Symposium MD5 : Fundamentals of Organic Semiconductors---Synthesis, Morphology, Devices and Theory

In this presentation, we seek to provide a rationalization of the impact that intermolecular arrangements and interactions at the polymer/fullerene interfaces have on the performance of bulk-heterojunction solar cells. We discuss the results of combined electronic-structure calculations and molecular-dynamics simulations both for representative systems reported in the literature and new systems synthesized in our Center. In particular, we examine:

(i) the propensity of the fullerene molecules to dock preferentially on top of the electron-poor moiety or electron-rich moiety of the polymer, as a function of the nature and location of the polymer side chains; and

(ii) the impact that the packing arrangements have on the energetic distribution of the charge-transfer interfacial electronic states and their localization/delocalization characteristics.

This work is supported by King Abdullah University of Science and Technology, in the framework of its Solar & Photovoltaics Engineering Research Center (SPERC) and its Collaborative Research Grant Program (Award CRG3-62140391), and by ONR-Global (Award N62909-15-1-2003).

While improved materials, processing protocols, and device designs have ushered organic electronic devices onto the commercial landscape, there remains a need to establish a thorough understanding of the intimate relationships among chemical and molecular structure, processing, solid-state packing, and the underlying physical processes that determine material performance. Through the development and application of multiscale, theoretical materials chemistry approaches, we seek to develop the chemical insight behind these relationships that is necessary to refine and offer novel design pathways for next generation organic semiconducting active layers. In this presentation, we will focus on how such models reveal the influence of seemingly modest changes in chemical structure on the processing and solid-state packing of organic semiconducting active layers.

The structure and electronic properties of a series of donor-acceptor organic molecules are explored using ab initio calculations to understand the behavior of polaron pairs at the interface of the donor-acceptor junction. Results suggest that one may be able to control polaron pair behavior based on the asymmetry of the donor molecule. This allows for control of thermodynamic losses as well as open circuit voltage. These calculations also revealed the probability distribution of the formation of different types of polaron pairs, especially, as it relates to deposition order (i.e. donor on top of acceptor versus acceptor on top of donor). The energetics of crystalline substrates with different surface terminations are mapped out using a single molecule of the partnering species. Accordingly, the interfacial structure and properties are different depending on whether the substrate is a donor or acceptor due to the incongruency between lattices and the disorder that develops in the contact layers of donor and acceptor.

Extending photoresponse into the near-infrared (NIR) is one clear route to improving performance of both panchromatic and transparent organic photovoltaics (OPVs). However, current demonstrations of NIR active OPVs have been limited by low open-circuit voltages (V_OC) and low external quantum efficiencies (EQE). One reason for this is the challenge of optimizing energy level alignment within the tightened tolerance of a small bandgap. In this work, we show that V_OC and EQE can be simultaneously enhanced in organic salt-based OPVs via single-step anion exchanges. We demonstrate that V_OC gains are due to improved energy level alignment and that energy levels can be finely tuned by alloying various anions. These effects can be exploited to optimize energy level alignment for abitrary donor-acceptor pairings with novel low bandgap organic salts, and we extend this method to several new molecules with unprecedented V_OC (for their spectral range) and photoresponse from 950 nm to as far as 1500 nm. This work bypasses the V_OC bottleneck which previously hindered small molecule-based photovoltaics, and presents an exciting path forward for the design of efficient multi- and single-junction transparent photovoltaics.

We used Density Functional Theory (DFT) calculations to study the electronic structure of organic mixed-stack charge-transfer crystals. We investigate the impact that the amount of nonlocal Hartree-Fock exchange (HFE) included in a hybrid density functional has on the geometry, the normal vibrational modes, electronic coupling and electron-vibrational couplings in these systems. The crystal geometry and the frequencies of the phonons are found to be only modestly affected by the amount of HFE. In contrast, the electronic couplings and electron-vibration couplings show a strong dependence on this value. We compare the DFT results with results obtained within the G₀W₀ approximation as a way of benchmarking the optimal amount of HFE needed in a given functional. The electronic structures of a series of donor-acceptor crystals have been investigated in detail. The charge-transport, ferroelectric and non-linear optical properties of several systems that were investigated will be discussed.

The fullerene-based organic solar cells (OSC) with a very minute amount of polymer “donor” have recently attracted intensive research interest due to their simple structure and unique electrical performance especially large Voc. However, charge transport mechanism in such devices with “donor” concentration much lower than the percolation threshold is still unclear. Some studies propose the high Voc is the Schottky barrier height between the anode electrode and the fullerene LUMO. On the other hand, other authors argue that Voc is enhanced because CT states, the major pathways for bimolecular recombination in bulk-heterojunction OSC devices, are significantly reduced in these active layers. Studies based primarily on current density-voltage (J-V) measurements are unable to differentiate different mechanisms. Here we apply impedance spectroscopy (IS), low-energy external quantum efficiency (EQE) spectroscopy, and photoluminescence (PL) dynamics in addition to J-V to study PCBM-based organic solar cells with varying P3HT concentrations. The experimental results are compared to 1D drift-diffusion modeling using SCAPS software.[1] Strong frequency-dependent capacitance behaviors are observed in devices with very low donor concentrations, indicating significant charge accumulation due to space charge limited as well as trap limited transport process. In addition, the role of exciton-polaron annihilation due to poor exciton dissociation and polaron collection will be probed by PL dynamic studies. Finally, impacts of p-type doping of active layer in the fullerene-based OSCs on hole mobility and internal field improvements will be discussed.

Reference:
[1] M. Burgelman, P. Nollet, S. Degrave, Modelling polycrystalline semiconductor solar cells, Thin Solid Films. 361 (2000) 527–532.

This project is sponsored by National Science Foundation DMR-1305893

Limited lateral conductivities of the photo-active materials used in organic thin-film solar cells necessitate the use of semitransparent electrodes for current collection and lateral current transport. Due to the tradeoff between electrical conductivity and optical transmission, which should not underrun 80%, typical values for the sheet resistances of semitransparent electrodes deposited on glass amount to 10–20 Ω/sq. The power loss due to Joule heating and the accompanying voltage drop caused by this sheet resistance increases with cell length in current transport direction and thus defines an upper limit for practical solar cell lengths. In the several approaches existing for calculation of the power loss, the semitransparent electrode layer is either modeled as one lumped resistance in series to the solar cell or as distributed resistance across the whole length of the solar cell in current transport direction. We present a quantitative comparison between these two concepts to investigate the direct influence on the optimal solar cell geometry and to discuss the capabilities as well as limitations of each model. Furthermore the impact of the results on the serial interconnection of a monolithic thin film organic solar cell module in terms of optimal cell lengths and cell interconnection distance is evaluated. The computational study presented here is based on the material system PCDTBT:PC70BM for the photo-active layer material as an example and commonly used semitransparent conductive electrodes: ITO deposited on glass as well as on PET foil and highly doped PEDOT:PSS named PH1000 on PET foil.

Energy levels in the mixed polymer-fullerene phase of bulk heterojunction solar cells are significantly shifted from their values in the pure materials [1]. These shifts are important for solar cell performance: they create energy cascades between the mixed phase and pure donor and acceptor phases, which have been shown to improve geminate splitting and suppress bimolecular recombination. This work investigates the origin of these energy level shifts and explains their effect on the charge transfer (CT) state energy and open circuit voltage (Voc).

We use regiorandom P3HT:PCBM as our model system. Regiorandom P3HT is amorphous, and when blended with PCBM, forms only one amorphous mixed phase, allowing us to study the energetics of the mixed phase without effects of polymer crystallites. We measure the polymer ionization potential (IP) and fullerene electron affinity (EA) as a function of blend composition using cyclic voltammetry (CV). The polymer IP monotonically increases in magnitude by around 400 meV and the fullerene EA by around 200 meV as the fullerene content in the blend is increased from 29% to 71%. The effective band gap measured by CV increases by around 300 meV.

