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Recent Submissions

  • Experimental Identification of the Second-Order Non-Hermitian Skin Effect with Physics-Graph-Informed Machine Learning.

    Shang, Ce; Liu, Shuo; Shao, Ruiwen; Han, Peng; Zang, Xiaoning; Zhang, Xiangliang; Salama, Khaled N.; Gao, Wenlong; Lee, Ching Hua; Thomale, Ronny; Manchon, Aurélien; Zhang, Shuang; Cui, Tie Jun; Schwingenschlögl, Udo (Advanced science (Weinheim, Baden-Wurttemberg, Germany), Wiley, 2022-11-13) [Article]
    Topological phases of matter are conventionally characterized by the bulk-boundary correspondence in Hermitian systems. The topological invariant of the bulk in d dimensions corresponds to the number of (d - 1)-dimensional boundary states. By extension, higher-order topological insulators reveal a bulk-edge-corner correspondence, such that nth order topological phases feature (d - n)-dimensional boundary states. The advent of non-Hermitian topological systems sheds new light on the emergence of the non-Hermitian skin effect (NHSE) with an extensive number of boundary modes under open boundary conditions. Still, the higher-order NHSE remains largely unexplored, particularly in the experiment. An unsupervised approach-physics-graph-informed machine learning (PGIML)-to enhance the data mining ability of machine learning with limited domain knowledge is introduced. Through PGIML, the second-order NHSE in a 2D non-Hermitian topoelectrical circuit is experimentally demonstrated. The admittance spectra of the circuit exhibit an extensive number of corner skin modes and extreme sensitivity of the spectral flow to the boundary conditions. The violation of the conventional bulk-boundary correspondence in the second-order NHSE implies that modification of the topological band theory is inevitable in higher dimensional non-Hermitian systems.
  • Excitons and light-emission in semiconducting MoSi2X4 two-dimensional materials

    Sun, Minglei; Re Fiorentin, Michele; Schwingenschlögl, Udo; Palummo, Maurizia (npj 2D Materials and Applications, Springer Science and Business Media LLC, 2022-11-07) [Article]
    Semiconducting two-dimensional materials with chemical formula MoSi2X4 (X = N, P, or As) are studied by means of atomistic ground- and excited-state first-principles simulations. Full-fledged quasi-particle bandstructures within the G0W0 approach substantially correct the electronic bandgaps previously obtained with hybrid-functional density functional theory and highlight the absence of lateral valleys close in energy to the conduction band minimum. By solving the Bethe–Salpeter equation, we show that the optical properties are dominated by strongly bound excitons with the absorbance and maximum short-circuit current densities of MoSi2P4 and MoSi2As4 comparable to those of transition metal dichalcogenides. Due to the presence of the outer SiX layers, the exciton binding energies are smaller than those generally found for transition metal dichalcogenides. Long radiative lifetimes of bright excitons, over 10 ns at room temperature for MoSi2As4, and the absence of band-nesting are very promising for application in efficient ultra-thin optoelectronic devices.
  • Fused-ring induced end-on orientation in conjugated molecular dyads toward efficient single-component organic solar cells

    Xia, Dongdong; Zhou, Shengxi; Tan, Wen Liang; Karuthedath, Safakath; Xiao, Chengyi; Zhao, Chaowei; Laquai, Frédéric; McNeill, Christopher R.; Li, Weiwei (Aggregate, Wiley, 2022-10-12) [Article]
    The molecular orientations of conjugated materials on the substrate mainly include edge-on, face-on, and end-on. Edge-on and face-on orientations have been widely observed, while end-on orientation has been rarely reported. Since in organic solar cells (OSCs) charge transport is along the vertical direction, end-on orientation with conjugated backbones perpendicular to the substrate is recognized as the ideal microstructure for OSCs. In this work, we for the first time obtained the preferential end-on orientation in a conjugated molecular dyad that contains a conjugated backbone as donor and perylene bisimide side units as acceptor. This was realized by introducing a fused-ring structure to replace linear terthiophenes with conjugated backbones, yielding F-MDPBI and L-MDPBI respectively. Surprisingly, a shifting trend of the molecular orientation from dominating edge-on in L-MDPBI to preferential end-on in F-MDPBI was observed. As a consequence, vertical charge carrier mobilities in F-MDPBI are one order of magnitude higher than those with preferential edge-on orientation, so single-component OSCs based on this molecular dyad as a single photoactive layer provided a power conversion efficiency of 4.89% compared to 1.70% based on L-MDPBI with preferential edge-on orientation.
  • Supermolecule-mediated defect engineering of porous carbons for zinc-ion hybrid capacitors

