Cofacial Versus Coplanar Arrangement in Centrosymmetric Packing Dimers of Dipolar Small Molecules: Structural Effects on the Crystallization Behaviors and Optoelectronic Characteristics

Shu Hua Chou, Hao Wei Kang, Shu Ting Chang, Kuan Yi Wu, Guillermo C. Bazan, Chien-Lung Wang*, Hong Lin Lin, Jung Hao Chang, Hao Wu Lin, Yu Ching Huang, Cheng Si Tsao, Ken Tsung Wong

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

9 Scopus citations

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 C 70 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.

Original languageEnglish
Pages (from-to)18266-18276
Number of pages11
JournalACS Applied Materials and Interfaces
Volume8
Issue number28
DOIs
StatePublished - 20 Jul 2016

Keywords

  • crystallinity
  • organic field effect transistors
  • organic solar cell
  • planar-mixed heterojunction
  • push-pull molecules
  • substitution effect

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