Category: Materials Thru Life

Scope of Research

My research interests center on organic semiconductors and functional composite materials regarding computer-aided materials design, sustainable synthetic protocols, solid-state morphology manipulation, and applications in integrated energy transduction devices. The state-of-the-art research efforts in organic semiconductors allow for relatively well-predicted and modulated molecule-level properties by virtue of synergic computational structure design and synthetic organic chemistry. Thinking from a material perspective, however, one needs to consider how the structure and property of a given isolated molecule relate to its aggregated polymorphs, and more importantly, how these polymorphs can be achieved controllably to reveal useful functionalities in favor of high-efficiency semiconducting devices modules.

The ultimate goals of my prospective research are to draw guideline correlations between material chemical structures and their bulk properties (e.g., optical, electronic, thermal, and mechanical) and device performances, and thus promote guidelines from materials design and commercialization. My research will be dedicated to addressing the challenges and understanding: (1) how a given material can be produced efficiently with a positive environmental impact, (2) to what extent molecular conjugation can be controlled in 2- or 3-dimensions and correlation to the consequent properties, (3) how well materials properties may be manipulated employing supramolecular chemistry and molecular self-assembly, (4) real-time visualization of materials bulk nanostructures using non-destructive techniques, and (5) how reliable materials design and morphology control would fulfill specific device requirements and long-term durability. With the aim for energy efficient research frontier, my current research proposals are in line with the demand from many federal agencies, such as DOE, ONR, DARPA, and NSF. I will also organize research and grant proposal collaborations with federal research institutions, especially with national lab facilities such as Advanced Light Source (ALS) and Stanford Synchrotron Radiation Lightsource (SSRL).

Two years of independent research at UC Santa Barbara and two years leading startup activity at UCLA equip me with rich experiences in research diversity and collaborations, as well as broad scope from fundamental science to rapid commercialization. I have been leading federal grant proposal preparation both at the academic and small business levels. My research perspectives can strengthen fundamental research and establish long-term collaborations both internally and with other institutions.

I would like to pursue my research goals by implementing the following three inherently related research perspectives, looking at:

(1) Molecular design of 2- and 3-dimensional (2D and 3D) organic semiconductors and how these molecular candidates can be prepared under environmentally friendly synthetic protocols;

(2) Delivering useful functionalities from aggregated molecular polymorphs through supramolecular chemistry;

(3) A synergic consideration of material design, processing, and device structure engineering to reveal the next generation of adaptive organic electronic devices.

2-Dimensional and 3-Dimensional Organic Semiconductors: Sustainable Catalysis, Structure-Property-Function Relationship, and Optoelectronic Application

Synthetic protocols provide means en route to a rich library of organic functional materials in favor of specific requirements for use in optoelectronic devices. Coupling reaction between arenes is one of the key steps in preparing organic p-conjugated materials. The common approaches, such as Suzuki and Stille reactions, require the use of Pd⁰ as a catalyst, e.g., Pd(PPh₃)₄ and Pd₂dba₃.^[1] Such Pd⁰ species often suffer from irreversible agglomerate during reaction thus leads to ceased reactivity, which also adversely affects the quality of the polymers.^[2] During my Ph.D. studies, I have developed a new synthetic pathway in realizing surfactant-free gold atomic clusters in boiling N,N-dimethylformamide (Figure 1A),^[3] which can further be adopted for other noble metals, e.g., platinum, palladium, and silver. The resulting metal clusters can be dispersed in water and show stability over months under ambient condition. A relevant study by Prof. Kawasaki has shown possible to incorporate such Pd⁰ clusters in Suzuki coupling reactions,^[4] which can be readily recycled without losing its catalytic activity. Considering organic semiconductors often require hash reaction conditions and toxic solvent environment, my intention is to optimize the synthetic protocol of such Pd⁰ clusters and to explore the potential application in promoting synthesis of conjugated materials in green reaction media under both Suzuki coupling and direct arylation reaction manners (Figure 1B).^[5] It is worth noting that such reaction manner may greatly simplify the post purification process of conjugated materials, e.g., with precipitation and extraction, rather than conventional chromatography, which in turn, reduces potential consumption of organic solvents. In a general perspective, these Pd⁰ clusters could potentially be employed in substitution of all current commercial Pd⁰ catalysts, by providing a more cost-effective and environmentally friendly yet high efficiency production. A successful prototype study will also open up opportunities to optimize systematically a variety of organic reactions that involve other transition metal catalysis.

