Symposium EP01—Liquid Crystalline Properties, Self-Assembly and Molecular Order in Organic Semiconductors

The molecular structure of the backbone of semiconducting polymers determines the electronic levels that are relevant for organic electronic devices. Sidechains are added to the backbone to enable solution-processing by improving solubility. While the sidechains are relatively isolated from the electronic structure of the backbone, they play a significant role in the final morphology and molecular packing in thin films of semiconducting polymers. We have examined how sidechains control the resulting microstructure in thin films of a series of high performance co-polymers. Using a combination of high resolution transmission electron microscopy (HR-TEM) and grazing incidence wide angle X-ray scattering (GIWAXS) we have determined connections between sidechain structures and the domain boundaries in ordered donor-acceptor polymers. HR-TEM reveals that the aspect ratio of the crystallographic repeat unit of the backbone and sidechain length helps to determine the possible structure of domain boundaries. We have also studied conjugated polymers with unique macrocyclic sidechains that reveal the photophysical behavior of conjugated backbones without the interference of aggregation. These investigations reveal that sidechains should be considered from the outset of the design of new semiconducting polymers.

The presence of unknown polymorphs within thin films is a frequently observed phenomena for organic electronic molecules. These polymorphs can have considerably different optical and electronic properties which influences the device performance in organic electronics and optoelectronics considerably. A first step of understanding the origin of substrate induced phases is the determination of the molecular packing. A combination of grazing incidence x-ray diffraction with molecular dynamics simulation is used for crystal structure solution. Recently, also single crystal diffraction is used for crystal structure solution of substrate induced phases. We selected a number of different molecules and demonstrate fundamental properties of the crystalline structure in relation to the bulk crystal structure. It is shown that confirmation of the molecular packing with the substrate surface is an important parameter, additionally the different polymorphs can show fundamental different packing motifs like herringbone packing and stacking of aromatic rings. In a subsequent step the importance of the crystal growth kinetics is presented which causes the substrate induced polymorph and finally the thermodynamic stability of these phases. The selected examples are molecules which forms van der Waals crystals like pentacene [1], benzothieno-benzthiophene based molecules [2] and oligothiophenes [3] but also examples from molecules with hydrogen bonds are discussed like dibromo-indigo [4] and dicyano-distyrylbenzene [5].

[1] S. Pachmajer et al., Self-Limited Growth in Pentacene Thin Films. ACS Appl. Mater. Interf. 9, 11977 (2017)
[2] R. Resel, et al. Polymorphism of terthiophene with surface confinement. IUCrJ 5, 304 (2018)
[3] N. Bedoya-Martínez, et al., DFT-assisted polymorph identification from lattice Raman fingerprinting. J. Phys. Chem. Lett. 8, 3690 (2017)
[4] M. Truger, et al., Surface-Induced Phase of Tyrian Purple (6,6′-Dibromoindigo): Thin Film Formation and Stability. Crystal Growth Design 16, 3647 (2016)
[5] S. Varghese, et al., Polymorphism and amplified spontaneous emission in a dicyano-distyrylbenzene derivative with multiple trifluoromethyl substituents: intermolecular interactions in play. Adv. Funct. Mater. 26, 2349 (2016)

Solution-processed organic thin-film transistors (OTFTs) have received enormous interest due to their great potential for realizing low-cost, lightweight and flexible electronics applications, such as displays, wearable sensors and electronic skins. Although several novel solution-processing techniques for achieving highly-crystalline organic semiconductor (OSC) films have been introduced to OTFTs, there still remains a challenging work to control thin-film properties, such as crystallinity, morphology and uniformity accompanying the rapid coating speed demanded for industrial-scale production. Here, I will present a new solution coating method that is compatible with a fast-moving roll-to-roll system by using a gap-controlled wire-bar and a bar-motion programming strategy. The developed method is highly practical and industry-friendly as well as feasible to fabricate high-quality OSC thin films in a large area at a high rate. In this method, we found that blending of high-crystalline semiconductor small molecules and an amorphous semiconducting polymer additive in an active ink could significantly improve the crystallinity of the small molecular semiconducting layer with the aid of efficient vertical phase separation during the process. Using the motion programmed bar-coated blend OSC thin films, we demonstrated large-area flexible single-crystal OTFT arrays with a superior charge carrier mobility, high uniformity, and high reproducibility. A crystallization and growth mechanism of the bar-coated films and structure-charge-transport properties will be discussed.

Understanding the impact of both the organic semiconductor design and processing conditions, on both molecular conformation and thin film microstructure is essential in achieving the required optical and electrical properties of organic electronic devices. For example, the charge carrier mobility in an organic field effect transistor can be significantly increased through appropriate optimization of the semiconductor molecular conformation and self-assembly. These aspects will be explored with an indacenodithiophene polymer as a case study. It is believed that conformational disorder reduces charge carrier mobility. In order to reduce potential conformational, and subsequently energetic, disorder due to rotation around single bonds along the polymer backbone, an aldol condensation reaction was explored, in which a bisisatin monomer reacts with a bisoxindole monomer to create an isoindigo repeat unit that is fully fused along the polymer backbone. This aldol polymerization requires neither metal containing monomers or transition-metal catalysts, opening up new synthetic possibilities for conjugated aromatic polymer design, particularly where both monomers are electron deficient. The condensation reaction locks the repeat units together with a carbon-carbon double bond link, eliminating free rotation of the repeat units and thus rigidifying the polymer conformation. Polymers with very large electron affinities can be synthesised by this method, resulting in air stable electron transport, demonstrated in solution processed organic thin film transistors. The rigid, planar nature of the backbone also facilitates extended delocalisation of both frontier molecular orbitals and a subsequently low bandgap. We present an electrical, optical and morphology characterisation of polymer thin films, illustrating structure-property relationships for this new class of polymers.

Liquid crystals are very attractive materials as not only display but also organic electronics applications because the nature of liquid crystals is self-organizing nature. In organic electronics applications, it is important that the molecules must be aligned for each device, e.g., lateral current flow on the substrate is required in an organic thin film transistor (OTFT) device. In the material research in OTFTs, a lot of soluble crystalline materials are proposed, however, it is difficult to fabricate uniform thin films and to solve trade-off problem between thermal durability and solubility for crystalline thin films.
In order to solve these problems, we proposed the utilization of the liquid crystallinity in the soluble crystalline OTFT materials on the basis of our previous research for liquid crystals as a crystalline organic semiconductor [1]. Liquid crystalline materials exhibit crystal phase up to 100 ^oC, i.e., liquid crystal phase shows over 100 ^oC. We propose a new fabrication technique for uniform and molecularly flat polycrystalline thin films by utilizing liquid crystalline thin films as a precursor for polycrystalline thin films [2]. In order to improve the thermal durability of the films, we propose to utilize new liquid crystalline materials exhibiting highly ordered liquid crystal phases such as smectic E (SmE) phase, which give film stability in the thermal processes thanks to solid-like nature of the highly ordered liquid crystals [3].
In this presentation, we will show the good processability, high thermal durability of the film up to 200 ^oC, and high mobility over 10 cm²/Vs in polycrystalline thin films of 2-phenyl-7-octyl-beozothienobenzothiophene (Ph-BTBT-10) as representative liquid crystalline materials [4]. Ph-BTBT-10 has highly and low ordered smectic phases of SmE and SmA, respectively. The polycrystalline thin films of Ph-BTBT-10 fabricated by spin-coating at 110 ^oC for SmE phase were very uniform without any crystal flakes and cracks and the surface of polycrystalline thin films existed terrace structure with molecular step of ca. 3 nm that corresponds to a molecular length of Ph-BTBT-10. The polycrystalline thin film of thermal stability is much improved. The thin films unchanged after the thermal stress at 200 ^oC, thanks to the solid-like nature of SmE phase up to 210 ^oC. The FET devices after thermal stress operated, whose mobility was over 2 cm²/Vs. These results show how effective the SmE phase is to improve thermal durability of OTFT in BTBT derivatives. Polycrystalline thin films of Ph-BTBT-10 show high mobility over 10 cm²/Vs after short thermal annealing at 120 ^oC at crystal phase. The polycrystalline thin films changed from monolayer to bilayer crystal structures after the thermal annealing and the utilization of bilayer crystal films via monolayer crystal films is good idea to realize for fabrication of uniform thin film by solution process and high mobility of bilayer crystal structure.
[1] H. Iino, and H. Hanna, Jpn. J. Appl. Phys. 45, L867 (2006).
[2] H. Iino, and H. Hanna, Adv. Mater. 23, 1748-1751 (2011).
[3] H. Iino, T. Kobori, and J. Hanna, Jpn. J. Appl. Phys. 51, 11PD02 (2012).
[4] H. Iino, T. Usui, and J. Hanna, Nat. Commun. 6, 6828 (2015).

Multiple exciton generation (MEG) through singlet fission (SF) is a spin allowed process whereby a singlet excite state is split into two triplet excitons. Inclusion of MEG chromophores into solar cells is raises the maximum theoretical efficiency of a solar cell form the Shockley-Queisser limit of 33% to around 45% by effectively harvesting the energy from high energy photons. SF has been reported and extensively studied in crystalline acenes, and more recently acene dimers to better understand the fundamental photophysics and materials requirements for SF. Incorporation of these SF materials in to functional solar cells, although demonstrating modest efficiency enhancements, have had limited success. In our efforts to produce higher efficiency printed organic solar cells we had the desire to incorporate solution processible SF materials in printed organic solar cells, however most of the reported SF materials are highly crystalline and either do not promote SF in the solid state or controlling crystallisation is difficult. Therefore, we were interested in developing a new range of SF materials that could be solution processed, promoted SF in the solid state, and were compatible with deposition methods used in printed organic solar cells.
We reasoned that materials designed to promote intra-molecular SF [1] would allow us greater control over the SF process. We also thought that a greater control over the pre-organisation of the chromophores would enhance SF yields. Design criteria outlined by Busby et al. [1] suggested an Acceptor-Donor-Acceptor (A-D-A) structure may promote SF, and control of the coupling between the acceptors (triplet hosts), and the donor could modify SF yield.
In our own work we have been examining A-D-A chromophores, such as BTR or BQR [2], as p-type organic semiconductors in organic solar cells. However, calculations indicated that the triplet energy levels in these materials were too high. Of interest to this study was that BTR and BQR have a rich phase space with high temperature nematic liquid crystalline phases. Could we use the self-organisation inherent if the BTR or BQR structure to promote self organisation in new SF A-D-A chromophores? In our preliminary studies we coupled the benzodithiophene (BDT) core in BTR or BQR with thiophene substituted diketopyrrolopyrrole (TDPP), which has a known triplet energy of around 1.0 eV. The new material BDT(TDPP)₂ has a singlet energy level of 1.88 eV and a measured triplet energy of 0.95 eV, which is on the borderline for the energy requirement for SF, that is E(S₁) ≥ 2xE(T₁).[3] We reported that BDT(TDPP)₂ shows modest SF in the solid state, perhaps due to the low singlet energy level.
With BDT(TDPP)₂ there was little evidence of liquid crystallinity or a rich phase space on coupling the TDPP units to the BDT core, and the singlet energy level was low. We therefore needed to redesign our intra-molecular SF host, with i) a singlet energy around 2.0 eV if TDPP was to be our triplet host, ii) stronger self-association through the core, and iii) solution processability. On reflection, we realised we have already described such a material where we had coupled TDPP to a fluorenyl substituted hexabenzocornene core (FHBC) as a p-type organic semiconductor.[4] The discotic liquid crystalline FHBC(TDPP)₂ material forms hexagonally packed columns and has a singlet energy level of 2.00 eV, however we did not report it's triplet energy. We report here our SF studies on FHBC(TDPP)₂ and demonstrate that a triplet yield of 150% in amorphous thin films, increasing to 170% in thermally annealed films.[5]

