The processing-structure-property nexus of organic semiconductors

Resume : Small molecular organic semiconductor crystals form interesting electronic systems of periodically arranged “charge clouds” whose mutual electronic coupling determines whether or not electronic states can be coherent over molecular distances. Recently, it turned out that band transport is realized in high-mobility organic semiconductor crystals though this situation is not common to all organic semiconductors. This presentation first focuses on the single-crystal molecular assembly of pentacene which does not exhibit full charge coherence at room temperature under atmospheric pressure. Hall coefficient, telling us the extent of the electronic coherence, is precisely measured for accumulated charge in pentacene single-crystal field-effect transistors at various temperatures with varied pressure. With the application of external pressure, the electronic coupling between pentacene molecules is continuously modified so that the extent of the intermolecular coherence grows with increasing pressure. In addition, it is demonstrated for newly synthesized decyl-dinaphthobenzodithiophene (C10-DNBDT) that room-temperature mobility is increased by the factor of 1.7 with the application of uniaxial strain with restricted molecular vibration. The results indicate the significant impact of the structural modification to the charge transport properties in organic semiconductor devices.

Resume : Polymer field-effect transistors (FETs) can achieve charge mobility in excess of 1 and 10 cm2/Vs for electrons and holes, respectively. Such performances are sufficient for a large range of applications of printed, light-weight and mechanically robust circuits, in diverse fields such as wearable electronics, smart packaging, and bio-electronics. Yet quantitative models describing charge transport in molecular solids are not available. While it is well established that films microstructure strongly affects performances in polymer FETs, the relation between structure and charge transport is still highly debated. In particular, recently developed, high mobility donor-acceptor copolymers offer evidences of strong resilience to disorder. In this framework, techniques capable of directly monitoring how charge is distributed when injected into a polymer film and how it correlates with the microstructure can be of strong help. In this contribution I will show that polarized charge modulation microscopy (p-CMM)[1,2] can selectively map the orientational order of the only conjugated segments that are probed by mobile charge in the few nanometer thick accumulation layer of polymer FETs. p-CMM offers the unprecedented opportunity to correlate, directly in a working device, electronic properties with structural information on those conjugated segments involved in charge transport at the buried semiconductor-dielectric interface of a FET. [1] Adv.Mater. 23(2011)5086 [2 ]ACS Nano 8(2014)5968

Resume : Thermoelectric devices convert thermal energy to electrical energy and by reciprocity the converse. These devices allow for capture of waste heat and for control of temperature without mechanical compressors. Organic semiconductors represent a promising class of thermoelectrics due to their simplicity in processing and potential performance due to their good electrical performance and relatively low thermal conductivity. A key challenge in achieving high performance organic thermoelectrics is control of their carrier concentration, which helps to set the thermopower and electrical conductivity. We will present recent work in our lab on doping of semiconducting polymers and small molecules to achieve both p-type and n-type conductivity. By studying several high carrier mobility semiconducting polymers, we have found that processing methods have a significant impact on thermopower. We will report temperature dependent measurements of thermopower and conductivity that help to reveal the nature of these changes. By comparing broad classes of materials, we have found a striking relationship between the thermopower and electrical conductivity that suggests directions for optimization of new materials.

Resume : Conjugated polymers are of great interest in low-temperature thermoelectric applications, as they can easily be solution processed, which enables large-area printing on flexible substrates. While the low thermal conductivity of polymers is of advantage, they also have rather low electrical conductivity. This drawback can be overcome by the addition of highly conductive fillers, like carbon nanotubes. These composites not only show increased electrical performance, but also allow tuning the sign of the Seebeck coefficient from positive to negative values, as is demonstrated herein. This way, both the p-type and the n-type legs of thermocouples can be fabricated from the same type of composite, thereby further simplifying the fabrication process, thus rendering possible prospective applications, such as power sources for wearable sensors and gadgets, for which price is an important factor. Here, we present results of experiments conducted on composites of regio-regular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT) and nitrogen-doped multi-walled carbon nanotubes (n-doped MWCNTs). Samples with a wide range of mixing ratios were drop-cast onto flexible PET substrates and the Seebeck coefficient S and electrical conductivity σ were measured in the plane of the substrate. σ shows percolative behaviour with threshold at about 5 wt% CNTs, as was observed previously for composites of undoped CNTs[1]. The Seebeck coefficient S exhibits an interesting difference in behaviour in the case of n-doped CNTs however. For low CNT content, S is positive, as would be expected for P3HT, even though the n-type CNTs percolate. Increasing the CNT content decreases S into negative values. The resulting power factor S2σ has two maxima, one near 20 wt% CNT content, where the composite shows p-type behaviour, and another near 80 wt% CNTs, where it acts as an n-type thermoelectric material. Using these composites, we illustrate the potential of the developed technology by demonstrating working, easy to process, thermoelectric generator prototypes, whose design capitalizes on the advantages that flexible substrates offer. References [1] Bounioux, C. et al. Thermoelectric composites of poly(3-hexylthiophene) and carbon nanotubes with a large power factor. Energy Environ. Sci. 6, 918–925 (2013).