Upon establishing reliable synthetic protocols, one intends to develop organic semiconductors that allow for charge transport along 2-dimension (2D), structurally alike graphene derivatives. Theoretical model (e.g., density functional theory, DFT) will be carried out to assist in molecule design and prescreen of electronic properties in a single-molecule level in the gas phase.^[6] Figures 1C and 1D illustrate examples

where conjugation extension is possible along various molecular edges. Carbon-carbon coupling and visible-light triggered ring-closing reactions are proposed to be implemented during materials synthesis. Energy band-gaps of these molecules can be well tuned by incorporating building blocks with varied electron affinities. As illustrated in Figure 2, chemical doping with boron or imide to a given 2D molecular framework will give rise to a local electron-deficient region (denoted as red), while nitrogen doping or pedant electron-rich moieties, e.g., thiophene or pyrrole, would create electron-rich areas (denoted as blue). Efficient 3-dimensional charge transport could thus be guaranteed, i.e., both along molecular backbone and through layered p-p stacking direction (Figure 2, lower right). The optical, electronic properties of these 2D molecular “sheet” and layered aggregates are to be studied in correlation with their structural features. Enlarged p-conjugation plane will potentially strengthen intermolecular interactions through electron delocalization. The resulting layer-by-layer stacking of these chromophores may initiate new opportunities for understanding fundamental questions (e.g., charge transport dynamics and possible pathways) in organic semiconductors research.

Molecular structure-property relationship has been a long-standing topic in material chemistry research. Much of current efforts are case-specific within each individual molecular structure and far less general in interpreting a guideline picture. A substantial further study is in urgent necessity to provide a more general relation that may direct structure design of a rich library of molecules. We would invest efforts to study systematically how changes in molecular structures affect materials properties, particularly in the regard of molecular length and symmetry. Molecular weights of conjugated polymers have proven deviating materials properties, for example charge carrier mobility, among several orders of magnitude. The challenges in drawing a clear conclusion from polymeric materials come from their statistic distribution of chemical structures as well as poor control over a certain molecular weight and polydispersity. Molecular materials, in contrast, possess well-defined chemical structures, given by controllable reaction selectivity in organic synthesis. A previous study done by Prof. Briseno showcases a series of poly-3-hexylthiophene analogues with precisely controlled molecular lengths.^[7] Similar protocol can be adopted for studying intrinsic structural features in a series of structurally well-defined electron-donor (D) and electron-acceptor (A) alternating molecular frameworks, as shown in Figure 1D. Structural precision in such D-A molecules give rise to a more quantitative information on how subtle chemical structure changes can impact material bulk properties, and even more relevant, how electron delocalizes within donor-acceptor structures and how far the delocalization can occur and stabilize, i.e., the effective saturation length for electron transitions. In addition, the effect of molecular symmetry on their properties will be found on the basis of the well-defined chemical structures in this molecular systems. A more comprehensive series of studies may provide critical information for chemists interested in structural transition from molecules to polymers, for physicists interested in how structural alteration may be translated in interpreting electron and energy transfer mechanisms, and for device engineers who are eager to reliable high performance device architectures.