Computational modelling studies using molecular dynamics [2] and classical microelectrostatics [3] have suggested that electrostatic interactions give rise to dipoles at the molecular interface between polymer and fullerene. We hypothesize that the electrostatic potential created by these interfacial dipoles shifts the energy levels of individual molecules in the blend. To test this hypothesis, we design a simplified electrostatic model to compute the effects of interfacial dipoles on polymer and fullerene energy levels. We fit the energy level shifts predicted by this model to experimentally measured values, using the magnitude of the interfacial dipole as the only fitting parameter. With this model, we show that the energy level shifts can be quantitatively accounted for by interfacial dipoles of comparable magnitude to permanent dipole moments in the molecules.

Despite large changes in the effective band gap, the measured CT state energy and Voc shift by only small amounts and show no trend. We show that energetic disorder in the mixed phase results in broadening of all the densities of states. During normal solar cell operation, only the low-energy tail of the CT density of states is filled, and effectively sets Voc. While changes in blend composition produce large changes in the centres of the energy levels, their low-energy tails are only slightly affected, resulting in Voc not varying by much.

In conclusion, this work shows how energy levels in the mixed phase are shifted by dipoles at the polymer-fullerene interfaces. Tuning these energy level shifts is likely to be an important part of future strategies aimed at improving the efficiencies of organic solar cells.

[1] J. Am. Chem. Soc. 2014, 40, 14078
[2] Adv. Mater. 2013, 25, 878
[3] J. Phys. Chem. C 2013, 117, 12981

Rubrene is a promising material for organic electronics and optoelectronics; it forms crystalline films with high hole mobility and efficient electroluminescence. Recent studies have shown that the electronic properties of rubrene films can be tuned by substrate-induced strain, suggesting a new route towards the design of more efficient devices. Here, we present a first-principles analysis of the strain-induced changes to the mechanical, electronic, and optical properties of rubrene crystals. Density functional theory and many-body perturbation theory studies predict changes in hole motilities in excellent agreement with electrical conductivity measurements when a strain consistent with the experiment is applied. Furthermore, we predict that the optical absorption and nature of low-energy excitons within the crystal can be tuned by an applied strain as low as 1%. This work utilized resources at the Center for Nanoscale Materials, supported by the U.S. Department of Energy under Contract No. DE-AC02-06CH11357.

Organicoptoelectronics, such as organicphotovoltaics (OPVs) and organic light emitting diodes (OLEDs), offer the promise of low-cost flexible solar cells, displays, and light sources that have the potential to be manufactured on large-area plastic substrates. One of the key elements for improving efficiencies in organic optoelectronics is finding suitable cathode electrode materials to replace the reactive low work function metals, such as calcium or magnesium, that are typically used to either inject electrons into or extract electrons from the lowest unoccupied molecular orbital (LUMO) of a given organic semiconductor.Polyoxometalates (POMs) are a well-known large group of clusters with frameworks built from transition metal oxo anions linked by shared oxide ions, first reported by Jöns Jacob Berzelius in 1826.One of the most intriguing properties of POM clusters is their high ability to accept a large number of electrons with minimal structural modifications, a property that could enable them to play an important role as excellent electron conductors in electronic devices.
We report here on the preparation of efficient electron conducting layers consisting of lacunaryPOM clusters spin coated from a water/methanol solution between the organic semiconductor and the cathode electrode surfaces for application inorganic optoelectronics. The photoelectron spectroscopy study reveals that these POMsexhibit a high degree of reduction which introduces delocalized electrons that can hop over the metal centers creating thus near Fermi level states that overlap sufficiently with the Fermi level of the metal cathode acting as a highly conductive path for electron conduction through the cathode interface. It is verified also that our method is applicable to a wide range of different organic semiconducting materials and that can be used in state-of-the-art high-efficiency organic electronic devices, including OLEDs and bulk heterojunction (BHJ) OPVs.In particular, OLEDs based on the greenemitting poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1’,3}-thiadiazole)] (F8BT) and a wide range of OPV devices based on photoactive layers composed of mixtures of a polymer donor, such as poly(3-hexylthiophene) (P3HT), poly[(9-(1-octylnonyl)-9H-carbazole-2,7-diyl)-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) or poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2- b :4,5- b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4- b ] thiophenediyl}) (PTB7), and a fullerene acceptor, such as [6,6]-phenyl C71butyric acid methyl ester (PC₇₁BM), exhibited a large enhancement in their efficiencies when used the reduced POMs as electron conduction layers.

Despite the many advances, OPV devices still fall behind their inorganic counterparts in terms of power conversion efficiency and longevity. Up to now, disordered blends of donor and acceptor molecules yield the best solar cells. However, every device is a compromise between small and large phases, which favor charge carrier generation and charge carrier transport, respectively. Additionally, optimizing processing conditions is a non-trivial task, which is usually done empirically. Hence, new materials often lag behind expectations obtained from chemical intuition. Furthermore, synthetic chemists open up new routes to synthesize even more molecules applicable to OPV meaning that the sheer number of compounds available renders the experimental selection of suitable compounds difficult. Hence, in-silico pre-screening of such new compounds is desirable.
Although in-silico screening of new compounds is less laborious than the experimental analogue, only a limited number of compounds can be treated due to the manual effort involved. Hence, we employ automation routines in order to minimize manual intervention on runtime-restricted systems that allow for easy screening of a large number of compounds in addition to accelerating calculations by the use of an efficient implementation of electron core potentials [Song et al., J. Chem. Phys., 2015, 143(1)]. Material properties that need to be predicted correctly by the aforementioned screening procedures include ionization potentials, electron affinities and exciton binding energies and absorption spectra both, in vacuum and in solution. This means that correct predictions of absolute frontier orbital energies and optical gaps are necessary in these environments. We could already show the fast reproduction of absorption spectra in some of these cases [Gampe et al., Chem. Eur. J., 2015, 21(20)].
Frontier orbital energies are very important molecular properties that decide whether or not a materials system can produce a photocurrent under irradiation as the difference between these energies is the main driving force behind exciton separation at the interface. However, commonly employed DFT calculations grossly underestimate the fundamental gap. Recently, it was suggested to use the class of range-separated hybrid functionals to overcome this issue by tuning or optimizing the range-separation parameter to the system at hand without the need for empirical data. This is done by minimizing the misfit to Koopman’s theorem and an analogous, inverse variant concerning the LUMO.
We employ this technique to reproduce experimental molecular properties for a variety of dyes applicable to OPV, such as porphyrins. We also compare the results obtained for different functionals. The automation routines we employ render this a feasible approach for research in the field of OPV.
We acknowledge funding from the Bundesministerium für Bildung und Forschung (BMBF FKZ 03EK3507) and the Deutsche Bundesstiftung Umwelt (DBU).