[1] Busby, E. et al. Nat. Mater. 2015, 14 (4), 426-33.
[2] Geraghty, P. B.; et al. Beilstein J. Org. Chem. 2016, 12, 2298-2314.
[3] Masoomi-Godarzi, S.; et al. Adv. Energy Mater. 2018. DOI: aenm.201801720R2
[4] Wong, W. W. H. et al. J. Mater. Chem. 2012, 22 (39), 21131-21137. DOI: 10.1039/c2jm34394k
[5] Masoomi-Godarzi, S.; et al. manuscript in preperation

Organic semiconductors have attracted increasing attention as low-temperature thermoelectric materials, offering the possibility of fabricating low-cost, large-scale and mechanically flexible thermoelectric modules. The thermal conductivity of organic semiconductors is intrinsically low, leaving the power factor as the most important parameter for optimization. The widely used strategy to optimize power factor, molecular doping, enables an increase in electrical conductivity, but usually at the cost of reducing the Seebeck coefficient. Molecular ordering, which is typically low in a doped organic semiconductor, can improve the electrical conductivity and, hence, the power factor. However, there is no universal strategy to induce a disorder-to-order transition in a doped organic film.

In this contribution, we demonstrate that the molecular order of pristine and doped fullerene derivatives can be greatly improved by a suitable choice of the side chain. We use a fullerene derivative with two oligo ethylene glycol side chains, PTEG-2. Annealing induces a significant ordering in thin films of this material. After cooling back to the room temperature, the formed ordered microstructure of PTEG-2 remains. GIWAXS data show that the molecular order is significantly enhanced after thermal annealing, leading to very favourable micro-structure. Owing to the increased molecular order, the electrical conductivity of doped PTEG-2 films is improved by a factor of 6 after thermal annealing resulting in conductivities of up to 7.5 S/cm. Additionally, this structural transition does not greatly influence the Seebeck coefficient, and leads to a power factor of up to 28 μWm^-1K^-2. Finally, doped PTEG-2 films are very stable during annealing at 150 degree Celsius, which is of importance in thermoelectric applications. Our work introduces an effective strategy for improving molecular order and thus n-type organic thermoelectric performance.

Organic field effect transistors (OFETs) have attracted high attention in the past decades for its potential in industrial applications such as flexible displays, organic bio-medical sensors, and flexible radio frequency identification (RFID) tags. In fact, among various OFET materials so far proposed, some of which exhibited high OFET mobility over a few cm²/Vs. However, FET performance have hardly been investigated in detail where practical device performance are concerned such as OFET characteristics under low voltage driving and high bias-stress stability.
In this study, we evaluated OFET performance fabricated with solution-processed polycrystalline thin films of liquid crystalline organic semiconductor of 2-phenyl-7-decyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) under the low voltage and high bias-stress conditions described above.
Ph-BTBT-10 OFETs (bottom-gate and bottom-contact FET with W/L of 350 μm/70μm) were fabricated with low-κ polystyrene/SiO₂ hybrid gate dielectric using highly ordered smectic E (SmE) liquid crystalline phase, which helps organic molecules form a well-aligned ccrystalline thin film during spin-coating process and gives it high thermal stability. The transfer characteristics were measured at drain voltage (V_ds = −0.1 V for linear regime and at V_ds = −2 V for saturation regime) under ambient air at room temperature. Despite the polycrystalline nature of Ph-BTBT-10 and low-κ gate dielectric, the solution-processed, bottom-gate, bottom-contact Ph-BTBT-10 OFET exhibits saturation mobility up to 4.9 cm²/Vs with the subthreshold swing of 79 mV/decade, which is very close to the theoretical limit of 60 mV/decade at room temperature [1].
On the other hand, we evaluated OFET bias-stress stability that depends on gate voltages as a function of operation time up to 10⁴ s. The negative gate bias-stress caused negative threshold voltage shift (△V_th) for Ph-BTBT-10 FET in ambient air, but △V_th rapidly decreases as the gate bias decreases and approaches to near zero when the gate bias went down to 9V in amplitude. In contrast, positive gate bias-stress causes negligible △V_th even with a relatively high bias voltage [2].
All these results conclude that Ph-BTBT-10 FET has excellent FET performance at a low voltage operation as well as excellent bias-stress stability in ambient air in the range of low to moderate operating voltages.

[1] M. Kunii, H. Iino, and J. Hanna, IEEE Elec. Dev. Lett., 37, 486, (2016).
[2] M. Kunii, H. Iino, and J. Hanna, Appl. Phys. Lett., 110, 243301 (2017).

We discuss the role of mesoscale order, electrostatic effects, defects, and roughness for charge splitting and detrapping at organic-organic interfaces. We show how inclusion of mesoscale order resolves the controversy between experimental and theoretical results for the energy-level profile and alignment in a variety of photovoltaic systems, with direct experimental validation [1,2]. We also show how one can predict open-circuit voltages of planar heterojunction solar cells, in excellent agreement with experimental data, based only on crystal structures and interfacial orientation. We explain how long-range molecular order and interfacial mixing generate homogeneous electrostatic forces that can drive charge separation and prevent minority carrier trapping across a donor-acceptor interphase [2]. Comparing several of small-molecule donor-fullerene combinations, we illustrate how tuning of molecular orientation and interfacial mixing leads to a trade-off between photovoltaic gap and charge-splitting and detrapping forces, with consequences for the design of efficient photovoltaic devices. By accounting for long-range mesoscale fields, we obtain the ionization energies in both crystalline [3] and mesoscopically amorphous systems with high accuracy [4,5,6,7].

References:

[1] C. Poelking, M. Tietze, C. Elschner, S. Olthof, D. Hertel, B. Baumeier, F. Wuerthner, K. Meerholz, K. Leo, D. Andrienko, Nature Materials, 14, 434, 2015
[2] C. Poelking, D. Andrienko, J. Am. Chem. Soc., 137, 6320, 2015
[3] M. Schwarze, W. Tress, B. Beyer, F. Gao, R. Scholz, C. Poelking, K. Ortstein, A. A. Guenther, D. Kasemann, D. Andrienko, K. Leo, Science, 352, 1446, 2016
[4] P. Kordt, J. J. M. van der Holst, M. Al Helwi, W. Kowalsky, F. May, A. Badinski, C. Lennartz, and D. Andrienko, Adv. Funct. Mater. 25, 1955, 2015
[5] C. Poelking, D. Andrienko J. Chem. Theory Comput., 12, 4516, 2016
[6] N. B. Kotadiya, A. Mondal, S. Xiong, P. W. M. Blom, D. Andrienko, G.-J. A. H. Wetzelaer, Adv. Elec. Mat, 2018
[7] P. Heimel, A. Mondal, F. May, W. Kowalsky, C. Lennartz, D. Andrienko, and R. Lovrincic, Nature Comm., 2018

Charge transport in organic semiconductors is strongly related to the molecular structure and the solid-state packing within the crystal. Reducing molecule aromaticity, while preserving conjugation length, can be effective in enhancing charge carrier mobility. One example of such a system is the diareno-fused antiaromatic compounds. In this study, we synthesized a series of dinaphtho-fused s-indacenes and evaluated their electronic and electrical properties using a combined computational and experimental approach. Three different substitutions of the linear-fused dinaphthoindacene (DNI) backbone were investigated, namely (n-octyldiisopropylsilyl)ethynyl (nODIPS), (triisobutylsilyl)ethynyl (TIBS), and (triisopropylsilyl)ethynyl (TIPS). Computational studies found high effective masses in the nODIPS and TIBS derivatives, suggesting that the charge transport is not effective in these materials. On the contrary, the TIPS substituted molecule yielded low effective masses for both holes and electrons, indicating that charge carrier mobility can be high in this compound. To test the electrical properties of these semiconductors, thin films of each derivative were electrically characterized in a top-gate bottom contact field effect transistor architecture. Gold and silver, in combination with several chemical treatments, served as source and drain electrodes, while spin-coated Cytop fluoropolymer was the dielectric. To improve thin film morphology and crystallinity, a solvent vapor annealing post-processing step was employed. The TIBS and nODIPS derivatives did not yield working devices, in agreement with the theoretical predictions. In the TIPS-substituted DNI, we measured hole mobilities as high as 2.25 cm²/Vs. We further investigated two more regioisomers, the syn and anti isomers, along with the linear TIPS compound. We found that the subtle variations in the backbone structure have led to changes in the electrical properties. The anti-isomer exhibited the highest measured mobility, at 7.1 cm²/Vs, in agreement with the low effective mass of 0.52 m₀, and over three times larger than the mobility measured in the linear isomer. We were not able to detect unipolar electron transport, in spite of the low electron effective mass of this compound (0.43 m₀), most likely because of inefficient injection from the high work function electrodes and trapping at the semiconductor/dielectric interface. Our results underline the importance of fine-tuning of the molecular structure in order to obtain high mobility organic semiconductors.