Resume : A microscopic understanding of charge carrier transport in polymeric semiconductors, the most suited class of materials for printed and flexible semiconductor devices, has been the central subject of organic electronics. Due to the presence of inherent disorder and thermally activated structural fluctuation, it is a fundamental challenge to realize band transport within the range of charge accumulation accessible in field-effect transistors. To our knowledge, there has been no report proving band transport in polymer organic transistors. In this work, we demonstrate the Hall effect and a slightly negative temperature dependence of the charge carrier mobility in solid-gate polymeric transistors based on CDT-BTZ, donor-acceptor copolymers with different length of sidechains (Hexadecyl = C16 and Eicosyl = C20). We thereby employ a unique method for the elaborate orientation of polymer chains on the stable surface of an ionic liquid. The Hall factor close to the ideal value, a maximum hole mobility of 8 cm2/Vs at room temperature, and negative temperature dependence indicate that the system is located in the transition region between the band and hopping transport and closeness to band transport differs with different length of sidechains.

Resume : -conjugated materials form an essential class of functional materials in plastic electronics. Thin film processing from solution or from the solid state is an important aspect to optimize device performances because of the strong correlations that exist between the way these materials order and orient on surfaces and their optical and electronic properties. Herein, we present recent results concerning large scale alignment by high temperature rubbing of molecular and macromolecular semi-conductors. This method has been successfully applied to alternated donor-acceptor copolymers (p(NDI2OD-T2), PCPDTBT), donor-acceptor co-oligomers and -conjugated organogelators. Transmission electron microscopy (dark field, low dose high resolution and electron diffration) provides a unique insight on the thin film structure and nanomorphology. It allows to draw correlations between morphology, structure, charge transport and optical properties in thin films.

Resume : We address the impact of the relative orientation between donor (D) and acceptor (A) molecules at the D/A heterojunction on the exciton dissociation. For this purpose, two-dimensional heterojunctions of diindenoperylene (DIP) and N,N´-dioctyl-3,4,9,10-perylene tetracarboxylicdiimide (PTCDI-C8) deposited onto SiO2/Si are grown, which exemplify two model interfaces with the π-staking direction either perpendicular or parallel to the interface.1 Aspects related to the nanomorphology of the heterojunctions and charge photogeneration are studied by scanning probe force methods and photoluminescence (PL) spectroscopy. We show that the exciton dissociation is influenced by the different relative molecular orientations of A and D. For the configuration with stronger orbital overlap between A and D at the interface, the exciton dissociation is dominated by recombination from an interfacial charge transfer state. This works provide a clear identification of molecular orientation as one of the significant factors governing exciton dissociation through interfacial states and outline the importance of the local structure at the A/D heterojunction for the charge photogeneration processes.

Resume : Semiconducting poly(3-alkylthiophene) nanofibers show remarkable optical and electrical properties and because of their high aspect ratio (linguini-like fibers) they are very suited to serve as organic one-dimensional high ways for charge carriers. Henceforth, they offer interesting perspectives for next generation optoelectronic applications. This contribution provides an overview of the preparation/processing and characterization of poly(3-alkylthiophene) nanofibers, and a discussion on nanofiber-based optoelectronic applications, ranging from bulk heterojunction solar cells to ‘one single fibre’-transistors and phototransistors.