Thin organic chromophore layers are the essential functional elements in devices for light to electric energy conversion and vice versa, i.e. in organic solar cells (OSCs) and organic light emitting diodes (OLEDs). In OLEDs the morphology of the active layer needs to be engineered for optimal directional characteristics of light emission while in OSCs the morphology of the active layer needs to be elaborately tuned for optimal power conversion efficiency. However, the morphology is generally challenging to target starting from molecular chromophore design. This is due to the fact that predictions of supra-molecular structures are still computationally very demanding at a high accuracy level and the formation of particular morphologies depends on various external physical, chemical and processing parameters. Therefore, various approaches have been developed to control the morphology. One promising approach to design defined supra-molecular structures that are thermodynamically stable is self-assembly. A prominent strategy to induce self-assembly is to introduce amphiphilicity to functional molecules. Beyond self-assembly, amphiphilicity additionally enables utilization of the Langmuir-Blodgett (LB) technique for superior control of molecular ordering and phase formation.
This presentation will focus on self-assembly, morphological and photonic properties of novel donor and novel acceptor molecules that were made amphiphilic. Using the LB-technique we are able to produce thin films of both donor and acceptor molecules with controlled morphology. The surface sensitive photothermal deflection spectroscopy[1-3] is not just applied to determine the UV-vis absorption spectra of even molecular monolayers, but also to monitor self-assembly on surfaces from solutions in situ. On the examples of C60-derivatives, but also merocyanines and novel thiazoles derivatives it is demonstrated how large the influence of the morphology on photonic properties might be and how to control morphologies by molecular assembling.[4]

1. Presselt, M., et al., Influence of Phonon Scattering on Exciton and Charge Diffusion in Polymer-Fullerene Solar Cells. Advanced Energy Materials, 2012. 2(8): p. 999-1003.
2. Presselt, M., et al., Sub-bandgap absorption in polymer-fullerene solar cells studied by temperature-dependent external quantum efficiency and absorption spectroscopy. Chemical Physics Letters, 2012. 542: p. 70-73.
3. Herrmann, F., et al., Influence of Interface Doping on Charge-Carrier Mobilities and Sub-Bandgap Absorption in Organic Solar Cells. The Journal of Physical Chemistry C, 2015. 119(17): p. 9036-9040.
4. Habenicht, S.H., et al., Tuning the Polarity and Surface Activity of Hydroxythiazoles – Extending Applicability of Highly Fluorescent Self-Assembling Chromophores to Supra-Molecular Photonic Structures. to be submitted, 2015.

The authors are grateful for financial support (BMBF FKZ 03EK3507).

Chirality plays a very important role in scientific and technological research fields with huge potential to be applied in key areas including chemical synthesis, pharmaceutics, catalysis, fundamental physics and biomedicals. However, the separation of chiral molecules is still a challenge for their practical use in chemical and pharmaceutical industries. The most widely used characterizations rely on the traditional methods of measuring the circular dichroism using circularly polarized light. Besides, chirality in nanostructures could potentially be very useful. However, the application of chiral nanostructures, particularly, organic chiral nanomaterials, is still in infancy. In this research, we have successfully synthesized chiral materials, i.e., CPDI-Ph derivatives, and self-assmbled them into nanowires. The prepared 1-D nanomaterials show excellent n-type organic field-effect transistor (OFET) performance and high photo-responsivity under monochromic light irradiations. More importantly, specific photo-responses were obtained from chiral semiconductors upon illuminating circularly polarized light on the active layer of the n-type OFETs, which demonstrates the first example for detecting circularly polarized light from n-type phototransistors based on photoactive 1-D nanomaterials.

The development of highly conductive conjugated polymers has been one of the major goals for organic electronics. Recently, conductivities of over 4000 S/cm was demonstrated for vapor phase polymerized poly(3,4-ethylenedioxythiophene):tosylate (PEDOT:Tos), rivaling ITO as the material of choice for transparent flexible electrodes. Substantial research has also been invested in the development of patterning techniques for such conductive polymers. These patterning techniques include laser cutting, ink jet printing, screen printing, transfer printing and photolithography. Each of these methods has its respective advantages and disadvantages. We report a method of patterning vapor phase polymerized PEDOT:Tos using UV-light and show how this new approach has several advantages over existing patterning techniques.
When patterning conductive polymers using photolithography, a mold of the pattern is first made using a photoresist. The polymer is then added to the mold after which the photoresist is dissolved in an organic solvent. This results in a step-like profile of the pattern edges. The procedure involves a number of steps and the chemicals used to strip the photoresist are often environmentally hazardous and can also damage the conductive polymer. Our approach is similar to that of photolithography in that the patterning is mediated by UV-light through a photomask. However, it has fewer processing steps and does not involve any photoresists or hazardous chemicals.
The patterning can either be done with a step-like profile of the pattern or having a continuous polymer film where the conductivity of the film is locally reduced. The reduction in conductivity is controlled by the radiation dose and can span over six orders of magnitude. The exposure also induces a change in the color of the polymer film. Line widths of 100 um can be done effortlessly and work is ongoing towards significantly smaller dimensions.
The simplicity and versatility of this novel patterning procedure show great promise for patterning of organic electronic circuits as well as electrochromic displays.

Donor–acceptor (D–A) semiconducting copolymers, PDPP-TVT-24 and PDPP-TVT-29, were synthesized by alternating diketopyrrolopyrrole (DPP) derivatives with difference alkyl side-chains (2-decylbutadecyl (24) and 7-decylnonadecyl (29)) and thiophene vinylene thiophene (TVT) for organic field-effect transistors (OFETs). PDPP-TVT-24 and PDPP-TVT-29 could form highly π-conjugated structures in as-spun films. In particular, the layer-like conjugated film morphologies could be developed via short-term thermal annealing above 150 °C for 10 min. The strong intermolecular interaction, originating from the fused DPP and D–A interaction, led to the spontaneous self-assembly of polymer chains within close proximity (with π-overlap distance of 3.55–3.70 Å) and formed unexpectedly long-range π-conjugation, which is favorable for both intra- and intermolecular charge transport. Unlike intergranular nanorods in the as-spun film, well-ordered layers in the 200 °C-annealed film could yield more efficient charge-transport pathways. The granular morphologies of the as-spun films produced a field-effect mobility (μ_FET) of 0.65 cm² V^–1 s^–1 (for PDPP-TVT-24) and 1.39 cm² V^–1 s^–1 (for PDPP-TVT-29) in OFETs based on a polymer-treated SiO₂ dielectrics, while the 200 °C-annealed films showed high μ_FET values of up to 3.4-3.7 cm² V^–1 s^–1. The difference of charge mobilities between PDPP-TVT-24 and PDPP-TVT-29 as-spun films might be originating from polymer chain flexibility with different alkyl side-chain length. The significantly high μ_FET observed for the thermally annealed copolymers films were mainly related to the extraordinarily layered π-conjugated crystals, owing to the strong intermolecular interactions and the spaced out side-chain arrangement that helped the polymer chains form well-interconnected crystal grains, thus providing efficient pathways for charge transport. The highly π-conjugated nature, excellent solution processability, electrical properties, and simple synthesis of PDPP-TVT-24 and PDPP-TVT-29 suggest that the copolymers can potentially be applied to high-throughput, roll-to-roll solution printing of low-cost OFET circuits and arrays.

Charge transport in organic semiconductors is typically described by hopping through localized electronic states in Gaussian density of states (GDOS). The efficiency of charge transport is determined by the position of Fermi level and the corresponding DOS. As the carriers fill the lower-lying states of the organic semiconductor, corresponding to a shift in the Fermi energy, additional charges will occupy states at relatively higher energies. Consequently, these additional charges on average require less activation energy to hop to neighbouring sites, resulting in a higher mobility with increasing carrier density. Carrier mobility in such devices can then be tuned by varying the pushing the Fermi level relative to the maximum of the DOS. However, there are limitations to how much the Fermi level can be shifted, primarily due to device breakdown considerations. As a result, the mobility in three terminal organic semiconductor based devices have been limited to low values.
We have examined the characteristics of Copper Phthalocyanine(CuPc) based organic field effect transistors in both the positive and negative drain-source voltage (V_DS) regime. Phthalocyanine based molecular materials are polycrystalline, thermally and chemically stable and are ideally suited for testing any carrier transport model. Moreover, they have the potential for application in organic solar cells, LEDs and field effect transistors. We find that the Fermi level can be moved closer to the maximum of the GDOS by applying a positive V_DS. In the positive V_DS regime, the I_DS-V_DS characteristics at zero gate voltage (V_G) are similar to the two terminal devices, but an external gate can still be used to modify the carrier density. We observe an increase of almost two orders of magnitude in the mobility in this regime, with mobility values comparable to those observed in single crystals of CuPc. The high mobility can be understood on the basis of a transition from hopping to band like transport, with an applied V_DS and V_G. Band mobilities calculated using first principle DFT calculations are found to be in good agreement with the observed experimental values.