During the past decade, many new candidates of organic semiconductors exhibiting not only performance (high mobility exceeding over 10 cm²/Vs) but also stability (thermal stability or lifetime etc.) or processability (printable or low-cost process) have been fabricated and developed significantly in response to various demands of device performance. However, there is no general solution to satisfy both performance and structural stability. Our approach to using liquid crystallinity is one of the candidates for the general solution. Introducing partial disorder of liquid crystallinity for the device fabrication is to accept the fluidity of the material and to provide the large- and uniform- thin film without structural defects or grain boundaries, which extrinsically cause serious problems for charge transport. On the other hand these partial disorder and fluidity provide static disorder ubiquitously distributed. For example, the degree of freedom in alkyl chain or flip-flop motion of p-conjugated core, which are typical behavior of “high-ordered” liquid crystal.
In our research, we focus on these static disorders which are inherent in liquid crystals. We discuss these static disorders using the theoretical estimation and the simulation compared with experimental results for phenyl naphthalene, biphenyl, the thiophene and BTBT derivatives exhibiting smectic E phase which is typical phase using for device process we are proposing. We found that these disorders are controlled by order parameters of liquid crystallinity because they are not extrinsic origin but intrinsic disorder caused by the degree of freedom originated by liquid crystallinity.
These typical values of disorder (typically ~50meV in energetic disorder) in liquid crystal are small compared with those in amorphous (> ~100meV). However, they are not ignored and their activation energy is comparable with polaron binding energy. Thus their transport behavior is the superposition of small polaron and Gaussian disorder effects. For understanding these phenomena, we modeled the polaron transport model considering with the disorder effect in the liquid crystal. We apply the quantum CT rate for our organic semiconductors. The model can explain the negative temperature dependence of mobility without using band-like model. We discuss critical temperature causes a transition from the polaron controlled to the disorder controlled.
These studies give us an insight into how we can control the liquid crystallinity for the fabrication of organic semi-conductive devices with high performance.

In introducing side chains into the phthalocyanine (Pc) molecule to induce liquid crystalline (LC) phases, there are two types of molecular structures, namely, a peripheral type and non-peripheral type. We have investigated the molecular packing and electronic transport characteristics of mesogenic Pc having alkyl side chains at non-peripheral sites, 1,4,8,11,15,18,22,25-octaalkylphthalocyanine (CnPc), and their families. These compounds exhibit very high long-range drift mobility up to 1.4 cm²/Vs evaluated by time of flight measurement and must be a candidate of organic semiconductor for printable electronics [1,2]. However, this fact is contrary to expectation. Namely, it can be easily predicted that the planarity of the Pc molecular plane might be reduced due to large steric hindrance between non-peripherally substituted chains and disordered molecular stacking undesirable for carrier transport is formed.
We have also demonstrated a high-efficient bulk-heterojunction (BHJ) solar cell based on C6PcH₂ and their family, which shows a high power conversion efficiency of 5.3% [3-5]. For the formation of the optimally phase-separated nano-structure for efficient carrier generation and transportation, the mesogenic properties should play an important role.
Using self-organizing characteristics of LC phase, we can fabricate a large area monodomain thin film of CnPc for electronic devices. A well-aligned uniaxial single crystal growth in large area of printed thin film was obtained by means of conversion process from LC phase in a supercooled state to crystal phase.
CnPc shows a crystal polymorphism and forms two different crystal structures (α-phase or β-phase) depending on the growth condition of the crystal. The high mobility of C6Pc reported so far is evaluated in the crystal structure of the β-phase. However, according to the theoretical prediction, higher mobility is expected in the α-phase [6]. We have demonstrated a large area single crystalline film of α-phase converted from spin-coated β-phase Pc film, which is based on the crystal polymorphism conversion triggered by the solvent vapor treatment [7,8].

Acknowledgment
This work was partly supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan (17K18882, 18H04514), and by Advanced Low Carbon Technology Research and Development Program from the Japan Science and Technology Agency (JST-ALCA)

References
[1] Y. Miyake, Y. Shiraiwa, K. Okada, H. Monobe, T. Hori, N. Yamasaki, H. Yoshida, M. J. Cook, A. Fujii, M. Ozaki, and Y. Shimizu, Appl. Phys. Express, 4, 021604 (2011).
[2] M. Yoneya, A. Miyamoto, Y. Shimizu, A. Fujii, and M. Ozaki, J. Phys. Chem. C, 119, 23852 (2015).
[3] T. Hori, Y. Miyake, N. Yamasaki, H. Yoshida, A. Fujii, Y. Shimizu, and M. Ozaki, Appl. Phys. Express, 3, 101602 (2010).
[4] Q.-D.Dao, T. Hori, K. Fukumura, T. Masuda, T. Kamikado, A. Fujii, Y. Shimizu, M. Ozaki, Organic Electronics, 14, 2628 (2013).
[5] Q.-D.Dao, K. Watanabe, H. Itani, L. Sosa-Vargas, A. Fujii, Y. Shimizu and M. Ozaki, Chem. Lett., 43, 1761 (2014).
[5] M. Yoneya, A. Miyamoto, Y. Shimizu, M. Ohmori, A. Fujii, M. Ozaki, Jpn. J. Appl. Phys., 56, 081601 (2017).
[6] T. Higashi, M. Ohmori, M.F. Ramananarivo, A. Fujii, and M. Ozaki, APL Mater., 3, 126107 (2015).
[7] Y. Anzai, T. Higashi, H. Kajii, A. Fujii, M. Ozaki, Organic Electronics, 60, 16 (2018).

Strong self-assembling nature of liquid crystals is fascinating in terms of thin film fabrication even as solid state to make the potential performance emerged in semiconducting properties such as carrier mobility. On the other hand, miscibility and phase separation are also interesting properties for liquid crystals, if spontaneous formation of nano-scale structure to make the systems perform high efficiency in exciton as well as charge migrations. In this talk, recent results we obtained on liquid crystalline composites as semiconducting materials and this is a trial to give a new methodology for control of electronic properties in molecular materials. Two case studies were carried out for calamitic systems and one is the combination of low-molecular weight liquid crystalline semiconductors (TNAT : dithienylnaphthalene derivatives) with the composites of the alkyl homologues exhibiting miscibility in liquid crystalline phase and the other is the composites of polymer liquid crystalline semiconductor (P3HT) and low-molecular weight one, TNAT. The composites of alkyl homologues exhibit miscibility even in highly ordered smectic-type mesophases and in the miscible regime, the observed drift mobility of charged carriers is just between the mobilities of single component materials, depending on the component ratio. However, the composite with non-mesogenic homologue (the shorter chain derivative) and mesogenic one is likely to enhance the carrier mobility, indicating that 2D-layer structure is stabilized by the addition of non-mesogenic homologue. The composites of P3HT with an alkyl-homologue of TNAT were studied for the second case. Phase separation was shown in a wider region of component ratio, though P3HT is well-known as also to liquid crystalline polymer of which melting point is >240 °C. The drift mobility measurements reveal that the mixing of a mesogenic TNAT with P3HT leads to the enhancement of carrier transport efficiency, even in the small component ratio of TNAT (> ca. 75 mole% as to the monomer unit of P3HT) and the comparable level of mobility is attained to the case of well-aligned polymer backbone of P3HT (~ 10^-1cm²/ Vs).

Organic crystalline thin films vertically oriented often are used as semiconductor layer in organic field effect transistors (OFETs), which can be fabricated by vacuum-evaporation and solution-processes of semiconductor materials such as spin-coating and drop-cast techniques. However, it is quite difficult to fabricate planarly-oriented crystalline thin films on the substrate, which are available for vertical devices such as light emitting diodes and solar cells where the current flows perpendicular to a film because there is no way to orient the semiconductor materials planarly.
In this study, we investigated fabrication of planarly-oriented polycrystalline thin films of organic semiconductors with the aid of self-organization of smectic liquid crystalline molecules and an overcoated orientation layer. We found that tens of nm thin films vertically oriented can be re-oriented from ‘‘homeotropic, or vertical’’ to ‘‘homogenous or planar’’ to achieve polycrystalline thin films planarly oriented after removing the over-coated orientation layer. Furthermore, we clarified the key factors to affect re-orientation of the films and uniformity and surface morphology of the resulting films, including conditions required for the re-orientation and properties of the orientation layer materials and liquid crystals [1].

[1] Yi-Fei Wang, Hiroaki Iino and Jun-ichi Hanna, Soft Matter, DOI: 10.1039/c7sm01303e (2017).

Benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives are very promising candidates in the field of solution processable organic semiconductors. In this work the surface crystallization behaviour of Ph-BTBT-10 is investigated. Thin films starting from the monolayer regime up to thick films were prepared by spin coating and physical vapour deposition. The films were investigated in terms of crystalline properties and thin film morphology by X-ray diffraction, X-ray reflectivity and Grazing Incidence X-ray diffraction, atomic force microscopy and optical microscopy. It is shown that by spin coating at low concentrations of 0.3 g/l an incomplete monolayer of standing molecules with the aromatic core pointing towards the substrate is formed. At higher concentrations between 0.5 g/l and 3 g/l strong island growth starts which leads to the formation of macroscopic islands with dendritic monolayers in-between them. Physical vapour deposition demonstrates a more homogenous distribution of the material forming terraces of multiple molecules. In thin films whether in solution (5 g/l) or PVD (300 Å) crystalline structures are observed in XRD. As well as the known bulk phase with a head to head arrangement of two molecules in an orthorhombic unit cell possessing a herringbone structure, also other polymorphic phases are present.

Recent advances in organic solar cell ingredients and processing have enabled fabrication of single devices with power conversion efficiencies in excess of 15%. However, such high efficiencies are not realized in commercial products because of the difficulty scaling up robust self-assembly of the active layer components. For mass-produced devices to be commercially viable, the factors governing self-assembly of structure, and how that structure determines charge transport must be understood. Investigating these factors is challenging due to the multiplex combinations of chemistries and processing protocols that are feasible. However, searching these combinations can be accelerated through the use of computational tools such as molecular dynamics and kinetic Monte Carlo. In this work, we develop an united-atom model for the benchmark donor polymer poly-(3-hexylthiophene), resulting in the most experimentally accurate structures to date. We then predict the self-assembly and charge transport under hundreds of processing variable combinations. We find that ordered structures obtained at low temperatures using good solvents allow for the highest charge transport rates. Furthermore, we find that polymer `tie-chains', which span between crystallites, are critical in preventing charges from becoming trapped within local structural features. We discuss modeling successes with more complex molecules and demonstrate promise for these tools in developing design rules for mass producing organic solar cells.

Ternary organic solar cells (OSCs) have attracted much research attention in the past few years, as ternary organic blends can broaden the absorption range of OSCs without the use of complicated tandem cell structures. Despite their broadened absorption range, the light harvesting capability of ternary OSCs is still limited because most ternary OSCs use thin active layers of about 100 nm in thickness, which is not sufficient to absorb all photons in their spectral range and may also cause problems for future roll-to-roll mass production that requires thick active layers.[1-2] Herein, we report a highly efficient ternary OSC obtained by incorporating a nematic liquid crystalline small molecule (named BTR) into a state-of-the-art PTB7-Th:PC71BM binary system. The addition of BTR into PTB7-Th:PC71BM was found to improve the morphology of the blend film with decreased π-π stacking distance, enlarged coherence length, and enhanced domain purity. This resulted in more efficient charge separation, faster charge transport, and less bimolecular recombination, which, when combined, led to better device performance even with thick active layers.[3] Moreover, it was also found that such a ternary strategy can largely reduce the processing temperature of PffBT2OD-4T:PCBM system by suppressing the aggregation of PffBT2OD-4T.[4] And the resulting ternary solar cells showed enhanced efficiency and stability than binary devices, mainly due to the optimized and stable morphology of ternary films. These works may provide novel solutions for preparing active layer for high-performance large area OSCs module device in the future.

