Symposium EN12 : Structure–Function Relationships and Interfacial Processes in Organic Semiconductors for Optoelectronics

In recent years, partially fluorinated organic semiconducting polymers have aroused a considerable interest, to be used as electron-donating material in organic photovoltaic solar cells and have led to a significant increase in power conversion efficiency.^[1-3] There are several reasons for this success, including:
a planarization of the polymer skeletons via weak bonds between the fluorine atoms and the neighboring atoms and heteroatoms that improves the charge transport and the π-stacking in fluorinated polymers,
the electro-deficient nature of the fluorine atom, which significantly stabilizes the energy levels of the frontier molecular orbitals.

For similar reasons, the chlorine atom has recently emerged as a suitable substitute for fluorine in some (macro)-molecular structures.^[4] However, its larger steric size and reduced electronegativity, as compared to fluorine, is found to significantly modulate its impact.
In this communication, we propose to clarify the architectural principles that govern the properties of fluorinated and chlorinated polymers. In particular, we will discuss the evolution of the structural and optoelectronic properties of two families of organic semiconducting polymers incorporating either of these two halogens, according to their positions and numbers. Finally, through additional molecular engineering work at the level of side chains (in particular via innovative siloxane chain functionalization), we will also discuss the role of these side chains on the polymers solubility, on their orientation on the substrate as well as on their miscibility with electron-accepting materials.

[1] N. Leclerc, P. Chávez, O. A. Ibraikulov, T. Heiser and P. Lévêque, Polymers, 2016, 8, 11.
[2] O. A. Ibraikulov, B. Heinrich, P. Chávez, I. Bulut, C. Ngov, O. Boyron, N. Brouckaert, S. Swaraj, K. L. Gerasimov, D. A. Ivanov, S. Mery, N. Leclerc, P. Lévêque and T. Heiser, J. Mater. Chem. A, 2018, 6, 12038.
[3] J. Yuan, Y. Zhang, L. Zhou, G. Zhang, H.-L. Yip, T.-K. Lau, X. Lu, C. Zhu, H. Peng, P. A. Johnson, M. Leclerc, Y. Cao, J. Ulanski, Y. Li and Y. Zou, Joules, 2019, 3, 1.
[4] S. Zhang, Y. Qin, J. Zhu and J. Hou, Adv. Mater., 2018, 1800868.

My talk will give an overview about our recent findings on structure-function relationships of n-type donor-acceptor copolymer films based on P(NDI2OD-T2).¹ On the molecular scale the role of 2,6- and 2,7-linked regioisomers in the polymer backbone for the preparation of regioregular (RR) and regioirregular (RI) systems is discussed.² A specificity of P(NDI2OD-T2) is its strong tendency for aggregation in solution. We find that with increasing regioirregularity the overall aggregation tendency goes down which is beneficial for applications where mixing with other compounds is necessary, e.g. for organic photovoltaics or for chemical doping.
On the other hand the strong aggregation tendency of the RR system is useful for the preparation of highly anisotropic structures. On the mesoscopic scale we give new insights in the structure formation upon solution deposition.³ A morphology analysis shows that the deposited films are based on characteristic fiber morphologies where the fiber and chain directions are coinciding. The involvement of liquid-crystalline phases, in particular nematic-like preordering in highly concentrated solutions is demonstrated. Solvent vapor annealing is shown to lead to large area alignment in spherulite-like superstructures with semi-crystalline structures. Particularly nice is the applicability of blade-coating to achieve highly anisotropic films where both the bulk and the surface morphology can be determined: whereas blade-coated films after 220°C annealing give evidence for face-on orientation in the bulk with segregated stacking in polymorph Form I, 300°C annealing leads to an edge-on orientation with mixed stacking in Form II.^3,4 All films have a top edge-on layer. Using these well-defined morphologies we can access detailed insights into electrochemical and chemical doping mechanisms.^5,6

1) Y. M. Gross, and S. Ludwigs, P(NDI2OD-T2) Revisited – Aggregation Control as Key for High Performance n-Type Applications, Synthetic Metals 2019, 253, 73.
2) Y.M. Gross, D. Trefz, R. Tkachov, V. Untilova, M. Brinkmann, G.L. Schulz, S. Ludwigs, Tuning Aggregation by Regioregularity in N-Type P(NDI2OD-T2) Donor-Acceptor Copolymers, Macromolecules 2017, 50, 5353.
3) D. Trefz, Y. M. Gross, C. Dingler, R. Tkachov, A. Hamidi-Sakr, A. Kiriy, C. R. McNeill, M. Brinkmann, S. Ludwigs, Tuning Orientational Order of Highly Aggregating P(NDI2OD-T₂) by Solvent Vapor Annealing and Blade Coating, Macromolecules 2019, 52, 43.
4) K. Tremel, F.S.U. Fischer, N. Kayunkid, R. Di Pietro, R. Tkachov, A. Kiriy, D. Neher, S. Ludwigs, M. Brinkmann, Charge Transport Anisotropy in Highly Oriented Thin Films of the Acceptor Polymer P(NDI2OD-T2), Adv. Energy Mater. 2014, 1301659.
5) Y. M. Gross, D. Trefz, C. Dingler, D. Bauer, V. Vijayakumar, V. Untilova, L. Biniek, M. Brinkmann, and S. Ludwigs, From Isotropic to Anisotropic Conductivities in P(NDI2OD-T2) by (Electro-)Chemical Doping Strategies, Chemistry of Materials 2019, 31, 3542.
6) D. Trefz, A. Ruff, R. Tkachov, M. Wieland, M. Goll, A. Kiriy, S. Ludwigs, Electrochemical Investigations of the N-Type Semiconducting Polymer P(NDI2OD-T2) and Its Monomer: New Insights in the Reduction Behavior, J. Phys. Chem. C 2015, 119, 22760.

In this presentation we report the development of novel organic semiconductors, as well as the process engineering, for mechanically flexible transistors and circuits. Particularly, we show that “ultra-soft” polymers comprising naphthalenediimides (NDI) units co-polymerized with “rigid” and “flexible” organic units can change how charge transport is affected by mechanical stress, demonstrating that polymer backbone composition is more important that film degree of texturing. This strategy enables to reduce the elastic modulus of the semiconducting film by >2-4x while retaining good charge transport characteristics. In addition, by fabricating polymer/polymer blends by shear techniques, it provides a new avenue to enhance charge transport and achieve excellent mechanical robustness, which is further increased by modification of the film morphology. Thus, we demonstrate that these materials can enable TFT-based circuits for ultra-flexible displays and sensors on plastics. Finally, new oxide-polymer blends can be used to fabricate high-rectification and stretchable diodes via the self-assembly/phase separation properties of polymers having different surface energy.

In the last years, non-fullerene acceptors (NFAs) have strongly burst into the field of organic photovoltaics (OPV) as alternative to fullerene derivatives. NFAs molecules not only have overcome the technical drawbacks associated to the fullerenes, but they also have made possible the increase of the efficiency over 16%¹ in single junction and over 17%² in tandem solar cells. Despite the extensive efforts made to achieve high efficiencies, more investigations are needed to further understand degradation mechanisms and hence improve their stability in order to make OPV technology a viable contender with technologies based on inorganic semiconductors for commercialization.
In that context, we developed three new ITIC-like NFAs with extended p-conjugated spacers between the donor and acceptor moieties. We present the synthesis and characterization of these innovative NFAs, the performances obtained in bulk-heterojunction solar cells, in addition to a comparative study of the degradation mechanisms. The fluorination of the acceptor moiety was employed as a strategy to improve the device performance³. The organic solar cells fabricated with the new NFAs combined with PCE12 as the electron donor exhibit, power conversion efficiencies comprised between 5-6 %, as preliminary results. Noticeably, the new NFAs allowed to reach V_oc over 1.1 V which is significantly higher than the V_oc obtained with the reference ITIC-based devices. Interestingly, one of the new NFA demonstrates remarkable stability compared to ITIC.
In this communication, the synthetic strategies, the optoelectronic properties of the molecules, the performances in devices and the degradation mechanisms under thermal and photochemical stress will be presented and discussed.


References:
[1]: Y. Cui et al., “Over 16% efficiency organic photovoltaic cells enabled by a chlorinated acceptor with increased open-circuit voltages” Nat. Commun. 10, 2515, 2019.

[2]: L. Meng et al., “Organic and solution-processed tandem solar cells with 17.3% efficiency,” Science, vol. 361, 1094, 2018.

[3]: T. J. Aldrich et al., “Fluorination Effects on Indacenodithienothiophene Acceptor Packing and Electronic Structure, End-Group Redistribution, and Solar Cell Photovoltaic Response”, J. Am. Chem. Soc, vol. 141, 3274, 2019.

Up to now, halochromism has mostly been demonstrated on chromophores absorbing in the UV-visible range. However, research in the field of Infra-Red (IR) technologies is in strong development, driven by technological needs in military and civilian applications, such as imaging, optical communications, energy or photodetectors.[1] Also, since 50 % of the solar energy falls into the IR spectral region, photovoltaic materials are under development to increase solar cells efficiency.[2] IR-materials are also synthesized for biosensing and bioimaging because IR light penetrates into tissues, the so-called “biological window”.[3] To design organic IR materials, the basic principle is to reduce the bandgap. Specifically, synthesis of electron donor–acceptor (D–A) alternating conjugated copolymers has demonstrated high potential to decrease the bandgap under 1.5 eV, leading to IR-absorbing or emitting materials.[4] For the moment, most of these organic materials showed a maximum absorption peaks falling in the first NIR optical window covering 750−1000 nm. Actually, the second NIR optical window covering 1000−1350 nm is more promising for biological applications due to its higher photothermal conversion and deeper tissue penetration.[5]
In this presentation, we will report the synthesis of a low bandgap copolymer based on the 2,5-diazapentalene (DAP) unit, derived from the diketopyrrolopyrrole (DPP) chromophore. We combine the strong acceptor DAP unit with the dithienosilole (DTS), a photostable electron donor [6] that allows the introduction of solubilizing alkyl chains onto the silicon atom. As the result of the polymerization, an IR-material was synthesized with a maximum absorption at 850 nm in chloroform solution. Upon protonation of the DAP unit with Brønsted acids or its complexation with Lewis acids, the maximum of absorption is further shifted up to 1100 nm (edge at 1500 nm) in the second NIR optical window. To the best of our knowledge, this halochromic behavior is the highest reported up to now. Using a combination of spectrophotometry, cyclic voltammetry and DFT calculations, we identified the Brønsted and Lewis adducts that are formed. We can demonstrate that this optical shift is correlated with a decrease of the copolymers bandgap associated to the decrease in the LUMO energy and enhancement of the pi-electron delocalization along the conjugated backbone, as revealed by the lowering of the bond length alternation.[7]
Acknowledgments: Agence Nationale de la Recherche (TAPIR project no. ANR–15-CE24-0024-02) and Région Nouvelle Aquitaine (TAMANOIR project no. 2016-1R10105-0007207) for their financial support. Pole Modélisation HPC facilities of the Institut des Sciences Moléculaires, co-funded by the Nouvelle Aquitaine region, as well as by the MCIA (Mésocentre de Calcul Intensif Aquitain) resources of the Université de Bordeaux and of the Université de Pau et des Pays de l’Adour for computer times.
[1] Myochin, T.; Kiyose, K.; Hanaoka, K.; Kojima, H.; Terai, T.; Nagano, T. JACS 2011,133(10), 3401
[2] a) Ameri, T.; Khoram, P.; Min, J.; Brabec, C. J., Adv. Mater. 2013,25(31), 4245; b) Hendriks, K. H.; Li, W.; Wienk, M. M.; Janssen, R. A. J. JACS 2014,136(34), 12130
[3] Croissant, J. G.; Zink, J. I.; Raehm, L.; Durand, J.-O., Adv. Healthcare Mater.1701248-n/a.
[4] Liu, C.; Wang, K.; Gong, X.; Heeger, A. J., Chem. Soc. Rev. 2016,45(17), 4825; b) Qi, J.; Qiao, W.; Wang, Z. Y., Chemical Record 2016, 1531
[5] He, Y.; Cao, Y.; Wang, Y., Asian J. Org. Chem. 2018,7(11), 2201
[6] Cheng, P.; Zhan, X., Chem. Soc. Rev. 2016,45(9), 2544
[7]Khelifi, W.; Awada, H.; Brymora, K.; Blanc, S.; Hirsch, L.; Castet, F.; Bousquet, A.; Lartigau-Dagron C., Macromolecules 2019, under revision

Designing stable open-shell organic materials through the modifications of the π-topology of molecular organic semiconductors have attracted recently considerable attention.¹ However, their uses as an active layer in organic field-effect transistors (OFETs) are very limited and the obtained hole and electron charge mobilities are around 10⁻³ cm² V⁻¹ s⁻¹.
Herein, we disclosed the synthesis of two peri-fused materials so-called tetraceno-tetracene (TT) and pentacenopentacene (PP) which have low band gap of 1.79 and 1.42 eV, respectively .² Their ground state natures have been investigated by different experiments including steady state absorption, electron spin resonance, superconducting quantum interfering device (SQUID) and variable temperature NMR along with DFT calculations. TT and PP have closed-shell and singlet open-shell structures in their ground state, respectively, and possess high stability. Their biradical characteristics were found to be 0.50 and 0.64. The origin of the open-shell character of PP is related to the concomitant opening of two tetracenes with the recovering of two extra aromatic sextets and a small HOMO-LUMO energy gap (gap < 1.5 eV). Thanks to the high stability, thin film OFET devices could be fabricated. In TG-BC configuration PP shows remarkably high hole mobility of 1.4 cm² V⁻¹ s⁻¹while TT exhibits a hole mobility of 0.77 cm² V⁻¹ s⁻¹. In configuration of BG-TC, ambipolar behaviors for both were obtained with hole and electron mobilities of 0.21 and 0.01 cm² V⁻¹ s⁻¹ for PP and 0.14 and 0.006 cm² V⁻¹ s⁻¹ for TT.
References
¹ Gopalakrishna, T. Y.; Zeng, W.; Lu, X.; Wu, J. Chem. Commun. 2018, 54, 2186.
² Jousselin-Oba, T.; Mamada, M.; Marrot, J.; Maignan, A.; Adachi, C.; Yassar, A.; Frigoli, M. J. Amer. Chem. Soc. 2019, DOI: 10.1021/jacs.9b03488.

As organic semiconductors attract increasing attention to application in diverse fields such as bioelectronics and artificial photosynthesis, understanding and improving their robust operation in a variety of challenging environments is a critical task. In this presentation, results from our lab are highlighted including our development of a morphology control strategy using conjugation-break spacers [1], demonstration of melt-processed small-molecule OPVs [2], and investigations of covalently-linked block[3] and network polymers[4], which afford tunable charge transport and solvent tolerance—making them promising for application as charge-transport interlayers in all-solution processed devices. In addition the application of bulk-heterojunctions to artificial photosynthesis via photoelectrochemical water splitting[5] will be discussed in terms of the requirements for material stability.

[1] A. Gasperini, X. A. Jeanbourquin, A. Rahmanudin, X. Yu, K. Sivula, Adv. Mater. 2015, 27, 5541.
[2] A. Rahmanudin, L. Yao, X. A. Jeanbourquin, Y. Liu, A. Sekar, E. Ripaud, K. Sivula, Green Chem. 2018, 20, 2218.
[3] A. Rahmanudin, L. Yao, A. Sekar, H.-H. Cho, Y. Liu, C. R. Lhermitte, K. Sivula, ACS Macro Lett. 2019, 8, 134.
[4] L. Yao, A. Rahmanudin, X. A. Jeanbourquin, X. Yu, M. Johnson, N. Guijarro, A. Sekar, K. Sivula, Adv. Funct. Mater. 2018, 28, 1706303.
[5] L. Yao, A. Rahmanudin, N. Guijarro, K. Sivula, Adv. Energy Mater. 2018, 1802585.

The use of novel organosilica materials embedding π-conjugated moieties as semiconductor into field effect transistors will be presented. For that purpose, the [1]benzothieno[3,2-b][1]benzothiophene (BTBT),¹ has been chosen as π-conjugated core, first functionalized with hydroxyl groups² and then modified with hydrolysable and cross-linkable triethoxysilyl moieties. After polycondensation, this compound forms a hybrid material composed of charge transport pathways as well as insulating layers (SiOx). However, overall, the material is found to be a semiconductor and can be incorporated into field effect transistors. Taking advantage of the solgel chemistry³ involved here, we built Hybrid Field Effect Transistors that are fully cross-linked with covalent bonds.⁴ Molecules are cross-linked to each other, covalently bonded to the silicon oxide dielectric and also covalently bonded to the gold electrode thanks to the use of an appropriate additional interfacial monolayer in between. This is the first report of fully covalent transistors. Those devices show impressive resilience against polar, aliphatic and aromatics solvents (even under sonication). This study opens the route towards a new class of hybrid materials to create highly robust electronic applications.

REFERENCES
[1] Izawa, T.; Miyazaki, E.; Takimiya, K. Molecular Ordering of High Performance Soluble Molecular Semiconductors and Re-evaluation of Their Field Effect Transistor Characteristics. Adv. Mater. 2008, 20, 3388-3392.
[2] Roche, G. H.; Tsai, Y.-T.; Clevers, S.; Thuau, D.; Castet, F.; Geerts, Y. H.; Moreau, J. J. E.; Wantz, G.; Dautel, O. J.The Role of H-bonds in the Solid State Organization of [1]Benzothieno[3,2-b][1]benzothiophene (BTBT) Structures : Bis(hydroxy-hexyl)-BTBT, as a Functional Derivative Offering Efficient Air Stable Organic Field Effect Transistors (OFETs).J. Mat. Chem. C, 2016, 6742-6749.
[3] Dautel, O. J.; Wantz, G.; Almairac, R.; Flot, D.; Hirsch, L.; Lere-Porte, J.-P.; Parneix, J.-P.; Serein-Spirau, F.; Vignau, L.; Moreau, J. J. E.Nanostructuration of Phenylenevinylenedi-Imide-Bridged Silsesquioxane: From Electroluminescent Molecular J-Aggregates to Photoresponsive Polymeric H-Aggregates.J. Am. Chem. Soc., 2006, 128, 4892-4901.
[4] Roche, G. H.; Thuau, D.; Valvin, P.; Clevers, S; Tjoutis, T.; Chambon, S.; Flot, D.; Geerts, Y. H.; Moreau, J. J. E.; Wantz G.; Dautel, O. J. π-Conjugated Organosilica Semiconductors: Toward Robust Organic Electronics. Adv. Electron. Mater. 2017, 1700218.

The effect of side chain length and type on the microstructure and organic field-effect transistor (OFET) performance of solution-processed naphthalene diimide (NDI) thin films will be discussed. Linear side chains with four (C4), five (C5), six (C6), eight (C8) and twelve (C12) carbon atoms are studied along with a branched ethyl hexyl (EH) side chain. Interestingly, relatively high mobilities of up to ~ 0.2 cm²/Vs are achieved for short (C4) and long (C12) side chains with linear chains of intermediate length and the branched side chain producing lower mobilities. The observed mobility trends are explained in terms of the competing influence of changes in crystal packing and changes in thin film morphology with changes in side chain length. Shorter side chains produce changes in the lateral stacking of NDI units which promote higher mobility while longer side chains produce solution-processed thin films with higher film quality evidenced by larger domain sizes and lower orientational disorder. Side chain length is also found to strongly modulate the molecular orientation of the NDI core, with high edge-on orientations observed for long chains, and tilted orientations for short chains. Thin film microstructure is investigated using a range of techniques including atomic force microscopy, grazing incidence wide-angle X-ray scattering and near-edge X-ray absorption fine-structure spectroscopy.

Molecules showing thermally activated delayed fluorescence (TADF) have emerged as a promising alternative to heavy-metal complexes for applications in high efficiency organic light emitting diodes (OLEDs).[1] To date the most successful TADF emitters have been designed with electron donor (D) and electron acceptor (A) units covalently linked with conjugational separation between the units. This separation can lead to a very small excited state singlet-triplet gap (ΔE_ST) on the order of a few meV. If ΔE_ST is small, a significant population of the triplet state that occupies upper vibrational levels is able to undergo reverse intersystem crossing (RISC), giving rise to delayed fluorescence. OLEDs fabricated with TADF emitters have shown impressive performances, sometimes with external efficiencies (EQE) above 30%.[2]
We will report how the efficiency of TADF can be controlled by precise molecular design of donor-acceptor linked molecules, with particular emphasis on the rigidity of the molecular framework and the dihedral angle between the donor and acceptor units. Representative building blocks include: phenothiazine, acridine and triazaangulene as donors, and dibenzothiophene-S,S-dioxide and 9,9-dimethylthioxanthene-S,S-dioxide as acceptor units. Synthesis, photophysical studies, theoretical calculations and high efficiency OLED data, including blue/deep-blue emitters will be presented, based on our on-going studies.[3-6]

References
[1] Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Nature 2012, 492, 234–238.
[2] Liu, Y.; Li, C.; Ren, Z.; Yan, S.; Bryce, M. R. Nat. Rev. Mater. 2018, 3, 18020.
[3] Nobuyasu, R. S.; Ward, J. S.; Gibson, J.; Beth A. Laidlaw, B. A.; Ren, Z.; Data, P.; Batsanov, A. S.; Penfold, T. J.; Bryce, M. R.; Dias, F. B. The influence of molecular geometry on the efficiency of thermally activated delayed fluorescence. J. Mater. Chem. C. 2019, DOI: 10.1039/c9tc00720b.
[4] Ward, J. S.; Kukhta, N. A.; dos Santos, P. L.; Congrave, D. G.; Batsanov, A. S.; Monkman, A. P.; Bryce, M. R. Carbon–Carbon Bonded Donor–Acceptor Molecules with Functionalized Azatriangulene Cores: Examples of Blue Upper-Triplet State Emitters, Chem. Mater. 2019, DOI: 10.1021/acs.chemmater.9b01184.
[5] Kukhta, N. A.; Batsanov, A. S.; Bryce, M. R.; Monkman, A. P. Importance of Chromophore Rigidity on the Efficiency of Blue Thermally Activated Delayed Fluorescence Emitters. J. Phys. Chem. C 2018, 120, 28564-28575.
[6] Chen, C.; Huang, R.; Batsanov, A. S.; Pander, P.; Hsu, Y-T.; Chi, Z.; Dias, F. B.; Bryce, M. R. Intramolecular Charge Transfer Controls Switching Between Room Temperature Phosphorescence and Thermally Activated Delayed Fluorescence. Angew. Chem., Int. Ed. 2018, 57, 16407-16411.

Acknowledgements. We thank EPSRC grant EP/L02621X/1 and EU Horizon 2020 Grant Agreement No. 732103 (HyperOLED) for funding.

Organic light emitting diodes (OLEDs) have been of great interest for display and lighting applications during decades and have been increasing to be utilized in smartphone and flat-panel display due to many advantages such as self-emission, high contrast, wide viewing angle, and so on.
From the OLED material point of view, improvement of emissive exciton generation probability is an important factor for higher efficiency. After the development of fluorescent materials, the phosphorescent materials have been introduced and have shown to achieve high efficiency. On the other hand, phosphorescent materials require expensive heavy metal such as Ir and Pt. Recently, thermally activated delayed fluorescence (TADF) materials have been developed as a new class of light emitting material where triplet excitons are converted into singlet excitons without heavy metals in phosphorescent materials.[1] Theoretically, an internal quantum efficiency (IQE) of 100%, the same as for phosphorescent materials, can be expected.
In order to study the optical properties of materials, we have employed the time dependent density functional theory (TDDFT), which is one of the most prominent and widely used methods for calculating excited states of various molecules, and it is recognized as a powerful tool for studying their electronic transition. In our calculations, the real-time and real-space (RSRT) techniques are employed in solving time dependent Kohn-Sham equation by the finite difference approach [3] without using explicit bases such as plane waves and Gaussian basis. Within the frame work of this approach, we can solve for the wave functions on the grid with a fixed domain, which encompasses the physical system of interests. Furthermore, we have applied the maximum entropy method (MEM) to the optical analysis of the time-series data from the real-time TDDFT. We have confirmed that the MEM technique provides higher resolution in fewer computational steps, compared to the conventional Fourier transform technique.
In this study, we have focused on the spectrum of TADF materials including materials based on DABNA[2], which has recently been studying intensively because of its narrow FWHM (full-width at half maximum) in its spectrum, that is, of its high color purity. Main focus would be to design new materials with small exchange energy required for TADF character and with good color purity, starting from the understanding of existing molecules and the effect of substituents by quantum chemical calculation. The peak emission wavelength seems to be well predicted. In the presentation, we will discuss the correlation between simulation and experiment in peak wavelength and also the spectrum shape.

REFERENCES
[1] K. Goushi, K. Yoshida, K. Sato, C. Adachi, Nat. Photonics, 6, 253(2012)
[2] T. Hatakeyama, K. Shiren, K. Nakajima, S. Nomura, S.Nakatsuka, K. Kinoshita, J. Ni, Y. Ono, T. Ikuta, Adv. Mater., 28, 2777(2016)
[3] N.Akino and Y.Zempo, MRS Advances, 1, 1773(2016)

Through transient optical and magnetic field measurements we have observed clear evidence for singlet exciton fission to triplet exciton pairs in a “butterfly-shaped” acene derivative, TIPS-tetrabenzopentacene (TTBP), that has excellent ambient stability imparted by the presence of triphenylene “wings”.

The synthesis and impressive stability of large acenes with triphenylene “wings”, including TTBP, have recently been reported. One property of acenes of great current interest is their ability to undergo singlet fission: that is for photoexcited singlet excitons to generate a pair of triplet excitons each of roughly half the energy of the singlet. The energy of these triplets could be utilised to reduce thermalisation losses in solar cells.

The core of the molecule to which the butterfly wings are attached is Triisopropylsilylethynyl-anthracene (TIPS-anthracene). The total effect of the two wings to the molecular energy levels is similar to that of extending anthracene by one regular benzene ring, i.e. TIPS-tetracene, which itself has previously been shown to undergo efficient singlet fission, albeit with poor ambient stability. Here, the triplet absorption spectrum of TTBP that we observe in transient absorption experiments is remarkably similar to that of TIPS-tetracene, indicating that the triplet exciton is concentrated on the central part of the TTBP molecule that is common to TIPS-tetracene, and the stabilising butterfly wings do not greatly affect the structure of the triplet. We posit that this is key to the surprising persistence of singlet fission.

In TIPS-tetracene, morphology plays a key role in the efficiency of singlet fission, particularly in the separation of the triplet-pair state, which has a broad red-shifted emission. In TTBP, we have observed a similar triplet-pair state emission, along with the characteristic triplet transient absorption spectra, and the magnetic field effect on photoluminescence yield, which is evidence of an equilibrium between singlet fission and triplet-triplet annihilation. The strength of these effects, and therefore the efficiency of singlet fission, can be tuned by film preparation, especially by the addition of an inert polymer. These findings represent significant progress towards stable and efficient singlet fission materials that can be used commercially to reduce losses in photovoltaics.

Singlet fission (SF) has the potential to significantly enhance solar cell performance if charges can be efficiently generated from the resulting triplet states. Here, we study charge separation from triplet excitons in polycrystalline pentacene using an electrochemical series of electron-acceptor molecules with varied reduction potentials, and follow the SF and charge transfer dynamics using a combination of transient absorption spectroscopy and time-resolved microwave conductivity. We find that even at the optimal driving force the rate constant for charge transfer from the triplet state is surprisingly small, (~10⁷ s^-1) when they are produced by singlet fission, but that triplets transfered into the film by a sensitizer dissociate much more rapidly.

Exciton-mediated photon upconversion (EXUC) can transform low-energy photons into higher-energy fluorescence under low-intensity illumination. A sensitizing species is necessary to generate triplet excitons from absorbed photons and transfer this energy to the triplet state of the acceptor molecules. Here, we demonstrate the use of a structurally rigid, norbornyl-bridged, tetracene homodimer (TIPS-BTc) as a triplet acceptor and emitter for EXUC in solution. Using an organometallic triplet sensitizer (PdPc(OBu)₈), incident photons of λ=730 nm were upconverted by 0.6 eV to emitted photons of λ=540 nm via triplet fusion (TF). By comparison with Stern-Volmer phosphorescence quenching experiments, we observe that TIPS-BTc can achieve brighter upconversion emission than monomeric controls (rubrene and tri-isopropylsilylethynyl tetracene (TIPS-Tc)) for the same number of extracted triplet excitons. We build a rate-equation model to simulate the concentration- and intensity-dependence of the upconverted emission at steady state, and extract rate constants for TF in the monomeric controls comparable to literature. However, for dimeric TIPS-BTc, our kinetic simulation cannot simultaneously reproduce the Stern-Volmer data and the concentration dependence of the threshold for max-efficiency upconversion. This result provides evidence that dimeric annihilators may offer pathways towards triplet fusion that are more efficient than uncorrelated diffusion, and could boost overall upconversion performance when paired with molecular engineering that advances the efficiency of triplet transfer. It also validates the strategy of designing rigid, electronically-decoupled molecular dimers for excitonic upconversion from known monomeric emitters, with the ultimate goal of reducing the collisional dependence of solution-phase exciton-mediated photon upconversion.

A large set of candidates for singlet fission, one of the most promising processes able to improve the efficiency of solar cells, are identified by screening a database of known molecular materials. The screening was carried out through a procedure exploiting quantum chemical calculations of excited state energies, carefully calibrated against a substantial set of experimental data. We identified ~200 potential singlet fission molecules, the vast majority of which were not known as singlet fission materials. The molecules identified could be grouped into chemical families enabling the design of further singlet fission materials using the hits as lead compounds for further explorations. Many of the discovered materials do not follow the current design rules used to develop singlet fission materials illustrating at the same time the power of the screening approach method and the need of developing new design principles.

Perylenediimids (PDIs) are well-known dyes that have attractive properties for a variety of applications in opt-electronic devices. These properties include strong absorption in the visible, high charge carrier mobility and excellent electron accepting properties. In addition, they also exhibit close to idea singlet/triplet energtics for singlet fission and triplet-triplet annihilation upconversion. Interestingly, the electronic properties of solid state materials based on these molecules strongly depend on the way the molecules are organized in the crystalline state. This is true for the optical properties, but also for transport of charges, excited state diffusion and for less common photophysical processes such as singlet exciton fission.

A strong relation has been found betwene the intermolecular packing of the individual packing in the solid state and the efficiency of singlet fission. This can be effectively tuned by changing the substitution pattern on the conjugated core of the molecule. Until now, most modifications involve introduction of different sidechains on the imid-nitrogen. In this work, we show a new approach by introducing bulky bromine atoms in directly in the aromatic core of the PDI, leading to a twisting of the core. This significantly affects the solid state packing but also the optoelectronic properties of the individual molecules.

We show that changes in the molecular packing induced by bromination have a strong effect on the temperature dependent photoluminescence, expressed as an activation energy. These effects are explained in terms of excimer formation for PDIs without bay-area substitution, which competes with singlet fission. Introduction of bromine atoms in the bay-positions strongly disrupts the solid-state packing leading to strongly reduced excitonic interactions and excimer formation. The suppression of this competing process leads to an ovrall higher yield of singlet fission, despite the expected smaller electronic coupling for singlet fission. This shows that optimizing the electronic coupling is not the only factor that is important in optimizing singlet fission yields, suppression of competing processes is also of prime importance.

The topic of Non-Fullerene Acceptors (NFAs) in the field of Organic Photovoltaics (OPV) has become tremendously important to industrial and academic research, as the rapid development of these materials has pushed the device power conversion efficiency over the 15% threshold.¹ An efficient chemical design of these molecules has led to a big surge in performance, achieving optimum optical absorptions and reduced voltage losses.² However, low electron mobilities still represent a major issue for the commercialization of large-scale solution-processed OPV devices.³ Researchers speculate that the highly anisotropic and two-dimensional conjugated structure of these molecules is critical to their organization, which in turn affects their electronic functions (e.g. carrier mobility).² In our industrial experience, we have also observed unique coating and thermal behaviour of these materials, suggesting the importance of their crystallinity.
Our work aims to provide direct insights into the crystallization of the materials and their charge transport. With this purpose, we compare a large body of newly identified non-fullerene single crystals and previously reported structures, including novel non-fullerene molecules. By meta-analysis, we identify common packing motifs, elucidate the role of the molecular fragments (sidechains, donor and acceptor groups), perform statistical analysis of close contacts and explore solvated host-guest complexes. Finally, the importance of crystal packing and topological connectivity on the charge transport is explored by linear-scaling Density Functional Theory calculations.
Experimental evidences of the influence of the solid-state organization on the charge carrier mobility and performance in OPV devices is the focus of our investigations that are currently underway.

References
1. J. Yuan, et al. Joule 2019, 3, 1140-1151.
2. C. Yan, et al. Nature Reviews Materials, 2018, 3, 18003.
3. J. Zhang, H. S. Tan, X. Guo, A. Facchetti and H. Yan, Nature Energy, 2018, 3, 720-731.

Among different dispersants of single-walled carbon nanotubes (SWCNTs), conjugated organic oligomers have the ability to interact strongly with SWCNTs and allow for effective dispersion in several organic solvents. Recently, we have carried out two computational investigations (Phys. Chem. Chem. Phys., 2017, 19, 28071 and J. Phys. Chem. C 2017, 121, 4692) on the intermolecular interactions between conjugated organic oligomers and SWCNTs in order to gain insight into an important process of the non-covalent dispersion of carbon nanotubes with short oligomers. These studies highlighted the fact that two additional factors, namely, the effect of the solvent and the carbon nanotube’s size on these interactions need further investigation. In this work (RSC Adv., 2018, 8, 30520), with the help of model compounds (which are molecular fragments of the short oligomers used in our previous investigations) we analyze the significance of these two factors. We employ three dispersion corrected density functional theory (D-DFT) approximations (B97D, wB97XD, and B3LYP-D3) to assess the effect of the DFT method and two basis sets (6-31G(d) and 6-31++G(d,p)) to assess the importance of using a higher basis set in our computations. The main focus of this work is to assess the effect of solvation and nanotube’s size on the structure, electronic properties, and binding energies of the respective pairs of model compounds and segments of carbon nanotubes. No significant differences are found between the results of (6,5) and (8,7) SWCNTs in either the geometrical parameters of interacting oligomers or the general tendency of wrapping of their long side chains (SCs) around the nanotubes. However, we find that the binding energies/atom between nanotubes and model compounds are larger for nanotubes with smaller diameter. The results of electronic properties also show that all model compounds interact more strongly with the (6,5) SWCNT than with the (8,7) SWCNT. Polar solvents such as chloroform lower binding energies relative to those obtained without a solvent or with non-polar solvents such as hexane. It appears that the presence of a solvent weakens the oligomer/nanotube interactions and, presumably, strengthens the oligomer/solvent and nanotube/solvent interactions to facilitate dispersion of SWCNTs.

Here, we developed a facile post-treatment method using ultraviolet irradiation that produced crystalline polymer nanofibrils in the solid film state. Ultraviolet irradiation over a few minutes effectively transformed the polymer chain conformational structure and promoted polymer chain extension and association in the film state. Brief ultraviolet irradiation of a thin film fabricated using a high boiling point solvent produced well-ordered molecular structures among the extended P3HT chains, whereas ultraviolet irradiation over longer times led to breaks in the chemical structures of the P3HT, resulting in shortened conjugation lengths. Conformational changes in the polymer main chain and resulting nanofibril morphologies induced by ultraviolet irradiation facilitated charge transport in organic transistors prepared using these films The relationship between the molecular structural order and the electrical characteristics of the films was used to determine the optimum ultraviolet irradiation time.

Organic light-emitting devices (OLEDs) are advantageous over conventional flat panel display. The success of OLEDs depends significantly on the performance of emitting molecules. Molecules displaying thermally activated delayed fluorescence (TADF) have emerged as promising dopant materials, because they are capable of harvesting all the electrogenerated excitons without relying on precious late transition metals. However, exploitation of the full potential of electroluminescence from TADF materials is retarded by short operational lifetime of devices.
We recently proposed the degradation mechanism involving exciton-mediated electron transfer between a host and a phosphorescent dopant within the emitting layers of OLEDs. The electron transfer results in the formation a radical ion pair (RIP), very vulnerable to degradation. RIP was found to be responsible for irreversible degradation of the emitting layer, and its lifetime was governed by the charge recombination kinetics.
We hypothesized an occurrence of the identical electron-transfer processes in OLEDs having a TADF dopant in place of a phosphorescent dopant. To verify our hypothesis, we investigated the degradation behaviors of the pairs of a TADF dopant, phenoxazine-1,3,5-triphenyltriazine (PXZ-TRZ), and a series of commercially available host materials, including 4,4′-bis(9-carbazolyl)-1,1′-biphenyl (CBP), 4,4'-bis(9-carbazolyl)-2,2'-dimethylbiphenyl (CDBP), N,N′-dicarbazolyl-3,5-benzene (mCP), 3,3'-bis(9-carbazolyl)biphenyl (mCBP), 1,3,5-tri(m-pyridin-3-ylphenyl)benzene (TmPyPB), and bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO). Electrochemical measurements predicted positive driving forces (–ΔG_eT, 0.69–1.47 eV) for electron transfer from the dopant to host exciton, which suggested the spontaneous formation of RIP between the TADF dopant and hosts. Annihilation of RIP by charge recombination was found to have very positive driving forces (–ΔG_cr, 2.62–2.76 eV).
To investigate the electron-transfer processes, we performed steady-state photophysical experiments. The addition of the dopant into a host solution resulted in fluorescence quenching of the host in a concentration-dependent manner. The rate constant for the bimolecular quenching was found to be proportional to –ΔG_eT, indicative of the formation of non-emissive RIP. This notion was supported by the observance of strong signals (g = 2.012) in photoinduced electron paramagnetic resonance spectra. The RIP accelerated the degradation of the materials under continuous photolysis, as evidenced by the rates of photodegradation greater than those determined for the individual compound. This result supported RIP being the key intermediate of the intrinsic degradation of TADF emitting layers.
Multi-layer electroluminescence devices with a configuration of ITO/DNTPD (60 nm)/BPBPA (20 nm)/PCzAC (10 nm)/host:PXZ-TRZ (30 nm : 10%)/DBF-Trz (5 nm)/ZADN (30 nm)/LiF (1 nm)/Al were fabricated and evaluated. Operation lifetime of the devices was quantitated as the time when the luminance decreased to 90% of the initial value (LT₉₀) under a constant current driving mode. LT₉₀ values were 13 h (CBP), 5.9 h (CDBP), 18 h (mCP), 27 h (mCBP), 0.11 h (TmPyPB), and 0.24 h (DPEPO). We found a positive linear correlation between LT₉₀ with –ΔG_cr for the ‘hole-transporting type’ hosts (i.e., CBP, CDBP, mCP, and mCBP). The significantly short LT₉₀s of the devices having ‘electron-transporting type’ host (i.e., TmPyPB and DPEPO) suggested the presence of a different degradation mechanism, because their –ΔG_cr were not very different from –ΔG_cr of ‘hole-transporting type’ hosts. The fast charge recombination rate minimized an accumulation of RIP. Collectively, our findings supported that the acceleration of charge recombination within RIP is crucial for device longevity. Our study revealed the importance of electrochemical control between a TADF dopant and a host for realizing long operation lifetimes of OLEDs.

