@article{mukherjee_gann_nahid_mcafee_herzing_delongchamp_ade_2021, title={Orientational Ordering within Semiconducting Polymer Fibrils}, volume={31}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.202102522}, abstractNote={Abstract Due to a general paucity of suitable characterization methods, the internal orientational ordering of polymer fibrils has rarely been measured despite its importance particularly for semi‐conducting polymers. An emerging tool with sensitivity to bond orientation is polarized resonant soft X‐ray scattering (P‐RSoXS). Here, P‐RSoXS reveals the molecular arrangement within fibrils (if type I or type II fibrils), the extent of orientation in the fibril crystal, and an explicit crystal‐amorphous interphase. Neat films as well as binary blends with a fullerene derivative are characterized for three different polymers, that are prototypical materials widely used in organic electronics applications. Anisotropic P‐RSoXS patterns reveal two different fibril types. Analysis of the q ‐dependence of the anisotropy from simulated and experimental scattering patterns reveal that neat polymer fibrillar systems likely comprise more than two phases, with the third phase in addition to crystal and amorphous likely being an interphase with distinct density and orientation. Intriguingly, the fibril type correlates to the H‐ or J‐aggregation signature in ultraviolet‐visible (UV–vis) spectroscopy, revealing insight into the fibril formation. Together, the results will open the door to develop more sophisticated structure‐function relationships between chemical design, fibril type, formation pathways and kinetics, interfacial ordering, and eventually device functions.}, number={28}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Mukherjee, Subhrangsu and Gann, Eliot and Nahid, Masrur Morshed and McAfee, Terry and Herzing, Andrew A. and DeLongchamp, Dean M. and Ade, Harald}, year={2021}, month={Jul} }
 @article{hoffman_mcafee_conrad_loth_anthony_ade_dougherty_2016, title={Intrinsic Charge Trapping Observed as Surface Potential Variations in diF-TES-ADT Films}, volume={8}, ISSN={["1944-8244"]}, DOI={10.1021/acsami.6b03886}, abstractNote={Spatial variations in surface potential are measured with Kelvin probe force microscopy for thin films of 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophenes (diF-TES-ADT) grown on SiO2 and silane-treated SiO2 substrates by organic molecular beam deposition. The variations are observed both between and within grains of the polycrystalline organic film and are quantitatively different than electrostatic variations on the substrate surfaces. The skewness of surface potential distributions is larger on SiO2 than on HMDS-treated substrates. This observation is attributed to the impact of substrate functionalization on minimizing intrinsic crystallographic defects in the organic film that can trap charge.}, number={33}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Hoffman, Benjamin C. and McAfee, Terry and Conrad, Brad R. and Loth, Marsha A. and Anthony, John E. and Ade, Harald W. and Dougherty, Daniel B.}, year={2016}, month={Aug}, pages={21490–21496} }
 @article{bondarev_popescu_younts_hoffman_mcafee_dougherty_gundogdu_ade_2016, title={Lowest energy Frenkel and charge transfer exciton intermixing in one-dimensional copper phthalocyanine molecular lattice}, volume={109}, ISSN={["1077-3118"]}, DOI={10.1063/1.4968821}, abstractNote={We report the results of the combined experimental and theoretical studies of the low-lying exciton states in crystalline copper phthalocyanine. We derive the eigen energy spectrum for the two lowest intramolecular Frenkel excitons coupled to the intermolecular charge transfer exciton state and compare it with temperature dependent optical absorption spectra measured experimentally, to obtain the parameters of the Frenkel-charge-transfer exciton intermixing. The two Frenkel exciton states are spaced apart by 0.26 eV, and the charge transfer exciton state is 50 meV above the lowest Frenkel exciton. Both Frenkel excitons are strongly mixed with the charge transfer exciton, showing the coupling constant 0.17 eV which agrees with earlier experimental measurements. These results can be used for the proper interpretation of the physical properties of crystalline phthalocyanines.}, number={21}, journal={APPLIED PHYSICS LETTERS}, author={Bondarev, I. V. and Popescu, A. and Younts, R. A. and Hoffman, B. and McAfee, T. and Dougherty, D. B. and Gundogdu, K. and Ade, H. W.}, year={2016}, month={Nov} }
