Bulletin of the American Physical Society
APS March Meeting 2024
Monday–Friday, March 4–8, 2024; Minneapolis & Virtual
Session D33: Organic Optoelectronics and Photonics IIFocus
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Sponsoring Units: DPOLY Chair: Youngmin Lee, New Mexico Tech Room: 102E |
Monday, March 4, 2024 3:00PM - 3:36PM |
D33.00001: The Role of Redox Doping in Organic Electronics and Opto-electronics Invited Speaker: Seth Marder Organic semiconductors and hybrid/organic materials have attracted interest for electronic applications due to their potential for use in low-cost, large-area, flexible electronic devices. Here we will report on recent developments pertaining to n-dopants that could impact the charge injection/collection processes in organic light emitting diodes, organic field effect transistors, and organic photovoltaic and hybrid organic/inorganic perovskite devices. I will highlight the application of n-doping for the development of electron injection layers for organic light emitting diodes (OLEDs), and their use for doping of electron transport materials which result in high conductivities and in some cases good thermoelectric performance. In the case of OLEDs, it will be shown that photoactivation (as illustrated in the cartoon at the right) can lead to stable doping of materials (i.e., the doping induced conductivity remains relatively constant over hundreds of hours) beyond the expected thermodynamic limit, which would be predicted based on an assessment of the effective reduction potential of the n-dopant and the reduction potential of the electron transport material. We will also highlight some of the differences between approaches based upon "dimeric" dopants vs. hydride donor dopants. |
Monday, March 4, 2024 3:36PM - 3:48PM |
D33.00002: Connecting optical absorption to doping in conjugated polymers Muhamed Duhandzic, Michael Lu Diaz, Subhayan Samanta, Dhandapani Venkataraman, Zlatan Aksamija |
Monday, March 4, 2024 3:48PM - 4:00PM |
D33.00003: Improving Blue Organic Light Emitting Diode Efficiency and Reliability using Purcell Effect-enhanced Tandem Emitters Claire Arneson, Haonan Zhao, Stephen R Forrest In recent years, organic light emitting diodes (OLEDs) have become ubiquitous, particularly in mobile display applications, such as smart watches and phones. While OLEDs have many desirable properties, including high color-rendering, high luminous efficacy, and flexibility, the presence of high energy triples in blue phosphorescent emitters can be detrimental to the operational lifetime of the device. Recently, experimental evidence has emerged to support the theory that using the Purcell effect to decrease the radiative lifetime of high energy triplets can improve the device operational lifetime by more than a factor 4. In this work, we use optical design to integrate a tandem blue OLED structure into a Purcell effect-enhanced cavity structure. With the increased luminance provided by the tandem structure and the lifetime enhancement provided by the Purcell effect, we demonstrate approximately a factor of three enhancement, compared to a Purcell effect-enhanced device with a single emissive layer, without sacrificing deep blue color quality. |
Monday, March 4, 2024 4:00PM - 4:12PM |
D33.00004: Investigating the structure-packing-mobility relationship of pure hydrocarbon host materials in OLEDs Kun-Han Lin, Yao-Yu Lee Due to the increasing demand for low-cost, large-scale, and flexible display products, organic light-emitting diodes (OLEDs) have attracted widespread attention. Understanding the structure-packing-mobility relationships (SPMR) of pure hydrocarbon (PHC) host materials is essential for developing high performance host materials and consequently improving the stability of blue OLED devices. |
Monday, March 4, 2024 4:12PM - 4:24PM |
D33.00005: Tight-binding approach describes polaron transport in organic semiconductors Vishal Jindal, Michael J Janik, Scott T Milner Excitons and polarons are key elements of the electronic and optical properties of π-conjugated semiconductors, which are the building blocks of organic electronic devices. For organic photovoltaics, conjugated molecules are designed as acceptors, while polymeric π-conjugated semiconductors such as poly(3-hexylthiophene) [P3HT] are used as donors. Charge transport in these materials is often described by thermally activated hopping, with a rate given by Marcus theory, which depends on interchain electronic coupling and activation barriers. Recently, we have shown that tight-binding models can predict the frontier orbitals and describe the energetics and structure of excitons on π-conjugated semiconductors. Here, we use a tight-binding polaron model to calculate activation barriers for interchain polaron hopping in amorphous P3HT. The barrier is a dielectric-stabilized transition state delocalized over chain A (initial state) and chain B (final state) with charge fraction f on chain A. We examine different initial and final chain configurations in amorphous P3HT to explore the heterogeneity of barrier states and activation energies. Our computationally efficient approach can be used to average over disorder and predict polaron hopping rates and charge carrier mobilities for various conjugated semiconducting materials. |