Parallel-like ternary blends provide a possible solution to one of the inherent limitations of organic solar cells: the relatively narrow absorption width of the conjugated polymers used in the active layer. This study focuses on selecting donor polymer pairs for ternary blends which will exhibit the characteristic composition dependent voltage, while extending the absorption and increasing the current, thereby improving the power conversion efficiency. In order to investigate the effect of a common backbone moiety on performance, two donor moieties and two acceptor moieties were mixed and matched to synthesize four polymers: poly(benzodithiophene-dithienyl-benzothiadiazole), poly(benzodithiophene-dithienylbenzotriazole), poly(naphthodithiophene-dithienyl-benzothiadiazole), and poly(naphthodithiophene-dithienybenzotriazole). These polymers were then paired in photovoltaic devices (with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as an electron acceptor) and their performance compared to binary devices made from each individual polymer. It was found that pairings in which the polymers shared a common donor moiety performed well relative to the binary blends, but pairings with either a common acceptor moiety or no common moiety performed poorly, as evidenced by a drop open-circuit voltage. Resonant soft X-ray scattering shows that each blend exhibits markedly different morphology, with P(NDT-DTBT) in particular dominating domain formation when included in a blend. In the most successful pairing, P(BnDT-DTBT) and P(BnDT-HTAZ), each polymer formed independent domains. For this set of polymers, in order to see parallel-like behavior it was essential that the ternary blend include a common donor moiety in the polymer backbone and exhibit independent, constructive domain formation. These insights will serve to guide future selection of polymer pairs for parallel-like ternary blends.

It is necessary to develop a method to control the electronic properties of single-walled carbon nanotubes (SWCNTs) for use in a wide range of areas such as electronics and photonics. For example, nitrogen doping into the SWCNTs is one of the methods for tuning their electronic properties, resulting in n-type SWCNTs in air. However, in previously-reported chemical vapor deposition using a nitrogen source, only nitrogen-doped (N-doped) defective SWCNTs can be synthesized and they do not show good conductivity. Here, we employed a defluorination-assisted nanotube substitution reaction, which was performed using ammonia gas, for the synthesis of N-doped crystalline SWCNTs that exhibit good conductivity and n-type conduction.
We synthesized highly crystalline SWCNTs (hc-SWCNTs) by the arc discharge method. The hc-SWCNTs were fluorinated at 250 °C using a gas mixture of F₂ (20%) and N₂ (80%) for 4 h. The fluorinated SWCNTs (F-SWCNTs) were characterized using X-ray photoelectron spectroscopy (XPS) and their C:F stoichiometry was determined to be CF_0.39. Next, The F-SWCNTs were reacted with a gas mixture of NH₃ (1%) and N₂ (99%) at temperatures of 300-600 °C for 30 min. The resulting samples were characterized using XPS, high-resolution transmission electron microscopy (HRTEM), and Raman scattering spectroscopy. We also measured the volume resistivity of the films of various nanotube samples and investigated their conduction properties in air by thermopower measurements.
The XPS spectra of all the samples subjected to the nitrogen-doping reaction indicated that the nitrogen atoms were introduced into the nanotube framework at concentrations of 1.38-3.04 at.%, which is as high as those reported in previous studies. The N_1s XPS spectra could be deconvoluted into several peaks, which corresponded to pyridinic-, pyrrolic-, and graphitic-type nitrogen bonds. Although the HRTEM images of these N-doped SWCNTs showed that their crystalline structure was poor, it was also clear that the nanotube frame of the former remained intact to a certain degree from Raman scattering spectra.
Since the N-doped SWCNTs retained the network of π-electron cloud, their volume resistivities were 1.9-3.7 Ω･cm, which are classified as semiconductors. The thermopower of N-doped SWCNTs synthesized at 400 °C was found to be negative in air, indicating that they exhibited n-type conduction. In contrast, the thermopowers of N-doped SWCNTs synthesized at temperatures greater than 400 °C were positive as with those of hc-SWCNTs. As the reaction temperature with ammonia gas increases, the concentration of graphitic-type nitrogen atoms gradually decreases and the pyridinic- and pyrrolic-type nitrogen atoms become more predominant. Thus, the graphitic-type nitrogen sites act as electron donors.

Organic semiconducting materials have a wide-variety of applications, including organic photovoltaics, but while there are many p-type materials in the literature, there are far fewer n-type materials reported. This research aimed to investigate derivatives of the textile dye 4,10-dibromoanthanthrone for use as the basis for n-type materials, due to its large conjugated system, absorptivity, ease of functionalization, and low cost. Conjugated peripheries were attached with the intention of tuning the absorbance, photoluminescence, orbital distributions, and energy levels to achieve optimal n-type character. Ultraviolet-visible spectroscopy and cyclic voltammetry were used to characterize the absorptivity, bandgaps, and orbital energy levels of the derivatives. All the derivatives have a very similar LUMO level, due to the large contribution from the common anthanthrone core, and the electron-donating or withdrawing character of attached units showed control over the HOMO energy levels. The geometry of the attachment point was also found to influence their electronic properties, giving an additional way to tune these systems.
Further tests are in progress to determine carrier mobility and photovoltaic efficiencies of the synthesized derivatives. If the electron mobilities are well matched with PCBM, these materials possess the solubility, synthetic ease, and absorptivity to be more feasible on a larger scale.

Organo-metal halide perovskite has good potential to be used as a light harvester in the solar cell due to its direct band gap, ambipolar transport, large absorption coefficients and high carrier mobility[1]. Both solution-processed planar and mesoporous perovskite solar cells are prone to hysteresis and current instabilities. Here, we firstly report the surface modification of scaffold layer by using metal oxide clusters. The metal oxide clusters decrease PbI₃^- crystallographic defects by passivating the iodide-rich trap sites on the surfaces during the perovskite self-assembly like PCBM[2,3]. Metal oxide clsuters used in this work can be also introduced to be electron acceptor materials showing tunable structural properties, favorable electron transporting properties, attractive optical properties in terms of high transparency in the visible, nanostructured morphology. Those advantages of metal oxide clusters make them promising materials in such studies. We observed the improvement of current stability and the loss of hysterises depending on the surface modification of scaffold layer.

Polycrystalline pentacene thin films afford a model semiconductor layer of organic thin-film transistors (OTFTs). Though enormous studies have been dedicated to investigate the charge carrier dynamics in pentacene OTFTs, whole understanding is still challenging, because of the nonuniform multi-domain structures composed of uniaxially-oriented dendritic microcrystal grains with a few μm in size. Here we report spatial distribution of accumulated charge carrier density in polycrystalline pentacene thin films as visualized by high-spatial resolution gate-modulation (GM) imaging technique.
We used polycrystalline pentacene thin films fabricated by vacuum evaporation technique. The prepared pentacene thin films were composed of pieces of microcrystal grains with the dimension of 10 µm. The high-resolution GM imaging measurements were conducted by using objective lens with high magnification power and high numerical apertures. We successfully obtained spatial distribution of charge carrier density at resolution less than 1 mm. The obtained images demonstrated inhomogeneous carrier distribution in the pentacene polycrystalline films: We observed regions showing small GM signal at around the grain boundaries, which indicates that the carrier density should be quite low due to the potential barriers against inter-grain carrier conduction. In contrast, regions showing high GM signals are observed within the microcrystal grains. Surprisingly, these regions can be divided into the two types; one is the region showing positive GM signal, while the other showing negative GM signal. We discuss the origin of these differences in terms of the spectral analyses of the obtained GM images acquired at different wavelengths, which could give a clue to understand the impact of local carrier conduction within and between the microcrystal grains in the device performance of polycrystalline pentacene TFTs.
[1] J. Tsutsumi, S. Matsuoka, T. Yamada, and T. Hasegawa, Org. Electron. 25 (2015).