    Zhang, Wenli; Yin, Jian; Jian, Wenbin; Wu, Ying; Chen, Liheng; Sun, Minglei; Schwingenschlögl, Udo; Qiu, Xueqing; Alshareef, Husam N. (Nano Energy, Elsevier BV, 2022-09-23) [Article]
    Zinc ion hybrid capacitors hold great potential for future energy storage that requires both high energy density and high power capability. However, the charge storage mechanism of porous carbon cathode is ambiguous in Zn2+ ion-containing aqueous solutions, albeit porous carbon usually stores charge by electric double-layer capacitance. Herein, we developed a supermolecule-mediated direct pyrolysis carbonization strategy to convert sustainable sodium lignosulfonate resources into three-dimensional highly heteroatom-doped porous carbons with large mesopores. Through this strategy, we obtained lignin-derived porous carbons with high heteroatom dopings (14.9 at% nitrogen and 4.7 at% oxygen) and relatively high specific surface areas. Furthermore, the nitrogen doping configurations were mainly edge-nitrogen dopants even under high pyrolysis temperatures (> 900 °C). Lignin-derived nitrogen-doped porous carbon showed a high gravimetric specific capacitance of 266 F g−1 with high rate capability, which is endowed by the increased surface pseudocapacitance. First-principles calculations and molecular dynamics simulations indicate that the edge nitrogen and oxygen dopants contribute to the reversible adsorption/desorption of zinc ions and protons. Pores size less than 6.8 Å can cause a significant diffusion energy barrier for the hydrated zinc ions, thus degrading the capacitance and rate capability.
  • In situ grown oxygen-vacancy-rich copper oxide nanosheets on a copper foam electrode afford the selective oxidation of alcohols to value-added chemicals

    Khan, Mustafa; Hameed, Asima; Samad, Abdus; Mushiana, Talifhani; Abdullah, Muhammad Imran; Akhtar, Asma; Ashraf, Raja Shahid; Zhang, Ning; Pollet, Bruno G.; Schwingenschlögl, Udo; Ma, Mingming (Communications Chemistry, Springer Science and Business Media LLC, 2022-09-12) [Article]
    Selective oxidation of low-molecular-weight aliphatic alcohols like methanol and ethanol into carboxylates in acid/base hybrid electrolytic cells offers reduced process operating costs for the generation of fuels and value-added chemicals, which is environmentally and economically more desirable than their full oxidation to CO2. Herein, we report the in-situ fabrication of oxygen-vacancies-rich CuO nanosheets on a copper foam (CF) via a simple ultrasonication-assisted acid-etching method. The CuO/CF monolith electrode enables efficient and selective electrooxidation of ethanol and methanol into value-added acetate and formate with ~100% selectivity. First principles calculations reveal that oxygen vacancies in CuO nanosheets efficiently regulate the surface chemistry and electronic structure, provide abundant active sites, and enhance charge transfer that facilitates the adsorption of reactant molecules on the catalyst surface. The as-prepared CuO/CF monolith electrode shows excellent stability for alcohol oxidation at current densities >200 mA·cm2 for 24 h. Moreover, the abundant oxygen vacancies significantly enhance the intrinsic indicators of the catalyst in terms of specific activity and outstanding turnover frequencies of 5.8k s−1 and 6k s−1 for acetate and formate normalized by their respective faradaic efficiencies at an applied potential of 1.82 V vs. RHE.
  • Lattice Instability and Ultralow Lattice Thermal Conductivity of Layered PbIF

