning at high load with the AFM tip could irreversibly modify the molecular packing in the monolayer and decrease its conductivity. In addition, by combining CS-AFM and high resolution AFM, we found evidence for a conduction anisotropy caused by the asymmetry of the herringbone packing of the monolayer. Molecular dynamics calculations confirmed the crystal structure determined by AFM as well as the direction of higher conduction found experimentally. [Preview Abstract] |
Tuesday, March 16, 2010 4:54PM - 5:06PM |
L16.00013: High performance encapsulation structures utilizing Russian Doll architectures Jimmy Granstrom, Michael Villet, Tirtha Chatterjee A Russian Doll encapsulation architecture utilizing pairs of free-standing barrier films and epoxy seals separated by nitrogen spacers is presented, enabling the use of low-cost epoxy to attach two or more free-standing barrier films to a substrate with improved barrier performance. The performance of various Russian Doll encapsulations was evaluated with the calcium thin film optical transmission test, showing improved performance of the Russian doll configuration relative to a non-nested barrier/spacer architecture, and demonstrating that water vapor transmission rates of 0.0021 g/(m$^{2}$, day) or below can be achieved with low-cost materials in this architecture. This WVTR correlates to a predicted lifetime of more than 10 years for inverted organic P3HT:PCBM bulk heterojunction solar cell modules fabricated and tested by Konarka Technologies (Lowell, MA, USA). [Preview Abstract] |
Tuesday, March 16, 2010 5:06PM - 5:18PM |
L16.00014: Cavity Enhanced Optical Absorption in Polymer Photovoltaics Brent Valle, Kenneth D. Singer, James Andrews Simulations using transfer matrix theory have been performed demonstrating enhanced optical absorption in the cavity formed by a polymer bulk-heterojunction active layer sandwiched between an aluminum cathode and indium tin oxide (ITO) anode. Under cavity resonance conditions, it is found that the absorption of the active layer can be both spectrally tuned via frequency pulling and/or strengthened by controlling the thickness of the ITO and active layer thicknesses. A thin, polymer active layer at the first cavity resonance, occurring at a thickness of 80 nm, is attractive due its lower series resistance, shorter charge extraction length, and higher electric fields. Characterization of polymer photovoltaic devices exploiting these cavity-enhanced structures will be reported and compared with calculations. [Preview Abstract] |
Tuesday, March 16, 2010 5:18PM - 5:30PM |
L16.00015: Modeling of series and parallel solar cell tandems Alexander Kuznetsov, Anvar Zakhidov Significant improvement of the solar cell efficiency is achieved by combining solar cells into tandems. Circuit analysis allows us to determine tandem parameters that guarantee its most efficient operation. In this study we apply the single-diode model to the analysis of solar cells connected in series and in parallel. The model explains how tandem efficiency relates to the efficiency of individual cells and suggests different ways of efficiency improvement in the series and parallel configurations. Thus, we predict that a parallel tandem shows highest efficiency when open circuit voltages of its individual cells are equal to each other. In a series configuration, short circuit currents and fill factors must be matched to achieve best device performance. If the parameters of individual solar cells remain unmatched, parallel connection usually results in higher tandem efficiency. We also analyze experimental results for several types of organic solar cells and explain the efficiency drop observed in tandems compared to the combined efficiency of the individual cells. [Preview Abstract] |
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