References
[1] Y. C. Jin, F. Huang et al. Adv. Mater. 2016, 28, 9811.
[2] Y. Jin, L. Ying, F. Huang et al. Adv. Energy. Mater. 2017, 1700944
[3] G. C. Zhang, F. Huang et al. J. Am. Chem. Soc. 2017, 139, 2387.
[4] M. Xiao, F. Huang et al. Nano Energy, 2018, 48, 53.

Organic thin-film transistors (TFTs) are devices in which the active layer is an organic semiconductor with a thickness of several molecular monolayers that form a homogenous, polycrystalline film [1]. The growth, morphology and stability of the semiconductor films on the surface of the gate dielectric are of particular interest, since the gate-field-induced charge-carrier channel is located in close vicinity to the semiconductor-dielectric interface. Using atomic force microscopy (AFM) and scanning electron microscopy (SEM), we have monitored the growth of thin films of the small-molecule semiconductor dinaphthothienothiophene (DNTT) [2,3] deposited by sublimation in vacuum onto a hybrid aluminum oxide/self-assembled monolayer (SAM) gate dielectric as a function of the film thickness. This was accomplished by depositing a DNTT film with a linear thickness gradient from 0 to 5 nm. At each point along the thickness gradient, i.e. for nominal semiconductor thicknesses between 0 and 5 nm, a significant temporal change in the stability and morphology of the semiconductor film was observed. Breuer et al. [4] have previously observed the phenomenon of post-deposition dewetting in DNTT films with a thickness of 2 nm, both on silicon dioxide and on silicon dioxide modified with an alkyl-silane SAM. They observed a transformation of the initially formed, nearly closed monolayer of DNTT molecules into separated islands of different height. This dewetting process was significantly less pronounced in thicker DNTT films (40 nm), but its potential influence on device stability needs to be nonetheless examined in more detail. We have therefore also fabricated bottom-gate, bottom-contact TFTs based on 2-nm- and 25-nm-thick DNTT films. We have observed a sharp decrease in the charge-carrier mobility in the TFTs with the 2-nm-thick DNTT films, from 0.2 cm²/Vs (15 mins after device fabrication) to 0.011 cm²/Vs (72 hours after device fabrication) and correlate this to post-depositional dewetting occurring in various stages. For comparison, the TFTs with the 25-nm-thick DNTT film are stable, showing a virtually constant carrier mobility of 1.1 cm²/Vs. Not only does this TFT architecture allow us to study the effect of dewetting on the characteristics and the stability of the semiconductor films and the TFTs, but it also opens up the possibility of new device designs if post-deposition semiconductor dewetting is successfully suppressed. This work also explores different strategies to that effect, such as post-deposition cooling. References: [1] R. Yi, et al., Sci. China Technol. Sci., 57, 1142, 2014. [2] K. Takimiya, et al., J. Am. Chem. Soc., 128, 12604, 2006. [3] U. Zschieschang, et al., Organic Eletronics, 12, 1370, 2011. [4] T. Breuer, et al., ACS Appl. Mater. Interfaces, 9, 8384, 2017.

Inverted thermal activation of charge mobility in polymer field effect transistors (FETs) has been recently observed in a few cases for holes and in a single case only for electrons. Such exciting observations lead us to foresee a future of polymer FETs beyond hopping transport limitations. Yet, there is a clear lack of understanding of the fundamental reasons leading to such transport regime and, as a consequence, of synthetic and processing guidelines. Here we make use of uniaxial molecular alignment and specific thermal treatments within the melting endotherm of a diketopyrrolopyrrole-tetrafluorobenzene-based electron transporting copolymer films to induce a favorable microstructure for electron transport. Thanks to such strategy we can achieve a single-crystal like, voltage independent mobility, with vanishing activation energy close to room temperature. We make use of extensive microstructural, electrical, electro-optical and theoretical investigations to rationalize the transition from a hopping to a band-like transport regime, and link it to the microstructure. Thermal barriers present in the disordered phase can be effectively suppressed in the direction of molecular alignment by a more efficient interconnection of the ordered phase following the increased crystals extension occurring during the thermal process.

For decades, liquid crystals (LCs) have found ubiquitous use in display technologies, operating as stimuli responsive polarizers largely based on electric-field induced ordering of calamitic (rod-shaped) LCs. Recently, the sub-class of multi-component, columnar phase liquid crystals (CLCs) has attracted increasing interest due to their broad potential as self-assembling and self-healing absorbers, semiconductors, and potentially room temperature ferroelectrics. These materials can exhibit synergistic charge-transfer (CT) behavior (e.g., band gap) that is tunable via simple mixing of donor/acceptor moieties, and the anisotropic nature of this CT absorption can be manipulated to form inherently dichroic films. Here, we take advantage of the functionality and chemical malleability of bi-component CLCs to develop an optically re-writable material (OWR) with fine control over dichroic properties. Films of mixed stack donor-acceptor columnar liquid crystals (DACLCs) comprised of diaminonaphthalene/ naphthalenediimide (DAN/NDI) complexes are subjected to laser-induced temperature gradients to direct columnar alignment. Fast laser scanning induces a sharp temperature gradient that leads to flash-cooling of the DACLC resulting in no long-range molecular order (isotropic). Slower scanning leads to a sustained thermal gradient that realigns the columnar director, and the subsequent polarization axis of the region, exactly in the direction of laser movement. Using this simple mechanism, the columnar alignment and hence CT-based dichroism can be patterned intentionally and repeatedly, as we demonstrate via fabrication of reconfigurable micro-polarizers and bar codes. By using chemically malleable precursors, this process can impact diverse applications including information storage and encryption via non-contact assembly and tunable persistence.

Semiconducting polymer films are promising candidates for emerging biological and electrochemical energy conversion and storage devices including but not limited to printable biological and chemical sensors, ion pumps, ionic-organic electronic ratchets, redox flow batteries, supercapacitors, electrochromics, and (photo-) electrochemical cells. Growing interest in these types of devices has led to the development of a number of polymeric and polyelectrolyte systems in an effort to control and understand the electronic-ionic transport mechanisms. In particular, focus on electroactive materials is targeting different morphologies (porous, dense, or heterogeneous) with microstructures ranging from crystalline to amorphous.

One major challenge continues to be the ability to measure ion transport coefficients in the presence of strong disorder, which can vary greatly as a function of electrochemical potential. Electrochemical impedance spectroscopy (EIS) is the most promising approach, in that interfacial and “bulk” transport events can be resolved as a function of both time and energy. However, in conductive polymeric systems, a wide distribution of energetic states and intercalated ions yields a complex environment that cannot be modeled accurately with simple circuit diagrams. Spectroelectrochemistry generally, and potential modulated spectroscopies specifically, combine the sensitivity of electrochemistry with this necessary specificity of optical spectroscopy by monitoring a potential-dependent optical change correlated to the charge transfer event of interest. Here we demonstrate potential modulated fluorescence (PMF) as a viable approach to monitor both polymer oxidation and ion migration in polymer films as a function of microstructure. Using poly(3-hexylthiophene) (P3HT) as a model system, we monitor the onset of ion diffusion at very low doping densities, just as the most ordered microstructure begins to relax from the most dense, neutral configuration to a more swollen, slightly doped configuration, yielding diffusion coefficients on the order of 10^-10 cm²/s. Ultimately, application of PMF to microstructure-dependent diffusion will allow chemically specific comparison of polymeric materials for organic electronics.

Typically semicrystalline conjugated polymers exhibit much higher mobility than their amorphous counterpart. The archetypal example is regio-random P3HT, with mobilities on the order of 10^-6cm²/V.s as compared to regio-regular P3HT with mobilities on the order of 0.1 cm²/V.s. The oft-cited reason for these differences is the fact that in polymer crystallites charges delocalize and take a partial 2D character, enabling higher mobility. The amount of delocalization and how it depends on structure and processing is however difficult to measure. I will show that charge modulation spectroscopy in the IR with model materials allows to determine the delocalization of the polaron in P3HT when aided by theory. In order to extract systematic trends we will use 100% regio-regular P3HT of well-defined molecular weights (PDI~1.1). These model materials allow to clearly discern different limiting factors in charge transport and how they depend on molecular weight. On one hand, short polymer segments crystallize well but have torsional disorder, which localizes the charges in 1D. As the molecular weight increases, backbone torsion decreases and charges delocalize in 2D. Eventually delocalization saturates and mobility increases are attributed to connectivity between crystallites. We extend this framework beyond model materials and study charge delocalization in high-performance donor-acceptor polymers and aligned polymers for stretchable electronics. In aligned films we are able to directly measure delocalization along the backbone and across molecules.