 @article{mcafee_hoffman_you_atkin_ade_dougherty_2016, title={Morphological, Optical, and Electronic Consequences of Coexisting Crystal Orientations in beta-Copper Phthalocyanine Thin Films}, volume={120}, ISSN={["1932-7455"]}, DOI={10.1021/acs.jpcc.6b05043}, abstractNote={The crystal structure of the β phase of copper phthalocyanine (CuPc) is a monoclinic herringbone, which is commonly created in thin films by either thermal annealing of α-CuPc at ∼300 °C or deposition on heated substrates. Of the several known CuPc crystal polymorphs, the β phase is of particular interest due to its thermodynamic stability. We observe three coexisting crystal orientations for thin films of β-CuPc to be (101), (1̅05), and (5̅02) using grazing incidence wide-angle X-ray scattering. Each of the three crystal orientations have distinct surface roughness, as measured by atomic force microscopy, but are electrostatically similar by Kelvin probe force microscopy. However, local optical properties, as measured by micro-UV–vis spectroscopy, are very different in the different domains.}, number={33}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={McAfee, Terry and Hoffman, Benjamin C. and You, Xiao and Atkin, Joanna M. and Ade, Harald and Dougherty, Daniel B.}, year={2016}, month={Aug}, pages={18616–18621} }
 @article{huston_wang_mcafee_loth_anthony_ade_conrad_dougherty_2015, title={Disruption of Molecular Ordering over Several Layers near the Au/2,8-Difluoro-5,11-bis(triethylsilylethynyl) Anthradithiophene Interface}, volume={15}, ISSN={["1528-7505"]}, DOI={10.1021/cg501621k}, abstractNote={The transition from an electrode-dominated ordered monolayer structure to a bulk-like thin film crystal structure of 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES-ADT) is observed to occur over at least the first four molecular layers near the electrode surface. Scanning tunneling microscopy studies of the growth of diF-TES-ADT on Au(111) show that the first two molecular layers assemble with aromatic planes parallel to the substrate surface. The monolayer structures are highly stable and well-ordered, while the bilayer structures are more loosely packed and poorly ordered. Subsequent diF-TES-ADT growth results in a more bulk-like layer containing standing up molecular configurations approaching the (001) crystal face as observed by grazing incidence wide angle X-ray scattering measurements. However, the third and fourth monolayers also show poor long-range ordering and an apparent height modulation that indicate significant strain effects from the substrate still persist.}, number={2}, journal={CRYSTAL GROWTH & DESIGN}, author={Huston, Shawn M. and Wang, Jiuyang and McAfee, Terry and Loth, Marsha and Anthony, John E. and Ade, Harald W. and Conrad, Brad R. and Dougherty, Daniel B.}, year={2015}, month={Feb}, pages={822–828} }
 @article{lin_chen_li_lai_yang_mcafee_jiang_li_liu_hu_et al._2015, title={High-Performance Non-Fullerene Polymer Solar Cells Based on a Pair of Donor-Acceptor Materials with Complementary Absorption Properties}, volume={27}, ISSN={["1521-4095"]}, DOI={10.1002/adma.201502775}, abstractNote={A 7.3% efficiency non-fullerene polymer solar cell is realized by combining a large-bandgap polymer PffT2-FTAZ-2DT with a small-bandgap acceptor IEIC. The complementary absorption of donor polymer and small-molecule acceptor is responsible for the high-performance of the solar-cell device. This work provides important guidance to improve the performance of non-fullerene polymer solar cells.}, number={45}, journal={ADVANCED MATERIALS}, author={Lin, Haoran and Chen, Shangshang and Li, Zhengke and Lai, Joshua Yuk Lin and Yang, Guofang and McAfee, Terry and Jiang, Kui and Li, Yunke and Liu, Yuhang and Hu, Huawei and et al.}, year={2015}, month={Dec}, pages={7299-+} }
