Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session J41: Focus Session: Polymers for Energy Storage and Conversion -- Structure in Organic Semiconductor Blends |
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Sponsoring Units: DMP DPOLY GERA Chair: Enrique Gomez, Pennsylvania State University Room: A115/117 |
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J41.00001: Organic devices. New perspectives provided from soft x-ray characterization Invited Speaker: Organic semiconductors are continuing to receive significant interest for application in photovoltaic cells, field-effect transistors and light-emitting diodes. Conjugated polymers in particular offer the convenience of solution processibility with the flexibility of materials design afforded by synthetic chemistry. One of the disadvantages of conjugated polymers is the complexity of their film structure that, while key for understanding and optimizing device performance, is difficult to characterize. Here I will present new insights into the structure of films based on conjugated polymers using synchrotron-based soft x-ray techniques. By exploiting molecular resonances near the carbon K-edge, soft x-ray techniques such as x-ray spectromicroscopy and resonant soft x-ray scattering afford enhanced material contrast with high spatial resolution. This enhanced material specificity has been exploited to reveal the complex, hierarchical structure of conjugated polymer blends used in polymer solar cells. Furthermore, we have recently demonstrated a significant degree of miscibility of fullerene derivatives used in high-efficiency polymer/fullerene blends calling into question the assumed paradigm of phase-separated, pure phases. The polarized nature of synchrotron radiation can also be exploited to probe local molecular orientation and order using soft x-rays. This facilitates mapping of domain orientation and molecular order important for understanding charge transport in polycrystalline polymer films used in field-effect transistors. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J41.00002: Systematic Multiscale Modeling of Polymers Roland Faller, David Huang, Beste Bayramoglu, Adam Moule The systematic coarse-graining of heterogeneous soft matter systems is an area of current research. We show how the Iterative Boltzmann Inversion systematically develops models for polymers in different environments. We present the scheme and a few applications. We study polystyrene in various environments and compare the different models from the melt, the solution and polymer brushes to validate accuracy and efficiency. We then apply the technique to a complex system needed as active layer in polymer-based solar cells. Nano-scale morphological information is difficult to obtain experimentally. On the other hand, atomistic computer simulations are only feasible to studying systems not much larger than an exciton diffusion length. Thus, we develop a coarse-grained (CG) simulation model, in which collections of atoms from an atomistic model are mapped onto a smaller number of ``superatoms.'' We study mixtures of poly(3-hexylthiophene) and C$_{60}$. By comparing the results of atomistic and CG simulations, we demonstrate that the model, parametrized at one temperature and two mixture compositions, accurately reproduces the system structure at other points of the phase diagram. We use the CG model to characterize the microstructure as a function of polymer:fullerene mole fraction and polymer chain length for systems approaching the scale of photovoltaic devices. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J41.00003: Exploiting solvent additives to introduce processability to organic solar cells James Rogers, Kristin Schmidt, Michael Toney, Guillermo Bazan, Edward Kramer Solution processable, highly efficient, organic photovoltaics typically consist of a two component donor-acceptor type system composed of a low bandgap conjugated polymer donor blended with a fullerene acceptor. Efficient charge extraction from these blends demands that donor and acceptor components form nanoscale phase separated percolating pathways to their respective electrodes. Although post deposition thermal annealing has been shown to degrade device performance in low bandgap polymer systems, the incorporation of a small concentration of solvent additive (e.g. diiodooctane) into the solution from which a bulk heterojunction solar cell is cast has been shown to nearly double device efficiency without the need for subsequent thermal annealing. In situ grazing incidence wide angle x-ray scattering measurements as a function of time after spin coating suggest that the role of additives is to induce nucleation of crystals of the polymeric component and to facilitate changes in the correlation length (size and/or perfection) of these crystallites during the film drying process. The resulting structural order in additive processed films suggests novel processing routes for existing organic photovoltaics. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J41.00004: Kinetics of Structure Formation in Polymer-Fullerene Solutions for Organic Photovoltaics Margaret Sobkowicz, Ronald Jones, R. Joseph Kline, Dean DeLongchamp Bulk heterojunctions (BHJs) composed of poly(hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) are promising active layers for organic photovoltaics. The nanoscale morphology of the BHJ is critical to the performance of solar devices because exciton diffusion, charge separation, and carrier transport require domains that have specific optimal sizes and connectivity. Processing parameters have been shown to influence the morphology and thus device performance. Because P3HT crystallization during film formation is the driving force for phase segregation, casting solution properties are vital to film electronic properties. Small angle neutron scattering (SANS) is an ideal measurement technique to study the blend morphology and phase formation in P3HT:PCBM solutions due to the large difference in neutron scattering length density between P3HT and PCBM, the length scale probed, and the excellent sensitivity of SANS to concentration fluctuations. In this work SANS and solution rheology are developed as characterization tools for organic photovoltaic materials. Scattering data from P3HT:PCBM solutions are correlated to gelation kinetics to develop a picture of the nanoscale organization and the influence of processing on morphology. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J41.00005: Molecular-Scale and Nanoscale Morphology of P3HT:PCBM Bulk Heterojunctions: Energy-Filtered TEM and Low-Dose HREM Lawrence Drummy, Robert Davis, Diana Moore, Michael Durstock, Richard Vaia, Julia Hsu The performance of bulk heterojunction organic photovoltaic devices is critically dependent on the morphology of the active layer. Here we describe the combination of two electron microscopy techniques to quantitatively examine the molecular level structure and mesoscopic domain morphology of the active layer of P3HT:PCBM bulk heterojunction solar cells. Energy-filtered transmission electron microscopy (EFTEM) revealed the nanoscopic, interpenetrating fibrillar structure of the phase separated blend, providing unique assignments of the P3HT-rich and PCBM-rich regions. Low-dose high-resolution electron microscopy (LD-HREM) provided direct images of the P3HT crystals and their orientation within the P3HT-rich domains. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J41.00006: Neutron Scattering Provides a New Model for Optimal Morphologies in Organic Photovoltaics: Rivers and Streams Mark Dadmun, Nathan Henry, Wen Yin, Kai Xiao, John Ankner The current model for the ideal morphology of a conjugated polymer bulk heterojunction organic photovoltaic (OPV) is a phase-separated structure that consists of two pure phases, one an electron donor, the other an acceptor, that form an interpenetrating, bicontinuous, network on the length scale of 10-20 nm. In this talk, neutron scattering experiments that demonstrate that this model is incorrect for the archetypal conjugated polymer bulk heterojunction, poly[3-hexylthiophene] (P3HT) and the fullerene 1-(3-methyloxycarbonyl)propy(1-phenyl [6,6]) C$_{61}$ (PCBM) will be presented. These studies show that the miscibility of PCBM in P3HT approaches 20 wt{\%}, a result that is counter to the standard model of efficient organic photovoltaics. The implications of this finding on the ideal morphology of conjugated polymer bulk heterojunctions will be discussed, where these results are interpreted to present a model that agrees with this data, and conforms to structural and functional information in the literature. Furthermore, the thermodynamics of conjugated polymer:fullerene mixtures dominate the formation of this hierarchical morphology and must be more thoroughly understood to rationally design and fabricate optimum morphologies for OPV activity. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J41.00007: Organic Photovoltaic Interfaces: Back Contact Study Brett Guralnick, Michael Mackay, Raul Lobo Charge transfer between the polymer and contact greatly affects organic photovoltaics' (OPV) performance. The processing conditions are key since depositing the contact incorrectly reduces the polymer cell efficiency by up to fifty percent. The back contact, typically aluminum, is thermally evaporated onto the OPV active layer which has long been suspected to be affected by the process. To analyze this, the aluminum layer was dissolved after deposition and the resulting surface was imaged with an atomic force microscope. A fast aluminum deposition rate pitted the polymer surface creating regions of high resistivity thereby reducing cell efficiency. The addition of a LiF blocking layer between the active layer and aluminum was found to eliminate pitting allowing faster deposition. Interestingly, thermally annealing the active layer prior to aluminum deposition was also found to eliminate pitting. Neutron reflectivity experiments were used to determine that the fullerene derivative, used as the electron acceptor in the active layer, migrated to the surface during the annealing step and apparently act as a shielding layer preventing damage. With this knowledge the optimum deposition conditions were determined and has led to the highest efficiencies from OPVs. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J41.00008: Long-range Order in Self-assembled Poly(3-alkylthiophene)-Diblock Copolymers Victor Ho, Bryan Boudouris, Rachel Segalman Poly(3-alkylthiophenes) (P3ATs) are used commonly as active layer components in plastic electronic devices due to their relatively high hole mobilities, low optical band gaps, and their ability to be processed from solution. To date, however, block copolymers containing these molecules as a functional component predominantly have shown nanofibrillar morphologies identical to that of the P3AT homopolymers due to the large thermodynamic driving force for crystallization. We show that by decreasing rod-rod interactions through rational side chain substitution, well-ordered ($e.g.$, lamellar and hexagonally-packed) geometries can be obtained with P3AT-containing diblock copolymers as evidenced by x-ray scattering and electron microscopy. Additionally, we demonstrate that the structural and optoelectronic properties of the P3EHT domains remain in place. The ability to pattern these functional macromolecules on the nanoscale opens many doors for advanced design of organic electronic active layers. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J41.00009: P3HT-based copolymers as interfacial compatibilizers in P3HT/PCBM system S. Michael Kilbey II, Jihua Chen, Xiang Yu, Kai Xiao, Mark Dadmun, Deanna Pickel, Bobby Sumpter To lower the interfacial tension and control the donor-acceptor phase separation in organic photovoltaic devices, a poly(3-hexylthiophene)-\textit{block}-poly(ethylene oxide) (P3HT-$b$-PEO) diblock copolymer compatibilizer was added to a binary blend of regioregular P3HT and the fullerene derivative 6,6-phenyl C$_{61}$ butyric acid methyl ester (PCBM). We systematically examined the ternary phase behavior of spin-coated films of P3HT/ P3HT-$b$-PEO/ PCBM before and after annealing with selected area electron diffraction, grazing-incidence X-ray diffraction, AFM, optical and transmission electron microscopy. Neutron reflectivity experiments were also carried out to study thermodynamic behaviors of P3HT/P3HT-$b$-PEO/PCBM trilayer films. The addition of 5{\%} P3HT-$b$-PEO (block molecular weights of 10kDa and 3kDa, respectively) to a 1:1 P3HT/PCBM blend reduces the size of P3HT-rich domains in P3HT/PCBM films by up to 40{\%} while the $\pi -\pi $ stacking of P3HT (i.e. (020) crystallinity) remains nearly unchanged. In addition we will discuss the effect of compatibilizer type, additive concentration, and thermal annealing conditions on power conversion efficiencies of compatibilized organic photovoltaic cells. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J41.00010: Cooperative Assembly of Hydrogen-bonded Block Copolythiophenes/Fullerene Blends for Photovoltaic Devices James J. Watkins, Ying Lin, Jung Ah Lim, Qingshuo Wei, Alejandro L. Briseno The current work provides a general approach to obtain reliable donor acceptor morphologies by H-bonding cooperative assembly and to achieve efficient photovoltaic devices with enhanced device stability. Herein, we utilize P3HT-based block copolymer (BCP), in which one block is P3HT and the other block is a P3HT derivative containing a poly(ethylene oxide) (PEO) oligomer side chain. This design both enables self-assembly of the devices via microphase segregation into lamellar, cylindrical or spherical morphologies depending on the relative volume fractions of the blocks and provides a means for establishing strong preferential interaction between fullerene derivatives containing hydrogen bond donating groups (such as COOH groups) and the PEO side chain. One advantage of this approach is excellent device stability due to the suppression of macrophase separation resulting from fullerene crystallization under harsh annealing condition. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J41.00011: Enhanced Photovoltaic Performance of All-Conjugated Poly(3-alkylthiophene) Diblock Copolymers Ming He, Wei Han, Jing Ge, Yuliang Yang, Feng Qiu, Zhiqun Lin Control of the ratio of blocks in the all-conjugated poly(3-butylthiophene)-b-poly(3-hexylthiophene) (P3BHT) diblock copolymer provides a simple route to precisely tune the molecular organization and nanoscale morphology in the resulting bulk heterojunction (BHJ) solar cells made of P3BHT/ PC$_{71}$BM. An attractive high PCE of 4.02 {\%} was found in P3BHT21 (i.e., P3BT/P3HT block ratio of 2:1 mol/mol)/PC$_{71}$BM, compared to that of 1.08 {\%} in P3BT and 3.54 {\%} in P3HT homopolymer-based devices. The enhanced performance is attributed to improved phase separation, interpenetrating pathway and the formation of crystalline domain size of 10.4 nm in the active layer; the latter also elucidated the importance of alkyl side-chain lengths in the molecular organization and final film morphology. In the P3BHT21/PC$_{71}$BM blend films, P3BT block facilitated the self-assembly of P3BHT chains into interpenetrating crystalline pathway for efficient charge transport, while P3HT block provided P3BHT chains with necessary flexibility to form improved phase separation at the nanoscale with maximum interfacial areas for charge generation. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J41.00012: Morphological Characterization of Low-Bandgap Crystalline Polymer:PCBM Bulk Heterojunction Solar Cells Haiyun Lu, Thomas Russell Understanding the morphology of polymer-based bulk heterojunction (BHJ) solar cells is key to improving device efficiencies. Blends of a low-bandgap silole-containing conjugated polymer, poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b;2',3'-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5'-diyl] (PSBTBT) with phenyl-C61-butyric acid methyl ester (PCBM) were investigated using different processing conditions. Scanning force microscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, dynamic secondary ion mass spectrometry and neutron reflectivity studies showed that thermal annealing did not induce obvious changes in the structure of the active layer. Grazing-incidence X-ray diffraction and small-angle neutron scattering showed that the crystallization of PSBTBT and segregation of PCBM occurred during spin coating, and a brief thermal annealing increased the ordering of PSBTBT and enhanced the segregation of the PCBM, forming domains with 10-nm in size, leading to an improvement in photovoltaic performance. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J41.00013: Donor-acceptor conjugated polymers used as electron acceptors in bulk heterojunction photovoltaics Christopher Bailey, Barney Taylor, Jianguo Mei, John Reynolds, John Henderson, Benjamin Leever, Michael Durstock Synthetic control over the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) has been of significant importance in organic photovoltaics due to the nature of charge separation in donor/acceptor blends. One technique for obtaining tunability of the HOMO and LUMO levels with polymer synthesis is to combine electron donating and electron accepting moieties separated by a conjugated linkage unit. This technique has been utilized to produce highly efficient devices reaching power conversion efficiencies above 8{\%} in polymer/fullerene blends. In this work, we report the characterization of poly(2,7{\-}divinylene fluorene{\-}co{\-}benzothiadiazole) (F10DVBT), and performs best as an electron acceptor when mixed with poly(3-hexylthiophene) (P3HT) with an open circuit voltage of 1.2V. A combination of morphological and photo-physical studies highlights interesting properties of this material and its interactions with P3HT. The donor-acceptor conjugated structure of F10DVBT appears to strongly affect the photocurrent of these devices, and may result from the interactions between intramolecular and intermolecular charge transfer processes. [Preview Abstract] |
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