    Yedukondalu, N.; Shafique, Aamir; Rakesh Roshan, S. C.; Barhoumi, Mohamed; Muthaiah, Rajmohan; Ehm, Lars; Parise, John B.; Schwingenschlögl, Udo (ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2022-09-02) [Article]
    Understanding the interplay between various design strategies (for instance, bonding heterogeneity and lone pair induced anharmonicity) to achieve ultralow lattice thermal conductivity (κl) is indispensable for discovering novel functional materials for thermal energy applications. In the present study, we investigate layered PbXF (X = Cl, Br, I), which offers bonding heterogeneity through the layered crystal structure, anharmonicity through the Pb2+ 6s2 lone pair, and phonon softening through the mass difference between F and Pb/X. The weak interlayer van der Waals bonding and the strong intralayer ionic bonding with partial covalent bonding result in a significant bonding heterogeneity and a poor phonon transport in the out-of-plane direction. Large average Grüneisen parameters (≥2.5) demonstrate strong anharmonicity. The computed phonon dispersions show flat bands, which suggest short phonon lifetimes, especially for PbIF. Enhanced Born effective charges are due to cross-band-gap hybridization. PbIF shows lattice instability at a small volume expansion of 0.1%. The κl values obtained by the two channel transport model are 20–50% higher than those obtained by solving the Boltzmann transport equation. Overall, ultralow κl values are found at 300 K, especially for PbIF. We propose that the interplay of bonding heterogeneity, lone pair induced anharmonicity, and constituent elements with high mass difference aids the design of low κl materials for thermal energy applications.
  • High-Efficiency Perovskite–Organic Blend Light-Emitting Diodes Featuring Self-Assembled Monolayers as Hole-Injecting Interlayers

    Gedda, Murali; Gkeka, Despoina; Nugraha, Mohamad; Scaccabarozzi, Alberto D.; Yengel, Emre; Khan, Jafar Iqbal; Hamilton, Iain; Lin, Yuanbao; Deconinck, Marielle; Vaynzof, Yana; Laquai, Frédéric; Bradley, Donal; Anthopoulos, Thomas D. (Advanced Energy Materials, Wiley, 2022-09-01) [Article]
    The high photoluminescence efficiency, color purity, extended gamut, and solution processability make low-dimensional hybrid perovskites attractive for light-emitting diode (PeLED) applications. However, controlling the microstructure of these materials to improve the device performance remains challenging. Here, the development of highly efficient green PeLEDs based on blends of the quasi-2D (q2D) perovskite, PEA2Cs4Pb5Br16, and the wide bandgap organic semiconductor 2,7 dioctyl[1] benzothieno[3,2-b]benzothiophene (C8-BTBT) is reported. The presence of C8-BTBT enables the formation of single-crystal-like q2D PEA2Cs4Pb5Br16 domains that are uniform and highly luminescent. Combining the PEA2Cs4Pb5Br16:C8-BTBT with self-assembled monolayers (SAMs) as hole-injecting layers (HILs), yields green PeLEDs with greatly enhanced performance characteristics, including external quantum efficiency up to 18.6%, current efficiency up to 46.3 cd A−1, the luminance of 45 276 cd m−2, and improved operational stability compared to neat PeLEDs. The enhanced performance originates from multiple synergistic effects, including enhanced hole-injection enabled by the SAM HILs, the single crystal-like quality of the perovskite phase, and the reduced concentration of electronic defects. This work highlights perovskite:organic blends as promising systems for use in LEDs, while the use of SAM HILs creates new opportunities toward simpler and more stable PeLEDs.
  • Experimental and Theoretical Investigation on the Temperature-dependent Optical Properties of Hybrid Halide Perovskites