Organic semiconducting polymers are typically rather stiff compared to traditional polymers, because the monomers are aromatic rings, and the deflection angle between successive rings is small. This leads to the possibility of significant nematic interactions between chains in an isotropic melt just from steric considerations. Working against this tendency are the solubilizing sidechains, which dilute the stiff backbones with flexible “grease”. In recent work, we have used atomistic simulations to predict the nematic coupling constant alpha for semiconducting polymers such as P3HT, and predicted the isotropic-nematic (IN) transition temperature. Our method does not rely on observing the nematic ordering transition with its attendant slow kinetics, but instead infers alpha from its amplifying effect on the elongation of chains in an isotropic melt in response to an applied external tension. Even when alpha is not large enough to result in a stable nematic phase, nematic interactions can amplify the aligning effect of an impenetrable surface, at which semiflexible chains tend to lie parallel. We have investigated this effect with coarse-grained MD simulations of melts near a wall, in which we observe how far nematic order extends from the wall into the bulk, as a function of chain stiffness. These results compare well to our predictions from self-consistent field calculations adapted to semiflexible chains. We have exploited this surface alignment effect to locate the IN transition, by observing how surface alignment propagates into a slab of polymer chains confined between walls, as the chain stiffness increases. Below the IN transition, surface-induced alignment grows into the bulk with a velocity proportional to the undercooling. By observing how the front propagates as we cool (or increase chain stiffness), we can accurately locate the transition.
Nematic order is important in semiconducting polymers because electronic transport in semiconducting polymers is inherently anisotropic. Carriers move readily along straight chains, can hop with relative ease between parallel chains in a crystal, and hop only with difficulty between obliquely oriented chains in an isotropic melt. Crystallization from a nematic phase may give better-organized crystalline domains with fewer amorphous regions, and may induce chain alignment even in the amorphous regions. In a separate thrust of our research, we have developed coarse-grained electronic models capable of describing carrier states and motion in crystalline and amorphous semiconducting polymers such as P3HT. Our approach uses tight-binding models, in which each ring moiety is a site, with hopping between adjacent sites. Parameters are tuned to agreement with state-of-the-art DFT calculations for small systems in well-defined geometries. Hopping matrix elements along chains are large for trans dihedrals, but fall off and vanish as dihedrals rotate to 90 degrees, breaking conjugation. Carriers in P3HT form polarons, in which the surrounding material reorganizes (“polarizes”) in response to the presence of the charge. The polaron size is set by a competition between electronic kinetic energy (which seeks to delocalize the charge) and the reorganization energy (which stabilizes a localized charge more effectively). The most important contribution to reorganization energy in these materials is dielectric polarization of nearby material, leading to polarons a few rings long. These polarons can move between chains a crystal, by hops described by Marcus theory. In the hopping barrier state, the polaron is midway between two degenerate locations, in a symmetric superposition of states. We use our tight-binding model to compute barriers for hopping between chains, in a crystal and an amorphous melt. The melt barrier is larger, because obliquely oriented chains do not share very effectively the same polarization field, which increases the reorganization energy.

Nanosegregated π-conjugated liquid crystals can be multi-functional soft materials in which an electronic function couples with optic, ionic, ferroelectric, and electrochemical activities. In this study, we demonstrate the coupling of electronic function with ionic transport and ferroelectricity in the columnar and chiral smectic C* (SmC*) phases, respectively. Strong electric field based on the polarization of electrolytes and spontaneous polarization of ferroelectrics can produce various electronic functions.
We have already reported thin film mixed conductors based on liquid crystalline (LC) perylene bisimide derivatives bearing oligosiloxane moieties and triethylene oxide chains.^[1,2] LC perylene bisimide derivatives bearing the two cyclotetrasiloxane rings and a triethylene oxide chain exhibited a columnar phase at room temperature. The LC spin-coated thin films were insolubilized by the in situ acid-induced ring-opening polymerization method. Moreover, triethylene oxide chains interact with ionic species which can penetrate into the polymerized thin films. Uniaxially aligned LC thin films can be produced by a friction transfer method. The thin films can be doped in aqueous solution of Na₂S₂O₄. The electrical conductivity along the columnar axis reached 1×10^-4 Scm^-1 in as-deposited and polymerized thin films. The anisotropy in the electrical conductivity was 100 and 10 in as-deposited and polymerized films, respectively.
In conventional photovoltaic effect, a local electric field at the p-n or Schottky junction promotes photocarrier generation and transport. The local electric field is produced by charge transfer at the junctions. In contrast, the internal electric fields originated from the spontaneous polarization in ferroelectrics contribute to the generation and transport of the photogenerated carriers in the anomalous or bulk photovoltaic effect.
Conventional ferroelectric liquid crystals (FLCs) have been electrically insulative. We synthesized phenylterthiophene derivatives bearing a chiral alkyl chain, which exhibit a SmC* phase. These FLCs indicated hole transport originated from the extended π-conjugated system. In the SmC* phase, UV light illumination induced photocurrent response under zero-bias state produced by the DC bias application prior to the UV light illumination.^[3] The photovoltaic effect was not observed in racemic mixtures, suggesting that the origin of this photovoltaic effect was molecular chirality.^[4] Moreover, the polarized state in the SmC* phase can be fixed in the ordered smectic phase which appears below the SmC* phase. Photocurrent response was enhanced in the ordered smectic phase.^[5]

References
[1] M. Funahashi, A. Sonoda, Dalton Trans., 42, 15987 (2013).
[2] M. Funahashi, Mater. Chem. Front., 1, 1137 (2017).
[3] Y. Funatsu, A. Sonoda, M. Funahashi, J. Mater. Chem. C, 3, 1982 (2015).
[4] A. Seki, Y. Funatsu, M. Funahashi, Phys. Chem. Chem. Phys., 19, 16446 (2017).
[5] A. Seki, M. Funahashi, Organic Electronics, 62, 311 (2018).

The molecule Ph-BTBT-C10 (2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene) is an asymmetric molecule, which shows a fully reversible soft crystalline phase transition from the bulk phase, which results in a thin film phase with high thermal stability as well as large carrier mobility in thin film transistors [1]. Thin-film samples are prepared by spin coating at different spin speeds and solution concentrations. The phase transition behavior as well as the crystal structure of the Smectic E (SmE) phase is studied in relation to the bulk structure, by in-situ Grazing Incidence X-ray Diffraction experiments, which were carried out using synchrotron radiation at different temperatures. Additionally, in-situ specular X-ray diffraction patterns are measured on in-house equipment to characterize the crystal structures as a function of temperature. While the phase transition can be monitored precisely by an observed peak shift, the exact molecular packing in the SmE cell is still not fully solved. Starting from Molecular Dynamic (MD) simulations [2] potential unit cells for the SmE phase are compared to experimental results. Distinct Bragg peaks in the diffraction pattern indicate an ordered structure in the out-of-plane as well as the in-plane direction of the sample, with very similar in-plane order as in the case of the bulk phase, but significant differences in out-of-plane order. Specific spin conditions lead to a disordered phase with broad diffraction peaks. This disordered phase cannot be transferred to an ordered state by temperature treatment.


[1] H. Iino, T. Usui, and J. I. Hanna, “Liquid crystals for organic thin-film transistors,” Nat. Commun., vol. 6, p. 6828, 2015.
[2] M. Yoneya, “Monolayer Crystal Structure of the Organic Semiconductor 7-Decyl-2-phenyl[1]benzothieno[3,2- b][1]benzothiophene,” J. Phys. Chem. C, vol. 122, pp. 22225–22231, 2018.

Molecular orientation and electronic structure at molecular donor/acceptor interfaces plays an important role in the performance of organic optoelectronic devices. The donor/acceptor interfaces within organic photovoltaics promote charge dissociation of the photo-generated excitons using the energy difference between donor and acceptor materials. π-conjugated organic semiconductor materials are widely used as donor and acceptor molecules, and so most organic semiconductors have planar or linear, π-conjugated, molecular structures. When these materials exist in a crystalline state, charge transport anisotropy exists along different crystallographic directions because of the directionality of π orbital overlap. Therefore, controlling the orientation of the organic molecules within organic films is important for improving the performance of organic optoelectronic devices. The interface between pentacene and C₆₀ is known as a model donor/acceptor interface at which excitons dissociate into free charge carriers. This study aims to analyze the effect of the molecular orientation of pentacene on the energy level alignment of a C₆₀/pentacene interface. An edge-on (standing-up) orientation is expected to produce significantly different results when compared with a face-on (lying-down) orientation, in which the π-orbitals of the PEN molecules will be directly exposed to the C₆₀ interface. Here, we show that graphene substrates can be used as templates for tuning the molecular orientation of pentacene, selectively driving the formation of either face-on or edge-on arrangements by controlling the temperature of the substrate during deposition [1]. The pentacene orientation is controlled using the same graphene substrate, such that the effect of different types of substrates on the resulting electronic structure (e.g. through the work function) could be eliminated. The electronic structure and morphology of the two resulting C₆₀/pentacene heterointerfaces were elucidated using ultraviolet photoelectron spectroscopy (UPS) and atomic force microscopy (AFM), respectively. While the C₆₀/pentacene (edge-on) interface exhibited vacuum level alignment, the C₆₀/pentacene (face-on) interface exhibited a vacuum level shift of 0.2 eV, which was attributed to the formation of an interface dipole that resulted from polarization at the C₆₀/pentacene boundary.

[1] T. Nishi, M. Kanno, M. Kuribayashi, Y. Nishida, S. Hattori, H. Kobayashi, F. von Wrochem, V. Rodin, G. Nelles, and S. Tomiya, Appl. Phys. Lett. 113, 163302 (2018).

The talk will give an overview of our latest results on how to use concepts from classical crystallization to guide structure formation of semicrystalline semiconducting polymer films on hierarchical length scales.
One of our work-horse systems is the n-type semicrystalline polymer poly{[N,N’-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5’-(2,2’-bithiophene)} (P(NDI2OD-T₂)). The strong tendency to aggregate in solution can be used to prepare highly anisotropic films. [1] Solvent vapor annealing [2] involving swelling and recrystallization of as-cast films allows to control nucleation densities and to grow spherulite-like superstructures. Interestingly, the spherulite superstructures result from radial growth of fibers with the fiber growth direction being the polymer chain direction. Simple blade coating from concentrated solutions even yields square centimeter large oriented films – again made of fibers - with dichroic ratios of 18 and charge transport anisotropies up to 14. Nucleation and growth mechanisms can be understood in terms of a reorientation of pre-formed aggregates in solution, suggesting nematic-like preorientations. These findings show completely different growth behavior than previously developed for the well-studied p-type polymer poly(3-hexylthiophene).[3]
The influence of regioregularity on aggregation behavior [4] and on chemical and electrochemical doping performance will be also discussed during the presentation.[5]
[1] D. Trefz, Y. Gross, C. Dingler, R. Tkachov, A. Hamidi-Sakr, A. Kiriy, C. McNeill, M. Brinkmann, S. Ludwigs, Manuscript submitted.
[2] C. Dingler, K. Dirnberger, S. Ludwigs, Macromolecular Rapid Comm. 2018 in print.
[3] G.L. Schulz, S. Ludwigs, Adv. Funct. Mater. 27, 2017, 1603083.
[4] Y.M. Gross, D. Trefz, R. Tkachov, V. Untilova, M. Brinkmann, G.L. Schulz, S. Ludwigs, Macromolecules 50, 2017, 5353.
[5] D. Trefz, A. Ruff, R. Tkachov, M. Wieland, M. Goll, A. Kiriy, S. Ludwigs, J. Phys. Chem. C 119(40), 2015, 22760.

Directred alignment of polymer chains plays an indispensable role in charge transport properties. In this aspect, liquid crystalline conjugated polymers possess significant advantages for the development of high-performance organic electronics due to their self-assembly characteristic for molecular alignment. Here, we designed and synthesized a lyotropic liquid crystalline conjugated polymer, CP1-P, containing photocleavable o-nitrobenzyl esters as side chains. A floating film transfer method was investigated to induce the polymer chain alignment via its liquid crystalline phase. An optimum amount of a high boiling point solvent in chloroform turned out to be an important factor to optimize the polymer alignment. The cleavable side chain design provided the solvent resistivity to the resulting film after the removal of the side chain without disrupting the alignment feature. The hole mobility along the polymer chain alignment direction in organic thin film transistors built by this method was 13-14 times higher than that in the direction perpendicular to the alignment.

