Lessons Learned in Organic Optoelectronics | Chemistry of Materials

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Lessons learned in organic optoelectronics Olle Inganäs Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01220 • Publication Date (Web): 09 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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Chemistry of Materials

Lessons learned in organic optoelectronics Olle Inganäs* Biomolecular and organic electronics, Dept. Physics, Chemistry and Biology Linköping University, 58183 Linköping, Sweden ABSTRACT: The contributions of Jean-Luc Brédas to the science of organic optoelectronics are immense, and so are the skills of communications in his talks and papers. They have been very influential and shaped the development of organic optoelectronics over a long time. This Festschrift contribution is a narrative of the impact of his work in my own scientific and technological studies, and a way of acknowledging his great influence. Thanks!

The geometry of conjugated molecules and polymers has a very direct impact on the electronic structure of these compounds. Thus the chemical structure, which sets the framework for quantum theoretical calculations of electronic structures, must be extended with the geometry of the molecule or polymer in real space, in order to fully account for the electronic structure. Even more, when several molecules or polymers are packed into a solid three dimensional material, interactions between them strongly affect both geometry and electronic structure. This sets the framework for electronic structure, electronic and optical properties and for electronic transport in the organic solid state. The contributions of Jean-Luc Brédas (JLB) in delineating the many facets of these physical phenomena have been guiding lights in the field of organic electronics over the last four decades, and I am happy to acknowledge the strong influence they have had in my own choice of topics and issues. How could I better express that debt than by recounting some of the studies where JLB’s work made a strong impact, through his contributions to the literature and discussion on organic solid state matter and organic optoelectronics?

labs making the substituted polythiophenes were that of Shu Hotta in Japan and Jan-Erik Österholm at Neste Oy in Finland. I was lucky to be part of a Scandinavian project with the Finnish company Neste Oy, and had early access to a full family of alkyl substituted polythiophenes. I discovered in the lab that thin films of spincoated poly(3-alkylthiophenes) (P3AT) would change colour when heated, only to return to their original colour upon cooling. This was the starting point for studies of thermochromism in substituted polythiophenes 1-2. The first explanation was that the torsion of the polythiophene’ main chain was affected by the size and location of alkyl chain anchored to thiophene in the 3 position. If torsion was increased by higher temperatures, it could be caused by the change of steric hindrance between alkyl chains, and between alkyl chains and main chain, as the side chain would expand by heating. The possibility that such a torsion would cause the observed optical changes was verified through quantum chemical calculations from JLB’s student Bérengère Themans 3, at that time visiting at Linköping University together with JLB. The role of the alkyl chains in the temperature induced change of electronic structure was elaborated in the infrared spectroscopy studies 4 5-7 from Giuseppe Zerbi in Politecnico di Milano, Italy, who did extensive studies verifying the molecular mechanism, where increase of temperature cause swelling of the side chain and thus causes increased steric hindrance. This process force the main chain out of planarity, and increase the bandgap, thus blueshifting the optical absorption. The side chain location and geometry, positioned along the main chain through random or regular patterns, also contribute to control the geometry, which are available for insertion of dopants, and required for the metallic state formation of the doped polymers. The desirability of processible synthetic metals was a common preoccupation of

Conformations of conjugated polymers and molecules--thermochromism and solvatochromism The conformational flexibility of macromolecules is the defining trait of polymer materials. With only a repetitive chemical structure of the mer in the polymer, how can so many different types of behavior be obtained? Where Flory in his majestic opus “Statistical Mechanics of Chain Molecules” describes the conformational flexibility of polymers chains, the impact of this flexibility on the electronic structure became most visible in the early studies of soluble conjugated polymers, in the mid 1980ies Among the first