    Alharbi, Ohoud K. (2022-08-30) [Thesis]
    Advisor: Schwingenschlögl, Udo
    Committee members: Laquai, Frédéric; Lanza, Mario
    Nowadays, studying materials for renewable energy applications are highly de- manded. Hybrid halide perovskites have proven to be promising materials for such technology since their first application in solar cells in 2008, with a power conversion efficiency of 2.7%. Since then, hybrid halide perovskites have proven their superior properties for light-absorbing devices. In this scope, studying the optical properties is ultimately essential. This work investigates the tempera- ture dependence of the optical spectra for formamidinium lead iodide/bromide perovskites (FAPb[IxBr1-x]3 (0 ≤ x ≤ 1) using spectroscopic ellipsometry mea- surements, empirical optical modeling, density functional theory, and molecular dynamics. Five FAPb[IxBr1-x]3 perovskite samples were fabricated by a hybrid processing technique. External Quantum Efficiency measurements reported an energy bandgap range between 1.58 eV and 1.77 eV for the resulted samples. Next, multi-angle spectroscopic ellipsometry measurements were applied with a temperature-controlled stage, allowing the variance of temperature from 25 ◦C to 75 ◦C. The results show a blue shift in the optical spectra at elevated tempera- tures. We then conducted a temperature-dependent empirical model that predicts the optical spectra for the sample of study at higher temperatures using input data of the spectra at room temperature. The model reports low mean squared errors which are less than ≈ 2 around the bandgap, and further development can be applied for better utilization. First-principles investigations were conducted on four FAPb[IxBr1-x]3 per- ovskite unit cells. Structural optimization was applied with assuming fixed angles of the lattice. Atomic configuration was chosen to achieve minimal ground state energies. Ab initio molecular dynamics simulations were applied to each opti- mized structures at target temperatures of 300 K and 350 K using Berendsen thermostat. The simulation time was 4ps with 1fs time step, and the electronic energy bandgap was calculated at each step using PBE functional. The simula- tions reported a rotational motion for the FA molecule that showed to be faster at 350 K, along with higher mean energy bandgap compared to the reported value at 300 K. The optical spectra were extracted using a snapshot from the resulted structures. Similar to the spectroscopic ellipsometry measurements, a temperature induced blue shift was reported. Overall, this work detects and predicts the temperature-dependent optical spectra and confirms the role of the atomic thermal motion. With further devel- opment, higher accuracy can be achieved along with broadening the materials of study for photovoltaic and optoelectronic applications.
  • A family of LixBy monolayers with a wide spectrum of potential applications

    Ren, Kai; Yan, Yuan; Zhang, Zhuoran; Sun, Minglei; Schwingenschlögl, Udo (Applied Surface Science, Elsevier BV, 2022-08-24) [Article]
    Using first-principles calculations and evolutionary structure search, we predict a series of kinetically and thermodynamically stable LixBy monolayers. While the Li2B2 monolayer (metallic) is found to be isotropic, the Li4B8 (metallic), Li2B6 (indirect bandgap semiconductor with a gap size similar to that of bulk Si), and Li2B12 (metallic) monolayers are anisotropic. Notably, the Li2B2 monolayer has a small Poisson's ratio, the Li2B6 monolayer achieves a high hole mobility of 6.8 × 103 cm2·V−1·s−1 (as required for high-speed electronic devices), and the Li2B12 monolayer is auxetic.
  • P3HT:PCBM polymer solar cells from a didactic perspective

