• Development of VO2 -Nanoparticle-Based Metal-Insulator Transition Electronic Ink

      Vaseem, Mohammad; Zhen, Su; Yang, Shuai; Li, Weiwei; Shamim, Atif (Advanced Electronic Materials, Wiley, 2019-03-13) [Article]
      The metal–insulator transition (MIT) phase change of vanadium dioxide (VO2) materials has facilitated many exciting applications. Among the various crystal phases of VO2, the monoclinic (M) phase is the only one that demonstrates low-temperature (≈68 °C) MIT behavior. However, the synthesis of pure VO2 (M) is challenging because various polymorphs, such as VO2 (A), VO2 (B), and VO2 (D), are also typically formed during the process. Furthermore, to achieve pure crystalline VO2 (M) phase, very long reaction times, up to 2–4 days, are typically required. In this work, an additional annealing step is introduced post nanoparticle preparation, which not only reduces the complete synthesis time from days to only 6 h but also removes the impure phases and helps in achieving the desired pure VO2 (M) phase. This work covers the complete synthesis and characterization details of such as-prepared nanoparticles. A VO2 (M)-nanoparticle-based ink is formulated for the inkjet printing of films with controlled thicknesses. The inkjet-printed films are investigated for their electrical conductivity with external stimuli such as temperature and electrical current. Finally, a fully printed antenna is devised that can change its frequency based on the different states of the VO2 film.
    • High-friction limits of Euler flows for multicomponent systems

      Huo, Xiaokai; Jungel, Ansgar; Tzavaras, Athanasios (IOP Publishing and the London Mathematical Society, 2019-03-10) [Article]
      The high-friction limit in Euler-Korteweg equations for fluid mixtures is analyzed. The convergence of the solutions towards the zeroth-order limiting system and the first-order correction is shown, assuming suitable uniform bounds. Three results are proved: The first-order correction system is shown to be of Maxwell-Stefan type and its diffusive part is parabolic in the sense of Petrovskii. The high-friction limit towards the first-order Chapman-Enskog approximate system is proved in the weak-strong solution context for general Euler-Korteweg systems. Finally, the limit towards the zeroth-order system is shown for smooth solutions in the isentropic case and for weak-strong solutions in the Euler-Korteweg case. These results include the case of constant capillarities and multicomponent quantum hydrodynamic models.
    • Fe(III) reduction and vivianite formation in activated sludge

      Wang, Ru; Wilfert, Philipp; Dugulan, Iulian; Goubitz, Kees; Korving, Leon; Witkamp, Geert-Jan; van Loosdrecht, Mark C.M. (Separation and Purification Technology, Elsevier BV, 2019-03-09) [Article]
      Kinetics of iron reduction, formation of vivianite and the microbial community in activated sludge from two sewage treatment plants (STPs) with low (STP Leeuwarden, applying enhanced biological phosphate removal, EBPR) and high (STP Cologne, applying chemical phosphate removal, CPR) iron dosing were studied in anaerobic batch experiments. The iron reduction rate in CPR sludge (2.99 mg-Fe g VS-1 h-1) was 3-times higher compared to EBPR sludge (1.02 mg-Fe g VS-1 h-1) which is probably caused by its 3-times higher iron content. Accordingly, first order rate constants in both sludges are comparable (0.06 ± 0.001 h-1 in EBPR vs 0.05 ± 0.007 h-1 in CPR sludge), thus potential rates in both sludges are comparable. The measured Fe(III) reduction rates suggest that all iron in STP Leeuwarden and STP Cologne can be turned over within 15 h and 44 h respectively. Mössbauer spectroscopy and X-ray diffraction (XRD) indicated vivianite formation within 24 h in both sludges. After 24 h, 53% and 34% of all iron were bound in vivianite in the EBPR and CPR sludge respectively. Next generation sequencing (NGS) showed that the microbial community in the CPR sludge comprised more genera with iron-oxidizing and iron-reducing bacteria. Iron reduction and vivianite formation commence once activated sludge is exposed to oxygen free conditions. Our study reveals that the biogeochemistry of iron in STPs is very dynamic. By understanding the interactions between iron and phosphate crucial processes in modern sewage treatment, such as chemical phosphate removal or phosphate recovery from sewage sludge, can be optimized.
    • Osmotically and thermally isolated forward osmosis – membrane distillation (FO-MD) integrated module