The electrical characteristics of thin-film transistors (TFTs) based on disordered semiconductors, such as hydrogenated amorphous silicon, metal oxides and conjugated organic semiconductors, are greatly influenced by the density and energetic distribution of localized electronic states in the energy gap of the semiconductor arising from chemical or structural defects. In the 1980s, Grünewald et al. developed a method to convert a measured transfer curve of a field-effect transistor to the underlying density-of-trap-states (trap DOS) function [1]. The Grünewald method has been employed with great success to calculate the trap DOS of organic TFTs based on various organic semiconductors and device architectures [2-4]. However, in all previous publications, a critical simplification in the choice of the boundary conditions for the Poisson equation was made, whereby the potential drop across the semiconductor layer was ignored. For TFTs with thick gate dielectrics, in which the applied gate-source voltage drops almost entirely across the gate dielectric, this simplification is indeed justified. However, for TFTs employed in real applications, low-voltage operation and thus thin gate dielectrics are critical prerequisites, in which case the above-mentioned simplification is no longer valid. Therefore, we have extended the Grünewald method to low-voltage TFTs by performing the derivation without this simplification. We then fabricated TFTs with thick and thin gate dielectrics (100 nm, 5.3 nm) and calculated the trap DOS from the TFT transfer curves using the original and the extended Grünewald method. Our results confirm the validity of the original Grünewald method for high-voltage transistors, but also the need for the extended Grünewald method for the reliable extraction of the trap DOS in low-voltage transistors with thin gate dielectrics. Furthermore, we developed a method to systematically control the surface roughness of the gate dielectric in our TFTs and then used the extended Grünewald method to investigate the correlation between the surface roughness of the gate dielectric and the trap DOS in the organic semiconductor layer and the implications of the surface roughness for the TFT performance. [1] M. Grünewald et al., Phys. Stat. Sol. B 100, K139, (1980); [2] W. L. Kalb et al., Phys. Rev. B 81, 035327 (2010); [3] P. J. Diemer et al., Appl. Phys. Lett. 107, 103303 (2015); [4] N. K. Za’aba et al., Org. Electronics 45, 174 (2017)

Grazing incidence X-ray diffraction (GIXD) is an experimental method frequently applied to characterize a wide variety of thin films and surfaces. Depending on the texture of the sample's crystallites, special care is required during the experimental measurements and successive data evaluation. For example, samples with large individual crystallites on a surface resulting in poor statistics or epitaxially grown samples with defined in-plane alignment of the crystallites require both a full sample rotation around the surface normal during data collection. This is necessary to obtain all information for phase, texture and epitaxy analysis. Moreover, this experimental approach opens new possibilities for advanced characterisation methods which in simple static experiments are inaccessible, e.g. the determination of the in-plane mosaicity. GIDVis is a software package developed in MATLAB for visualization and analysis of GIXD data in general but mostly used for thin films. It is specialized on data obtained during a sample rotation around the surface normal, i.e. a rotating GIXD experiment. Due to the geometrical independent character of the underlying algorithms implemented, most features can be used independently from the input data type. GIDVis allows the user to perform detector calibration, data stitching, intensity corrections, standard data evaluation (e.g. cuts and integrations along specific reciprocal space directions), crystal phase analysis, etc. Further, these features are not limited to a single experimental setup, data from mostly all diffraction setups can be processed. To fully take advantage of the measured data in the case of sample rotation, pole figures can be calculated from the experimental data for any q value covered. As an example, GIDVis is used here to investigate the epitaxial alignment of pentacenequinone (P2O) crystallites on a Au(111) surface, illustrating the importance of the sample rotation. GIDVis is available online free of charge (http://www.if.tugraz.at/amd/GIDVis/).

A well-defined hierarchical superstructure based on benzene-1,3,5-tricarboxamide (3AZO-BTC) macrogelator was newly designed and synthesized for constructing randomly dispersed liquid crystal physical gels (LCPGs) to fabricate rewritable electro-optic devices. 3AZO-BTC in the nematic LC medium constructed the three-dimensional networks by nanophase separation of 3AZO-BTC from nematic LC as well as hydrogen-bonding. The light scattering state of the LCPGs systems was converted to the transmittance state at a low voltage (22 V), whereas the conventional polymer-based systems can be switched at a relatively high voltage (around 100 V). It was realized that the self-assembled 3AZO-BTC nanocolumns (only 1 wt%) which were finely dispersed in the nematic LC medium with good affinity, can induce a lower driving voltage. Furthermore, the trans-to-cis photoisomerization of azobenzene moiety can lead to tuning the optical state. The nematic gel of LCPGs at opaque mode was certainly changed to a nematic sol state at transmittance mode exposing UV light. We expect a new way of the remote-controllable and rewritable electro-optical device of dual-responsive macrogelator. This work was supported by BK21 Plus program, the BRL 2015042417, Mid-Career Researcher Program (2016R1A2B2011041).

Asymmetric azobenzene macrogelator (BPAZ) was successfully synthesized by linking the biphenyl-based (BP) moiety and the photo-responsive azobenzene (AZ) molecule. On the basis of the microscopic, thermal, spectroscopic experimental results, it was realized that BPAZ in solid state formed a layered structure mainly driven by π-π interaction. The asymmetric molecular shape of BPAZ played a key role for stabilizing the layer structure. Due to nanophase separation and intermolecular hydrogen bond, BPAZ formed three-dimensional (3D) network in organic solvent. Reversible sol-gel transition was often occurred by the trans-to-cis photoisomerization of azobenzene. Surprisingly, upon exposing UV light, organogels were transformed to the layered structure and solvents were squeezed out, similar to a “wringing out of cloths” phenomenon. Wringing behavior of BPAZ can be applied in sensors, light shutters and generators. This work was supported by the BK21 plus, BRL2015042417, and Mid-Career Researcher Program (2016R1A2B2011041) of Republic of Korea.

Benzene-1,3,5-tricarboxamide (BT) containing photo-responsive azobenzene (AZ) moieties at the periphery of the BT core (BT-AZ) was newly synthesized for the fabrication of remote controllable films. BT-AZ was self-assembled to nanocolumns mainly driven by the intermolecular hydrogen-bonds between BTA cores, and further self-organized laterally to form the low-ordered hexagonal columnar liquid crystal (LC) phase below the isotropic temperature. Upon cooling, a lamello-columnar crystal phase (K_Cr) was emerged via a highly ordered lamella-columnar LC phase. Due to the hierarchical structures by molecular self-assembly, free-standing films were formed by BT-AZ solution. By tuning the wavelength of light, the shape and color of the three-dimensional (3D) networked thin films were remote-controlled by the conformational changes of AZ moieties in the BT-AZ. The demonstrations of remote-controllable 3D actuating and rewritable films with the self-assembled hierarchical BT-AZ films will be of great help in contributing to the advanced flexible optoelectronic devices. This work was supported by BK21 Plus program, the BRL 2015042417, Mid-Career Researcher Program (2016R1A2B2011041) of South Korea.

Perylenediimide-based reactive mesogen (PDI-RM) was newly synthesized for the patterned and oriented thin film polarizer (TFP). The self-assembly behavior and anisotropic optical properties of PDI-RM were investigated using scattering, spectroscopic and microscopic analyses. Based on the results of polarized optical microscopy and X-ray diffraction, it was realized that PDI-RM formed a lyotropic chromonic liquid crystal (LCLC) phase which was originated from the balance between attractive force (π-π stacking in the self assembled nanocolumn) and repulsive force (ionic repulsion between the self-assembled nanocolumns). The uniaxially oriented film was fabricated by mechanical shearing of PDI-RM solution (60 wt% solution in water) on the flexible polymer substrate and by photopolymerizing. The polarization of PDI-RM TFP was estimated by polarized UV-vis spectroscopy. PDI-RM TFP exhibited the property of E-type polarizer with a polarizability of 99.81% at 491 nm (λ_max). Microscopically patterned TFP was also fabricated by photolithography process. Flexible TFP showed excellent mechanical and chemical stability. Patterned and flexible TFP can drive the flexible devices market to a new level. This work was supported by BK21 Plus program, the BRL 2015042417, Mid-Career Researcher Program (2016R1A2B2011041) of South Korea.

To develop an advanced thermal interface material (TIM) which can effectively dissipate the problematic heat in the electronic devices, rod-shaped reactive mesogen (RM) was newly synthesized as a building block for the thermally conductive liquid crystal (LC) films. Based on the combined techniques of spectroscopy, scattering and thermal analyses, it was verified that the synthesized RM formed a smectic A (SmA) mesophase between crystalline and isotropic (I) phases. The thermal conducting LC films were fabricated by the photopolymerization of RM in both LC and isotropic phases. Regardless of the molecular packing structures, the thermal conducting LC films exhibited outstanding heat transfer properties over 1 W/mK that surpass the conventional polymeric materials. In addition, the heat conducting performance of LC film can be amplified in a certain direction by the uniaxial orientation of LC molecules in the network. The uniaxially oriented LC film exhibited significant heat guiding performance as TIM than that of conventional organic materials. The highly thermal conducting LC films can be used as an advanced thermal managing material for electronic devices. This work was supported by BK21 Plus program, the BRL 2015042417, Mid-Career Researcher Program (2016R1A2B2011041).

Organic electronics offer the premise of low-cost, printable electronic devices with alternative mechanical, optoelectronic, and chemical properties not afforded by inorganic semiconductors. One of the major limitations continues to be lower conductivity associated with their highly disordered systems. The predominant approach is to use chemical additives (i.e. dopants) that undergo charge transfer with the matrix organic semiconductor to increase carrier concentration and conductivity. Yet, broad application of doping has been limited as precise control has remained elusive, due to partial electron transfer events and/or phase segregation of dopant. Alternatively, electrochemical doping precisely varies conductive polymer properties with only an applied voltage on a thin film in an electrolyte. This allows for direct monitoring of the conductivity and holes/monomer in situ to evaluate critical structure-property relationships. As a proof of concept, and comparison to known chemical doping strategies, the model system poly(3-hexylthiophene) (P3HT) was held at different doping (oxidizing) potentials and subsequent doping effects were characterized using a combination of electrochemical and spectroscopic methods. Conductivity was demonstrated to vary by ~10⁴ from ~2x10^-4 S/cm for the undoped film exposed to electrolyte to 0.35 S/cm for the highly doped film in solution, with solid state conductivity being retained at 0.24 S/cm. UV-vis absorbance data confirms the presence of both polaronic and bipolaronic levels in the solution doping, although the bipolaronic species are not retained in the solid state. Complementary grazing incidence wide angle x-ray scattering (GIWAXS) demonstrates a voltage-dependent swelling of the film with increasing oxidation potential in the alkyl stacking direction, demonstrating preferential ion uptake while retaining the desired π-π stacking necessary for high electronic transport. These results demonstrate a promising alternative path to increasing conductivity of organic semiconductor films and could eliminate the synthetic bottleneck associated with designing dopants for individual semiconductor systems.