    Alam, Shahidul; Anand, Aman; Islam, Md Moidul; Meitzner, Rico; Djoumessi, Aurelien Sokeng; Slowik, Josef; Teklu, Zekarias; Fischer, Peter; Kästner, Christian; Khan, Jafar Iqbal; Schubert, Ulrich S.; Laquai, Frédéric; Hoppe, Harald (Journal of Photonics for Energy, SPIE-Intl Soc Optical Eng, 2022-08-23) [Article]
    Here, we studied the influence of pre- and post-thermal annealing on the performance of polymer:fullerene bulk heterojunction solar cells using the conventional architecture, comprising a conjugated polymer, namely, poly(3-hexylthiophene-2,5-diyl) (P3HT) and a fullerene derivative [6,6]-phenyl-C60-butyric acid methyl ester (PC60BM) as a photoactive layer. The non-annealed active layer device exhibited a power conversion efficiency of <1 % , which was significantly lower than the pre- and post-annealed devices. To investigate the impact of pre- and post thermal annealing on the natural morphological state of the polymer, regiorandom (P3HT-I) and regioregular (P3HT-II) type P3HT were compared in photoactive layers. In general, P3HT-I is amorphous, whereas P3HT-II is semi-crystalline. Changes in solar cell performance were associated with changes in carrier extraction efficiencies influenced by the annealing conditions. The charge photogeneration processes were investigated using spectroscopic techniques, including electroluminescence, steady-state, and time-resolved photoluminescence spectroscopy. Finally, to explore the morphological changes upon annealing, atomic force microscopy and electroluminescence imaging measurements were performed on films and solar cells, respectively.
  • Tuneable Poisson's ratio of monolayer GeS and Ge2SSe

    Jangir, Arun; Ho, Duc Tam; Schwingenschlögl, Udo (Extreme Mechanics Letters, Elsevier BV, 2022-08-19) [Article]
    A tuneable Poisson's ratio is desirable to extend the applications of 2D materials. We demonstrate that Poisson's ratio of monolayer GeS depends approximately linear on the applied strain and electric field strength. In contrast, monolayer Ge2SSe is subject to abrupt switching between positive and negative values (sign reversal) at particular critical values of the strain and electric field strength. The tuneability of Poisson's ratio is attributed to the interplay of different bending stiffnesses. A structural transition is identified as origin of the sign reversal in the case of monolayer Ge2SSe.
  • Vertical Stratification Engineering of Insulating Poly(aryl ether)s Enables 18.6% Organic Solar Cells with Improved Stability

    Han, Jianhua; Xu, Han; Paleti, Sri Harish Kumar; Wen, Yuanfan; Wang, Jianxiao; Wu, Yuanwei; Bao, Feng; Yang, Chunming; Li, Xiangyu; Jian, Xigao; Wang, Jinyan; Karuthedath, Safakath; Gorenflot, Julien; Laquai, Frédéric; Baran, Derya; Bao, Xichang (ACS Energy Letters, American Chemical Society (ACS), 2022-08-11) [Article]
    Control of the preferred vertical phase distribution of the components in the photoactive layer is of vital importance to improve the device performance and stability of organic photovoltaics (OPVs). Herein, a universal sequential-deposition strategy is demonstrated to realize optimal vertical phase distribution by employing orthogonal solvents and synergistically integrating ternary/quaternary compositions consisting of insulating heat-resistant poly(aryl ether)s as the third/fourth component in a photoactive layer. The selective segregation of poly(aryl ether)s in the middle of the active layer enhances molecular packing of the photovoltaic materials and improves the charge transport and extraction properties. A maximum efficiency of 18.6% is achieved for PM6:BTP-eC9:PC71BM:poly(aryl ether)-based quaternary solar cells, with photo/thermal stability that is better than that of devices without poly(aryl ether). This work provides an effective approach for achieving stable and efficient OPVs with expected topology by tuning molecular packing behaviors and the vertical segregation of multicomponents in photovoltaic layers.
  • Large Spin Coherence Length and High Photovoltaic Efficiency of the Room Temperature Ferrimagnet Ca2FeOsO6 by Strain Engineering

