@article{yan_collins_gann_wang_ade_mcneill_2012, title={Correlating the Efficiency and Nanomorphology of Polymer Blend Solar Cells Utilizing Resonant Soft X-ray Scattering}, volume={6}, ISSN={["1936-086X"]}, DOI={10.1021/nn204150f}, abstractNote={Enhanced scattering contrast afforded by resonant soft X-ray scattering (R-SoXS) is used to probe the nanomorphology of all-polymer solar cells based on blends of the donor polymer poly(3-hexylthiophene) (P3HT) with either the acceptor polymer poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (F8TBT) or poly([N,N′-bis(2-octyldodecyl)-11-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-12-bithiophene)) (P(NDI2OD-T2)). Both P3HT:F8TBT and P3HT:P(NDI2OD-T2) blends processed from chloroform with subsequent annealing exhibit complicated morphologies with a hierarchy of phase separation. A bimodal distribution of domain sizes is observed for P3HT:P(NDI2OD-T2) blends with small domains of size ∼5–10 nm that evolve with annealing and larger domains of size ∼100 nm that are insensitive to annealing. P3HT:F8TBT blends in contrast show a broader distribution of domain size but with the majority of this blend structured on the 10 nm length scale. For both P3HT:P(NDI2OD-T2) and P3HT:F8TBT blends, an evolution in device performance is observed that is correlated with a coarsening and purification of domains on the 5–10 nm length scale. Grazing-incidence wide-angle X-ray scattering (GI-WAXS) is also employed to probe material crystallinity, revealing P(NDI2OD-T2) crystallites 25–40 nm in thickness that are embedded in the larger domains observed by R-SoXS. A higher degree of P3HT crystallinity is also observed in blends with P(NDI2OD-T2) compared to F8TBT with the propensity of the polymers to crystallize in P3HT:P(NDI2OD-T2) blends hindering the structuring of morphology on the sub-10 nm length scale. This work also underscores the complementarity of R-SoXS and GI-WAXS, with R-SoXS measuring the size of compositionally distinguishable domains and GI-WAXS providing information regarding crystallinity and crystallite thickness.}, number={1}, journal={ACS NANO}, author={Yan, Hongping and Collins, Brian A. and Gann, Eliot and Wang, Cheng and Ade, Harald and McNeill, Christopher R.}, year={2012}, month={Jan}, pages={677–688} }
 @article{wang_lee_hexemer_kim_zhao_hasegawa_ade_russell_2011, title={Defining the Nanostructured Morphology of Triblock Copolymers Using Resonant Soft X-ray Scattering}, volume={11}, ISSN={["1530-6992"]}, DOI={10.1021/nl2020526}, abstractNote={The morphologies of a poly(1,4-isoprene)-block-polystyrene-block-poly(2-vinyl pyridine) (IS2VP) copolymer were investigated using resonant soft X-ray scattering (RSoXS) together with scanning force microscopy, small-angle X-ray scattering, and electron microscopy. Differences in the nanoscopic morphologies in the bulk and thin film samples were observed arising from the competition between segmental interactions between the blocks and the substrate and the surface energies of each block. Using soft X-rays at selected photon energies to isolate the scattering contribution from different polymer blocks, RSoXS unambiguously defined the complex morphology of the triblock copolymer. In the bulk sample, two nested, hexagonal arrays of P2VP and PI cylindrical microdomains residing in the PS matrix were observed. The cylindrical microdomains of one component were found to be located at the interstitial sites of the hexagonal array of the other component that has the larger d spacing. In solvent-annealed thin films with 40 nm in thickness, a hexagonal array of core-shell microdomains of P2VP cores with PS shells that reside in a PI matrix were observed.}, number={9}, journal={NANO LETTERS}, author={Wang, Cheng and Lee, Dong Hyun and Hexemer, Alexander and Kim, Myung Im and Zhao, Wei and Hasegawa, Hirokazu and Ade, Harald and Russell, Thomas P.}, year={2011}, month={Sep}, pages={3906–3911} }
 @article{yan_swaraj_wang_hwang_greenham_groves_ade_mcneill_2010, title={Influence of Annealing and Interfacial Roughness on the Performance of Bilayer Donor/Acceptor Polymer Photovoltaic Devices}, volume={20}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.201001292}, abstractNote={Through controlled annealing of planar heterojunction (bilayer) devices based on the polyfluorene copolymers poly(9,9-dioctylfluorene-co-bis(N,N′-(4,butylphenyl))bis(N,N′-phenyl-1,4-phenylene)diamine) (PFB) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) we study the influence of interface roughness on the generation and separation of electron–hole pairs at the donor/acceptor interface. Interface structure is independently characterized by resonant soft X-ray reflectivity with the interfacial width of the PFB/F8BT heterojunction observed to systematically increase with annealing temperature from 1.6 nm for unannealed films to 16 nm with annealing at 200 °C for ten minutes. Photoluminescence