Monday, March 4, 2024 4:24PM - 4:36PM |
D33.00006: Engineering Strong Exciton-Photon Coupling via Molecular Orientation in Organic Microcavities Yicheng Liu, Russell J Holmes Exciton-polaritons are formed by strong coupling between the exciton in a semiconductor and a confined cavity photon mode. Organic semiconductors are attractive for exciton-polariton formation due to their large exciton binding energy and oscillator strengths. Indeed, prior work has led to demonstrations of polariton condensation, lasing, and applications in light-emitting and detecting devices. To enhance potential applications, a better understanding of how to engineer coupling strength at a material level is needed. This study focuses on tuning the exciton-photon interaction through intentional control over molecular transition dipole moment (TDM) orientation in the glassy, organic thin film absorber. Specifically, ultrastrong coupling is demonstrated in a metal reflector microcavity containing a thin film of 4, 4'-bis[(N-carbazole)styryl]biphenyl(BSB-Cz) with a Rabi splitting greater than 1.0 eV. Building upon prior literature showing the tunability of molecular orientation with substrate temperature during film deposition, BSB-Cz optical microcavities have been used to show that tuning molecular orientation via processing conditions can lead to a ~20% variation in the Rabi splitting. These results will be discussed in the context of absorber design rules for tuning polariton behavior, and in informing potential applications. |
Monday, March 4, 2024 4:36PM - 4:48PM |
D33.00007: A nanoscale view of PM6 and Y6 bulk and interfacial structures Christine L Mahajan, Enrique D Gomez, Scott T Milner Control of device performance within organic photovoltaics requires a nano to molecular scale description of the device morphology. Here we describe the bulk and interface morphology of PM6:Y6 systems, enabled through virtual site coarse grained molecular dynamics. In neat film simulations, PM6 remained amorphous, while Y6 assembled into highly ordered structures. In intimately mixed blends, Y6 and PM6 cluster into segregated domains. When blend simulations started from a completely segregated state, concentration profiles show Y6 first penetrated the PM6 domain, followed by PM6 chain swelling, resulting in a mixed phase interface a few nanometers wide. After several microseconds of simulation the two blend configurations were similar in their structure. Our coarse-grained method still preserves molecular details, and we capture these details by calculating the fraction of pi-stacked conjugated rings between PM6-PM6, Y6-PM6 and Y6-Y6. Our morphology description provides details on the molecular conformations integral to understanding exciton and polaron transport and exciton separation at the interface. |
Monday, March 4, 2024 4:48PM - 5:00PM |
D33.00008: Exciton dissociation in Y6-based nonfullerene organic solar cells: a nonadiabatic molecular dynamics study BIN LIU, Ding Pan Y6-based nonfullerene organic solar cells (OSCs) have achieved an outstanding power conversion efficiency (PCE) of over 19% due to the low energy loss and high exciton dissociation efficiency with a small energy offset. However, the exciton dissociation mechanism is still under debate. It is unclear why a small energy offset can lead to efficient exciton dissociation in nonfullerene systems, but causes significant charge recombination in fullerene ones. Here, we applied nonadiabatic molecular dynamics simulations to study the charge transfer dynamics in both donor:Y6 and donor:C60 crystalline systems. Based on our simulations, we proposed a five-step charge transfer process in nonfullerene systems, which is consistent with experimental findings. We found that the efficient exciton dissociation with a small energy offset can be attributed to the charge redistribution occurring on both the polymer and the Y6 backbones, driven by the interactions between the donor polymer and the Y6 molecule. This mechanism significantly reduces the Coulomb attraction in the local excitions; however, it is not observed in fullerene OSC systems. Our findings provide a fundamental basis for the further development of novel OSC materials with the potential for achieving even higher PCE. |
Monday, March 4, 2024 5:00PM - 5:12PM |