    Rout, Paresh Chandra; Schwingenschlögl, Udo (Advanced Science, Wiley, 2022-07-21) [Article]
    The influence of epitaxial strain on the electronic, magnetic, and optical properties of the distorted double perovskite Ca2FeOsO6 is studied. These calculations show that the compound realizes a monoclinic structure with P21/n space group from −6% to +6% strain. While it retains ferrimagnetic ordering with a net magnetic moment of 2 μB per formula unit at low strain, it undergoes transitions into E-antiferromagnetic and C-antiferromagnetic phases at −5% and +5% strain, respectively. It is shown that spin frustration reduces the critical temperature of the ferrimagnetic ordering from the mean field value of 600–350 K, in excellent agreement with the experimental value of 320 K. It is also shown that the critical temperature can be tuned efficiently through strain and that the spin coherence length surpasses that of Sr2FeMoO6 under tensile strain. An indirect-to-direct bandgap transition is observed at +5% strain. Localization of the valence and conduction states on different transition metal sublattices enables efficient electron–hole separation upon photoexcitation. The calculated spectroscopic limited maximum efficiency of up to 33% points to excellent potential of Ca2FeOsO6 in solar cell applications.
  • Charge transfer mediated triplet excited state formation in donor-acceptor-donor BODIPY: Application for recording of holographic structures in photopolymerizable glass

    Mikulchyk, Tatsiana; Karuthedath, Safakath; De Castro, Catherine S. P.; Buglak, Andrey A.; Sheehan, Aimee; Wieder, Aaron; Laquai, Frédéric; Naydenova, Izabela; Filatov, Mikhail A. (JOURNAL OF MATERIALS CHEMISTRY C, Royal Society of Chemistry (RSC), 2022-07-18) [Article]
    Donor–acceptor–donor BODIPY triads bearing anthracene or pyrene as electron donating subunits were prepared through a stepwise synthesis. Photoinduced electron transfer and formation of long-lived triplet excited states via spin–orbit charge transfer intersystem crossing (SOCT-ISC) was studied by steady-state and ultrafast pump-probe spectroscopy and further supported by DFT computations. New BODIPYs were found to form triplet states and sensitize singlet oxygen in both polar and non-polar solvents which is unusual for photosensitizers operating via SOCT-ISC. BODIPY-anthracene triad (ABA) was used as a photosensitizer component in a photopolymerizable glass that was prepared by a four-step sol–gel process. ABA in combination with N-phenylglycin (NPG) showed the ability to initiate a free-radical polymerization of methacrylate monomers under 532 nm irradiation thus allowing for holographic recording of diffractive structures. High diffraction efficiency (up to 87%) obtained for ABA-NPG containing glass as compared to a reference diiodo-BODIPY (I2BDP) demonstrates for the first time that heavy-atom-free SOCT-ISC photosensitizers can efficiently operate in the solid state.
  • Selectivity in single-molecule reactions by tip-induced redox chemistry

    Albrecht, Florian; Fatayer, Shadi P.; Pozo, Iago; Tavernelli, Ivano; Repp, Jascha; Peña, Diego; Gross, Leo (Science, American Association for the Advancement of Science (AAAS), 2022-07-14) [Article]
    Controlling selectivity of reactions is an ongoing quest in chemistry. In this work, we demonstrate reversible and selective bond formation and dissociation promoted by tip-induced reduction-oxidation reactions on a surface. Molecular rearrangements leading to different constitutional isomers are selected by the polarity and magnitude of applied voltage pulses from the tip of a combined scanning tunneling and atomic force microscope. Characterization of voltage dependence of the reactions and determination of reaction rates demonstrate selectivity in constitutional isomerization reactions and provide insight into the underlying mechanisms. With support of density functional theory calculations, we find that the energy landscape of the isomers in different charge states is important to rationalize the selectivity. Tip-induced selective single-molecule reactions increase our understanding of redox chemistry and could lead to novel molecular machines.
  • Triplet-triplet annihilation up-conversion sensitizes metal oxide nanoparticles for photocatalytic reactions