      Kim, Youngjin; Li, Sheng; Francis, Lijo; Li, Zhenyu; Valladares Linares, Rodrigo; Alsaadi, Ahmad Salem; Abu-Ghdaib, Muhannad; Son, Hyuk Soo; Amy, Gary L.; Ghaffour, NorEddine (Environmental Science & Technology, American Chemical Society (ACS), 2019-03-08) [Article]
      In this study, we propose a novel module design to integrate forward osmosis (FO) and membrane distillation (MD). The two processes are sealed in one module and operated simultaneously, making the system compact and suitable for a wide range of applications. To evaluate the system under large-scale module operating conditions, FO and MD experiments were performed separately. The effect of draw solution (DS) temperature on the FO performance was first assessed in terms of flux, reverse salt flux (RSF), and specific RSF (SRSF). While a higher DS temperature resulted in an increased RSF, a higher FO flux was achieved, with a lower SRSF. The influence of DS concentration on the MD performance was then investigated in terms of flux and salt rejection. High DS concentration had a slightly negative impact on MD water vapor flux, but the MD membrane was a complete barrier for DS salts. The FO-MD integrated module was simulated based on mass balance equations. Results indicated that initial DS (MD feed) flow rate and concentration are the most important factors for stable operation of the integrated module. Higher initial DS flow rate and lower initial DS concentration can achieve a higher permeate rate of the FO-MD module.
    • Role of carbonate burial in Blue Carbon budgets

      Saderne, Vincent; Geraldi, N R; Macreadie, P I; Maher, D T; Middelburg, J J; Serrano, O; Almahasheer, H; Arias-Ortiz, A; Cusack, Michael; Eyre, B D; Fourqurean, J W; Kennedy, H; Krause-Jensen, D; Kuwae, T; Lavery, P S; Lovelock, C E; Marba, N; Masqué, P; Mateo, M A; Mazarrasa, I; McGlathery, K J; Oreska, M P J; Sanders, C J; Santos, I R; Smoak, J M; Tanaya, T; Watanabe, K; Duarte, Carlos M. (Nature Communications, Springer Nature, 2019-03-07) [Article]
      Calcium carbonates (CaCO3) often accumulate in mangrove and seagrass sediments. As CaCO3 production emits CO2, there is concern that this may partially offset the role of Blue Carbon ecosystems as CO2 sinks through the burial of organic carbon (Corg). A global collection of data on inorganic carbon burial rates (Cinorg, 12% of CaCO3 mass) revealed global rates of 0.8 TgCinorg yr-1 and 15-62 TgCinorg yr-1 in mangrove and seagrass ecosystems, respectively. In seagrass, CaCO3 burial may correspond to an offset of 30% of the net CO2 sequestration. However, a mass balance assessment highlights that the Cinorg burial is mainly supported by inputs from adjacent ecosystems rather than by local calcification, and that Blue Carbon ecosystems are sites of net CaCO3 dissolution. Hence, CaCO3 burial in Blue Carbon ecosystems contribute to seabed elevation and therefore buffers sea-level rise, without undermining their role as CO2 sinks.
    • Electroactive microorganisms in bioelectrochemical systems