 @article{ma_yang_jiang_carpenter_wu_meng_mcafee_zhao_zhu_wang_et al._2015, title={Influence of processing parameters and molecular weight on the morphology and properties of high-performance PffBT4T-2OD:PC71BM organic solar cells}, volume={5}, DOI={10.1002/aenm.201570126}, abstractNote={The detailed morphology of high efficiency polymer (PffBT4T-2OD) based organic solar cells is investigated by Wei Ma, Harald Ade, He Yan and co-workers in article number 1501400. It is found that the median domain sizes of PffBT4T-2OD:PC71BM blends processed at different temperatures/spin rates are nearly identical, while the average domain purity and the molecular orientation relative to polymer:fullerene interfaces can be significantly changed by the processing conditions.}, number={23}, journal={Advanced Energy Materials}, author={Ma, W. and Yang, G. F. and Jiang, K. and Carpenter, J. H. and Wu, Y. and Meng, X. Y. and McAfee, T. and Zhao, J. B. and Zhu, C. H. and Wang, C. and et al.}, year={2015} }
 @article{ma_reinspach_zhou_diao_mcafee_mannsfeld_bao_ade_2015, title={Tuning Local Molecular Orientation-Composition Correlations in Binary Organic Thin Films by Solution Shearing}, volume={25}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.201500468}, abstractNote={A general impact of solution shearing on molecular orientation correlation is observed in polymer:fullerene organic solar cells in which one of the components forms fibrils or aggregates. Further investigation with polarized soft X‐ray scattering reveals that solution shearing induces more face‐to‐face orientation relative to the interface of two components compared to spin‐coating. This impact is shearing speed dependent, that is, slow shearing speed can induce more face‐to‐face orientation than a fast shearing speed. These results demonstrate that solution shearing is an effective method to control the relative molecular orientation. Solution shearing can also modify the domain size and average composition variations.}, number={21}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Ma, Wei and Reinspach, Julia and Zhou, Yan and Diao, Ying and McAfee, Terry and Mannsfeld, Stefan C. B. and Bao, Zhenan and Ade, Harald}, year={2015}, month={Jun}, pages={3131–3137} }
 @article{kitchen_awartani_kline_mcafee_ade_brendan t. o'connor_2015, title={Tuning Open-Circuit Voltage in Organic Solar Cells with Molecular Orientation}, volume={7}, ISSN={["1944-8244"]}, DOI={10.1021/am508855s}, abstractNote={The role of molecular orientation of a polar conjugated polymer in polymer-fullerene organic photovoltaic (OPV) cells is investigated. A planar heterojunction (PHJ) OPV cell composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) is used as a model system to isolate the effect of the interfacial orientation on the photovoltaic properties. The molecular orientation of the aggregate P3HT relative to the PCBM layer is varied from highly edge-on (conjugated ring plane perpendicular to the interface plane) to appreciably face-on (ring plane parallel to the interface). It is found that as the P3HT stacking becomes more face-on there is a positive correlation to the OPV open-circuit voltage (V(OC)), attributed to a shift in the highest occupied molecular orbital (HOMO) energy level of P3HT. In addition, the PHJ OPV cell with a broad P3HT stacking orientation distribution has a V(OC) comparable to an archetypal bulk heterojunction (BHJ) device. These results suggest that, in the BHJ OPV cell, the hole energy level in the charge transfer state is defined in part by the orientation distribution of the P3HT at the interface with PCBM. Finally, the photoresponses of the devices are also shown to have a dependence on P3HT stacking orientation.}, number={24}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Kitchen, Brent and Awartani, Omar and Kline, R. Joseph and McAfee, Terry and Ade, Harald and Brendan T. O'Connor}, year={2015}, month={Jun}, pages={13208–13216} }