    Gonzalez Lopez, Sandra Patricia; Gorenflot, Julien; Murton, Patrick; Moser, Maximilian; McCulloch, Iain; Laquai, Frédéric (FUNDACIO DE LA COMUNITAT VALENCIANA SCITO, 2022-07-11) [Presentation]
    Many metal oxide semiconductors, such as TiO2, require UV excitation in order to drive photocatalytic reactions. However, UV photons account for only 3-5% of the solar spectrum, and thus strategies to extend the range of harvested photons into the visible wavelength range are required. One potential way is to use triplet-triplet annihilation photon up-conversion (TTA-UC). TTA-UC systems generate higher energy photons from lower energy, low intensity, and non-coherent excitation. In this work, we synthesized several 9,10-diphenylanthracene derivatives and attached them to different wide-bandgap metal oxide nanoparticles, TiO2, ZrO2, and CeO2, which were then dispersed in solutions containing triplet sensitizers. Triplet energy transfer leading to triplet-triplet annihilation photon up-conversion and charge generation were probed through steady-state and transient spectroscopy techniques such as photo-induced absorption (PIA), time-resolved photoluminescence (trPL), and transient absorption (TA) spectroscopy. We characterized these TTA-UC systems as films and suspensions and applied them in different photocatalytic processes as proof of concept.
  • The Energetic Frontiers of Non-Fullerene Organic Solar Cells

    Bertrandie, Jules; Han, Jianhua; De Castro, Catherine S. P.; Yengel, Emre; Gorenflot, Julien; Anthopoulos, Thomas D.; Laquai, Frédéric; Sharma, Anirudh; Baran, Derya (FUNDACIO DE LA COMUNITAT VALENCIANA SCITO, 2022-07-11) [Presentation]
    The design and operation of organic photovoltaic devices (OPVs) rely primarily on the electronic structure of organic semiconductors (OSCs). For instance, the dissociation of the exciton at the donor-acceptor (D-A) interface is determined by the energy level offset between the highest energy occupied molecular orbital (HOMO) and the lowest energy unoccupied orbital (LUMO) between the donor and the acceptor. Consequently, the measurement technique and its accuracy to determine the frontier molecular energies of OSCs are critical for designing efficient devices. Several techniques, such as cyclic voltammetry (CV), ultraviolet photoelectron spectroscopy (UPS), low-energy inverted photoelectron spectroscopy (LE-IPES), or photoelectron spectroscopy in air (PESA), are used to measure the ionization energy (IE) and the electron affinity (EA) of OSCs. CV is more commonly used in OPV field for its accessibility and its simple operation. However, differences between the absolute energy levels measured by CV and photoelectron spectroscopy (PES) have been reported. The range of energy level values for identical material measured with CV shows the uncertainties and the inconsistency of the measurement protocol of this method. Moreover, direct measurement of the EA can be challenging when measured with CV, and indirect methods are used to estimate it by calculation using the IE and the optical gap. Since the optical gap been usually lower than the transport gap (difference between the IE and EA of a same material), that kind of approximation can lead to contradictory conclusion. Several high performing blends have been reported to have negligeable energy offset, but other studies are showing that an IE offset of 0.5eV is necessary for efficient charge transfer. In this work, we highlight the differences in the IE and EA values for different commonly used OSCs between CV and UPS/LE-IPES measurements. The work on OPV devices made with different D-A blends shows correlation between the energetic properties of the pristine donor and non-fullerene acceptor (NFA) and the OPV devices’ parameters. When measured with PES methods, the photovoltaic gap Epv, difference between the IE of the donor and the EA of the acceptor, corelate better with the open-circuit voltage (Voc), compared to CV measurements. This led to a good path to predict the maximum Voc of OPV devices. A further study on blends like PM6:Y6 reported as high-performing blends with low IE offset, based on CV measurement, reveals a bigger energy offset when measured with PES techniques. In contrast, devices with low IE offset measured with PES methods demonstrate a non-efficient charge generation which can be observed with their low short-circuit current (Jsc). This may bring the community to rethink the behaviors and the correct designs of OPVs. This work establishes a solid base for reliably evaluating mater
  • The Energetic Landscape in Non-fullerene Acceptor Organic Solar Cells Determines the Device Performance

