title: Photoprecursor Approach Enables Preparation of Well-Performing Bulk-Heterojunction Layers Comprising a Highly Aggregating Molecular Semiconductor.
abstract: Active-layer morphology critically affects the performance of organic photovoltaic cells, and thus its optimization is a key toward the achievement of high-efficiency devices. However, the optimization of active-layer morphology is sometimes challenging because of the intrinsic properties of materials such as strong self-aggregating nature or low miscibility. This study postulates that the "photoprecursor approach" can serve as an effective means to prepare well-performing bulk-heterojunction (BHJ) layers containing highly aggregating molecular semiconductors. In the photoprecursor approach, a photoreactive precursor compound is solution-deposited and then converted in situ to a semiconducting material. This study employs 2,6-di(2-thienyl)anthracene (DTA) and [6,6]-phenyl-C71-butyric acid methyl ester as p- and n-type materials, respectively, in which DTA is generated by the photoprecursor approach from the corresponding α-diketone-type derivative DTADK. When only chloroform is used as a cast solvent, the photovoltaic performance of the resulting BHJ films is severely limited because of unfavorable film morphology. The addition of a high-boiling-point cosolvent, o-dichlorobenzene (o-DCB), to the cast solution leads to significant improvement such that the resulting active layers afford up to approximately 5 times higher power conversion efficiencies. The film structure is investigated by two-dimensional grazing-incident wide-angle X-ray diffraction, atomic force microscopy, and fluorescence microspectroscopy to demonstrate that the use of o-DCB leads to improvement in film crystallinity and increase in charge-carrier generation efficiency. The change in film structure is assumed to originate from dynamic molecular motion enabled by the existence of solvent during the in situ photoreaction. The unique features of the photoprecursor approach will be beneficial in extending the material and processing scopes for the development of organic thin-film devices.
authors.affiliation: Graduate School of Materials Science, Nara Institute of Science and Technology , 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan.
authors.forename: Mitsuharu
authors.initials: M
authors.lastname: Suzuki
authors.affiliation: Department of Organic Device Engineering, Yamagata University , 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan.
authors.forename: Yuji
authors.initials: Y
authors.lastname: Yamaguchi
authors.affiliation: Department of Organic Device Engineering, Yamagata University , 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan.
authors.forename: Kohei
authors.initials: K
authors.lastname: Takahashi
authors.affiliation: Department of Organic Device Engineering, Yamagata University , 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan.
authors.forename: Katsuya
authors.initials: K
authors.lastname: Takahira
authors.affiliation: Japan Synchrotron Radiation Research Institute , 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan.
authors.forename: Tomoyuki
authors.initials: T
authors.lastname: Koganezawa
authors.affiliation: Department of Applied Chemistry and Environment, Kwansei Gakuin University , 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan.
authors.forename: Sadahiro
authors.initials: S
authors.lastname: Masuo
authors.affiliation: Department of Organic Device Engineering, Yamagata University , 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan.
authors.forename: Ken-ichi
authors.initials: K
authors.lastname: Nakayama
authors.affiliation: Graduate School of Materials Science, Nara Institute of Science and Technology , 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan.
authors.forename: Hiroko
authors.initials: H
authors.lastname: Yamada
keywords: morphology control; organic solar cells; photoprecursor approach; solution process; α-diketones