Organic semiconductors (OSCs) have gained significant attention as active components in various (opto)electronic devices due to low-cost manufacturing, ease of processing, mechanical flexibility, and versatility in chemical synthesis. These attractive properties are a direct consequence of the weak intermolecular interactions of van der Waals type characteristic of these materials. On the other hand, the weak interactions make OSCs susceptible to the formation of localized electronic states in the band gap, which can trap charge carriers at different timescales. Charge carrier trapping is an important phenomenon in electronic devices and its impact on device performance and long-term stability has been extensively studied. However, determining the origin, concentration, and composition of electronic traps in organic semiconductors, as well as their spatial and energetic distribution, is not trivial and remains elusive. In this work, we probe the trap density of states (DOS) using organic field-effect transistor measurements and quantify their energy distribution in response to applying bias stress. DOS analysis provides direct evidence for the creation of a discrete electronic trap in a solution-processed OSC, namely tri(n-hexyl) silylethynyl benzodithiophene (TnHS BDT) trimer, in response to repetitive electrical measurements performed under ambient conditions. The trap DOS spectrum evaluated over the course of bias stress measurements (total 500 min) shows the dynamics of the trap formation, which starts with an initial broadening of the DOS after 10 min, followed by the appearance of a discrete peak located at ca. 0.3 eV above the valence band edge after 20 mins of bias stressing. The peak is absent in measurements performed under vacuum. The correlation of the trap DOS spectrum with the time evolution of device metrics, including mobility, subthreshold swing, and the threshold voltage, suggests that the generated electronic trap occurs at the interface between the OSC and the dielectric. Density functional theory (DFT) calculations are employed to elucidate the nature of the trap and the mechanism of formation. This work marks as the first of its kind in experimentally detecting electronic traps generated as a result of bias stress and concurrently determining their precise nature by using a combination of trap DOS analysis and DFT calculations.

Organic field-effect transistors (OFETs) possess the accessibility of flexible, large-area, multi-signal detection, and bio-compatibility. For their applications in the biomimetic field, another requirement is the low work-voltage. Usually, the bio-signals are much more difficult to capture compared to the common chemical signals since their magnitudes are very small. Meanwhile, considering that the bio-system is relatively fragile and complicated, the detection method is thus limited. Especially, for the in-situ and non-invasive detection, not only the low work-voltage is required, low/non toxicity and high selectivity are also essential. Here, we mainly focus on the application of OFETs in the field of non-invasive blood glucose detection by introducing the porous film in the OFET device to increase the sensitivity of the OFET device. Usually, the detection of the liquid analyte is realized by the electrochemical transistor since their unique property of integrating the electrochemical gate. The report of using the OFET to detect the liquid analyte is rare. We designed to take advantage of the interface between the organic semiconductor and the dielectric in OFET to achieve the detection of glucose.

Organic semiconductor materials are attracting increasing interest for catalytic hydrogen generation [1]. Despite this, very few studies [2] attempt to deconvolute the factors (structural, optical, and electronic) that affect photocatalytic performance.

Here we investigate a collection of novel linear conjugated polymer photocatalysts which exhibit high rates of hydrogen photo-evolution from water in the presence of a sacrificial electron donor. Unlike most organic photocatalysts, these materials are solution processable, allowing for greater control of structure to explore the structure-activity relationship. These polymers show a positive correlation between the density of hydrophilic groups and the rate of hydrogen evolution. Using transient absorption spectroscopy (TAS) and spectroelectrochemistry, we observe that catalytic activity is correlated with the density of photogenerated electrons in the polymer. Using electronic structure and molecular dynamics calculations, we then investigate the effect of different hydrophilic groups on charge transfer through their role in structuring the liquid environment close to the polymer backbone. We find that the impact of the polar groups on the dielectric properties of the local liquid environment is critical for controlling the driving energy for charge generation.

Charge generation in both polymer particles and thin films is also highly dependent on their physical microstructures. Hydrogen evolution and TAS measurements suggest that catalytic activity exists not only at the polymer-liquid interface but also within the structures, indicating that the materials are partially permeable to the liquid environment. Using photoluminescence spectroscopy, atomic force microscopy and TAS, we show that this activity can be explained by a combination of solvent penetration and exciton diffusion. We demonstrate that this understanding can be used to control the structure of the photocatalyst and enhance catalytic performance. Together with the control of the solvent environment, this work provides design rules for improved polymer photocatalyst behaviour.

[1] Bai, Y. et al. Accelerated Discovery of Organic Polymer Photocatalysts for Hydrogen Evolution from Water through the Integration of Experiment and Theory. Journal of the American Chemical Society 141 (22), 9063-9071. (2019).
[2] Sachs, M. et al. Understanding structure-activity relationships in linear polymer photocatalysts for hydrogen evolution. Nature Commun. 9, 4968 (2018).

The power conversion efficiency of non-fullerene organic solar cells develops dramatically in recent years. Although tremendous efforts have been dedicated to the design and synthesis of new small molecular acceptors, less is known on how the molecular order and aggregation of these small molecular acceptors will affect the device performance. A high structure order is a desired to achieve high charge transport property, however, it can also induce excessive phase aggregation and consequently reduce the exciton dissociation efficiency to reduce device performance. We have discovered that heating-induced molecular ordering and aggregation can be an effective approach. The asisiatance of heating during film casting can suppress the formation of large-scale spherilite from INPIC-4F non-fullerene acceptor, but encourage the formation of p-pstacks to reduce phase separation to results an efficiency over 13%. Heating also convert COi8DFIC from edge-on and flat-on oriented lamellae to face-on H- and J- type p-pstacks, which broadens the light absorption spectrum and reduces the contact between donor and acceptor molecules for efficient exciton dissociation. An high efficiney of 13.8% was achieved in PTB7-Th:COi8DFIC binary solar cells. Heating-induced-aggregation emerges as a new strategy to optimize the morphology of non-fullerene solar cells for high peformacne.

Nonfullerene organic solar cells have been attracting significant attention in the past several years. It is still challenging to achieve high-performance flexible nonfullerene organic solar cells. Nonfullerene acceptors are chemically reactive and tend to react with the low-temperature processed low-work function interfacial layers, such as polyethylenimine ethoxylated (PEIE), which leads to the “S” shape in the current-density characteristics of the cells. In this work, we deactivate the chemical interaction between the nonfullerene active layer and the polymer interfacial layer of PEIE by increasing its protonation. The PEIE processed from aqueous solution shows more protonated N⁺ than that processed from isopropanol solution, observed from X-ray photoelectron spectroscopy. Nonfullerene solar cells (active layer: PCE-10:IEICO-4F) with the protonated PEIE interfacial layer show an efficiency of 13.2%, which is higher than the reference cells with a ZnO interlayer (12.6%). More importantly, the protonated PEIE interfacial layer processed from aqueous solution does not require a further thermal annealing treatment (only processing at room temperature). The room-temperature processing and effective work function reduction enable the demonstration of high-performance (12.5%) flexible nonfullerene organic solar cells.

Owing to their high conductivity, transparency, flexibility, and compatibility with solution processes, silver nanowire (AgNW) networks have been widely explored as a promising alternative to indium tin oxide (ITO). However, while AgNW networks have been successfully integrated as transparent electrodes in a range of optoelectronic devices, their susceptibility to corrosion and thermal instability remain limiting factors for widespread adoption. To impart stability, hybrid electrodes that combine AgNW with an encapsulating material such as graphene or semiconducting oxides have been explored, although they have relied on costly processes such as chemical vapor deposition (CVD) or atomic layer deposition (ALD). Graphene oxide (GO) is an appealing barrier material due to its tunable properties, robustness under a range of conditions, and solution-processability. However, thus far the processing methods to fabricate GO/AgNW hybrid structures have been largely limited to various forms of spray deposition that can only be used on flat substrates with optimized wettability and require expensive equipment for scale-up. Furthermore, many aspects of the chemical and electrical stability of GO/AgNW hybrid electrodes have either not been fully demonstrated or remain poorly understood.
In this study, we propose a scalable and economically viable process involving electrophoretic deposition (EPD) to fabricate a highly stable hybrid transparent electrode with a sandwich-like structure, where the AgNW network is covered by GO films on both sides. The use of EPD allows facile control over the thickness of the GO film, leading to tunable transparency. The newly developed process allows the conductive transparent film to be transferred to an arbitrary surface after deposition and demonstrates excellent sheet resistance (15 Ω/sq) and transmittance (75% at 550 nm) on glass substrates. Unlike bare AgNW networks, the hybrid electrode was found to retain its original conductivity under long-term storage at 80°C. This chemical resilience can be explained by the absence of major silver corrosion products for the AgNW encapsulated by GO as indicated by X-ray photoelectron spectroscopy (XPS). In-situ electrical ramping and resistance measurements up to 20V were conducted in order to assess the electrical stability of our electrodes. The results indicate a novel stabilization mechanism enabled by the presence of GO that prevents abrupt divergence of the resistance to the MΩ range experienced by bare AgNW networks. Finally, the application of our hybrid electrode in a full device is demonstrated by solar cell fabrication and characterization.

We report clear evidence from transient optical measurements and transient electron spin resonance that a family of derivatives of indolonaphthyridine thiophene (INDT) supports singlet exciton fission, and show excellent ambient stability.

Singlet fission offers the potential for the generation of multiple excitons from one photon in organic semiconductors, which could reduce losses to thermalisation in solar cells by exploiting the relatively low energy of the triplet exciton in certain organic materials. This is a characteristic reserved for only a handful of organic molecules due to the atypical energetic requirement for low energy excited triplet states. The canonical systems on which much singlet fission research is focused - pentacene, tetracene and their derivatives - have such conveniently low triplet energies due to their large diradical nature, greatly compromising their ambient stability. In order to move towards photostable singlet-fission-enhanced solar cells, there is a need for materials that undergo efficient singlet fission at no expense to their stability.

To this end, we have used Baird’s rule of excited state aromaticity to manipulate the singlet-triplet energy gap and create novel singlet fission candidates. We achieved this through the inclusion of a [4n] 5-membered heterocycle, whose electronic resonance promotes aromaticity in the triplet state, stabilizing its energy relative to the singlet excited state. Using this theory, we have designed a family of derivatives of INDT with highly tunable excited state energies which are highly stable and clearly undergo rapid singlet fission at morphological “hot spots”. Not only do we access an original class of singlet fission materials, they also exhibit excellent ambient stability, imparted due to the delocalized nature of the triplet excited state. Spin-coated films retained over 80% activity after several weeks of exposure to oxygen and light, whilst analogous films of TIPS-pentacene showed full degradation after four days, showcasing the excellent stability of this class of singlet fission scaffold. Extension of our theoretical analysis to almost ten thousand candidates reveals an unprecedented degree of tuneability and several thousand potential fission-capable candidates, whilst clearly demonstrating the relationship between triplet aromaticity and singlet-triplet energy gap, confirming this novel strategy for manipulating the exchange energy in organic materials.

Through transient absorption spectroscopy, transient photoluminescence, and transient electron paramagnetic resonance, we can reveal a full picture of the dynamic evolution of excitons in these new materials. Singlet fission, an inherently intermolecular process producing two triplet excitons localized on separate molecules, is highly dependent on morphology. We have found that when tuning the energetics by altering the electron-withdrawing substituent groups on the INDT core, the solid state morphology also changes. We find that with a more H-aggregate-like morphology, with a blue shift of absorption onset and relative suppression of 0-0 absorption peak, singlet fission yields are enhanced. In recent months, we have made further synthetic progress, which has allowed additional exploration of structure-function relationships down two avenues. Firstly, by editing the length of solubilizing chains, which modifies packing, we can explore different morphologies without affecting the relative energies of the localized singlet and triplet. Secondly, by the synthesis of covalent INDT dimers, bridged with phenyl- and fluorene-linkers, we can vary the coupling and conjugation across an isolated pair of molecules. Both of these pathways offer new insights to how local organization and molecular environment can affect singlet fission, exciton-exciton annihilation, and other critical processes in these films which go on to dictate their performance in optoelectronic devices.

Semiconducting polymers are promising for the application of wearable electronics through blending with rubbery elastomers to achieve the softness and stretchability similar to that of biological skin. However, the inherent tear-resistant and self-healing properties of skin are still unexplored areas for electronic active blends. Here, we present the first tear-resistant semiconducting material using a carefully designed blend system, consisting of conjugated polymers and a high-performance elastomer. This blending system demonstrated excellent mechanical performance with the ultra-low modulus (1 MPa), high stretchability (800% strain), and good electronic property with charge carrier mobility of 0.5 cm²V^-1s^-1. The charge mobility of the blend system is stable at ambient condition and retains at 70% of charge mobility at 100% strain. More importantly, the semiconducting material is insensitive to precut notches and self-healable at room temperature for two pieces of cut films upon contact within seconds on the water surface. The elastomer has low oxygen and water permeability, allowing better device stability over months of operation, which can be generally applied to a variety of semiconducting polymers to achieve improvements in the mechanical performances for the application of wearable electronic devices.

For the reduction of contact resistances as well as shunting currents in organic solar cells it is important to control the two at the interface towards the respective charge collecting electrodes. Many conductive and non-conductive materials have been investigated so far and several of them offer already good solutions. However, the underlying mechanisms for selective charge extraction are not often well understood. We have systematically varied dipole moments anchored in side-chains of small organic non-conductive molecules as well as non-conductive polymers. We observed a clear correlation between the polarity and various photovoltaic parameters. We present a model on how to achieve proper alignment of dipole moments in that charge extraction materials for improved device function and performance.

Optoelectronic devices with semiconducting single-walled carbon nanotubes (s-SWNTs) as the absorbing layer require the photogenerated excitons to diffuse to a heterojunction to drive dissociation into charge carriers. However, current device efficiencies are hindered by a short transverse exciton diffusion length of about 5 nm which in turn limits film thickness and number of photons absorbed. In this work, I have created wrinkled s-SWNT films by depositing a thin layer of nanotubes on a pre-strained stretchable substrate and subsequently releasing so the film wrinkles on itself and effectively increases the optical absorption pathlength. We then use ultrafast 2D white-light (2DWL) spectroscopy to study the underlying photophysics of these wrinkled films. Our 2DWL technique uses a broadband supercontinuum spanning from the visible to NIR to collect information on femtosecond timescales. This technique allows us to monitor the factors that influence device performance such as exciton lifetime, coupling between nanotubes, and the effects of strain on individual tubes. Furthermore, by depositing a bilayer of s-SWNT with C60 as an electron accepting layer on the elastomeric substrate and allowing it to wrinkle, we create a bulk-like heterojunction architecture with high interfacial area to drive exciton dissociation. These wrinkled s-SWNT/C60 layers are incorporated as the active layer in stretchable phototransistors and their device performance is evaluated.

Recent years have seen significant improvements in the performances of organic electronic devices based on conjugated polymers. Among the methods of forming a polymer thin film, the dip coating technique has an advantage that a uniform large-scale polymer thin film having a low roughness can be manufactured. However the solvent with a high boiling point produced inhomogeneous film coverage with dewetted regions on the dip-coated polymer film. In this study we presented a systematic study of a simple post dip-coating method for controlling the structural and electrical characteristics of dip-coated polythiophene thin films through direct exposure to various solvents. Simply dip-coating of polythiophene thin film into various solvents with different solubility parameter improved the intermolecular order in the films, and the polymer FET device performance. When the polythiophene thin film was dip-coated into various solvents before the film is completely dried, the molecular rearrangement was occurred depending on the solubility of the solvent. The small difference in the solubility parameter between the polythiophene and the solvent in post-treatment process induced the strong rearrangement of the molecular arrays and the production of nanowires. The structural and electrical characteristics of dip-coated polythiophene thin films or thin films dip-coated into various solvents were investigated and compared.

Organic electronic devices consist of stacked organic as well as inorganic materials and the device performance is strongly influenced by the interfaces between the layers. The investigation of charge transport across these interfaces is a major key to the basic understanding of the fundamental mechanisms in organic electronics. Thin films of certain metal oxides are known to be excellent charge-selective interlayers in optoelectronic devices due to their outstanding properties such as high optical transparency, suitable energy level alignment and high electron mobility [1-3].
In this study, solution-processed tin oxide (sSnOx) as promising electron transport layer was analyzed by infrared spectroscopy. This non-destructive method gives insight into the chemical composition of the thin film and can monitor orientation of characteristic vibrations or functional groups. We studied the influence of annealing temperature on bulk and surface properties of thin sSnOx. During subsequent annealing from room temperature up to 400°C, the tin chloride precursor is converted into tin oxide. The infrared spectroscopic investigations show the decrease of tin hydroxide and water in the sSnOx films and the higher crystallinity with increasing annealing temperature. Moreover, the phonon modes of sSnOx in the far-infrared region confirm the conversion from tin oxide into tin dioxide between 350°C and 400°C. Furthermore, we discovered that the solvent of the precursor solution has a crucial influence on the film morphology and the conductivity of sSnOx.
Together with the results of photoelectron spectroscopy, atomic force microscopy and conductivity measurements a better understanding of the electronic, chemical and morphological properties can be achieved which is important for the improvement of device performance.

References:
[1] E.H. Anaraki al., "Highly efficient and stable planar perovskite solar cells by solution-processed tin oxide." Energy & Environmental Science, 2016 9 (10), 3128-3134
[2] S.Hietzschold, S.Hillebrandt, F.Ullrich, J.Bombsch, V.Rohnacher et al., "Functionalized Nickel Oxide Hole Contact Layers: Work Function versus Conductivity." ACS Applied Materials and Interfaces, 2017, 9 (45), 39821-39829
[3] F.Ullrich, S.Hillebrandt, S.Hietzschold, V.Rohnacher et al., "On the Correlation between Chemical and Electronic Properties of Solution-Processed Nickel Oxide." ACS Applied Energy Materials, 2018, 1 (7), 3113–3122

Charge transfer cocrystals have interesting applications in the field of organic electronics due to their unique optical and electronic properties. We have studied cocrystals formed from phenothiazine (PTZ) and 7,7,8,8 tetracyanoquinodimethane (TCNQ) as electron donor and acceptor molecules respectively. PTZ:TCNQ cocrystals were found to readily grow using the controlled evaporative self-assembly (CESA) method, in which crystals nucleate at the receding contact line of an evaporating solution. The resulting cocrystals formed an array of long, well-aligned wires that could be easily incorporated into field effect transistors (FETs). Here, we present measurements of carrier mobility in PTZ:TCNQ FETs. In addition, we show that the device performance as a function of temperature agrees with a hopping model of charge transport in the material. Finally, we have investigated the effects of photoexcitation on charge transport in the devices.

Photosynthetic complexes achieve remarkable spatial control of densely-packed pigments using a protein scaffold to organise multiple organic semiconductors. This promotes strong electronic coupling between adjacent chromophores yielding efficient energy transport and charge separation. In contrast, man-made organic solar cells lack natures precision and are typically limited to a pseudo-random blend mixture.^[1] To overcome the limitations of poor assembly we attach acenes, rylene diimides, porphyrins, and push-pull chromophores to DNA.

Inspired by how nature uses DNA to encode the structure of peptides, our method allows the unique assembly of monodisperse, hetero-aggregated semiconducting structures; transcribed by Watson-Crick base-pairing. The DNA sequence programs the final aggregate superstructure and give us unparalleled control over an energy landscape encompassing many different molecules. We outline our efforts using on-resin synthetic routes to overcome the incompatibility of large, hydrophobic aromatic molecules used in organic electronics, with hydrophilic DNA.^{[2, 3]} Subsequent DNA hybridisation then forms a scaffolded assembly where a photoexcitation coherently extends over several building blocks.

Using 10-fs time resolution transient absorption spectroscopy we track energy transport and charge separation through a nanoscale heterojunction and energy cascade, where the number of electron donors and acceptors have been predefined by the DNA sequence. Furthermore by instead assembling with acene materials we can control the extent of singlet fission as a function of the number of molecules electronically coupled.

[1] D. Baran, T. Kirchartz, S. Wheeler, S. Dimitrov, M. Abdelsamie, J. Gorman, R. Ashraf, S. Holliday, A. Wadsworth, N. Gasparini, P. Kaienburg, H. Yan, A. Amassian, J. Brabec, J. Durrant, I. McCulloch, Energy Environ. Sci., 2016, 9, 3783.
[2] J. Gorman, R. Pandya, J. Allardice, M. Price, T.Schmidt, R. Friend, A. Rao, N. Davis, J. Phys. Chem., 2019, 6, 3433.
[3] M. Price, A. Paton, J. Gorman, I. Wagner, G. Laufersky, K. Chen, R. Friend, T. Schmidt, J. Hodgkiss, N. Davis, Chem. Commun., 2019, (in press).

Water/Ethanol-processed all-polymer solar cells (eco-APSCs) with superior efficiency and stability were developed for the first time by synthesizing a series of aqueous-soluble naphthalene diimide (NDI)-based polymer acceptors P(NDIDEG-T), P(NDITEG-T), and P(NDITEG-T2). The polymer acceptors were designed by using the backbones of NDI-bithiophene and NDI-thiophene in combination with non-ionic hydrophilic oligoethylene glycol (OEG) side chains that facilitate processability in water/ethanol mixtures without causing any electronic trap sites. While all three polymers exhibited semi-crystalline properties and sufficient solubility in the aqueous medium, the P(NDIDEG-T) polymer with shorter OEG side chains was the most crystalline, enabling the fabrication of efficient eco-APSCs with the maximum power conversion efficiency of 2.15%. To date, this is the highest efficiency reported for devices based on water/ethanol-soluble conjugated materials. Furthermore, these eco-APSCs were fabricated under ambient atmosphere by taking advantage of the eco-friendly aqueous process and, importantly, the devices exhibited outstanding air-stability without encapsulation, as evident by maintaining more than 90% of the initial PCE in air after 4 days. These results provide important guidelines for the design of electroactive polymers for sustainable fabrication of organic electronics.

The authors report the development of a desirable aqueous process for ecofriendly fabrication of efficient and stable organic field-effect transistors (eco-OFETs) and polymer solar cells (eco-PSCs). Intriguingly, the addition of water to ethanol was found to remarkably improve the solubility of oligoethylene glycol (OEG) side chain-based electroactive materials (e.g., the highly crystalline conjugated polymer PPDT2FBT-A and the fullerene monoadduct PC₆₁BO₁₂). A water–ethanol cosolvent with a 1:1 molar ratio provided an increased solubility of PPDT2FBT-A from 2.3 to 42.9 mg mL⁻¹ and that of PC₆₁BO₁₂ from 0.3 to 40.5 mg mL⁻¹. Due to the enhanced processability, efficient eco-OFETs with a hole mobility of 2.0 × 10⁻² cm² V⁻¹ s⁻¹ and eco-PSCs with a power conversion efficiency of 2.05% were successfully fabricated. In addition, the eco-PSCs fabricated with water–ethanol processing were highly stable under ambient conditions, showing the great potential of this new process for industrial scale application. To better understand the underlying role of water addition, the influence of water content on the thin-film morphologies and the performance of the eco-OFETs and eco-PSCs were studied. Additionally, it was demonstrated that the application of the aqueous process can be extended to a variety of other OEG-based material systems.

Since phosphorescent organic light-emitting diodes (PhOLEDs) have four times higher quantum efficiency than fluorescent OLEDs in that they can utilize both singlet and triplet excitons, they are focused on the display market and widely used. Many studies have been conducted to enhance the performance of PhOLEDs, but satisfactory efficiency and lifetime have not been achieved for blue emitters. In order to improve the efficiency of the blue PhOLED, a hole blocking layer (HBL) can be introduced between the electron transporting layer (ETL) and emissive layer (EML). In general, the highest occupied molecular orbital (HOMO) energy level of the ETL is not much lower than that of the EML. Holes reaching the EML from an anode can be migrated into the ETL, lowering their efficiencies. Therefore, it is necessary to introduce a HBL with deep HOMO energy level. In addition, if the triplet energy (E_T) of ETL is lower than that of EML, the triplet excitons created in EML can be quenched at the interface between EML and ETL. Therefore, a HBL should have high E_T as well as deep HOMO energy level.
In this work, Novel hole blocking materials (HBMs) based on 2,6-disubstituted dibenzo[b,d]furan and dibenzo[b,d]thiophene segments, 3,3',3'',3'''-(dibenzo[b,d]furan-2,6-diylbis(benzene-5,3,1-triyl))tetrapyridine (26DBFPTPy) and 3,3',3'',3'''-(dibenzo[b,d]thiophene-2,6-diylbis(benzene-5,3,1-triyl))tetrapyridine (26DBTPTPy), are rationally designed and synthesized for high-performance blue PhOLEDs for the first time. Computational simulation is used to investigate the optimal structure, orbital distribution, and physicochemical property of both molecules. Thermal, optical, and electrochemical analysis show that 26DBFPTPy and 26DBTPTPy possess high thermal stability, deep HOMO energy level (-7.08 and -6.91 eV), and E_T (2.75 and 2.70 eV). Blue PhOLEDs with 26DBFPTPy or 26DBTPTPy as a HBL exhibit low turn-on voltage (3.0 V) and operating voltage (4.5 V) at 1000 cd m^-2. In addition, the blue PhOLEDs with 26DBFPTPy or 26DBTPTPy show superior external quantum efficiency (24.1 and 23.6%) and power efficiency (43.9 and 42.7 lm W^-1). They also show a very small efficiency roll-off of about 8.5% from 100 to 1000 cd m^-2. Details of the correlation between the structure of organic molecules and their properties with OLED performance will be presented.

It is essential to tune the carrier injection barrier between organic semiconductors and electrodes in present-day organic electronics, including organic light-emitting diodes (OLEDs), organic thin-film transistors (OTFTs), organic solar cells (OSCs), and perovskite solar cells (PSCs). Although efficient hole injection can easily be achieved by using stable materials, the most commonly used materials for electron injection are reactive alkali metals, which make the organic devices unstable, especially on flexible substrates. Thus, the development of a novel electron injection layer without the use of such reactive materials is necessary for future flexible electronics. Recently, a stable amidine derivative named 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) has been proposed to be effective for controlling the electronic behavior in organic devices. When a DBU-doped layer was used as an electron transport layer in PSCs and an n-channel layer in OTFTs, the performance of these organic devices was enhanced owing to the increase in the conductivity and the Fermi level shift [1]. We focused on the high electron-donating property of DBU, and we evaluated a similar amidine-type derivative as the additive of the electron injection layer in OLEDs.
In the present study, the effect of two amidine-type derivatives on the electron injection efficiency was investigated by adding them to the interlayer of inverted OLEDs (iOLEDs) [2,3]. We fabricated iOLEDs in the following device configuration: ITO cathode/ZnO/interlayer/emitting layer/hole transporting layer/hole injection layer/Al anode. The interlayer, which plays a key role in electron injection, consists of two materials: one is the electron-transporting host and the other is the amidine-type derivative [1]. The host material used in the interlayer was a boron compound, which is suitable for demonstrating operationally stable iOLEDs [3]. DBU and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) were used as the amidine-type derivative. We observed from the current density (J)–voltage (V)–luminance (L) characteristics of the fabricated iOLEDs that the electron injection property was improved by adding each amidine-type derivative to the interlayer. In addition, the DBN-added interlayer exhibited a better electron injection property than the DBU-added interlayer when these amidine-type derivatives were added at the same concentration. The driving voltage of the iOLED with the DBN-added interlayer was about 1.0 V less than that of the iOLED without DBN added to the interlayer. Moreover, the optimized iOLED with a DBN-added interlayer exhibited higher operational stability than the iOLED without DBN added to the interlayer, which suggests that the operational stability of DBN is high. To investigate the mechanism of the efficient electron injection resulting from adding DBN, we measured the energy-level alignment at the ITO/ZnO/interlayer (with or w/o DBN) by ultraviolet photoelectron spectroscopy (UPS). The line shape of the spectrum did not change substantially upon the addition of DBN, whereas a shift toward a higher binding energy and a change in the surface work function were detected for the DBN-added interlayer. A relatively large Fermi level shift of about 0.46 eV was observed by adding DBN. Thus, the electron injection barrier can be significantly reduced by adding DBN, and electrons can be effectively injected. In the previous study on DBU in PSCs, a similar Fermi level shift was observed, which was attributed to the electron transfer from DBU to the host. However, electron transfer from DBN to the host was not clearly observed in this study. Thus, we concluded that the origin of the improved electron injection property in iOLEDs is different from that in PSCs.

[1] L. Hu et al., Adv. Funct. Mater. vol.27, p.1703254 (2017).
[2] K. Morii et al., Appl. Phys. Lett. vol.89, p.183510 (2006).
[3] H. Fukagawa et al., Adv. Mater. vol.30, p.1706768 (2018).

Electron transport materials (ETMs)play a key role in determining OLED performances, such as driving voltage, efficiency and operation lifetime.In this work, we newly designed and developed a series of chrysene-based ETMs, namely BnTPyCs (n=3,4) modified with terpyridine moieties. BnTPyCs showed high T_m of over 390°C. An OLED with a structure of [ITO (130 nm)/triphenylamine containing polymer: 4-isopropyl-4’-methyldiphenyl-iodoniumtetrakis(pentafluorophenyl)borate (PPBI) (20 nm)/N,N’-di(1-naphthyl)-N,N’-(1,1’-biphenyl)-4,4’-diamine (NPD) (20 nm)/Ir(ppy)₃12 wt% doped 3,3-di(9H-carbazole-9-yl)biphenyl (mCBP)/2-(3’-(dibenzo[b,d]thiophen-4-yl)-[1,1’-biphenyl]-3-yl)-4,6-diphenyl-1,3,5-triazine (DBT-TRZ) (10 nm)/20 wt% 8-quinolinolato lithium (Liq) doped-ETL (40 nm)/Liq (1 nm)/Al (100 nm)] were fabricated. The device exhibited maximum EQE of 18% with long operation lifetime at 50% the initial luminance (LT₅₀) of over 250 hours at the initial luminance of approximately 10,000 cd cm^–1(current density: 25 mA cm^–2),which was approximately 2 times longer than that with B3PyPC^[1]and slightly longer than that with anthracene-based ETM (ZADN)^[2](current density: 25 mA cm^–2).

Reference: [1] T. Watanabe et al., Chem Lett. 2019, 48,457. [2] J. R. Cha, C. W. Lee, M. S. Gong, New J. Chem., 2015, 39, 3813-3820.

Organic-Photovoltaics (OPVs) can potentially provide a less energy intensive means of harnessing solar energy. However, optimum OPV performance depends on understanding the Process-Structure-Property (PSP) correlation in organic semiconductors. The working of Bulk-Heterojunction (BHJ) OPVs is such that the morphology plays a key role in device performance¹.

It is experimentally observed that there is an optimal blend ratio of p and n-type organic semiconductor as well as annealing time with respect to device characteristics. In this work, we attempt to understand the characteristics of morphology that maximize the device performance by developing a theoretical framework. We first established process-structure correlations by generating a range of morphologies with different blend ratios of P3HT (p-type organic semiconductor) and PCBM (n-type organic semiconductor) for various annealing times. The morphologies were generated using phase-field simulations. In the phase-field model, the free-energy of polymer-fullerene blend was modelled using the Flory-Huggins function. Since the polymer-fullerene blend undergoes Spinodal decomposition, Cahn-Hilliard formulation was used to model the free-energy functional.

Secondly, we developed effective electronic properties of the morphologies which allows us to characterize the performance of each of the obtained morphologies. The structure-property correlation was derived using diffuse interface approach. The application of diffuse interface model to complex morphologies as are typically encountered in BHJ devices is simpler and numerically more robust than the classical sharp interface model used in semiconductor industry. This completes the theoretical PSP linkage which allows the optimization of the process parameters for device applications. We found that the necessary condition for efficient OPV device is bi-continuous network of donor and acceptor phases as that leads to percolating channels for electrons and holes to their respective electrodes. The percolation of the morphology was found out using Hoshen-Kopelman algorithm. Amongst the percolating morphologies, the ones with higher interfacial area lead to higher short circuit current density (J_sc) and eventually to higher efficiency since open-circuit potential (V_oc) is a weak function of morphology. Therefore, the key to an efficient solar cell is optimisation of the two important length scales involved in the working mechanism of OPV i.e., exciton dissociation and carrier conduction through the morphology to the respective electrodes. Exciton dissociation is governed by interfacial area. Efficient carrier conduction results from transportation of the electron and hole to their respective electrode via a well-connected network of phases with minimal leakage of current and recombination of charge carriers. Hence, efficient carrier conduction is governed by higher percolation fraction of the morphology. Another important step during device fabrication is annealing the device after production. It is experimentally observed that initially during annealing, J_sc increases and then decreases thus resulting in an optimal anneal time. The initial increase in J_sc corresponds to existence of better interface between electrode and active layer². The depreciation in J_sc stems from coarsening of morphology resulting in lower exciton dissociation. The depreciation in J_sc has been verified numerically by our simulations.
1. Jackson, N. E., Savoie, B. M., Marks, T. J., Chen, L. X. & Ratner, M. A. The Next Breakthrough for Organic Photovoltaics? J. Phys. Chem. Lett. 6, 77–84 (2015).
2. Chen, Dian, et al. "P3HT/PCBM bulk heterojunction organic photovoltaics: correlating efficiency and morphology." Nano letters 11.2 (2010): 561-567.

Recent advances in organic electroluminescence benefit from molecular emitters capable of harvesting electrogenerated excitons. The successful emitters include room-temperature phosphorescent complexes of late transition metals and pure organic molecules exhibiting thermally activated delayed fluorescence. These compounds, however, suffer from quenching of triplet excitons due to slow rates during the processes of radiative transition or exciton spin flip. To overcome this limitation we have proposed and investigated an exciton-harvesting strategy which employs exergonic, El-Sayed rule-allowed reverse intersystem crossing (ES-rISC) from the triplet n-π^* transition state to the singlet π-π^* transition state as the key process of exciton collection.
In order to demonstrate our strategy, we designed and synthesized a series of blue fluorescent organic molecules (SY1-6) based on a 9,10-diphenylanthracene (DPA) scaffold. DPA was chosen because the large exchange energy between the singlet and triplet π-π^* transition states would suppress non-emissive loss of triplet exciton through internal conversion. Carbonyl units, including acetyl (SY1), benzoyl (SY2), 4-acetylphenyl (SY3), 4-benzoylphenyl (SY4), 3-acetylphenyl (SY5), and 3-benzoylphenyl (SY6) were introduced into DPA, and provided the triplet n-π* transition state higher than the singlet π-π* transition state in energy. Our structural control aimed at achieving exergonic, rapid conversion of triplet excitons into fluorescent singlet excitons through ES-rISC from the triplet n-π* transition state to the singlet π-π* transition state.
The validity of our strategy was supported by quantum chemical calculations which predicted exothermicity of ES-rISC of the SY compounds. The steady-state photoluminescence spectra revealed that blue fluorescence emissions were attributable to the singlet π-π* transition state of DPA. Photoluminescence quantum yield as high as 0.93 was achieved. The applicability of the exciton harvest by ES-rISC was examined by evaluating the performance of organic light-emitting devices incorporating SY compounds as dopants. Devices containing DPA were also fabricated, and served as a singlet exciton only control. The maximum external quantum efficiency of 4.8% was recorded for the SY3 device. This value was higher than that of the control device even though DPA had a photoluminescence quantum yield greater than SY3. Exciton utilization efficiency of the SY3 device was determined as large as 46%, being a two-fold enhancement from the DPA device (~26%). This enhancement was ascribed to exciton harvest by ES-rISC. Control experiments rebutted any contribution from E- or P-type delayed fluorescence and orientation effects on the improvement and unambiguously confirmed the contribution of ES-rISC. We hope that our research will provide useful guidance to designing molecules capable of rapid harvesting of excitons for high-efficiency electroluminescence.