      Logan, Bruce E; Rossi, Ruggero; Ragab, Ala'a; Saikaly, Pascal E (Nature Reviews Microbiology, Springer Nature, 2019-03-07) [Article]
      A vast array of microorganisms from all three domains of life can produce electrical current and transfer electrons to the anodes of different types of bioelectrochemical systems. These exoelectrogens are typically iron-reducing bacteria, such as Geobacter sulfurreducens, that produce high power densities at moderate temperatures. With the right media and growth conditions, many other microorganisms ranging from common yeasts to extremophiles such as hyperthermophilic archaea can also generate high current densities. Electrotrophic microorganisms that grow by using electrons derived from the cathode are less diverse and have no common or prototypical traits, and current densities are usually well below those reported for model exoelectrogens. However, electrotrophic microorganisms can use diverse terminal electron acceptors for cell respiration, including carbon dioxide, enabling a variety of novel cathode-driven reactions. The impressive diversity of electroactive microorganisms and the conditions in which they function provide new opportunities for electrochemical devices, such as microbial fuel cells that generate electricity or microbial electrolysis cells that produce hydrogen or methane.
    • Photoelectrochemical hydrogen generation using graded In-content InGaN photoelectrode structures

      Ohkawa, Kazuhiro; Uetake, Yusuke; Velazquez-Rizo, Martin; Iida, Daisuke (Nano Energy, Elsevier BV, 2019-03-07) [Article]
      We have improved the InGaN/GaN heterointerface to achieve higher energy conversion efficiency by replacing a uniform InGaN layer with a graded In-content InGaN layer. Even In0.08Ga0.92N/GaN heterostructure has a large conduction band offset, which is large enough to suppress the photocurrent in the photocatalytic system. The graded In-content InGaN structures were grown by metalorganic vapor-phase epitaxy by changing the TMIn flow rate gradually. X-ray reciprocal space mapping confirmed the graded structures. The graded InGaN/GaN structure significantly increased photocurrent and H2 generation by 50% and more compared with the conventional uniform InGaN/GaN structures.
    • A Salty Coral Secret: How High Salinity Helps Corals To Be Stronger

      Gegner, Hagen M.; Voolstra, Christian R. (Frontiers for Young Minds, Frontiers Media SA, 2019-03-07) [Article]
      Corals are mysterious animals that have been around for ages. They are the creators of beautiful reefs. Sadly, the reefs that we love to look at are in danger of overheating and are disappearing from our planet. While this sounds bad, not all corals are affected the same way by warm seawater. Corals from the Red Sea seem to be more resistant to higher temperatures than are corals from other regions. Red Sea reefs are thriving in seawater that is hotter than that in other places. But what is their secret? What makes Red Sea corals stronger and more heat resistant? We know that Red Sea corals not only handle the incredibly high temperatures, but also deal with high salinity (saltiness). This connection between high salinity and high temperature made us wonder: can we find evidence that high salinity makes corals stronger?
    • Numerical Approximation of a Phase-Field Surfactant Model with Fluid Flow

      Zhu, Guangpu; Kou, Jisheng; Sun, Shuyu; Yao, Jun; Li, Aifen (Journal of Scientific Computing, Springer Nature, 2019-03-07) [Article]
      Modeling interfacial dynamics with soluble surfactants in a multiphase system is a challenging task. Here, we consider the numerical approximation of a phase-field surfactant model with fluid flow. The nonlinearly coupled model consists of two Cahn–Hilliard-type equations and incompressible Navier–Stokes equation. With the introduction of two auxiliary variables, the governing system is transformed into an equivalent form, which allows the nonlinear potentials to be treated efficiently and semi-explicitly. By certain subtle explicit-implicit treatments to stress and convective terms, we construct first and second-order time marching schemes, which are extremely efficient and easy-to-implement, for the transformed governing system. At each time step, the schemes involve solving only a sequence of linear elliptic equations, and computations of phase-field variables, velocity and pressure are fully decoupled. We further establish a rigorous proof of unconditional energy stability for the first-order scheme. Numerical results in both two and three dimensions are obtained, which demonstrate that the proposed schemes are accurate, efficient and unconditionally energy stable. Using our schemes, we investigate the effect of surfactants on droplet deformation and collision under a shear flow, where the increase of surfactant concentration can enhance droplet deformation and inhibit droplet coalescence.
    • The Strategic Location of Glycogen and Lactate: From Body Energy Reserve to Brain Plasticity