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that time, when synthetic metals was one of the keywords for this field of science. It turned out that the semiconducting state of electronic polymers is of much larger interest today, but this was to become. The thermal instability of the doped poly(3-alkylthiophenes) 8 9was a severe problem hindering processing, and processing in the melt and in the solution phase was the attraction for the new class of fully soluble electronic polymers. With the conflicting request for space for dopants and alkyl side chains, we noted that if dopants would be found to the side of the main chain, they could compete for the space of side chains, and thus be kicked out of position by thermal processing of the material. A more plausible location for the dopants would be in an equatorial position, where side chains would be of lesser consequence. The first studies of the x-ray structure of poly(3alkylthiophenes) were appearing at this time 10 11-12, and evidence was found for a lamellar ordering where main chains were separated by alkyl chains in one direction, and with a thiophene-thiophene distance of 3,8 Å in the zdirection. At the time, the efforts to remove the thermal instability included pruning of the side chain gallery, by making random or regular copolymers where thiophene and alkylthiophene were mixed to leave free space in between alkyl side chains along the main chain polythiophene. Qibing Pei did this chemical polymerization and obtained materials with higher stability in the doped state13 14. While the issue of the specific location of dopants in highly doped polyalkylthiophenes may have mainly academic interest now, the crystal geometry of the polyalkylthiophenes and the possibility of interdigitation of side chains, as well as the mode of insertion of fullerenes in P3AT’s have found much attention since then, as the consequences for charge generation and charge transport in photovoltaic functions of P3AT’s and related materials have collected much study.

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photoluminescence quantum yield of many of these in the solid form was a consequence of the quenching due to interactions between main chains. By diluting with octylphenyl substituents, we could separate the main chain also in the solid state and improve the quantum yield. This polymer also went into one of the first optically pumped lasers 20 21, in a microcavity geometry with a thin slice of the optical polymer located in between two dielectric Bragg mirrors, and could be driven into lasing conditions by optical pumping. Control of the optical properties by design of side chains and positioning of side chains thus gave full access to bandgap and colour over the full visible range. Conjugated polyelectrolytes biodetection

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Early generations of soluble conjugated polymer were soluble in hydrocarbon solvents, matching the solubility properties of alkyl side chains first used in substituted conjugated polymers. The possibility of attaching more polar side chains which can allow solubility in polar solvents was demonstrated by Fred Wudl 22, in synthesis of alkylsulphonated polythiophenes. The set of conjugated polyelectrolytes have grown tremendously and is now an important element in the development of organic bioelectronics. It is the compatibility of conjugated polyelectrolytes with biological environments, read aqueous environments, that give the chance to make molecular contact between conjugated polymers and biomolecules, biomembranes, cells and tissue. A first and most intimate contact was that formed in polythiophene substituted with zwitterionic amino acid side chains23, later also other amino acids24 . Here the electronic properties of the main chain polythiophene directly in contact with an amino acid give the possibility of interaction with other biomolecular species found in water. This demonstration came long after the synthesis of the first amino acid side chain polythiophene23, and later focused on the interaction with the most important polyelectrolyte found on Earth, DNA25. We demonstrated that DNA interacting with the polymer would cause a change of colour in emission. The change of electronic structure demonstrated in this behavior could be due to changes of the conformation of the single chain interacting with DNA; more importantly, it could be due to changes of packing in the hydrophobic polythiophenes interacting with each other, and with the hydrophobic interior of the DNA chain26-27. Similar kind of phenomena, where electronic structure is changed due to conformational change of the main chain with changes of packing, were observed with an extended family of substituted polythiophenes and oligothiophenes, and also extended to interactions with long and short DNA and RNA chains. The possibility to use the colour change as a probe of DNA hybridization was demonstrated25, 28, and even indications of single mismatch in DNA sequences was obtained. Turning to the next big class of biological macromolecules in polyelectrolyte format, proteins, revealed

Designing bandgaps by chemical and geometrical structure The possibility to design the geometry and optical bandgap by choice and pattern of substitution of a main chain polythiophene was an inspiration for making emitter materials for OLEDs. Polythiophenes with alkyl and alkoxy side chains were designed for emission from the blue to the near infrared optical spectrum 15. They could be combined with other materials for making white light in one of the first white organic OLEDs 16, and they could be blended into a single layer sandwich structure, where the emission colour varied with the applied voltage 17. While our choice of polymer family for this function had the limitation of a low quantum yield for photoluminescence, and therefore also for electroluminescence in devices, the versatility of synthesis enabled a rapid throughput from concept to device. With the model of steric hindrance, due to alkyl side chains affecting the main chain torsion, Mats Andersson at Chalmers University of Technology, Göteborg, made blue, green 18, red and NIR emitting 19 polythiophenes. The low

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that detection of misfolded proteins was possible, through simple colour changes of polythiophenes and oligothiophenes in contact with these proteins29. Further studies using better defined oligothiophenes30-31, compared to the polythiophenes with varying polydispersity and defects, showed that interaction of the oligoelectrolytes with biological polyelectrolytes made detection of biomolecule function possible.