Resume : Secondary electrons (SE) can be emitted as result of the interaction with light (as in organic photo-voltaic, (OPV)) or by interaction with an electron beam, as exploited in a scanning electron microscopes (SEMs). SEMs are known for their ease of use, fast image collection, and simple or no specimen preparation. Therefore the SEM appears to be an ideal tool for the fast assessment of morphology provided, that low electron beam energies and currents can be used for imaging so that no conductive coating is needed. It is recognised that active layer morphology on different length scales is critical to OPV performance, in particular life time but mapping local variations across multiple length scales usually requires a multitude of techniques and specialist equipment. Here we show for different OPV blend systems how an SEM can provide high contrast images of OPV active layers and give local morphology information from the micron-scale to sub-nanometre scale in a few minutes by imaging using selected regions of the SE Emission spectrum or using low energy back scattered electrons. By comparing modelled SE spectra with experimental data we show that electron affinity and trap density strongly affect the shape of SE Emission spectra, thus providing an opportunity for mapping such properties by energy filtered low voltage SEM.

Resume : In polymeric semi-conducting materials, both molecular and crystalline orientations determine optical, electronic and opto-electronic properties in thin films since these properties are by essence highly anisotropic.(1) Therefore, it is of interest to control the contact plane of crystalline domains in the active layers as it determines, for instance, the direction of facile charge transport in the cases of OPV and OFET. Recently, mechanical rubbing has been applied to prepare highly oriented films of a large palette of pi-conjugated polymers over large area. Both, the temperature of the films during rubbing and the molecular weight distribution of the polymer determine the level of orientation.(2) In this work, we demonstrate the precise control of lamellar periodicity by varying the rubbing and annealing temperatures of poly(3-alkythiophene) (P3HT and P3BT) thin films. The nanostructures, observed by HR-TEM, can be clearly correlated to the optical properties of the rubbed thin films. Higher processing temperatures affect the lamellar structure of the polymers causing a longer exciton delocalization and reducing therefore the excitonic coupling constant in a P3AT crystal. The effect of lamellar thickness on charge transport properties is also discussed in this presentation. (1) Brinkmann M. et al. Macromol. Rapid. Comm. 2014, 35, 9. (2) Biniek L. et al. Macromolecules 2014, 47, 3871.

Resume : A hierarchical nanotube array composed of the novel conducting polymer, poly(hydroxymethyl 3,4-ethylenedioxythiophene) (PEDOT-OH), was successfully synthesized via a template-free electrochemical polymerization technique. At first step, PEDOT-OH was synthesized under a dichloromethane-based atmosphere; and thereby the one-dimensional (1D) nanotube array was formed on fluorine-doped tin oxide substrate. At second step, PEDOT-OH was further assembled onto the as-prepared film under another acetonitrile-based atmosphere, and thereby the honeycomb PEDOT-OH was decorated on the PEDOT-OH nanotube array to form a hierarchical structure. Accordingly, several PEDOT-OH films with various morphologies, including honeycomb, nanotube array, and hierarchical structures, were obtained; they aimed to work as the electro-catalytic counter electrodes for iodine/triiodine (I-/I3-) reduction in dye-sensitized solar cells (DSSCs). For a hierarchical film, PEDOT-OH nanotube array provides fast 1D charge transfer passage, while the decorated honeycomb PEDOT-OH gives the extended electro-active sites. Therefore, the DSSCs with the hierarchical PEDOT-OH as the counter electrode achieve the highest power conversion efficiency (PCE) of 8.0%; the PCE is comparable to that of the cell with traditional expensive Pt counter electrode (8.11%). Thus, PEDOT-OH can be considered as a good replacement of Pt, due to several advantages of high efficiency, low cost, and simple fabrication process.