      Cali, Corrado; Tauffenberger, Arnaud; Magistretti, Pierre J. (Frontiers in Cellular Neuroscience, Frontiers Media SA, 2019-03-06) [Article]
      Brain energy metabolism has been the object of intense research in recent years. Pioneering work has identified the different cell types involved in energy production and use. Recent evidence has demonstrated a key role of L-Lactate in brain energy metabolism, producing a paradigm-shift in our understanding of the neuronal energy metabolism. At the center of this shift, is the identification of a central role of astrocytes in neuroenergetics. Thanks to their morphological characteristics, they are poised to take up glucose from the circulation and deliver energy substrates to neurons. Astrocyte neuron lactate shuttle (ANLS) model, has shown that the main energy substrate that astrocytes deliver to neurons is L-Lactate, to sustain neuronal oxidative metabolism. L-Lactate can also be produced from glycogen, the storage form of glucose, which is exclusively localized in astrocytes. Inhibition of glycogen metabolism and the ensuing inhibition of L-Lactate production leads to cognitive dysfunction. Experimental evidence indicates that the role of lactate in cognitive function relates not only to its role as a metabolic substrate for neurons but also as a signaling molecule for synaptic plasticity. Interestingly, a similar metabolic uncoupling appears to exist in peripheral tissues plasma, whereby glucose provides L-Lactate as the substrate for cellular oxidative metabolism. In this perspective article, we review the known information on the distribution of glycogen and lactate within brain cells, and how this distribution relates to the energy regime of glial vs. neuronal cells.
    • Impact of grazing, resource availability and light on prokaryotic growth and diversity in the oligotrophic surface global ocean

      Teira, Eva; Logares, Ramiro; Gutiérrez-Barral, Alberto; Ferrera, Isabel; Varela, Marta M; G Morán, Xosé Anxelu; Gasol, Josep M (Environmental Microbiology, Wiley, 2019-03-06) [Article]
      The impact of grazing, resource competition and light on prokaryotic growth and taxonomic composition in subtropical and tropical surface waters was studied through 10 microcosm experiments conducted between 30°N and 30°S in the Atlantic, Pacific and Indian oceans. Under natural sunlight conditions, significant changes in taxonomic composition were only observed after the reduction of grazing by sample filtration in combination with a decrease in resource competition by sample dilution. Sunlight exposure significantly reduced prokaryote growth (11±6%) and community richness (14±4%) compared to continuous darkness but did not significantly change community composition. The largest growth inhibition after sunlight exposure occurred at locations showing deep mixed layers. The reduction of grazing had an expected and significant positive effect on growth, but caused a significant decrease in community richness (16±6%), suggesting that the coexistence of many different OTUs is partly promoted by the presence of predators. Dilution of the grazer-free prokaryotic community significantly enhanced growth at the level of community, but consistently and sharply reduced the abundance of Prochlorococcus and SAR11 populations. The decline of these oligotrophic bacterial taxa following an increase in resource availability is consistent with their high specialization for exploiting the limited resources available in the oligotrophic warm ocean. This article is protected by copyright. All rights reserved.
    • Fiddler crab bioturbation determines consistent changes in bacterial communities across contrasting environmental conditions

      Booth, Jenny Marie; Fusi, Marco; Marasco, Ramona; Mbobo, Tumeka; Daffonchio, Daniele (Scientific Reports, Springer Nature, 2019-03-06) [Article]
      Ecosystem functions are regulated by compositional and functional traits of bacterial communities, shaped by stochastic and deterministic processes. Biogeographical studies have revealed microbial community taxonomy in a given ecosystem to change alongside varying environmental characteristics. Considering that stable functional traits are essential for community stability, we hypothesize that contrasting environmental conditions affect microbial taxonomy rather than function in a model system, testing this in three geographically distinct mangrove forests subjected to intense animal bioturbation (a shared deterministic force). Using a metabarcoding approach combined with sediment microprofiling and biochemistry, we examined vertical and radial sediment profiles of burrows belonging to the pantropical fiddler crab (subfamily Gelasiminae) in three contrasting mangrove environments across a broad latitudinal range (total samples = 432). Each mangrove was environmentally distinct, reflected in taxonomically different bacterial communities, but communities consistently displayed the same spatial stratification (a halo effect) around the burrow which invariably determined the retention of similar inferred functional community traits independent of the local environment.
    • Dimensions of Blue Carbon and emerging perspectives

