Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Q38: Focus Session: Organic Electronics and Photonics -- Morphology in polymer-based solar cells |
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Sponsoring Units: DMP DPOLY GERA Chair: Garry Rumbles, National Renewable Energy Laboratory Room: A130/131 |
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q38.00001: Structural measurements of polymer-fullerene blend films for organic photovoltaics Invited Speaker: Organic photovoltaic (OPV) technology has the potential to greatly lower the cost of solar cell fabrication by enabling ink-based deposition of active layers. In bulk heterojunction (BHJ) OPV devices, the power conversion efficiency critically depends on the distribution of the polymer absorber and the fullerene electron acceptor (e.g., the blend morphology). I will describe measurement methods to probe the structure of OPV devices, with a focus on the morphology of the BHJ layer. For example, the vertical distribution of absorber and electron acceptor in BHJ films follows segregation behavior similar to that of miscible polymer blends. The top (air) interface becomes rich in the polymer absorber, whereas the bottom interface composition depends on the substrate surface energy. Thin film transistors fabricated from BHJs can therefore exhibit ambipolar or hole-only transport depending on the dielectric, because of different interfacial segregation. We extend these results to practical photovoltaic devices by comparing BHJs cast upon hole transport layers that have similar work functions but different surface energies. This study includes the application of variable angle spectroscopic ellipsometry (VASE) to BHJ films, and emphasizes the importance of absorber anisotropy and vertical heterogeneity in the optical model. Additional results will describe the nanometer-scale structure in the BHJ interior. The application of solid-state nuclear magnetic resonance (SS-NMR) can reveal details about the segregation of absorber and acceptor in a BHJ film. Nanoscale BHJ morphology information can also be collected using tomographic transmission electron microscopy (TEM). Together these measurements allow us to reveal a detailed picture of BHJ morphology, explain how the morphology originates from materials and processing choices, and relate the morphology to device performance and stability. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q38.00002: Tuning Rod-Rod Interactions in Poly(3-alkylthiophene) Derivatives Bryan Boudouris, Victor Ho, Rachel Segalman Poly(3-alkylthiophene) (P3AT) derivatives are used commonly in polymer semiconducting applications. However, during the coating of P3AT thin films strong intermolecular interactions generally lead to the formation of semiconducting fibers. This prevents the formation of long-range ordered domains and complicates analysis of structure-property relationships in P3AT-containing devices (e.g., organic photovoltaic cells). Here, we show rod-rod interactions can be controlled by rational polythiophene side chain design. The effects of side chain passivation are evidenced by a depressed melting temperature and the presence of a liquid crystalline region. We show also that while the rod-rod interactions are lowered significantly in a polythiophene derivative with a branched side chain relative to straight chain P3ATs, the optoelectronic properties remain approximately constant. Importantly, this reduced melting temperature allows for the real-time evolution of a P3AT crystal structure at room temperature to be monitored on an experimentally convenient time scale. These structural data correlate well with field-effect charge carrier mobility measurements and provide a path for studying the mechanism of ordering in plastic electronics. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q38.00003: Polymer/Polymer Heterojunctions for Ambipolar Charge Transport in Organic Electronics Felix Kim, Selvam Subramaniyan, Samson Jenekhe Understanding of charge transport in polymer semiconductor heterojunctions is of basic interest in developing high-performance organic optoelectronic devices based on multicomponent polymer semiconductors. We report ambipolar charge transport in thin films of layered heterojunctions and bulk heterojunctions of solution-processable unipolar polymer semiconductors. Selective solubility of the polymer semiconductors, poly(thiazolothiazole)s and ladder-type poly(benzobisimidazo-benzophenanthroline), in organic and acidic solvents enabled the sequential deposition or blending of the polymer semiconductors. Charge carrier mobilities of 0.001-0.01 cm2/Vs were observed for both electrons and holes in the polymer/polymer heterojunction field-effect transistors. Thin film deposition and processing with various solvents are effective to improve charge-carrier mobilities by a factor of 100-1000. We have investigated the effects of the processing methods on morphology, and photophysical and charge transport properties of the polymer semiconductor heterojunctions. Integrated circuits and solar cells based on the polymer semiconductor heterojunctions are also demonstrated. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q38.00004: The role of donor-acceptor intermixing in the performance of polymer-polymer OPVs Eleni Pavlopoulou, Stephanie Lee, Chang Su Kim, Yueh-Lin Loo, Zhihua Chen, Antonio Facchetti, Michael F. Toney We investigated the effect of donor-acceptor intermixing in bulk-heterojunction active layers on device performance of polymer-polymer organic photovoltaics (OPVs). Poly(3-hexylthiophene) (P3HT) was blended with poly{\{}[N,N'-bis(2-octyldodecyl) -naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene){\}} (PNDI) and P3HT/PNDI films were spin-cast from dichrolobenzene, a good solvent for PNDI; chlorobenzene, a good solvent for P3HT; and xylene, a bad solvent for both. The short-circuit current densities and device efficiencies vary with casting solvent quality; devices with active layers cast from xylene exhibit the highest efficiencies while those cast from dichlorobenzene the lowest. Grazing Incidence X-ray Diffraction show that intermixing on a molecular scale increases with decreasing dissolution of the polymers in the parent solutions. Accordingly, increasing intermixing enhances device efficiencies. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q38.00005: Phase purity in organic solar cells Brian Collins, Eliot Gann, Lewis Guignard, Xiaoxi He, Christopher McNeill, Harald Ade To date, the device function of organic bulk heterojunction solar cells has been commonly interpreted to arise from two interpenetrating, phase-separated donor and acceptor materials with charge separation of excitons occurring at discrete interfaces. However, little attention has been paid to phase purity and the consequences of a mixed phase on the operation of devices. To probe this possibility and its implications, the miscibility of common fullerenes in (3-hexylthiophene) (P3HT) and a number of new low bandgap polymers including PCDTBT have been measured directly via x-ray absorption spectroscopy in a scanning transmission x-ray microscope on films brought to thermodynamic equilibrium. A mixed amorphous phase is always observed, along with a fullerene-rich phase and possibly a pure crystalline polymer phase if the polymer is able to crystallize. For example, grazing incidence x-ray scattering shows no intercalation of fullerenes into P3HT crystallites, while amorphous portions of the polymer contain $\sim $20 wt.{\%} of the fullerene. In fact, all systems tested to date have failed to exhibt a pure amorphous polymer phase, suggesting that the device paradigm of pure phases and discrete interfaces requires modification. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q38.00006: Diffusion rates and crystallization of phenyl-C61-butyric acid methyl ester in poly(3-hexylthiophene) L. Guignard, B. Collins, J. Seok, H. Ade Bulk heterojunction (BHJ) solar cells based on poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) are an important model system for studying organic solar cell operation. Recent experiments reveal that PCBM is partially miscible in the amorphous regions of P3HT [1-3], implying that P3HT:PCBM devices have three phases. The miscibility depends on temperature and regioregularity of the P3HT. To better understand the influence of P3HT regioregularity and molecular weight on the the P3HT:PCBM system, diffusion rates of PCBM are determined as a function of regioregularity of P3HT and temperature with visible light microscopy by analyzing the growth of the PCBM depletion region near PCBM crystals or agglomerates. Lower diffusion constants are found for less crystalline regiorandom P3HT than highly regioregular P3HT. The shape and growth behavior of PCBM crystal or agglomerate is also found to vary greatly. \\[4pt] [1] B. Watts\textit{ et al.}, Macromolecules \textbf{42}, 8392 (2009) \\[0pt] [2] B. A. Collins\textit{ et al.}, J. Phys. Chem. Lett \textbf{1}, 3160 (2010) \\[0pt] [3] J. W. Kiel\textit{ et al.}, Phys. Rev. Lett. \textbf{105}, 168701 (2010) and N. D. Treat\textit{ et al.}, Adv Funct Mater [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q38.00007: Structural evolution in polythiophene-fullerene mixtures Enrique Gomez, Derek Kozub, Kiarash Vakhshouri The morphology of organic semiconductor mixtures employed as the active layer of organic solar cells is a result of the complex interplay between the crystallinity of the constituents and the chemical incompatibility. Given that device performance can depend critically on the morphology of the active layer, efforts aimed identifying at the critical parameters for the structure formation process are important for the development of high-performance devices. We demonstrate that polythiophene-fullerene mixtures are partially miscible and that the crystallization of the electron donor drives the characteristic length scales of the structure. By modeling fullerene as a solvent for polythiophene, we have estimated the Flory-Huggins interaction parameter from measurements of the melting point depression of polythiophene. The miscibility between poly(3-hexylthiophene) (P3HT) and fullerene at P3HT volume fractions greater than 0.4 leads to a severe suppression of the crystallization of fullerene. Our efforts have enabled us to develop a hypothesis for the structure formation process in polythiophene/fullerene mixtures. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q38.00008: Miscibility Study of PCBM/P3EHT Organic Photovoltaics via Small Angle Neutron Scattering Wen Yin, Bryan McCulloch, Rachel Segalman, Mark Dadmun Organic photovoltaics (OPV) attracted considerable interest as lightweight, inexpensive, and easily processable replacement of inorganic photovoltaics. Current results indicate that the morphology of these photovoltaic materials is essential to their solar energy conversion efficiency but a detailed and fundamental understanding is absent. In this paper, the miscibility and structure of P3EHT/PCBM composites with varying PCBM loading level are investigated via small angle neutron scattering (SANS). With P3EHT having a melting temperature below 100\r{ }C, SANS experiments of the blends are conducted above the melting point to unequivocally determine the miscibility of PCBM and P3EHT without the added complexity of polymer crystals. Our SANS results show that blends with 20 and 50 wt{\%} PCBM exhibit dramatically larger scattering at low-Q regime relative to 10 and 15wt{\%} PCBM samples. This result implies that the miscibility limit of PCBM and P3EHT lies between 15:85 and 20:80. Further analysis is underway to correlate these results to OPV efficiency. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q38.00009: Characterization and Improvement on the Morphology in Polymer-Based Solar Cells Hao Shen, Wenluan Zhang, Brett Guralnick, Michael Mackay, Brian Kirby, Charles Majkrzak Polymer-based solar cells are promising for their cost-effective solar energy, yet this technology is still far from practical application owing to its low energy conversion efficiency. It has been known that the morphology in the active layer, or the nano-scaled intermixing between the polymer and fullerene derivative, is critical to the device performance. We have quantitatively measured the morphology in one of the most-studied polymer-based solar cells consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), by means of neutron and x-ray scattering techniques. In particular, the effects of thermal and co-solvent-assisted annealing on the PCBM cluster formation and vertical distribution are characterized. Basing on the observations, we are proposing a new design of solar cell architecture to approach a more controlled morphology in the active layer, by utilizing a thermodynamically-driven assembly of fullerenes onto the surface of silica microspheres. This presentation will focus on its application in the P3HT:PCBM system. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q38.00010: Investigation of fullerene concentration profile in polymer based solar cell by using magnetic contrast neutron reflectivity Wenluan Zhang, Brian Kirby, Michael Mackay Poly(2,5-bis(3-tetradecyllthiophen-2-yl)thieno[3,2-b] thiophene) (pBTTT) has recently caused great interest as the electron donor in organic photovoltaics. It was demonstrated that fullerene molecules intercalate between side-chains of this semiconducting polymer creating a stable crystalline structure, so, a large concentration of fullerene must be used relative to the polymer to promote phase separated electron conductive pathways. We used state-of-the-art neutron reflectivity methods, with the application of magnetic contrast variation, to investigate the concentration profile of [6,6]-phenyl-C$_{71}$- butyric acid methyl ester(PC$_{71}$BM) in order to understand the internal structure within the active layer. The PC$_{71}$BM concentration profile greatly depends on the weight ratio of polymer to fullerene. XRD and other data are also used to show the morphology change of active layer. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q38.00011: Correlation of Structure and Roughness with Fabrication Conditions of P3HT-PCBM Bilayer Interfaces with X-Ray Reflectometry Stuart Kirschner, Ming-Ling Yeh, Nathaniel Smith, Howard Katz, Daniel Reich Organic semiconductors, including poly(3-hexylthiophene) (P3HT) and polymer-phenyl-C61-butyric acid methyl ester (PCBM), are considered as promising materials for applications such as photovoltaics, transistors, sensors, thermoelectrics, optoelectronics, and magnetoelectronics. In many cases, the interface plays a crucial role in device performance and in determining the origins of many effects. In this research, neat bilayers of P3HT-PCBM, and PCBM blended with polystyrenes, were studied with X-ray reflectometry (XRR), atomic force microscopy, and ultraviolet-visible spectroscopy. A polymer with a high atomic number element was included to improve the scattering length density contrast, and provided improved XRR resolution. A mobility of order 10$^{-4}$ cm$^{2}$/V*s was maintained. The effect of different annealing, solvent, spin coating, and other fabrication conditions, was explored. Applicability of XRR to study interface characteristics, in these systems, will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q38.00012: Polymer Solar Cells, Deconstructed Yueh-Lin Loo, Jongbok Kim, He Wang, Stephanie Lee, Zelei Guan, Antoine Kahn Soft-contact lamination and delamination has enabled us to construct polymer solar cells for testing, and deconstruct them subsequently for structural characterization of the active layers and electronic characterization of relevant charge transfer interfaces. We have thus been able characterize buried active layers and interfaces that are otherwise inaccessible. Structural characterization post-device fabrication and testing reveals simultaneous crystallization of the polymer donor and the electron acceptor in the once-buried bulk-heterojunction active layers to be responsible for photocurrent enhancement in these devices. Electronic characterization of the active layer-bottom electrode interface reveals an electronic band gap of 1.5 eV, attributable to the difference between the ionization potential of the polymer donor and the electron affinity of the electron acceptor. This value is significantly larger than the band gap predicted by examining the energy levels of the individual constituents, likely due to the presence of interfacial dipoles when the polymer donor and the electron acceptor are intimately mixed. [Preview Abstract] |
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