Daekyoung Yoo¹, Younggul Song¹, Jingon Jang¹, Youngrok Kim¹,
Seok-Heon Jung², Jin-Kyun Lee², Takhee Lee<sup>1

1</sup>Department of Physics and Astronomy, Seoul National University, Seoul 08826, Korea
²Department of Polymer Science and Engineering, Inha University, Incheon 22212, Korea
*E-mail : [email protected]



Recently organic electronics has attracted great attention due to its material variety, easy and low cost fabrication, flexibility, and spin-coating process. In particular, organic non-volatile memory (ONVM) has been spotlighted for data-storage applications. Most ONVM devices so far have been fabricated with very large cell size of ~100 µm. The reason for this limitation is that the standard chemicals and solvents for conventional photolithography that are necessary to produce microscale patterns damage the existing organic layers during the photolithographic development and lift-off process. In this presentation, I will report microscale three-dimensionally integrated ONVM devices with a 7 µm × 7 µm memory cell size in a 32 × 32 crossbar structure on a silicon substrate by orthogonal photolithography using highly fluorinated photoresist and development solvent. Because highly fluorinated photoresist material has orthogonal property to the most organic polymers, there is no damage for organic memory layers during orthogonal photolithography [1, 2]. Our three-dimensionally integrated microscale ONVM devices showed promising memory performance with good endurance cycle and stability with long retention time over 10⁴ s for the vertically stacked layers [3]. And, the operative memory cells in each layer showed stable and uniform electric characteristics. Our research suggests that the realization of three-dimensional integrated microscale ONVM may enable the production of functioning organic electronic devices in practical configurations such as flexible organic devices with high cell density.

[1] J. K. Lee, M. Chatzchristidi, A. A. Zakhidov, P. G. Taylor, J. A. Defranco, H. S. Hwang, H. H. Fong, A. B. Holmes, G. G. Malliaras, C. K. Ober, J. Am. Chem. Soc. 130, 11564 (2008).
[2] Y. Song, J. Jang, D. Yoo, S.-H. Jung, S. Hong, J.-K. Lee, T. Lee, Org. Electron. 17, 192 (2015).
[3] D. Yoo, Y. Song, J. Jang, W.-T. Hwang, S.-H. Jung, S. Hong, J.-K. Lee, and T. Lee, Org. Electron. 21, 198 (2015).

Electrical characteristics of printed OTFT(Organic Thin Film Transistor) include the on-off ratio and the field effect mobility. They are affected by various factors and one of the major ones is surface roughness of the gate dielectric layer. This paper presents a treatment process called calendering that was applied to the surface of gate dielectric layer for reducing surface roughness. Calendering has been used to improve smoothness of substrates by using a heating roller and a nip roller. During a calendering process, a substrate moving through the rollers becomes subject to the heat and the nip pressure.
Bottom-gate, bottom-contact, TIPS-pentacene OTFT samples were fabricated on PET substrates. Gravure printing (first gate electrode and second gate dielectric layer), inkjet printing (third source/drain electrode layer), and pipetting(fourth active layer) were used for fabrication. As for materials, silver ink(Paru Inc.) was used for printing the gate and source/drain electrode layers and BaTiO₃(Paru Inc.) for the gate dielectric layer and TIPS-pentacene for the active layer were used.
In this study, calendering process parameters include three factors(temperature, speed, pressure) with three levels(high, middle, low). The levels of temperature are 100 °C, 80 °C, and 60 °C. For the speed, they are 6 m/min, 4 m/min, and 2 m/min. The levels of pressure are 123 kN/m, 82 kN/m, 41 kN/m. Taguchi method was used to reduce experimental sets from 3³ (27 sets) to nine sets and optimize the calendering process parameters.
As a result of Taguchi method, an optimal set of calendering process parameters has been obtained: 100 °C, 4 m/min, 123 kN/m. Both temperature and pressure contribute equally, 35% each, and speed contributes 30% to the electrical characteristics of printed OTFT. The on-off ratio and the field effect mobility of the samples from the optimal parameters are 11000 and 0.008 cm²/Vs, respectively. This shows five times improvement in the on-off ratio and 8% improvement in the field effect mobility, compared to non-calendered samples.

Substrates can significantly affect the electronic properties of organic semiconductors. Charge transfer to/from the substrate can lead to the formation of an interfacial doped region with different electronic properties from the bulk. Recently, we discovered that high work function MoO_x hole transport layer (HTL) could induce p-doping in the active layer and decrease the photocurrent of organic photovoltaic (OPV) devices.¹ This finding directly contradicts the current belief that high work function HTL improves the OPV performance by increasing the built-in field. Here we report how electron transport layer (ETL) modify doping in the OPV active layer and the resulting device behaviors. Different doping type and concentration are observed in P3HT:PCBM or P3HT:ICBA devices made on ETLs of ZnO or polyethylenimine (PEI). This phenomenon is ascribed to the interplay between the ETL-induced n-type doping and the p-type nature of P3HT donor. The amount of ETL-induced n-type doping depends on the relative difference between the work function of ETL and the negative integer charge transfer state (E_ICT^-) of the acceptor. Excessive n-type doping from low work function ETL is shown to be deleterious to OPV device performance. The device behaviors are understood by combined experimental and modeling investigations, including capacitance-voltage (C-V) measurement, Hall effect measurement, light-biased external quantum efficiency (EQE) measurement, transfer matrix method (TMM) simulation, and drift-diffusion simulation. These results establish a criterion to select ETL according to the electronic properties of OPV active layer, and shed light on ways to optimize other organic electronic devices.
Reference:
1. Nano Lett. 2015, DOI: 10.1021/acs.nanolett.5b03473.
This project is sponsored by National Science Foundation DMR-1305893.

Pentacene has emerged as benchmark organic molecular semiconductors because of its relatively high carrier mobility and a typical surface morphology. Charge transport properties of Pentacene have been investigated by a joint experimental and theoretical study. The growth of the Pentacene thin film on the substrate shows mainly two different polymorphic phases, bulk phase and thin film phase. Thin film phase is crucial for the charge transport in two terminal and three terminal devices such organic Schottky diodes and organic thin film transistors (OFETs), respectively. Under optimized growth conditions, Pentacene thin films show typical diffusion limited aggregation (DLA) plus mound growth and devices based on Pentacene thin film grown under these conditions exhibit higher mobility.
However, intermolecular disorders present in thin films limit the charge transport in two terminal and three terminal devices. Intrinsic charge transport properties of Pentacene in thin film phase have been studied using first principle density functional theory (DFT) calculations. In the limit of strong electron–phonon coupling, the charge transport in organic semiconductors can be considered as an intermolecular hopping process. In theoretical framework, the charge transfer in adjacent molecular dimmers can be modelled as a self-exchange reaction process and charge transfer rate is determined by two microscopic parameters, transfer integral and reorganization energy. Reorganization energy describes the strength of hole (electron) vibration coupling of a charge localized on a single molecule and depends on its chemical structure whereas transfer integral depends on the overlapping between the molecular orbitals of two adjacent molecules hence on the molecular packing in the crystal structure. From XRD study, it is known that when Pentacene is deposited on flat substrate, Pentacene molecules stand almost vertically on the substrate, arranged in herringbone manner in ab-plane. Intrinsic mobilities of Pentacene in different planes have been calculated using DFT calculations. In two terminal devices, charge transport occurs along c-axis and mobility calculated in this direction has been found to be 4.45 x 10^-5 cm²/Vs. However, charge carrier mobility of ~2cm²/Vs has been obtained along herringbone direction in ab-plane which is relevant for charge transport in three terminal devices. Theoretically calculated mobilities are comparable with experimental values of mobility of 1.3 x 10^-5 cm²/Vs and 0.3cm²/Vs in two and three terminal devices, respectively. Mobility in two terminal devices is lower by four orders of magnitude than in three terminal devices due to the strong electronic couplings between molecular dimers located in ab- plane and relatively weak coupling between the layers (c axis).