    Laquai, Frédéric (FUNDACIO DE LA COMUNITAT VALENCIANA SCITO, 2022-07-11) [Presentation]
    In bulk heterojunction organic solar cells, the energetic landscape at the donor-acceptor interface provides the driving force for charge separation. The mechanism leading to efficient charge separation in fullerene-based blends has been intensively investigated, however with the recent advent of high-efficiency non-fullerene acceptors (NFAs) now surpassing 19% power conversion efficiency, the previous findings have to be revisited for NFA-based systems. In this presentation, I will discuss our latest insights into the photophysical processes governing charge separation, recombination, and energetic (voltage) losses in novel NFA-based systems studied by steady-state and advanced transient spectroscopy techniques. I will address the question, how the interfacial energy offsets control exciton dissociation and charge separation in binary and ternary blends of polymer or small molecular donors with novel NFAs, including photoactive layers using state-of-the-art Y-type acceptors. Generally, it appears that it is primarily the ionization energy (IE) offset that limits the exciton-to-charge transfer (CT) state conversion in many low-bandgap NFA-based systems, while the subsequent separation of the CT state into free charges is barrier-less. Sizeable IE offsets of 0.4-0.5 eV are required to ensure quantitative exciton-to-CT state conversion. The underlying reasons of this limitation, their implications for future donor and acceptor material design strategies, and novel computational (in-silico) approaches to material design will be discussed.
  • Photophysics of Defect-Passivated Quasi-2D (PEA)2PbBr4 Perovskite Using an Organic Small Molecule

    Khan, Jafar Iqbal; Gedda, Murali; Wang, Mingcong; Yengel, Emre; Kreß, Joshua A.; Vaynzof, Yana; Anthopoulos, Thomas D.; Laquai, Frédéric (ACS Energy Letters, American Chemical Society (ACS), 2022-07-07) [Article]
    2D Ruddlesden–Popper perovskites are promising candidates for energy-harvesting applications because of their tunable optical properties and ambient stability. Moreover, they are solution-processable and compatible with scalable manufacturing via various printing techniques. However, such methods often induce large degrees of heterogeneity because of poorly controlled crystallization. We address this issue by blending the well-known 2D perovskite (PEA)2PbBr4 with an organic small molecule, C8-BTBT. Terahertz (THz) absorption and temperature-dependent photoluminescence (PL) spectroscopy studies revealed changes in the photophysical properties of the perovskite without affecting its structural integrity upon adding C8-BTBT. The inclusion of trace amounts of C8-BTBT results in defect passivation both at perovskite platelet boundaries and at surfaces, as indicated by increased carrier lifetimes and substantially increased photoluminescence quantum yields (PLQY). This improves the responsivity of photodetectors using the 2D perovskite as an active layer. Our study highlights a straightforward strategy for fabricating high-quality 2D perovskites via large-area processing techniques.
  • An effective method of reconnoitering current–voltage (IV) characteristics of organic solar cells

    Meitzner, Rico; Madalaimuthu, Jose Prince; Alam, Shahidul; Islam, Md Moidul; Peiler, Sebastian; Anand, Aman; Ahner, Johannes; Hager, Martin D.; Schubert, Ulrich S.; Zou, Yingping; Laquai, Frédéric; Hoppe, Harald (Journal of Applied Physics, AIP Publishing, 2022-07-05) [Article]
    Current–voltage ( IV) characterization is the most fundamental measurement performed on solar cells. This measurement is commonly used to extract basic solar cell parameters, such as open circuit voltage, short circuit current density, fill factor, and power conversion efficiency. We were able to obtain a fast tool to find defective behavior using Simulation Program with Integrated Circuit Emphasis simulations and generate an understanding of which device property can create such defective behaviors by analyzing the second derivative of IV curves.

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