Materials properties correlate directly to the molecular conjugation, which guarantees pathways for intramolecular electron communication. The capability to modulate conjugation structures within a given molecule through external treatments will thus provide tunable material electronic properties without introducing extra synthetic efforts. Interchangeable conjugated and non-conjugated molecular structures can happen with careful design of chemical structures, two of which are depicted in Figure 3. One proposed molecular segment contains a thiophene-fused pentalene backbone with two N,N-dimethylaniline (DMA) pendant moieties. Conjugation along molecular backbone is not accessible in the neutral state. However, oxidation can lead to the formation of a quinoid structure that allows for conjugation along both backbone and pedant DMA units (Figure 3, left). Conjugation switch in another system, shown in Figure 3, right, can be readily modulated by photo irradiation. These molecular segments, when built into a conjugated polymer or molecule, are able to show well-regulated behaviors (e.g., optical and electronic properties) by means of external stimuli, which could enable smart device applications relevant to energy conversion and light emitting applications.

Manipulation of Material Structure and Property via Supramolecular Approaches

On the pursuit of high-performance optoelectronic devices, light-weight and flexibility are emerging as urgent demands. In realizing such needs, organic materials are of great interests being the next generation semiconductor with respect to the inorganic counterparts. Recent efforts suggest that device performance of organic materials is comparable, sometimes even superior to the inorganic-based devices. The current challenge remains that, it is often impossible to guarantee a reliable device performance from a given organic material. The reason behind is the fact that how molecules aggregate to form functional bulk cannot be well predicted and controlled. The possible solution that could enable general optoelectronic applications of organic materials falls to how one can manipulate isolated molecules to form reproducible bulk structures in a controllable manner. This is particularly important for the state-of-the-art solution-processing technique (e.g., spin casting, inkjet printing, and blade coating, etc.).^[8]

While synthetic methodologies can raise a large variety of molecular structures, supramolecular chemistry and molecular self-assembly provide necessary means to organize material nanostructures beyond the single-molecule realm. This part of research will initiate the design of materials with consideration of molecular shape and functional pending groups for intermolecular recognitions. Molecule “ring” and “wire” can be built with an alternating donor and acceptor conformation (Figure 4A). The pedant side chains are designed to promote solubility and both polar and nonpolar functionalities. Molecular internal dipole moments can also be modulated accordingly, which is believed to direct molecular aggregation preference in the solid state.^[9] Dielectric constant of the intrinsic

materials or the surrounding solvent media is another important parameter in consideration. The foci of studying these molecules are to monitor how the above-mentioned parameters may induce intermolecular interactions when an isotropic solution is in the process of concentrating (or drying). Limited voids between molecules along with driving force from p-p stacking may induce molecular aggregates to form a semi-stable phase that possesses a certain degree of molecular ordering, which is also known as lyotropic-like liquid crystalline phase.^[10] Such semi-stable phases may be revealing in translating molecule level packing to macroscopic device functions.

To understand the transition from solution to solid state of a given molecular material is critical for practical application. Molecular self-assembly can be optimized by controlling a number of molecular intrinsic parameters (e.g., molecular geometry, surface energy, and dipole moment) as well as external environmental factors (e.g., solvent polarity and temperature) that can be optimized in directing molecular self-assembly of interest (Figure 4B).^[11] In particular, interfacial surface energy engineering may induce preferable molecular orientation during film formation, which can be quantitatively described as polar and nonpolar surface energies.^[12] A combination of contact angle measurement and structural characterization techniques (e.g., X-ray scattering and electron microscopy) will be employed to draw correlation between molecular orientation and surface energy. Molecular materials can also

self-assemble, based on control over electrostatic dipole moment, solution polarity, and temperature, into either kinetically or thermal dynamically stable nanostructures (e.g., nanowires and micelles). These pre-organized nanostructures are expected to maintain their superstructures when transitioning to solid states. A combination of characterization tools will be implemented to provide direct visual evidence, and even real-time tracing of structure evolutions. Figure 4C shows geometries that in-situ spectroscopic ellipsometry^[13] and X-ray scattering^[14] are applied while (1) thin films are being treated under solvent or thermal annealing, (2) molecules self-assemble during film formation, and (3) the formation of lyotropic liquid crystals during solvent evaporation of a concentrated given solution.