Polymer solar cells are promising alternatives for conventional energy sources. Due to the lower cost of manufacturing, easy processing and flexibility of polymers, solar cells made of organic materials may compete with current inorganic solar cells. In the present article we have characterized Dye:PCBM bulk heterojunction solar cells. A synthesized dye D (2,2'-[1,4-henylenebis(azo)]bis-1Hpyrrole) of low band gap characterized. It was soluble in solvents like tetrahydrofuran THF).Using D and PCBM we prepared blend solution. We have prepared as cast as well blend films on ITO plates using spin coating unit. Blend films were annealed at 373K for 10 minutes. As cast, blend and annealed films were characterized using Keithley electrometer, Uv-Vis spectrophotometer and Atomic force microscopy.

Keywords: Bulk heterojunction solar cells; Dye; I-V characteristics, Uv-Vis; AFM.

This article explores how the widely used molecular dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) affects the doping and morphology of the self-doped organic hole transport layer (HTL) poly(thiophene-3-[2-(2-methoxy-ethoxy)ethoxy]-2,5-diyl) (S-P3MEET). It is found that the molecular dopant additive competes for doping sites with the covalently attached dopants on the polymer. This competition results in large changes in the morphology and doping level of the polymer with little change in the conductivity. Calorimetry measurements were performed to reveal that the F4TCNQ interacts strongly with the side chains of the S-P3MEET, increasing the melting temperature of the side chains by 30 °C with 5 wt% dopant loading. The thermal stability of the doping is also investigated to determine whether doping with F4TCNQ is feasible in a device. Experiments with poly-3-hexylthiophene (P3HT) as the fluorescent reporter layer show that F4TCNQ binds much stronger in S-P3MEET than P3HT and only little F4TCNQ is found in the P3HT layer after annealing. In combination with reflectometry measurements, we show that F4TCNQ binds in S-P3MEET with annealing to 150 °C even though the sublimation temperature for neat F4TCNQ is 80 °C. In contrast, F4TCNQ is shown to slowly diffuse out of P3HT at room temperature. We attribute this difference in binding with the F4TCNQ molecules to the ability of the polar ethyl-oxy side chains of the S-P3MEET to orient around the charged dopant molecule and thereby to stabilize its position. This study suggests that polar side chains are a design element in organic electronic materials that can be used to increase the thermal stability of molecular dopants.

It is generally understood that the widely used p-type dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) has low solubility in common solvents, which restricts further development of solution-processed and mass-produced organic electronics applications. In this work, we introduce solubility control into molecular dopants based on the F4TCNQ structure by replacing cyano groups with alkyl esters. UV-vis-NIR and cyclic voltammetry measurements show increased solubility of mono- and di-ester substituted dopants with only modest changes to acceptor strength. With the use of UV-vis-NIR, photoluminescence, and conductivity measurements, we demonstrate that the new dopants are able to successfully p-dope poly(3-hexylthiophene-2,5-diyl) (P3HT). Mono-ester substituted dopants show only slightly reduced electron affinity (EA) relative to F4TCNQ, but greater doping effectiveness due to increased miscibility with P3HT. In contrast, di-ester substituted dopants undergo a dimerization reaction before assuming their doped states, which may help anchor dopants into position post deposition, thus decreasing the negative effect of dopant diffusion and drift. We conclude that increased dopant solubility/miscibility not only simplifies film fabrication but also increases the effectiveness of doping in solution-cast polymer films. The ester modification is an effective synthetic route to achieve solubility/miscibility control in TCNQ dopants.

Solution-processable π-conjugated small molecule donors and acceptors have seen their efficiency characteristics improving rapidly in bulk-heterojunction (BHJ) solar cells over the past recent years. One of the most successful approaches relies on the use of π-delocalized sequences of electron-donor and -acceptor motifs that can mimic the wide-range visible absorption spectra of π-conjugated polymers, while adopting particularly high molecular ordering patterns and optimum morphologies in BHJ thin-films. Combining optimizations of (i) π-conjugated sequences,[1] side-chain substituents,[2] functional end groups,[3] and (ii) solution-processing and post-deposition conditions, including the use of processing additives[4] and thermal or solvent annealing steps,[5] the power conversion efficiencies (PCEs) of small molecule donors in BHJs with fullerene acceptors has quickly improved from ca. 4%[6] to >9%[7]. Likewise, some recently reported π-extended small molecule acceptors are showing to be promising systems in combination with polymer donors, with PCEs that have rapidly increased to >8%[8]. At this point in time, the efficiency of small molecules and polymers are comparable in BHJ solar cells, and yet the morphologies, phase composition and the texture of small molecule and polymer thin films are uniquely different – differences that are inherent to the well-defined, finite molecular structure of small molecules. Meanwhile, specific end group modifications[3] and covalent connections[9] between small molecules can direct their self-assembly and induce distinct, improved morphological and efficiency patterns in BHJ solar cells. Our recent developments emphasize some key design principles that can significantly impact the film-forming properties, morphologies and charge transport properties of π-extended small molecules in thin film devices.[10]

[1] P. Bäuerle et al. Angew. Chem. Int. Ed. 2012, 51, 2020; J. Roncali et al. Adv. Mater. 2014, 26, 3821; T.-Q. Nguyen et al. Chem. Mater. 2011, 23, 470.
[2] P.M. Beaujuge et al. 2015, Submitted
[3] P.M. Beaujuge, J.M.J. Fréchet et al. Adv. Mater. 2011, 23, 5359.
[4] A.J. Heeger et al. Nat. Mater. 2012, 11, 44.
[5] Y. Cao et al. Adv. Funct. Mater. 2015, 25, 3514; Y. Chen et al. J. Am. Chem. Soc. 2014, 136, 15529.
[6] T.-Q. Nguyen et al. Adv. Funct. Mater. 2009, 19, 3063.
[7] D.J. Jones et al. Nat. Commun. 2015, 6, 6013; Y. Chen et al., JACS 2015, 137, 3886.
[8] H. Yan et al. Energy Environ. Sci. 2015, 8, 520; Z. Wang et al. J. Am. Chem. Soc. 2015, 137, 11156; C. Nuckolls et al. Nat. Commun. 2015, 6, 8242.
[9] W. Li et al. Chem. Commun. 2014, 50, 7720; K. Sivula Adv. Mater. 2015, 27, 5541
[10] P. Wucher, P.M. Beaujuge et al. 2015, Submitted

Molecular doping has been hypothesized to fill trap states and improve charge carrier mobility in organic bulk heterojunction (BHJ) solar cells [1–3]. However, the addition of dopant ions and excess charges to the film can also create additional trap states and increase recombination. Here, we analyze these effects in two model systems and investigate their impact on device performance. We study both p-doped and n-doped PCDTBT:PC71BM and P3HT:PC61BM solar cells using F4TCNQ and benzimidazoline-radical dimer( (DMBI)2) respectively.

We find that p-type doping slightly increased the power conversion efficiency (PCE) from 5.87% to 6.30% for PCDTBT cells, and from 3.68% to 4.03% for P3HT cells. However, in the majority of doped organic BHJ films, charge carrier mobility and lifetime decrease while the number of trap states increases. We measure the electron and hole mobility separately using metal-insulator-semiconductor charge extraction by linearly increasing voltage (MIS-CELIV) and demonstrate that at low doping ratios the hole mobility initially drops for F4TCNQ-doped polymers in BHJ blend films. At higher doping concentrations, the mobility only increases at the cost of lower shunt resistance in the cell. A similar trend is observed for (DMBI)2 doped fullerenes. In both types of doping, the minority carrier mobility drops at higher dopant concentrations. These findings agree well with models by Arkhipov and Neher [4,5], and Coulombic traps formed by dopant ions are likely responsible for this non-monotonic effect of doping on mobility.

