Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session F04: Organic Electronics IFocus Live
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Sponsoring Units: DPOLY FIAP DMP Chair: Stephanie Lee, Stevens Inst of Tech; Takuji Adachi, Univ of Geneva |
Tuesday, March 16, 2021 11:30AM - 12:06PM Live |
F04.00001: First-Principles Theory for Understanding Excitons in Stacked Organic Assemblies Invited Speaker: Sahar Sharifzadeh Organic semiconductors are tunable light absorbers with promise as solar energy conversion materials, with their efficiency highly dependent on the nature and energy of electron-hole pairs or excitons formed upon light absorption. Excitons in these materials are controlled by the interplay between inter- and intra-molecular electronic as well as vibrational interactions, which is not yet well-controlled in devices. Here, we utilize first-principles theory to investigate the excitonic properties of stacks of functionalized PTCDI DNA base surrogates as a model system to study inter- and intra-molecular interactions. We apply time-dependent density functional theory (TDDFT), along with a Franck-Condon analysis of vibronic effects, to finite stacks of molecules that have been recently synthesized. We determine that the intra- and inter-molecular interactions result in distinct vibrational, electronic, and optical properties. Additionally, by combining TDDFT with a recently developed time-resolved non-adiabatic dynamics approach, we show that stacking increases the efficiency of non-radiative relaxation dynamics from a high excitonic state to the lowest energy exciton. For a periodic assembly of PTCDI, many-body perturbation theory predicts a bandstructure with significant bandwidth (~ 0.8 eV), consistent with strong inter-molecular electronic interactions, and several spatially delocalizated low-energy optically excited-states. By incorporating electron-phonon interactions, we find that at T = 300K, the optical absorption is altered from T = 0 K due to allowed indirect transitions, while exciton delocalization and binding energy, a measure of intermolecular electronic interactions, remains constant. Overall, this work demonstrates that excitonic properties can be modified via inter-molecular electronic and vibrational interactions. |
Tuesday, March 16, 2021 12:06PM - 12:18PM Live |
F04.00002: Porous, aligned polymer films for improved organic electrochemical transistors Lucas Flagg, Dean DeLongchamp, Lee Richter We study the role of porosity and molecular order on the performance of poly(3-hexylthiophene-2,5-diyl) (P3HT) based organic electrochemical transistors (OECTs). We utilize the small molecule 1,3,5 trichlorobenzene (TCB) as a crystallizable solvent to induce directional crystallization during blade coating. Removal of the TCB results in a porous layer of semiconducting polymer. We find that films cast with TCB show a greater than 5x improvement in device performance compared to neat P3HT films. Additionally, at high TCB loading ratios, we observe significant alignment of the polymer resulting in polarization dependent absorption. In these aligned films, we find significant anisotropy in OECT performance depending on coating direction. When the coating direction is parallel to the current direction in the transistor, the current is more than 10x greater than when the coating direction is perpendicular to the current direction. Overall, these results illustrate the role of porosity in improving OECT performance, even for hydrophobic polymers. Furthermore, this work demonstrates a means of fabricating highly anisotropic OECTs. |
Tuesday, March 16, 2021 12:18PM - 12:30PM Live |
F04.00003: Directing Organic Semiconductor Crystal Orientation and Shape using Nanoconfining Scaffolds Aida Alaei, Kai Zong, Kaustubh Asawa, Tseng-Ming Chou, Alejandro L. Briseño, Chang-Hwan Choi, Stephanie S. Lee Because optoelectronic properties can vary significantly along different crystallographic directions, directing the orientation of organic semiconductor crystals is critical. Here we demonstrate a method to dictate crystal orientation by (mis)matching the crystal dimensionality with the geometry of the nanoconfining scaffolds. Specifically, nanoconfining scaffolds that limit crystal growth in one or two dimensions were used to confine the solution-phase crystallization of 1D needlelike triisopropylsilylethynyl pyranthrene (TIPS-PY) and 2D platelike perylene. For systems in which the crystal dimensionality matched the scaffold dimensionality, control over crystal orientation was observed in one dimension for TIPS-PY and two dimensions for perylene. For systems in which the dimensionalities were mismatched, orientation control was limited to a single dimension. Intriguingly, platelike perylene crystals grown within cylindrical nanopores were found to form unconventional T-shaped single crystals with concave 90o corners. Inside the nanopores, crystal growth was confined in two directions, forcing crystals to grow in a needlelike shape. At the scaffold surface, however, crystal growth became unrestricted and fast growth along two directions resumed. |