Post chemical modification of known materials upon film formation can be useful for tuning material property without disturbing the existing bulk structures, such as Diels-Alder, thiol-ene “click” reactions, and electrophilic addition. All these manners would afford nondestructive modification of film optical and electrochemical properties. Figure 5 illustrates two possible ideas where Lewis acid can be used to modify molecular charge transfer states, with electrophilic addition reactions to either azulene or electron-rich alkyne groups. The emergence of an extra charge transfer complex, which is perpendicular to the donor-acceptor conjugation direction, allows one to be able to modulate electron transitions independently along these two directions. Of particular relevance is that, with respect to organic solar cell application, such versatile “chemical doping” strategies provide possibility to create panchromatic organic materials. In particular, a strong D-A pair along the molecular main chain can create a narrow optical energy gap (e.g., 1.2 eV), while the pedant D’-A’ pair may be modulated in order to generate a photon absorber at higher energy edge (e.g., 1.8 eV). The complementary photon absorption profiles can pave a path toward a significantly higher power conversion efficiency in organic thin-film solar cells. A systematic study will be carried out in order to look into to what degree changes of molecular structures and properties can be affected by stoichiometry of Lewis acid addition. Original synthesis and post modification of material structures will provide a powerful toolbox to assist in understanding fundamental research questions on how molecules aggregate, and, ultimately, draw general guidelines for reliable device performance and future material design.

Flexible Organic Energy Transduction Devices and Integrated Systems

Streamline material design, synthesis, and structural manipulation will certainly enable high-performance candidates for use in semiconducting devices to address energy and environmental concerns, e.g., photon-electricity conversion, photosynthesis, and organic spintronics. Biomimetic processes relevant to energy harvesting, conversion and storage have been long dominated by inorganic semiconductors mainly due to their high dielectric constant and charge carrier mobility.^[15] In fact, the structural diversity of organic semiconductors hold great potential in precisely tune molecular frontier orbital energy levels, i.e., the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which can enable candidates as substitution for inorganic counterparts.

As an example, an ideal light-harvesting material would require an efficient overlap with solar spectrum. Current research focuses on solar energy conversion is mostly dominated by using the classic two-component bulk heterojunctions.^[16] Fullerene derivatives are frequently used to promote efficient exciton dissociation that eventually drive an electric circuit. Thinking from a different perspective, molecular materials can be designed to favor functional single-component organic photovoltaic devices, shown in Figure 6A, as an ideal-structure organic solar cell. The proposed molecule should have an energy band-gap of 1.5 eV, corresponding to an absorption onset of c.a. 830 nm. Molecular geometry will be optimized to elevate efficient intermolecular electron communication. An ambipolar charge transport characteristic is adequate for transport of both electrons and holes. Moreover, a strong internal dipole moment will be an important design factor to guarantee a long-living charge separated state.^[17] A working energy diagram is shown in Figure 6A, right. Considering the relatively short exciton diffusion length (< 20 nm) of common organic semiconductors, it is reasonable to fabricate nanowire structures of ZnO along the substrate normal. The length and diameter of such nanowires will be optimized in maximizing device efficiencies (Figure 6A, middle). The combination of molecular materials with ambipolar charge transport behaviors and plasmonic effect from nanostructured cathode/anode buffer layers could potentially open the door to fullerene-free solar cell devices, and, more importantly, greatly simplified solid state structure that can be easily modeled and interpreted. Another important intention is to evaluate these organic semiconductors in spin valves and collaborate with theorists and physicists in guiding materials design principle to reveal the next-generation more efficient and robust solar cells.