Resume : Semiconductor quantum dots have great attention due to their tunable band gap depending on crystal size. These unique properties make them attractive materials for many applications such as electronics solar cells and medical applications.[1] [2] Core, core/shell or alloyed forms of those materials have been reported by many authors. [3] In this work, we investigated the composition effect on electrochemical properties of CdSeS alloys. For this purpose, gradient alloys of CdSeS nanocrystals were synthesized and their electrochemical properties were investigated. Cyclic voltammetry technique was used for electrochemical analysis. The redox potentials show some differences depending on chalcogenide composition in nanocrystals. The reduction potentials of CdSeS nanocrystals shift to more negative potentials by increasing Se/S ratio. The results showed that, the band position of nanocrystals are tunable depending on the chalcogenide composition. References: [1] Carver B. M., “Intro into the Engineering and Science of Nanotechnology-Quantum dots”, 2006, Course no: 0909-504-06. [2] Manna L., Scher E. C., Alivisatos A. P., J. Am. Chem. Soc., 2000, 122, 12700–12706. [3] Jinjin L., Wanting Y., Yunchao L., Louzhen F., Yongfang L., Electrochemical studies of the effects of the size, ligand and composition on the band structures of CdSe, CdTe and their alloy nanocrystals, Phys.Chem.Chem.Phys., 2014, 16, 4778 Acknowledgement: We thank to TUBITAK (PN:109T881) for financial support.

Resume : Cross-linked films of conjugated polymers have been a topic of significant attention in the past few decades.1 The conjugated polymers are easy processability since they are soluble and which enable them to be used in many commercial areas.2 Solution processability property allows for easy spin-coating, spraying, and inking of films directly onto a variety of rigid and flexible substrates. However, this property leads to problem in construction multilayer and patterned devices, which can greatly improve device performance.3 Cross-linked conjugated polymer films propose an alternative to solve this problem.4 In this work, a series of thiol-ene cross-linkable conjugated materials with tunable band gap were synthesized. Optical and electrochemical properties were carried out by using UV-vis, Flurescence spectroscopy and cyclic voltammetry, respectively. The morphology of crosslinked polymer film surfaces was studied by AFM. Particle size was increased after crosslinking and the RMS (root mean surface) roughness was observed as 10 nm. Finally, multilayer OLED devices was fabricated and characterized. The results were compared with standard OLED devices and discussed. Aknowledgement: We thank to TUBITAK (PN:113Z255) for financial support. 1.Bayerl, M, etal. Macromol. Rapid Commun. 1999, 20 (4), 224−228. 2.Helgesen, M. et al. J. Mater. Chem. 2010, 20 (1), 36−60. 3.Zhao, Wet al, M. Adv. Funct. Mater. 2004, 14 (8), 783−790. 4.Png, R. Q. et al.Nat. Mater. 2010, 9 (2), 152−158.

Resume : In this work, a theoretical design addressed to the synthesis of new conjugated co-polymers for organic photovoltaic devices, based on tetrazine, benzothiadiazole, dithiophene and thiophene moieties was applied. The theoretical study is aimed to explore which combination of molecules is the most promising taking into account as first step the intrinsic electronic properties and successively on their correlations with the nanostructure of the organic semiconductors. In particular, the band gap energy and the open circuit voltage simulated values are the parameters to be taken in account for the selection of the potential material for solar cell devices. Preliminary results shows that the most suitable combination that gives rise to low band gaps, higher absorption coefficients and with interesting offset of the HOMO and LUMO energy levels is the bridge-linked copolymer which alternates the dithiophene (D) and the benzothiazole (A) moieties, likely due to their push-pull effect D->A.

Resume : We investigate the impact a low volatile additive on the structure formation of PCPDTBT:PC71BM blends using a combined GIXD/reflectometry setup that allows for a simultaneous determination of the crystallization dynamics and the solvent composition of the film. If processed from pure DCB solutions, PCPDTBT does not exhibit crystalline features due to fullerene clusters that impede dense chain packing. In the presence of 3wt% ODT we observe the crystallization of PCPDTBT in several stages, which is interrelated with the aggregation behavior of the fullerene component. We determine that 3wt% ODT dissolve 0.09wt% PC71BM leaving 2.91wt% “free PC71BM” in solution and therefore suggest that upon the addition of ODT the fullerene component does not entirely dissolve. In the late stages of drying, where mainly ODT evaporates, further aggregation of PCBM reduces the (100) ordering of the polymer component.