Early attempts to build polymer/polymer junctions as materials for photovoltaics in the early 1990’ies were based on polythiophene/polythiophene combinations. Simple bilayers could be operated, but gave low photovoltages and very poor photocurrents. As the combination of fullerenes and conjugated polymers was demonstrated around 1992 50, the very efficient quenching of the photoluminescence of the polymer donor by the fullerene acceptor became the dramatic crucial experiment to verify charge transfer. This very intuitive observation, reported first for polyparaphenylenevinylene (PPV) polymers and later for polythiophene polymers, was extensively used to select materials for efficient charge generation. With the demonstration of high photocurrent from P3HT/fullerene blends51 induced in devices under operation, a very strong interest appeared to develop these materials for real world solar modules.

The interaction of the electronic polyelectrolytes with the protein insulin under misfolding, showed that both kinetics and shape of the protein fibrils formed in the protein misfolding could be manipulated by the presence of the probe molecule32. Further studies demonstrated that these probe molecules could also be relevant in vivo, as a drug for influencing the development of protein misfolding diseases. As these diseases are rapidly increasing in the aging populations of the affluent world, the need for drugs and detectors is very large.

With the observation of high bandgap, narrow optical absorption and low photovoltages in P3AT/fullerene blends, and the known poor thermal stability of the P3ATs, we focused on developing new polymer systems where low bandgaps, thermal stability and sufficient electronic transport could be obtained. We used the alternating copolymers of fluorene with donor-acceptor-donor comonomers (APFOs), to manipulate the electronic structure52. The notion of alternating comonomers for manipulating the electronic structure has been proposed by Havinga and others at an earlier phase, and we were quite successful in modulating the electronic structure through choice of comonomers, and in particular by modifying the strength of the acceptor part of the donor-acceptor-donor comonomer.

Further developments of this class of poly- and oligoelectrolytes of thiophene has been done by Peter Nilsson, who have combined them with biological and biochemical investigations. Investigations on the different types of misfolded protein deposits in pathological tissue using well defined oligomers enable classifications of stages and types of deposits33-36. Some compounds act as antiprion agents40-41 in mice, opening pathways for treatment of prion disease, a special form of protein misfolding diseases. These developments also include synthesis of oligoelectrolytes that detect cellulose in biological matrixes, such as biofilms42 and plants43, but also in technical processes of paper and pulp processing44. The family of biological macromolecules possible to detect is thus extended with carbohydrates, beyond that of DNA, RNA and proteins.

With the high bandgap version of such materials, we could obtain 1.1 V photovoltage, also moderate photocurrents, in the blends with fullerenes53. While the first generations gave a solar power conversions close to that of the P3HT/fullerenes at that time, it rapidly became clear that decreasing the bandgap to increase optical overlap with solar spectrum did not give a sufficient reward in photocurrent, with the inevitable reduction of photovoltage. This was mainly due to the poor optical absorption in the near infrared range for the APFOs. Our transfer matrix models of optical power dissipation in multilayer thin films54 really predicted that we would have to find donor polymers, where the optical absorption increased at least linearly with the increase of wavelength, in order to collect the optical power in the red and infrared part of the solar spectrum55.

In a parallel route, the assembly of materials using interactions between poly and oligoelectrolytes based on the thiophene main chain and synthetic polypeptides 45 46 gave chiral supermolecules from non-chiral constituents, and interactions with biological nanofibrils of misfolded insulin protein and metallic conjugated polyelectrolytes can give conducting nanowires47. Through stretch aligned DNA interacting with such metallic conjugated polyelectrolytes, electrochemical transistors could be constructed48. The modes of interactions between charged biopolymers and charged conjugated polyelectrolytes include the electrostatic, van der Waals and hydrophobic, and opens rich possibilities for materials construction49.

Regular donor-acceptor copolymers were synthesized and fulfilled this request of steep absorption increase with increasing wavelengths. They also delivered higher photocurrents, and high internal quantum efficiencies for charge generation. This was the time of bulk heterojunctions reaching PCEs of 5-7 %, indicating that there was more to win than the limit found for P3HT/PCBM around 5 %.