Tuesday, March 16, 2021 12:30PM - 12:42PM Live |
F04.00004: Phase Separation Dynamics in an Annealed Polymer:Fullerene Blend Analyzed using the Critical Point Model of the Permittivity Rafi Shikler, Netaniel Hai In this work we show for the first time how spectroscopic ellipsometry can be used to analyze the dynamics of morphological changes that take place during the annealing of a film of two bench-mark materials used for organic solar cells that are the most reported on in the literature, poly(3-hexyl-thiophene 2.5-diyl) (P3HT) and [6,6] phenyl C61 Butyric acid methyl ester (PCBM). Morphological changes within the homopolymer and the blend during annealing process manifest themselves in the optical spectra of the film and are qualitatively attributed to the structural changes. In contrast, we demonstrate that parametric description using the critical point model gives additional information that was previously only accessible by x-ray diffraction based techniques. We analyzed the dimensionality of the excitons as extracted from the model and found a one-to-one correspondence with observed peaks in the x-ray spectra. We show that a preferable morphology for the organic blend, in the sense of the highest dimension of excitons, was found after 20 minutes of annealing treatment at 140°C. Our results indicate to the immense potential of using a macroscopic measurement technique, ellipsometry, to analyze the dynamic of microscopic changes in the active layer morphology. |
Tuesday, March 16, 2021 12:42PM - 12:54PM Live |
F04.00005: Marangoni-Coupled Alignment in a Semiconductor-Elastomer Blend for Stretchable Electronics Peter Dudenas, Eliot H Gann, Lee Richter, Dean DeLongchamp Semiconducting polymer-elastomer blends are a promising material class for flexible and stretchable electronics due to their superior mechanical performance and ability to maintain charge transport at large strains. The semiconducting polymer phase segregates into nanofibrils upon spin- or blade-coating and under certain conditions these fibrils can be aligned for anisotropic charge transport. Here, we report on a long-wavelength Marangoni instability in the coating process that leads to significant variation in the local semiconducting polymer fibril alignment. AFM, resonant soft x-ray scattering (RSoXS) and near-edge x-ray absorption fine structure (NEXAFS) quantify the morphology and these measurements are correlated to thin-film transistor data to characterize the impact on charge transport. Quantifying and potentially controlling coating instabilities is important for device optimization, especially in the scale-up process. |
Tuesday, March 16, 2021 12:54PM - 1:06PM Live |
F04.00006: The Impact of Illumination on the Photoluminescence and Depth Profile of MEH-PPV/dPS Thin Films Joshua Moncada, Tanguy Terlier, Ilia Ivanov, Rafael Verduzco, Mark Dadmun This study examines the changes in photoluminescence and film structure due to white light exposure during annealing of conjugated polymer blend thin films. Previous studies in our group have shown that annealing similar polymer blend thin films above the polymers’ Tg in a dark or illuminated environment alters the film structure. These structural changes also impact the photoluminescence (PL) activity of the film. The present work investigates the correlation between light exposure, film morphology and photoluminescence for four films, blends of MEH-PPV and polystyrene at 5, 20, 35, and 50% MEH-PPV. The measured PL of the films showed variation in PL activity dependent on illumination during annealing at 125 °C. Samples that are exposed to light during annealing exhibited lower PL. Time-of-Flight Secondary Ion Mass Spectroscopy provides data to correlate the film’s depth profile to the observed PL. Concurrently, small angle neutron scattering investigates the morphological changes in these films with illumination. These studies indicate that layering of the film dominates the changes in PL activity at low loadings of MEH-PPV, while at higher loadings in-plane morphological changes direct the observed PL. |
Tuesday, March 16, 2021 1:06PM - 1:18PM Live |
F04.00007: Relationship between Charge-transfer State Electroluminescence and the Degradation of Organic Photovoltaics Claire Arneson, Xinjing Huang, Xiaheng Huang, Dejiu Fan, Yongxi Li, Mengyuan Gao, Long Ye, Harald W Ade, Stephen Ross Forrest Previous studies of organic photovoltaics have related open-circuit voltage to charge transfer state emission efficiency using a theory based on detailed balance. We modify this theory by distinguishing between electroluminescent external quantum efficiency and external charge transfer emission efficiency, and use it to quantify the degradation of archetype vacuum- and solution-processed devices. The presence of aging behavior is included using an empirical factor, m, whose value varies for degradation occurring within, m = 1, and outside, m > 1, of the photoactive heterojunction. It is observed that m = 1.02 ± 0.01 for vacuum-processed devices, and m = 2.93 ± 0.09 for solution-processed devices. We conclude that vacuum-processed devices exhibit degradation within the heterojunction while solution-processed devices exhibit degradation in regions peripheral to the heterojunction. This characterization suggests that the reliability of solution-processed devices can be extended by improving the stability of layers and interfaces external to the heterojunction. |
