Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Y33: Focus Session: Organic Electronics and Photonics - Morphology and Structure I |
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Sponsoring Units: DMP Chair: Penpeng Zhang, Michigan State University Room: 341 |
Friday, March 22, 2013 8:00AM - 8:12AM |
Y33.00001: Molecular simulation studies of morphology in blends of conjugated polymers and fullerene derivatives for organic photovoltaic applications Eric Jankowski, Hilary Marsh, Arthi Jayaraman The device efficiency of organic solar cells is dependent on the microstructure of the active layer, which is typically a mixture of conjugated polymer electron donor molecules and fullerene based acceptor molecules. Active layer morphology can be tuned by choosing these acceptor and donor components that self-assemble into thermodynamically stable structures and by choosing processing conditions that facilitate the formation of equilibrium structures or that ``trap'' the active layer in an optimal metastable configuration. We present the results of molecular dynamics studies of model conjugated polymers and fullerene derivatives performed on GPUs. We show that the ordered structures that are self-assembled from initially disordered configurations depend strongly upon the strength of the attractions between acceptor and donor molecules, the relative amounts of each component, and the architecture of the donor molecules. Further we quantify the relaxation times and suggest processing strategies for obtaining optimal morphologies for charge transport. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y33.00002: Correlation of Fullerene Structure to its Miscibility in P3HT and OPV Function Mark Dadmun, Huipeng Chen, Jeff Peet The miscibility of four fullerenes, bis-PCBM, ICBA, Thio-PCBM and PC$_{\mathrm{70}}$BM in poly(3-hexylthiophene) (P3HT) is determined by neutron reflectivity by monitoring the intermixing of P3HT and fullerene bilayers with thermal annealing. The miscibility limit of these fullerenes in P3HT ranges from 11{\%} to 26{\%}, where the bis-adduct fullerenes exhibit a lower miscibility in P3HT than singly functionalized fullerenes. The correlation of miscibility to device performance indicates that sufficient polymer/fullerene miscibility is crucial to rationally optimize organic photovoltaic active layers. Low miscibility of conjugated polymer and fullerene in the amorphous phase decreases the probability of exciton dissociation and enhances the recombination of free charge-carriers. Moreover, the results indicate that the average surface-to-surface distance between fullerenes must be less than approximately of 5-7 {\AA} to minimize charge traps and allow sufficient charge transport in the mixed phase to improve photovoltaic performance. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y33.00003: Mixing-Induced Anisotropic Correlations in Molecular Crystalline Systems: Rationalizing the Behavior of Organic Semiconductor Blends Katharina Broch, Antje Aufderheide, Jiri Novak, Alexander Hinderhofer, Alexander Gerlach, Rupak Banerjee, Frank Schreiber Binary mixtures of organic semiconductors (OSCs) have recently become an important field of research, as they find applications in opto-electronic devices [1]. In these systems, the mixing (intermixing vs. phase separation) and ordering behavior is crucial, since it affects the optical and electronic properties. We present a comprehensive study of binary mixtures of the three prototypical OSCs pentacene (PEN), perfluoropentacene (PFP) and diindenoperlyene (DIP) in all possible combinations [1,2]. Using X-ray reflectivity and grazing incidence X-ray diffraction we investigate the stuctural properties of the mixed films as well as their impact on the optical spectra obtained by spectroscopic ellipsometry. For PEN:DIP we find an anisotropic ordering behavior, comparable to that observed in some liquid crystals, which is fundamentally new for OSCs [2]. The influence of sterical compatibility and the strength of the intermolecular interactions on the mixing and ordering behavior in the different blends will be discussed by extending a conventional mean-field model [1]. Finally, we discuss general rules for the targeted preparation of blends of OSCs. [1] A. Hinderhofer and F. Schreiber, Chem. Phys. Chem., 13, 628 (2012); [2] A. Aufderheide et al., Phys. Rev. Lett., 109, 156102 (2012) [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y33.00004: Domain compositions in the active layer of low band gap polymer/fullerene solar cells strongly affect device performance Sameer Vajjala Kesava, Zhuping Fei, Martin Heeney, Cheng Wang, Alexander Hexemer, Enrique Gomez We have characterized the morphology of mixtures of a germole-containing polymer, poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]germole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PGeBTBT), and PCBM using Resonance Soft X-ray Scattering (RSOXS) and Energy-Filtered Transmission Electron Microscopy (EFTEM). PGeBTBT belongs to cyclopentadithiophene-based polymer family with a band gap of 1.5 eV. Analyses of RSOXS data and EFTEM images have shown that the volume fraction of polymer in the fullerene matrix enveloping PGeBTBT fibers ($\sim$10 nm diameter) decreases with increasing overall composition of PCBM. Furthermore, PGeBTBT/PCBM devices demonstrate a correlation between the short circuit current and the purity of the PCBM-rich phase. We hypothesize that the relationship between PCBM domain composition and device performance is related to charge recombination, where increasing the polymer content suppresses charge transport thereby increasing the transit time. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y33.00005: The importance of domain purity for performance in P(NDI2OD-T2)-based all-polymer solar cells revealed by resonant x-ray scattering Harald Ade, Brian Collins, Marcel Schubert, Steffen Roland, Robert Steyrluethner, Zhihua Chen, Antonio Facchetti, Dieter Neher The nanostructure of bulk heterojunction organic solar cells has long been recognized as critical to their performance. To date, the primary morphological characteristics under investigation have been the level and nature of crystallinity of the materials. Yet the recent and wide-spread measurement of molecular mixing and diffusion of the electron donor and acceptor materials in amorphous regions has focused attention on the non-crystalline portions in these films as well. Here we investigate both aspects using x-ray diffraction and resonant scattering techniques to measure crystallinity and the domain sizes and purities, respectively, of devices based on P3HT:P(NDI2OD-T2) blends. The repercussions of the nanostructure is revealed in measurements of exciton bandwidth and photoluminescence quenching. We find that through variation of solvent blends and film drying conditions can significantly alter domain size and purity. This results in significant increases in device performance that correlate with increased domain purity and exciton bandwith of the P3HT crystals. This strongly indicates that molecular mixing of these materials is detrimental to performance in harvesting solar energy. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y33.00006: Probing the morphology of novel non-fullerene based bulk heterojunction solar cells Gregory Su, Toan Pho, Fred Wudl, Edward Kramer, Michael Chabinyc Organic semiconductors are promising for low-cost, large-area electronics such as organic photovoltaics (OPVs). OPVs require an active layer that is an intimate mixture of an electron donor, usually a conjugated polymer, and an electron acceptor, typically a fullerene. While fullerene-based OPVs show high efficiencies, the inability to tune its electronic levels limits the open circuit voltage, so alternative acceptors are desirable. Here, we report on blend films consisting of a polymer donor, poly(3-hexylthiophene) (P3HT), and a novel acceptor, decacyclene triimide (DTI), that display good solar power conversion efficiencies (PCE) as-cast. The PCE of these blends decreases significantly with thermal annealing, unlike P3HT:fullerene blends. NEXAFS spectroscopy and grazing incidence wide angle X-ray scattering suggest the PCE decrease is due to the formation of hexagonally packed DTI columns with an in-plane $\pi$-$\pi$ stacking direction, resulting in poor electron transport out-of-plane. Polarized resonant soft X-ray scattering suggests that the $\pi$-$\pi$ stacking directions are orthogonal at interfaces between DTI and P3HT domains. These results demonstrate the importance of blend morphology in OPV efficiency and key differences between DTI-based and fullerene-based blends. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y33.00007: Tuning polymer/inorganic blend morphology using pyridine terminated poly(3-hexylthiophene)s: Novel ligands for potential OPV applications W. Michael Kochemba, S. Michael Kilbey II, Deanna L. Pickel, Bobby G. Sumpter End-functional pi-conjugated polymers are promising materials for the improvement of organic electronic devices due to their high hole mobility and ease of processability. Here we describe a ``materials by design'' approach to create 2- and 3-pyridyl, end-functionalized poly(3-hexylthiophene)s (P3HTs) that possess the capacity to ligate semiconductor quantum dots (SQDs). The replacement of native ligands on the SQD surface by pyridyl-terminated P3HTs provides the opportunity to manipulate the morphology of polymer/inorganic blends created by dispersing the P3HT-ligated SQDs in a P3HT matrix. TEM imaging and small angle x-ray scattering were used to assess the morphological traits of the blends as a function of ligand type, processing condition, and matrix molecular weight, which in general show that the P3HT ligands improve dispersion of the nanoparticles upon thermal annealing. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y33.00008: Determination of the Crystallinity of Semicrystalline Poly(3-hexyl thiophene) by Means of Wide Angle X-Ray Scattering Jens Balko, Ruth Lohwasser, Mukundan Thelakkat, Michael Sommer, Ovidiu Pascui, Kay Saalwaechter, Thomas Thurn-Albrecht Poly(3-hexyl thiophene) (P3HT) is a common polymer semiconductor, often used as material or component in organic field effect transistors or solar cells. The crystallinity of this semicrystalline material is among other parameters governing the electronic mobility. However, at present there is no routine method available to determine an absolute value for the crystallinity, and the values given in the literature e.g. for the enthalpy of melting vary by a factor of three. Wide Angle X-Ray Scattering (WAXS) probes the crystals as well the amorphous parts of the sample. We present an approach for the determination of the crystallinity based on the evaluation of WAXS intensities at low scattering vectors emanating from the amorphous regions. The result is used for a calibration of the melting enthalpy (34 J/g) that can serve as a reference value for more convenient calorimetric techniques and compared to the results of recent NMR investigations. We discuss the crystallinity for a number of chemically well-defined samples, with different molecular weight and a typical commercial sample with broad molecular weight distribution. Despite the high crystallinities of 60 to 80{\%} the crystallites exhibit a large amount of disorder. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y33.00009: Structural and Morphological Analysis of Poly(3-hexylthiophene ) at Surfaces and Interfaces Yeneneh Yimer, Mesfin Tsige The structure and morphology of semiconducting polymers such as Poly(3-hexylthiophene) (P3HT) at surfaces and interfaces have significant influence over the performance of organic solar cell devices. Because charge-carrier generation, transport to and collection at the electrodes depend on the material properties of P3HT - themselves controlled by factors including packing, orientation and environment - analysis of the mechanisms that contribute to efficient charge generation and minimization of recombination is necessary. Using molecular dynamics simulation, we have investigated the structural properties and morphological evolution of P3HT at different surfaces and interfaces. We have also investigated the dependence of those properties on temperature, chain length, and interfacial energies. The morphology of P3HT is correlated to efficient charge transport. Using our analyses, we have attempted to elucidate these correlations, which should help lead to optimization of the morphology of P3HT in devices in the pursuit of increasing the efficiency of polymeric devices. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y33.00010: Conjugated backbone orientation variation in high mobility regioregular PT based copolymers Louis Perez, Lei Ying, Guillermo Bazan, Edward Kramer The synthesis of novel solution processable conjugated polymers is an active field of study due to the potential to fabricate low cost, high though-put electronic devices such as organic field effect transistors (OFET). A regioregular copolymer based on cyclopenta[2,1-$b$:3,4-$b'$]dithiophene (CDT) and pyridal[2,1,3]thiadiazole (PT) structural units has been prepared by using polymerization reactions involving reactants specifically designed to avoid random orientation of the asymmetric PT heterocycle. Compared to it's regiorandom counterpart, the regioregular polymer exhibits a two orders of magnitude increase in hole mobility from 0.005 to 0.6 cm$^{\mathrm{2\thinspace }}$V$^{\mathrm{-1}}$ s$^{\mathrm{-1}}$. A combination of X-ray scattering techniques were employed to quantitatively access the degree of orientation and crystallinity in thin films (15-20 nm) that matched device architecture. We examined the backbone orientation dependence as a function of depth via grazing incidence wide angle X-ray scattering (GIWAXS) and found significant differences in the backbone stacking orientation between the regiorandom and regioregular copolymers. These experiments suggest the backbone regularity leads to significant differences in the structural arrangement and it is another important design criteria to consider in the design of new conjugated copolymers with asymmetric structural units. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y33.00011: Poly(3-hexylthiophene) Brush-Modified Interfaces for Control of Active Layer Morphology and Properties S. Michael Kilbey, W. Michael Kochemba, Deanna Pickel, Jose Alonzo Tailoring the morphology of donor-acceptor blends based on conjugated polymers and fullerenes is an essential part of optimizing the power conversion efficiency of organic photovoltaic (OPV) devices. While a variety of studies have demonstrated the importance of the nanoscale morphology of donor-acceptor blends on efficiency, a clear understanding of the links between morphology, processing, interfacial structure and device-level properties is yet to emerge. Here we turn to well-defined layers of end-tethered poly(3-hexylthiophene) (P3HT) chains as modifiers, or buffer layers, that straddle the inorganic/organic interface and exert control over the morphology of donor-acceptor blends. In addition to improving device performance characteristics, ostensibly due the presence of surface dipoles brought about by confinement, P3HT brushes affect the penetration of the fullerene derivative, 6,6-phenyl-C61-butyric acid methyl ester, PCBM, into the brush as well as the morphology of bilayers and blends of P3HT and PCBM coated atop the brushes. The role of molecular weight, chain grafting density, and thermal aging and light cycling on these behaviors will be highlighted. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y33.00012: Rod-Coil Copolymer as Efficient Compatibilizer for Thermally-Stable Polymer Solar Cell H.J. Kim, K. Paek, H. Yang, B.J. Kim Improving the thermal stability of polymer solar cells (PSC) is important for the future application of these devices since any heat generated by solar irradiation could be detrimental to the performance as a result of the relatively low Tg of polymers and the strong immiscibility of components in the active layer. Herein we have developed new type of compatibilizers having two different blocks of conjugated polymer and poly(2-vinyl pyridine)(P2VP). The P2VP and fullerene are mixed together by supramolecular interaction resulting conjugated polymer-P2VP copolymers act as a compatibilizer reducing the interfacial tension between the two dissimilar components of the PSC. Our compatibilizer successfully suppresses the macrophase separation of donor and acceptor blended films made of either singly functionalized PCBM or bisadduct fullerene derivatives. P3HT/o-xylene-C60-bissadduct fullerne(OXCBA) blended device containing 10 vol percent of compatibilizer shows an average efficiency higher than 4.3 percent after 60 h annealing at an elevated temperature of 150'C. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y33.00013: Percolating bulk-heterostructures from neutron reflectometry and small angle scattering data Daniel Olds, Phillip Duxbury We present a novel algorithm for efficiently calculating the simulated small angle scattering data of any discretized morphological model of arbitrary scale and resolution, referred to as the distribution function method (DFM). Unlike standard SAS fitting methods, the DFM algorithm allows for the calculation of form factors and structure factors from complex nanoscale morphologies commonly encountered in many modern polymeric and nanoparticle based systems, which have no exact analytical corollary. The computational efficiency of the DFM algorithm suggests it's use in morphological model refinement. We will present a number of simple examples to demonstrate the accuracy and limits of the algorithm, followed by an example of incorporation of the DFM algorithm into reverse Monte Carlo structural refinement of bulk-heterojunction two-phase morphologies, such as those commonly found in organic photovoltaic devices. We will show that morphological features introduced via direct incorporation of experimental neutron reflectometry and SANS data to the models has a direct effect on the results of device simulations. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y33.00014: In-situ Neutron Scattering Determination of 3D Phase-Morphology Correlations in Fullerene -Polymer Organic Photovoltaic Thin Films Alamgir Karim, David Bucknall, Dharmaraj Raghavan, Bobby Sumpter, Scott Sides The tunability of the morphology and structure of conjugated polymer-fullerene bulk heterojunctions (BHJs) is being investigated through synthesis of new materials, novel processing strategies and advanced characterization (experimental and computational). We are using this integrated approach to test currently poorly understood fundamental issues in organic photovoltaic (OPV) performance relating to structure-property and very importantly processing relationships. Using model conjugated polymer-fullerene systems, we are investigating how the phase morphology of the BHJs correlate with OPV efficiency. A range of fullerenes is being investigated that include a number of new derivatives that we have synthesized. We are currently investigating the use of surface energy confinement and block copolymer templating to control both phase domain segregation and orientation relative to the film normal to allow us to test morphology-device efficiency hypotheses in OPVs. Using both neutron scattering and computational modeling we have developed important correlations that establish relationships between the polymer-fullerene miscibility, phase domain orientation and interfacial behavior with the corresponding photoelectronic properties. [Preview Abstract] |
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