      Lovelock, Catherine E; Duarte, Carlos M. (Biology Letters, The Royal Society, 2019-03-06) [Article]
      Blue Carbon is a term coined in 2009 to draw attention to the degradation of marine and coastal ecosystems and the need to conserve and restore them to mitigate climate change and for the other ecosystem services they provide. Blue Carbon has multiple meanings, which we aim to clarify here, which reflect the original descriptions of the concept including (1) all organic matter captured by marine organisms, and (2) how marine ecosystems could be managed to reduce greenhouse gas emissions and thereby contribute to climate change mitigation and conservation. The multifaceted nature of the Blue Carbon concept has led to unprecedented collaboration across disciplines, where scientists, conservationists and policy makers have interacted intensely to advance shared goals. Some coastal ecosystems (mangroves, tidal marshes and seagrass) are established Blue Carbon ecosystems as they often have high carbon stocks, support long-term carbon storage, offer the potential to manage greenhouse gas emissions and support other adaptation policies. Some marine ecosystems do not meet key criteria for inclusion within the Blue Carbon framework (e.g. fish, bivalves and coral reefs). Others have gaps in scientific understanding of carbon stocks or greenhouse gas fluxes, or currently there is limited potential for management or accounting for carbon sequestration (macroalgae and phytoplankton), but may be considered Blue Carbon ecosystems in the future, once these gaps are addressed.
    • OFDM-IM Based Dual-Hop System Using Fixed-Gain Amplify-and-Forward Relay with Pre-Processing Capability

      Dang, Shuping; Li, Jun; Wen, Miaowen; Mumtaz, Shahid; Zhang, Zhenrong (IEEE Transactions on Wireless Communications, Institute of Electrical and Electronics Engineers (IEEE), 2019-03-06) [Article]
      Orthogonal frequency-division multiplexing with index modulation (OFDM-IM) has recently attracted much researchers’ attention due to its superior spectrum efficiency and reliability compared to the traditional OFDM. Cooperative decode-and-forward (DF) relaying has been incorporated with OFDM-IM, which provides a higher energy efficiency and better network coverage. However, it might not be feasible in realistic applications owing to the high system complexity and transmission delay rendered by complex decoding and channel estimation procedures. Therefore, in this paper, we propose a fixed-gain (FG) amplify-and-forward (AF) relay assisted OFDMIM system, which does not need to perform complex decoding and channel estimation at the relay, but only requires a preprocessing capability at the relay, e.g., cyclic prefix (CP) removal and re-insertion. Therefore, the system complexity can be reduced and the forwarding delay as well as power consumption caused by processing at the relay also decline. We analyze the average outage probability, block error rate (BLER), and achievable rate of the proposed system and verify all analysis by numerical results. The proposed FG AF relay assisted OFDM-IM provides a simple solution to the implementation of OFDM-IM in new network paradigms, where nodes are simple, power-limited and/or complexity-limited.
    • Temperature Dependence of the Urbach Energy in Lead Iodide Perovskites