Geometry and charge separation in organic photovoltaics

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A paper 56 from Mondal and JLB influenced my thinking heavily at this time and drove us to make alternating copolymers with simpler structures. As noted in this paper, we can expect that the overlap of wave function of the ground and excited states of the donor polymer sets the oscillator strength for optical transitions. By reducing the complexity of the donor-acceptor-donor comonomer to just an acceptor, higher symmetry would be possible and expected. And this was an approach that delivered the desired behavior, as demonstrated in the blue thiophenequinoline copolymers (TQ) 57 and the thiophene-isoindigo copolymers58-59, that became favorite objects of study in my lab, after synthesis by Ergang Wang and coworkers at Chalmers University of Technology.

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studied for the second generation of quantum technology. While the debate is still ongoing, there has also been suggestions that quantum coherence may play a key role in the OPV 69 70, or might use quantum coherence for better performance. JLB is contributing in this discussion 71. We have recently discovered a signature of vibronic coherence in the charge generation process in a particular bulk heterojunction, a ternary blend of two polymer donors and a fullerene derivative. This signature is only found in some stoichiometries of the ternary blend, with the higher photocurrent, and is visible as an oscillating amplitude of the transient absorption for the charged species. This is extending for the first few hundreds of femtoseconds, and has a period of ≈95 fs, corresponding to a vibrational mode of ≈370 cm-1. We measure a strong Raman vibration around this wavenumber in the ternary blends. DFT calculations indicate that this is a translational breathing mode of one of the donor polymers. The calculations have been done by Mathieu Linares, and can best be visualized by the dynamics of motion, as evidenced in a video file (link). This is the first and only image in this paper, and illustrates also how far the advances in quantum theory and molecular visualization has taken us over the last decades.

The high photovoltage was sacrificed in low bandgap polymers, but we could still obtain photovoltages in donor/fullerene blends with donor polymers extending absorption up to 1200 nm. The orthodoxy at this time predicted that the driving energy, an energy gap between donor and acceptor LUMO orbitals, generally assigned to be 0.3 eV, would be necessary for overcoming the Coulombic attraction of separated charges. As we had already demonstrated moderate charge generation capability in APFO blends where measured the gap between donor LUMO and acceptor LUMO to be smaller than what our electrochemical analysis could discern52, 60, we knew that this was an erroneous analysis. The mechanism of charge generation thus needed deeper study.

It thus appears that the conversion of the excited state to a charge separated state is coupled to electronic and vibrational coherence, as documented in these oscillations. Whether this hypothesis is valid or not will take more studies to evaluate, but it can be a source of inspiration for the design of novel donor and acceptor materials, as well as a source of problems for quantum theory to further develop the understanding of how charges are generated from excited molecules in bulk heterojunctions.

The thermodynamic analysis of donor/acceptor bulk heterojunctions was pursued by Koen Vandewal, who after his PhD at Hasselt University came to Linköping for a postdoc. The nature of the charge transfer state at the donor/acceptor interface and the connection to the photovoltage is his contribution 61 62 63, and through that we now better know that the photovoltaic bulk heterojunctions fall under the same analysis of steady state thermodynamics due to Shockley and Queisser. As the character of that charge transfer state is the obvious target for so many quantum chemical calculations, sometimes integrating with the molecular mechanics to describe the donor and acceptor interface dynamic, JLB has been a strong voice in arguing the complexity of the trajectories from excited states to fully separated charges64. Our understanding of that process is still incomplete, in my view, and may incorporate physics that goes beyond the electron transfer theories of Marcus. The impact of this complexity is visible in the demonstration of efficient charge generation with aligned orbitals, and negligible driving force65. A recent contribution from JLB fortifies my impressions66.

The interplay between theory, chemical design and device studies is essential for progress in all of these areas of study of organic solids and organic optoelectronics, So many of the developments occur by surprising discovery or serendipity, later to be rationalized by modelling and theory, and turning in the next generation into more refined design of molecules and polymers. The need for the communicators of science bridging these diverse topics is great, and JLB has been one of the strongest voices, bringing detailed analysis and developing this into conceptual synthesis in the intellectual realm. Many thanks!