 @article{li_abrecht_yang_roland_tumbleston_mcafee_yan_kelly_ade_neher_et al._2014, title={Mobility-Controlled Performance of Thick Solar Cells Based on Fluorinated Copolymers}, volume={136}, ISSN={["0002-7863"]}, DOI={10.1021/ja5067724}, abstractNote={Developing novel materials and device architectures to further enhance the efficiency of polymer solar cells requires a fundamental understanding of the impact of chemical structures on photovoltaic properties. Given that device characteristics depend on many parameters, deriving structure-property relationships has been very challenging. Here we report that a single parameter, hole mobility, determines the fill factor of several hundred nanometer thick bulk heterojunction photovoltaic devices based on a series of copolymers with varying amount of fluorine substitution. We attribute the steady increase of hole mobility with fluorine content to changes in polymer molecular ordering. Importantly, all other parameters, including the efficiency of free charge generation and the coefficient of nongeminate recombination, are nearly identical. Our work emphasizes the need to achieve high mobility in combination with strongly suppressed charge recombination for the thick devices required by mass production technologies.}, number={44}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, author={Li, Wentao and Abrecht, Steve and Yang, Liqiang and Roland, Steffen and Tumbleston, John R. and McAfee, Terry and Yan, Liang and Kelly, Mary Allison and Ade, Harald and Neher, Dieter and et al.}, year={2014}, month={Nov}, pages={15566–15576} }
 @article{mcafee_gann_guan_stuart_rowe_dougherty_ade_2014, title={Toward Single-Crystal Hybrid-Carbon Electronics: Impact of Graphene Substrate Defect Density on Copper Phthalocyanine Film Growth}, volume={14}, ISSN={["1528-7505"]}, DOI={10.1021/cg500504u}, abstractNote={Graphene has long been recognized as a potential replacement for indium tin oxide as a transparent conducting substrate that may not only be cheaper to manufacture but also may provide mechanical flexibility and templating for preferential organic film growth. Here, we report the discovery that the thin film growth mode and crystal structure of copper phthalocyanine (CuPc), a prototype organic semiconductor, is extremely sensitive to even atomic-scale defects (e.g., steps) on the graphene surface and that high quality films can be grown with a well-defined crystal orientation that should be favorable for optimized solar cell applications. The initial growth involves flat-lying copper phthalocyanine molecules in a triclinic brickstone crystal with (012̅) orientation. Thicker films on pristine graphite, as well as thin films on lower quality graphene, show an orientational transition to the flat-lying (112̅)-oriented brickstone, which nucleates near film defects and grows in more compact 3D islands. The thickness dependent transition between these two flat-lying crystal orientations is sensitive to substrate defect density since this determines the extent to which initial island elongation along the substrate can alleviate strain in the film. The sensitivity of copper phthalocyanine film morphology demonstrates the extreme importance of graphene substrate quality to controlling organic film growth. The high crystallinity and optimal molecular orientation achieved here implies a new driving force for high performance organic optoelectronics based on substrate-controlled crystal engineering.}, number={9}, journal={CRYSTAL GROWTH & DESIGN}, author={McAfee, Terry and Gann, Eliot and Guan, Tianshuai and Stuart, Sean C. and Rowe, Jack and Dougherty, Daniel B. and Ade, Harald}, year={2014}, month={Sep}, pages={4394–4401} }
 @article{mcafee_gann_ade_dougherty_2013, title={Thermally Induced Dewetting in Ultrathin C-60 Films on Copper Phthalocyanine}, volume={117}, ISSN={["1932-7447"]}, DOI={10.1021/jp4067372}, abstractNote={The evolution of thermally annealed ultrathin fullerene–C60 layers on copper phthalocyanine is examined by atomic force microscopy and near-edge X-ray absorption fine structure spectroscopy. Annealing causes C60 films to dewet the copper phthalocyanine substrate surface via lateral surface mass transport. Coarsening of C60 clusters is observed that creates mounds that exceed the nominal C60 thickness by more than an order of magnitude and is consistent with surface diffusion-mediated mass transport. Implications for thermal morphology control in organic solar cells, such as the destabilization of multilayered C60:CuPc with individual layers only ∼5–10 nm thick, are discussed.}, number={49}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={McAfee, T. and Gann, E. and Ade, H. and Dougherty, D. B.}, year={2013}, month={Dec}, pages={26007–26012} }