Nanocrystallization Process, Structure, and Properties of Charge-Transfer Complex Prepared by the Reprecipitation Method
Authors: Takeda Masaki , Sharber Markus Clark , Stadler Philipp , Yumusak Cigdem , Sariciftci Niyazi Serdar , White Matthew Shuette , Furis Madalina , Yoshida Tsukasa , Matsui Jun , Masuhara Akito 
Published: February 2020

Charge-transfer (CT) complex crystals composed of organic electron donor (D) and acceptor (A) have been extensively studied because of their attractive properties such as metallic conductivity, photoconductivity, ambipolar charge transport, and high career mobility. Numerous research efforts have been made for their use in devices such as organic field effect transistors and organic photocells. However, bulky single crystals of CT complexes are difficult to handle, specifically to be made into a form of a thin film needed for solar cell applications.

　Nano/micro crystallization is a promising path to overcome the above mentioned problem. However, the growth of high-quality nanocrystals is imperative for photoconductivity but often hampered, since CT complexes are poorly soluble and vapor pressures of donor and acceptor molecules are imbalanced[1-2].

　Here we presente the “reprecipitation method” as facile synthesis route to obtain high quality organic CT complex nanocrystals. The process of nanocrystallization can be explained in four steps[3]; 1) A small amount of a solution of the target compound in its good solvent is injected into a large amount of its poor solvent under vigorous stirring. 2) The tiny droplets formed by the injection diffuse into the poor solvent as the two solvents are chosen to be miscible. 3) Nucleation and crystal growth of the target compound proceed due to its supersaturation at the boundary of good/poor solvents. 4) As the good solvent is completely diffused, nanocrystals are stably dispersed in the poor solvents. All these events are expected to occur in a short time (within tens of milliseconds[4]) and thus nanocrystals with kinetically stable structures are often obtained, which are different from thermodynamically stable single bulk crystal structures[5].

　In this work, we have successfully applied the reprecipitation method to dibenzotetrathiafulvalene (DBTTF)-tetracyanoquinodimethane (TCNQ) and tetrathiafulvalene (TTF)-TCNQ to obtain its nanocrystal. The DBTTF-TCNQ nanocrystals were prepared by typical reprecipitation method from D-A mixed solution. The nanocrystals prepared in such a way have been found to possess kinetically stable crystal structure, different from the bulk material. Moreover, a direct current device employing a polycrystalline thin film of the nanocrystals prepared by simple filtration an equivalent photosensitivity (354.57 μA/W) with that of a bulk crystal (417.14 μA/W) despite of the presence of many grain boundaries, indicating a favorable molecular stacking in the nanocrystals for career transport. In addition, the TTF-TCNQ nanocrystals were prepared by two-step reprecipitation method. TTF (TCNQ) solution was injected to TCNQ (TTF) nanoparticle water dispersion. The TTF-TCNQ nanocrystals were formed when the nanoparticles reacted and the dispersion color was changed immediately from yellow to black.

　Details of morphology, characteristic and crystallization process of the nanocrystals will be discussed in presentation.

Reference

[1] M. Hiraoka, T. Hasegawa, T. Yamada, Y. Takahashi, S. Horiuchi, and Y. Tokura, Adv. Mater.
19, 3248 (2007).

[2] Y. Takahashi, T. Hasegawa, Y. Abe, Y. Tokura, and G. Saito, Appl. Phys. Lett.
88, 073504 (2006).

[3] K. Shito, N. Ito, and A. Masuhara, Jpn. J. Appl. Phys.
54, 06FK05-1 (2015).

[4] J. Mori, Y. Miyashita, D Oliveira, H. Kasai, H. Oikawa, and H. Nakanishi, J. Cryst. Growth
311, 553 (2009).

[5] L. Huang, Q. Liao, H. Fu, J. Ma, and J. Yao, J. Mater. Chem.
20, 159 (2010).