Tuesday, March 16, 2021 1:18PM - 1:54PM Live |
F04.00008: Resonant Tender X-ray Diffraction for Disclosing the Molecular Packing of Paracrystalline Conjugated Polymer Films Invited Speaker: Christopher McNeill The performance of optoelectronic devices based on conjugated polymers is critically dependent upon molecular packing; however the paracrystalline nature of these materials limits the amount of information that can be extracted from conventional X-ray diffraction. In this presentation it will be shown that resonant diffraction (also known as anomalous diffraction) is able to provide new molecular packing information about conjugated polymer films. Resonant diffraction effects will be demonstrated for several common sulfur-containing polymers whereby rapid changes in diffraction intensity are observed as the X-ray energy is varied across the sulfur K-edge. These changes – combined with theoretical calculations – make it possible to discriminate between different packing geometries, which opens up a new way to unlock important microstructural information about conjugated polymer thin films for which only a handful of diffraction peaks are typically available. |
Tuesday, March 16, 2021 1:54PM - 2:06PM Live |
F04.00009: Impact of the primary structure of organic semiconductors on amorphous topology and crystal morphology Chad Snyder The amorphous topology and crystal morphology are are believed to be critical parameters for semicrystalline organic semiconductor device performance. Amorphous topology describes the level and type of chain connectivity between crystallites. Crystal morphology describes the size, shape, and connectivity of crystals. In this presentation, we show how much of the above can be influenced and quantified through the primary structure of the organic semiconductors, i.e., molar mass, molar mass distribution, and comonomers/defects. |
Tuesday, March 16, 2021 2:06PM - 2:18PM Live |
F04.00010: Morphology-performance Correlation in Near-Infrared Organic Solar Cells with Efficiency Exceeding 9% Obaid Alqahtani, Brian Collins, Devin Grabner, Victor Manuel Murcia, Zhipping Kan, Tongle Xu, Tainan Duan Organic solar cells (OSCs) are attractive due to their tunable properties and rising efficiency, >18%. Recently synthesized electron-accepting molecules for OSCs show efficient absorption near IR and PCE~10% when paired with a polymer donor. Here, we utilized X-ray techniques to probe their morphology. Our x-ray, TEM, SEM and AFM results show that small-molecule domains undergo drastic change in packing and phase separating beyond the optimal processing condition, i.e. solvent additive as a plasticizer. We found that the additive increases domain size and packing, suggesting reduced charge recombination and enhanced charge transport, i.e. favorable for performance. However, beyond 1% additive, the performance significantly decreases. We attribute that to large domains reducing exciton-dissociation efficiency. We found that the elemental substitution of fluorine for chlorine in the small molecules have similar trends in performance and morphology. Such a thorough characterization provides sufficient knowledge to correlate morphology with performance, also gives insights into the morphological effects of chemical and processing modifications of organic materials for optoelectronic applications. |
Tuesday, March 16, 2021 2:18PM - 2:30PM Live |
F04.00011: Real-time analysis of mixed ion and electron transfer in a conjugated polymer during the doping and de-doping RATUL MITRA THAKUR, Alexandra Easley, Shaoyang Wang, Yiren Zhang, Christopher K Ober, Jodie L. Lutkenhaus Conjugated polymers have promising applications in electronics and energy storage due to the polymer’s tunable conductivity and redox activity. For example, the conductivity of poly(3-hexylthiophene) (P3HT) is heavily dependent upon the doping level and the dopant type. This feature becomes especially important when considering P3HT or similar conjugated polymers for devices that require switching between electronic states (conductive vs insulating). In this talk, the mechanism of mixed ion-electron transfer studied using electrochemical quartz crystal microbalance with dissipation monitoring is discussed. During cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) experiments, the mass change of the P3HT electrode is monitored in real-time. This leads to an understanding of how the doping level and ion transfer behavior change with current and voltage. Specific doping levels were then correlated with the conductivity of the polymer using in-situ conductance experiments. This effort gives valuable insight into the nature of mixed ion-electron transfer during the doping-de-doping process. |
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