      Ledinsky, Martin; Schönfeldová, Tereza; Holovský, Jakub; Aydin, Erkan; Hájková, Zdeňka; Landová, Lucie; Neyková, Neda; Fejfar, Antonín; De Wolf, Stefaan (The Journal of Physical Chemistry Letters, American Chemical Society (ACS), 2019-03-06) [Article]
      To gain insight into the properties of photovoltaic and light-emitting materials, detailed information about their optical absorption spectra is essential. Here, we elucidate the temperature dependence of such spectra for methylammonium lead iodide (CH3NH3PbI3), with specific attention to its sub-band gap absorption edge (often termed Urbach energy). On the basis of these data, we first find clear further evidence for the universality of the correlation between the Urbach energy and open-circuit voltage losses of solar cells. Second, we find that for CH3NH3PbI3 the static, temperature-independent, contribution of the Urbach energy is 3.8 ± 0.7 meV, which is smaller than that of crystalline silicon (Si), gallium arsenide (GaAs), indium phosphide (InP), or gallium nitride (GaN), underlining the remarkable optoelectronic properties of perovskites.
    • Metathesis of Functionalized Alkane: Understanding the Unsolved Story

      Tretiakov, Mykyta; Lebedev, Yury; Samantaray, Manoja; Saidi, Aya; Rueping, Magnus; Basset, Jean-Marie (Catalysts, MDPI AG, 2019-03-05) [Article]
      For the first time, we developed a method which enables a functionalized alkane to be metathesized to its lower and higher homologues. For this metathesis reaction, we used [(≡Si-O-)W(CH3)5] as a catalyst precursor and 9-hexyl-9H-carbazole as a reactant.
    • Waveguiding and SERS Simplified Raman Spectroscopy on Biological Samples

      Valpapuram, Immanuel; Candeloro, Patrizio; Coluccio, Maria; Parrotta, Elvira; Giugni, Andrea; Das, Gobind; Cuda, Gianni; Di Fabrizio, Enzo M.; Perozziello, Gerardo (Biosensors, MDPI AG, 2019-03-04) [Article]
      Biomarkers detection at an ultra-low concentration in biofluids (blood, serum, saliva, etc.) is a key point for the early diagnosis success and the development of personalized therapies. However, it remains a challenge due to limiting factors like (i) the complexity of analyzed media, and (ii) the aspecificity detection and the poor sensitivity of the conventional methods. In addition, several applications require the integration of the primary sensors with other devices (microfluidic devices, capillaries, flasks, vials, etc.) where transducing the signal might be difficult, reducing performances and applicability. In the present work, we demonstrate a new class of optical biosensor we have developed integrating an optical waveguide (OWG) with specific plasmonic surfaces. Exploiting the plasmonic resonance, the devices give consistent results in surface enhanced Raman spectroscopy (SERS) for continuous and label-free detection of biological compounds. The OWG allows driving optical signals in the proximity of SERS surfaces (detection area) overcoming spatial constraints, in order to reach places previously optically inaccessible. A rutile prism couples the remote laser source to the OWG, while a Raman spectrometer collects the SERS far field scattering. The present biosensors were implemented by a simple fabrication process, which includes photolithography and nanofabrication. By using such devices, it was possible to detect cell metabolites like Phenylalanine (Phe), Adenosine 5-triphosphate sodium hydrate (ATP), Sodium Lactate, Human Interleukin 6 (IL6), and relate them to possible metabolic pathway variation.
    • A Chemically Well-Defined, Self-Assembling 3D Substrate for Long Term Culture of Human Pluripotent Stem Cells

      Loo, Yihua; Chan, Yun S.; Szczerbinska, Iwona; Tan, Bobby C. P.; Wan, Andrew C.A; Ng, Huck Hui; Hauser, Charlotte (ACS Applied Bio Materials, American Chemical Society (ACS), 2019-03-01) [Article]
      Clinical applications of human pluripotent stem cells (PSCs) are limited by the lack of chemically well-defined scaffolds for cell expansion, differentiation and implantation. In this study, we systematically screened various self-assembling hexapeptides to identify the best matrix for long-term 3D PSC culture. Lysine-containing Ac-ILVAGK-NH2 hydrogels maintained best the pluripotency of human embryonic and induced PSCs even after 30 passages. This peptide matrix is also compatible with the use of xeno-free and defined differentiation media. By exploiting its stimuli-responsive sol-gel transition, arrays of encapsulated PSCs can be bioprinted for large-scale cell expansion and derivation of miniaturized organoid cultures for high-throughput screening.
    • De Novo Variants Disrupting the HX Repeat Motif of ATN1 Cause a Recognizable Non-Progressive Neurocognitive Syndrome

