Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session S34: Organic Electronics I: Microstructure and Mechanical PropertyFocus
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Sponsoring Units: DPOLY FIAP DMP Chair: Stephanie Lee, Stevens Inst of Tech Room: 506 |
Thursday, March 5, 2020 11:15AM - 11:51AM |
S34.00001: Advances in Non-Fullerene Organic Photovoltaics Invited Speaker: Samson Jenekhe Recent progress in molecular engineering of organic small-molecule and polymer semiconductors and in engineering of the blend active layer morphology has pushed the state-of-the-art organic photovoltaic (OPV) device efficiency above 15%, narrowing the gap with theoretical values. In this talk I will discuss our work in both of the main classes of OPV devices: (i) all-polymer solar cells (all-PSCs) composed of both donor and acceptor polymers; and (ii) blends of a small-molecule non-fullerene acceptor (NFA) with a donor polymer. We have found that binary blends of new random copolymer acceptors with a known donor polymer can spontaneously self-organize into optimal bulk heterojunction morphologies without the use of any solution processing additives; thereby, enabling highly efficient all-PSCs with low energy loss and near unity internal quantum efficiency. Another example is our finding of the different roles of charge transport, blend morphology, and blend photophysics in dictating the photovoltaic properties of NFA/polymer blends, highlighting the importance of fused-ring NFAs in suppressing geminate and bimolecular recombination and enabling improved photocurrent and fill factor. |
Thursday, March 5, 2020 11:51AM - 12:03PM |
S34.00002: Revealing the Microstructure of the Active Layer of Ternary Organic Solar Cells using Energy-Filtered TEM Ismail Alperen Ayhan, Enrique D Gomez Fullerene-free ternary organic solar cells have emerged as a promising class of devices with higher efficiencies when compared with solar cells that encompass binary active layers. Compared with traditional fullerene acceptors, non-fullerene acceptors have stronger absorption in the visible and near-infrared, which helps to generate high photocurrent for solar cells. Nevertheless, controlling phase separation and domain size in the photoactive layer morphology is crucial to enable efficient charge separation, transport and collection. Here, we demonstrate high-efficiency devices by combining the organic semiconductors with complementary light absorption, using PTB7-Th as donor, and IDIC, COi8DFIC, Eh-IDTBR, and O-IDTBR as acceptors. The combination of Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS), Energy-Filtered Transmission Electron Microscopy (EFTEM), Resonant Soft X-ray Scattering (RSoXS) reveals the impact of crystallization on the mesoscale morphology. We show that ternary systems composed of compatible nonfullerene acceptors exhibit fibril-like nanostructures in the active layer, leading to devices with efficiencies in excess of 10%. The morphology suggests that compatibility between two of the three components is important for maximizing performance in devices. |
Thursday, March 5, 2020 12:03PM - 12:15PM |
S34.00003: Solution Processed Organic and Inorganic Transistors – Application in Inverter Circuits John Barron, Alec Pickett, James Glaser, Suchismita Guha In the fabrication of organic field-effect transistors (FETs), the usage of high κ dielectrics lowers the operating voltage and enhances device performance. Polymer ferroelectrics open the path not just for high κ values but allow processing of the films via electrical poling. Poled ferroelectric dielectrics in p-type organic FETs is seen to improve carrier mobility and reduce parasitic current when compared to unpoled devices using the same dielectric. Furthermore, n-type inorganic FETs using solution processed ZnO have shown improved performance when the semiconductor layer is subjected to a UV-ozone treatment. FETs were fabricated using the ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) as the dielectric insulator and the electron donor-acceptor polymer diketopyrrolopyrrole-dithienylthieno[3,2-b]thiophene (DPP-DTT) for the semiconductor layer, with devices showing mobilities as high as 0.4 cm2/Vs when electrical poling was utilized. ZnO thin film FETs were fabricated with SiO2 as the dielectric layer, yielding carrier mobilities on the order of 10-2 cm2/Vs. Both p- and n-type FETs were used in a voltage inverter circuit, showing promising characteristic gain. |
Thursday, March 5, 2020 12:15PM - 12:27PM |
S34.00004: Electric field-induced second harmonic generation imaging of organic thin-film devices PAYAL BHATTACHARYA, Oksana Ostroverkhova, Ping Yu, Suchismita Guha The application of an electric field is an effective mechanism for breaking the symmetry in otherwise symmetric molecular systems. Electric-field induced second harmonic generation (EFISHG) provides a tool for investigating chromophore orientation in nonlinear optical (NLO) materials and understanding transport at the semiconductor-dielectric interface in thin film transistors. A microscopic imaging platform in the reflection geometry has been developed for measuring second harmonic generation (SHG) and EFISHG signals from organic thin-film devices. The polymer poly(vinylidene fluoride) (PVDF), with its low glass transition temperature, provides a medium for orienting embedded NLO molecules under the application of external fields. Malonitrile based NLO chromophores were mixed with PVDF to form thin films. An in-plane electric field upwards of 6×105 V/m yields a strong EFISHG signal. The SHG signal is seen to increase with the magnitude of the applied in-plane field and decreases upon the application of a vertical electric field. We further apply this technique to small molecule based organic transistors, where the application of a source-drain potential breaks the symmetry and allows the investigation of carrier transport using an EFISHG signal. |