quenching measurements confirm the increase in interface area by the three-fold increase in the number of excitons dissociated. Under short-circuit conditions, however, unannealed devices with the sharpest interface are found to give the best device performance, despite the increase in interfacial area (and hence the number of excitons dissociated) in annealed devices. The decrease in device efficiency with annealing is attributed to decreased interfacial charge separation efficiency, partly due to a decrease in the bulk mobility of the constituent materials upon annealing but also (and significantly) due to the increased interface roughness. We present results of Monte Carlo simulations that demonstrate that increased interface roughness leads to lower charge separation efficiency, and are able to reproduce the experimental current-voltage curves taking both increased interfacial roughness and decreased carrier mobility into account. Our results show that organic photovoltaic performance can be sensitive to interfacial order, and heterojunction sharpness should be considered a requirement for high performance devices.}, number={24}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Yan, Hongping and Swaraj, Sufal and Wang, Cheng and Hwang, Inchan and Greenham, Neil C. and Groves, Chris and Ade, Harald and McNeill, Christopher R.}, year={2010}, month={Dec}, pages={4329–4337} }
 @article{swaraj_wang_yan_watts_lüning jan_mcneill_ade_2010, title={Nanomorphology of Bulk Heterojunction Photovoltaic Thin Films Probed with Resonant Soft X-ray Scattering}, volume={10}, ISSN={1530-6984 1530-6992}, url={http://dx.doi.org/10.1021/nl1009266}, DOI={10.1021/nl1009266}, abstractNote={The bulk nanomorphology of organic bulk heterojunction devices, particularly of all-polymer devices, is difficult to characterize due to limited electron density contrast between constituent materials. Resonant soft X-ray scattering can overcome this problem and is used to show that the morphologies in chloroform cast and subsequently annealed polyfluorene copolymer poly(9,9'-dioctylfluorene-co-bis(N,N'-(4,butylphenyl))bis(N,N'-phenyl-1,4-phenylene)diamine) (PFB) and poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT) blends exhibit a hierarchy of length scales with impure domains in as-cast films. With annealing, these domains first become purer at the smallest length scale and only then evolve in size with annealing. Even optimized cells using present fabrication methods are found to have a dominant domain size much larger than the exciton diffusion length. The observed morphology is far from ideal for efficient solar cell operation and very different from those achieved in high-efficiency fullerene-based devices. This strongly implies that lack of morphological control contributes to the relatively poor performance of the all-polymer PFB/F8BT devices and may be problematic for all-polymer devices in general. Novel processing strategies will have to be employed to harness the full potential these high open circuit voltage devices offer.}, number={8}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Swaraj, Sufal and Wang, Cheng and Yan, Hongping and Watts, Benjamin and Lüning Jan and McNeill, Christopher R. and Ade, Harald}, year={2010}, month={Aug}, pages={2863–2869} }
 @article{wang_garcia_yan_sohn_hexemer_nguyen_bazan_kramer_ade_2009, title={Interfacial widths of conjugated polymer bilayers}, volume={131}, DOI={10.1021/ja905293m}, abstractNote={The interfaces of conjugated polyelectrolyte (CPE)/poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) bilayers cast from differential solvents are shown by resonant soft X-ray reflectivity (RSoXR) to be very smooth and sharp. The chemical interdiffusion due to casting is limited to less than 0.6 nm, and the interface created is thus nearly “molecularly” sharp. These results demonstrate for the first time and with high precision that the nonpolar MEH-PPV layer is not much disturbed by casting the CPE layer from a polar solvent. A baseline is established for understanding the role of interfacial structure in determining the performance of CPE-based polymer light-emitting diodes. More broadly, we anticipate further applications of RSoXR as an important tool in achieving a deeper understanding of other multilayer organic optoelectronic devices, including multilayer photovoltaic devices.}, number={35}, journal={Journal of the American Chemical Society}, author={Wang, C. and Garcia, A. and Yan, H. P. and Sohn, K. E. and Hexemer, A. and Nguyen, T. Q. and Bazan, G. C. and Kramer, E. J. and Ade, Harald}, year={2009}, pages={12538-} }
 @article{swaraj_wang_araki_mitchell_liu_gaynor_deshmukh_yan_mcneill_ade_2009, title={The utility of resonant soft x-ray scattering and reflectivity for the nanoscale characterization of polymers}, volume={167}, ISSN={["1951-6355"]}, DOI={10.1140/epjst/e2009-00946-3}, journal={EUROPEAN PHYSICAL JOURNAL-SPECIAL TOPICS}, author={Swaraj, S. and Wang, C. and Araki, T. and Mitchell, G. and Liu, L. and Gaynor, S. and Deshmukh, B. and Yan, H. and McNeill, C. R. and Ade, H.}, year={2009}, month={Feb}, pages={121–126} }