      Palmer, Elizabeth E; Hong, Seungbeom; Al Zahrani, Fatema; Hashem, Mais O; Aleisa, Fajr A; Ahmed, Heba M Jalal; Kandula, Tejaswi; Macintosh, Rebecca; Minoche, Andre E; Puttick, Clare; Gayevskiy, Velimir; Drew, Alexander P; Cowley, Mark J; Dinger, Marcel; Rosenfeld, Jill A; Xiao, Rui; Cho, Megan T; Yakubu, Suliat F; Henderson, Lindsay B; Guillen Sacoto, Maria J; Begtrup, Amber; Hamad, Muddathir; Shinawi, Marwan; Andrews, Marisa V; Jones, Marilyn C; Lindstrom, Kristin; Bristol, Ruth E; Kayani, Saima; Snyder, Molly; Villanueva, María Mercedes; Schteinschnaider, Angeles; Faivre, Laurence; Thauvin, Christel; Vitobello, Antonio; Roscioli, Tony; Kirk, Edwin P; Bye, Ann; Merzaban, Jasmeen; Jaremko, Łukas; Jaremko, Mariusz; Sachdev, Rani K; Alkuraya, Fowzan S; Arold, Stefan T. (The American Journal of Human Genetics, Elsevier BV, 2019-02-28) [Article]
      Polyglutamine expansions in the transcriptional co-repressor Atrophin-1, encoded by ATN1, cause the neurodegenerative condition dentatorubral-pallidoluysian atrophy (DRPLA) via a proposed novel toxic gain of function. We present detailed phenotypic information on eight unrelated individuals who have de novo missense and insertion variants within a conserved 16-amino-acid
    • Spectroscopic Investigation of the Effect of Microstructure and Energetic Offset on the Nature of Interfacial Charge Transfer States in Polymer: Fullerene Blends

      Dimitrov, S D; Azzouzi, M; Wu, J; Yao, J; Dong, Y; Tuladhar, P Shakya; Schroeder, B C; Bittner, E R; McCulloch, Iain; Nelson, J; Durrant, J R (Journal of the American Chemical Society, American Chemical Society (ACS), 2019-02-27) [Article]
      Despite performance improvements of organic photovoltaics, the mechanism of photoinduced electron-hole separation at organic donor-acceptor interfaces remains poorly understood. Inconclusive experimental and theoretical results have produced contradictory models for electron-hole separation in which the role of interfacial charge-transfer (CT) states is unclear, with one model identifying them as limiting separation and another as readily dissociating. Here, polymer-fullerene blends with contrasting photocurrent properties and enthalpic offsets driving separation were studied. By modifying composition, film structures were varied from consisting of molecularly mixed polymer-fullerene domains to consisting of both molecularly mixed and fullerene domains. Transient absorption spectroscopy revealed that CT state dissociation generating separated electron-hole pairs is only efficient in the high energy offset blend with fullerene domains. In all other blends (with low offset or predominantly molecularly mixed domains), nanosecond geminate electron-hole recombination is observed revealing the importance of spatially localized electron-hole pairs (bound CT states) in the electron-hole dynamics. A two-dimensional lattice exciton model was used to simulate the excited state spectrum of a model system as a function of microstructure and energy offset. The results could reproduce the main features of experimental electroluminescence spectra indicating that electron-hole pairs become less bound and more spatially separated upon increasing energy offset and fullerene domain density. Differences between electroluminescence and photoluminescence spectra could be explained by CT photoluminescence being dominated by more-bound states, reflecting geminate recombination processes, while CT electroluminescence preferentially probes less-bound CT states that escape geminate recombination. These results suggest that apparently contradictory studies on electron-hole separation can be explained by the presence of both bound and unbound CT states in the same film, as a result of a range of interface structures.