Thursday, March 5, 2020 12:27PM - 12:39PM |
S34.00005: Structure of vapor-deposited glasses within a few nanometers of organic-organic interfaces is bulk-like Marie Fiori, Kushal Bagchi, Michael Toney, Mark Ediger Vapor-deposited (PVD) glasses are used in OLED devices as <50 nm layers, each deposited onto the layer below it. The bulk structure of a >100 nm PVD glass depends on the temperature at which it was deposited; it is unaffected by the substrate. The distance over which the substrate perturbs the structure of a PVD glass is unknown. In this work, we prepare PVD films made of 60 alternating 10 nm layers of organic semiconductors, DSA-Ph and Alq3, creating 60 interfaces. The large number of organic-organic interfaces allows us to access the X-ray scattering of the structure near the organic substrate. GIWAXS of films with 60 interfaces is virtually identical to the sum of the scattering of thick films of the pure components, indicating that the organic substrate perturbs the glass structure for <2 nm. When deposited at or below 0.8 Tg, we find that the DSA-Ph molecules at organic interfaces lie parallel to the substrate. We expect that this is true for all PVD glassy films of rigid, flat molecules like DSA-Ph, ensuring good π-π orbital overlap at the organic-organic interface, allowing for efficient charge transport across the interface and into the semiconductor layer. |
Thursday, March 5, 2020 12:39PM - 12:51PM |
S34.00006: Polymer light-emitting diodes with an emitting layer based on a nano-confined semiconducting polymer blend Anielen Ribeiro, Katharina Landfester, Paul Blom, Jasper Michels Blending a visible light-emitting organic semiconductor with an insulator alleviates the trap-limited nature of the electron current. Solution-processed organic light emitting diodes (OLEDs) comprising such a blend as emissive layer exhibit a two-fold increase in luminous efficiency with only 10% semiconductor. This low content of semiconductor can only be achieved with polymeric emitters. Hence, polymer-LEDs are considerably more cost-effective than devices based on small molecular emitters. However, due to the reduced translational entropy, polymers typically exhibit a low miscibility. In principle, macro-phase separation can be avoided if the molecular weight is kept low, which is however disadvantageous due to low charge carrier mobilities. An alternative strategy is to impose a nano-confinement. We prepare aqueous nanodispersions of poly(p-phenylenevinylene)-based emitters, blended with polystyrene. Macro-phase separation is fully suppressed, as confirmed by scanning force microscopy. Despite the fact that fabricating OLEDs from aqueous nanoparticle dispersions poses considerable risks of strong field inhomogeneity and high leakage currents, we succeed in fabricating such devices highly reproducibly and at efficiencies similar to the non-particle based reference devices. |
Thursday, March 5, 2020 12:51PM - 1:03PM |
S34.00007: Sub-Turn-on Exciton Quenching Modulated by Spontaneous Orientation Polarization in Organic Light-Emitting Devices John Bangsund, Jack R Van Sambeek, Russell J Holmes Many common materials used in organic light-emitting devices (OLEDs) show preferred molecular orientation in thin films. In molecules with a permanent dipole moment, this can lead to spontaneous orientation polarization (SOP) of the film and can generate a large interface charge. This polarization charge is compensated in a device by the accumulation of charge carriers, typically holes, in the adjacent layer. While it has been speculated that these accumulated charges can quench excitons and accelerate degradation, no direct demonstration of these effects has been made. Here, we show that electron transport layers exhibiting SOP induce substantial exciton quenching prior to device turn-on, reducing the peak external quantum efficiency by ~20% relative to layers without SOP. In addition, we find that the accumulated charges accelerate degradation of emitter photoluminescence efficiency under optical pumping. We further show that SOP can be eliminated by substrate heating during deposition, thereby improving device efficiency and stability. These findings confirm that SOP is an important material parameter that must be considered when optimizing efficiency and lifetime of OLEDs. |
Thursday, March 5, 2020 1:03PM - 1:39PM |