D33.00009: Theory of Purcell effect enhancement on the operational lifetime of phosphorescent organic light-emitting devices Haonan Zhao, Boning Qu, Stephen R Forrest The short operational lifetime of blue phosphorescent organic light-emitting devices (PHOLEDs) remains a major challenge in organic electronics. The device degradation processes include triplet-polaron annihilation (TPA) and triplet-triplet annihilation (TTA) that elevate the excited state energy to significantly greater than the intramolecular bond strength thus resulting in molecular fragmentation and exciton quenching. Previous studies show that the Purcell effect enhanced by polaritons increases Ir-based blue PHOLED operational lifetime up to 5.6 X that of conventional devices, matching a power law of m = 1.7-2.4 between Purcell factor (PF) and device lifetime. Here, we propose a model to explain the mechanism of Purcell effect enhancements by linking the transient current-voltage and triplet exciton dynamics to the device degradation process. By combining the exciton profiles and the bimolecular rate coefficients, the model elucidates the power law dependence between PF and device lifetime. From both photonic and electronic perspectives, this model provides a convenient tool for designing stable PHOLEDs for practical applications. |
Monday, March 4, 2024 5:12PM - 5:24PM |
D33.00010: Impact of Doping on Absorbance Tails and Power Conversion Efficiency of Photovoltaics Andrew Tolton, Zlatan Aksamija Efficient and affordable photovoltaics are critical to widespread adoption of renewable energy. Photoelectric conversion efficiency (PCE) is limited by a few key losses—radiative, resistive, and recombinative. Doping is a common strategy for improving electronic properties in other devices. However, doping introduces positional and energetic disorder, particularly in organic materials, which has been shown to create Urbach tails in the absorbance spectrum. These absorbance tails increase radiative losses, significantly reducing PCE. Because of this, doping is often shown to be harmful to PCE, and the optimal doping concentration in photovoltaics remains an open question. We numerically compute the PCE from absorbance and show that doping is not always harmful to PCE and, in fact, can even improve it. While dopant-induced disorder always broadens the tails in the Density of States (DOS), its effect on absorbance tails depends on the position of the Fermi level. At high doping concentrations, the Fermi level can be positioned inside of one DOS tail, filling the trap states in the tail and "cutting off" its contribution to the absorbance. This mitigates the absorbance tail, creating a step-like absorbance spectrum that results in a high PCE. This is particularly exciting for intrinsically disordered materials such as organic photovoltaics, whose intrinsic absorbance tails are an obstacle to improving PCE. Additionally, heavy doping increases electrical conductivity, decreasing open-circuit voltage losses. In our work, we have identified conditions at which doping can increase the PCE. |
Monday, March 4, 2024 5:24PM - 5:36PM |
D33.00011: Singlet fission dynamics in organic compounds containing hetero-atom linkers Moshe R Chesler, Sumitendra Mazumdar Singlet Fission (SF) is a spin-allowed multichromophore process in which a singlet exciton reached optically undergoes internal conversion to a bound triplet-triplet spin biexciton, which may further dissociate into two free triplets, each of which may contribute to the photoconductivity of an organic solar cell. SF has been explored in a wide variety of organic compounds wherein seemingly slight differences between otherwise similar compounds can lead to vast differences in SF dynamics. This has practical applications for SF-based materials that may potentially be used in the design and production of next generation solar cells. In recent years, research has shifted from intermolecular (xSF) to intramolecular (iSF) compounds wherein the former systems are characterized by through-space charge transfer contrasted with the through-bond dynamics that describe the latter. Many iSF compounds under active investigation are dimers of tetracene or pentacene in which the chromophores are connected by one of various types of linkers. Having investigated the sensitive dependence on connectivity in systems in which the linking monomer is itself an acene, we now present many-body calculations on the low-lying electronic states of iSF systems containing linkers characterized by multiple atomic species. With differning numbers of pi electrons from atoms in the unit as well as alternative conjugation patterns, these systems present a variety of features that have strong affects on SF dynamics and present novel opportunities for exploration of the relevant many-body wavefunctions and potential design of new SF-based solar cells. |
Monday, March 4, 2024 5:36PM - 5:48PM |
D33.00012: Engineering of polymerized small molecule acceptors for all-polymer solar cells: insights from DFT calculations Diego Sorbelli, Giulia Galli All-polymer solar cells (all-PSCs) with polymerized small molecule acceptors (PSMAs) offer high mechanical and morphological stability, but they usually have low efficiency due to the availability of a small number of high-performance acceptor polymers, challenges in morphology control and fast charge recombination. Usually, most PSMAs feature small molecule acceptors polymerized using a thiophene linker. However, modifications of the π-linker at the molecular level can help improving their electronic and morphological properties. |
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