Lyotropic liquid crystalline materials (as designed in previous section) tend to form textured domains in concentrated suspensions. Molecular alignment or crystal seeds induced directional crystallization may be applied to drive large-area crystalline films (Figure 6B).^[18] Anisotropic charge transport behavior is expected when applying bias either along or perpendicular to the p-p stacking direction. Charge carrier (electron or hole) mobility obtained along the p-p stacking (denoted as m₁) is likely several orders of magnitude higher than that measured at 90° with respect to the alignment direction. The anisotropy in crystallites orientation and electronic properties correlates principally to molecular intrinsic structures. The understanding obtained from processing one given molecule can provide useful information on how one should optimize the current molecular framework, or design new structures to tackle specific issue.

One of the challenges researchers are currently facing is the rising environmental impact from material production and processing, during which highly toxic solvents, such as chloroform, chlorobenzene, and iodoalkane, are often used. Moving toward large-scale production and application will certainly increase health risk brought by the leftover toxins. There are increasing efforts that people start noticing the importance of greener processing of organic materials.^[19] Well-gifted from structural tunability, organic conjugated materials can be designed to be compatible with low-toxic solution processing. Figure 7 shows molecular systems that will be considered for possible water soluble organic semiconductors, by introducing ionic functionalities either along the conjugated backbone (left) or as pedant side groups (right). The highly planar and electron-delocalized structures can foster electron communication when transitioning from solution to solid state. Positive charges within the polymer backbone are likely to promote a more electron-deficient local environment, hence increase the chance for interchain interaction with the near donor moieties. A stronger charge transfer state in the aggregated morphology is expected to expand the photon response approaching the infrared region, which could also find utilities in bioimaging and tracing of biological processes. A structural design with focus on the side chains can give rise to a block copolymer (Figure 7, right) with both crown ether and ionic groups. The strong coordinating tendency between crown ether and alkali metal cation can transform the as-proposed structure into an ion-dissociated conformation. The Coulombic interaction from the oppositely-charged side chains will probably enable strong interchain aggregation even under aqueous environment. In addition, these ionic organic semiconductors can be found useful also as interfacial modifiers when incorporated in between organic layers and metal/semiconductor electrodes, which are possible to tune work functions, surface energy, and doping state.

An efficient and durable organic photovoltaic device module is not ideal solution to our energy demands. One obvious reason is that these devices heavily rely on available light source. Practical application requires one to consider, more importantly, how the electric energy generated from light harvesting can be stored or transformed into chemical substances for potential future use.^[20] Photon-initiated electron generation in inorganic semiconductors has been an emerging technology in mimicking natural photosynthesis for efficient carbon capture and fuel generation.^[21] Organic semiconductors have been paid much less attentions due to its poorer charge separation efficiency and competitive charge recombination; results mostly from an overall low dielectric constant. However, organic materials possess apparent benefits over the inorganic counterparts in the aspects of processability, production cost, and structural diversity. Much of these properties of organic semiconductors can be well predicted and modulated during material preparation. Figure 8 illustrates an example of an artificial photosynthesis system that utilizes organic semiconductor as light absorber for exciton generation. H₂O and CO₂ are chosen to be oxidation and reduction source chemicals in order to

evaluate the device performance and help understanding the basic operation processes unique to the intrinsic organic semiconductors.

An early attempt will be initiated by designing organic dye molecules with the intention of narrow band-gap (~1.4 eV), appropriate molecular orbital energy levels (E_HOMO = -5.1 eV, E_LUMO = -3.7 eV), and a large internal electrostatic dipole moment. The molecular design will be performed by employing density functional theory. A systematic study of structure-performance correlation will be followed up in optimizing the power conversion efficiency of the photoelectrochemical synthesis cell. Particular research emphasis will be put forward in identifying new molecular structures as substitutions to the existing ruthenium complexes,^[15] applying the device in catalyzing organic oxidation and reduction reactions for organic methodology and material preparation, and creating possible cooperation with microorganisms in bioelectronics applications.