Vacuum thermally-evaporated (VTE) small molecule organic photovoltaics (OPVs) have proven to be efficient and intrinsically stable. However, the high thermalization losses impedes the progress of VTE-grown small molecule OPVs. Ternary blend OPVs can improve light absorption and reduce energy losses beyond that of binary blend OPVs, but the difficulties in optimizing the morphology of three component device active regions, results in ternary systems that have heretofore exhibited performance inferior to analogous binary OPVs. Here, we introduce “a bi-ternary” OPV comprising two individual binary, bulk heterojunctions fused at a planar junction without component intermixing. In contrast to previous reports where the open circuit voltage (V_OC) of a conventional, blended ternary cell lies between that of the individual binaries, the V_OC of the bi-ternary OPV is that of either one of the constituting binaries depending on the order in which they are stacked relative to the anode. Additionally, dipole-induced energy-level realignment between two constituting binaries was observed only when using d–a–a′ dipolar donors in the photoactive heterojunction. The optimized bi-ternary single junction OPV shows improved performance compared to the two constituent binary OPVs, achieving PCE = 10.6 ± 0.3% under AM 1.5G 1 sun (100 mW/cm²) simulated illumination, with V_OC = 0.94 ± 0.01 V, a short circuit current density, J_sc = 16.0 ± 0.5 mA cm⁻² and a fill factor, FF = 0.70 ± 0.01. These results open new avenues for achieving highly efficient thermally evaporated, small molecule organic solar cells.

Organic photodiodes (OPD) are an emerging candidate as photosensors because of their wavelength tunability from Ultraviolet (UV) to Near Infrared (NIR), and processability at low-temperature over large area and variety of substrates [1]. Numerous applications can benefit from OPDs such as biomedical signal detection, imaging, communication, proximity sensing, etc. In particular, broadband OPD’s having response extended to NIR could be useful in optical communications, remote control and environmental monitoring [2].

OPDs can be solution-processed and hence can be fabricated using printing methods e.g. blade coating, which combined with a roll-to-roll process would offer high-throughput production over large areas. Further, the use of flexible substrates would enable applications in wearables as flexible OPDs could increase the comfort of use and quality of the acquired signal.

In this work, blade coated OPD’s have been fabricated with a bulk heterojunction (BHJ) containing a polymer donor material and a novel non-Fullerene acceptor (NFA). The donor material used in this work, PBDTTT-EFT, is proven to result in high efficiency and long lifetime of the resultant devices [3, 4]. The acceptor is an Indacenodithipophene derivative NFA (EHIDTBR) [5] having an absorption spectrum extended beyond red light wavelength. Unlike most conventional OPD’s where BHJ contains Fullerene acceptors because of their efficiency of charge separation [6], an NFA has been used in this work. Because, as opposed to Fullerene acceptors NFA’s can offer good absorption at visible and/or NIR wavelengths. As the donor and acceptor have somewhat complementary absorption spectrum the devices in current work exhibit fairly uniform broadband spectral response extending up to 750 nm. And, use of blade coating method which is compatible with roll-to-roll printing ensures device scalability.

OPD devices fabricated with the photoactive layer comprising PBDTTT-EFT polymer donor material and small molecule (EHIDTBR) NFA shows EQE as high as 55% with the simple conventional device architecture. One of the reasons behind this performance is favorable miscibility of the two components of the BHJ that results in excellent blend morphology [5]. For similar reasons, the dark or reverse leakage current is also small, on the order of 100 nAcm^-2 and comparable to state of the art devices. Overall, the devices show great promise as flexible printed photodetectors.

References
[1] Kang-Jun Baeg, Maddalena Binda, Dario Natali, Mario Caironi, and Yong-Young Noh. Organic light detectors: photodiodes and phototransistors. Adv. Mater.'13
[2] Fobao Huang, Xin Wang, Kun Xu, Yuanlong Liang, Yingquan Peng, and Guohan Liu. Broadband organic phototransistor with high photoresponse from ultraviolet to near-infrared realized via synergistic effect of trilayer heterostructure. J. Mater. Chem.'18
[3] Andrew J. Pearson, Paul E. Hopkinson, Elsa Couderc, Conrad Domanski, Mojtaba Abdi-Jalebia, and Neil C. Greenham. Critical light instability in CB/DIO processed PBDTTT-EFT:PC71BM organic photovoltaic devices. Org. Elec.'16
[4] Wenchao Huang, Eliot Gann, Lars Thomsen, Cunku Dong, Yi-Bing Cheng, and Christopher R. MacNeil. Unraveling the morphology of high-efficiency polymer solar cells based on the donor polymer PBDTTT-EFT. Adv. Energy Mater.'15
[5] Derya Baran, Nicola Gasparini, Andrew Wadsworth, Ching Hong Tan, Nimer Wehbe, Xin Song, Zeinab Hamid, Weimin Zhang, Marios Neophytou, Thomas Kirchartz, James J. Brabec, Christoph R. Durrant, and Iain McCulloch. Robust nonfullerene solar cells approaching unity external quantum efficiency enabled by suppression of geminate recombination. Adv. Energy Mater.'18
[6] Qana A. Alsulami, Banavoth Murali, Yara Alsinan, Manas R. Parida, Shawkat M. Aly, and Omar F. Mohammed. Remarkably high conversion efficiency of inverted bulk heterojunction solar cells: from ultrafast laser spectroscopy and electron microscopy to device fabrication and optimization. Adv. Energy Mater.'16

Controlling of crystalline structures of conjugated polymers (CPs) within nanostructured block copolymer (BCP) domains is crucial to enhance their electrical properties. However, it has been a challenge due to the intrinsic incompatibility between crystallization and phase-separation process of CP-based BCPs. Herein, we demonstrate solvo-microwave annealing as an effective method for producing highly ordered thin film structures of poly(3-dodecylthiophene)-block-poly(lactic acid) (P3DDT-b-PLA) polymers in very short processing time (~ 3 min). The conventional thermal annealing process even with long annealing time (~24h) resulted in incompatibility between ordered nanostructures and crystallization of the conjugated polymer. In contrast, the solvo-microwave annealing with the combination of heat and solvent vapor treatment was employed to grant enhanced chain mobility and solvent interaction with the π-π structure of the conjugated polymer. As a result, this process rapidly generated highly-ordered nanostructures with very few defects within 3 min and, at the same time, the high-crystalline intermolecular ordering of P3DDT blocks was obtained. Furthermore, we demonstrate the successful transfer of highly-ordered P3DDT structures after the removal of PLA domain, which is important for potential nanolithography and electronic applications.

In this work was prepared a mesoporous titanium dioxide films (TiO₂-mp) with sol-gel method, using different percentages of the polymer polyvinylpyrrolidone (PVP), and they were deposited by the spin-coating technique. It was done several thermal treatments from 120 to 600 °C for 1 and 3 hours. At temperatures of 500 °C. With a x-ray diffraction (XRD), anatase phase with tetragonal structure with crystal size of 19.5-21 nm was identified. The top view morphology of the films showed a porous range size between 15-30 nm. It was estimated for the first time the volumetric porosity of the material by the volume porosity average with the values of the refractive index. The porosity estimated showed a result between 40.3% and 43-6%, the revolutions per minute (rpm) varied from 1000-2500. These films of mp-TiO₂ were used as a scaffold and electro transport layers for 3 types of solar cells:

A) sensitized solar cells with quantum dots. For this type of solar cells, it was prepared heterojunctions of antimony sulfide-selenide (Sb₂(S_xSe_1-x)₃) in solid solution as the absorbing material and cadmium sulfide (CdS) as a sensitized layer deposited by the successive ionic layer adsorption and reaction (SILAR) technique and the (Sb₂(S_xSe_1-x)₃) with chemical bath deposition. It was shown by increasing the number of CdS layers the photovoltaic performance was improved by the reduction of the Sb₂O₃ and promoting a better nucleation with the (Sb₂(S_xSe_1-x)₃) at the chemical bath. The best photovoltaic performance was obtained with a solar cell with 30 cycles of CdS (CdS-30), a voltage of 434 mV, current density of 9.73 mA/cm² and an efficiency of 1.69%. B) As well were prepared Sb₂S₃ solar cells and
C) Perovskite solar cells (PVK), several PVK solar cells were fabricated varying the hole transport material, for that reason it was used the Poly(3-hexylthiophene-2,5-diyl) (P3HT) and the spiro-MeOTAD and a variation of the percentage of the PVP was used. For the solar cells with P3HT it was used a 20% of PVP and the variation of the rpm at the spin-coating process. With this was possible to correlate the thickness, the volume porosity and the efficiency of the device. It was found for solar cells prepared at 1500 and 2000 rpm, it was obtained a 203 and 168 of thicknesses and efficiencies of 1.24% and 2.14% respectively. The best solar cell with this configuration and 1000 rpm showed a 308 nm of thickness as well 41.6% of porosity. This one showed a current density of 17.15 mA/cm2, generating a 6.29% of efficiency. By changing the HTL for the spiro-MeOTAD, as well increasing the porosity of the material it was possible to obtained a device with a current density of 24.08%, a fill factor of 49.22 and an efficiency of 11.37%.

As an important part of the improvement of the electrical and morphological characteristics it will be done a plasma treatment.

Conjugated polymers are promising materials for flexible electronics and energy-relevant applications due to their combination of semiconducting properties, mechanical flexibility and ease of melt and solution processing. Chain length and dimensions in solution can be key factors in influencing the nanoscale and macroscale properties of polymers during processing including crystal architecture, solubility, phase transition temperatures, rheological response, and charge transport efficacy. These, in turn, have been shown to affect performance in various devices, including transistors, photovoltaics and light emitting diodes. Nevertheless, accurate characterization of the conjugated polymers in solution remains a challenge. Conjugated polymers exhibit unique relationships between coil size and contour length due to their semiflexible nature, while also exhibiting low solubilities relative to traditional polymer systems. This means that methods which rely on measurements relative to flexible polymer standards, such as gel permeation chromatography (GPC), can produce inaccurate results for semiflexible polymers. Using absolute measurement techniques such as static light scattering (SLS) and small-angle neutron scattering (SANS), we determine chain length, conformation, and persistence length for a multitude of conjugated polymer systems. Based on these measurements, we have found that the freely-rotating model (FRM) can accurately predict the persistence length for many conjugated polymers. Using this data, we then show that GPC can be made to produce accurate results for conjugated polymers with a range of persistence lengths and molecular weights. We demonstrate this using two additional in-line detectors: a viscometer in order to employ a universal calibration, as well as a differential refractometer to quantify sample loss in the columns. Using these techniques, conjugated polymer size and solution behavior can be accurately and precisely characterized.

Until recently fullerenes were the commonly used electron acceptor in Organic photovoltaics (OPVs) and had several drawbacks including low absorption in visible wavelengths and phase separation leading to poor performance. Since 2011 major efforts are given on the development of non-fullerene electron acceptors (NFAs) to fabricate fullerene free solar cells. High extinction coefficient is essential for OPV materials to minimize the active layer thickness. We have been therefore investigating the incorporation of BODIPY, a strongly absorbing organic dye, into polymer backbone for design of newer OPV materials. Though BODIPY polymers have been reported for donor applications, to our best knowledge, BODIPY based NFAs are scarce in literature.
Herein we present the design of two new BODIPY based D-A polymers (P7 and P8) with fluorene as the donor subunit. The meso substituent on BODIPY, in the acceptor units, is methylphenyl in case of P7 and trifluoromethyl in case of P8. The polymers were characterized using NMR, GPC, UV-vis spectroscopy and Cyclic volatmmetry. A lower band gap of 1.6 eV is observed for the trifluoromethylated polymer P8 compared to 1.8 eV in P7. The trifluoromethyl group is seen to improve the electron mobility of the polymer P8 by two orders compared to P7. Further electron acceptor application of these polymers was investigated. Using time resolved microwave conductivity experiments, a number of donor polymers with varying energy offset with P7 and P8 were screened. Photoinduced electron transfer was observed from the donor polymer PTB7-Th into these polymers. Subsequently all polymer solar cells were fabricated with PTB7-Th:P7/P8 using 1:1 blend ratios. The trifluoromethylated polymer P8 performs better than the methylated counterpart P7. The initial set of devices show low efficiency (~ 0.6 %) but high V_OC of ~ 0.85 V. Further optimization of device structure is underway to improve the performance.

In an organic solar cell, in addition to electrodes and the active layer, it is necessary to introduce interfacial layers to optimise the transport of charge carriers, and consequently the efficiency. In direct and inverted architectures, a layer of PEDOT-PSS (poly(ethylene dioxythiophene) doped with poly(styrene sulfonate)) is commonly used mainly to help the collection of holes.
In direct devices, the acidic character of PEDOT-PSS tends to promote the degradation of layers in contact, in particular the ITO (Indium Tin Oxide) electrode. The strategy presented here is to avoid this degradation by replacing the layer of PEDOT-PSS by a thin layer of P3HT (poly(3-hexylthiophene) grafted on ITO. We developed an efficient method to graft P3HT on various surfaces, including ITO, confirmed by XPS measurements [1]. This modified electrode was used to prepare direct solar cells and compared to the one with PEDOT-PSS. Devices stored in air without encapsulation with the grafted P3HT as hole transporting layer exhibited the highest stability.
In inverted solar cells, the problem of chemical degradation of ITO is no longer observed as PEDOT-PSS is deposited on the active layer, on the rear side. While the electrical failure mechanisms in such devices have been thoroughly investigated, little is known about their mechanical stability, which is as important and critical to ensure long term reliability [2]. The characteristic thin films stresses of each layer provide the mechanical driving force for delamination of weak interfaces, leading to a loss of device integrity and performance [3]. In this study [4], we developed a technique to probe weak layers or interfaces in thin multilayer devices, establishing a new set-up for the so-called probe tack making it similar to a pull-off test [5]. The technique has been extended varying both active layers, using different p-type low bandgap polymers for the active layer in combination with two different PEDOT:PSS formulations (Clevios^TMHTL Solar and HTL Solar 2).After mechanical tests, the upper and lower surfaces have been characterized by contact angle and XPS to locate the fracture point, which is dependent on the active layer and PEDOT-PSS formulations.
Acknowledgments: Part of the results were funded in the framework of the European Union Seventh Programme (FP7/2011 under grant agreement ESTABLIS n° 290022) and of ANR-13-JS09-0014-01 INSTEP.
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[4] A. Gregori, A. Tournebize, S. Schumann, H. Peisert, R. C Hiorns, T. Chassé, C. Lartigau-Dagron, A. Allal, Sol. En. Mat. & Sol. Cells2018, 25, 174
[5] ASTM D4541-0

In a molecular photovoltaic device, charge separation and energy conversion result from the evolution of a photogenerated exciton into a charge separated state, in competition with recombination to ground. The efficiency of charge separation is a function of the molecular packing and energy level alignment near the interface, and of disorder in these properties. Understanding the effects of chemical structure, packing, energetics and disorder on the competition between charge separation and recombination helps to identify the factors controlling device photovoltage and ultimately conversion efficiency. Here, we address the factors controlling generation efficiency and photovoltage in molecular donor: acceptor solar cells using a combination of electrical, spectroscopic and structural characterisation techniques along with numerical models. We adapt a recent model [1] of the effect of recombination losses on open-circuit voltage (V_oc) to incorporate properties of the intermediate charge-transfer state and show how control of these properties, through choice of materials and control of processing, could benefit V_oc and device performance [2]. In particular we find that the hybridisation of charge-transfer and local exciton states appears to benefit Voc by reducing non-radiative voltage losses [3]. We attempt to develop an improved modeling framework that allows for the evolution of charge states in the molecular environment of the interface. We use our results to consider the importance of chemical structure, phase behaviour and microstructure of the binary system in controlling actual performance and the ultimate limitations placed on solar to electric conversion by the molecular nature of the materials.

References
[1] J. Benduhn et al., Nature Energy (2017). DOI: 10.1038/nenergy.2017.53
[2] M. Azzouzi et al, Physical Review X (2018). DOI: 10.1103/PhysRevX.8.031055
[3] F. Eisner et al, J. Am. Chem. Soc (2019). DOI: 10.1021/jacs.9b01465

The power conversion efficiency of organic photovoltaic (OPV) devices has recently crossed 15%, thanks to the advent of non-fullerene acceptors, improved material and interface engineering and better understanding of energy loss mechanisms. In recent years, consolidated experimental and theoretical efforts led to empirical design rules, that mandate small frontier orbital energy difference between donor and acceptor molecules to reduce the non-radiative energy loss, one of the major bottlenecks in OPVs. Although charge transfer (CT) states mediated recombination in organic donor-acceptor (D-A) interfaces are considered to be the leading non-radiative loss pathway in highly efficient OPVs, the exact mechanisms of such loss processes are not well understood. Most studies in the literature to date on CT mediated loss processes are performed on D-A blends that feature very small frontier orbital energy offsets with CT spectral features buried under and possibly hybridized with the local molecular energy state spectrum. To conclusively determine the role of frontier orbital energy offset in CT mediated non-radiative energy loss mechanisms, we need to identify D-A systems with spectrally well resolved CT states. On that note, in this work, we demonstrate a class of D-A systems that has ~2 eV broad CT external quantum efficiency (EQE) spectra, originating from the large optical gap of the D-A molecules and very low donor-HOMO acceptor-LUMO gap at the interface (i.e. very large frontier orbital energy offset). Being the exact opposite of the material systems recommended by the efficient OPV design rules, these devices are expected to have severe non-radiative recombination loss and thus are ideal candidate to study loss mechanisms in-depth.

We have identified six D-A systems that exhibit ~2 eV broad CT state spectra, despite their dissimilar molecular structures and HOMO-LUMO energy levels. To investigate the origin of the broad CT spectrum in detail, we have chosen N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB)-1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN) based bulk-heterojunction (BHJ) D-A system, which demonstrates three distinct sub-gap peaks in its EQE spectrum. We have performed comprehensive electrical and optical characterization, along with molecular dynamics and quantum chemical simulations to conclusively prove that the lowest two peaks in the EQE are indeed originating from distinct electronic CT state transitions. We have also found that the lowest energy CT state is non-emissive and acts as a significant non-radiative loss path, possibly due to the large vibrational overlap with the ground state, and as expected from the energy gap law of organic materials. This efficient non-radiative loss pathway in NPB:HAT-CN BHJs results in a photon-energy dependent internal quantum efficiency spectrum and strong voltage bias dependent EQE, unlike most high-performing OPV devices.

Structural dynamics of the active layer of organic photovoltaic (OPV) devices is known to influence their performance and lifetime. By structural dynamics, it is meant dynamics in its larger extent, covering vibrational as well as diffusive and rotational dynamics in such materials.
We study the structural dynamics of blends of various compositions of poly(3-hexylthiophene-2,5-diyl (P3HT) and Phenyl-C61-butyric acid methyl ester (PCBM) as a function of temperature, using a combination of inelastic neutron scattering (INS), quasi-elastic neutron scattering (QENS), molecular dynamics simulations (MD) and density function theory (DFT). We use the deuteration technique for a contrast variation purpose to access not only the dynamics of the polymer but also the dynamics of PCBM within the blends. We observe, on the picosecond [1] and nanosecond timescales, that the faster polymer dynamics is increasingly frustrated upon blending with higher PCBM concentration, while the slower fullerene dynamics is promoted upon blending with P3HT. We find a good agreement between the simulated and experimental data on this broad timescale.
The MD simulation suggests that in the amorphous mixtures of P3HT and PCBM, P3HT is partially wrapping around PCBM and we speculate that the partial wrapping of P3HT around PCBM is responsible for the relatively high miscibility of PCBM with P3HT.[2] To further probe this specific interaction, we carry out INS experiment on blends of regiorandom-P3HT:PCBM of different compositions. INS allows us to explore molecular vibrations as well as lattice vibrations without selection rule and covers the entire Brillouin zone. By using a combination of MD, molecular and periodic DFT calculations, we reproduce the full INS spectra (10-3500 cm^-1) and validate our wrapping hypothesis.

References:
[1] A. A. Y. Guilbert et al., J. Phys. Chem. Lett. 7, 2252 (2016).
[2] A. A. Y. Guilbert et al., J. Phys. Chem. B, 121, 9073 (2017).

Organic photovoltaics (OPVs) are considered one of the most promising cost-effective options for utilizing solar energy in high energy/weight or semi-transparent applications. Recently, the OSC field has been revolutionized by the development of novel non-fullerene small molecular acceptors with efficiencies now reaching 16% in many systems. The device stability and mechanical durability of non-fullerene OPVs have received less attention and developing devices with both high performance and long-term stability remains challenging, particularly if the material choice is restricted by roll-to-roll and benign solvent processing requirements and desirable ductility requirements. Yet, morphological and mechanical stability is a prerequisite for OPV commercialization. Here we report our current understanding of the phase behavior of OPV mixtures and the relation of phase behavior to performance, processing needs (e.g., kinetic quench), and morphological stability via meta-stability or vitrification. A large range of miscibility (from hyper-miscibility to strong hypo-miscibility) is observed, including complex temperature dependence that can be a complex mixture of upper- and lower critical solution temperature behavior for both the binodal and the liquidus. The measurements presented should help to create molecular structure-function relationships that would allow some predictive guidance on how desired phase behavior and vitrification properties can be targeted by specific chemical design.

There is no doubt that the recent emergence of the non-fullerene electron acceptors has rejuvenated the field of organic photovoltaics. The current single junction record is now 15.7% and was achieved with an electron-deficient-core-based fused ring non-fullerene (Y6) in combination with a spectrally matched bespoke donor system [1]. Greater than two decades of reliance on fullerenes as the ‘only n-type in town’ is essentially broken although they still have a significant role to play in organic (and indeed perovskite) optoelectronics.
One critical element of the advances made in fullerene-based organic solar cells is a deep understanding of the device electro-optics – the means by which incident light interacts with the cell junction, creates a free carrier distribution profile, and those free carriers are extracted via various transport pathways. This has led to advanced optimization concepts in junction and interlayer design, manipulation of the external quantum efficiency, minimization of parasitic recombination and voltage loss, and structure-property relationships that guide the development of manufacturable thick junctions [2]. The question is – can we reapply the same electro-optical physics for the non-fullerenes, or must we start again?
In my talk I will address this most important of questions, detail some specific areas requiring thought and describe some of our recent work seeking to begin at least, this new electro-optical journey.
[1] Yuan et al. Joule, 3(4), 1140 (2019).
[2] Meredith & Armin, Nature Communications, 9:5261 (2018).

One of the puzzles in the field of organic photovoltaic cells (OPVs) is the experimentally deduced low binding energy despite the fact that simple coulomb based arguments ( ) would predict binding energy of at least E_b=500meV for intrachain excitons and 150-200meV for charge transfer excitons. Such high binding energies are not in line with devices achieving power conversion efficiency (PCE) of 14% or internal quantum efficiency (IQE) that is almost 100%.
There have been several suggestions for the reduced effective binding energy as delocalization, disorder, entropy, as well as a combination of entropy and disorder. By entropic contribution, one means that the more dissociation paths there are, the more likely the exciton is to dissociate, and this can be translated into lower binding energy using the entropy gained in the dissociation process. The process becomes less obvious when disorder is introduced as one has to account for the fact that energetic disorder creates a spatial distribution of the local environments. Namely, each CT experiences a different environment through which it dissociates, and the challenges are in translating the overall processes into effective binding energy. We will describe an improved entropy-disorder model that resolves deficiencies of earlier models that predicted the effective binding energy to become negative at high disorder. To test our model we performed voltage and temperature dependent analysis of charge generation in NFA (PTB7-th:ITIC) based device. We find good agreement between modeling and experiment. Specifically, the activation energy of the dissociation efficiency is about 10meV which translates to an effective binding energy of about ~80meV, at room temperature. Using the experimental and theoretical analysis we will discuss the implications for higher efficiency materials.

The interface between electron-donating and electron-accepting materials in organic photovoltaic (OPV) devices is commonly probed by charge-transfer (CT) electroluminescence (EL) measurements to estimate the CT energy, which critically relates to device open-circuit voltage. Despite the lack of a quantitative CT-EL model at disordered organic heterointerfaces and lack of quantitative proof, it is generally assumed that during CT-EL injected charges recombine at close-to-equilibrium energies in their respective density-of-states (DOS).

Combining optical and electrical experiments on a wide range of materials with numerical 3D kinetic Monte Carlo simulations of the entire OPV device we explicitly quantify that CT-EL instead originates from higher energy DOS site distributions far above DOS equilibrium energies. We present an experimentally verified and quantitative model of charge-transfer electroluminescence at donor/acceptor interfaces that reconciles the inconsistencies present in the literature. Our CT-EL model quantitatively and simultaneously accounts for charge transport physics in an energetically disordered DOS and molecular vibration induced broadening, using experimentally measured energetic disorder values and spectroscopically determined phonon-mode energies governing CT luminescence.

The lowest-energy CT states are in fact situated ~180-570 meV below the 0-0 CT-EL transition, enabling photogenerated carrier thermalization to these low-lying DOS sites when the OPV device is operated as a solar cell rather than as a light-emitting diode (LED). We reveal how DOS sites relevant to sub-gap photovoltaic action and CT-EL relate to the total DOS. The DOS sites sampled during CT-EL are not necessarily the same as those sampled during charge transport and sub-gap absorption measurements.

The non-equilibrium site distribution governing CT-EL rationalizes the experimentally observed weak current-density dependence of CT-EL and poses new research questions on reciprocity relations relating light emission to photovoltaic action and regarding minimal attainable photovoltaic energy conversion losses in OPV devices.

To efficiently harvest photoinduced current in organic solar cells by extending the coverage of the solar spectrum, many non-fullerene (NF) electron acceptors with complementary absorption spectra are synthesized, which indeed enables the power conversion efficiency of organic solar cells up to 14% in single junction and 17% in tandem devices.
There have been intensive studies on electron transfer from electron donors to various fullerene acceptors in fullerene based organic solar cells. However, the mechanism of efficient hole transfer from non-fullerene acceptors (or hole donor, denoted as D_h) to molecular or polymeric electron donors (or hole acceptors, denoted as A_h) is still not fully understood although considerable contribution from non-fullerene D_h materials to photoinduced current in non-fullerene solar cells. There are several reports on high performance non-fullerene organic solar cells with very small or even close to zero LUMO or/and HOMO offsets between two photoactive components, which challenges the theories on charge generation derived in fullerene based organic solar cells. Currently, to increase the efficiency of organic solar cells, more attention is paid on generating charge with minimized photo-voltage loss by matching energy levels of donors and acceptors. To understand the mechanism of non-fullerene organic solar cells, charge transport and extraction at electrodes also need to be considered.

Here we will present our study on the mechanism of efficient hole transfer from D_h to A_h in non-fullerene organic solar cells. Based on the results, efficient hole transfer from D_h to A_h is not only depends on the offset of HOMOs of D_h and A_h, but also charge transport and extraction at electrodes. Therefore, a comprehensive strategy including both charge generation and extraction is desired to further enhancing the PCE of the organic solar cells.

Eliminating the excess energetic driving force in organic solar cells leads to a smaller energy loss and higher device performance; hence, it is vital to understand the relation between the interfacial energetics and the photoelectric conversion efficiency. In practice, however, mixing morphology of the donor (D) and the acceptor (A) in bulk heterojunction has a large impact on photovoltaic properties, which prevents us from investigating the minimum energetic driving force at D/A interface for efficient charge generation.
In this study, we systematically fabricate 16 planar heterojunctions of four donor polymers and four acceptors and investigate the relation between state energies at D/A interface and resulting charge generation efficiency. A thin film of donor polymer was transferred onto an acceptor film, forming planar heterojunction, so as to eliminate the effect of morphology. The state energies of singlet excited state, charge transfer state, and charge separated state were experimentally quantified. The charge generation efficiency from donor exciton and acceptor exciton were evaluated using external quantum efficiency combined with transfer-matrix optical modeling with making a simple assumption about exciton diffusion.
The charge generation efficiency and its electric field dependence correlated with the energy difference between the singlet excited state and the interfacial charge transfer state. The threshold energy difference is 0.2 to 0.3 eV, below which the efficiency stated dropping and the charge generation became electric-field dependent. In contrast, the charge generation efficiency did not correlate with the energy difference between the charge transfer and the charge separated states, indicating that the binding of the charge pairs in the charge transfer state is not the determining factor for the charge generation.

[Reference]
K. Nakano, Y. Chen, B. Xiao, W. Han, J. Huang, H. Yoshida, E. Zhou, and K. Tajima, “Anatomy of the energetic driving force for charge generation in organic solar cells”, Nat. Commun. 10, 2520 (2019).

A significant breakthrough in the efficiency of organic solar cells (OSCs) has been achieved due to the synthesis of new materials offering a plethora of donor:acceptor combinations. However, the device performance is interlinked in a complex way not only with the properties of the individual components but also its strong dependence on the blend’s microstructure and phase morphology. Consequently, identifying high-performing donor:acceptor combinations has so far been an intricate process nearly uniquely relying on tedious and time-consuming trial-and-error materials selection methods. To overcome this unsustainable approach we developed a methodology that rapidly elucidates how blend composition and processing conditions affect the final blend morphology and microstructure. We demonstrate that transient absorption spectroscopy (TAS), vapor phase infiltration (VPI) and differential scanning calorimetry (DSC) measurements can be jointly harnessed to fast-screen OSC blends. VPI infuses inorganic materials into an organic matrix by exposure to gaseous precursors that diffuse into the film and in-situ convert to an inorganic product. In BHJ films, the diffusion process proceeds selectively through domains with high free-volume leading to inorganic deposition selectively along the diffusion paths. The diffusion network is easily visualized with electron microscopy. Using this labelling approach to spatially map OSC BHJ is in concept similar to the staining approach used to image low contrast biological. The spatially mapping of the phase morphology of organic solar cells via VPI giving insights into the size, shape, distribution and connectivity of specific domains, resolved through fast and straightforward HRSEM characterization, the information obtained on the general phase behavior as well as the phase purity (or at least degree of order) via DSC, and the indirect correlations on the fine structure of, e.g., the intermixed phases and phase-pure domains, provided by the charge separation dynamics from TAS, can, when combined, be used to identify best working compositions and deliver understanding why specific deposition methodologies do not lead to well-performing devices.

The distributed bulk heterojunction in organic solar cells is the result of a delicate nanoscale blend of electron donating (D) and accepting (A) material resulting in different interfacial varietals. These include interfaces between aggregated pure D and A phases, mixed interphases exhibiting significantly different energetics and other interfaces between an aggregated phase and the mixed interphase. Revealing the complex energy landscape that results in real-space can be extremely challenging, yet it locks the secrets of light harvesting of organic solar cells. Considerable debate remains about the nature of interfaces and the importance of sharp D-A interfaces versus the mixed interphase on charge generation and extraction. In this talk, we discuss recent developments in the direct imaging of the real-space energetic landscape of BHJ layers via scanning tunneling microscopy and spectroscopy (STM/STS) measurements. This approach is shown to work on BHJ films directly usable in OPV devices and reveal several heterointerfaces with different energetics that surround aggregated D, A and mixed (M) phase domains. We reveal the coexistence of three types of heterointerfaces within the BHJ and, in total, four types of D-A intermolecular interactions exhibiting different energetics. These scenarios are universally present in classical and modern BHJs exhibiting a wide range of power conversion efficiencies (PCE, ca. 3.5 – 10.8%; P3HT:PCBM, P3HT:O-IDTBR and PCE11:PCBM). We provide accurate HOMO/LUMO energy diagrams for D/M, A/M and D/A heterointerfaces in these systems and discuss the most likely pathways for charge generation and recombination.

Understanding various sources of energy loss in organic photovoltaic (OPV) cells is important for achieving high efficiency. In this work we explore energy losses at the interface between a vacuum deposited tetraphenyldibenzoperiflanthene (DBP) donor and C₇₀ as acceptor bulk heterojunction (BHJ) active layer and a MoO₃ anode buffer through experimental and computational methods. Material composition at the organic/anode buffer interface is independently controlled to distinguish the energy loss due to the organic/anode buffer interface from that in the BHJ. Interface energy loss is attributed to charge transfer between the organic active layer and anode buffer layer. Monte Carlo simulations are used to quantitatively evaluate the magnitude of of the interfacial energy loss and its effect on device performance. A strategy is proposed to reduce this source of loss. Specifically, a thin interlayer is inserted between the organic active layer and anode buffer layer, leading to an increase of up to 6% in power conversion efficiency compared with conventional devices. Finally, basic principles are provided for the design of the interface layer.

Intermolecular charge transfer states at the interface between electron donating (D) and accepting (A) materials are critical for the operation of organic solar cells but can be employed also in organic light emitting diodes. Non radiative decay of the charge transfer state is dominant in state of the art D/A organic solar cells and is responsible for large voltage losses as well as electroluminescence external quantum yields in the 0.01–0.0001% range. In contrast, the electroluminescence external quantum yield reaches up to 16% in D/A organic light emitting diodes. In our work we show that proper control of charge transfer state properties allows simultaneously an high photovoltaic and high emission quantum yield within a single system, demostrating that efficient photogeneration of free carriers and a high electroluminescence quantum yield do not necessarily need to be mutually exclusive in organic semiconductors. This leads to ultralow emission turn on voltages as well as significantly reduced voltage losses upon solar illumination, as low as 0.37eV. These results unify the description of the electro optical properties of charge transfer states in organic optoelectronic devices and encourage the use of organic D/A blends in energy conversion applications involving visible and ultraviolet photons, for example for indoor application and multijunction solar cells.

Charge carrier traps are generally highly detrimental for the performance of semiconductor devices. Unlike the situation for inorganic semiconductors, detailed knowledge about the characteristics and causes of traps in organic semiconductors is still very limited and surprisingly few systematic investigations to the energetics of traps in organic semiconductors have been conducted.

Here, we systematically investigate the JV characteristics from hole- and electron-only thin-film devices for a large range of organic semiconductors. Trap energies and densities are extracted directly from the JV curves and, more accurately, using a method based on the logarithmic slope of the JV curves. For all investigated materials, the hole and electron trap distributions are found to be centered at a more or less constant energy offset of 0.3–0.4 eV above the HOMO and below the LUMO level, respectively.¹ For typical preparation and measurement conditions the total trap density is around 0.5–1×10²³ m^-3 for both electrons and holes. We experimentally identify water absorbed in nano-voids as cause. Using density functional theory (DFT) calculations we show that direct interaction of H₂O molecules with the conjugated backbone cannot explain specific conformation locks nor the finding of a generic trap. Instead, we show that electrostatic interaction with an ensemble of H₂O molecules that for example are enclosed in a nanoscopic void in the film provides a stabilization of both electron and hole polarons on different model systems of conjugated molecule. The mechanism has a broad relevance as it does not rely on any specific interactions with, or properties of the active material like conjugation length, mobility or transport mechanism, and is shown to affect electron and hole transport in a similar manner. The prerequisite of having nano-voids in the molecular morphology suggests this trapping mechanism to be of lesser relevance for single crystals, but also suggests processing routes to suppress the mechanism; an example of the latter is demonstrated.

The current findings are in marked contrast to earlier works that concluded that the electron trap level is about constant and centered at an energy of ∼3.6 eV below the vacuum level.² For p-type materials, trap formation has previously been argued to be due to a specific hydrogen bonding interaction of a single water molecule water affecting the torsional potential energy profile of the bond connecting the donor and acceptor subunits of a D-A-type polymer.³

References:
1. Zuo, G., Linares, M., Upreti, T. & Kemerink, M. General rule for the energy of water-induced traps in organic semiconductors. Nat. Mater. 18, 588–593 (2019).
2. Nicolai, H. T. et al. Unification of trap-limited electron transport in semiconducting polymers. Nat. Mater. 11, 882–887 (2012).
3. Nikolka, M. et al. High operational and environmental stability of high-mobility conjugated polymer field-effect transistors through the use of molecular additives. Nat. Mater. 16, 356–362 (2016).

Small-molecule organic semiconductors are used in a wide spectrum of applications, ranging from organic light emitting diodes to organic photovoltaics. Presently, quantitative ab-initio models to assess the influence of these molecule-dependent properties, including the influence of dopants, are lacking. Here, we present a scale-bridging model, which can quantitatively describe an increasing number of experimentally relevant properties of these materials. The model consists of a multi-step procedure, incorporating single molecule parameterization by ab-initio method, generation of atomistic morphologies, DFT based electronic structure calculations yielding site energies, energy disorder, electronic couplings and reorganization energies. These parameters are used in KMC simulations or analytic models to address key properties of the materials. In this talk, I will focus on disorder-compensation effects that provide a quantitative understanding of doping in these materials. While well understood for inorganic materials, the mechanism of doping-induced conductivity and Fermi level shift in organic semiconductors remains elusive despite recent research efforts. In microscopic simulations with full treatment of many-body Coulomb effects, we reproduce the Fermi level shift in agreement with experimental observations. We find that the additional disorder introduced by doping can actually compensate the intrinsic disorder of the material, such that the total disorder remains constant or is even reduced at doping molar ratios relevant to experiment. In addition to the established dependence of the doping-induced states on the Coulomb interaction in the ionized host-dopant pair, we find that the position of the Fermi level is controlled by disorder compensation. Secondly, I will address bimolecular exciton quenching processes such as triplet-triplet annihilation (TTA) and triplet-polaron quenching, which play a central role in phosphorescent organic light-emitting diode (PhOLED) device performance and are therefore an essential component in computational models. We performed virtual photoluminescence experiments on a prototypical PhOLED emission materials and arrive at a phenomenological TTA quenching rate comparable to experimental results experiments in the low-intensity limit. Finally, I will discuss efforts to computionally predict novel pure ETL materials with three orders of magnitude higher mobility than their precursors and elucidate the molecular mechanism of doping these materials with kinetic Monte-Carlo simulations. The availability of first-principles based models to compute key performance characteristics of organic semiconductors may enable in-silico screening of numerous chemical compounds for the development of highly efficient opto-electronic devices.