To further examine the changes in trap states, we performed transient photocurrent and charge extraction measurements. PCDTBT:PC71BM and P3HT:PC61BM turn out to behave oppositely. The former has an increased number of trap states while in the latter trap states are filled by dopant-induced charges. We conclude that in order to achieve positive trap-filling outcomes the dopant must dope efficiently and cause minimum morphology change. Caution must be taken when examining the effects of solution-processed doping in thin organic solar cells.

[1] Y. Zhang et al., Adv. Mater. 25, 7038 (2013).

[2] Y. Xiao et al., ACS Appl. Mater. Interfaces 7, 13415 (2015).

[3] F. Deschler et al., Phys. Rev. Lett. 107, 127402 (2011).

[4] V. Arkhipov et al., Phys. Rev. B 72, 235202 (2005).

[5] P. Pingel and D. Neher, Phys. Rev. B 87, 115209 (2013).

Molecular doping of solution processed organic semiconductors is routinely applied in various devices such as organic light emitting diodes (OLEDs). Nevertheless, numerous recent publications reveal new insights on doping mechanisms¹ and charge transfer², emphasizing the need for further investigations. In this work, the impact of structural order in p-type doping of poly(3-hexylthiophene) (P3HT) with 2,3,5,6- tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is studied. We show that for doped films formed from the low-boiling-point solvent chloroform (CF), the conductivity is significantly higher than for layers from high-boiling-point chlorobenzene (CB). To understand this phenomenon, we perform electron diffraction studies with a transmission electron microscope (TEM) to obtain structural information. Infrared (IR) spectroscopy and UV-Vis absorption spectroscopy enable us to connect our findings from TEM to the electronic properties of the doped material.
Surprisingly, P3HT-films from CF exhibit a strong increase in order upon doping, especially in terms of π-π-stacking. On the other hand, this stacking direction does not alter in a comparable manner in doped films from CB. Changes in characteristic stacking distances give evidence that the degree of intercalation of F4TCNQ in P3HT crystallites also varies between films from the two solvents. In addition, our results show that layers from CB in contrast to layers from CF incorporate a noticeable amount of pure P3HT crystallites. Optical spectroscopy reveals stronger and red-shifted polaron absorption for films from CF. This observation matches the higher order compared to films from CB, which is additionally supported by analysis of vibronic features of P3HT from UV-Vis spectra. Furthermore, IR absorption features from the charged dopant molecules also show different signatures. The combination of these methods leads to the conclusion of a varying distribution of the dopant F4TCNQ throughout the film depending on the solvent in use.
Our findings imply that the structural order is already predefined in solution. As literature suggests, dopant concentration³ and temperature¹ of the solution affect the molecular order. We extend this view to the effects of solvent characteristics such as polarity, studying the examples of chlorobenzene and chloroform. Finally, we propose a model to link features observed in UV-Vis spectra measured on P3HT:F4TCNQ in solution to our findings for films from both solvents.

References
¹Wang et al., Physical Review B, 2015, 91(8), 085205
²Méndez et al., Nature Communications, 2015, 6, 8560
³Duong et al., Organic Electronics, 2013, 14(5), 1330–1336

Polystyrene-random-poly(pentafluorostyrene) pStPFS copolymer series were synthesized using a radical polymerization, and then spun-cast on a SiO₂ gate dielectric for pentacene semiconductor. Surface energies (γ) of the fluorinated polymers decreased from 46.9 to 24 mJ m^-2 with an increase in the PFS mole%. As the surface energy differences between pentacene (with a (001) crystal plane of 38 mJ m^-2) and the polymers became larger, small-sized pentacene grains were dominant on the polymer-treated dielectrics, instead of terrace-like crystals. Organic field-effect transistors (OFETs) based on the resulting pentacene films showed large variations in field-effect mobility (µ_FET) from 0.82 to 0.28 cm² V^-1 s^-1. Additionally, a density optimization of the fluorinated moieties in the copolymer could effectively improve gate bias stress stability of these OFETs on the polymer-coated SiO₂ surfaces during a long-term operation, without any severe degradation of μ_FET. The introduction of 25 mole% PFS-loaded fluorinated polymer (with γ of 34.7 mJ m^-2) onto a oxide dielectric could induce charge-transport favorable grains of pentacene, and minimize the affinity (or trapping) of injected charges in operating OFETs, in comparison to the extensively fluorinated polymers.

A high molecular-weight donor-acceptor (D-A) copolymer, pDfTBT-4T, poly(5,8-di[3-octyldodecanthiopen-5-yl]-dithieno[3',2':3,4:2",3":5,6]benzo[1,2-c][1,2,5]-thiadiazole)-alt-2,2'-bithiophene) including alternating dithieno[3',2':3,4;2",3":5,6] benzo[1,2-c][1,2,5]thiadiazole (DTfBT) and 3-octyldodecyl tetrathiophene (4T) was synthesized by a Stille coupling polymerization. We found that pDTfBT-4T had a molecular number average weight of 176 kgmol^-1 and formed unexpectedly strong interchain aggregates in dilute solutions at room temperature, which was similar to those in as-spun thin solid films. pDTfBT-4T thin films were spun-cast from a warm dilute chlorobenzene solution on hydrophobic polymer-treated SiO₂ dielectrics. Some were further annealed at various temperatures (T) for 10 min to enhance π-overlapped structures as charge-carrier transport paths for use in organic field-effect transistors (OFETs). The resulting ordered domains in the annealed films were tuned from nano-rods to -sheets with an increasing in T, which also provided enhanced crystal orientation. Because of the DTfBT-driven co-planarity of the polymer backbone and strong intermolecular interactions between the D-A copolymer, pDTfBT-4T based OFETs yielded a hole mobility up to 1.45 cm²V^-1s^-1 as well as negligible hysteresis and excellent negative bias stability.

Polymeric semiconductors inherently have many advantages for applications in next-generation flexible electronics such as mechanical flexibility, solution processibility, low-cost, large area fabrication. Although many polymeric semiconductors show their high electrical and mechanical properties, their critical drawback is poor stability under various environmental conditions. In general, polymeric semiconductors can be easily damaged by various chemicals, which significantly limits their applications in practical applications. In this work, we utilized a high-performance polymer semiconductor with transparent graphene electrodes for the fabrication of transparent organic field-effect transistors (OFETs) showing ambipolar electrical properties and high robustness against various chemicals. The OFET devices showed both high-performance and superior stability upon exposure to various organic solvents. The prepared OFETs were applied to FET-type sensors for detecting chemical vapors. In addition, a transparent 10 × 10 FET sensor array device was fabricated, which showed uniform electrical properties and high photoresponsivity. Our approach provides a viable way for the fabrication of flexible and transparent FET-type sensors using polymeric semiconductors.