Resume : In this work we investigated the behaviour of standard and inverted polymer solar cells by impedance spectroscopy (IS) analysis. The photoactive layer is a blend film of poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b']dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiopene)-2,6-diyl] (PBDTTT-C) and [6,6]-phenyl C71 butyric acid methyl ester ([70]PCBM). The standard cells sequence is ITO/PEDOT:PSS/PBDTTT-C:[70]PCBM/Ca/Al and the inverted cells one is ITO/ZnO/PBDTTT-C:[70]PCBM/MoO3/Ag. All the devices were characterized by UV-VIS spectroscopy, IS, IV light, IV dark and quantum efficiency measurements. In particular we used IS technique to check the conductance and the capacitance response of a device when an ac voltage is applied as a function of the frequency. We performed also Capacitance and Conductance measurements fixing the frequency and changing the applied voltage to have an understanding about the change in the diode performance during the operating voltage scan and to better compare the IS measurements with the IV one. We investigate the charge injection across the organic–organic interfaces, the effects of the charge injection rate and the series resistances introduced by the electrodes. We can establish that the inverse stack in solar cells is a very good solutions not only to solve partially the degradation in organic solar devices (no use of calcium) but also to reduce the loss current, to improve the diode switch on and the built in voltage.

Resume : We firstly report on a novel polymer hybride solar cell structure based on POM-P3HT mixture. Different ratio of POM/P3HT (polyoxometalate/poly(3-hexylthiophene)) was used to investigate the influence on device performance. The efficiency increases by increasing POM ration in blend. The quantum efficiency was found to be between 0.3% and 1.5 % depending on blend composition. Thermal treatment also influenced the device performance due to microphase seperation. All these parameters have been optimized and presented.

Resume : In bulk heterojunction (BHJ) organic photovoltaics (OPVs), electron donating and electron accepting materials form a complex network of discrete and distributed heterointerfaces and charge transport pathways in the photoactive layer where critical photo-physical processes occur. However, we have insufficient knowledge about the structural properties of these interfaces due to their 3-dimensional arrangement and the paucity of techniques to measure local order and purity. The presentation will review the use of synchrotron radiation based methods that can uniquely measure critical structural parameters, which includes molecular orientation correlations relative to donor/acceptor heterojunctions [1]. Using polarized resonant soft X-ray scattering [2], the degree of molecular orientation, an order parameter that describes face-on (+1) or edge-on orientation (-1) relative to these discrete heterointerfaces, can be determined. By manipulating the degree of molecular orientation through choice of molecular chemistry and processing solvent characteristics, the importance of this structural parameter on the performance of BHJ OPV devices and charge transfer dynamics can be demonstrated. We will furthermore show how compositional variations can be related to polymer crystal size [3] and how mobility and purity can relate to charge extraction and thus in turn to device performance [4,5]. A complete description of actual morphologies and theoretical modeling yet to be developed for OPVs will have to take these factors into account. Devices with efficiencies up to 10.8% will be discussed [6]. 1. J. R. Tumbleston et al., Nature Photonics 8, 386 (2014). 2. B. A. Collins et al., Nat. Mater. 11, 536 (2012). 3. W. Ma at al, Advanced Materials 26, 4234 (2014) 4. S. Albrecht et al. J. Physical Chemistry Letters 5, 1131-1138 (2014). 5. W. Liu et al., J. Am. Chem. Soc. 136, 15566-15576 (2014). 6. Y. Liu et al., Nature Communications 5, 5293 (2014)

Resume : We report the design and investigation of >50 fullerene derivatives whose electron affinity is lowered considerably due to the electron donating nature of a single organic addend attached to the fullerene cage. The electrochemical properties of the fullerene derivatives were investigated in solution and in thin composite films (with P3HT and PCDTBT). The best representatives of the designed families of compounds showed cathodic shifts of the first reduction potentials as large as 100-120 mV compared to [60]PCBM. It was revealed that decrease in the electron affinity of the designed fullerene derivatives is related to the through-space electronic interactions of electron donating alkoxy groups with the fullerene cage. The application of the designed fullerene derivatives as n-type components in organic bulk heterojunction solar cells led to increase in the open circuit voltage by 100-170 mV compared to the reference devices based on [60]PCBM. Few compounds provided even higher open circuit voltages (up to 770-780 mV with P3HT) than bis-PCBM bearing two organic addends on the fullerene cage. Moreover, using the best materials improved also the power conversion efficiency of organic solar cells where P3HT and PCDTBT were used as electron donor components. To our best knowledge this is one of the first examples of the fullerene derivatives with reduced electron affinity which outperform PCBM in solar cells comprising non-crystalline conjugated copolymers such as PCDTBT.