In selective solvents, poly(3-hexylthiophene) (P3HT)-based block copolymers (BCPs) assemble into crystallized nanowire (NW) structures due to the strong crystalline interactions of the rigid P3HT blocks. Herein, we report the effect of the crystalline interactions of the P3HT block on the self-assembled structure of the P3HT-based BCPs in solvent by modulating the regioregularities (RRs) of P3HT. We synthesized a series of P3HT-block-poly(2-vinylpyridine) (P3HT-b-P2VP) copolymers that had the same molecular weights but various RRs, ranging from 55 to 95%, and we investigate their behavior in a binary solvent mixture. We observed a remarkable transition from the well-expected 1D NW structure to micelle structures (i.e., spherical micelles) depending on RR. The well-defined NW structures were produced from the P3HT-b-P2VP copolymers with high RR, i.e., 95%. For the RR of 55% in the BCPs, crystallization of the P3HT moiety was suppressed so that only micelle structures were formed. Remarkably, between RR values of 75% and 65% in the BCPs, we observed 1D nanostructures, which are considered to be intermediate morphologies between NWs and micelles. Even though they had distinctive characteristics, such as larger widths and heights, less crystalline properties, and different positions of the P2VP chains than conventional NWs, they still possessed partial crystallinity, which was different from the amorphous assemblies of RR of 55% BCPs. Therefore, we demonstrated the substantial relationships between the solution assembly behaviors and the crystalline interactions of conjugated semicrystalline–coil BCPs, which can be used as important guidelines for designing versatile BCP nanostructures.

During the last years the molecule dioctyl-benzothienobenrothiophene (C8-BTBT-C8) has attracted large attention, since thin film transistors based on this organic semiconductor show outstanding device performance. The high quality of the C8-BTBT-C8 films may be one of the key features for the successful application. The presence of a liquid crystalline smectic state at elevated temperatures together with a highly defined crystalline state at room temperature allows special film preparation methods of high structural perfectness. Thin films of C8-BTBT-C8 in the monolayer (thin films) and in the multilayer regime (thick films) are prepared by spin coating, a method far from thermodynamic equilibrium. Heat treatment of the films results in strong islanding (for thick films). The structure of the organic films is investigated by temperature-dependent in-situ methods using atomic force microscopy, x-ray reflectivity, and grazing incidence x-ray diffraction [1]. Already at temperatures few degrees below the phase transition from the crystalline state to the smectic A state (T = 381.5 K), the formation of a single monolayer is observed. The molecular rearrangement takes 30 minutes until the monolayer formation is completed. In case of thick films an increase of the temperature in defined steps induces discrete monolayer formation as bi-layer, triple-layer, etc. Up to 5-layer structures are observed. The reversibility of the discrete layer formation reveals that thermodynamic equilibrium states are observed. Non-equilibrium structures can be stabilised by rapid cooling from the respective temperature [2]. Explanation of these observations are given in terms of the classical film formation models using surface energy of the used silicon oxide surface and the variation of the surface energies / surface tension of the molecule C8-BTBT-C8.

[1] M. Dohr, et al., Reversibility of temperature driven discrete layer-by-layer formation of dioctyl-benzothieno-benzothiophene films. Soft Matter 13, 2322 (2017)
[2] M. Dohr, et al., Dynamics of monolayer – islands transitions in 2,7-dioctyl-benzothieno-benzthiophene thin films. ChemPhysChem 14, 2554 (2013)

Pentacene is one of the most basic materials in organic thin-film transistors. The compound is known to form a polycrystalline thin film, in which the molecular long axis is oriented perpendicular to the substrate surface, i.e., end-on orientation. The face-on oriented thin film, on the other hand, where the molecular plane is parallel to the substrate, has never been found on an inert substrate represented by SiO₂. As a result, the face-on orientation has long been believed to be generated only on specific substrates such as a metal single crystal.
A low-temperature (LT) deposition technique is a candidate for obtaining the metastable face-on crystalline film on an inert substrate. In a low temperature region (<223 K) of the substrate, however, pentacene molecules have long been believed to take a randomly oriented amorphous structure.
In the present study, the LT film of pentacene is re-investigated by using p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) together with two-dimensional grazing incidence X-ray diffraction (2D-GIXD). The combination of the cutting-edge techniques readily reveals that the face-on phase is definitely realized as the dominant component. In short, the present study shows that the conventional schematic of the “amorphous” structure should be revised.

2-Decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10), a liquid crystalline material, has been recently reported by Hanna et al as a promising p-type organic field-effect transistor (OFET) material.¹ Ph-BTBT-10 exhibits remarkable air stability and ultra-high mobility (μ_max = 14.7 cm²/Vs) in spin-coated device. Here we report that fabrication and evaluation of Ph-BTBT-10-based OFET devices by using vacuum deposition method, and thin-film structure analysis of the deposited films by grazing-incidence X-ray diffraction (GIXD) using synchrotron radiation.

All vacuum-deposited Ph-BTBT-10-based devices exhibited typical p-channel FET characteristics. OFET mobilities were quite dependent on the thermal annealing treatment regardless of self-assemble-monolayer (SAM) treatment. An ODTS-treated device without thermal annealing demonstrated good OFET performance with a hole carrier mobility of 1.42 cm²/Vs. Moreover, after thermal annealing in air at 120 °C for 5 min, the device showed a large increase in drain current (I_DS) and the highest transport performance with a hole carrier mobility of 14.0 cm²/Vs.

The crystal structures of Ph-BTBT-10 thin films were analyzed by thermal in situ 2D-GIXD at different substrate temperatures. The Ph-BTBT-10 films at RT and 60 °C showed a similar series of peaks assignable to a monolayer structure with a d-spacing of 27 Å. When the substrate temperature was increased to 120 °C, the diffraction peaks clearly changed, which suggests a transformation from a monolayer structure (d = 27 Å) into a bilayer structure with a d-spacing of 54 Å. Since the phase transition temperature to smectic E (SmE) mesophase is 144 °C,¹ a series of peaks assignable to SmE were observed when heated to 180°C. In addition, a mixed layer of monolayer and bilayer structures appeared when the Ph-BTBT-10 thin film was rapidly cooled from 180 °C to RT. This result indicates that the cooling speed might be a significant factor in forming a well-uniformed crystalline thin film structure. Based on these results, Ph-BTBT-10 can be handled through vacuum deposition method, and the phase transition from the monolayer to the bilayer structure can occur in the same way which occurs for solution processed Ph-BTBT-10 thin films.¹
Finally, we demonstrated top-ranked FET performances through vacuum deposition process. This presentation will also include fabrication and evaluation of Ph-BTBT-10-based device via dip-coating method.

1) H. Iino, T. Usui, J. Hanna, Nat. Commun. 2015, 6, 6828.

In an organic light-emitting diode (OLED) device, the spatial distribution of dopant molecules in the emissive layer host-guest blend impacts key performance characteristics. Specifically, aggregation of dopant molecules is expected to exacerbate exciton annihilation processes associated with efficiency roll-off and device degradation. We have developed atom probe tomography (APT) techniques that allow us to create a chemically sensitive three-dimensional reconstruction of molecular positions (i.e. nanometer-scale spatial resolution with sub-Dalton mass discrimination). APT allows us to study dopant aggregation in OLED emissive layers based on a blend of multiple molecular constituents.

We have used spatial statistics analysis of APT data to show that Ir(ppy)₃ aggregates in a number of commonly used hosts; here, we investigate the effects of aggregation in an mCBP:Ir(ppy)₃ emissive layer blend. By controlling substrate temperature during deposition, we show that we can control the spatial distribution of the dopant in the film. Hole mobility increases over two orders of magnitude in this system as substrate temperature increases from -50°C to 50°C (mCBP T_g = 97°C), which is consistent with the Gaussian Disorder Model and the assumption that holes are transported on the Ir(ppy)₃ Highest Occupied Molecular Orbital (HOMO). We also show that the device triplet-triplet annihilation (TTA) rate increases with substrate temperature, as expected with stronger guest aggregation. Understanding how guest aggregation affects exciton dynamics and annihilation in an OLED emissive layer is an important step in maximizing device efficiencies and reducing the effects of long-term degradation.

Patterning structures at the scale of the wavelength of light is central to photonic device fabrication. Whilst standard patterning techniques such as e-beam lithography can be used to pattern the topography of many materials, orientation, configuration and conformation can also be used to control the optical properties of molecular materials. An example of conformation control is formation of the β-phase in the organic semiconductor poly(9,9-dioctylfluorene) (PFO), in which polymer chain segments adopt a rigid, planar structure when subjected to particular solvent interactions. The photophysics of this phase has been studied in great detail, with particular focus on the presence of a new long wavelength absorption peak, which results in a significant change in refractive index within the optical gap (10.9% at 450nm). The index change offers potential for photonic element fabrication, in a fashion that mimics dielectric metamaterial structuring on a molecular length scale. Spatial patterning of the structure defines the photonic elements, with millimeter scale patterning shown in 2009, followed by micron scale in 2012, and nanoscale in 2015. We now report a further study on the use of Dip Pen Nanolithography (DPN) - an AFM derived single step writing process - as a technique to pattern nanoscale structures and discuss their suitability for photonic device applications.