In recent years, copper(I) thiocyanate (CuSCN) has been established as a high-potential semiconductor with an impressive range of opto/electronic applications demonstrated, including thin-film transistors (TFTs), organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), extremely thin absorber (ETA) solar cells, and now perovskite solar cells (PSCs). Belonging to the group of coordination polymers, CuSCN has an extended 3D network structure formed by the ambidentate SCN ligand linking the Cu centers together. This gives rise to the electronic structure that leads to its unique properties of optical transparency (wide band gap) and good hole transport (dispersed valence band).
Due to its excellent semiconducting properties and wide-ranging applications, it is of great interest to investigate other derivatives of CuSCN. For coordination polymers, ligand coordination is a common method to modify the structure. Interestingly, CuSCN is routinely processed from dialkyl sulfide-based solutions [Adv. Mater. 2013, 25, 1504; Adv. Energy Mater. 2015, 5, 1401529; Science 2017, 358, 768], and in fact the coordination of these sulfide ligands with Cu center in CuSCN can lead to different 1D structures [Polyhedron 2016, 114, 252]. Moreover, N-containing aromatic ligands have also been reported to react with CuSCN to yield colored compounds with optical absorption/emission variable in the visible range [Inorg. Chem. 2011, 50, 7239]. These examples show the structural versatility and adjustable electronic properties of CuSCN.
In this work, we report a survey of 25 structures of CuSCN-ligand (L) complexes and, for the first time, their electronic structures calculated by density functional theory (DFT). Four groups of ligands were investigated: dialkyl sulfides, aliphatic cyclic amines, aromatic imines, and aromatic diimines. We observed that the dimensionality of the Cu-SCN network decreased with the increasing CuSCN:L ratio. In general, the 3D network of the parent compounds (α- and β-CuSCN) was reduced to 2D sheet or 1D ladder structures when the CuSCN:L ratio was 1:1 and further to 1D zigzag chain or helical chain at 1:2 ratio and 0D monomer at 1:3 ratio. Aromatic diimine ligands resulted in bridged 2D or bridged 1D structures. Importantly, we found the relationship between the Cu-SCN network dimensionality and the width of the top Cu valence band (VB) as well as the energy gap between the Cu and SCN states. With the reducing dimensionality, the Cu VB width also decreases whereas the Cu-SCN energy gap increases. Based on the band structure and the total and partial density of states (DOS) analyses, hole transport prevailed in the 2D structures but became limited in the 1D structures and were completely localized in the 0D structure. Aromatic ligands generated states between the Cu and SCN energy states due to their lower-energy π* states compared to the SCN π* level. As a result, the combination of 2D Cu-SCN network (CuSCN:L ratio less than or equal to 1:1) and aromatic ligands can be employed to tune the electronic properties of CuSCN; we propose that the 2D network should be kept to provide hole transport while using the aromatic ligands to modify the optical absorption/emission properties. In addition, one particular case of the bridging 4,4’-bipyridine (4,4’BPy) ligand was found to exhibit dispersed states both at the valence band and the ligand-dominant conduction band as well as a small fundamental band gap; we speculate that the complex of CuSCN and 4,4’BPy could display ambipolar charge carrier transport properties, expanding further the possible applications of CuSCN-based materials. This work provides a comparative study on the effects of ligand coordination on the structural and electronic properties of CuSCN and could serve as guidelines for further development of semiconductors based on coordination polymers.

Intense light-field application to solids produces enormous/ultrafast non-linear phenomena such as high-harmonic generations and attosecond charge dynamics. They are distinct from conventional photonics. However, main targets have been limited to insulators and semiconductors, although theoretical approaches have been made also for correlated metals and superconductors. Here, in layered organic conductors and a superconductor, anomalous non-linear photonics driven by a nearly single-cycle strong electric field of >10 megavolts /cm are observed as a dynamical localization [1-3], a stimulated emission [4] and a harmonic generation (HG). It should be emphasized that they are enhanced by critical fluctuations near a metal-to-insulator transition temperature and a superconducting transition temperature. In particular, observation of the characteristic stimulate emission in an organic superconductor Kappa-(ET)₂Cu[N(CN)₂]Br on an ultrafast timescale of 10 fs clarifies that the Coulomb repulsion plays an essential role in the superconductor [4].
Moreover, second harmonic generation (SHG) in Kappa-(ET)₂Cu[N(CN)₂]Br (in which spatial inversion centers exist) shows that the non-perturbative light-driven current can break the inversion symmetry. Temperature dependence, polarization dependence, and carrier-envelope phase (CEP) dependence of the SHG in the organic superconductor will be discussed.

[1] T. Ishikawa, K. Yonemitsu, S. Iwai et al., Nature commun. 5, 5528(2014).
[2] Y. Naitoh, K.Yonemitsu, M. Dressel, S. Iwai et al., Phys. Rev. B93, 165126(2016).
[3] Y. Kawakami, K. Yonemitsu, S. Iwai et al., Phys. Rev. B95, 201105(R)(2017).
[4] Y. Kawakami, T. Sasaki, H. Yamamoto, K. Yonemitsu, S. Iwai et al., Nature Photon. 12, 474(2018).

The communication will display our recent progress Organic Photovoltaics (OPV). Solution-processed organic solar cells are continuously showing increasing performances other the recent years with record power conversion efficiency exceeding 15%. They represent a more and more promising technology with a growing interest from companies and markets. The challenge is surely to maintain the global research efforts in materials development to increase efficiency and stability but also to enable a realistic cost-competitve transfer from lab to fab. Our efforts to increase the stability of OPV cells will be exposed including device engineering and materials developments for thermally stable devices. We have recently shown that impurities such as photo-oxidized fullerene or photo-oxidized polymers (P3HT or PTB7) are responsible for an impressive loss in device performances even in very low amounts. This finding is raising issues for large area manufacturing where organic semiconductors are commonly processed in ambient conditions.
The cost of materials is also a critical when it comes to scale up solar cell production. If the best materials are achieving high PCE, it is crucial that these materials remain as low cost as possible thanks to simplified chemical structures and straightforward synthesis routes and purification steps. If a few simple and efficient polymers are currently emerging, we have been investigating a series of bio-inspired solution-processable compounds derivated from curcuminoids enabling one-pot, one-step synthetized organic semiconductors for OPV.
Finally, large area OPV modules are currently facing two major challenges. (a) The need to be light activated which becomes difficult when the flexible product uses packaging sheets acting as UV filter to promote long lived devices. (b) The fast decay of efficiency within the first hours of usage under illumination; this so-called burn-in effect is still not fully understood and should be overcome. On these two issues, this communication will bring some fundamental understandings and guidelines for more efficient and stable OPV cells.

Organic photovoltaic stands as one of the most promising clean energy technologies. However, its commercial availability still presents some challenges that have not yet been overcome. To improve the cost effectiveness of the organic solar cell active layer, our group has recently developed a series of N-annulated perylene diimide (PDI) derivatives acting as electron acceptors, one of these is today commercially available. (1) The resulting fullerene-free photovoltaic devices present a high power conversion efficiency, making them a viable alternative to the more traditional fullerene-containing solar cells. (2) Considering that these molecules can be mass-produced, they are excellent candidates for slot-die coating of large area solar cells. Herein, we present the structure-property relationships of these compounds (3) along with their utility as electron acceptors in bulk heterojunction organic photovoltaic. We also discuss the upscaling results of our efforts towards printing large-scale organic solar cells using slot-die coating. References: (1) Sigma-Aldrich, PDI-DPP-PDI, https://www.sigmaaldrich.com/catalog/product/aldrich/901143); (2) McAfee, S.M., Dayneko, S.V., Josse, P., Blanchard, P., Cabanetos, C., Welch, G.C., Chem. Mater., 2017, 29, 1309; (3) Tintori, F., Laventure, A., Welch, G.C., Soft Matter, 2019, Advance article, 10.1039/C9SM00716D.

The optoelectronic and charge transport properties of printed semiconductor nano-layers (e.g. polymer donors and acceptors, hybrid perovskites) is mainly controlled by the structural morphology and crystallization dynamics during their fabrication in functional device architectures (as Organic Photovoltaics, Perovskite Photovoltaics, etc). Despite the numerous advances reported on the structure-property relationships on these materials by lab-scale solution-based methods, their reliable manufacturing on flexible substrates by large scale roll-to-roll (R2R) printing processes is accompanied by numerous challenges, such as the formation of structural inhomogeneities, and non-reproducible properties (optical, electrical, structural) and interface quality over large areas. The above limitations provide significant obstacles for R2R printing to meet the requirements for reliable large scale manufacturing of high performance Organic Electronic devices on flexible substrates for commercial applications.

In this work, we present an intelligent in-line optical characterization methodology based on the combination of robust in-line Spectroscopic Ellipsometry (SE) and in-line Raman Spectroscopy (RS) for the investigation of the formation mechanisms and morphology of printed semiconductor nanolayers (e.g. polymer donors as PBDB-T, non-fullerene acceptors as ITIC, and hybrid perovskites based on MAPbI₃), on flexible polymer substrates by R2R pilot-to-production lines. In addition, we report on the development of intelligent modelling procedures that can extract from single very fast optical in-line measurements of significant insights from the printed semiconductors properties such as film quality, morphology (surface roughness, voids, pinholes etc), degradation traces and film thickness. By this approach, we demonstrate an optimized fabrication process of fully printed large scale flexible OPV and PPV devices with improved charge transport properties and performance that exceeds 6.5%, and significant device-to-device reproducibility. [1,2] Finally, this methodology will open the way for the reliable scalable nano-manufacturing of high performance flexible electronic devices by large scale processes.

[1] EU Project SmartLine (www.smartline-project.eu)
[2] EU Project CORNET (www.cornet-project.eu)

Over the last decade, organic solar cells (OSCs) have become a promising technology for next generation solar cells combining novel properties such as light weight, flexibility, or color design with large-scale manufacturing with low environmental impact. However, the main challenge for OSC will be the transfer from lab-scale processes to large-area industrial solar cell fabrication. High efficiencies in the field of OSCs are mainly achieved for devices fabricated under inert atmosphere using small active areas, typically below 0.2 cm². So far, a small lab scale devices have now reached performances above 17% [1].

In this light, inkjet printed organic solar cells and modules with large area were demonstrated. Inkjet printing allows direct patterning of four layers, including the top electrode, offering full freedom of design without the use of masks or structuring by hardware. Inkjet printed large area (>1 cm²) organic solar cells with power conversion efficiency exceeding 6.5 % deposited from environmentally friendly solvents in an air atmosphere are demonstrated using the same printer. To prove the great advantage of inkjet printing as a digital technology allowing freedom of forms and designs, large area organic modules with different artistic shapes were demonstrated
keeping high performance.

The good module performance at low illumination make our OPV modules good candidates for indoor applications, field in full expansion thanks to the Internet of Things (IoT).

Reported results confirm that inkjet printing has high potential for the processing of OPV, allowing quick changes in design as well as the materials.

[1] L. Meng et al., Organic and solution-processed tandem solar cells with 17.3% efficiency, Science 10.1126/science.aat2612 (2018)

In the field of energy generation, several emerging technologies using new PV materials and innovative device concepts have appeared in the last years: Dye solar cells (DSC), Quantum Dot (QDSC), organic photovoltaics (OPV), and perovskite solar cells (PSC). Among these technologies, PSC are the most promising ones, since they have drastically increased their efficiency up to 24.2 % in just a few years, while DSC have reached 11.9 %, OPV 15.6 % and QDSC 16.6%(according to NREL certified efficiencies chart) [1].
The main characteristic of these technologies is that most of the parts of the devices can be processed from solution, allowing to realize light-weight, flexibile, transparent, conformable, roll-to-roll compatible, potential low costs devices.
Among these technologies, OPV has seen in the last few year a fast rise in efficiency thanks to the introduction of new materials such as low band gap polymer donors and non-fullerene acceptors [2,3] allowing to reach the actual record efficiency of 16.5% [4].
Such optimal performance are strongly correlated to the use of spin coating technique to ensure reproducible and homogeneous films and chlorinated solvents that help the suitable nanoscale morphology. This procedure, however, is not industry compatible since spin coating does not allow large scale production and chlorinated solvents are poorly tolerated in workplaces since they are harmful towards environment and human health [5, 6, 7]. In this talk, we will present a possible route toward the realization of large area, high-performing inverted polymer solar cells where all the layers, apart from TCO, have been realized via the spray coating technique with green solvents [8,9]. In particular, we will show via atomic force microscopy (AFM) analysis, how this technique affects the morphology at the interface between photoactive layer (PAL) deposited through a non-chlorinated solvent (ortho-xylene) and hole and electron trasporting layers processed from alcohol based solvents. We will consider the interface between PAL and an electron transporting layer (ETL) fabricated using zinc oxide nanoparticles coated with polyethylenimine ethoxylated (PEIE). For the interface between PAL and the hole transporting layer (HTL)/anode three different combination will be shown: i) a mixture of two commercial poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) formulations (CPP:PH1000), ii) an anhydrous PEDOT:PSS (A-PEDOT) solution developed in our laboratory. The electrical performance in terms of PCE of small area devices (10 mm²) realized using these two typologies are 1.7% in the case of PH1000 and 3.6% anhydrous PEDOT:PSS when the devices were illuminated from ITO side. A study of scaling up of the solar cells will be presented for a fully spray coated organic photovoltaic modules (active area: 12 cm²) realized with structure ITO/ZnO-PEIE/PTB7:PC₇₀BM/V₂O₅/A-PEDOT. For this module, a power conversion efficiency 0.8% is achieved, while large area modules with metallic anode on top PCE of 3%. Finally, a practical application of our solar module is reported.
References
[1]https://www.nrel.gov/pv/cell-efficiency.html (accessed June 2019)
[2]S. Zhang, L. Ye, J. Hou , Adv. Energy Mater. (2016), 1502529
[3]L.Duan, et al. Sol. Energy Mater. Sol. Cells, Vol. 193, pp. 22-65 (2019)
[4]Y.Cui et al. Nature Communications 10, 2515 (2019)
[5]S. Zhang, L. Ye, H. Zhang, J. Hou , Mater. Today, 19, 9, (2016), 533-543
[6]L. La Notte, et al. Energy Technology, 2, 9-10 (2014) 786-791.
[7]G.Susanna et al. Solar Energy Materials and Solar Cells, vol. 134, pp.194-198 (2015)
[8]A. Reale et al. Energy Technology, 3, 4 (2015) 385-406.
[9]G Polino, et al.Energy technology, (2018) https://doi.org/10.1002/ente.201800627

In the field of optoelectronics, Poly (3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) is the most common and successful commercial conductive polyelectrolyte. Its properties such as high transparency in the visible range, high electrical conductivity, excellent chemical and physical stability, high ductility, good film-forming properties, and high work function enable multiple applications in the field of electronics. The work function is one of the main parameters which control the functionality of an electronic device. Properly tuning the work functions within semiconductor devices is very crucial in terms of performance and may be strongly influenced by unmonitored processing conditions. For example, PEDOT:PSS is highly hygroscopic in nature, which may impact on its functionality. In the present work, we have investigated the influence of the thermal post-production treatments and relative humidity during film casting or storage on the work function of PEDOT:PSS films made from various commercial formulations. We find clear trends between the processing conditions and absolute work function as obtained with a carefully calibrated Kelvin Probe setup. As a conclusion, we can suggest suitable processing parameters for a wide range of formulations and targeted electronic properties.

Electrode buffer layer plays an important role on the performance of polymer solar cell by facilitating the flow of charges via better energy level alignment at the organic/electrode interface. Transition metal oxide material such as MoO₃ was found as the good material for anode buffer layer due to high conductivity and high stability. In general, the MoOx layer fabricated by wet chemical method needs a post-thermal treatment around 200 °C in air to archive better quality thin film. However, the usage of high temperature makes the transition metal oxide hard to employ in flexible electronics where flexible substrate can’t stand with the temperature above 100^oC. In this work, we developed annealing free modified s-MoOx and have applied on bulk heterojunction (BHJ) polymer solar cell based on a blend of p-type polymer PTB7-Th and fullerene derivative PC₇₁BM. The polymer solar cell with non-annealed c-MoOx hole transport layer prepared by conventional sol-gel method showed poor efficiency of 1.37%. If the c-MoOx layer was annealed at 200^oC before active layer deposition, the efficiency was recovered to normal range that typically shown in conventional structure PTB7-Th and PC₇₁BM solar cells. However, the polymer solar cell with our modified-MoOx (m-MoOx) prepared using bis(2,4-pentanedionato)molybdenum(VI)dioxide (MoO₂(acac)₂) exhibited higher efficiency without high temperature annealing. The solar cell with m-MoOx showed higher efficiency and better stability comparable with that of device with PEDOT:PSS hole transport layer. Note that even PEDOT:PSS also needs annealing process around 120^oC normally. Furthermore, annealing free m-MoOx allows successful fabrication of inverted PTB7-Th:PC₇₁BM solar cells with solution processable MoOx.

c-MoOx: MoOx with conventional sol-gel method

Semiconducting polymers (SPs) have received enormous attention due to low-cost scalable solution-processing, and the potential for creating light-weight flexible electronic devices. Most working electronic devices consist of several layers of material, each having a specific optical and/or electronic function. One universal design constraint for complicated device architectures, like organic field effect transistors (OFETs), organic photovoltaics (OPVs) and red-green-blue organic light emitting diode (OLED) displays is that they require multiple components patterned laterally and vertically to operate. Currently, many of these components are comprised of non-flexible inorganic materials. In order to move towards flexible all organic electronic devices, there is a need to develop high precision vertical and lateral patterning methods that are compatible with solution processing techniques. This study demonstrates an additive solution process for depositing multiple layers of semiconducting polymer (SP) films by controlling film solubility with molecular dopants. During multi-layer deposition the bottom layers are exposed to a series of solvent environments that swell the SP films. We use neutron reflectometry (NR) to quantify the film thickness change and solvent content during solvent exposure in a single poly-3-hexylthiophene (P3HT) layer. The film thickness increases by 40-80% with exposure to good solvents. Four layer thin-films composed of alternating protonated and deuterated P3HT layers were additively coated from solution. NR measurements reveal high individual layer purity and that extensive solvent soaking induces no mixing between layers. This facile process enables additive layering of mutually soluble SP films and can be used to design novel electronic device architectures.

Efficient indoor light harvesters introduce a new design paradigm to Internet of Things (IoT) devices to maximize their ability to process, sense, and communicate data.(1) We here implement an adaptive wireless sensor node powered by dye-sensitized solar cells (DSCs) with an adapted sensitizer combination.
Co-sensitizing organic dyes enhances the photovoltage of DSCs based on the Cu^II/I(tmby)₂ redox couple up to 1080 mV and the power conversion efficiency above 11.5% under simulated sunlight. By adding optical density in the spectral region of 400 – 500 nm, the additional dye efficiently compensates for competitive absorption in the Cu^II/I(tmby)₂ electrolyte. Cosensitized light harvesting systems maintain a photovoltage of 910 mV when lowering the illumination to 1000 lux of fluorescent light, with the power output over 100 µW cm^-2 translating to an unprecedented conversion efficiency of 34.0% for ambient light. Subtle drying of the Cu^II/I(tmby)₂ electrolyte in ambient environment leads to ‘Zombie’ solid-state DSCs with an amorphous solid hole transport material that maintain power output for high stability.(2–4)
We demonstrate a working example of a self-powered wireless ’IoT’ device (light harvesting area 8 cm²) to power a microcontroller board with supercapacitor as energy buffer. Indoor light harvesters will lead to a new generation of energy-harvesting IoT.

1. M. Freitag et al., Dye-sensitized solar cells for efficient power generation under ambient lighting. Nat. Photonics. 11, 372–378 (2017).
2. H. Michaels et al., Copper Complexes with Tetradentate Ligands for Enhanced Charge Transport in Dye-Sensitized Solar Cells. Inorganics. 6 (2018), doi:10.3390/inorganics6020053.
3. M. Freitag et al., High-efficiency dye-sensitized solar cells with molecular copper phenanthroline as solid hole conductor. Energy Environ. Sci. 8 (2015), doi:10.1039/c5ee01204j.
4.I. Benesperi, H. Michaels, M. Freitag, The researcher’s guide to solid-state dye-sensitized solar cells. J. Mater. Chem. C. 6 (2018), doi:10.1039/c8tc03542c.

Semiconductor-based catalysts can convert solar energy into chemical fuels such as hydrogen, hydrogen peroxide, or hydrocarbons produced via carbon dioxide reduction. Long overlooked due to stability concerns, some organic semiconductors have recently emerged as promising electrocatalysts and photocatalysts for operation in aqueous environments. We have found that organic semiconductors have, in general, a high selectivity for the two-electron reduction of oxygen to hydrogen peroxide. We find this occurs on numerous organic semiconductors and conducting polymers in a pH range from 1 to 12. As cathodic catalysts, organic semiconductors can demonstrate impressive stability. The possibilities of solar energy conversion into the high-energy molecule H₂O₂ enabling carbon-neutral energy storage in liquid form, in contrast to gaseous H₂, will be discussed. Finally, I will cover recent results on purely photocatalytic systems, where hydrogen peroxide is produced reductively via one- or two-electron mechanisms, while various substrates are oxidized, including water. Organic semiconductors have potential to become a powerful class of intrinsic catalysts, tunable by molecular design.

Organic mixed ionic/electronic conductors have gained considerable interest in bioelectronics, power electronics, circuits and neuromorphic computing. These organic, often polymer based, semiconductors rely on a combination or ionic transport, electronic transport, and high volumetric charge storage capacity. Despite recent progress and a rapidly expanding library of new materials, the understanding of stability and transport/coupling of ionic and electronic carriers remain largely unexplored. We highlight recent synthetic and processing approaches used to tailor electrochemical device properties and stability, as well as new opportunities enabled by such advances. Our understanding of critical processes in electrochemical devices further requires us to study these materials in device-relevant conditions, fully considering the effects of ions and solvent on microstructure and transport. To this end, we report on recent efforts towards structure-property relations in high performance organic mixed conductors using ex-situ, in-situ, and operando scattering and spectroscopic techniques.

The development of large area ionizing radiation detection system is a crucial task in several areas of human society such as nuclear waste management, citizens security, radiotherapy or personal protection devices. Despite the excellent detecting performance exhibited by the inorganic materials (e.g. a-Se, CZT…), the increasing quest for flexible, portable, low cost and low power consumption sensors pushed the scientific community to look for alternative materials and technologies able to fulfill these new requirements. Electronic devices based on organic materials have already demonstrated to be a promising alternative to achieve a novel class of direct, flexible, portable and low-cost ionizing radiation detectors [1][2]. In particular, the excellent direct X-ray detection performance exhibited by solution-processed flexible organic thin film transistors (OTFTs) based on 100 nm thick microcrystalline (e.g. 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene)) active layer has been recently reported [3][4]. This class of detectors exhibits the highest sensitivity to the ionizing radiation among the other polymeric devices thanks to a photoconductive gain mechanism. This effect is based on the role of minority carrier trap states and it leads to an inner amplification of the photocurrent induced by the high-energy ionizing radiation.
However, in organic thin film detectors high-energy photon absorption is challenging because of the small active volume and the small cross section of interaction between radiation and low-Z elements of organic materials.
In order to enhance the radiation detection performance of this novel class of organic sensors, different strategies exist.
One possibility could be the optimization of the photoconductive gain effect by increasing the density of minority carrier trap states, e.g. varying the morphology of the organic films and the interfaces of the stacked electronic devices. An alternative approach is to increase the radiation capture cross section by tuning the organic material properties through small molecule tailoring, i.e. by means of the addition of high-Z atoms into the basic molecular structure of the material. In detail, by synthesizing new solution-processable organic molecules derived from TIPS-pentacene and 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT), with Ge-substitution in place of the Si atoms, we demonstrate boosted X-ray detection performance, reaching higher sensitivity values (up to 9*10⁵ µC Gy^-1 cm^-3) and better charge transport, with respect to TIPS-pentacene-based detectors [5].
[1] A.Ciavatti et al., Adv. Mater., 27, 7213 (2015)
[2] H.M. Thirimanne et al., Nat. Commun., 9, 2926 (2018)
[3] L.Basiricò et al., Nat. Commun., 7, 13063 (2016)
[4] S. Lai et al., Adv. Electron. Mater., 3, 1600409 (2017)
[5] A.Ciavatti et al., Adv. Funct. Mater., 1806119 (2018).

The recently-discovered water-enabled self-healing ability of conducting polymer polyethylenedioxythiophene doped with polystyrene sulfonate (PEDOT:PSS) makes it a candidate for healable electronics. A wetted PEDOT:PSS film can transform into an autonomic healable conductor, but its autonomic healing ability disappears after the water evaporates¹. In this work, we reveal an eternal autonomic electrically self-healable conductor through the addition of polyethylene glycol (PEG), demonstrating a fast healing response time and high healing efficiency. We also exhibit that the addition of glycerol can further enhance the conductivity while retaining the autonomic healing behavior. We performed systematic experiments to investigate and explain the underlying mechanisms of autonomic self-healing behavior of PEDOT: PSS films. This work provides a simple method to realize an autonomic healable conductor and paves the way for developing PEDOT:PSS based self-healing electronics for flexible and stretchable bioelectronic applications.
References
1) S. Zhang, F. Cicoira, Water Enabled Healing of Conducting Polymer Films. Adv Mater 2017, 29 (40).

The electric conduction in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is significantly affected and controlled by the morphology of the polymers [1]. The clarification of the electronic states governing the electric conduction is an important issue; the electronic state of the PEDOT:PSS can be probed by not only infrared spectroscopy but also terahertz spectroscopy [2] and time-resolved photoluminescence measurements [3]. In this study, we control the electronic states of the PEDOT:PSS with an ionic liquid and observe the change of the electronic states by Raman scattering spectroscopy.
PEDOT:PSS films were fabricated on ITO/CaF₂ substrates. We put an ionic liquid on the PEDOT:PSS films. Between the electrode under the PEDOT:PSS film and another electrode on the same substrate, we applied voltage and controlled the doped state. The change in the electronic states was monitored with not only infrared absorption bands but also the broad structure in the Raman scattering spectra. The detail of this broad Raman structure will be discussed in comparison with the electronic Raman scattering signals observed in an organic complex α-(BEDT-TTF)₂I₃ [4].

[1] T. Unuma, M. Yoshikawa, A. Nakamura, and H. Kishida, Appl. Phys. Express 9, 051601 (2016).
[2] T. Unuma, K. Fujii, H. Kishida, and A. Nakamura, Appl. Phys. Lett. 97, 033308 (2010).
[3] T. Koyama, A. Nakamura, and H. Kishida, ACS Photonics 1, 655 (2014).
[4] A. Ito, Y. Nakamura, A. Nakamura, and H. Kishida, Phys. Rev. Lett. 111, 197801 (2013).

Reduced graphene oxide (rGO) reduced by functionalized hydrazine from graphene oxide (GO) is a promising alterative material to PEDOT:PSS as hole transport layer (HTL) because rGO based organic solar cells (OSCs) show much better device stability than PEDOT:PSS based OSCs with similar device performance. However, to date, it has not been systematically investigated about effect of sheet size of rGO on the hole transporting properties. In addition, sheet size of rGO can affect the thin film uniformity as well as the elctrical properties of the HTL. The thin film uniformity of HTL has a considerable influence on the device performance in inveted OSCs, because HTLs directily contact with the top metals. Therefore, in this study, we evaluate the size effect of rGO sheets on their thin film uniformity and hole tranporting properties. In atomic force micoscopes analysis, the root mean square roughness of rGO films was decreased from 5.7 to 3.2 nm as the size of the sheets decreased meaning that the film uniformity of rGOs was improved. The hole transporting properties of rGOs having different sheets size were investigated by anlysis of J-V characteristics in the light and dark, J_ph-V_eff curves, the plots of J_sc and V_oc with respect to the light intensity, etc. The small size rGO based HTL, which have more uniform film than large size rGO based HTL, show the more efficient hole extraction and electron blocking abblity due to the improved interface contact with active layer and top metal (Ag), resulting in suppressed carrier recombination at the inferfaces. Thus, a higher power conversion efficiency of 9.07%, corresponding to an approximately 20% efficiency improvement compared with that of a large size rGO HTL (7.65%), was achieved by small size rGO HTL.

The p-type organic molecules of zinc phthalocyanine (ZnPc) adapt a standing-up (edge-on) molecular orientation on the indium-tin-oxide (ITO) substrate. On copper iodide (CuI) substrates, ZnPc molecules take on a molecular orientation lying flat on the substrate (flat-on) owing to π−d orbital interactions between the ZnPc molecules and the CuI. Earlier, Taima et al., (Appl. Phys. Lett., 2012, 100, 233302) reported that flat-on molecular orientation of the ZnPc films enhance the light absorption coefficient and charge carriers transport, resulted improved solar cell performance. When edge-on ZnPc films was heated at 200 °C, crystal phase changes from metastable α-ZnPc phase to stable β-ZnPc phase, resulted the long wavelength shift of the light absorption region and enhance in absorption coefficient. (J Mater Sci: Mater Electron, 2008, 19, 482486) Herein, we report an efficient approach to further increase of the light absorption by changing crystal phase from α-ZnPc phase to β-ZnPc phase through the heating of ZnPc on CuI substrate, followed by enhancing the performance of organic photovoltaic devices. We reveal that that when the ZnPc films was adapted edge-on orientation on the ITO substrate, the long wavelength was shifted owing to the phase transition from α-ZnPc to β-ZnPc. The phase transition of the ZnPc film improved the quantum efficiency near 650 nm to 750 nm. The improvement of the power conversion efficiency for the resultant solar cell is mainly caused by an increase in short circuit current (J_sc) from 2.56 to 3.50 mA/cm². These improvements are caused by the α-phase to β-phase change of ZnPc on the ITO, whose molecular orientation is the edge-on orientation. In contrast, as the ZnPc films was altered flat-on orientation on the CuI substrate, the long wavelength was further shifted in terms of the phase change from α-ZnPc to β-ZnPc. In addition, molecular stacking of the ZnPc/CuI film yields a higher light absorption coefficient. From these findings, we expect higher carrier transport efficiency and light absorption coefficient could be attributed to the higher J_sc, fill factor and open circuit voltage of the resultant solar cells.

A steady-state photoconductivity setup capable of taking sensitive external quantum efficiency (sEQE) measurements was constructed. This experimental setup records sEQE spectra used in determining the Charge Transfer (CT) state energy, the S₁ energy of the system, and the DE between the two, the driving force.

These energy-determining steps have thus far been used in two studies. One study in which electron and hole transfer rates were assessed in different Polymer:Non-Fullerene Acceptor (NFA) blends as a result of varying driving force. The driving forces were varied by using different donor polymers (P3HT, PCDT-BT, PBTTT, J61), with the same NFA (m-ITIC), and determined in the EQE setup. Further work with these systems went on to show that charge transfer remained a sub-picosecond process with near zero driving force. This study dispels the commonly-held belief that there is a trade-off between high-lying CT states which allow for high open-circuit voltages and high driving force which allows for faster charge transfer rates.

In a second study, a Polymer:NFA series in which the donor polymer (P3HT) remained constant and the NFA was varied (ITIC, ITIC-4F, or ITIC-DM), was able to decouple the driving force and CT state energy. While the energy of the CT state varied, the driving force remained relatively constant. Further studies will probe the impact of CT state energy levels on organic photovoltaic performance independent of driving force. This study touches on the effects of CT state energy and how this contributes to increasing open-circuit voltages and over-all power conversion efficiencies.

There is a growing interest in organic electronic devices based on conducting polymers. Most of the conducting polymers show unipolar conductivity based on either p-type (hole transport) or n-type (electron transport). Organic electronic devices e.g., organic light-emitting diodes (OLEDs), organic field effect transistor (OFETs), organic solar cells (OSCs) are functioned by both the p-type and n-type materials. Although there are a lot of polymers which are being used as the p-type materials, due to the stability issue of the n-type polymers, the usual n-type materials are generally limited to the small molecules e.g., fullerene and their derivatives. However, the benzimidazo-benzophenanthroline ladder polymer (BBL) is a stable polymer which has an electron-affinity (~4.0 eV) in the same range of fullerene. Polymer-blend made of both n-type and p-type polymers can act as a bipolar conducting material by showing both the hole transport and electron transport properties.

Here, in this study we consider a polymer blend system consists of poly(3,4-ethylenedioxythiophene) (PEDOT) as a p-type material and BBL as an n-type material where the two polymers are stacked by pi-pi interaction and electrostatic potential. We studied the electronic structures, optical properties, excitation energy transfer of the PEDOT/BBL blend material using ground-state and time-dependent density functional theory.

We found that the addition of positive charges in the system leads ~90% of positive charges to be collected by PEDOT. On the other hand, upon addition of negative charges, ~90% negative charges are collected by BBL. In this way, the material shows an excellent charge collection property. Therefore, this system can efficiently transport the positive and negative charges towards the opposite electrodes in an electronic device e.g., OLED, OSC. The electronic structure calculation shows the unoccupied states are localized on the BBL chain and occupied states are localized on the PEDOT chain. Moreover, the ionization potential of PEDOT is close to the electron affinity of the BBL. Thus, BBL as an acceptor material could be a good counterpart of donor PEDOT to replace the polymer/fullerene OSC to all polymer OSC. Oxidation and reduction of the blend material show oxidized PEDOT and reduced BBL, respectively, by forming positive polarons in PEDOT and negative polarons in BBL. The photo-induced electronic energy transfer calculation shows that the material can also be used as a hetero-junction in organic solar cells to dissociate the photo-induced exciton and consecutively, to transfer the free charges to the counter electrodes. The strong electronic coupling between the photo-excited PEDOT and charge-transferred states indicate an enhanced charge separation process. The obtained electronic coupling value of the PEDOT/BBL blend material is higher than the usual average coupling value of the available materials (e.g., polymer/fullerene).

In summary, this study shows BBL has the capability to replace the conventional n-type (acceptor) material, fullerene in order to fabricate all-polymer electronic devices. The PEDOT/BBL blend is a new bipolar conducting polymeric material which can efficiently be used in many energy application e.g., OLEDs, OFETs and OSCs.

Graphene with its sp² hybridized surface can function as a growth template for novel nano-microstructure, especially for organic semiconductors. We find that the doping tunability and atomic thickness of graphene allow the organic semiconductor ad-molecules on its surface to “feel” and interact with the doping charge carriers. Such interactions vary the molecular surface dynamics and thus the assembly and growth of the organic semiconductor crystals. For high quality organic semiconductor thin films and clean heterostructures of semiconductor-graphene, we unravel a rule-of-thumb that is tuning graphene’s electronic structure so that the interactions between graphene and ad-molecules are loose enough for free motion of these ad-molecules. The thin films grown at such conditions not only provide favorable pathways for both charge-carriers and exciton transports but form nearly ideal Schottky interface with graphene, both important for organic electronics.

Solution–processed polymer bulk heterojunction organic photovoltaic (BHJ-OPV) devices have gained serious attention during the last decade. They are one of the leading next generation photovoltaic technologies for low cost power production [1]. The active layer of a polymer solar cell consists of a thin solid film of an electron donor blended with an electron acceptor. The morphology of the active layer is one of the important factors for the solar cell performance. To control the morphology, one needs to understand the morphology formation on a molecular level.
The active layer of these solar cells is often produced by spincoating in the research laboratory, while dip-coating is an alternative that offers better control over the process parameters. Some of the most important parameters in dip-coating are the viscous drag, the gravitational force, and the capillary force. The viscous drag makes the liquid go upward with the substrate upon withdrawal. This force is proportional to the liquid viscosity and the withdrawal speed. Gravity causes the solution to move downwards.
To gain a deeper knowledge on the morphology formation in the active layer, being a consequence of the partial phase separation that occurs during the evaporation of the solvent [2-5]. In order to slow down the phase separation and, hence, get a better picture of the early stages of this process, we have chosen to prepare thin active layer films under microgravity conditions [6]. Under microgravity conditions, Marangoni flow will be the dominant source of convection during solvent drying.
Here we try to understand the effects of gravity on the film morphology for system for co-polymer (TQ1) and a fullerene derivative (PC₇₀BM). The ratio between the donor and the acceptor, as well as the processing solvent, were varied. Films prepared at microgravity conditions and at Earth conditions are compared to each other. We have chosen two commonly used solvents, i.e., chlorobenzene (CB) and ortho-dichlorobenzene (oDCB) and their fluorinated counterparts fluorobenzene and ortho-difluorobenzene (FB and oDFB, respectively). The choice of solvents allows a comparison of the influence of the solvents’ density, viscosity, and surface tension on the morphology. From our experiments, there is evidence for differences in structure between thin films at 1g and those prepared at microgravity conditions.