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

Coherence has been widely discussed in the biophysics community as a possible element of photosynthetic systems 67 68, operating at Earth temperature rather than the low temperatures where quantum coherence properties are now

A video file(MPEG) show the outcome of DFT calculations of Raman modes in a donor polymer; the mode visualized is the one that corresponds to the element of vibrational coherence observed

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Chemistry of Materials 10. Winokur, M. J.; Spiegel, D.; Kim, Y.; Hotta, S.; Heeger, A. J., Structural and absorption studies of the thermochromic transition in poly (3-hexylthiophene). Synth. Met. 1989, 28, 419-426. 11. Gustafsson, G.; Inganas, O.; Osterholm, H.; Laakso, J., X-Ray-Diffraction And Infrared-Spectroscopy Studies Of Oriented Poly(3-Alkylthiophenes). Polymer 1991, 32, 15741580. 12. Prosa, T. J.; Winokur, M. J.; Moulton, J.; Smith, P.; Heeger, A. J., X-ray structural studies of poly (3alkylthiophenes): an example of an inverse comb. Macromolecules 1992, 25, 4364-4372. 13. Pei, Q.; Jarvinen, H.; Osterholm, J. E.; Inganas, O.; Laakso, J., Poly[3-(4-Octylphenyl)Thiophene], A New Processible Conducting Polymer. Macromolecules 1992, 25, 4297-4301. 14. Pei, Q. B.; Inganas, O.; Osterholm, J. E.; Laakso, J., Electrically Conducting Copolymers From 3-Octylthiophene And 3-Methylthiophene. Polymer 1993, 34, 247-252. 15. Andersson, M. R.; Berggren, M.; Inganas, O.; Gustafsson, G.; Gustafsson-Carlberg, J. C.; Selse, D.; Hjertberg, T.; Wennerstrom, O., Electroluminescence from substituted poly (thiophenes): from blue to near-infrared. Macromolecules 1995, 28, 7525-7529. 16. Berggren, M.; Gustafsson, G.; Inganas, O.; Andersson, M. R.; Hjertberg, T.; Wennerstrom, O., WhiteLight From An Electroluminescent Diode Made From Poly[3(4-Octylphenyl)-2,2'-Bithiophene] And An Oxadiazole Derivative. J. Appl.Phys. 1994, 76, 7530-7534. 17. Berggren, M.; Inganas, O.; Gustafsson, G.; Rasmusson, J.; Andersson, M. R.; Hjertberg, T.; Wennerstrom, O., Light-Emitting-Diodes With Variable Colors From Polymer Blends. Nature 1994, 372, 444-446. 18. Berggren, M.; Gustaffson, G.; Inganas, O.; Andersson, M. R.; Wennerstrom, O.; Hjertberg, T., Green Electroluminescence In Poly-(3-Cyclohexylthiophene) LightEmitting-Diodes. Adv. Mater. 1994, 6, 488-490. 19. Berggren, M.; Gustafsson, G.; Inganas, O.; Andersson, M. R.; Wennerstrom, O.; Hjertberg, T., Thermal Control Of Near-Infrared And Visible Electroluminescence In Alkyl-Phenyl Substituted Polythiophenes. Appl. Phys. Lett. 1994, 65, 1489-1491. 20. Granlund, T.; Theander, M.; Berggren, M.; Andersson, M.; Ruzeckas, A.; Sundstrom, V.; Bjork, G.; Granstrom, M.; Inganas, O., A polythiophene microcavity laser. Chem. Phys. Lett. 1998, 288, 879-884. 21. Granlund, T.; Theander, M.; Berggren, M.; Andersson, M.; Ruzeckas, A.; Sundstrom, V.; Bjork, G.; Granstrom, M.; Inganas, O., A polythiophene microcavity laser (vol 288, pg 20, 1998). Chem. Phys. Lett. 1999, 310, 577-577. 22. Patil, A. O.; Ikenoue, Y.; Wudl, F.; Heeger, A. J., Water soluble conducting polymers. J. Am.Chem. Soc. 1987, 109, 1858-1859.

intransient spectroscopy studies of a ternary donor/donor/fullerene blend. The calculations and visualization come from Mathieu Linares, KTH, Stockholm,

AUTHOR INFORMATION Corresponding Author * Olle Inganäs, professor emeritus, Biomolecular and organic electronics, Dept.Physics, Chemistry and Biology, Linköping University, S-581 83 Linköping, Sweden

ACKNOWLEDGMENT Mathiue Linares at KTH, Stockholm did the DFT calculations and visualization shown in the video file, and is gratefully acknowledged. Critical reading of the manuscript from Feng Gao and Qingzhen Bian is much appreciated, as well in formatting the manuscript. Funding from the Knut and Alice Wallenberg Foundation of a personal Wallenberg Scholar for me is much appreciated.

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Lessons Learned in Organic Optoelectronics | Chemistry of Materials

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