Session Chair: Kazuo Takimiya, RIKEN Center for Emergent Matter Science
10:30am - 11:00am Invited
Molecular Singlet Fission in π-Electron Materials
Dirk M. GULDI
University of Erlangen, Germany
Singlet Fission (SF) is a spin-allowed process to convert one singlet excited state into two triplet excited states, namely a correlated triplet pair. The ability to effectively implement SF processes in solar cells could allow for more efficient harvesting of high-energy photons from the solar spectrum and allow for the design of solar cells to circumvent the Shockley-Queisser performance limit. Indeed, several recent studies have demonstrated remarkably efficient solar cell devices based on SF.
In the present work, we show unambiguous and compelling evidence for unprecedented intramolecular SF within a set of pentacene dimers at room temperature and in dilute solution. To this end, pump-probe experiments are employed and complemented by theoretical calculations using high level ab-initio multireference perturbation theory methods. The observed triplet quantum yields reach remarkable values, which are as high as 180%. The latter should be compared to triplet quantum yields of 10% as they are typically found in pentacene derivatives due to slow intersystem crossing dynamics. Here, we demonstrate that the proximity and sufficient coupling through bond in pentacene dimers is sufficient to induce intramolecular SF. As a consequence, two triplets are generated on the same molecule. This work constitutes a conceptional breakthrough, in that it documents the great potential of synthetically tailored acenes to surpass the 30% Shockley-Queisser limit and its impact in terms of easing device fabrication.
11:00am - 11:30am Invited
New Design and Thin Film Transistor Applications of N-Type Semiconducting Polymers
Tokyo Institute of Technology, Japan
The past decade has witnessed significant achievements in the field of flexible and printable organic electronics, such as polymer solar cells and thin film transistors. The main driving force is the rapid progress in semiconducting polymers. My group has pursued novel n-type semiconducting polymers. The initial attempt was to synthesize fullerene (C60) polymers, since C60 was a well-known n-type semiconductor. Based on the unique molecular design, solution-processable linear C60 polymers with the C60 content exceeding 40wt% were successfully synthesized by azide-alkyne click polymerization. However, they did not show good transistor performances despite the relatively high C60 content. Next, we developed highly cyanated conjugated polymers by utilizing a [2+2] cycloaddition-retroelectrocyclization reaction with tetracyanoethylene (TCNE). Postfunctionalization of aromatic polyamines by this reaction enabled the formation of n-type energy levels, and there was a clear correlation between the energy levels and transistor performances. Recently, we started a new research program of benzobisthiadiazole (BBT)-based semiconducting polymers.The polarity and mobilities of many types of BBT polymers were examined by fabricating thin film transistors. It was shown that designing p-spacers and heteroatom substitutions on the BBT unit were effective methods of determining the polarity, but the mobilities were largely dependent on the polymer packing orientations on a substrate.
11:30am - 12:00pm Invited
Two Dimensional Crystals of Organic Semiconductors
Chunhui XU1, Huanli DONG2, Wenping HU1,2
1Tianjin University, China; 2Institute of Chemistry, Chinese Academy of Sciences (CAS), China
Large scale single crystalline two-dimensional (2D) crystals are promising for electronic and optoelectronic devices. Very recently, 2D crystals of organic semiconductors (2DCOS) have attracted attention since their unique features in tailoring functionalities by molecular-design for large-area and low-cost flexible optoelectronics. However, growing large-size 2DCOS in controllable ways and transferring them onto technologically important substrates, remain key challenges in this field. Here we make the breadkthrough, reporting a facile, yet general and effective method to grow 2DCOS up to centimeter size which could be transferred to any substrate efficiently. This novel method is named “solution-epitaxy” due to its unique features of solution process to epitaxy 2DCOS. The heart cut of this method can be divided into two steps. The first is to self-assemble micrometer-sized 2DCOS on water surface, and then epitaxy them into centimeter sized 2DCOS with thickness of several molecular layers. The general applicability of this method for the growth of 2DCOS is demonstrated by nine organic semiconductors with different molecular structures served as model systems, and can be further extended to more complicated conjugated materials. More attractively, the 2DCOS by “solution-epitaxy” are facilely transferred to substrate for devices because of the unique feature of crystal growth by solution process. Organic field-effect transistors (OFETs) based on the 2DCOS demonstrated high performance, confirming the high quality of the 2DCOS. Our methodology provides a novel and convenient route to grow and transfer high quality 2DCOS in large area, overcoming the bottlenecks of organic 2D crystals, opening the prospect of 2DCOS in organic and flexible optoelectronics.