References
[1] Brabec, Christoph J., et al., Adv. Mater., 2010, 22(34), 3839-3856
[2] van Stam J., et al., Organic Photovoltaics XVII. International Society for Optics and Photonics, 2016, 9943, 99420D
[3] Lindqvist, C., et al., Materials, 2018, 11, 2068
[4] van Stam, J.; Lindqvist, C.; Hansson, R.; Ericsson, L.; Moons, E. Fluorescence and UV/VIS absorption spectroscopy studies on polymer blend films for photovoltaics.Proc. SPIE,2015, 9549, 95490L:1-9, 10.1117/12.2188618.
[5] van Stam, J.; Ericsson, L.; Deribew, D.; Moons, E. Morphology in Dip-Coated Blend Films for Photovoltaics Studied by UV/VIS Absorption and Fluorescence Spectroscopy. Proc. SPIE,2018, 10687, 10687A:1-10, 10.1117/12.2306857.
[6] The 70^thESA Parabolic Flight Campaign

Organic polymers are promising materials for the design of active layers of chemical sensors. In this context, polyfuran (PF) derivatives have not been extensively investigated, mainly due to typical drawbacks that have been recently overcome by using appropriate lateral substituents [1]. In the present study we employ electronic structure calculations (based on DFT approach) and molecular dynamics (MD) simulations (based on ReaxFF reactive force field) to evaluate the reactivity of branched PF derivatives and identify promising systems for chemical sensing. Condensed-to-atoms Fukui indexes (CAFI) were employed to identify the most reactive sites on the oligomers structure. The chemical sensing abilities of the most promising systems were evaluated via MD simulations in the presence of distinct gaseous analytes. The results indicate the derivatives PF-CCH and PF-NO2 (i.e. CCH and NO2 as side groups) as the most promising systems for chemical sensor applications, which present higher reactivity on the most accessible sites. An interesting correspondence between DFT and MD results was also identified, suggesting the plausibility of using CAFI parameters for the identification of improved materials for chemical sensors.

Reference:
[1] Sheberla D, Patra S, Wijsboom YH, et al (2015) Conducting polyfurans by electropolymerization of oligofurans. Chem Sci 6:360–371.

The outstanding mechanical properties and transparency of graphene make it promising as transparent conducting electrode for flexible organic light-emitting diodes (OLEDs). However, its work function (WF ∼4.5 eV), like that of ITO, is relatively high and typically unsuitable for use as a cathode in OLEDs. Surface doping of graphene by adsorption of reducing organic and metal-organic molecules has been shown to be able to tune the electronic properties of graphene without forming covalent bonds.^[1-2] Such molecular doping method preserves the structure of the graphene lattice and is less perturbing than substitutional doping or alkali metal deposition.^[1,3] Here we present a study and application^[4] of surface n-doping of graphene with the molecular reductant (pentamethylcyclopentadienyl) (1,3,5-trimethylbenzene)ruthenium dimer ([RuCp*Mes]₂).^[5] Photoemission spectroscopy, contact-potential measurements, and Hall-effect measurements confirm the dopant-induced changes in the electronic properties of the graphene layer. The graphene WF is reduced from 4.5 eV to a remarkably low value of 2.6 eV upon deposition of 1 nm of the molecular reductant [RuCp*Mes]₂ and UV photo-activation, which enhances the electron-donation process.^[4] The majority carriers of graphene switch from holes to electrons upon doping and the sheet resistance is reduced by 25%, despite the moderate decrease in carrier mobility. Photoemission spectroscopy and carrier-transport measurements are combined to investigate doping-induced changes in the electronic structure of the interface between graphene and phenyldi(pyren-2-yl)phosphine oxide (POPy₂), a low electron-affinity (2.2 eV) electron-transport material used in OLEDs. Surface doping with 1−2 nm of [RuCp*Mes]₂ reduces the electron injection barrier between graphene and POPy₂ by more than 1 eV, enhancing electron injection into POPy₂ by several orders of magnitude. Graphene/POPy₂/Al diodes with doped graphene cathodes exhibit reasonable stability in both nitrogen and air. This investigation allows a better understanding of the interface between graphene and an organic transport layer. These results represent a significant step toward the use of graphene as a transparent cathode for organic devices in general and for OLEDs in particular.
[1] Mansour, A. E. et al. Facile Doping and Work-Function Modification of Few-Layer Graphene Using Molecular Oxidants and Reductants. Adv. Funct. Mater. 2017, 27, 1602004.
[2] Pinto, H.; Markevich, A. Electronic and Electrochemical Doping of Graphene by Surface Adsorbates. Beilstein J. Nanotechnol. 2014, 5, 1842−1848.
[3] Wang, Y. et al. Nitrogen-Doped Graphene and Its Application in Electrochemical Biosensing. ACS Nano 2010, 4, 1790−1798.
[4] Zhang, F. et al. Molecular Reductant-Induced Control of a Graphene-Organic Interface for Electron Injection, Chem. Mat. 2019 (available online) DOI: 10.1021/acs.chemmater.9b0056
[5] Guo, S. et al. n-Doping of Organic Electronic Materials Using Air-Stable Organometallics, Adv. Mat. 2012, 24, 699

Organic semiconductors have attracted interest over the last decades for their easily tunable optoelectronic properties. Organic solar cells, which utilize organic electron donor and acceptor materials to convert sunlight into electricity, have proven their potential as valuable additions to the field of photovoltaics for their low cost and lightweight. Currently, one of the main limitations of organic solar cells are the relatively large voltage losses of often more than 600 meV between the optical gap and the open-circuit voltage due to radiative and nonradiative recombination. In addition, driving forces required to separate strongly bound excitons are a major contribution to the large overall voltage losses. To fully understand free energy losses in organic solar cells, the influence of donor and acceptor energy levels and microstructure contributions to the charge transfer state need to be understood.

In this work we investigate voltage losses as a function of donor energy levels and donor concentration. We use vacuum deposition techniques to pair the commonly used electron acceptor C₆₀ with Zinc phthalocyanine (ZnPc) or its fluorinated derivatives (F₄ZnPc, F₈ZnPc, F₁₆ZnPc) in a bulk heterojunction. Fluorination of ZnPc has been shown to simultaneously shift the HOMO and LUMO energy levels away from vacuum, thereby keeping the energy of the first allowed singlet transition approximately constant, but directly affecting the charge transfer state energy of the donor:acceptor system. The concentration of donor molecules is sequentially varied from the “dilute” case of 5% donor molecules in 95% acceptor molecules to a 1:1 ratio of donor:acceptor.

Sensitive external quantum efficiency and sensitive electroluminescence measurements are performed to determine the energy of the charge transfer state and reorganization energy as a function of donor energy levels and donor concentration. The effect of changing charge transfer state energies is correlated with the performance and open-circuit voltage of the devices obtained through current-voltage measurements. Our measurements are further enhanced through X-ray measurements of the microstructure of the active layers to obtain a full picture of the effect of changing energy levels and donor concentration on the device performance.

Two diketopyrrolopyrroles (DPPs) and three rylenes (NDI, dPyr PDI, and dEO PDI) were combined to form six hierarchical superstructures that assemble as a result of orthogonal H-bonding and π●●●π stacking. The individual components and the DPP–NDI as well as DPP–PDI pairs were cast into films, and their superstructures were interrogated by electron microscopy and advanced spectroscopy. All six superstructures feature different geometries, causing subtle changes in the solid-state packing of the DPPs. Changes in inter-DPP stacking that are scaffolded by the adjacent rylenes have a subtle impact on both the excited-state dynamics and on activating new pathways such as singlet fission (SF). Our studies demonstrate the unique benefits of combinatorial supramolecular assembly in exploring the impact of structure on advanced light management in the form of SF to afford triplet quantum yields, which are as high as 65% for a correlated pair of triplets and 15% for an uncorrelated pair of triplets.

Organic semiconductors are amongst the most important materials in the field of organic electronics. They are used for various devices applications like transistors and photovoltaics. The hole-transporting p-type organic semiconductor is most commonly used for these applications, while electron-transporting n-type organic semiconductors lag far behind because they suffer from moisture and air instability, as well as low electron mobility.¹
In this research, we synthesized a small-molecule n-type organic semiconductor, 2,6-Dibromo-N,N’-bis(2-ethoxyethyl-2-(2-(2-methoxyethoxy)ethoxy)acetate)-1,4,5,8-naphthalenetetracarboxylicdiimide, (gNDI-Br₂). Due to the redox ability of gNDI-Br₂², it is an electroactive material and can be utilized for organic electrochemical transistors (OECTs). OECTs have a channel material which is an electroactive organic semiconductor and its conductivity changes by the influx of ions from the electrolyte. Reduction and oxidation reactions with metal ions in electrolytes such as sodium or potassium ions occurs at the tetracarboxylic oxygen atoms in gNDI-Br₂ molecules. Due to the conjugated structure of this organic semiconductor, it reacts electrochemically and maintains its structure with the change in its conductivity. In addition, gNDI-Br₂ is easily solution processable because it is soluble in common organic solvents, such as chloroform, due to the ethylene glycol side chains attached to the NDI. Furthermore, these ethylene glycol side chains increase the free volume among the semiconducting molecules, providing gaps for the permeation of the metal ions.
The advantage of this gNDI-Br₂ small-molecule organic semiconductor is the improvement of its electrical property due to its ordered structure. It has less distorted structure compared to semiconducting polymers such as p(gNDI-gT2)³ or poly(benzimidazobenzophenanthroline) (BBL)⁴. Due to the coplanar structure of gNDI-Br₂, it allows for a more organized molecular arrangement, unlike polymeric n-type semiconductors. On account of this more orderly orientation, gNDI-Br₂ shows better electron mobility than previously reported n-type organic semiconductors, which enhances device performance. We will present the material, electrical and electrochemical properties of gNDI-Br₂ based on the different techniques like Electrospray (ESI-TOF) mass spectrometry, Nuclear magnetic resonance spectroscopy (NMR), X-ray diffraction (XRD), Atomic force microscopy (AFM), Scanning electron microscopy (SEM), and Electrochemical cyclic voltammetry (ECV).

Reference
1. J. T. E. Quinn, J. Zhu, X. Li, J. Wang, and Y. Li: Recent progress in the development of n-type organic semiconductors for organic field effect transistors. J. Mater. Chem. C5(34), 8654 (2017).
2. Y. Shi, H. Tang, S. Jiang, L. V. Kayser, M. Li, F. Liu, F. Ji, D. J. Lipomi, S. P. Ong, and Z. Chen: Understanding the Electrochemical Properties of Naphthalene Diimide: Implication for Stable and High-Rate Lithium-Ion Battery Electrodes. Chem. Mater.30(10), 3508 (2018).
3. A. Giovannitti, C. B. Nielsen, D.-T. Sbircea, S. Inal, M. Donahue, M. R. Niazi, D. A. Hanifi, A. Amassian, G. G. Malliaras, J. Rivnay, and I. McCulloch: N-type organic electrochemical transistors with stability in water. Nat. Commun.7(1), 13066 (2016).
4. H. Sun, M. Vagin, S. Wang, X. Crispin, R. Forchheimer, M. Berggren, and S. Fabiano: Complementary Logic Circuits Based on High-Performance n-Type Organic Electrochemical Transistors. Adv. Mater.30(9), 1704916 (2018).

In this study, we firstly investigate the effects of the molecular weight (MW) of a green-solvent processable semiconducting polymer (asy-PBTBDT) on photovoltaic performance and thermal stability of the devices. The asy-PBTBDT with high MW (132 kDa) has high μh values (4.91 × 10⁻³ cm² V⁻¹ s⁻¹ without dopants and 5.77 × 10⁻³ cm² V⁻¹ s⁻¹ with dopants) as a result of an increase in π–π stacking with higher MW than low MW asy-PBTBDTs (27 and 8 kDa). The high MW asy-PBTBDT with high μh achieves the highest power conversion efficiencies of 18.2% and 20.0% for the non-doped and doped states in PerSCs, respectively, and 5.7% in eco-friendly processed PSCs. Moreover, as the MW of asy-PBTBDT increases, the glass transition temperature increases, indicating an effective decrease of the thermally-induced morphological degradation in the photovoltaic devices. Likewise, an increase in the chain density along with MW induces the good robustness against humidity and oxygen. This material has important industrial significances because of its environmental processability, reproducibility, stability and efficiency of edible solvents.

Morphologies and optoelectronic/mechanical properties of semiconducting polymers are highly affected by polymer backbone configuration. In this study, thiophene units are introduced into the main backbone chain of semiconducting polymer in either a regular (PffBT-T4) or a random (PffBT-RT4) copolymerization to compare the performance of which semiconducting polymers are suitable in developing more efficient and flexible polymer solar cells. The unit composition ratio of the main chain is same, so both polymers show very similar energy levels. However, PffBT-RT4 has lower crystallinity than PffBT-T4 due to its random arrangement. As a result of the microstructure analysis, PffBT-RT4 shows π- π stacking distance shorter than PffBT-T4. Therefore, Since short distance benefits in charge transport, PffBt-RT4 shows higher space-charge-limited current mobility, and has a higher power conversion efficiency (PCE; 8.84%) than PffBT-T4 (7.25%) In addition, the PffBT-RT4 shows good performance in green solvent without additives (7.23%). Moreover, PffBT-RT4 maintains stable efficiency even after bending cycle, compared to PffBT-T4. Therefore, this study demonstrates that the random arrangement is a promising donor-acceptor based semiconducting polymer design strategy for efficient, flexible and green solvent processable polymer solar cells

Here, the effects of non-uniform energetic and spatial distributions of molecular electronic states in active layer on the open-circuit voltage (V_oc) of organic solar cells are investigated. Model devices are fabricated using active layers that consist of two sublayers. The sublayers have different donor-acceptor blend compositions. The different compositions offer large change in molecular energy levels of the donor and the acceptor materials. Large difference in the V_oc is observed depending on the thicknesses and the stacking order of the sublayers. Optical and electrical analyses suggest that the V_oc in these devices is not determined by the energy levels of the spatial region where the photocurrent is generated, and that the V_oc is not determined by the relative amount of different CT states within the active layer. Instead, we suggested that the energy levels of the regions where the electrons and holes are populated dominantly are important in determining the V_oc. This study would provide a new insight in deep understanding of the origin of V_oc in organic solar cells, especially for the systems that have spatial variation of energy levels of organic semiconductors within the active layer.

In this study, we introduce an evaporative assembly method which can control the growth and align of the crystalline material to achieve high-quality organic semiconductor patterns via one-step printing process. Organic semiconductor patterns that has a high crystallinity property and its uniform alignment are essential to realizing their application into the high-performance organic field-effect transistors (OFETs). Organic materials have flexible and lightweight properties compare to metallic compounds. Moreover, the recent development of electrical performances emphasized its importance for next-generation electric devices. However, there are obstacles to pattern crystals into large-area, print circuit cost-effectively.
Various patterning methods (e.g., dip coating, slot-die coating, blade coating, inkjet, and electrohydrodynamic jet printing) have been studies, however, there are some problems. In the case of dip coating, it is hard to make highly integrated patterns because the entire substrate is immersed in a semiconductor solution. Lithography needs tons of steps until patterns are made in place and it is hard to control the direction of the crystal. Inkjet and electrohydrodynamic jet printing show slow patterning speed. Flow coating system fulfills requirements i) simplification of patterning steps, ii) accuracy of patterning on the desired position, iii) speed for patterning process and iv) improving crystallinity and alignment of crystal over the large area.
A flow coating technique is a kind of an evaporative self-assembly method. This versatile method utilized the phenomenon of the “coffee ring effect” coupled to the confined convective flow with a controlled stick-slip motion. When the liquid is placed on the solid substrate, the different evaporation rate between the edge and center of the droplet induces migration of solution to the three-phase contact line and deposits solute. Then, intended stick-slip motions determined the distance and width of patterns. Furthermore, flow coating confines the evaporation of the solvent at the solution guiding blade and it gives shear stress on the solution. It induces one-directional coffee ring effect along the blade and the given shear stress determines the direction and morphology of patterns. Different amount of shear stress was given under the various speed of flow coater. It enhances the directional crystallization of the semiconducting layer and electrical properties.
In this study, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) was chosen as a crystalline organic semiconductor and deposited by the solution shearing process. The relationships between shearing of the solution and characteristic property of crystal were determined with p-OM, pUV, GIWAX, and NEXAFS. One-directional solution shearing caused a crystal aligned morphology, anisotropic pUV absorption spectrums. The intensity of crystal was differed by coating speed and verified its tendency through X-ray diffraction.
Furthermore, flow coating equipment is controlled by the computer system and each condition can be tuned properly. It is possible that adjusting coating condition in demand during the process with any kind of materials. Therefore, this study is essential for cost-effective printing techniques to achieve high-performance OFETs with high-quality organic semiconducting crystals.

We report on the development of beta-ray detectors using organic photodiodes (OPD). Beta-ray detectors are used for surface contamination monitoring at nuclear power plants and similar facilities. Various organic radiation detectors for x-ray have been reported but few examples of radiation detectors for charged particles (alpha and beta particles) have been reported. OPDs are advantageous for beta-ray detection in view of their high beta-gamma ray detectability ratios, since organic semiconductors are composed of light elements and therefore expected to show low gamma-ray detection efficiency. We examined indirect- and direct-conversion organic beta-ray detectors and observed Sr-90 and Co-60 beta particles with the respective detectors and a readout circuit. The indirect conversion detectors are composed of an OPD and a scintillator, which converts beta rays into green light. To realize direct-conversion radiation detectors, the organic semiconductor layer must be much thicker than that of conventional organic photodiodes. Therefore, by forming an organic semiconductor layer of 50 μm thickness, we fabricated OPDs with the following structure: ITO electrode/ buffer layer/ P3HT:PCBM/ Al electrode. We also examined photoconductive properties of the OPDs. The OPD with the 50 μm-thick organic semiconductor layer showed high external quantum efficiency of more than 60% at 530 nm wavelength. In addition, molecular doping in the thick organic semiconductor layer brought about a remarkable lowering of the rising voltage of the photocurrent. In the presentation, we will discuss the relationship between the properties of the radiation detection and the photoconductivity of the OPDs.

Doping has been a key topic in organic electronics since the demonstration of a metallic conductivity in doped pi-conjugated polymers. However, it has been difficult to prevent dopant-induced structural and energetic disorder, while maintaining a high conductivity. We have developed an efficient doping method based on solid-state diffusion of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane(F₄-TCNQ) in poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT)which exhibited a high conductivity of 200 S/cm and one of the highest Hall mobilities for conducting polymers [1]. The observed coherent charge transport in the doped PBTTT couldbe attributed to a preserved microstructural order upon doping. We have recently employed this doping method to enhance charge injection properties in organic field-effect transistors (FETs) [2]. A selective contact doping of bottom-gated PBTTT FETs by solid-state diffusion of F₄-TCNQ achieved a significantly lower contact resistance. In addition, a post-doping treatment on contact-doped PBTTT FETs was shown to be an effective way of improving the device stability. Furthermore, an extra degree of freedom in the choice of gate dielectric material with the bottom-gate structure enabled a low-voltage operation in our contact-doped PBTTT FETs, demonstrating a potential for designing stable and low-power organic electronic devices by utilizing doping of conjugated polymers.

References:
[1] K. Kang, S. Watanabe, K. Broch, A. Sepe, A. Brown, I. Nasrallah1, M. Nikolka, Z. Fei, M. Heeney, D. Matsumoto, K. Marumoto, H. Tanaka, S. Kuroda and H. Sirringhaus, Nat. Mater. 15, 896 (2016).
[2] Y. Kim, S. Chung, K. Cho, D. Harkin, W. -T. Hwang, D. Yoo, J. -K. Kim, W. Lee, Y. Song, H. Ahn, Y. Hong, H. Sirringhaus, K. Kang and T. Lee, Adv. Mater. 31, 1806697 (2019).

Polymer solar cells have recently experienced a renewal of interest thanks to power conversion efficiencies above 14% for both single and tandem configurations of heterojunction, mostly ascribed to recent use of new non-fullerene acceptors.[1,2] However, even though the photoactive blend governs the maximum efficiency of the solar cell, the interfacial layers sandwiching the photoactive layer are of equal importance as they insure different important roles such as the efficient charge carrier extraction together with major implication in the stability of the whole device . In this work, we report new organic-inorganic hybrid materials composed of diaminopyridine combined with halides and either bismuth[3] or antimony[4] metals that were studied as novel materials for solution-processed electron extraction layers in polymer solar cells in order to improve the stability of these devices.
We describe the synthesis of the materials by crystallization and the different characterizations by X-ray diffraction, infrared and Raman spectroscopies, optical absorption and photoluminescence measurements. The energy band gap of these materials was found to be close to the one used in classical interfacial layers [5] of some organic solar cells, such as ZnO. Therefore, the hybrid materials were solution-processed on top of two different low bandgap polymers (PTB7 or PTB7-Th) as donor materials mixed with PC70BM fullerenes as the acceptor. By optimizing optical, electrical, and morphological properties of these new wide bandgap materials, bulk heterojunction solar cells with conversion efficiency close to 10% were obtained with all-solution-processed interlayers in regular device structure thanks to efficient hole blocking behavior leading to fill factor up to 73%. Importantly, these hybrid electron extraction layers also strongly improved the device stability in air compared to solar cells processed with ZnO interlayers, and showing an additional increase in solar cell stability under illumination. We believe that, together with the compatibility of facile low-temperature solution processing, these novel hybrid high bandgap materials open interesting opportunities towards use in low-cost high-efficiency stable solar cells. Especially, the synthesis of bismuth/antimony and other metal-based organic-inorganic materials may be a promising strategy to further improve the electronic properties and stability of devices.


[1] Li, H.; Xiao, Z.; Ding, L.; Wang, J. Thermostable Single-Junction Organic Solar Cells with a Power Conversion Efficiency of 14.62%. Sci. Bull. 2018, 63 (6), 340.
[2] Che, X.; Li, Y.; Qu, Y.; Forrest, S. R. High Fabrication Yield Organic Tandem Photovoltaics Combining Vacuum- and Solution-Processed Subcells with 15% Efficiency. Nat. Energy 2018, 3 (5), 422.
[3] Fredj, D.; Pourcin, F.; Alkarsifi, R.; Kilinc, V.; Liu, X.; Ben Dkhil, S.; Boudjada, N. C.; Fahlman, M.; Videlot-Ackermann, C.; Margeat, O.; et al. Fabrication and Characterization of Hybrid Organic–Inorganic Electron Extraction Layers for Polymer Solar Cells toward Improved Processing Robustness and Air Stability. ACS Appl. Mater. Interfaces 2018, 10 (20), 17309.
[4] Fredj, D.; Pourcin, F.; Alkarsifi, R.; Kilinc, V.; Liu, X.; Ben Dkhil, S.; Boudjada, N. C.; Fahlman, M.; Videlot-Ackermann, C.; Margeat, O.; et al. A new antimony-based organic-inorganic hybrid material as electron extraction layer for efficient and stable polymer solar cells. Submitted.
[5] Dkhil, S. Ben; Duché, D.; Gaceur, M.; Thakur, A. K.; Aboura, F. B.; Escoubas, L.; Simon, J.-J.; Guerrero, A.; Bisquert, J.; Garcia-Belmonte, G.; et al. Interplay of Optical, Morphological, and Electronic Effects of ZnO Optical Spacers in Highly Efficient Polymer Solar Cells. Adv. Energy Mater. 2014, 4 (18), 1400805.

Properties of organic semiconductors are highly dependent on the chemical composition, the manner of the molecular organization and the electronic properties of the individual components. At the same time, the intermolecular electronic communication among neighboring molecules defines the photoinduced charge generation, separation and transport properties. Here, we design new, highly ordered organic semiconductor materials integrating the diverse organic molecules into surface-anchored metal-organic frameworks (SURMOFs). Even if metal-organic frameworks are known to be poor conducting materials, we show that their semiconducting properties can be significantly tuned due to the control of their nano/microscale material morphology and structure with the understanding of their physicochemical properties both by theory and experiment.

We use quantum mechanical calculations in the density functional theory formalism to investigate the photophysical properties and the mechanism of charge transport in the SURMOF materials made of anthracene, diphenylethynyl-anthracene, naphthalenediimide and porphyrin linkers. We calculate the transfer rates using the semi-classical Marcus theory approach based on the electronic coupling
elements and the reorganization energies between the adjacent MOF linkers. Using the in-house developed ab initio method, we explain highly efficient excited-state transport properties of porphyrin SURMOF [1, 2] and predict the photoinduced charge transport in polycyclic aromatic hydrocarbon based MOF as a function of the type of organic linker. We show the reason of the improved conductivity as derived from the spatially ordered structure of a material [3] and confirm the charge transfer anisotropy through the detailed analysis of the transfer rates for electron and hole transport. Finally, we prove that ab initio calculations enable valuable predictions of new promising candidates for semiconducting organic materials made in spatially ordered fashion in the SURMOF. Our data demonstrates the feasibility of MOF-based crystal engineering approaches that can be universally applied to tailor the photophysical properties of organic semiconductor materials.

References:
[1] M. Adams, M. Kozlowska, N. Baroni, M. Oldenburg, R. Ma, D. Busko, A. Turshatov, G. Emandi, M. O. Senge, R. Haldar, C. Wöll, G. U. Nienhaus, B. S. Richards, I. A. Howard, Highly efficient 1D triplet exciton transport in a palladium-porphyrin based surface-anchored metal-organic framework, ACS Appl. Mater. Interfaces, 2019, 11, 15688-15697.
[2] R. Haldar, A. Mazel, M. Krstić, Q. Zhang, M. Jakoby, I. A. Howard, B. S. Richards, N. Jung, D. Jacquemin, S. Diring, W. Wenzel, F. Odobel, C. Wöll, A de novo strategy for predictive crystal engineering to tune excitonic coupling, Nat. Commun., 2019, 10, 2048-2054.
[3] X. Liu, M. Kozlowska, T. Okkali, D. Wagner, T. Higashino, G. Brenner-Weiß, S. M. Marschner, Z. Fu, Q. Zhang, H. Imahori, S. Bräse, W. Wenzel, C. Wöll, L. Heinke, Photoconductivity in Metal-Organic Framework Thin Films, Angew. Chem. Int. Ed, accepted, doi: 10.1002/ange.201904475.

Metal-organic frameworks are highly ordered structure nanoporous materials with many different properties which can be tailored. In this context, surface-anchored metal-organic frameworks (SURMOFs) are promising subclass of MOFs which offers appealing possibility to tune properties for each specific application. Here, we present recent results of such nanomaterials with focus on photovoltaic activity, photostability and charge transport as well as photoluminescence (PL).
One of the most promising tuning approach is integration of different organic linkers into MOFs with specific properties. Due to their large photoabsorption coefficients for visible light, porphyrin derivatives are widely used in light-harvesting applications, such as photovoltaics and photocatalysis. We use quantum mechanical calculations in order to explain highly efficient excited-state transport properties of porphyrin SURMOF with Pd-coordinated organic linkers [1]. The calculated transfer rates are consistent with experimentally obtained rates, which result in micron-range exciton diffusion length in this MOF. First principle calculations are also applied to understand mechanism of charge transport in Zn-coordinated porphyrin-containing SURMOFs with embedded fullerene molecules [2]. The improved photoconductivity is shown to be derived from the spatially continuous network of donor and acceptor domains in MOF material. Owing to the fact that the porphyrin properties can be tailored by advanced organic chemistry and MOF properties can be tuned for the specific applications, ab initio calculations enable valuable predictions of new promising candidates for light harvesting.

In molecular solids, the intense PL observed for solvated dye molecules is often suppressed by nonradiative decay processes introduced by excitonic coupling to adjacent chromophores. To achieve unprecedented PL quantum yields for MOF crystalline NDI-based materials through predictive crystal engineering and tuning of excitonic coupling we have developed a strategy to avoid this undesirable PL quenching by optimizing the chromophore packing [3]. We integrated the photoactive compounds into metal-organic frameworks (MOFs) and tuned the molecular alignment by introducing adjustable “steric control units” (SCUs). We determined the optimal alignment of core-substituted naphthalenediimides (cNDIs) to yield highly emissive J-aggregates by a computational analysis. Then, we created a large library of handle-equipped MOF chromophoric linkers and computationally screened for the best SCUs. A thorough photophysical characterization confirmed the formation of J-aggregates with bright green emission, with unprecedented photoluminescent quantum yields for crystalline NDI-based MOF materials. This data demonstrates the viability of MOF-based crystal engineering approaches that can be universally applied to tailor the photophysical properties of organic crystaline MOF materials.

References:
[1] M. Adams, M. Kozlowska, N. Baroni, M. Oldenburg, R. Ma, D. Busko, A. Turshatov, G. Emandi, M. O. Senge, R. Haldar, C. Wöll, G. U. Nienhaus, B. S. Richards, I. A. Howard, Highly efficient 1D triplet exciton transport in a palladium-porphyrin based surface-anchored metal-organic framework, ACS Appl. Mater. Interfaces, 2019, 11, 15688-15697.
[2] X. Liu, M. Kozlowska, T. Okkali, D. Wagner, T. Higashino, G. Brenner-Weiß, S. M. Marschner, Z. Fu, Q. Zhang, H. Imahori, S. Bräse, W. Wenzel, C. Wöll, L. Heinke, Photoconductivity in Metal-Organic Framework Thin Films, Angew. Chem. Int. Ed, accepted, doi: 10.1002/ange.201904475.
[3] R. Haldar, A. Mazel, M. Krstić, Q. Zhang, M. Jakoby, I. A. Howard, B. S. Richards, N. Jung, D. Jacquemin, S. Diring, W. Wenzel, F. Odobel, C. Wöll, A de novo strategy for predictive crystal engineering to tune excitonic coupling, Nat. Commun., 2019, 10, 2048-2054.

Organic small molecule solar cells are used as a test bed to investigate the influence of film morphology on the density of charge-transfer (CT) states. CT states are considered as precursors for charge generation and their energy (E_CT) as the upper limit for the open-circuit voltage (V_OC) in organic donor/acceptor solar cells. In this study the morphological influence of using a crystalline donor diindenoperylene (DIP) versus an amorphous donor tetraphenyldibezoperiflanthene (DBP) with almost identical ionization energy is investigated. As acceptor material the widely used fullerene C₆₀ is used.
By combining electrical measurements with optical spectroscopy and low-energy ultraviolet photoelectron spectroscopy (LE-UPS) with different excitation energies, a comprehensive picture is obtained that describes how morphology and the connected density of states (DOS) influence the device performance and the spectroscopic signature of CT states. Especially for the crystalline donor material DIP strong exponential tail states reaching far into the gap are observed in the UPS spectra, which can be related to the presence of grain boundaries. A voltage dependent filling of these states is identified as the origin of a blue shift of the electroluminescence spectra with increasing applied voltage.
Different approaches to the description of the reduced electroluminescence (EL) and reduced external quantum efficiency (EQE) spectra, as proposed by Vandewal¹, Burke² and Kahle³, are compared. This is done to verify the necessity of static disorder in the description of CT emission and absorption spectra of organic solar cells. Despite the fact that both donors yield almost identical E_CT (and, thus, the same open-circuit voltage) the Stokes shift between EL and EQE spectra and, concomitantly, the width of the CT DOS varies by more than a factor of 2. We discuss this observation in terms of the donor-acceptor reorganization energy (λ) as well as an additional contribution of static disorder (σ). Remarkably, the more crystalline donor DIP shows a signifant contribution of the latter, while for the amorphous DBP this additional term is not required. This highlights the importance of film morphology in organic solar cells.
[1] K. Vandewal, K. Tvingstedt, A. Gadisa, O. Inganäs, and J. V. Manca, Physical Review B 81, 125204 (2010).
[2] T. M. Burke, S. Sweetnam, K. Vandewal, and M. D. McGehee, Advanced Energy Materials 5, 1500123 (2015).
[3] F. J. Kahle, A. Rudnick, H. Bässler and A. Köhler, Materials Horizons, 5(5), 837-848 (2018).

Organic Semiconductor (OS) is an emerging class of energy materials, for its several advantages over its inorganic counterpart, such as large area, flexibility, low cost, and most importantly their environment-friendly manufacturing process. Computational designing and study of new semiconducting organic molecules have come up as a great support in this regard. Push-pull type small organic molecule has recently gained a huge scientific research owing to their remarkable charge transfer properties, high non-linear optical response, reduced HOMO-LUMO gap and hence broad range of absorption spectrum, air stability etc. which collectively promotes this class of molecules as potential candidate for non-linear optical devices, OFETs and organic solar cells. In this effort, we therefore rationally designed a promising Donor(D)-π-Acceptor(A) (i.e, push-pull) type molecule trans-4-nitro-4'-dimethylamino-α-aminostilbene (NNDMNH₂), by 'backbone engineering' from a standard NLO (Non Linear Optical) dye. We predicted its crystal structure starting from the experimental crystal structure of another stilbene derivative and calculated the charge transport properties, electronic band structure, gas phase linear and non-linear optical properties. We also did the Hirshfeld surface analysis and plotted the molecular electrostatic potential to get insight into the structure-property correlation. We found that this new organic semiconductor owns a high charge carrier mobility of ~ 7.46 cm²/Vs for hole and 2.14 cm²/Vs for electron, together with desirable electronic and linear and non-linear optical properties revealing NNDMNH₂ as a potential candidate for the optoelectronic devices.

Until now, organic semiconductors have failed to achieve high performance in highly integrated, sub-100 nm transistors. Consequently, single-crystalline materials such as single-walled carbon nanotubes, MoS₂ or inorganic semiconductors are the materials of choice at the nanoscale. Here we show, using a vertical field-effect transistor design with a channel length of only 40 nm and a footprint of 2 × 80 × 80 nm², that high electrical performance with organic polymers can be realized when using electrolyte gating. Our organic transistors combine high on-state current densities of above 3 MA cm⁻², on/off current modulation ratios of up to 108 and large transconductances of up to 5,000 S m⁻¹ [1]. Given the high on-state currents at such large on/off ratios, our novel structures also show promise for use in artificial neural networks, where they could operate as memristive devices with sub-100 fJ energy usage.

[1] Lenz et al. Nat. Nanotechnol., 14, 579–585 (2019)

Some of the highest charge transporting conjugated polymers to date such as Indacenodithiophene-co-benzothiadiazole (IDT-BT) have shown weak to no crystallinity which is puzzling as it deviates from traditional evidence that the higher order of semi-crystalline polymers is needed for high charge mobility. Stiffer conjugated polymers can have liquid crystalline phases, and many of the most common high mobility semi-crystalline conjugated polymers, such as PBTTT, have liquid crystallinity. Using oscillatory shear rheology, X-ray scattering, DSC and polarized optical measurements we have investigated the morphology and phase behavior of IDT-BT as a function of temperature. We have identified the backbone and hexadecyl side chain glass transition temperatures (Tg’s), discovered two phase transitions that are likely disordered liquid crystalline transitions as well as evidence of biphasic behavior. We have also fabricated IDT-BT field effect transistors and annealed them at different temperatures in the disordered phases to track how mobility changes in regard to these phases. We hypothesize that because polymer chains are not restricted to a certain packing structure dictated by crystallization, chains in a liquid crystalline polymer are able to adopt conformations that can maximize interchain charge coupling.

The development of high-performing organic photodiodes (OPDs) is presently an intense field of research motivated by its potential applications in the field of imaging, health-care, or environmental monitoring. Yet only a limited number of publications report on fabrication routes that enable fully-printed device architectures facilitating the cost-efficient integration of OPDs in novel sensing applications. One main difficulty lies in the hindrance of device functionality through the successive deposition of the required multi-layer stack. The ink deposition can lead to disruption of film morphology, intermixing of materials and short circuits, reducing the figures of merit such as spectral responsivity, detectivity, or detection speed. To circumvent this problem an orthogonal solvent approach is commonly used. However, as the number of device layers increases, this approach exponentially restricts the palette of materials with suitable optoelectronic properties.
In this work, we present a route to overcome the need of orthogonal solvent systems utilizing Aerosol-jet printing for the fabrication of OPDs. By fine adjustment of the aerosol properties in terms of droplet temperature, solvent concentration and material flow rate, we gain control of the film drying dynamics without negatively affecting the underlying layers. Following this approach, we print a P3HT blocking layer on top of a P3HT:PC₆₀BM bulk-heterojunction from the same solvent system. The P3HT blocking layer spatially separates the lowest unoccupied molecular orbital (LUMO) of the acceptor from the electrode effectively reducing the flow of electrons from the contact. By following this approach, we demonstrate a reduction of the dark current from 176 mA/cm² in a device without the blocking layer down to 0.2 mA/cm² at 4 V reverse bias. This leads to a significant increase of the specific detectivity of the device up to 1.2E¹¹ Jones, ~3 orders of magnitude higher than that of the reference device.


Eckstein et al., “Fully Digitally Printed Image Sensor Based on Organic Photodiodes“. Advanced Optical Materials 6, Nr. 5 (2018): 1701108. https://doi.org/10.1002/adom.201701108.
Strobel et al., “Semiconductor:Insulator Blends for Speed Enhancement in Organic Photodiodes“. Advanced Electronic Materials 4, Nr. 10 (2018): 1700345. https://doi.org/10.1002/aelm.201700345.
Strobel et al., “Non-Fullerene-Based Printed Organic Photodiodes with High Responsivity and Megahertz Detection Speed“. ACS Applied Materials & Interfaces 10, Nr. 49 (12. Dezember 2018): 42733–39. https://doi.org/10.1021/acsami.8b16018.