The joint efforts from organic chemistry and materials science create a unique perspective to further the research advancement in the next generation of organic semiconductors. I dedicate my research to the generalization of organic materials in optoelectronic device applications, exploring new molecular materials for high-performance, easily reproducible, and environmentally durable semiconducting device modules. Moreover, environmentally friendly and degradable organic product is an emerging must. Upfront research efforts are necessary to bring the attention of sustainability to our worldwide research community.

References

1. Carsten, F. He, H. J. Son, T. Xu, L. Yu. Chem. Rev. 2011, 111, 1493.
2. T. Nielsen, K. Bechgaard, F. C. Krebs. Macromolecules 2005, 38, 658.
3. Liu, C. Li, J. Xu, J. Lv, M. Zhu, Y. Guo, S. Cui, H. Liu, S. Wang, Y. Li. J. Phys. Chem. C 2008, 112, 10778.
4. Hyotanishi, Y. Isomura, H. Yamamoto, H. Kawasaki, Y. Obora. Chem. Commun. 2011, 47, 5750.
5. G. Mercier, M. Leclerc. Acc. Chem. Res. 2013, 46, 1597.
6. Liu, Y. Sun, B. B. Y. Hsu, A. Lorbach, L. Qi, A. J. Heeger, G. C. Bazan. J. Am. Chem. Soc. 2014, 136, 5697.
7. Zhang, N. S. Colella, F. Liu, S. Trahan, J. K. Baral, H. H. Winter, S. C. Mannsfeld, A. L. Briseno. J. Am. Chem. Soc. 2013, 135, 844.
8. C. Krebs, J. Fyenbo, M. Jorgensen. J. Mater. Chem. 2010, 20, 8994.
9. J. Takacs, Y. Sun, G. C. Welch, L. A. Perez, X. Liu, W. Wen, G. C. Bazan, A. J. Heeger. J. Am. Chem. Soc. 2012, 134, 16597.
10. -G. Kim, E. J. Jeong, J.W. Chung, S. Seo, B. Koo, J. Kim. Nat. Mater. 2013, 12, 659.
11. Liu, Y. Li. Dalton Trans. 2009, 6447.
12. E. Widjonarko, P. Schulz, P. A. Parilla, C. L. Perkins, P. F. Ndione, A. K. Sigdel, D. C. Olson, D. S. Ginley, A. Kahn, M. F. Toney, J. J. Berry. Advanced Energy Materials 2014, DOI: 10.1002/aenm.201301879.
13. V. Madsen, K. O. Sylvester-Hvid, B. Dastmalchi, K. Hingerl, K. Norrman, T. Tromholt, M. Manceau, D. Angmo, F. C. Krebs. J.Phys. Chem. C 2011, 115, 10817.
14. D. Treat, C. G. Shuttle, M. F. Toney, C. J. Hawker, M. L. Chabinyc. J. Mater. Chem. 2011, 21, 15224.
15. M. Schultz, T. P. Yoon. Science 2014, 343, 1239176.
16. Dennler, M. C. Scharber, C. J. Brabec. Adv. Mater. 2009, 21, 1323.
17. Carsten, J. M. Szarko, H. J. Son, W. Wang, L. Lu, F. He, B. S. Rolczynski, S. J. Lou, L. X. Chen, L. Yu. J. Am. Chem. Soc. 2011, 133, 20468.
18. Diao, B. C. Tee, G. Giri, J. Xu, D. H. Kim, H. A. Becerril, R. M. Stoltenberg, T. H. Lee, G. Xue, S. C. Mannsfeld, Z. Bao. Nat. Mater. 2013, 12, 665.
19. Irimia-Vladu, M. Soc. Rev. 2014, 43, 588.
20. DeBlase, C. R.; Silberstein, K. E.; Truong, T. T.; Abruna, H. D.; Dichtel, W. R. Am. Chem. Soc. 2013, 135, 16821.
21. Kenney, M. J.; Gong, M.; Li, Y.; Wu, J. Z.; Feng, J.; Lanza, M.; Dai, H. Science 2013, 342, 836.