S34.00008: Influence of Acceptor Type and Polymer Molecular Weight on the Mechanical and Photovoltaic Properties of Polymer Solar Cells Invited Speaker: Bumjoon Kim The mechanical robustness of polymer solar cells (PSCs) is of great importance to ensure the long-term stability and enable their use as power-generators in flexible and stretchable electronics. Here, we present a comparative study of the mechanical properties of small-molecule acceptor (SMA)-based, polymer acceptors (PA)-based, and fullerene-based PSCs. We chose ITIC, P(NDI2OD-T2), and PCBM as three representative acceptor materials and blended them with the same polymer donor. To understand the difference between the mechanical properties of SMA-based and PA-based PSCs, we control the number-average molecular weight (Mn) of P(NDI2OD-T2) from 15 to 163 kg mol-1 in all-PSCs. The high Mn PA-based-PSCs exhibited a high strain at fracture of 31.1%, which is 9- and 28-fold higher than those of SMA-PSCs and PCBM-PSCs, respectively. The superior mechanical robustness of all-PSCs is attributed to using a PA above the critical molecular weight (Mc), which produces tie molecules and polymer entanglements that dissipate substantial mechanical strain energy with large plastic deformation. The connectivity between the crystalline domains generated by PA tie chains leads to high charge mobilities and photovoltaics performances of all-PSCs. Also, this feature explains very high donor:acceptor composition tolerance of all-PSCs in the photovoltaic and mechanical performances. Therefore, our work highlights the importance of incorporating high Mn PAs above the Mc for producing the PSCs with excellent mechanical robustness and device performance. |
Thursday, March 5, 2020 1:39PM - 1:51PM |
S34.00009: Mechanically-durable high performance OPVs using semi-interpenetrating networks Zhiqi Hu, Changxu Sun, Joshua Jackson, Tanguy Terlier, Rafael Verduzco Organic photovoltaic (OPVs) devices offer a number of unique advantages over conventional single crystal silicon solar cells, including simple and low-cost fabrication and reduced weight. However, OPV devices exhibit poor durability to mechanical deformations. Here, we study the use of an elastic semi-interpenetrating network to improve the mechanical durability of the active layer of OPV devices based on the high-performance PBDBT-2F:ITIC donor:acceptor blend. The elastic interpenetrating network is synthesized in situ after solution deposition of the donor:acceptor blend along with reactive thiol-ene small molecules. We systematically investigate the effects of strain on the network-stabilized active layer structure and show that network-stabilized devices outperform pristine devices above a critical bending strain and number of bending deformations. The elastic interpenetrating network suppresses crack formation and improves durability to high-curvature and repeated bending deformations. This work describes a general route to high performance flexible devices and detailed design parameters that influence performance. |
Thursday, March 5, 2020 1:51PM - 2:03PM |
S34.00010: Mechanically-Robust and High-Performance Thin Film Transistors with Regioregular-block-Regiorandom Poly(3-hexylthiophene) Copolymers Hyeonjung Park, Boo Soo Ma, Jin-Seong Kim, Youngkwon Kim, Hyeong Jun Kim, Donguk Kim, Hongseok Yun, Junghun Han, Felix Sunjoo Kim, Taek-Soo Kim, Bumjoon Kim We develop mechanically-robust and high-performance organic thin film transistors (OTFTs) based on poly(3-hexylthiophene) (P3HT) regioblock copolymers (block-P3HTs). These block-P3HTs consist of regioregular (rre) and regiorandom (rra) P3HTs, where the crystalline rre block allows efficient charge transport while the amorphous rra block provides mechanical robustness and inter-domain connection. A series of block-P3HTs, having different molecular weight (Mn) of rra blocks with fixed Mn of rre blocks, are prepared and they all exhibit high hole mobility due to the formation of well-developed edge-on crystallites. In addition, mechanical robustness of block-P3HT thin films is remarkably enhanced with the longer rra block, finally leading to 30.2% of elongation at break, which is 100 times higher than rre P3HT homopolymer. In particular, the noticeable enhancement of both elongation at break and toughness is observed between the Mn of rra block, 8 and 20 kg mol-1, indicating that the critical molecular weight of rra P3HT plays an important role in mechanical response of the block-P3HT thin films. This study provides good strategies for high-performance soft electronics to improve the mechanical properties of electroactive materials without disruption of optoelectrical properties. |
Thursday, March 5, 2020 2:03PM - 2:15PM |
S34.00011: Multiscale-ordered, highly stretchable polymer semiconductor through nanoconfinement Jie Xu Semiconducting polymers have been developed as a key component to enable skin-like wearable electronics, but their electrical performance must be improved while maintaining good mechanical stretchability. To substantially improve their charge carrier mobility under strain, we reported a novel and genetic approach that combines advanced solution processing with nanoconfinement effect to achieve multi-scale ordering and high stretchability in conjugated polymers. Here, the increased polymer chain dynamics under nanoconfinement significantly reduces the modulus of the conjugated polymer and largely delays the onset of crack formation under strain. As a result, our fabricated semiconducting film can be stretched up to 100% strain without affecting mobility. By using solution shearing with a patterned microtrench coating blade, macroscale alignment of conjugated-polymer nanostructures was achieved along the charge transport direction. In conjunction, the nanoscale spatial confinement aligns chain conformation and promotes short-range π–π ordering, substantially reducing the energetic barrier for charge carrier transport. As a result, the mobilities of stretchable conjugated-polymer films have been enhanced up to threefold and maintained under a strain up to 100%. |
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