Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Z33: Focus Session: Organic Electronics and Photonics - Morphology and Structure II |
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Sponsoring Units: DPOLY Chair: Brian Collins, National Institute of Standards and Technology Room: 341 |
Friday, March 22, 2013 11:15AM - 11:27AM |
Z33.00001: Correlating polymer solution conformation and thin film nanostructure: Implications for BHJ processing Rajeev Dattani, Alisyn Nedoma, Natalie Stingelin, Jenny Nelson, Joao Cabral We study the solution properties of polymer-fullerene mixtures by a combination of dynamic light scattering, viscometry, small angle neutron scattering and microscopy. Specifically, the kinetics of polymer conformation (Rg and Rh) and interaction changes are mapped as function of polymer-particle concentration, overall concentration in solution and age. A model system of polystyrene and C60 fullerene was selected for this study, in addition to the P3HT/PCBM pair, which is currently explored in photovoltaic applications. The solution properties show a clear correlation to the resulting thin film nanostructured composite morphology. Our future work will further link it to bulk heterojunction solar cell performance. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z33.00002: Chemical Effects in Solution on the Formation of Film Morphology in Bulk Heterojunction Organic Solar Cells Jong Kuk Koh, Won Tae Choi, Kookheon Char A novel method to control the active layer morphology of bulk heterojunction(BHJ) organic solar cells will be presented in this study. The effect of solvent quality, chemical effect in solution, on the morphology of poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) active layer has been investigated. The solubility of solvent can be controlled by mixing other types of additional solvents (additives) to the P3HT:PCBM blend solution, which could adjust the association and/or solvation characteristics for both P3HT and PCBM solutes in mixed solvents. As a result, the control over the solubility has a definitive effect on the film morphology. We report a new additive, 2-chlorophenol, which could drive P3HT to have more association character and, PCBM to have more solvation character in the mixed solvent. Higher P3HT crystallinity was achieved due to more association character in the presence of 2-chlorophenol. Also, the higher solvation character of PCBM leads to the reduced size of PCBM agglomerates, as confirmed by SANS measurements. Based on these results, P3HT:PCBM BHJ solar cell devices were fabricated, with maximum power conversion efficiency of 3.24{\%}, which is 43{\%} enhancement when compared with the reference. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z33.00003: Effect of solvent annealing on phase separation of donor/acceptor species in organic mixtures Miriam Cezza, Qian Shao, Shy-Hauh Guo, Raymond J. Phaneuf Studies on phase separation of mixtures of tetranitro zinc- phthalocyanine (tn-ZnPc) and [6,6]-phenyl-C$_{61}$-butyric acid methyl ester (PCBM) were performed in which we controlled the evaporation rate of the solvent (chloroform). Phase-contrast AFM analysis reveals that slowing down the evaporation rate of the solvent facilitates the nucleation of the donor component, and the two components phase-separate. The size of the molecular agglomerates and single small particles decreases for slow solvent evaporation and the density of small particles per unit area increases by an order of magnitude over the range studied. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z33.00004: Precise Structural Development and its Correlation to Function in Conjugated Polymer: Fullerene Thin Films by Controlled Solvent Annealing Huipeng Chen, Sheng Hu, Huidong Zang, Bin Hu, Mark Dadmun The structural evolution and function of solvent processed poly(3-hexylthiophene):[6,6]-phenyl-C$_{\mathrm{61}}$-butyric acid methyl ester (P3HT:PCBM) bilayers with controlled exposure to ortho-dichlorobenzene solvent vapor is examined. Different from thermal annealing, where the structure develops (P3HT crystallization and PCBM phase separation) in seconds, solvent vapor annealing provides more precise morphological control and a more detailed picture of the competing processes that drive the structural development. This work shows that P3HT crystallization and PCBM phase separation occur in different stages with solvent annealing. The interdiffusion of PCBM and P3HT and crystallization of P3HT occurs in the first stage, while in the second stage, the phase separation of PCBM from P3HT and agglomeration of PCBM occurs. Therefore, the sequential nature of these processes clearly documents that the phase separation of PCBM from P3HT \textit{is not} driven by P3HT crystallinity, but by the thermodynamic driving force of mixing (the miscibility limit of PCBM in P3HT) Correlation of the morphology to device performance indicates that both sufficient P3HT crystallization and PCBM phase separation are crucial in the optimization of the morphology of the active layer. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z33.00005: Controlling donor/acceptor interface structure by processing solvents in organic solar cells Wei Ma, Long Ye, Gann Eliot, Jianhui Hou, Harald Ade The nature of the interface structure between donor and acceptor are known to be critical for fullerene-based solar cells, yet have not been widely studied due to limitations of common characterization techniques. We show that processing solvents are an effective way to control the interface structure (sharp, fractal, or diffuse) of the active layer and thus impact device performance. Six different solvents or solvent mixtures are used as processing solvents in PDPP3T with PC$_{71}$BM blends to investigate the impact of solvents on interface properties. Interface roughness is revealed by analysing the scaling of high-q data of resonant soft x-ray scattering profiles. We find that with the presence of DIO, rough interfaces are always observed. While rough interfaces provide shorter average distances for excitons to reach donor/acceptor interfaces, they also enhance recombination and are thus not ideal. When CF is used as one component, a sharp or slightly diffuse interface is induced. However, over-pure domains (especially mixed CF with DIO) are also created that seem to negatively impact performance. Overall, the mixture of ternary yields the highest PCE of 6.7{\%}. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z33.00006: A comparative study of the morphology of flow and spin coated P3HT:PCBM films Jose Chapa, Alamgir Karim Polymer solar cells are attractive due to the possibility of using cheaper materials and processing techniques for mass production of solar panels. Previous methods of fabricating polymer solar cells are suitable in laboratory conditions but are not scalable for industrial production. In this study, thin films of the photoactive blend of poly(3-hexylthiophene) (P3HT) and fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were prepared by flow coating, which is suitable for industrial manufacturing of solar cells. P3HT:PCBM blends were cast from different solvents, and the morphology of flow coated and spin coated films was compared. The surface morphology and optical properties of P3HT:PCBM films were characterized with optical microscopy, AFM, and UV-vis absorption spectroscopy. The degree of P3HT order was higher in flow coated films, as compared to spin coated films. Films flow coated using chloroform solutions had a higher thermal stability and an enhanced degree of phase separation as compared to spin coated films. Flow coated films from chlorobenzene solutions had a lower thermal stability and a smaller length scale of phase separation. This study demonstrates that flow coating is a suitable alternative technique for fabricating polymer solar cells. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z33.00007: Microstructure of self-assembled all-conjugated donor-acceptor block copolymers for organic solar cells Michael Brady, Sung-Yu Ku, Justin Cochran, Craig Hawker, Edward Kramer, Michael Chabinyc All-conjugated diblock copolymers (CBCPs), with donor and acceptor blocks, form intriguing alternatives to polymer/fullerene bulk heterojunction (BHJ) blends as low-cost photovoltaics. BHJs comprise a phase-separated thin film microstructure, in which chemically distinct domains of donor and acceptor enable exciton dissociation at their interface and transport of free charges through continuous n- and p-type paths to the electrodes. GIWAXS, AFM, soft X-ray spectroscopy (NEXAFS), and resonant scattering (RSoXS) are used to probe the structure of films of CBCPs that have an electron-donating P3HT block and an electron-accepting poly-(diketopyrrolopyrrole-terthiophene) (DPP) block. Thermal annealing after casting causes these CBCP films to form ordered domains on the scale of the exciton diffusion length, with ca. 50 nm in-plane lamellar spacings, with crystallites of each block present. GIWAXS diffraction peaks from the (100), (200), and (300) alkyl chain stacking planes for crystals of each block show (h00) orientation toward the out-of-plane direction, with the (010) pi-stacking vectors in the film plane. CBCP processing-structure studies have enabled the control of chain ordering and orientation at both length scales, and thus the formation of optimal BHJ morphologies. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z33.00008: Controlled Domain Swelling for Block Copolymer-Based Solar Cells Alisyn Nedoma, Rajeev Dattani, James Bannock, Paul Westacott, Joao Cabral Block copolymers seem ideally suited materials for solar cells because they self-assemble to form highly-ordered domains on the same length scale as the diffusion length of an exciton. Success has thus far been limited by the tendency of block copolymers to disorder at low loadings of fullerene; a consequence of Timmerman's Rule whereby preferential interactions between the fullerene and one block of the copolymer tend to destabilize the microstructure. We present a method for balancing the volumetric swelling of one block by swelling the other block with a commensurate amount of the homopolymer. This technique is demonstrated for a model polymer system and extended to a conjugated rod-coil block copolymer. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z33.00009: Observation the Nanoscale Blending Morphology of P3HT:PCBM Bulk-Heterojunction by Energy-Filtered TEM and Contrast Transfer Function. Nopporn Rujisamphan, Ismat Shah The efficiency of bulk-heterojucntion organic solar cells is strongly related to the blending morphology of donor and acceptor materials. By understanding the intermixed morphology would improve device performance. Herein, we present the ways to improve contrast images in the transmission electron microscopy of P3HT:PCBM. In general, TEM images took at the focus point gives one low contrast. We take advantage of the contrast transfer function (CTF) to improve contrast images in bright field TEM. By changing the defocus values, the fibril structure of the P3HT is obviously observed and distinguished. In order to observe the nanoscopic blending morphology, fibril size, and distribution of those fibrils, we carry out the energy filtered TEM (EFTEM). The energy window centered at 19 eV with the slit width energy of 7 ev is selected for looking only P3HT domain. In contrast, the energy window is centered at 25, and 30 eV for observing PCBM domains with the same slit width energy for a comparison. When used the window at 19 ev, we are able to clearly observe the P3HT fibril structure with the diameter and the length of $15\pm 1$ nm and $51\pm 20$nm, respectively. The diameter size of those fibrils did not change even in the annealed samples implying that the PCBM diffused only into an amorphous region of P3HT. The distribution of those fibrils seemed to be homogeneous without any preferred direction. Together with XRD results, we found that in only one P3HT fibrils, there are 40 pi-pi stacking layers with 9 layers parallel to the fibril length. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z33.00010: Cross-sectional nanoscale morphology and interfacial band alignment of phase-separated polymer/fullerene by scanning tunneling microscopy and spectroscopy M.C. Shih, Y.P. Chiu, B.C. Huang, C.C. Lin, S.S. Li, C.S. Chang, C.W. Chen The efficiency of organic films based on poly(3-hexylthiophene) (P3HT) and methanofullerene derivative (PCBM) was shown to be strongly dependent on the crystalline order inside. Through the suitable annealing process, the well-crystallized organic P3HT:PCBM films can be fabricated to enhance their charge transport. To further improve the efficiency of photo-induced charge separation and transport as well as the corresponding photocurrent, more detailed electronic information at both interfaces of the donors/accepters and photoactive-layer/electrode will be essential. In this work, cross-sectional scanning tunneling microscopy and spectroscopy were employed to investigate the interfacial properties of P3HT:PCBM films. The vertical phase distribution and local electronic structures across the interfaces of substrate/organic film and P3HT/PCBM are obtained at the atomic resolution. These electronic structures also provide direct observations of the interfacial band alignments, suggesting the possible carrier transport mechanism of P3HT:PCBM organic films. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z33.00011: Enhanced Photocurrent in a Photovoltaic Cell involving a Nonconjugated Conductive Polymer, Poly($\beta $-pinene) M. Sangal, G. Telang, M. Thakur Photovoltaic cells have been fabricated using titanium dioxide/doped poly($\beta $-pinene)/carbon on ITO glass-substrates. Photocurrents and photo-voltages for different intensities of light (from a white illuminant light bulb, emission at 300-700 nm) have been measured. Use of iodine-doped nonconjugated conductive polymer film has led to significant enhancement of photocurrent compared to previous reports which included a different cell structure with undoped polymer-C$_{60}$ composites. A maximum photocurrent of about 0.3 mA was observed for a light intensity of about 5mW/cm$^{2}$. The maximum photo-voltage as observed was about 0.6 V for the same light intensity. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z33.00012: Molecular Imaging of Ultrathin Pentacene Films: Evidence for Homoepitaxy Yanfei Wu, Greg Haugstad, C. Daniel Frisbie Ultrathin polycrystalline films of organic semiconductors have received intensive investigations due to the critical role they play in governing the performance of organic thin film transistors. In this work, a variety of scanning probe microscopy (SPM) techniques have been employed to investigate ultrathin polycrystalline films (1-3 nm) of the benchmark organic semiconductor pentacene. By using spatially resolved Friction Force Microscopy (FFM), Kelvin Probe Force Microscopy (KFM) and Electrostatic Force Microscopy (EFM), an interesting multi-domain structure is revealed within the second layer of the films, characterized as two distinct friction and surface potential domains correlating with each other. The existence of multiple homoepitaxial modes within the films is thus proposed and examined. By employing lattice-revolved imaging using contact mode SPM, direct molecular evidence for the unusual homoepitaxy is obtained. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z33.00013: Understanding the growth of nanoscale organic semiconductors: the role of substrates Mina Yoon, Kai Xiao, Kendal W. Clark, An-Ping Li, David Geohegan, Bobby Sumpter, Sean Smith Our recent studies have demonstrated how substrates can be used to control the synthesis of nanoscale organic semicorductors. In particular, we study the growth mechanism of oriented crystalline organic nanowires consisting of M-TCNQF4 (M=Cu or Ag) from vapor-solid chemical reaction (VSCR). Our experimental and theoretical study combining time-resolved in situ X-ray diffraction and first-principles atomistic calculations indicate that the selectivity of different metals to induce nanowire growth depends strongly upon effective charge transfer between the organic molecules and the metal substrates. Understanding how to control the VSCR growth process may enable the synthesis of novel organic nanowires with axial or coaxial p/n junctions for organic nanoelectronics and solar energy harvesting. Another example is the growth of another promising organic semiconductor, CuPc assemblies on graphene(s) and Si substrates, where we investigate the role of the substrates in controlling the orientational arrangement of the molecules and their growth modes. Our theoretical study supports the various experimental observations from STM, TEM, and GIXS. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z33.00014: Temperature Dependent Anisotropic Step-Flow Growth of Metal Phthalocyanine on Silicon Studied by Scanning Probe Microscopy Sean Wagner, Richard Lunt, Pengpeng Zhang Control of highly ordered organic molecular thin films is currently of intense interest for integration into modern electronics due to the tunable nature of organic molecules. Here, we study the initial growth of archetypal zinc phthalocyanine (ZnPc) and copper phthalocyanine (CuPc) on the deactivated Si(111) surface. Using scanning probe microscopy (SPM), we demonstrate access to a new quasi-epitaxial anisotropic step-flow growth for both ZnPc and CuPc with a \textit{single} dominant long-range ordered relationship between the organic crystalline film and the substrate, uniquely distinct from inorganic epitaxial step-flow growth. This growth mode is largely attributed to the molecular diffusion and preferential nucleation at step edges enabled by the deactivated Si surface. We demonstrate the transition of growth modes by varying substrate temperature during deposition, altering the balance between diffusion and step- and island- nucleation rates. Access to the anisotropic step-flow growth offers new potential for the integration of highly-ordered organic thin films in silicon-based electronics. [Preview Abstract] |
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