(Invited) Solution-Processed Reorganization of Semiconductor Nanocrystals in Thin Films
Authors: Sagawa Takashi 
Published: July 2021

Colloidal nanocrystals (NCs) have been attracted because of their strongly size-dependent luminous characteristics and solution processability. As prepared colloidal NCs covered with some bulky and long-chain organic ligands such as oleic acid, oleylamine (OAm), trioctylphosphine oxide, and so on are mostly dispersed in some solvents such as hexane, toluene, and so on. After preparation of thin films from the dispersed solutions of NCs, reduced inter NC distance (viz. NCs coming closer) with highly ordered alignment will lead to enhanced electronic coupling between adjacent NCs resulting in better wave function overlap and charge transport of the conducting solid materials.
In this context, remarkable improvement of the alignment of the particles through solution-processed reorganization of AgInS2 quantum dots (QDs) covered with OAm by cast method was confirmed by bright-field TEM observation [1,2]. The interplanar spacing of the QDs was 0.33 nm, which corresponds to (112) crystal planes of the tetragonal phase of AgInS2.
On the other hand, it was found that PbS colloidal QDs can be packed densely and infiltrated efficiently into the interrod space of vertically aligned ZnO nanowire arrays through convective deposition method, which is one of the meniscus coating techniques [3] as compared with spin-coating process, which brings many cracks and voids in the obtained thin films. As a result, a record power conversion efficiency of 9.92% has been demonstrated for ordered bulk heterojunction structured solar cells that are compatible with low-temperature and scalable manufacturing processes.
While comparisons of perovskite CsPbBr3 NCs grafted on the surface of silica nanohelices as templates in dispersed state in toluene and dried state on quartz glass substrate revealed that the interparticle distance becomes shorter after the dryness by cast method from the dispersed state and the closely and helically packed NCs [4]. Tuning the conditions of solution-processed reorganization of CsPbBr3 NCs is crucial for reducing the inter NC distance with highly ordered alignment in the cast films with large circular dichroism and circularly polarized luminescence signals.
On the basis of the above findings, reorganization of NCs with highly controlled crystal orientation and inter NC distance in thin films has deep influence on specific spectral characteristics and photovoltaic performances.
[1] Kaewprajak, A.; Kumnorkaew, P.; Sagawa, T. Silver-indium-sulfide quantum dots in titanium dioxide as electron transport layer for highly efficient and stable perovskite solar cells. J. Mater. Sci.: Mater. Electron., 2019, 30, 4041-4055.
[2] Kaewprajak, A.; Kumnorkaew, P.; Sagawa, T. Improved photovoltaic performance and device stability of planar heterojunction perovskite solar cells using TiO2 and TiO2 mixed with AgInS2 quantum dots as dual electron transport layers. Org. Electron., 2019, 69, 26-33.
[3] Shi, G.; Kaewprajak, A.; Ling, X.; Hayakawa, A.; Zhou, S.; Song, B.; Kang, Y.-w.; Hayashi, T.; Altun, M. E.; Nakaya, M.; Liu, Z.; Wang, H.; Sagawa, T.; Ma, W. Finely interpenetrating bulk heterojunction structure for lead sulfide colloidal quantum dot solar cells by convective deposition. ACS Energy Lett., 2019, 4, 960-967.
[4] Liu, P.; Chen, W.; Okazaki, Y.; Battie, Y.; Brocard, L.; Decossas, M.; Pouget, E.; M.-Buschbaum, P.; Kauffmann, B.; Pathan, S.; Sagawa, T.; Oda, R. Optically active perovskite CsPbBr3 nanocrystals helically arranged on inorganic silica nanohelices. Nano Lett., 2020, 20, 8453-8460.