title: Cofacial Versus Coplanar Arrangement in Centrosymmetric Packing Dimers of Dipolar Small Molecules: Structural Effects on the Crystallization Behaviors and Optoelectronic Characteristics.
abstract: Two D-π-A-A molecules (MIDTP and TIDTP) composed of an electron-rich ditolylamino group (D) and an electron-deficient 5-dicyanovinylenylpyrimidine (A-A) fragment bridged together with indeno[1,2-b]thiophene (IDT) were synthesized. These molecules provide an opportunity to examine in-depth the impact of side-chain variations (methyl vs p-tolyl) on the crystallization behaviors, solid-state morphology, physical properties, and optoelectronic characteristics relevant for practical applications. X-ray analyses on single-crystal structures indicate that methyl-substituted MIDTP forms "coplanar antiparallel dimers" via C-H···S interactions and organizes into an ordered slip-staircase arrays. In contrast, p-tolyl-bearing TIDTP shows "cofacial centrosymmetric dimers" via π-π interactions and packs into a less-ordered layered structures. The X-ray diffraction analyses upon thermal treatment are consistent with a superior crystallinity of MIDTP, as compared to that of TIDTP. This difference indicates a greater propensity to organization by introduction of the smaller methyl group versus the bulkier p-tolyl group. The increased propensity for order by MIDTP facilitates the crystallization of MIDTP in both solution-processed and vacuum-deposited thin films. MIDTP forms solution-processed single-crystal arrays that deliver OFET hole mobility of 6.56 × 10(-4) cm(2) V(-1) s(-1), whereas TIDTP only forms amorhpous films that gave lower hole mobility of 1.34 × 10(-5) cm(2) V(-1) s(-1). MIDTP and TIDTP were utilized to serve as donors together with C70 as acceptor in the fabrication of small-molecule organic solar cells (SMOSCs) with planar heterojunction (PHJ) or planar-mixed heterojunction (PMHJ) device architectures. OPV devices based on higher crystalline MIDTP delivered power conversion efficiencies (PCEs) of 2.5% and 4.3% for PHJ and PMHJ device, respectively, which are higher than those of TIDTP-based cells. The improved PCEs of MIDTP-based devices are attributed to better hole-transport character.
authors.forename: Shu-Hua
authors.initials: SH
authors.lastname: Chou
authors.affiliation: Department of Materials Science and Engineering, National Tsing Hua University , Hsinchu 30013, Taiwan.
authors.forename: Hao-Wei
authors.initials: HW
authors.lastname: Kang
authors.affiliation: Department of Applied Chemistry, National Chiao Tung University , Hsinchu 30010, Taiwan.
authors.forename: Shu-Ting
authors.initials: ST
authors.lastname: Chang
authors.affiliation: Department of Applied Chemistry, National Chiao Tung University , Hsinchu 30010, Taiwan.
authors.forename: Kuan-Yi
authors.initials: KY
authors.lastname: Wu
authors.affiliation: Center for Polymers and Organic Solids, University of California at Santa Barbara , Santa Barbara, California 93106-5090, United States.
authors.forename: Guillermo C
authors.initials: GC
authors.lastname: Bazan
authors.affiliation: Department of Applied Chemistry, National Chiao Tung University , Hsinchu 30010, Taiwan.
authors.forename: Chien-Lung
authors.initials: CL
authors.lastname: Wang
authors.affiliation: Department of Materials Science and Engineering, National Tsing Hua University , Hsinchu 30013, Taiwan.
authors.forename: Hong-Lin
authors.initials: HL
authors.lastname: Lin
authors.affiliation: Department of Materials Science and Engineering, National Tsing Hua University , Hsinchu 30013, Taiwan.
authors.forename: Jung-Hao
authors.initials: JH
authors.lastname: Chang
authors.affiliation: Department of Materials Science and Engineering, National Tsing Hua University , Hsinchu 30013, Taiwan.
authors.forename: Hao-Wu
authors.initials: HW
authors.lastname: Lin
authors.affiliation: Institute of Nuclear Energy Research , Taoyuan 32546, Taiwan.
authors.forename: Yu-Ching
authors.initials: YC
authors.lastname: Huang
authors.affiliation: Institute of Nuclear Energy Research , Taoyuan 32546, Taiwan.
authors.forename: Cheng-Si
authors.initials: CS
authors.lastname: Tsao
authors.affiliation: Institute of Atomic and Molecular Science, Academia Sinica , Taipei 10617, Taiwan.
authors.forename: Ken-Tsung
authors.initials: KT
authors.lastname: Wong
keywords: crystallinity; organic field effect transistors; organic solar cell; planar-mixed heterojunction; push−pull molecules; substitution effect