 @article{wang_coifman_2008, title={The effect of lane-change maneuvers on a simplified car-following theory}, volume={9}, ISSN={["1558-0016"]}, DOI={10.1109/TITS.2008.928265}, abstractNote={This paper investigates the linearity of empirically observed spacing-speed relations for various drivers in the context of car-following theory and how lane-change maneuvers perturb the relation. It is shown that the impacts of lane-change maneuvers are not balanced, and the response time to an exiting vehicle is much longer than the response time to an entering vehicle. This accommodation imbalance will propagate upstream, and as discussed herein, it appears to be a source of speed and flow fluctuations (or oscillations) within a queue. This paper is motivated by Newell's simplified car-following theory, namely, that during congested periods, the trajectory of a given vehicle is essentially identical to the preceding vehicle's trajectory, except for a translation in space and time. One of the basic assumptions in Newell's presentation is that spacing and speed are linearly related. While other researchers have found macroscopic evidence supporting Newell's theory, they have also found that it fails in the presence of frequent lane-change maneuvers. This paper takes a microscopic approach, employing vehicle trajectory data. This paper provides support for Newell's assumed linear relation between spacing and speed over a large range of speeds when vehicles are not impacted by lane-change maneuvers. It also offers a possible explanation for the degraded performance of Newell's theory in the presence of heavy lane-change maneuvers. Although the focus is on Newell's simplified car-following theory, the empirical results of this paper have similar implications for many other car-following theories as well.}, number={3}, journal={IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS}, author={Wang, Chao and Coifman, Benjamin}, year={2008}, month={Sep}, pages={523–535} }
 @article{wang_araki_watts_harton_koga_basu_ade_2007, title={Resonant soft x-ray reflectivity of organic thin films}, volume={25}, ISSN={["0734-2101"]}, DOI={10.1116/1.2731352}, abstractNote={At photon energies close to absorption edges in the soft x-ray range, the complex index of refraction, n=1−δ−iβ, of organic materials varies rapidly as a function of photon energy in a manner that strongly depends on the chemical moieties and functionalities present in the material. The authors present details of how these molecular structure specific variations in the complex index of refraction can be utilized to enhance and tune the contrast in reflectivity experiments of organic films. This near edge contrast enhancement mimics the specific contrast achieved through deuterium labeling in neutron reflectivity (NR). This relatively new x-ray approach, resonant soft x-ray reflectivity (RSoXR), thus combines aspects of NR and conventional x-ray reflectivity (XR), yet does not require special chemical procedures. The capabilities of RSoXR are exemplified using a number of polymeric bi- and multilayers. Furthermore, a direct comparison of RSoXR to conventional x-ray reflectivity and NR for polystyrene and poly(methyl methacrylate) bilayers verifies that RSoXR is an excellent alternative tool for the characterization of organic thin films. The influence of the longitudinal and transverse coherence properties as well as the divergence of the x-ray or neutron beam on the capabilities and limitations of each reflectivity variant is discussed.}, number={3}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Wang, Cheng and Araki, Tohru and Watts, Benjamin and Harton, Shane and Koga, Tadanori and Basu, Saibal and Ade, Harald}, year={2007}, pages={575–586} }
 @article{wang_araki_ade_2005, title={Soft x-ray resonant reflectivity of low-Z material thin films}, volume={87}, ISSN={["1077-3118"]}, DOI={10.1063/1.2136353}, abstractNote={Soft x-ray resonant reflectivity, a method for low-Z materials that combines aspects of neutron reflectivity and x-ray reflectivity, is presented. Resonant reflectivity provides enhanced and selective sensitivity to specific chemical moieties near the absorption edges of constituent elements and was demonstrated through the characterization of a bilayer polymer thin film. The relative reflectivity of a particular interface could be tuned by adjusting the incident photon energy near the carbon 1s absorption edge. The resulting chemical specificity is analogous to using deuteration as a tracer or marker in neutron reflectivity, but without requiring special sample synthesis or preparation.}, number={21}, journal={APPLIED PHYSICS LETTERS}, author={Wang, C and Araki, T and Ade, H}, year={2005}, month={Nov} }