Conjugated polymers (CP) have gained considerable attention due to their brilliant applications in electronic devices and optoelectronics. We designed and synthesized thiazolo[5,4-d]thiazole (TT) and benzo[1,2-b:4,5-b]dithiophene (BDT) planar fused rings containing donor-acceptor type conjugated polymers (viz., poly[thiazolo[5,4-d]thiazole thiophene-2-yl bis(dodecyloxy)benzo[1, 2-b:4,5-b]dithiophene] (PTDBT), poly[thiazolo[5,4-d]thiazole thiophene-2-yl 2-dodecyl-2H benzo[d][1,2,3]triazole (PTDBDT) and poly[thiazolo[5,4-d]thiazole thiophene-2-yl 9,9-dihexyl-9H-fluorene] (PTOF) with improved optoelectrical features. The molecular engineering of fused hetero-cyclic thiazolo[5,4-d]thiazole based π-conjugated polymers via introduction of substituted donor-acceptor units as backbones could enhanced the optoelectrical features. Conjugated polymers showed bright luminescent properties in yellow-red region of the electromagnetic spectrum having emission maxima in the range of 595-887 nm. PTDBT, PTDBDT and PTOF showed homogeneous morphologies with root-mean-square (RMS) roughness of ca. 0.255 nm, 0.464 nm and 0.303 nm, respectively. Band gaps obtained from cyclic voltammetric analysis are 2.27 eV, 2.04 eV and 2.4 eV, indicating the semiconducting nature of the synthesized systems. The impedance studies of these materials are done for a comprehensive understanding of their capacitive behavior in terms of the components of complex impedance. These all results indicate the application of synthesized systems as active layers in optoelectronics.

Conjugated polymers undergo structural changes upon electrochemically-driven ion insertion. The nature of these structural changes influences both the kinetics of doping and electronic mobility of the doped state, impacting the operation of many technologies that rely on the dual ionic/electronic conductivity of conjugated polymers, such as organic bioelectronics, neuromorphic computing, and supercapacitors. Typically, the polymer crystal lattice changes continuously to accommodate ions. We find that the lattice can also expand discontinuously, resulting in phase separation between ion-rich and ion-poor regions of the film. This phenomenon is routinely observed in inorganic materials, yet the flexible nature of conjugated polymers typically prevents phase separation from occurring. Using a combination of ex situ and in situ grazing incidence wide angle X-ray scattering (GIWAXS), as well as observations of ion distributions and migration with photoinduced force microscopy (PiFM), we characterize the phase-separated doped state of poly[2,5-bis(thiophenyl)-1,4-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)benzene] (PB2T-TEG), a conjugated polymer with glycolated side chains. We find that phase separation has important implications for polaron transport within conjugated polymers, as lattice expansion rather than polaron diffusion can restrict transport.

Adding n-dopants to the electron transport layer (ETL) of an organic light emitting diode (OLED) increases its conductivity and improves electron injection into the emissive layer of the device. However, n-doping becomes very challenging when dealing with large gap, low electron affinity (EA ≤ 3 eV) ETLs used in green or blue OLEDs. In this work, we address the problem of solution n-doping of such a material, i.e. the polymer F8BT [poly(9,9-dioctylfluorene-alt-benzothiadiazole)], via incorporation of the air-stable n-type dimeric dopant [RuCp*Mes]₂ [1]. Characterization of the undoped F8BT with ultra-violet and inverse photoelectron spectroscopy (UPS and IPES) places the highest occupied molecular orbital (HOMO) at 5.8 eV and the lowest unoccupied molecular orbital (LUMO) at ~2.8-3.0 eV below vacuum level. As the dopant is incorporated into the F8BT film, the energy levels shift by 0.6 eV, pushing the Fermi level of the material closer to the LUMO. n-Doping with [RuCp*Mes]₂ results from the dimer cleavage mechanism described in previous work, whereby an initial electron transfer to the host is immediately followed by cleavage and release of a stable 18-e^- monomer cation and a highly reducing 19-e^- monomer [1,2]. In the case of the low EA F8BT, as was shown previously for other low EA hosts [1], photoactivation (here with a 375 nm UV LED) is required to initiate the first electron transfer. The conductivity of the F8BT film is improved by four orders of magnitude. The doped F8BT film is incorporated as an electron transport layer in a TPBi:Ir(ppy)₃-based OLED and found to improve the luminance by over three orders of magnitude, bringing the external quantum efficiency (EQE) to about 14%. The utility of these powerful, air-stable n-dopants, previously demonstrated in the context of vacuum processing of low EA organic electronic materials [2,3], is therefore now also established for solution-based doping of an OLED ETL polymer with an EA as low as 2.8-3.0 eV.

[1] Guo, S. et al., n-Doping of Organic Electronic Materials Using Air-Stable Organometallics, Adv. Mat. 2012, 24, 699.
[2] Lin, X. et al., Beating the thermodynamic limit with photoactivation of n-doping in organic semiconductors. Nat. Mater. 2017, 16, 1209.
[3] Qi, Y. et al., Solution doping of organic semiconductors using air-stable n-dopants Appl. Phys. Lett. 2012, 100, 083305.

Semiconducting single-walled carbon nanotubes (s-SWCNTs) have been intensively studied as the components of various photovoltaic cells due to several advantages such as spectral tunability, giant aspect ratio, chemical robustness, hydrophobicity, and absence of charge-transfer (CT) states. In the previous study, it was reported that the heterojunctions between s-SWCNTs and perylene diimide (PDI)-based electron acceptors produce long-lived charge separated states whose lifetimes are more than 1.5 µs. Besides the potential of PDI-based electron acceptors to substitute fullerene-based electron acceptors, this study emphasized the significance of the molecular geometries of PDI-based electron acceptors which result in molecular aggregation and the associated charge delocalization in the acceptor phase. However, there is no clear understanding yet for why the charge carriers for these heterojunctions live so long. For instance, it was not determined how much of the charge carriers generated at these heterojunctions are free or trapped. Moreover, the nature of the charge recombination process from these heterojunctions was not yet revealed to be either monomolecular-like or bimolecular-like. In this study, we explored the nature of charge carriers for the heterojunctions between two singly chiral s-SWCNTs and two different PDI-based electron acceptors by combining two effective spectroscopic techniques: transient absorption (TA), probing charge carriers spectrally, and time-resolved microwave conductivity (TRMC), probing free charge carriers only.

Two PDI-based electron acceptors, hPDI2-pyr-hPDI2 and Trip-hPDI2, were synthesized and coated on (6,5) or (7,5) s-SWCNT films to form donor-acceptor heterojunctions. To compare the charge carrier characteristics from these heterojunctions to those from s-SWCNT/fullerene acceptor heterojunction, C₆₀ was also deposited on singly chiral s-SWCNT films. The charge recombination kinetics across the singly chiral s-SWCNT/PDI-based acceptor heterojunctions, deduced from TA and TRMC studies, are well-matched, indicating that most charge carriers from these heterojunctions are free, not trapped. These studies also presented that the charge carrier dynamics from s-SWCNT/PDI-based acceptor heterojunctions remain similar over four orders of magnitude in the absorbed photon fluences of PDI-based acceptors. However, TRMC studies of s-SWCNT/C₆₀ heterojunctions revealed that the fast decay dynamics at early delay time contributes more significantly at high C₆₀ absorbed photon fluences than at low fluences. It is attributed that the fast bimolecular charge recombination processes such as charge collision become more probable at high C₆₀ absorbed photon fluences. The substantially smaller molecular size of C₆₀ than those of PDI acceptors may induce the greater degrees of donor-acceptor intercalation, resulting in the lower probability of the charge carrier extraction out of the donor-acceptor interface at early stage. For each s-SWCNT heterojunction, the charge recombination decay time constants from TRMC studies are independent of absorbed photon fluences of electron acceptors, and this fluence independence indicates that most of free charge carriers from s-SWCNT heterojunctions recombine ‘pseudo’-monomolecularly. The unconventionally high free charge carrier generation and strong suppression of bimolecular charge recombination from these s-SWCNT heterojunctions may be attributed to the high carrier mobility and good charge delocalization in s-SWCNTs.

These photophysical studies provide the fundamental understandings of the charge generation process in s-SWCNT-based heterojunctions and how different electron acceptor materials can influence the nature of charge generation with respect to the heterojunction energetics and molecular orientations. The results can inform rational design strategies for s-SWCNT-based optoelectronic applications.

Silver is an important material for electrodes in high-performance organic optoelectronic devices due to its high reflectivity and low parasitic absorption loss at visible wavelengths. However, the electronic work function of Ag is not ideal for use as either the cathode or anode in many organic optoelectronic devices. Here, we investigate the formation of an ultrathin surface oxide layer on a Ag electrode and its impact on hole injection into an organic conjugated polymer semiconductor. The surface oxide is formed by exposing the Ag electrode to a low-power O₂/Ar plasma and it changes the electrical properties of the pure Ag electrode. We study the morphology and the chemical composition of the Ag electrode surfaces after different plasma treatment times through X-ray photoelectron spectroscopy, scanning electron microscopy and dark-field optical microscopy. After plasma exposure the surface oxide is composed of both AgO_x and Ag₂CO₃. As plasma exposure time increases from 1 s to 7 s, the fraction of AgO_x increases while Ag₂CO₃ decreases gradually. Both the turn-on voltages and barrier heights of poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) hole-only devices decrease with partial oxidation of the Ag electrode surfaces indicating that the work function of the Ag surface is increased by the surface oxide. F8BT hole-only devices with a thin MoO₃ layer and a thicker AgO_x layer on the electrode are found to be stable compared to thinner surface oxides and untreated Ag electrode surfaces [1].
[1] Zhongkai Cheng, Yan Wang, Deirdre M. O’Carroll. Influence of partially-oxidized silver back electrodes on the electrical properties and stability of organic semiconductor diodes. Organic Electronics 70, 179-185 (2019).

Abstract: We report novel solution-processable highly efficient hole injection materials, metal-doped phosphomolybdic acids (M-PMAs). PMA includes MoO₃ units and works as a hole injection material as reported in the previous papers [1, 2]: however, thermal reduction of PMA at high temperature is indispensable for realizing high hole injection capability, and the thermal treatment is not desired for cost-effective manufacturing. In this work, we reduced PMA chemically by using reactions with various kinds of metals including Mg, Al, Ti, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ag, Sn, Sb, and W metals. Powders of the metals were added to yellow-colored PMA solutions, and the reaction between PMA and the metals proceeded at room temperature to give the chemically reduced blue-colored M-PMA solutions. The M-PMAs were applied to hole injection layers in organic light-emitting devices with a structure of [ITO/M-PMA or thermally reduced PMA (rPMA) (10 nm) / N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (30 nm) / 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl: 8 wt% tris(2-phenylpyridinato)iridium(III) / bis(8-hydroxy-2-methylquinoline)-(4-phenylphenoxy)aluminum (10 nm) / tris(8-hydroxyquinolinato)aluminium (40 nm) / 8-quinolinolato lithium (1 nm)/Al (100 nm)]. Here, the rPMA film was formed by baking at 200°C of a PMA film, and has been already known to show high hole injection capability similar to well-known poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) [1]. On the other hand, the M-PMA films developed in this work were formed without any post-baking process. Among the various kinds of metals, the Mn, Co, Cu, Zn, Mo, and Sb-PMA-based devices showed similarly low driving voltages to the rPMA-based device. Moreover, the M-PMAs-based devices showed much longer operation lifetime than the rPMA-based device. Thus, we succeeded in the development of post-treatment free, highly efficient, highly stable hole injection materials.

References: [1] S. Ohisa, S, Kagami, Y.-J. Pu, T. Chiba, J. Kido, ACS Appl. Mater. Interfaces, 2016, 8, 20946. [2] S. Ohisa, K. Endo, K. Kasuga, M. Suzuki, T. Chiba, Y.-J. Pu, J. Kido, 2018, 57, 1950.

It is reported the synthesis of two bis-benzodithiophene (BDT₂)-based small molecules, SM1 and SM2, and the effects of solvent vapor annealing (SVA) on their molecular ordering, optical, and photovoltaic properties. Although SM1 and SM2 have similar backbones, they exhibit different photophysical and electrochemical properties, charge carrier mobilities, and morphologies in blend films. SVA profoundly affected the molecular structure and photovoltaic properties. Solvent vapor annealing remarkably enhanced the photovoltaic properties of SM2, increasing the power conversion efficiency (PCE) from 1.47% to 8.66% by inducing a favorable molecular orientation, a miscible morphology, and enhanced mobility. The relationship between the morphologies of the active layers comprising SM1 or SM2 with PC₇₁BM submitted to SVA and the device performances were elucidated using 3D transmission electron microscope tomography.

Organic inorganic hybrid perovskites (OIHP) is the most promising material to achieved high power conversion efficiency (PCE) at low cost. The high-quality optoelectronic properties in combination with solution-based preparation methods are responsible for the currently certified PCE record of 24.2%, which is close to the PCE of single crystal silicon solar cells (26.1%). The properties of the perovskite film are direct related to film morphology, composition and crystalline structure, thus a clear understanding of how and when the intermediate and the perovskite phases are forming, as well the distribution of these multiple phases in the bulk and grains boundaries are important questions to be addressed in order to improve perovskite film properties and consequently the PCE of the devices. In this presentation, we will summarize our most recent results using in situtime-resolved grazing incidence wide angle x-ray scattering (GIWAXS) and synchrotron infrared nanospectroscopy (nano-FTIR). GIWAXS experiments allowed us to understand the influence of the relative humidity, type of solvent and time to drop the antisolvent during the preparation of mixed cation perovskite films. We also identified intermediates formed before and during the spin coating process of mixed cations precursor solutions. Nano-FTIR technique was applied for the first time on OIHP. Our results revealed a spatial heterogeneity of the vibrational signal, which are associated to different chemical composition.
Colloidal perovskite nanocrystals (PQD) and nanoplates are very interesting for LED, however, the most used synthetic method to prepare them relies on a mixture of oleylamine (OLA) and oleic acid (OA) as surfactants. We will discuss an amine-free synthesis that utilizes tetraoctylammonium halides (TOAX) for preparation of OA capped CsPbX₃PQDs without the need of post-anion exchange methods. As an alternative to cubic nanocrystals, we will present our recent results on the formation of perovskite nanoplates (PNP) using SnX₄ (X = Cl, Br, and I) salts as the halide source. The dynamic equilibrium between OLA, OA and the formation of a complex with Sn(IV) dictate the morphology and size, allowing us to prepare a range of efficient 2D quantum well materials.

Highly efficient halide perovskite solar cells (PSCs) can only be cost-competitive if their operational stability is ascertained. Defect control and passivation in the halide perovskite absorber and at the interface with the transport layers, is crucial for stability improvement. Defects act as recombination centres and, under continuous light irradiation can reduce the stability of the solar cell. In this presentation we report our latest results on the engineering of defects at the oxide/perovskite interface and within the halide perovskite structure. We will show the application of binary and complex oxides as ETL. We will give special attention to the application of ferroelectric oxides, such as PZT, where O_vac are employed for the creation of defect-dipoles responsible for photo-carrier separation and current transport, evading device degradation. We will also show a new method to passivate defects in the halide perovskite thin film, as a result we are able to fabricate PSC with > 21 % efficiency and 1000 h stability under 1 sun continuous light irradiation. Understanding the mechanism of defect passivation in PSCs materials and interfaces can facilitate the development of highly stable PSCs.

For last decades, organic electronic materials have been widely used for realizing large-area flexible device applications, such as organic field-effect transistors (OFETs), organic light-emitting diodes, and optoelectronic sensors fabricated by low-cost and non-vacuum solution processing. Because of their inherent weakness of environmental instability, many researches have been conducted to improve the reliability of organic devices for further practical applications. Specifically, water-based threats could significantly degrade the electrical performances of organic materials, therefore, one of the key challenges is to integrate effective protection layers against external threats in daily conditions. In this context, introducing superhydrophobic layers has been regarded as an attractive solution because their excellent water-repellency can be helpful to protect organic devices from harmful dusts or water-based threats efficiently. In our previous study, we reported that organo-compatible superhydrophobic protection layers could improve the environmental stability of OFETs [1]. However, the employed TiO₂ nanoparticle-based superhydrophobic layers having a thickness over 10 μm can be a critical hurdle to be utilized in optoelectronic applications due to their opaqueness [1]. For providing transparency to the nanoparticle-based protection layers, the control of their thickness and surface roughness should allow the enhanced transmittance via reduced light scattering. Specifically, because the high surface roughness is a key parameter for maintaining the superhydrophobicity, the investigation on the optimized conditions for achieving transparent superhydrophobic surfaces is highly desirable as well as organo-compatible low-temperature processing.
In this presentation, we will report a facile method to realize a transparent superhydrophobic layer onto organic phototransistors through an organo-compatible solution process. TiO₂ nanoparticles coated with fluorinated silane molecules are dispersed in a highly fluorinated solvent, so we can introduce a superhydrophobic surface onto organic devices directly. The optimized transmittance over 90 % in the visible wavelength region is attributed to the controlled roughness parameters of our superhydrophobic layer. The transparent superhydrophobic layers exhibit good water repellencies without critical delamination issues even after or during bending and stretching tests. Moreover, flexible organic phototransistors with the transparent superhydrophobic layers show self-cleaning abilities from excellent water repellency, therefore harmful contaminants on the surface can be eliminated by dropping water droplets while preserving their optoelectronic characteristics. This work can provide a key pathway to implement a transparent self-cleaning layer for more reliable organic optoelectronic devices.

[1] D. Yoo, Y. Kim, M. Min, G. H. Ahn, D. H. Lien, J. Jang, H. Jeong, Y. Song, S. Chung, A. Javey, T. Lee, ACS Nano, 12, 11062 (2018).

Interfacial engineering has been identified as a fundamental strategy for maximizing the performances and stability of organic solar cells. To this aim, conjugated polyelectrolytes (CPEs) received increasing attention thanks to their ability of improving the performances of organic solar cells through processing from orthogonal solvents. Most of the CPEs have been utilized as cathode modifiers while their use as anode interfacial layer (AIL) materials was less frequently reported. Recently it was demonstrated that self-doped anionic CPEs are a promising PH-neutral alternative to PEDOT:PSS AIL for organic solar cells¹. Actually the library of efficient CPEs anode modifiers has enlarged,² helping to identify the key CPEs characteristics for effective anode interfacial engineering ³<!--![endif]---->. In spite of these advancements, the use of CPEs AILs in organic photovoltaics has been limited to devices prepared with conventional direct architecture, while their application in devices with inverted geometry hasn’t yet been addressed.
In this view we have prepared three conjugated copolymers modifying by chemical design conjugated backbone or alkyl-sulfonate side chain architecture. Their functional behavior as anode modifiers in inverted P3HT:PCBM₆₁ solar cells is herein investigated.
The results indicate that anionic CPEs can be effectively applied for anode interfacial engineering in inverted organic solar cells. Comparable photovoltaic performances to a classical thermally evaporated inorganic MoOx AIL have been achieved. Some insights and guidelines for future development of efficient CEPs anode modifiers for inverted devices are discussed.

1. Zhou, H.et als, Advanced Materials 2014, 26, 780
2. Xu, B. et als., Advanced Energy Materials 2018, 8, 1800022
3. (a) Mai, C.-K. et als, Angewandte Chemie International Edition 2013, 52, 12874; (b) Cui, Y. et als, Chemistry of Materials 2018, 30, 1078

Organic semiconductors with conjugated electron systems are currently intensively investigated for many novel electronic and optoelectronic applications. Their key advantages are flexibility, low cost, and low resource usage since the mostly carbon-based materials are fabricated in nano-meter scale thin film devices. Recently, we have introduced the novel principle of band structure engineering into organic semiconductors [1]: Long range Coulomb interactions allow to continuously tune the energy levels, offering to finely adjust energy levels without synthesis of new molecules. We have now directly proven the electrostatic nature of these phenomena by experiments which show the strong influence of these quadrupole effects at interfaces [2]. This also dramatically changes the interface energetics of organic solar cells, depending on quadrupole moments and molecular orientation. Recently, we have furthermore shown in a detailed study of the conductivity of doped organic semiconductors that the quadrupole interacts play an important role in the activation energy of ground state charge transfer pairs [3]. This opens a new path for the optimum design of host-dopant interactions to achieve the highest possible conductivities.


Band Structure Engineering in Organic Semiconductors, M. Schwarze et al., Science 352, 1446 (2016)
Impact of molecular quadrupole moments on the energy levels at organic heterojunctions, M. Schwarze et al, Nature Communications 10, 2466 (2019)
Molecular parameters responsible for thermally activated transport in doped organic semiconductors, M. Schwarze et al., Nature Mater. 18, 242 (2019)

In order to make rational improvements of the charge transport properties of organic semiconductors, a thorough understanding of this rather complex process is needed. Charge carrier transport in organic semiconductors is usually described as a series of events where a charge carrier hops from one localised state to the next. In this type of description, the disordered nature of such systems leads to a broad distribution of site energies. As a result, the mobility of charge carriers increases as more charge carriers are introduced unless the number of charges is relatively low [1].

While it has been recognised that Coulomb interactions between dopants and charge carriers are important, carrier-carrier interactions in doped organics have received less attention. In a doped organic semiconductor, however, the number of (free and bound) charge carriers equals the number of reacted dopants. As a result, both types of interactions are of importance for a proper description of the transport properties of doped organic semiconductors. However, it has been proposed that carrier-carrier interactions is significantly weaker than the effect of Coulomb interactions between dopants and charge carriers [2].

In this contribution, we use a kinetic Monte-Carlo model to study the transport properties in the presence of dopants and including both carrier-carrier and carrier-dopant interactions. In particular, we can simulate up to the relatively high doping densities that are experimentally relevant even though numerically challenging. We find that the density of states (DOS) of such systems shows a pseudo-gap at the Fermi level due to Coulomb interactions between the charge carriers. Such Coulomb gaps are a manifestation of the carrier-carrier interactions and have been predicted to exist in hopping systems [3], but are usually washed out at anything other than very low temperatures. In contrast, we observe these pseudo-gaps at room temperature, which is a consequence of the low dielectric constant and highly localised charges that are typical of organic semiconductors. We wish to point out that the pseudo-gap is a sole consequence of carrier-carrier Coulomb interactions and is not a result of the dopants. This has profound implications for electrical conductivity as the pseudo-gap limits charge carrier transport at high doping densities. The corresponding (absolute) Seebeck coefficient remains approximately constant in this regime.

Experimentally, we observe that the electrical conductivity of a large number of organic semiconductors shows a maximum: Upon increased doping levels the conductivity decreases and the (absolute) Seebeck coefficient remains approximately constant. This type of behaviour is commonly attributed to changes in the microstructure as a consequence of doping. However, we find that even when using vapour doping, where there are no observable changes in the microstructure, this behaviour persists. The experimental findings match the Monte-Carlo data (drop in conductivity and saturation of the Seebeck coefficient at high doping) very closely, which implies that the Coulomb pseudo-gap is at the root of the limited conductivity at high doping densities.

As a major consequence of our findings, we stress that both dopant-charge carrier as well as carrier-carrier interactions need to be considered when desiging new materials for efficient and highly conductive doped organic semiconductors and thermoelectrics.

[1] W. F. Pasveer, J. Cottaar, C. Tanase, R. Coehoorn, P. A. Bobbert, P. W. M. Blom, D. M. de Leeuw, and M. A. J. Michels, Phys. Rev. Lett. 94, 206601 (2005).
[2] V. I. Arkhipov, P. Heremans, E. V. Emelianova, H. Bässler, Phys. Rev. B. 71, 045214 (2005).
[3] A. L. Efros and B. I. Shklovskii, J. Phys. C: Solid State Phys. 8, L49 (1975).

Doping of organic conjugated materials has been proven to be crucial to achieve high conductivities and thus the exploration of their fundamental properties is essential to maximize their potentials for applications in electronic devices. This has motivated the intensive study of the long-range transport properties of doped organic materials that are governed by weak intermolecular interactions, however their intrachain motion has been much less explored. Thus, we have used THz spectroscopy to gain information about the local transport properties of doped organic conjugated polymers in the non-excited state and access the carriers nanoscale mobility. We show that these measurements can serve as a tool to determine the doping efficiency of doped organic materials. Due to the short THz pulse duration, we selectively probe the intrachain motion of the mobile charge carriers in doped organic materials. Moreover, we aim to give an insight on how these properties are affected by the strong interactions between the charge carriers formed on the conjugated backbone and the charged dopants necessary to counterbalance the organic material and the impact of the formation of partial charge transfer complexes or structural disruption by external dopant molecules on their local transport properties.

Doping is a key technology in semiconductor physics to tune bulk and interfacial electronic properties. In organic semiconductors, efficient doping may be dominated by different effects, which are presently not well understood and cannot be explained by standard semiconductor models. An improved microscopic picture would therefore be highly desirable.
In our contribution, we will discuss and compare some examples of our current research comprising n-doped bulk systems, organic-organic interfaces and metal-organic ones where we have analyzed the effect of doping in microscopic detail from density functional theory to the statistical description of the resulting Fermi level position. A consistent description is emerging from these different systems. Our discussion finally includes a comparison to various experimental results.

Molecular p- and n-doping of organic semiconductors has been key for the successful commercialisation of OLEDs and promises to significantly advance the field of organic solar cells and transistors. As recent reports have shown, in-depth comprehension of the doping process is required to guide the design of improved semiconductor materials and molecular dopants [1,2]. In this context, electron paramagnetic resonance (EPR) spectroscopy is a technique ideally suited to study molecular doping as it is sensitive to the nature and dynamics of the paramagnetic species generated upon introduction of dopant molecules into films of organic semiconductors [3].

In this contribution, EPR spectroscopy and electrical measurements are carried out to investigate p-type doping of a layer of ZnPc (host) with the acceptor F6-TCNNQ (dopant) at different dopant concentrations (0 - 5 wt.%) and temperatures (80K – 280K). The samples are fabricated through thermal evaporation under vacuum to achieve a high control over the composition, the thickness and the morphology of the thin films.
The EPR analysis performed at different dopant concentrations and at room temperature reveals the presence of two different paramagnetic species distinguished by two different g-tensors. We attribute these two species to the ZnPc positive polaron and the F6-TCNNQ anion. The quantification of the number of paramagnetic species in the film allows us to provide an estimate of the doping efficiency and to clarify the ground-state charge transfer mechanism underlying charge generation in doped films. Furthermore, from the EPR lineshape analysis, we delve into the microscopic transport dynamics of the polarons. The results are in line with conductivity measurements.
The analysis performed at different temperatures discloses a further wealth of information on the temperature dependence of the charge transfer process and the thermally-activated transport properties of polarons and elucidates the role of trap states in the framework of the multiple trap-and-release (MTR) model [4]. The results are supported by electrical conductivity measurements performed at different temperatures from 100 to 280K.

With the aim to further validate our investigation, we are currently expanding our work to different energy level offsets between the host and the dopant. Taking advantage of the band structure engineering reported by Schwartze et al. [5], we are using different blended host materials, ranging from ZnPc to fluorinated derivatives FxZnPc (x= 4, 8, 16), co-evaporated with F6-TCNNQ (dopant). The investigation will unravel how the doping process depends on energetic offset and will help us to define novel design rules to control molecular doping.

[1] I. Salzmann et al. Acc. Chem. Res. 2016, 49, 370-378.
[2] M. L. Tietze, et al. Adv. Funct. Mater. 2015, 25, 2701-2707.
[3] Aránzazu Aguirre, et al. Phys. Chem. Chem. Phys. 2008,10, 7129-7138.
[4] K. Marumoto, et al. Phys. Rev. B, 2011, 83, 075302.
[5] M. Schwarze, et al. Science 2016, 352, 1446-1449.

Organic semiconductors typically exhibit a rich variety of ordered phases. Orientation occurs during processing, often by passing through lyotropic liquid crystalline phases, and dried films exhibit orientation at many length scales. Liquid crystallinity appears almost ubiquitous in organic semiconductors we have studied, including behaviors similar to nematics, smectics, or sanidics. Uniaxial orientation where the conjugated plane exhibits a preferential “face on” or “edge on” orientation is the most commonly characterized aspect of orientational order, and mature techniques such as near edge X-ray absorption fine structure (NEXAFS) spectroscopy and grazing incidence X-ray diffraction (GIXD) are frequently employed to measure it. Significant challenges remain, even for this seemingly simple aspect of orientational order. For example, these approaches are often found to be insufficient when films exhibit a complex depth profile of orientation, composition, or both.

As the materials diversity of organic semiconductors increases thanks to achievements in synthetic chemistry, and as the complexity of formulation, blending, and coating increases as process design knowledge matures, there is a commensurate need for more advanced approaches to characterizing orientational order. Biaxial orientation is increasingly observed, where “face on” or “edge on” preferences are combined with an in-plane preference imparted by a substrate or processing vector. Domain-relative orientations are also been observed, with correlations in domain-to-domain orientation thought to influence charge transport.

I will describe some emerging approaches to measure orientational order in organic semiconductors. Resonant soft X-ray reflectivity (RSoXR), a method capable of measuring thin film depth profiles using the spectroscopic principles of NEXAFS, will be discussed. I will also summarize progress on in-situ spectroscopic and scattering-based structure measurement techniques that are combined with ex-situ probes including imaging and image analysis to determine how orientation originates during the processing of organic semiconductors. These new measurements will support science-based approaches for orientation control and optimization, helping to maintain the rapid pace of organic semiconductor technology development in an era where increasing complexity renders matrix-style empirical approaches too inefficient to contemplate.

This presentation focuses on recent advances in growth control and oriented crystallization of (semi)-conducting polymers for organic electronic applications. The performance and lifetime of organic electronic devices are critically dependent on the morphology of the active layers and structural order of the materials. For instance both molecular and crystalline orientations of the polymers determine optical and charge transport properties in thin films since these properties are by essence highly anisotropic.

Particular emphasis will be given to the progress made in high-temperature rubbing of conjugated polymers films. This effective large scale alignment method can orient a large palette of semiconductors with n- or p-type character without the use of alignment substrate.¹ High degrees of crystallinity and in-plane alignment can be obtained which provide well-defined electron diffraction patterns essential for structure refinement. The concurrent roles of polymer molecular weight distribution and rubbing temperature (T_R) on the in-plane orientation are rationalized for P3HT and PBTTT.^2a Correlations are drawn between nanomorphology/crystallinity on one side and charge transport and optical properties on the other side. It is shown that the exciton bandwidth in P3HT crystals is determined by the length of the average planarized chain segments in the crystals. The high alignment and crystallinity observed for T_R > 200 °C cannot translate to high hole mobilities parallel to the rubbing because of the adverse effect of amorphous interlamellar zones interrupting charge transport between crystalline lamellae of semi-crystalline P3HT.^2b As opposite, hole mobilities along the polymer chains of rubbed PTB7 films are observed to be 6 times higher than the non-rubbed films.³ This is due to the smectic- like character of this alternated donor-acceptor copolymer. Interestingly high-Tr rubbing and post annealing process of PTB7 provide well-defined electron diffraction pattern. Combined with DFT calculation, this helps refining the structure and the chain conformation of this benchmark electron donor polymer for OPV application.
In a second part of this presentation, we show that soft doping of aligned P3HT yields highly oriented conducting polymer films with anisotropic charges and thermal conductivities. The thermoelectric properties are enhanced along the rubbing direction. The unique in-plane orientation in such conducting polymer films helps rationalizing the mechanism of redox doping.⁴



References.
1. M. Brinkmann et al. Macromolecular Rapid. Comm. 2014, 35, 9.
2. a) L. Biniek et al. Macromolecules 2014, 47, 3871. b) A. Hamidi-Sakr et al. Adv. Funct. Mater. 2016, 26, 408.
3. L. Biniek et al. Adv. Electron. Mater. 2018, 1700480
4. A. Hamidi-Sakr et al. Adv. Funct. Mater. 2017, 1700173.

The electrical properties of organic thin film transistors (OTFTs) are strongly influenced by the structural characteristics and thin film morphology. Understanding the growth dynamics that lead to preferential molecular orientation and changes in crystal structure remain an important factor in the design of new high-performance OTFTs as well as in their fundamental studies.
In this contribution, we have investigated the thin film growth behavior of 5,5′-Bis(naphth-2-yl)-2,2′-bithiophene (NaT2) and 5,5′′-bis(naphth-2-yl)-2,2′:5′,2′′-terthiophene (NaT3) during the deposition process using real-time grazing incidence X-ray diffraction (GIXRD) in situ. The thin films were prepared by vacuum sublimation atop various substrates, including monolayer graphene on 90 nm SiO₂, in a custom-built ultra-high vacuum (UHV) chamber with a 360° cylindrical beryllium window that allows for in situ X-ray measurements within realistic deposition time scales. The crystal structure analysis revealed that the preferred orientation of the molecules were dictated by the substrate and that the unit cell underwent significant changes when transitioning from monolayer to multilayer structure. The changes of the unit cell were not readily observed when measuring thin films of corresponding thicknesses ex situ, suggesting that the molecules undergo further re-organization/relaxation upon terminating the deposition. From the evolution of the crystal structure, a connection to the film growth mode and kinetics is made. These findings are rationalized based on the surface energies of the studied substrates and supplemented with AFM and helium ion microscopy. Finally, the crystal structure and morphology are correlated with charge transport properties of the final thin films.

The evolution of organic electronics has now reached the commercial phase, with the recent market introduction of the first prototypes based on organic transistors and organic solar cell modules fabricated from solution. Understanding the impact of both the organic semiconductor design and processing conditions, on both molecular conformation and thin film microstructure has been demonstrated to be essential in achieving the required optical and electrical properties to enable these devices. Polymeric semiconductors offer an attractive combination in terms of appropriate solution rheology for printing processes, mechanical flexibility for rollable processing and applications, but their optical and electrical performance requires further improvement in order to fulfil their potential. Synthesis of conjugated aromatic polymers typically involves carbon coupling polymerisations utilising transition metal catalysts and metal containing monomers. This polymerisation chemistry creates polymers where the aromatic repeat units are linked by single carbon-carbon bonds along the backbone. In order to reduce potential conformational, and subsequently energetic, disorder due to rotation around these single bonds, 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. 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. We present an electrical, optical and morphology characterisation of polymer thin films, illustrating structure-property relationships for this new class of polymers.

Carrier mobility in conjugated polymers continues to increase with recent reports of field-effect mobilities exceeding 10 cm²/V.s. Charge transport is intrinsically dependent on processes occurring across multiple lengthscales. In order to access order parameters at the molecular scale we use charge modulation spectroscopy combined with theory. Furthermore, we study the mesoscale organization of polymers using a new technique in the transmission electron microscope. These techniques are used on homopolymers and donor-acceptor copolymers. Such multiscale studies of microstructure are instrumental in guiding our understanding of charge transport in conjugated polymers.

Doping is a fundamental strategy to increase the carrier density and conductivity of organic electronic layers for opto-electronic devices. The model system for p-doping is poly(3-hexyl)thiophene (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ), whereby increases in conductivity occur through integer charge transfer (ICT) to form mobile carriers in the P3HT. Extensive work in the organic electronics field has revealed the interplay between film microstructure, the opto-electronic properties, and conductivity.

This talk asks the simple question: How stable are ICT states in doped films? An alternate undesired reaction pathway is formation of a partial charge transfer complex (CPX), which results in a localized, trap-like state for the hole on P3HT. Starting first with our recent observation that both ICT and CPX states exist simultaneously in a single film, this talk will focus on thermodynamic and kinetic pathways that lead to conductivity loss in F₄TCNQ-doped P3HT films. Both reaction chemistries and microstructural effects will be discussed.

The advent of solution-processed organic semiconductors makes it possible for the realization of heterojunction phototransistor architectures with the feasibility to break through the photodetection limitations in conventional organic photodetectors such as the trade-off in balancing the separation, transport, and recombination of photogenerated charges. The appealing progress was reported in recently with the well-tailored device structure in which the light absorption and carrier transport take place in separately optimized bulk heterojunction (BHJ) photoactive layer and conduction channel, respectively. However, a critical issue hindering the further improvement of this type of device is that the solution-processed upper photoactive layer would degrade the underneath channel layer or induce high-density defect states at their interface, which would significantly lower the carrier mobility and affect the charge transfer process. In this presentation, we will present an ultrafast and scalable approach to fabricate large-area and high-performance organic phototransistors in ambient condition by using solution-floating method (SFM) to prepare both the high-quality channel layer and the BHJ photoactive layer. Based on this approach, the highly smooth and uniform conjugated polymer films with precise control of film thickness at a monolayer level can be prepared, offering the potential to realize low-cost, large-area and mass-manufacturable optoelectronic devices. In addition, the solution-floated organic films can theoretically be transferred to any kind of substrate. With this SFM process, we fabricated the hybrid-layered phototransistors by sequential transferring of channel layer and BHJ photoactive layer on rigid or flexible substrates. And we further demonstrate that carrier mobility was significantly boosted as compared to the spin-coated counterpart, and the high mobility can be well maintained after the covering of the BHJ photoactive layer. Resultantly, high photodetection performance can be expected for the phototransistors due to its excellent light absorption efficiency, highly-efficient charge transfer and high conductivity enabled by the high-quality organic films and interfaces. More importantly, employing SFM to prepare high-quality and well-controlled organic semiconductor film is not only technologically promising for high-performance organic phototransistors, but also opens up new opportunities control charge transfer and electronic transport properties for more efficient optoelectronic devices in general.

The study of semiconducting polymer deformation mechanics is a challenging area that needs to be thoroughly understood for flexible and stretchable electronics to become a reality. The main challenge lies in handling and measuring these materials, which at the device scale are a mere 100 nm in thickness or less. The Gu research group has specialized in the pseudo free standing tensile test to characterize the thin film mechanical property while supported in a water bath, however there is a question as to what effect, water has on these films. Here, we developed a new free-standing (in air) tensile platform to not only confirm the effect of water on sub-100nm films, but also establish a route for in-situ tensile characterization to probe the morphological change upon deformation. This is accomplished through what our group has termed Supported Shear Release, a gentle transfer method capable of obtaining macroscopic sub-100 nm thin films, which has enabled the tensile characterization of polystyrene and poly(3-hexylthiophene-2,5-diyl) films down to thicknesses of 38 and 80 nm respectively. We have registered negligible differences between the modulus obtained from on water and in air measurements, leading us to conclude that water plays an insignificant role in these hydrophobic films despite other factors such as confinement, which remain evident in the polystyrene system. Our current work is to utilize this newly developed free-standing tensile platform for in-situ UV-vis characterization of mechanochemical polymers. Upon tensile strain, these polymers exhibit a transition marking the appearance of conjugation which is detectable with UV-Vis spectroscopy. Thus, enabling a thorough understanding of the structure property relationships for these systems as well as establishing this novel tensile platform as a multimodal characterization tool for the in-depth understanding of thin film deformation mechanics across multiple length scales.