Resume : It is has been shown that bulk heterojunction solar cells can reach higher efficiencies and fill factors if controlling the morphology by applying solvent vapor annealing (SVA) treatments[1]. By this method, the solvent vapor penetrates into the blend re-organizing the donor-acceptor arrangement. In the first part of this work, the effect of SVA is addressed for the system formed by a conjugated A-D-A type oligothiophene as donor in combination with PC60BM and PC71BM, being observed an increase of power conversion efficiency from near 1% to 6% upon SVA in chloroform. To understand this effect, the microstructure before and after SVA has been investigated by grazing incidence X-ray scattering (GIXD) for different incident angles thus obtaining depth-resolved structural information. The most remarkable effect of the solvent annealing is the increase of the π-π oligomers packing. Secondly, we focus on the modulation of the ITO’s workfunction (WF) by functionalization with self assembled monolayers (SAMs) of different phosphonic acids. Kelvin probe force microscopy has been used for measuring the impact of the SAMs on the WF of the substrates and conductive atomic force microscopy for a nanoscale characterization of the electrical homogeneity of the substrates. The electrical performance of OPV devices with these modified ITO substrates has been characterized with a blend of PBDTTT-CF and PC71BM as the active layer [1] Cordula D. Wessendorf et al. Adv. Energy Mater. 2014, 14002

Resume : The performance of bulk heterojunction solar cells is critically dependent on processing conditions. Processing conditions influence both charge transport and recombination, but often in opposite ways: improvements in one parameter can be accompanied by deterioration of others. Even subtle changes in the processing conditions are known to affect the fill-factor (FF). Here, we show that such variations in the FF are due to shifts in the competition between extraction and recombination of charge carriers. We empirically demonstrate the precise relationship between this competition and the FF for a wide variety of donor/acceptor combinations following different processing protocols. We measure charge carrier mobilities and bimolecular recombination rates using a combination of steady-state and transient extraction techniques. These quantities are used to estimate the extraction and recombination times. The ratio between the extraction and recombination times, which we call theta, indicates whether the majority of charge carriers recombine or can be extracted from the solar cell. If all the FFs of the solar cells studied are plotted versus theta, the data collapse onto one universal curve. This shows that the competition between extraction and recombination can be quantified by theta. Our results rationalise the observed changes in FF upon varying the processing conditions even when this leads to competing changes in transport and recombination.

Resume : In this work, we report the application of a sol-gel derived ZnO thin film, doped with group III elements (Al, Ga or In) as a buffer layer for high efficiency inverted polymer solar cells (PSCs). ZnO films are widely used in such devices because they have a relatively high electron mobility, high transparency and environmental stability. The group III element-doped ZnO precursor was prepared by dissolving zinc acetate and ethanolamine in the 2-methoxyethanol in presence of a group III salt (i.e. Al(NO3)3, Ga(NO3)3 or InCl3). Doped ZnO thin films were then deposited on indium tin oxide (ITO)/glass substrates by spin coating the above solution. Inverted polymer solar cells with the configuration ITO/ZnO/photoactive layer/MoOx/Ag were realized in order to investigate the performance of ZnO thin film. The photoactive layer is a blend of Poly[(4,8-bis-(2-ethylhexyloxy)-benzo(1,2-b:4,5-b′)dithiophene)-2,6-diyl-alt-(4-(2-octanoyl)-3-fluorothieno[3,4-b]thiophene-)-2-6-diyl)] (PBDTTT-CF) and [6,6]-phenyl C71 butyric acid methyl ester ([70]PCBM) (1:1.5 w/w). We made a comparative study of the photovoltaic behavior of devices with ZnO films doped with different elements. All the devices were characterized by UV-VIS spectroscopy, IV light, IV dark and quantum efficiency measurements. The best device reached a power conversion efficiency of 9%.