UV/vis spectroscopy is an important tool to characterize photoactive molecules for the use in organic light emitting diodes and photovoltaics. The shape and asymmetry of absorption or emission bands assigned to individual electronic transitions are mostly determined by vibrational progression. In cases where just a few vibrational modes are coupled to electronic transitions vibrational satellites might possess considerable intensities and need to be considered for correct assignment of electronic transitions.[1] This discrimination of electronic transitions and vibrational satellites on the basis of experimental data might be very laborious but can be facilitated significantly by ab initio modelling of the vibrational progression.
In this work, we employ different models for the description of vibrational progression to three model systems. For a Ru-polypyridyl complex, we broadened the TDDFT stick spectra based on the Huang-Rhys factors, which were obtained by a fit of experimental data. This rather inexpensive semi-empirical method[2] involves experimental data and can be expanded to treat aggregate effects – the latter is exemplified in a study on a polymer film. In a third example to be discussed in this contribution, we employed the nuclear ensemble method to get a detailed assignment of the spectral features of a tetrapyrrole beyond the Franck-Condon approximation. The three case-studies allow for assessing the applicability of the different approaches to account for vibrational progression or electron phonon coupling in given research questions.
JP and BD want to thank the Nagelschneider Foundation for financial support.

1. Beenken, W., et al., Molecular Structures and Absorption Spectra Assignment of Corrole NH Tautomers. The Journal of Physical Chemistry A, 2014. 118(5): p. 862-871.
2. Preiss, J., et al., How Does Peripheral Functionalization of Ruthenium(II)–Terpyridine Complexes Affect Spatial Charge Redistribution after Photoexcitation at the Franck–Condon Point? ChemPhysChem, 2015. 16(7): p. 1395-1404.

We fabricated the vertical Schottky barrier (SB) transistors based on the graphene–organic semiconductor (p–type pentacene or n–type N,N′–dioctyl–3,4,9,10–perylenedicarboximide (PTCDI–C₈)) heterostructures. The Schottky barrier formed at the graphene–organic semiconductor heterojunction could be modulated by the application of the gate voltage. In order to investigate the graphene doping on the electrical properties of the vertical SB transistors, we introduced a novel backside–doping method. This doping method enabled the effective tuning of the work function of the graphene while maintaining the surface properties of the graphene electrodes, and thus resulted in the enhanced crystalline structures of the overlying organic semiconductors compared with typical topside–doping method. The graphene doping affected strongly the charge injection at the heterojunction and resulting transistor performances including the device on current density and on–off current ratio. The p− and n−type devices based on the optimized backside−doped graphene electrodes yielded a high current density of 20mAcm^–2 and a high on–off current ratio of ~10³. Furthermore, the complementary inverters were successfully fabricated by assembling p−type and n−type vertical SB transistors. Both proposed backside doping method and systematic investigation of the doping effect open up new opportunities for realizing future organic electronics.

We report on the application of gate-modulation (GM) imaging technique in rapid and collective inspection of organic thin-film transistor (OTFT) array operations.^[1] The method allows visualizing charge carriers accumulated in the OTFT array by time-translational differential image sensing with the use of a charge coupled device (CCD) sensor. The feature makes it possible to visualize the dead pixels, broken channels, or distributed device performance in the OTFT array.
A possible future role of the OTFTs in so-called "ubiquitous electronics" is that any plastic surfaces are decorated electronically by arraying a large number of OTFTs (10⁶-10⁷) to function as active backplanes for displays or sensors. This feature now raises the challenge of developing a rapid and collective inspection technique for OTFT arrays composed of such large numbers of devices, as the inspection may take an extremely long time if conventional electrical measurements are used. The issue is particularly important for solution-processed OTFTs that require arduous process optimization. Charge-modulation spectroscopy (CMS) is known as a technique to optically detect accumulated charge carriers in OTFTs.^[2] This method involves observing slight changes in the optical absorption spectrum due to the charge accumulation by the lock-in technique with applying an AC gate bias. The obtained CMS spectra can provide microscopic information on the carrier states. However, the technique has not yet been established for the collective inspection of OTFT arrays to find dead pixels or to investigate distributed device characteristics.
We here developed GM imaging technique in which a CCD area image sensor is introduced into CMS, thus adopting the lock-in technique for each pixel detection.^[1] This technique enabled two-dimensional spatial mapping of charge carriers accumulated in an OTFT within a realistic time scale of about 10 min. The method could be a new powerful tool for collective inspection of OTFT arrays to find dead pixels or to investigate distributed device characteristics. We successfully demonstrated the GM imaging of time-developing and equilibrium charge carriers by time-differential image sensing with the use of a CCD area sensor. We show that it is possible to visualize the dead pixels or broken channels by the observation of the equilibrium charges. Additionally, the serial images for the time-developing charge images allow correlating the response time of the OTFTs with the field-effect mobility, thus enabling inspection of the distributed device performance of OTFT arrays. These unique advantages of GM imaging techniques should be quite useful for the rapid and collective inspection of OTFT arrays composed of large numbers of pixels.

References: [1] J. Tsutsumi, S. Matsuoka, T. Yamada, and T. Hasegawa, Org. Electron. 25, 289 (2015). [2] P. J. Brown, H. Sirringhaus, M. Harrison, M. Shkunov, R. H. Friend, Phys. Rev. B 63, 125204 (2001).

Over past two decades, interest in organic electronics, organic field effect transistor (OFETs) and organic solar cells (OSCs), has increased due to their cheap, light-weight, and mechanical flexibility compared to inorganic counterparts. To improve organic electronic device performance, research on new materials for semiconducting layers has been required. Among others, conjugated polymers have good film formability and compared to conjugated small molecules and oligomers. In order to improve optical and electronical properties of conjugated polymer, push-pusll structure was employed with alternating structure of electron donating units (D) and accepting units (A) in conjugated backbone. Through this concept, energy level and optical bandgap of conjugated polymers were easily tuned by changing D-A units. In this study, (E)-1,2-bis(3-alkylthiophen-2-yl)ethane, simply thienylene vinylene (TV), was used as donating units which is one of the promising molecular moiety for organic electronics due to its rigid and planar moiety. Vinyl groups between thiophenes make the conjugated backbone highly coplanar as suppressing steric hinderance caused from alkyl substituent on 3-position of thiophene. Polythienylevinylene (PTV) derivatives showed good charge carrier mobilities in OTFTs due to their tendency to form crystalline structures for efficient charge carrier transport. In some cases, despite substitution of alkyl side chains on thiophene, PTV derivatives seem to have limited solubility in common processing solvent. Thus, alkoxy-substitued benzooxadiazole (BO) and benzothiadiazole (BT) moieties were used as acceptor units not only for reducing optical bandgap but also for maintaining good solution processability. In this presentation, four D-A type conjugated polymers, BO8, BT8, TBO12 and TBT12, were designed and synthesized by Stille polymerization. These polymers showed ambipolar charge carrier transporting characterstics, and among others, TBT12 exhibited 0.77 cm²/Vs for hole mobility and 0.01 cm²/Vs for hole mobility. Photovoltaic cells based on TBO12:PC₇₁BM showed an open-circuit voltage of 0.58 V, a power conversion efficiency of 4.7% and a short circuit current of 11.4 mA/cm².

Replacing phosphorescent materials by efficient Thermally Activated Delayed Fluorescence (TADF) materials will eventually lower cost with the potential of clearing stability issues. Especially, blue dyes among the three primary colors have been considered as the most crucial ones due to their important role in generating white light with good color purity. TADF materials have the donor-connection unit-acceptor structure to have small overlap between the HOMO and the LUMO to minimize the singlet-triplet energy gap. However, it seems quite challenging to search for an appropriate combination of donor and acceptor moieties to achieve good color purity and high EL efficiency for blue fluorescent OLEDs at the same time. Carbazole and dimethyl dihydroacridine have been mainly used as the donor moieties and sulfone oxide and triazine as the acceptor unit in the blue TADF molecules. Even a highly efficient blue fluorescent OLED of 20.6% was reported recently yet the OLED displayed sky-blue light with the CIE coordinates of (0.19, 0.35).
In this article, an efficient blue fluorescent OLED with an unprecedented high EQE of 22.3% was fabricated by doping DTPDDA in a mixed co-host system. The device emitted deep blue with the CIE coordinates of (0.149, 0.197). This is the first report demonstrating the both deep blue emission of y