Small-molecule organic semiconductors are well studied for applications from cell phones to solar cells due to their electronic properties, flexibility and lower manufacturing cost. Charge transport in these organic materials is normally localized among a π-conjugated core of carbon atoms, making charge transport highly anisotropic, with higher electron transport in the plane of the π-π bond. Thus, slight differences in π-π stacking can significantly change this transport. As a result, researchers are interested in understanding how to manipulate the processing of organic semiconductor crystals and how to control the formation of specific polymorphs with desirable charge transport properties.

One molecule receiving such attention is the class of core-substituted naphthalene diimide molecules (c-NDI). In this presentation, we studied core-chlorinated naphthalene tetracarboxylic diimide (NTCDI) where the chlorinated core has as an electron-withdrawing effect to lower the LUMO energy level and improve chemical stability by discouraging interactions with oxidants.¹ The Loo group found that using different solvents during the solvent-vapor annealing of NTCDI thin-film transistor resulted in thermodynamically reversible α and β-phases.² Here, we have used ab initio density functional theory and molecular dynamics calculations to study how solvent choice (acetone, chloroform, dichloromethane, toluene) affects NTCDI processing. Some force field parameterization was necessary for this study, largely missing dihedrals. For the DFT, we used HF-3c and a continuum solvation model based on charge density.³

We found “boat-” and “chair-” like conformations of NTCDI molecules in solvents that cannot be observed experimentally and showed that it has a high-energy barrier that prevents inter-conformer conversion. We calculated the binding energy between two NTCDI molecules immersed in solvent and determined that the tendency of NTCDI aggregation was low in chloroform/dichloromethane, intermediate in acetone and high in toluene. To ensure that entropic as well as enthalpic considerations were made we calculated the free energy of the affinity between two NTCDI molecules in solvent to better understand solvent-NTCDI phenomena. NTCDI molecules preferred to be solvated in chloroform as the free energy decreased when the distance between NTCDI increased. In contrast, in acetone, NTCDI monomers as well as aggregated complexes co-existed, evidenced by two local free energy minima at both short and long distances. Importantly, although our binding energy calculation indicated that NTCDI shows the strongest binding in toluene, it does not necessarily aggregate in toluene. This is because the toluene-NTCDI molecules tend to form a sandwich-like complex in solution and π-π interactions between NTCDI and toluene aromatic carbons played an key role here. We also observed strong differences among the radial distribution function of solvent-NTCDI interactions as a result of solvent choice; e.g., the first maximum in g(r), around 4 Å, for toluene-NTCDI is significantly higher than in other solvents. All these findings validate experimental results obtained by our collaborators, Dr. Geoffrey Purdum, Dr. Yuen-Lin Loo, and others. But they also offer exquisitely detailed atomic-scale insights, which cannot be observed experimentally and which will help the community understand the small-molecule organic semiconductor processing of thin-film transistor and help researchers achieve more control over preferable crystal structures.

Conjugated polymers possess a wide range of desirable properties including accessible band gaps, plasticity, tunability, mechanical flexibility and synthetic versatility making them attractive for use as active materials in organic photovoltaics. In particular, push-pull copolymers, which are made of alternating electron-rich and electron-deficient moieties, are increasingly used due to their broad optical absorption, ease of tunability of relevant orbital energy levels (HOMO/LUMO and band gap) and increased charge transfer between the monomer units. However, the existence of a large number of possible copolymers makes the screening for ideal copolymers for OPVs through first-principles strategies computationally intensive. Connecting small molecule (monomer,dimer) or homopolymer properties to copolymer properties helps avoid expensive first-principles calculations on the copolymer itself to determine its electronic properties. Several studies utilizing high throughput computational screening for novel OPV materials have focused on calculating small molecule properties. Revisiting these calculations with a reliable model to predict copolymer band structures eliminates the need for a first-principle calculation. This is useful for day to day predictions and aids the screening for novel materials. Therefore, this work aims to build a recipe to construct copolymer band structures from the monomers involved while avoiding expensive first-principle calculation on the copolymer itself. For this, we use the Tight Binding (TB) approximation to build models, with parameters determined using Density Functional Theory (DFT) calculations on monomers/homopolymers. These models are then used to construct copolymer valence and conduction band structures and validated by comparing them to the DFT calculated electronic structures of the copolymers. In addition, we discuss how such band structures can be qualitatively constructed from monomer HOMOs and LUMOs; available in materials databases, and a simple estimate of the electronic coupling between monomers of the same kind.

The conducting polymer poly(3,4-ethylenedioxythiophne) (PEDOT) is widely used as channel material for organic electrochemical transistors (OECT), due to its mixed ionic-electronic conductivity, chemical stability and biocompatibility. The study presented here aims to explore and optimize the fabrication of OECTs by electropolymerization of the monomer, EDOT, bridging the gap between microfabricated source and drain electrodes. Various types of dopants, namely poly(styrenesulfonate), perchlorate and tetrafluoroborate were used. The channel length and the deposition time were also varied to control the channel dimensions. These parameters were chosen to study their impact on overall film morphology and uniformity, which are known to strongly affect electrical properties, and hence, transistor behaviour. The electropolymerization, which is a relatively simple technique, could therefore be used to fabricate OECTs in bioelectronics.

There is an increasing demand for high-power electronic components and optoelectronic devices with low losses and high efficiency. The III-nitride semiconductor materials have demonstrated great potential for high-power, high-frequency, and high-temperature applications because of their remarkable and wide-ranging electronic and physical properties. For example, In_xGa_1-xN alloys have optical bandgaps ranging from infrared to ultraviolet, between 0.7 eV and 3.4 eV. A recently explored area of InGaN research is in grading the composition of the alloy to achieve such novel properties as polarization induced p-type doping. However, these graded composition alloys do not yet have well-characterized metal contact materials. Thus, we will present a study of low resistance ohmic contacts characterized by using the transmission line method, on graded layers of InGaN grown on semi-insulating GaN. Carrier concentration and mobility are subsequently characterized in the graded films by temperature-dependent Hall-effect measurements.

Polymeric conjugated materials are very promising for developing future soft material-based semiconductors, conductors, electronic and optoelectronic devices due to their inherent advantages such as lightweight, flexible shape, low-cost, ease of processability, ease of scalability, etc. There are numerous ways to tune material properties via post-processing treatments such as annealing, solvent additives, or doping. One challenge of solution processed organic thin film electronics is the repeatable and well-defined morphological control of the resulting thin films. In this study, we observed that the addition of para-substituted halogenated benzenes to poly-3-hexy-thiophene (P3HT) solutions can affect the morphological properties of P3HT thin films. These additives have been shown in literature to exhibit halogen bonding interactions with other small molecules. Here we seek to use this interaction to see if this interaction can be used to promote polymer self-assembly on the nanometer-scale. Techniques such as UV/Vis spectroscopy, Raman spectroscopy, X-ray diffraction, and Atomic Force Microscopy were used to probe the evolution of the film morphology as the additives were added to the solutions. This study could reveal new knowledge and insights of approaches that would enhance the solid-state morphology of thin films for high performance electronic applications.

Polymeric conjugated materials are very promising for developing future soft material-based semiconductors, conductors, electronic and optoelectronic devices due to their inherent advantages such as lightweight, flexible shape, low-cost, ease of processability, ease of scalability, etc. There are a number of ways to tune material properties via post-processing treatments such as annealing, solvent additives, or doping. One challenge of solution processed organic thin film electronics is the repeatable and well-defined morphological control of the resulting films after deposition. In this study, we observed how the addition of diiodo-alkanes to poly-3-hexy-thiophene (P3HT) solutions effect the formation of resulting thin films.These additives have various boiling points that will affect the drying properties of cast thin films. We also sought to observe if any halogen bonding interaction between iodine and sulfur occurs and if this interaction can be used to promote polymer morphological self-assembly. Techniques such as UV/Vis spectroscopy, Raman spectroscopy, X-ray diffraction, and Atomic Force Microscopy were used to probe the evolution of the film morphology as the additives were added to the solutions. This study may result in a new technique that can be used to enhance the solid-state morphology of thin films for high performance electronic applications.

Conjugated polymers like polyacetylene, polypyrrole, polythiophene and polyaniline have been studied extensively in terms of their bulk optical and electrical properties due to their potential for various applications in optoelectronic devices. These polymers have various desirable characteristics such as low processing cost and solubility in common solvents, in addition to possessing useful physical properties like light weight and flexibility. The most exciting feature of these polymers is the delocalization of “pi” bonds along the chain axis that makes them intrinsically conductive. The versatility of conjugated polymer materials is increasing due to the introduction of additional electro-active behavior via doping. Doped conjugated polymers have the potential to exhibit electrical conduction that approaches that of metals, which gives rise to altered optical transparency. These optical and electrical properties are of interest for both photonic and organic electronic devices. In recent history, investigations of the nanoscale properties of matter have proven to be fruitful in terms of understanding and exploiting unique characteristics of materials. Studying nanoscale properties of doped conjugated polymers could provide more information on the distribution of dopant sites, which is necessary for improving the charge conduction in these polymers.
In this study, we focus on comparison of nanoscale topological, mechanical and electrical properties of poly(3-hexylthiophene) (P3HT) and doped P3HT thin films. P3HT was spin coated onto an indium-tin oxide (ITO) coated glass substrate followed by electrochemical doping using tetrabutylammonium perchlorate (TBAP). Atomic force microscopy (AFM) was used to perform topography and phase imaging (tapping mode) of both undoped and doped P3HT and their crystallinity was compared to assess the effects of doping. There was a clear loss in crystallinity caused by doping, according to the phase images, and it was found that domains of varying crystallinity were not correlated with film topography. The mechanical properties of the thin films were studied by performing fast force mapping (FFM) using an AFM tip with a lower spring constant and adhesion to the polymer surface. Young’s modulus was calculated from FFM and a relation was established between the modulus and crystallinity for both undoped and doped P3HT at the nanoscale. The crystalline domains in both the polymer thin films exhibited higher Young’s modulus when compared to the amorphous ones. Conductive AFM imaging (CAFM) was employed to perform scans of the local current distribution of the polymer films with the application of a bias voltage. The current values increase by at least 3 orders of magnitude on going from undoped to doped samples. Also, evidence that nanoscale crystalline polymer domains conduct more current than nearby amorphous ones was found by simultaneously acquiring topography FFM and CAFM scans.

Recently, pi-conjugated organic molecules have been attracted a great attention due to their low-temperature processability on flexible substrates and low-cost applications via graphic arts printing techniques. For enabling the high performance electronic and optoelectronic devices, both electron and hole charge carrier injection have to be efficiently controllable. Here, we report an efficient electron injection layer for high-performance n-type organic field-effect transistors (OFETs) using solution-processed reducing agents. The reductive interlayer plays many different roles depending on an energy level alignment between the reduction potential of a reducing agent and frontier molecular orbitals of organic semiconductors. Basically it induces better electron injection properties so that electron mobility is significantly improved. Moreover, the reducing agent could induce doping effect by direct charge transfer to the semiconductors with the high lowest unoccupied molecular orbital. Operation modes of ambipolar OFETs could also be changed by completely regulating the hole current, thereby initial ambipolar OFETs converts to n-type only unipolar mode device. Those interlayers increase memory capacity of non-volatile transistor-type memory via changing the operation modes from unipolar to bipolar.

Organic solar cells (OSCs) are an important eco-friendly technology to produce clean and renewable energy, using flexible, low-cost, and lightweight devices. Poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunctions have stood out among the active layer compositions due to its prominent power conversion efficiency [1]. Experimental data have shown that the chosen conditions for the manufacture processing - as solvent evaporation rate and thermal annealing - can lead to different morphologies. It is well known that the bulk heterojunction microstructure directly influences on the OSC performance. However, due to the amorphous nature of these materials, the physicochemical characterizations are still limited [2]. In this work, we have performed atomistic molecular dynamics simulations aiming to estimate the morphological characteristic of the P3HT:PCBM blend. The evaporation process is reproduced in the simulations, and solvent removal has been carried out in a multistep process until the film formation [3]. At first, we simulated three systems at room temperature: (i) a P3HT film, (ii) a PCBM film, and (iii) a P3HT:PCBM bulk heterojunction. After that, in order to investigate the temperature influence on the arrangements of the backbone chains for the final film morphology, the simulations of these systems were performed under thermal annealing. We evaluated the structural properties of the oligomer backbone chains on both, P3HT film and bulk heterojunction. In addition, we analyzed the distribution of PCBM in the bulk heterojunction, which behavior has a good agreement with experimental results available in the literature.
Acknowledgements:
UFOP, INEO/INCT, CNPq, CAPES and FAPEMIG.
References:
[1] Minh Trung Dang , Lionel Hirsch , et al., Chemical Reviews, 101021 (2013)
[2] J. A. Reinspach, Y. Diao et al., ACS Appl. Mater. Interfaces 8, 1742 (2016)
[3] R. Ramos, M.F. Siqueira et al., Phys. Chem. Chem. Phys. 17, 20530 (2015)

Organic semiconductors are now widely employed in electronic and optoelectronic devices due to their solution processability and compatibility with flexible substrates. ¹ The Meyer Neldel (MN) Rule or isokinetic (IK) law means that a set of comparable samples exhibit kinetic behavior independent of activation energy at an IK or MN temperature.² It is obeyed by the electron or hole mobility, µ, in many organic semiconductors, including pentacene³, fullerene⁴, N-Alkyl Perylene Diimides,⁵ etc. The existence of MNR is still not widely known. Further, despite considerable evidence in its favor, the multi-excitation entropy (MEE) model of its origin is still frequently contested. We fabricated organic field effect transistors (FETs) based on regioregular poly(3-hexylthiophene-2,5-diyl) (RR P3HT) of three different values of molecular weight, MW, and studied the dependence of the mobility upon the gate bias, V_g, and temperature, T. For all values of MW, we found that in the range 200 to 290 K (low T), the µ of the FET carriers exhibits an exponential dependence upon 1/T for different V_g, with an MN temperature about 290K. Between this temperature and 350K (high T), µ increases more rapidly than it did below, and is independent of V_g. We believe that, in the low T regime, the polaronic carriers hop along the polymer backbone chain. We attribute the high temperature behavior to the alpha relaxation of the polymers, that is to the movement of the backbone chains. We believe that in this regime, the polaronic carriers move with the mobile chains.

Reference
1. F. Cicoira and C. Santato, Organic Electronics: Emerging Concepts and Technology, Wiley-VCH Verlag GmbH and Co. KGaA, 2013, Germany.
2. A. Yelon, et al., Multi-excitation entropy: its role in thermodynamics and kinetics, Rep. Prog. Phys., 69, 1145-1194, 2006.
3. E. J. Meijer, et al., The Meyer–Neldel rule in organic thin-film transistors, Appl. Phys. Lett., 76, 3433, 2000.
4. J. C. Wang and Y. F. Chen, The Meyer–Neldel rule in fullerenes, Appl. Phys. Lett., 73, 948 1998.
5. R. J. Chesterfield, et al., Organic Thin Film Transistors Based on N-Alkyl Perylene Diimides: Charge Transport Kinetics as a Function of Gate Voltage and Temperature, J. Phys. Chem. B, 108, 19281-19292, 2004.

Organic solar cell efficiencies can be boosted by replacing fullerene acceptors with strongly-absorbing dye molecules, namely, non-fullerene acceptors (NFAs). In fact, even pristine non-fullerene acceptors can serve as efficient exciton charge converters, thereby enhancing the total solar cell efficiency. In this presentation, we explain why exciton dissociation into a charge transfer (CT) state and the splitting of the CT state into charge separated (CS) state are efficient in pristine NFAs.
First, we consider exciton dissociation into particular CT states. Evaluation of dielectric solvation of excited and CT states shows that there is enough driving force for the transition, due to a stronger dielectric stabilization of charges as compared to the localized excited state. We then evaluate electrostatic potential at a disordered interface between NFA domains with different acceptor orientations and show that it provides electrostatic potential bending, sufficient to overcome the Coulomb binding energy of the CT state. Both effects can be traced back to molecular quadrupole moments and their long-range contributions to solid-state ionization energies and electron affinities.
The present study demonstrates that the electrostatic configuration of NFA molecules is important for both stages of charge conversion and suggests several design rules for NFAs with efficient charge separation in photovoltaic applications [1,2].
[1] N. Gasparini, A. Markina et. al. “Efficient exciton to charge conversion in a single-component organic solar cell”, in preparation, 2019.
[2] A. Markina, et. al. “Molecular design of acceptors for non-fullerene organic solar cells”, in preparation, 2019.

Organic semiconductors (OSCs) are promising materials for low-cost, flexible, large-area production of printed electronic devices. In this context, molecular doping allows controlling the electrical properties of OSCs, offering a powerful tool to improve the performances of different electronic devices such as organic light-emitting diodes, solar cells, and field effect transistors [1]. Despite the progress in the fundamental understanding of the doping mechanism and processing techniques, stability aspects of p-doped OSCs have received little attention [2]. Contrary to the extensive literature on the air stability of n-doped OSCs, which presents challenges due to the low ionization energies of efficient molecular n-dopants [3-4], to the best of our knowledge, there are no studies on the air stability of ultra-thin p-doped OSCs. However, this is an important requirement for the compatibility of p-doped OSCs with real device applications in printed electronics.

Here, we present a first experimental investigation on the air stability of p-doped OSCs. For this study, we chose the widely used molecular p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) and molybdenum tris[1-(trifluoroethanoyl)-2-(trifluoromethyl) ethane-1,2-dithiolene] (Mo(tfd-COCF₃)₃). Poly(3-hexylthiophene) (P3HT) and poly[(4,8-bis-(2-ethylhexyloxy)-benzo(1,2-b:4,5-b’)dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl )-thieno[3,4-b]thiophene-)-2-6-diyl) (PBDTTT-c) were chosen as OSCs.

When exposed to ambient air, all the p-doped layers investigated in this work exhibited a significant decrease in electrical conductivity and UV-Vis-NIR dedoping signatures, independent of the polymer:dopant system and of the dopant concentration. The layer thickness, varying from 15-200 nm, presented a non-negligible impact over the kinetics of the doping degradation. We highlighted the detrimental impact of moisture and/or O₂(H₂0)_n hydrated complexes by evaluating the p-doping stability under different atmospheres (ambient air, anhydrous air and nitrogen). Using X-ray Photoelectron Spectroscopy (XPS) we were able to assign the p-doping instability in ambient air to changes in the oxidation state of the dopant molecules and some degradation signatures. Deeper investigations on the impact of water in the stability of neutral and ionized p-dopant molecules are carried out by Nuclear Magnetic Resonance (NMR) Spectroscopy, Electron Paramagnetic Resonance (EPR) and UV-vis-NIR to better elucidate on the possible degradation mechanism. This work invites to reconsider the requirements for air-stable p-doping of OSCs as a first step towards air-processable thin p-doped layers.

[1] Salzmann, I. & Heimel, G. Journal of Electron Spectroscopy and Related Phenomena 204, 208–222 (2015).
[2] Jacobs, I. E. & Moulé, A. J. Advanced Materials 29, 1703063 (2018).
[3] Tietze, M. L. et al. Advanced Functional Materials 26, 3730–3737 (2016).
[4] de Leeuw, D. M., Simenon, M. M. J., Brown, A. R. & Einerhand, R. E. F. Synthetic Metals 87, 53–59 (1997).

Recently, non-fullerene acceptors have received increasing attention for use in polymer-based bulk-heterojunction organic solar cells, as they have demonstrated considerably improved photovoltaic performances over classical polymer-fullerene blends. Part of the success of these materials has been attributed to an increased contribution to the overall absorption of the solar cells, thanks to relatively low bandgaps in these materials. In this study a systematic comparison between two acceptor materials, the classical fullerene-derivative PCBM and the non-fullerene-acceptor 2,2′-[[6,6,12,12-Tetrakis(4-hexylphenyl)-6,12-dihydrodithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene-2,8-diyl]bis[methylidyne(3-oxo-1H-indene-2,1(3H)-diylidene)]]bis[propanedinitrile] (ITIC) was performed in combination with a statistically substituted anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylenevinylene)s (PPE–PPV) copolymer (AnE-PVstat), as well as with two other commonly applied materials called Poly[[4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c′]dithiophene-1,3-diyl]] polymer (PBDB-T) and Poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)], Poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT).
We find several cases in which the polymer-fullerene blend does function properly in photovoltaic terms; however, when exchanging the fullerene-derivative with ITIC, photovoltaics function seizes down dramatically. While cyclic voltammetry derived energy levels suggest successful charge transfer from the polymer to the ITIC, blend films and photovoltaic studies indicate the opposite. Numbers of spectroscopic methods like ultraviolet-visible spectroscopy (UV-Vis), photoluminescence spectroscopy (PLS), electroluminescence spectroscopy (ELS), energy-resolved electrochemical impedance spectroscopy (ER-EIS), transient absorption spectroscopy (TAS), fourier-transform infrared spectroscopy (FTIR), electron spin resonance spectroscopy (ESR) as well as quantum chemical calculations have been used for finding an explanation.

Semiconducting polymers (SPs) have received a lot of attention in recent years due their potential for creating low cost solution processable electronic devices. The SP conductivity can be tuned by several orders of magnitude by molecular doping. The molecular dopant 2,3,5,6-tetrfluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) has been extensively explored as a p-type dopant for poly-(3-hexyltiophene) (P3HT). With the synthesis of novel high mobility conjugated copolymers with higher ionization potentials, such as diketopyrrolopyrrole (DPP)-based copolymers, there is a need to develop stronger molecular dopants. Recent studies have reported stronger molecular dopants than F4TCNQ, however, most of them present poor solubility that makes them unable to be use for solution processing.
In this work we present the synthesis and characterization of a new soluble dopant with high electron affinity (-5.5 eV) 2,2',2″-(cyclopropane-1,2,3-triylidene)-tris(cyanoacetate) (TMCN3-CP). Using sequential solution processing we demonstrate that conjugated copolymers such as PDPP-4T, PDPP3T, and PDPP-T-TT-T achieve high conductivities when they are p-type doped with TMCN3-CP. To our knowledge, this is the first report of sequential solution doping of these high ionization energy polymers.

Semiconductors are the subject of much investigation as next-generation materials for organic electronic devices. The ability of these materials to transport charges is a key factor limiting the performance of these devices. Charge carriers in conjugated polymers are localize by disorder and polaronic effects. As such, charge transport in these materials is often described as non-adiabatic hopping transport, with a rate given by Marcus Theory. An important variable which determines the temperature dependence of Marcus rate is the activation energy barrier for polaron hopping. We use a tight-binding polaron model, in which charge carriers are stabilized by nuclear reorganization and by dielectric polarization to map out the potential energy curve for polaron hopping, and thus determine barriers and rates relevant to intrachain and interchain charge hopping processes in P3HT crystalline lamellae and amorphous melts. We find that the barriers largely depend on polarization effects, and on the electronic coupling between the initial and final location, which penalize hopping events involving longer distances. We also find that transport is limited by interchain hopping in P3HT crystals and amorphous melts. Along the chains, transport is fast and relatively insensitive to dihedral disorder. Our predicted barriers and rates are in excellent agreement with experiment.

Solution-processed organic thin-film transistors (OTFTs) have received extensive attention over the past decade due to their noteworthy features: low-material consumption, low-cost manufacturing, and large-area processability. However, hidden behind the virtues is potential damage to environment and human health by using detrimental organic solvents that are not adaptable for future industrial scale. Here, we propose a water-based polyimide (PI) as gate dielectric layer for green chemistry to substitute harmful aprotic solvents that emit toxic vapors during drying process. We find that a water-based precursor shows considerably better compatibility with facile thin film fabrication than a N-methyl-2-pyrrolidone-based precursor. To adopt the water-based precursor facilitates not only escape from a hindrance in humid air through hydrolytic stability buy also use of less thermal energy through faster imidization. As a result, pentacene-based TFTs using water-based PI spin-coated in relative humidity of 43 % exhibit average mobility of 0.132 cm² V^-1 s^-1, on/off current ratio of over 10⁵, and high yield nearly 100 %. Under the same conditions, on the other hand, OTFTs using NMP-based PI exhibit poorer device performance with lower yield as low as 50 %. The results indicate that the newly designed water-based PI shows a viable path to development of electronic packages using harmless processing with less sensitivity to ambient atmosphere.

In this work, the role of side-chain bulk on the acceptor unit in a Donor-Acceptor (DA) polymer on charge generation and power conversion efficiencies in PCBM blend bulk heterojunction solar cells is investigated through the design and synthesis of novel polymer families and device fabrication. The PCE11 repeat unit is used as a high performing solar polymer whose acceptor unit lacks a sidechain. It was hypothesized that the lack of a substituent on the acceptor unit contributed to better fullerene docking on the acceptor units and facilitated more efficient charge transfer from the DA polymer to the fullerene, enhancing OPV performance (Graham, et. al., J. Am. Chem. Soc. 2014). Previously, we reported a family of PCE11 analog polymers designed with comparatively small methyl substituted acceptor units for use as donor phase materials in bulk heterojunction (BHJ) OPV applications. These polymers make use of common acceptor moieties thieno [3,4-c]pyrrole-4,6-dione (TPD), diketopyrrolopyrrole (DPP), and isoindigo (iI) with methyl sidechains to provide sterically unhindered sites for enhanced interactions with a molecular acceptor (MA), either PC₇₁BM or ITIC in this study. The resulting polymers were utilized in BHJ OPV devices where the highest performing DA polymer, P(T4-TPD-M), obtained a maximum PCE of 7.5% with PC₇₁BM and 4.0% with ITIC. These results indicate that minimally sized methyl sidechain derivatives of TPD, DPP, and iI can be useful acceptor moieties in DA polymers. As a further effort to probe the effect of acceptor unit bulk on power conversion efficiency, a family of TPD-based polymers with sidechains of increasing bulk, but similar solubility, was synthesized. Devices were fabricated with these polymers under similar conditions to ensure the only variable was the side chain bulk on the acceptor unit. Photophysics studies followed device fabrication to study the nature of charge separation in the respective polymer systems . By using these polymers with precisely controlled side chain bulk, a connection between acceptor side chain bulk sterics and charge transport and solar cell performance is probed.

In recent years, ternary organic solar cells (t-OSCs) have offered the potential of high-power conversion efficiency due to enhanced photon absorption or energy level complimentary materials as the second donor or acceptor based on a single bulk heterojunction architecture. For well promoting the development of t-OSCs, we fabricated ternary blend OSCs with two donors, including one low bandgap PBDTTT-EFT, one wide bandgap polymer PCDTBT, and one inorganic acceptor PC₇₁BM. The incorporation of a wide bandgap PCDTBT polymer as a third component in to the binary blends improve fill factor of 71.62 % with high PCE of ~9 % and high J_sc (16.55 mAcm^-2) of t- OSCs were observed than those of the binary OSCs with enhanced light absorption, efficient energy transfer and better blend morphology. This increased PCE attributed to not only non-radiative Forster resonance energy transfer (FRET) and enhancement of absorption between two donor polymers but also the formation of a bicontinuous interpenetrating network in ternary blends. The addition of a wide bandgap polymer into binary blends open up a new door for enhancing device performance in a simple, effective and promising way.

Halogenation, particularly fluorination, is commonly used to alter the energetics, stability, and morphology of organic semiconductors. In the case of organic photovoltaics (OPVs), fluorination of electron donor molecules or polymers at appropriate positions can help improve PV performance. In this contribution, we present insights into how halogenation influences the bulk solid-state energetics of model anthradithiophene (ADT) materials, their interfacial energetics with C₆₀, and the energetics of various ADT:C₆₀ blend compositions using ultraviolet photoelectron spectroscopy, external quantum efficiency measurements of charge-transfer (CT) states, and density functional theory calculations. In agreement with previous work, nonhalogenated ADT molecules show higher energy CT states in blends with C₆₀ and lower energy CT states in ADT/C₆₀ bilayers. This trend is reversed in the halogenated ADT/C₆₀ systems, with the CT state energies of ADT:C₆₀ blends lieing at lower energies than those in the bilayers. In bulk-heterojunction photovoltaics, the lower energy CT states present in the mixed phase with the halogenated ADT derivatives will likely decrease the probability of charge separation and increase charge recombination. These less favorable energy landscapes observed upon halogenation suggest that the benefits of fluorination observed in many OPV material systems may arise primarily from morphological factors.

Triphenylamines are an important class of donor building blocks in the field of organic electronics. Crucial electronic properties such as triplet energy (E_T), HOMO/LUMO levels and donor strength can be tuned via modifications of its molecular structure e.g. planarization. Specifically, the donor strength decreases with increased planarization, with the completely planar indolo[3,2,1-jk]carbazole (ICz) even exhibiting weak acceptor characteristics. In this contribution an increase in donor strength of the building block, while retaining full planarity, is presented. This is achieved by the incorporation of electron rich thiophene into the scaffold. Furthermore, a fine tuning of the electronic properties depending on the substitution positions is realized. A synthetic approach towards the target molecules employing Ullmann condensation as well as CH activation will be presented. Furthermore, the electrochemical as well as photophysical characterizations of the novel building blocks will be discussed.

We experimentally study organic dye thin films under mechanical strain using engineered microstructures and extract mechanical properties that are essential to the design of next-generation strain based excitonic devices. Such mechanically strained architecture provides a simpler and energy efficient platform to achieve directed energy transport in organic materials. In our measurements, we estimate the elastic modulus of the thin film of red luminescent dye 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM), embedded in Tris-(8-hydroxyquinoline) aluminum (Alq₃) matrix to be within 3.6-7.2 GPa. Reported values in literature lie in the range of 1- 100 GPa [1] [2] and so, our results demonstrate a new non-destructive characterization approach based on solid state solvation effect [3] that greatly reduces the range of uncertainty in the measurement of elastic modulus of organic thin films. The effect of strain gradient on energy transport in organic thin films is currently under investigation.
Our device platform consists of buckled silicon di-oxide (SiO₂) microbeam patterned using optical lithography and reactive ion etching, followed by deposition of 30 nm Alq₃:DCM. 50 nm 2,2′,2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) thin film is evaporated to protect the active layer during the subsequent XeF₂ gas phase etching used to release the microbeams. Due to internal compressive stress in the oxide, the microbeams are buckled upwards, inducing tensile strain on the organic thin film. The buckling was quantified using optical white light interferometry, to extract the tensile strain on the film.
The Photoluminescence (PL) of the strained organic thin films was characterized using a 450 nm excitation laser in a nitrogen (N₂) environment. We observed gradually increasing blueshift in the PL emission towards the center of the microbeam, with the maximum blueshift occurring at the center of the beam. The spectral blueshift is attributed to a change in local molecular concentration that changes the local electric field surrounding the DCM molecules.

We fitted the observed spectral blueshift with respect to the applied tensile strain, using a simple linear equation:

E_Emission = Bε + C; Here, E_Emission is the emission energy of DCM from PL measurement, B and C are fitting parameters, ε is the tensile strain on the thin film.

and estimated the elastic modulus of the organic thin film to be within 3.6-7.2 GPa. By engineering the strain at microscale, we have successfully measured mechanical property on an organic thin film that are vital to the development of wearable and flexible energy conversion devices such as photovoltaics and light emitting diodes.

References:
1] C. J. Chiang, S. Bull, C. Winscom, and A. Monkman, Org. Electron. Physics, Mater. Appl. 11, 450 (2010).
[2] N. Bakken, J. M. Torres, J. Li, and B. D. Vogt, Soft Matter 7, 7269 (2011).
[3] C. F. Madigan and V. Bulović, Phys. Rev. Lett. 91, 1 (2003).

We prepare CuPc/C₆₀ organic thin-film solar cell structure and investigate the photo-excited carrier lifetime by using time of flight method to reveal the carrier transport dynamics of the structure.
The structure of the sample was ITO/CuPc(30 nm)/C₆₀(40 nm)/BCP(5 nm)/Al. These layers were deposited on SiO₂ substrate by using vacuum evaporation. After the depositions, the sample was taken out into the atmosphere, and then, it was immediately sealed with epoxy resin. Optical absorption spectra for each thin-film layer were measured by ultraviolet to visible light absorption spectrometer (JASCO V-630). Current density-voltage characteristics of the sample were measured by using YOKOGAWA GS820 source measure unit with conventional two-terminal method. The spectral sensitivity characteristics were measured using a lock-in amplifier (Stanford Research SR830) and a xenon lamp as the irradiation source. The transient photocurrent properties were investigated irradiating third harmonic (532 nm) of pulsed Nd-YAG laser light (Quantel CFR200, pulse width: 7 ns) and the current decay characteristics were observed on a screen of digital oscilloscope. All measurements were carried out at room temperature and in air.
The obtained carrier lifetime (transit time) was 58 μs. This value is much longer than that of estimated lifetime from the mobility of thin-fillm layers. This means the carrier transport cannot be described by a general carrier drift model. In addition, the carrier lifetime become short (5 μs) under white halogen light illumination. The results suggest that the photo-excited carrier transport is affected by trapping levels in the organic layers.

Copper(I) thiocyanate (CuSCN) has become a well-known transparent semiconductor with wide-ranging applications, such as organic photovoltaic cells¹, perovskite solar cells² and organic light-emitting diodes³. CuSCN is a coordination polymer with a 3D extended network; its unique properties of wide band gap and good hole mobility are resulted from its electronic structure which consists of Cu 3d states in the valence band and SCN states in the conduction band. With the objective to develop novel semiconductors based on coordination polymers, we have recently reported on tin(II) thiocyanate [Sn(NCS)₂], which was found to exhibit high optical transparency and promising hole transport states of Sn 5s in the valence band⁴. In this work, we expand the concept of modifying the metal centers of the thiocyanate compounds further by synthesizing mixed-metal thiocyanates via mechanochemical reactions. Starting from CuSCN which has already exhibited excellent semiconducting properties, we experimented with a number of thiocyanate compounds of other metal ions and found that Zn(II), Co(II) and Ni(II) thiocyanates were suitable candidates. After mechanical grinding CuSCN, Zn(SCN)₂, Co(SCN)₂ and Ni(SCN)₂, the powder X-ray diffraction (PXRD) showed that their diffraction peaks appear in the same position but become significantly broadened. This suggested that their lattice structures were sufficiently soft to allow amorphization but at the same time stable enough to retain the same phases. This showed the potential that these compounds could be mixed and reacted using mechanochemical processes. Subsequently, CuSCN and the thiocyanate compounds of Zn(II), Co(II), and Ni(II) were mixed by the solvent-free ball-milling process to realize the homogeneous metal ion combination without an environmental issue from toxic organic solvents. The PXRD results of the final compounds were completely different from the starting thiocyanate materials, suggesting the successful synthesis of new structures of mixed-metal thiocyanates. Analyzing the powder samples with the energy dispersive X-ray spectroscopy (EDX) under a scanning electron microscope (SEM) showed homogeneous distribution of the mixed metal ions. Other characterization results from Fourier-transform infrared spectroscopy (FTIR) and thermogravimetric/differential thermal analyses (TGA/DTA) also confirmed different physical properties of the new compounds compared to the starting materials. In particular, TGA/DTA results showed that some of the compounds display melting behavior at temperatures around 300 °C; this could lead to thiocyanate-based inorganic semiconductors that can be processed into films form melt. Further studies on the electronic properties and applications of these compounds are ongoing. This work shows that the solvent-free mechanochemical processes are promising for the synthesis of new coordination polymers as well as expand the material library of semiconductors based on coordination polymers.

Coordination polymers feature a large library of materials; however, their applications in electronic devices have been mostly unexplored. One recent example that has been demonstrated as a novel semiconductor with extensive device applications is copper(I) thiocyanate (CuSCN).^[1-3] We have been developing other coordination polymer semiconductors and recently reported on tin(II) thiocyanate [Sn(NCS)₂] which exhibited a wide band gap and promising hole transport properties due to the dispersed Sn 5s electronic states at the top of the valence band giving rise to a small hole effective mass.^[4] Another promising aspect of Sn(NCS)₂ is that it can be solution-processed from common solvents with high polarity, such as alcohol-based solvents. This is in contrast to CuSCN which is normally deposited from alkyl sulfides or aqueous ammonia which are either malodorous or corrosive. Herein, we report the optimization of solution-based thin-film processing of Sn(NCS)₂ and its application as a hole transport interlayer in organic photovoltaics (OPVs). We found that the thin-film formation of Sn(NCS)₂ was not straightforward due to its strong tendency to form clusters and islands resulting in discontinuous and inhomogeneous films. These films led to the absence of conduction pathway in the planar direction and short-circuits in the vertical direction. To resolve this issue, we dried the films at room temperature under inert atmosphere for an extended period of time after the spin-coating step and consistently obtained high-quality Sn(NCS)₂ thin films. These films were highly transparent due to its wide band gap (3.98 eV) and extremely smooth with a root-mean-square (RMS) surface roughness of 0.4 nm on glass substrates and 1.6 nm on ITO substrates as measured by the UV-visible-near IR spectroscopy and atomic force microscopy (AFM), respectively. According to the photoelectron spectroscopy (PES) results, the valence band maximum of Sn(NCS)₂ was found at -6.9 eV, a deep value compared to other hole transport materials. However, its Fermi level was determined by the Kelvin probe measurements to be ~4.5 eV allowing the formation of ohmic contact with ITO substrates. As such, Sn(NCS)₂ thin films were employed as the hole-transporting/electron-blocking interlayer in OPVs based on PTB7-Th:PC₇₀BM as the active light-absorbing layer. OPV cells yielded impressive power conversion efficiencies (PCE) up to 7.98% (J_sc= 16.06 mA cm², V_oc = 0.80 V, FF = 0.62) with a device area of 0.181 cm². The average PCE from 10 devices was 7.5%. This remarkable result demonstrates the potential of Sn(NCS)₂ in thin-film electronic applications and provides a strong foundation for further development of semiconductors based on coordination polymers.