Resume : Solution-processing is extensively used in laboratory-based production of the photoactive layer in bulk heterojunction (BHJ) organic photovoltaic (OPV) devices and has been the preferred way to fabricate the record efficiency OPV devices reported in recent years. The current understanding of BHJ formation via common solution-processing methods, such as spin-coating stems essentially from post-deposition investigations of the film, while our understandings of aggregation, crystallization and phase separation with respect to processing conditions and formulations are qualitative at best. Yet, the successful manufacturing of efficient BHJ OPV devices via scalable manufacturing processes, such as blade coating and slot-die coating, requires more quantitative understanding of BHJ formation in state-of-the-art spin-coated solar cells. Recent advances have made it possible to monitor the solution thinning as well as optical properties and x-ray scattering during the formation of BHJ thin films during spin-coating1-3. We will provide several recent examples to show how in situ investigations of solution-processing of polymer:fullerene and oligomer:fullerene blends helps to understand the critical roles of formulation and processing conditions in the solution-to-solid phase transformation and establish the processing-structure-property-performance nexus central to this symposium. Moreover, the lessons learned about BHJ formation during spin-coating help to achieve highly efficient solar cells, the positive characteristics of which can be emulated and transferred to more scalable and cost-effective processes, such as blade-coating. 1. K. W. Chou, B. Yan, R. Li, E. Q. Li, K. Zhao, D. H. Anjum, S. Alvarez, R. Gassaway, A. Biocca, S. T. Thoroddsen, A. Hexemer, A. Amassian, “Spin-cast bulk heterojunction solar cells: A dynamical investigation” Adv. Mater. 25, 1923-1929 (2013). 2. L. A. Perez, K. W. Chou, J. A. Love, T. S. van der Poll, D.-M. Smilgies, T.-Q. Nguyen, E. J. Kramer, A. Amassian, G. C. Bazan, “Solvent additive effects on solution processable small molecule crystallization and structural evolution in bulk heterojunction solar cells probed in situ during spin-casting”, Adv. Mater. 25, 6380-6384 (2013). 3. M. Abdelsamie, K. Zhao, M. R Niazi, K. W. Chou, A. Amassian, “In situ UV-Visible absorption during spin-coating of organic semiconductors: A new probe for organic electronics and photovoltaics”, J. Mater. Chem. C 2, 3373-3381 (2014).

Resume : A number of recent studies point to the importance of solution and thin-film ordering effects in solution-processable π-conjugated polymers for bulk-heterojunction (BHJ) solar cells with fullerene acceptors such as PCBM. In many instances, π–aggregation governs the self-assembly of the polymer donor – a key aspect that may ultimately impact nanoscale thin-film morphology and the phase-separation pattern of the BHJ. Significant differences in phase-separation patterns are known to influence BHJ solar cell performance, and in order to curb these effects, small-molecule processing additives, such as 1-chloronaphthalene (CN) and 1,8-diiodooctane (DIO), are now commonly used in the optimization of polymer-PCBM BHJ blend morphologies. In poly(benzo[1,2-b:4,5-b’]dithiophene–thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) and wide-bandgap analogs blended with PCBM, the formation of π–aggregates in solution and in thin-films depends upon i) the combination of side-chain substituents and ii) the functional groups appended to the main chain, and the use of processing additives can be critical in the optimization of the BHJ morphologies. Furthering our understanding of how the main chain substitution pattern mediates the interplay between polymer donor and fullerene acceptor is a critical step as we look to continue improving BHJ solar cell efficiencies.

Resume : Crystallinity, domain size, and grain boundaries strongly affect charge carrier transport in solution-processed organic semiconductor thin films. While solvent and thermal annealing as well as additives such as DIO and CN have been established as tools for controlling these aspects, their capacity to tune morphology is limited. Hence more robust approaches are needed. Here a novel strategy to control neat thin-film and bulk heterojunction blend morphology is demonstrated using a flexible linker (FL) approach. By polymerizing short fully-conjugated segments with flexible aliphatic chains, polymers offering independent tunability of the conjugation length and chain rigidity are demonstrated. For FL-pBTTT, a remarkable effect on the chain self-assembly is revealed that affords control between distinct thin-film morphologies and allows new insights into the important factors that direct microstructure and control charge transport in the thin film.[1] Moreover, our FL approach is found to offer a unique method to control small-molecule systems. Using FL-DPP(TBFu)2 as an additive in DPP(TBFu)2 based OFETs and OPVs provides control over domain size, and long-range charge transport, while also significantly improving the thermal stability of the thin-film morphology. These results are examined in the perspective of crystal nucleation and growth dynamics to offer further insight. [1] A. Gasperini, S. Bivaud, K. Sivula, Chem. Sci. 2014, 5, 4922-4927.