Fabrication of electronic devices based on organic semiconductors at the fundamental level can be considered a problem of engineering of polycrystalline materials. Tuning the charge transport properties in such materials requires controlling of size, perfection, distribution, and orientation of crystalline domains in the semiconducting films¹. Those parameters can be controlled through film formation and post-processing conditions, such as thermal annealing. While the casting parameters (solvent, temperature etc.) exert an influence on the primary crystallization and formation of polycrystalline system, the annealing induces the further growth of crystalline domains, increases perfection of crystal packing but may also trigger phase transitions of the ordered phases crystallized from solution². Insufficient insight into the processes occurring upon the annealing may lead to erroneous conclusions related to properties - especially when samples reveal a complex phase behavior like the one observed in the case of aromatic, conjugated molecules or polymers. These materials may show multiple packings in crystalline phases as well as multiple thermally-induced transitions between crystalline and liquid crystalline phases before reaching the isotropic state³. Different packing of molecules and hence different geometrical overlap of aromatic moieties may affect charge transport properties⁴. Thus in our research we concentrated on relationships between packing of molecules in the ordered domains and electronic properties of materials.
The study was focused on the blends of poly(3-hexylthiophene) (P3HT) with butyl- or hexyl- or octyl-substituted naphthalene diimides (NDI). In our experimental approach we have applied differential scanning calorimetry (DSC) to determine phase transition points and X-ray diffraction (XRD) to gain insights into the structure of molecular assemblies in the system. These studies were supplemented with images from scanning electron microscopy (SEM) providing information about the morphology of the blends. In order to correlate the structure and morphology with charge transport properties, the blends were tested in organic field effect transistors (OFETs).
Results of our experiments indicated that packing of molecules in crystalline phases and mesophases as well as phase transition temperatures were related to molecular architecture of NDI and compositions of the blends. XRD and DSC data were used to determine phase diagrams for the blends of P3HT with NDI. It was found that decreasing content of NDI in the blends caused a notable decrease of the isotropization temperature of the NDI component. The melting point of P3HT also significantly drops as its content in the blend decreases. The profile of the isotropization points of the P3HT:NDI blends plotted as a function of composition reveals a clear minimum resembling the eutectic point. In some of the blends, we observed additional transitions that may suggest formation of new crystalline phases occurring neither in pure P3HT nor pure NDI. The studies of the blends by means of scanning electron microscopy indicated that compositions of the blends also exerted an influence on the crystalline morphology. The studies electronic properties in the model field-effect transistors (OFETs) revealed that the blends with the uniform, fine-grained polycrystalline morphologies enable balanced ambipolar charge carrier transport. This, once again, indicates that the fine tuning of the blends composition is crucial for engineering of organic electronic devices.

References
[1] Brady M. A. et al., Soft Matter, 7, 11065 (2011)
[2] Chlebosz D. et al., Dyes and Pigments, 140, 491 (2017)
[3] Yuan Y. et al., Polymer, 105, 88 (2016)
[4] Kakinuma T. el al., Journal of Materials Chemistry C, 1, 5395 (2013)

The work was supported by National Science Centre, Poland through the grant DEC-2016/22/E/ST5/00472

In the past decade, significant progress has been made in the fabrication of organic photovoltaic devices (OPVs), predominantly due to important improvements of existing materials and the creation of a wealth of novel compounds. Many challenges, however, still exist. Real understanding of what structural and electronic features determine, for instance, the short-circuit current (Jsc), open-circuit voltage (Voc) and fill factor are still lacking; and the role of charge transfer states and which charge transfer states are critical for efficient charge generation are still debated. Here we attempt to obtain further insight of relevant structure/processing/performance interrelations using classical polymer processing ‘tools’. We present a survey on the principles of structure development of this material family and how it can be manipulated, with focus on how to control the phase morphology and important interfaces (molecular and between different phase regions). Goal is to tailor and tune the final ‘morphology’ towards establishing correlations with relevant device characteristics. Examples are given based on polymer:fullerene solar cells as well as solution-processed perovskite structures.

Nanostructured liquid-crystalline (LC) assemblies forming columnar, smectic, and bicontinuous cubic phases can provide 1D, 2D, and 3D nano-channels capable of transport of ion and electron [1,2]. We have developed these nanostructured functional soft materials. Potential applications of these materials as electrolytes electrochemical devices, luminescence materials, and semiconductors have been shown. Here we report design, self-assembled structures, and functions of nanostructured LC materials exhibiting ion and electron transport properties. Nanostructured Ionic liquid crystals bearing imidazolium and ammonium moieties and liquid crystals complexed with ionic species exhibit 1D, 2D, and 3D ion transport properties. They can be used for electrolytes for lithium ion batteries [3.4] and photovoltaics [5]. They also show stimuli responsive properties such as conductivity switching [6] and photo-control of ion transport directions [7]. Electron transport LC materials have also been developed [8,9]. Supramolecular hydrogen-bonded electron transport materials exhibit ion recognition properties [9].
References:
[1] T. Kato, M. Yoshio, T. Ichikawa, B. Soberats, H. Ohno, M. Funahashi, Nat. Rev. Mater., 2, 17001 (2017).
[2] T. Kato, J. Uchida, T. Ichikawa, T. Sakamoto, Angew. Chem. Int. Ed. 57, 4355 (2018).
[3] J. Sakuda, E. Hosono, M. Yoshio, T. Ichikawa, T. Matsumoto, H. Ohno, H. Zhou, T. Kato, Adv. Funct. Mater., 25, 1206 (2015).
[4] T. Onuma, E. Hosono, M. Takenouchi, J. Sakuda, S. Kajiyama, M. Yoshio, T. Kato, ACS Omega, 3, 159 (2018).
[5] D. Högberg, B. Soberats, R. Yatagai, S. Uchida, M. Yoshio, L. Kloo, H. Segawa, T. Kato, Chem. Mater., 28, 6493 (2016).
[6] B. Soberats, M. Yoshio, T. Ichikawa, X. Zeng, H. Ohno, G. Ungar, T. Kato, J. Am. Chem. Soc., 137, 13212 (2015).
[7] B. Soberats, E. Uchida, M. Yoshio, J. Kagimoto, H. Ohno, T. Kato, J. Am. Chem. Soc., 136, 9552 (2014).
[8] T. Yasuda, T. Shimizu, F. Liu, G. Ungar, T. Kato, J. Am. Chem. Soc., 133, 13437 (2011).
[9] K. P. Gan, M. Yoshio, Y. Sugihara, T. Kato, Chem. Sci., 9, 576 (2018).

Organic semiconductors have attracted attention due to their relative ease of fabrication, cost economy, and uniquely tunable optical and electronic properties via functionalization of the molecules. These properties in the solid state are determined both by the molecular structure and molecular packing, and thus it is important to understand the contribution of each factor to the resulting performance of the material, although separating these contributions remains challenging. Towards resolving the challenge, we present a systematic study of the optical and optoelectronic properties of single crystals of fluorinated anthradithiophene (ADT) functionalized with various side groups R (diF R-ADT) that control the molecular packing.

The optical properties of isolated diF R-ADT molecules are dominated by those of the backbone of the molecule, irrespective of the side groups R. In the solid state, however, different side groups R enable different crystal packing structures, and variation of R allows one to isolate the crystal packing as an experimental variable. The best known diF R-ADT derivative is that with R = TES (triethysilylethynyl), which exhibits a 2D “brick-work” molecular packing motif (Z = 1) enabling hole mobility of up to ~7 cm²/(Vs). Here, we analyze several diF R-ADT crystalline systems with Z≥1 and a broad variety of packing motifs including 1D “sandwich-herringbone”, 1D “sandwich-brick-work” and 1D “stack” motifs.

By quantifying the absorption and photoluminescence characteristics of these diF R-ADT crystals as functions of polarization and temperature, we elucidated molecular packing-dependent structure and dynamics of excitons. Closely spaced π-π stacking in these crystals show evidence of strong admixtures of both Frenkel and Charge-Transfer (CT) excitons, while inter-stack interactions give rise to distinctly polarized Davydov components with the Davydov splitting strongly dependent on the crystal structure. Molecular packing-dependent formation of excimers was also observed. Next, we investigated the influence of crystal structure on light-matter coupling by incorporating crystalline diF R-ADT into microcavities. Selective interactions with the cavity, depending on the nature of the excited state, were observed and quantified.

The optoelectronic properties of diF R-ADT crystals were established using measurements of time-resolved and steady-state photocurrent. More than an order of magnitude difference was observed in the peak photocurrents at sub-nanosecond time-scales after photoexcitation, depending on the molecular packing, due to differences in intrinsic charge carrier mobilities. Although shallow charge trap energies with a characteristic energy of ~40-50 meV were similar for diF R-ADT crystals with drastically different packing motifs, differences in the charge trapping and detrapping dynamics were observed. Considerably better electronic performance was achieved in the TES derivative with the 2D “brick-work” packing as compared to, for example, the TSBS (TSBS = tri-sec-butylsilylethynyl) derivative with the 1D “sandwich-herringbone” packing motif.

Unraveling the orientation of emissive dye molecules within organic light-emitting diodes (OLEDs) is of crucial importance to achieve high performing devices. Within the last years vast progress has been made in understanding and predicting the preferential alignment of organic dye molecules using both experimental and computational methods. The most common technique to determine the orientation of the emissive species within a thin film is angular dependent photoluminescence spectroscopy. However this method can only extract the second moment of the transition dipole moment distribution with respect to the film surface, leaving several possible configurations of the investigated molecule within a three dimensional space.^[1,2]
To overcome this limitation we applied a second measurement technique, probing the permanent dipole moment and thus giving information of a second axis within the molecular frame. This method itself already allows for a prediction of the dye alignment. Further, if combined with the optical measurements, the properties of two molecular axes can be deduced ^[3][1]
We applied this novel method to two well-known Ir-complexes, fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy)₃)and its heteroleptic counterpart Bis(2-phenylpyridine)(acetylacetonate)iridium(III) (Ir(ppy)₂(acac)). The orientation of the emissive transition dipole moment of both molecules has been determined several times, revealing the preferential alignment of the Ir(ppy)₂(acac) molecule, while the homoleptic compound showed isotropic behavior.^[4] Nevertheless, the only predictions of the molecular alignment made so far, rely on computational methods such as molecular dynamics simulations. ^[5]
After carefully taking the amorphous nature of organic guest host systems into account, the application of the combined method revealed the preferential alignment of the heteroleptic compound, which is in agreement with earlier predictions. Furthermore the homoleptic dye molecule was confirmed to be isotropically oriented within a certain threshold of measurement uncertainty.
The application of this novel technique is not only limited to transition metal complexes in organic guest-host systems but can also be applied to any polar film within organic optoelectronic devices.

[1] T. Morgenstern, M. Schmid, A. Hofmann, M. Bierling, L. Jäger, W. Brütting, ACS applied materials & interfaces 2018, 10, 31541.
[2] M. J. Jurow, C. Mayr, T. D. Schmidt, T. Lampe, P. I. Djurovich, W. Brutting, M. E. Thompson, Nature materials 2016, 15, 85.
[3] L. Jäger, T. D. Schmidt, W. Brütting, AIP Advances 2016, 6, 95220.
[4] P. Liehm, C. Murawski, M. Furno, B. Lüssem, K. Leo, M. C. Gather, Appl. Phys. Lett. 2012, 101, 253304.
[5] C.-K. Moon, K.-H. Kim, J.-J. Kim, Nature communications 2017, 8, 791.