[1] P. Pattanasattayavong, N. Yaacobi-Gross, K. Zhao, G. O. N. Ndjawa, J. Li, F. Yan, B. C. O’Regan, A. Amassian, T. D. Anthopoulos, Adv. Mater. 2013, 25, 1504.
[2] N. Arora, M. I. Dar, A. Hinderhofer, N. Pellet, F. Schreiber, S. M. Zakeeruddin, M. Grätzel, Science. 2017, 358, 768.
[3] W. Y. Sit, F. D. Eisner, Y. H. Lin, Y. Firdaus, A. Seitkhan, A. H. Balawi, F. Laquai, C. H. Burgess, M. A. McLachlan, G. Volonakis, F. Giustino, T. D. Anthopoulos, Adv. Sci. 2018, 5, 1.
[4] C. Wechwithayakhlung, D. M. Packwood, J. Chaopaknam, P. Worakajit, S. Ittisanronnachai, N. Chanlek, V. Promarak, K. Kongpatpanich, D. J. Harding, P. Pattanasattayavong, J. Mater. Chem. C 2019, 7, 3452.

Copper(I) thiocyanate (CuSCN) has been demonstrated as a promising coordination polymer semiconductor with extensive opto/electronic applications owing to its p-type characteristic with high hole mobility, excellent optical transparency, and solution processability. CuSCN has been used as a transparent p-type active channel layer in thin-film transistors (TFTs), usually exhibiting filed-effect hole mobility (μ^FE_h) on the order of 0.01 cm² V^-1 s^-1.^[1,2] In order to challenge p-type metal oxide semiconductors, further optimization of the film processing is required to improve the hole transport properties. A few techniques have been reported to enhance μ^FE_h of CuSCN films leading to higher TFT performance; one of the novel examples is the molecular doping of CuSCN with C₆₀F₄₈ to increase the hole concentration and fill the trap states, enhancing μ^FE_h by tenfold.^[3] However, the use of such dopant may not be widely applicable. Herein, we report a simple and versatile approach of using anti-solvent treatment that can enhance μ^FE_h in solution-processed layers of CuSCN by fivefold. Acetone (Ace), tetrahydrofuran (THF), methanol (MeOH) and isopropyl alcohol (IPA) were employed as anti-solvents and spin-coated onto CuSCN films which had been prepared from diethylsulfide (DES) solution also by spin-coating. The treated films exhibited similar electronic properties to the neat films, i.e., an optical band gap of 3.9 eV and a valence band maximum at -5.5 eV. Interestingly, the anti-solvent treatment demonstrated a significant effect on CuSCN film morphology with the root-mean-square (RMS) roughness of the films decreased from 4 nm to 1.7, 1.5 and 1.3 nm after treated with MeOH, Ace and THF, respectively, due to the reduction in CuSCN grain size. This analysis was also corroborated by the results of the extended X-ray absorption fine structure (EXAFS) of S K-edge X-ray absorption spectroscopy, which was employed to study the coordination between Cu and S. THF-treated film showed the highest number of surface Cu-S coordination that resulted from CuSCN grain size reduction. As a result, TFT measurements revealed a significant improvement of μ^FE_h in THF-treated CuSCN film to 0.05 cm² V^-1 s^-1 accompanied by a decrease in the turn-on voltage (V_on) and trap density. In addition, Ace- and MeOH-treated devices could also raise μ^FE_h to 0.024 cm² V^-1 s^-1 and 0.012 cm² V^-1 s^-1_, respectively. The trend in the mobility was also further substantiated by microwave conductivity measurements that also showed the mobility of untreated films to increase with anti-solvent treatment in the order: MeOH < Ace < THF. This study shows that the anti-solvent treatment can be used to significantly improve film morphology and hole transport properties in CuSCN thin films. This simple method relies on common chemicals and standard procedures, and thus can be applicable to a wide range of thin-film opto/electronic device processing.

Reference
[1] N. Wijeyasinghe, T. D. Anthopoulos, Semicond. Sci. Technol. 2015, 30, 104002.
[2] P. Pattanasattayavong, V. Promarak, T. D. Anthopoulos, Adv. Electron. Mater. 2017, 3, 1.
[3] N. Wijeyasinghe, F. Eisner, L. Tsetseris, Y. H. Lin, A. Seitkhan, J. Li, F. Yan, O. Solomeshch, N. Tessler, P. Patsalas, T. D. Anthopoulos, Adv. Funct. Mater. 2018, 28, 1.

Blend films of conjugated polymers have been attracting attention as an active layer of organic electronic devices. Nanostructures inside the blend films play essential roles in determining the overall device performance. For instance, the ordered regions of the conjugated polymer could support charge transport while the amorphous counterpart blocks it due to the sizeable energetic barrier. However, it is difficult to expect the nanoscale self-assembly tendency of conjugated polymer chains. Therefore, a systematic study on the structure in the blend films is still limited so far. In this study, we conduct experiments to measure and identify the change in the conformation and aggregation structures of a conjugated polymer, which arises characteristically by blending with another conjugated polymer by the spin-coating method.
Blend films composed of a low-bandgap polymer (PCPDTBT) and a wide-bandgap polymer (P3M4HT) were prepared by spin-coating from the Chlorobenzene solution which dissolves the polymers with different weight ratios. For these blend films, UV-Vis absorption spectroscopy and Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were performed to characterize structures of the PCPDTBT dispersed in the P3M4HT matrix. Both of the vibrational structure from UV-Vis absorption spectra and the scattering intensity from GIWAXS cleared that fraction of the ordered PCPDTBT aggregates relative to the amorphous PCPDTBT counterpart increases by blending. Furthermore, the GIWAXS analysis reveals that the correlation length of the PCPDTBT chain ordering increased in both its alkyl-stacking and chain backbone directions. These structural ordering of PCPDTBT was enhanced with the decreasing the PCPDTBT weight ratio in the blend film.
Our result concludes that blending of conjugated polymers by spin-coating causes an enhancement of ordering of the polymer chains rather than the corresponding neat film. These changes in the aggregation structure of conjugated polymer chains are beneficial for the improvement of charge transport and the device performances.

Owing to their excellent transconductance property, organic electrochemical transistors (OECT) have been found to be an attractive choice for various kinds of transducers such as biological sensors and neuromorphic devices. Historically, attempts to enhance the ionic-electronic transport focussed on additives that improved the PEDOT packing resulting in better electronic transport. However, the densification of PEDOT domains deleteriously affects the ionic transport. Here, we present a novel approach to engineer these intricately interrelated properties to simultaneously enhance these figures of merits (μC*) through structural reorganization of the domains with the help of an ionic liquid [1]. Further, owing to their excellent stability, we evidence its application in point of care devices using a self-powered OECT for sensing applications. Our work highlights for the first time, a highly efficient, ambient stable perovskite photovoltaic (PV) powered OECT, whose transconductance (~60.50 μS) is nearly unaffected by the incident light intensity (0.13 – 100 mW/cm²) owing to the excellent balanced electron-hole transport and a nearly non-existent trap assisted recombination processes in the solar cell. Our findings therefore demonstrate the great potential of PV-OECT device for various indoor and outdoor luminance conditions.

1. X. Wu, A. Surendran, J. Ko, O. Filonik, E. M. Herzig, P. Müller-Buschbaum, and W. L. Leong, "Ionic-Liquid Doping Enables High Transconductance, Fast Response Time, and High Ion Sensitivity in Organic Electrochemical Transistors," Adv. Mater. 1805544 (2018).

The non-fullerene organic solar cells (NFOSCs) has recently have emerged as a promising photovoltaic technology due to their potential in large area printable solar cells with low costs. The typical film casting for the photoactive blend is spin-coating, but it is not suitable to be utilized in actual industries. Thus, the printing technique such as blade-coating has been demonstrated as an efficient method of high-throughput large area coating for NFOSCs. The morphology generation kinetics in the blade-coated blend films are totally different to that of the spin-coating because the amount of residual solvents, the concentration of the photoactive components, and the drying time of the as-cast wet films. In this work, we systematically investigated the influence of the solvent additive (1,8-diiodooctane) and substrate temperature on the blend film morphology, and their correlations with charge transport and photovoltaic properties were analyzed. We also figured out the optimal condition for the blade-coated NFOSCs that surpasses the spin-coated devices.

Organic molecules producing thermally activated delayed fluorescence (TADF) have emerged as promising dopant materials for electroluminescence. The emergence benefits from the capability of TADF molecules to harvest all the electrogenerated excitons through a thermal equilibrium between singlet and triplet excitons. However, utility of organic light-emitting devices (OLEDs) having TADF dopants is yet to be fully exploited because they suffer from short operation lifetime. The short lifetime of the devices was ascribed to intrinsic degradation of TADF molecules during normal device operation. Various studies have been conducted to identify the chemical mechanism underlying the intrinsic degradation of TADF molecules. Previous studies revealed the occurrence of irreversible bond rupture of TADF compounds upon the formation of exciton or trap of charge carriers. However, the chemical mechanisms underlying the degradation have not been fully elucidated.
To understand the degradation mechanism, we investigated the molecular factors that governed the stability of a series of TADF molecules. Archetype TADF molecules having a 1,3,5-triphenyltriazine (TRZ) acceptor and different cyclic donors, including phenoxazine (PXZ), 10H-phenothiazine (PTZ), 9,10-dihydro-9,9-dimethylacridine (ACR), 10,11-dihydro-5H-dibenzo[b,f]azepine (AZP), and carbazole (Cbz) were synthesized. Our synthetic control aimed at semi-quantitative assessments of the molecular parameters, including an exchange energy, excited-state energy and lifetime, and a bond dissociation energy. Degradation behaviors were monitored during continuous photoillumination (white light, 1.7 × 10^-7 einstein s^-1) of an Ar-saturated THF solution of the dopant (500 μM). The photolysis yielded degradation byproducts after the cleavage of the C-N bond connecting the donor and acceptor moieties. We observed that the degradation rate (k_dgr) depended on the identity of the cyclic donor unit.
Multi-layer OLEDs having the donor-TRZ dyad TADF molecules were constructed with a configuration of ITO/DNTPD (60 nm)/BPBPA (20 nm)/PCzAC (10 nm)/mCBP:dopant (30 nm : 10%)/DBF-Trz (5 nm)/ZADN (30 nm)/LiF (1 nm)/Al. Operation lifetime was determined when the luminance reached 90% of the initial value (LT₉₀) under a constant current driving mode. A positive linear relationship was found between LT₉₀ of the devices and the intrinsic stability (1/k_dgr) of the TADF dopants. This linearity provided firm evidence that intrinsic stability of the materials was directly linked to device longevity.
Unexpectedly, we found that the excited-state properties, such as excited-state energy and lifetime, were not the governing factors of both the stability of TADF materials (i.e.,1/k_dgr) and the device longevity (i.e., LT₉₀). The two stability parameters increased in proportion with the ratio between the quasi equatorial and the quasi axial forms of the cyclic donors in the TADF molecules. X-ray crystal structure analyses and quantum chemical calculations performed at the CAM-B3LYP level of theory supported this notion. Cleavage of the C-N bond was predicted to be exergonic from the singlet intramolecular charge-transfer (¹ICT) transition state of the quasi axial form. On the contrary, the bond cleavage from the ¹ICT transition state becomes endergonic in the quasi equatorial form. Our finding established that the equilibrium between the conformers determines intrinsic stability. We believe that our research will provide useful insights into the molecular strategies to achieving prolonged device lifetime.

An ideal solar cell requires efficient carrier generation as well as minimized carrier recombination loss. Recombination loss in organic photovoltaics (OPVs) mostly originates from non-radiative recombination. Recent studies have demonstrated efficient OPV blends with low non-radiative recombination loss by: 1) reducing the energy difference between the local exciton of inherently bright acceptor materials and donor/acceptor charge-transfer (CT) states to achieve wavefunction mixing and luminescence intensity borrowing; and 2) demonstrating increased photoluminescence yield due to radiative non-geminate carrier recombination.

In the widely accepted carrier dynamics scheme of organic photovoltaics, photogenerated excitons on the acceptor and donor molecules may form charge-transfer states at the donor/acceptor interface, a portion of which then dissociate and generate free carriers. Charge carrier recombination occurs via the CT states. Thus, it naturally follows that increasing the radiative efficiency of the CT states is critical to reduce the overall energy loss in OPVs. We herein propose a new route to reduce non-radiative recombination loss by using intrinsically luminescent system with charge-transfer characteristics—thermally activated delayed fluorescence (TADF) exciplex, as the carrier generation interface in OPVs. We demonstrate a model TADF OPV system with maximum of 10% external quantum efficiency and 40% internal quantum efficiency with ~2% electroluminescence quantum efficiency at 1-Sun injection carrier density. Moreover, photoluminescence efficiency spectrum obtained by scanning through excitation wavelengths and measuring PL quantum yield matches with the photoaction spectrum, indicating that the TADF exciplex is responsible for charge generation, consistent with the OPV carrier generation scheme. We observed fluence-dependent micro-second photoluminescence decay in this system, suggesting radiative non-geminate recombination of photogenerated carriers. Our preliminary results show promise for a proof-of-concept highly emissive and charge-generating OPV device. Furthermore, the large binding energies of excitons and charge-transfer states induce significant energy losses in organic solar cells and poses questions on the equilibrium or the lack thereof between charge carriers and CT states. We aim to understand carrier dynamics in the TADF model OPV blend by directly monitoring charge carrier decay via transient photovoltage experiments and correlating it with photoluminescence emission decay.

AgNW(Silver nanowire)-based transparent electrodes are considered as the most suitable alternative ITO(indium tin oxide) due to excellent electrical, optical and mechanical properties. But, when we apply AgNW-based transparent electrode to optoelectronic devices such as LCD(liquid crystal display), OLED(organic light emitting diode), organic solar cells, a patterning process is essential. Generally, AgNW-based electrodes are patterned by a photolithographical technique and a laser direct technique. Although it is possible to fabricate very fine pattern shapes through photolithographical technique and laser direct patterning method, these methods are required complex processing and environmental hazard. Therefore, researching of a novel patterning method for AgNW-based electrode is indispensable. In this study, we control the surface energy of substrate by UV-Ozone treatment system, so we control the adhesion between the substrate and AgNW to form the patterned AgNW-based electrode. And we apply a patterned AgNW-based electrode to organic solar cells.
Hydrophilic treatment such as UV-Ozone, oxygen plasma can increase the surface energy of substrate, thus adhesion between substrate and materials can be increased due to absorption mechanism. So, we control the adhesion between the substrate and AgNWs through UV-Ozone treatment system. When we treat UV-Ozone on substrate, the adhesion between the substrate and AgNWs is strong. So, AgNWs remains on the substrate when the photocurable polymer is peeled off. On the other hand, when we do not treat UV-Ozone on substrate, the adhesion between the substrate and AgNWs is weak. So that, we can fabricate embedded AgNWs-based electrode. For that reason, we fabricate patterned AgNW-based electrode by adhesion patterning method. SEM(scanning electron microscopy) and EDS(energy dispersive spectrometry) mapping are used to confirm patterned line(500, 300, 100, and 50μm). When we apply patterned AgNW-based transparent electrode to organic solar cells, we get the good characteristics of organics solar cells.
As a result, we can control adhesion between the substrate and AgNWs by UV-Ozone treatment system. So, we can fabricate patterned AgNW-based electrode. When AgNW-based electrode is applied to organic solar cells, we confirm that organic solar cells with patterned AgNW-based electrode are exhibited similar or better performance than organic solar cells with ITO transparent electrode.

In this work we deal with an instance of the general problem
occurring when optimising multi-component materials: can components be optimised
separately or the optimisation should occur simultaneosly? We investigate this
problem from a computational perspective in the domain of donor-acceptor pairs
for organic photovoltaics, since most experimental research reports
optimisation of each component separately. We use a collection of organic
acceptors and donors we recently analysed [Kuzmich et al., Energy
Environ. Sci., 2017, 10, 295, Padula et al., Mater. Horiz.,
2019, 6, 343] to train non-linear Machine Learning
models of different families to predict the power conversion
efficiency of donor-acceptor pairs, considering computed electronic and
structural parameters of both components.
The trained models are then used to predict photovoltaic performance for
donor-acceptor combinations for which experimental data are not available in
the data set. We critically assess data structure, and the
usefulness of the trained models by predicting some donor-acceptor pairs that
recently appeared in the literature, and we propose the best combinations as
worth investigating experimentally.

While countless donors and acceptors for TADF emitters have been reported, there is still a thirst for efficient and deep red emitter compared to blue and green emitters because of the potential of brightness and color gamut. One of noticeable acceptor core, 1,8-naphthalimide (NI), has great electron withdrawing property due to its carbonyl group and unique low excited state level, which sounds out about applicability for high-efficiency red TADF emitter. From this, some of orange TADF emitters with outstanding light emitting performance using NI as acceptor core were reported^1,2, but there exists possibility for way longer wavelength range of emission to become a deep red TADF emitter.
In this work, deeper and more efficient red TADF emitter using NI as acceptor was realized through two tracks, using phenoxazine as more electron-donating donor moiety and introducing trifluoromethyl group as auxiliary electron-withdrawing group to N-phenylnaphthalimide. Three newly synthesized materials and PNIDMAC (which is equal to 6AcBIQ in the already reported paper¹) were fully characterized and applied as emitter in OLED device. Among the target molecules, diCF3PNIDMAC, with two equivalence of CF₃ group substituted into phenyl ring as electron withdrawing group (EWG) exhibited deep red emission with a photoluminescence peak at about 615nm and high PLQY up to 47%, maintaining TADF characteristic of the D-A molecule based on NI. OLED device using diCF3PNIDMAC as emitter featured EQE of nearly 13% with long-wavelength emission of electroluminescence peak greater than 600nm and CIE coordinate (0.56,0.43) when mCPCN is used as host material.
Reference: 1. Dyes and Pigments 2017, 144, 212-217. / 2. Advanced Materials 2018, 30, 1704961.

Ternary organic solar cells, a single active layer comprising three different components; have been demonstrated to be one of the most efficient ways to approach high-performance organic solar cells. But nevertheless, most of the ternary organic solar cells were characterized by the steady-state measurements, which are helpful but inadequate to fully understand the underlying charge carrier behavior at a short time scale. In this work, a comparison of the steady-state and time-dependent measurements was used to investigate the functionality of non-fullerene electron acceptors in ternary organic solar cells. The steady-state measurements indicate that non-fullerene electron acceptors can enlarge the absorption range of photoactive layer, suppress charge carrier recombination, reduce charge carrier transfer resistance and thereby increases photocurrent in ternary organic solar cells. The time-dependent measurements demonstrate that short charge carrier extraction time and high charge carrier mobility are responsible for enhanced photocurrent in ternary organic solar cells. Our studies provide a comprehensive method understanding underlying of enhanced efficiency of ternary organic solar cells.

Extensive research has been done in recent years for the development of fluorescent small organic molecules with aggregation-induced emission (AIE). The application of such solid-state organic luminogens has been widely used in organic light-emitting diodes (OLEDs), biological probes and fluorescent sensors due to their potential enhancement of emission in either solid states or in an aggregated state provided by the aqueous media. In order to produce efficient AIE materials, organic π-conjugate fluorophores play a crucial role with varying the photophysical and electrochemical properties which are strongly dependent on linking position of π-conjugates as well as the design of building blocks. It is believed that there are ample possibilities to modify these characteristic properties to successfully tune their optoelectronic properties. To increase the efficiency of an OLED device, it is desirable to develop a molecule with twisted conformation to generate AIE-active material which not only prevents the π-π stacking but also block the quenching channels. Carbazole being explored as one of the favourite synthon for materials chemists because of its unique characteristics such as rigid molecular structure, amorphous nature, inexpensive of the starting material, good chemical stability assisted by its fully aromatic system, excellent thermal and photochemical stability, good hole transporting ability and high triplet energy (~ 2.9 eV). Apart from these attractive features, due to its electron-rich nitrogen atom carbazole framework can allow large structural diversity to modulate the functional properties of the materials systematically. Therefore, carbazole based functional materials have been exemplified as a promising family for optoelectronic applications such as organic light-emitting diodes (OLEDs), solar cells (OPVs), field-effect transistors (OFETs) and in molecular sensors. It is important to note that multi-functionalization of mono and di-substituted dicyanovinyl carbazole derivatives are rarely reported in the literature. On the basis of above stated discussion, a series of carbazole based derivatives with different functional groups comprising of mono- and disubstituted dicyanovinyl functionalities (MD1, MD2, DD1, DD2) have been designed, synthesized and systematically characterized to effectively modulate the aggregation-induced emission effect by changing the linking position of carbazole and explore the simple structure-function relationship phenomenon. Dyes consist of N-phenylcarbazole (MD1 and DD1) and N-butylcarbazole (MD2 and DD2) as donor substituted at the various position of central carbazole with dicyanovinyl group as acceptor. The substitution effect of different donors on optical, electrochemical and thermal properties on carbazole-dicyanovinyl derivatives is unraveled. The dyes MD2 and DD2 showed red-shifted absorption and higher molar extinction coefficient as compared to MD1 and DD1 attributed to extended conjugation tendency of 3-substituted N-butylcarbazole as compared to N-phenyl linked carbazole. The photoluminescence studies revealed the role of auxiliary chromophores loading at C1, C6 and C8 positions of the central carbazole moiety in the excited state. All the compounds showed positive solvatochromism in excited states attributable to intramolecular charge transfer. The higher thermal stability of all the dyes reveals rigidity of carbazole derivatives. Interestingly, all the dyes in particular dyes consist of N-phenylcarbazole substituents (MD1 and DD1) showed aggregation-induced emission properties in THF-H₂O mixtures attributable to increased distortion in molecular configuration and restricted π–π stacking among adjacent molecules. OLED device was fabricated based on these dyes as dopant emitter along with a host (CBP) and experiencing the best performance in a doped device of MD2 with 3 wt% having EQE of 4.2% (current efficiency of 13.1 cd A^-1) and maximum brightness (L_max) of 6180 cd/m².

This paper reported correlation between surface morphology and work function in nanoscale. Surface nanoscale-morphology of an aluminum film was controlled using colloidal lithography. After the nanopatterning, the averaged work function was decreased from 4.24 eV to 3.94 eV. In addition, high resolution (resolution of 200 nm) Kelvin prove measurement revealed that there was work function difference of 0.31 eV between the top and bottom of the prepared nanospikes. These results indicate to develop stable low work function electrodes.

Low work function materials are essential for electrodes of electronic devices, for example organic light emitting diodes and organic solar cells. Generally, such low work function materials show high reactivity and are unstable, causing surface property changes when they are used in devices. On the other hand, some research reported that work function can be controlled by adjusting the surface roughness with the same material. In general, increment of surface roughness of a metal film causes reduction of the work function based on effect of the dipole barrier, as reported by Saito. However, detailed correlation between surface nanoscale-morphology and work function has not been clarified. Hence, in this study, we studied correlation between surface morphology and work function “in nanoscale” using an aluminum film. The aluminum film was patterned by colloidal lithography, and the work function of the film was evaluated in both millimeter and nanometer-level.
An aluminum film of 100-nm-thick was prepared on a silicon substrate by ion-beam sputtering. In order to analyze correlation between surface morphology and work function, surface of the aluminum film was nanostructured via colloidal lithography. The detailed fabrication process is as follows. Firstly, polystyrene nanoparticles (100 nm in diameter) dispersed as colloidal water solution were regularly aligned on the aluminum film by spin-coating. Then, the nanoparticles were slightly etched by oxygen plasma to separate each nanoparticle. The aluminum surface was etched to form nanospikes using etchant of acids mixture (phosphoric acid, acetic acid, and nitric acid) through the nanoparticles as a mask. Finally, the polystyrene nanoparticles were removed by oxygen plasma. In addition, an aluminum film without nanopatterns was also prepared as a reference. The work functions of the samples were measured by Kelvin probe method. We evaluated not only averaged work function of about 3 mm² area, but also nanoscale work function, resolution of 200 nm.
From atomic force microscopy evaluations, we observed that the aluminum film having nanospikes was successfully fabricated with 5.8 nm in arithmetical mean roughness and 49.5 nm in maximum peak to volley height. In the millimeter-level observation, the work function of the aluminum film with the nanospikes was decreased to 3.94 eV compared to that without the nanospikes (4.24 eV). This result indicated that work function of the aluminum film was decreased with increasing the surface roughness, as previously reported. On the other hand, in the nanometer-level observation, the work function changing was obtained along with the surface nanoscale-morphology changes. The maximum difference of the work function between the top and bottom of the nanospike was 0.31 eV. It is a first time to show the relationship between surface morphology and work function in nanoscale to the best of our knowledge. This result implies that electrons are easier to be emitted from top of the nanospikes rather than bottom of them. This is a promising step for developing stable low work function electrodes.

Ag₂S thin film can absorb the sun light from ultraviolet (UV) to near infrared (NIR) region due to its direct narrow band gap (Eg = 0.9 eV ~ 1.1 eV). For such a narrow band gap semiconductor, the related photoelectric conversion efficiency (PCE) can reach to about 30% in an ideal solar cell. It can be a promising semiconductor for next generation solar cell application, even the PCE is still low.
To obtain high PCE, the photoinduced charge carrier recombination at the interface of heterojunction or at the electrode surface must be reduced. Some works have been reported addressing this consideration, such as doping element to tune the Fermi level of Ag₂S, using electron transfer layer between the electrode and Ag₂S thin film and introducing intermediate layer to form cascade band structure. Despite these efforts, the Ag₂S thin film based solar cells are still featuring low PCE. One of the problems is how the electrode/Ag₂S interface affecting the photoinduced charge carrier recombination has not been well studied. Therefore, we should furtherly check out the photoinduced charge carrier transportation and recombination in mechanism. To our knowledge, the Schottky barrier will generate at the interface of n-type semiconductor and electrode, once the Fermi levels are different. The band is bending upward at each side of n-type semiconductor and form a “V-type” band structure in electrode/n-type semiconductor/ p-type semiconductor structur. The photoinduced charge carrier recombination will happen at the inappropriate interface of electrode and n-type semiconductor. To achieve higher photoelectric performance, such recombination at the electrode surface should be suppressed. Here, FTO surface was modified by SnS₂ layer to tune the Fermi level of FTO, which was deposited by thermal evaporation and post thermal annealing treatment. Photoelectrochemical method was carried out to evaluate the variation of charge carrier transfer resistance between the Ag₂S/ FTO interface. KPFM technique was used to study the Fermi level alignment of Ag₂S/TFO interface. Transient surface photovoltage (TSPV) technique has been employed to investigate the photoinduced charge carrier transportation and recombination between the Ag₂S/ TFO interface. Basing on these results, a more clear charge carrier recombination mechanism at the Ag₂S/ TFO interface is obtained, which may give more opportunities to furtherly increase power conversion efficiency of new type photoelectric system.

Recently, non-fullerene acceptors have attracted significant attention for organic photovoltaics (OPVs) due to their frontier energy levels and absorption spectra tunability and good photo/chemical stability rather than fullerene based acceptors. Moreover, with the superior compatibility with several donor polymers exhibiting well-aligned energy levels, complementary absorption and appropriate morphology, outstanding PCEs over 15% have been achieved. ^[1] However, most of the high performance non-fullerene acceptors are A-D-A type based on linear core structure which contains electron donating core unit and two electron deficient end-capping groups. Although the 2-dimentional (2D) star-shaped molecule has potential to exhibit good efficiency in solar cells, such as improved molar absorption coefficient ^[2] and face-on oriented columnar packing structures enhancing charge mobility in a vertical direction, ^[3] little studies have been done. In particular, a number of p-type star-shaped materials which contain electron-rich polycyclic aromatic hydrocarbon structures have been studied, ^[4] but the advancement of star-shaped n-type materials is still lagging behind.

In this study, triazine-based n-type 2D star-shaped molecules were designed and synthesized. Intramolecular non-covalent columbic interactions and rigid molecular structure were introduced to induce the structural planarity for enhanced the packing property and the D-A type structures are designed to control the optical/electrical properties with a push-pull effect. The optical/electrochemical properties were monitored by UV-Visible absorption spectroscopy and cyclic voltammetry, and the film morphology and crystallinity were analyzed by grazing-incidence wide-angle scattering and differential scanning calorimetry. It has also been applied as an n-type active layer material for OPVs, and will be presented in detail.


References
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2. Y. Lin, P. Cheng, Y. Li, and X. Zhan, Chem. Commun., 48, 4773 (2012)
3. T. Yasuda, T. Shimizu, F. Liu, G. Ungar, and T. Kato, J. Am. Chem. Soc. 133, 13437 (2011)
4. T. Wohrle, I. Wurzbach, J. Kirres, A. Kostidou, N. Kapernaum, J. Litterscheidt, J. C. Haenle, P. Staffeld, A. Baro, F. Giesselmann, and S. Laschat, Chem. Rev., 116, 1139 (2016)

Characterizing the density of states (DOS) width accurately is critical in understanding the charge-transport properties of organic semiconducting materials as broader DOS distributions lead to an inferior transport. From a morphological standpoint, the relative densities of ordered and disordered regions are known to affect charge transport properties in films; however, a comparison between molecular structures showing quantifiable ordered and disordered regions at an atomic-level and its impact on DOS widths and charge transport properties has yet to be made. In this work, the DOS distribution widths of two model conjugated polymer systems are characterized using three different techniques. A quantitative correlation between energetic disorder from band bending measurements and charge transport is established, providing direct experimental evidence that charge carrier mobility in disordered materials is compromised due to the relaxation of carriers into the tail states of the DOS. Distinction and quantification of ordered and disordered regions of thin films at an atomic-level was achieved using solid-state NMR spectroscopy. An ability to compare solid-state films morphologies of organic semiconducting polymers to energetic disorder, and in turn charge transport, can provide useful guidelines for applications of organic conjugated polymers in pertinent devices.

Organic semiconductors offer many advantages for energy conversion, saving and storage applications. However the underlying processes that govern electrical processes such as charge transport, the separation of photogenerated charge, and carrier recombination, are not well-understood. New experimental and theoretical approaches to correlate molecular structure with electronic processes in organic semiconductors are valuable for gaining insights into smart design for this class of materials. In this talk I will discuss our recent results on coupling electrical and optical spectroscopies to study structure-function relationships in organic semiconductors, such as polaron formation and transport in donor-acceptor polymers for photovoltaics and the influence of molecular morphology on the performance of piezoelectric polymers. By combining optical spectroscopy with device physics and theoretical modeling, we are able to identify relevant aspects of molecular structure and morphology that determine the electronic response of organic semiconducting films.

The development of third generation photovoltaics is massively based on the use of materials displaying a heterogeneous morphology at the meso or nanoscopic scales, or nanostructured by purpose. Whatever the technology, understanding how the nanostructure impacts the electronic transport and photocarrier dynamics is vital for device optimization. This is especially true for organic bulk heterojunctions (BHJs) based solar cells, whose performance continue to be improved over time thanks to the joint efforts of chemists, physicists and material scientists. Despite fierce competition with hybrid perovskites, organic photovoltaics have indeed still a stay in the competition, thanks in particular to the introduction of novel non-fullerene acceptors. As in the case of their fullerene-based elders, optimizing the film morphology and phase composition of novel donor-acceptor blends remains the key to achieve an efficient photocarrier generation, to balance properly the carrier mobilities and to minimize the losses by recombination.
Since their introduction in the 1980s, scanning probe microscopy (SPM) techniques have been continuously improved, and have reach the ability to map not only the surface topography but also a huge range of local functional mechanical, chemical, electronic and optical properties at the nanoscale. Several advanced modes of the atomic force microscope (AFM) are nowadays commercialy available from SPM manufacturers, and are (or could be) widely used in routine by the OPV community to characterize the BHJs morphology and phase composition (for instance by quantitative nanomechanical imaging) and their optoelectronic properties (by photo-conducting AFM or surface photovoltage imaging). Some others are still confidential, but will probably experience strong growth in the coming years. In particular, time-resolved electrostatic and potentiometric variants of the AFM [1-6] hold great promise, for they are developed with the aim to map the photo-carrier dynamics and lifetime at the nanoscale.
In this communication, we will present the works undertaken in our group to implement time-resolved imaging modes of the charge dynamics based on the Kelvin Probe Force microscope (tr-KPFM for time-resolved KPFM) combined with non contact AFM under ultrahigh vacuum. Two complementary modes will be introduced: KPFM under frequency-modulated illumination [5] and pump-probe KPFM [6], both operated in spectroscopic data cube mode. These techniques have been applied to model BHJ systems including PTB7-PCBM and NFA-based blends, which phase composition was pre-characterized by quantitative nanomechanical imaging in peak force AFM. It will be shown that in these systems, the surface photovoltage decays probed by tr-KPFM originates from trap-delayed process. Our results stress also the crucial need to combine several KPFM modes to disclose the full temporal range of dynamical process that can coexist in these materials.

[1] Takihara, M.; Takahashi, T.; Ujihara, T. Appl. Phys. Lett. 2008, 93, 021902.
[2] Shao, G.; Glaz, M. S.; Ma, F.; Ju, H.; Ginger, D. S. ACS Nano 2014, 8, 10799−10807.
[3] Cox, P. A.; Glaz, M. S.; Harrison, J. S.; Peurifoy, S. R.; Coffey, D .C.; Ginger, D. S. J. Phys. Chem. Lett.2015 6 2852-2858.
[4] Collins, L.; Ahmadi, M.; Wu, T.; Hu, B.; Kalinin, S. V.; Jesse, S. ACS Nano 2017, 11, 8717-8729.
[5] Pablo A. Fernández Garrillo, P. A.; Borowik, L.; Caffy, F.; Demadrille, R.; Grévin, B.; ACS Appl. Mater. Interfaces2016, 8, 4531460-31468.
[6] Murawski, J.; Graupner, T.; Milde, P.; Raupach, R.; Zerweck-Trogisch, U.; Eng, L. M. Journal of Applied Physics 2015, 118, 154302.

Transmission electron microscopy (TEM) of conjugated polymers has remained a challenge because resolution is limited by the electron dose the sample can handle. We have characterized the effects of beam damage on poly(3-hexylthiophene) (P3HT), poly(3-dodecylthiophene-2,5-diyl) (P3DDT), and poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3’’’-di(2-octyldodecyl)-2,2’;5’,2’’;5’’,2’’’-quaterthiophene-5,5’’’-diyl)] (PffBT4T-2OD) via electron diffraction and electron energy-loss spectroscopy (EELS). Critical dose D_C values were calculated from the decay of diffraction and low-loss EELS peaks as functions of dose rate, temperature, beam size, and the addition of antioxidants. At room temperature, D_C first increases then decreases with increasing dose rate, whereas at cryogenic conditions this dose rate dependence is less pronounced and the overall D_C increases; these results suggest that the main mechanism for beam damage in conjugated polymers is diffusion of free radicals. Thus, we show with both Dc experiments and high-resolution TEM that the addition of free radical scavengers such as butylated hydroxytoluene (BHT) mitigates beam damage at room temperature. We also observe that D_C increases with decreasing beam size, suggesting that convergent beam techniques are amenable for beam-sensitive materials. Thus, we demonstrate the use of scanning nanodiffraction to map π-π stacking in several conjugated polymers.

Printing technology is set to enable the high-throughput low-cost and customized fabrication of optoelectronic and sensors devices. For this goal to become a reality, this functional printing approach should encompass a material process development which enables high device performance and industrial compatibility, thus enabling a facile transfer of research results into consumer applications.

In this contribution, I will present the fabrication of fully printed organic photodiodes (OPDs) by digital printing techniques (e.g. aerosol and inkjet). The devices show mechanical flexibility, semitransparency and an excellent reproducibility. Moreover, I will demonstrate their integration into fully printed OPD 256-pixel arrays. This was performed through a micro-patterning process based on printed dewetting structures that guide the self-alignment of the functional inks, improving printing resolution, registration accuracy and performance reproducibility. Secondly, I will discuss the use of the ink formulation as way to access and tailor material optoelectronic properties. In this direction, we have focused on the control of the molecular order in organic semiconductors though an inkjet printing process. This enabled us to deposit functional polymers with a high degree of alignment and explore its use in polarization sensitive OPDs. Finally, I will outline our recent efforts in the fabrication of inkjet printed OPDs based on novel non-fullerene acceptors (NFAs). The devices show a photo-response up to 800nm and reach record responsivities of 400mA/W as well as cut-off frequencies surpassing 2MHz. Furthermore, we demonstrate the successful decoupling of the optical and rheological properties by using a visibly transparent polymer donor and color-selective NFAs. This approach offers spectral flexibility without the need for a variation in process parameters. The choice of NFA enabled devices with color selectivity in the ranges of 400-600nm and 500-800nm.