Resume : Liquid crystals can be aligned in response to an electric field and conjugated polymers act as good optical absorbers, both classes of organic material are well known from two rather different research areas [1]. Blends of these materials benefiting from their typical characteristics have as yet not been deeply pursued. Here we present the investigation of a blend in which F8BT acts as optically active part and 5CB as nematic liquid crystal aligned by an external electric field in order to obtain a new functional material, where a field assisted modulation allows the tuning of the optical properties. To unveil the mechanisms involved in the interaction of the two materials we performed ultrafast pump-probe characterization with and without applied electric field. This field enables the variation of inter- and/or intra-chain interaction. By applying the electric field the formation of a novel positive transient absorption band is observed, generally assigned to the stimulated emission (SE)[2], which increases proportionally to the intensity of the electric field. The results are explained as a change of the orientation and the nanostructure domains of the polymer. Moreover, the SE dynamics have lifetimes longer than the neat polymer: this is a very important condition for optoelectronic and lasing applications. [1] P . G . de Gennes. Oxford Clarendon, 1974; J. Clark et al. Nature Photonics, 4, 2010. [2] M.A. Stevens et al. J. Physical Review B, 63, 2001

Resume : We use redox hybrid materials (Au NPs, 2 and 10 nm in diameter, functionalized by ethylene dioxythiophene derivatives, AuNP/EDOT) deposited between 2 planar electrodes (2 μm apart) and electropolymerized in situ to growth a continuous polymer PEDOT with embedded AuNPs. We demonstrate that the current-voltage curves (IVs) exhibit two interesting behaviors : a NDR (negative differential resistance) behavior and a memory behavior. When we sweep the voltage back and forth between -40V and 40V, we clearly and repeatedly observe a peak in the IVs Two almost symmetric peaks are observed at +/- 5-7 V. The amplitude of the peak depends on the voltage sweep rates, while Vpeak remains in the same range. A blank experiment (PEDOT without AuNP) does not show any NDR effect. Thus, this NDR feature can be ascribed specifically to this hybrid AuNP/redox material. Using a different voltage sweep sequence, a memory effect is also demonstrated. with an on/off current ratio of about 1E3 to 1E4. The time stability of the on and off states is quite good (tested up to 7h) demonstrating a non-volatile memory. The memory effect is also preserved when repeating the cycles (100 for this test), while in this latter case we have observed an increase of the off current with the number of cycles. In conclusion the combination of a memory and NDR effects in the same device is unique and may be interesting for specific applications (e.g. neuro-inspired circuits with learning capabilities).

Resume : Defects in organic solar cells impact strongly on their performance by trapping of the charge carriers in different parts of the cells. It has been demonstrated that they play a prime role in the degradation process of devices [1]. The formation of defects in organic devices has been studied by several techniques but many aspects of the mechanism have not been elucidated so far. In particular, for inverted structures the use of oxide layer either on the anode or the cathode side may constitute supplementary sources of defects. We have investigated defects in inverted structure using P3HT:PCBM blend as an active materials. We applied the charge based Deep Level Transient Spectroscopy (Q-DLTS) to devices of structure ITO/ZnO nanorods/P3HT:PCBM/PEDOT:PSS/WO3/Au. The energy conversion efficiency of the inverted solar cells is ~ 3.3%. The Q-DLTS spectra show two distinct charge peaks, which correspond to two different relaxation mechanisms. The high relaxation time mechanism is similar to that observed in standard devices [1], which was attributed to the defects of the P3HT:PCBM blend. These defects comprise several energy levels in the range of 10 to 450 meV. The low relaxation time peak, which corresponds to a shallow trap level, is assigned to interface defects, probably originated from zinc oxide contact with the blend. The characteristics of the defects are comparable to those found in ZnO crystal. [1] T. P. Nguyen, C. Renaud, F. Reisdorffer, L. Wang, Journal of Energy Photonics 2 (2012) 021013-1/12, Degradation of PCBM :P3HT organic photovoltaic cells and structure changes as determined by defect investigations