title: Theoretical rationalization for reduced charge recombination in bulky carbazole-based sensitizers in solar cells.
abstract: The search for greater efficiency in organic dye-sensitized solar cells (DSCs) and in their perovskite cousins is greatly aided by a more complete understanding of the spectral and morphological properties of the photoactive layer. This investigation resolves a discrepancy in the observed photoconversion efficiency (PCE) of two closely related DSCs based on carbazole-containing D-π-A organic sensitizers. Detailed theoretical characterization of the absorption spectra, dye adsorption on TiO2, and electronic couplings for charge separation and recombination permit a systematic determination of the origin of the difference in PCE. Although the two dyes produce similar spectral features, ground- and excited-state density functional theory (DFT) simulations reveal that the dye with the bulkier donor group adsorbs more strongly to TiO2, experiences limited π-π aggregation, and is more resistant to loss of excitation energy via charge recombination on the dye. The effects of conformational flexibility on absorption spectra and on the electronic coupling between the bright exciton and charge-transfer states are revealed to be substantial and are characterized through density-functional tight-binding (DFTB) molecular dynamics sampling. These simulations offer a mechanistic explanation for the superior open-circuit voltage and short-circuit current of the bulky-donor dye sensitizer and provide theoretical justification of an important design feature for the pursuit of greater photocurrent efficiency in DSCs. © 2017 Wiley Periodicals, Inc.
authors.affiliation: Center for Organic Electronic and Alternative Energy, Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Ubon Ratchathani University, Ubon Ratchathani, 34190, Thailand.
authors.forename: Yaowarat
authors.initials: Y
authors.lastname: Surakhot
authors.affiliation: Department of Chemistry, Advanced Materials Science and Engineering Center, and Institute for Energy Studies, Western Washington University, Bellingham, Washington, 98225.
authors.forename: Viktor
authors.initials: V
authors.lastname: Laszlo
authors.affiliation: National Nanotechnology Center, National Science and Technology Development Agency, Klong Luang, Pathumthani, 12120, Thailand.
authors.forename: Chirawat
authors.initials: C
authors.lastname: Chitpakdee
authors.affiliation: School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Wangchan, Rayong, 21210, Thailand.
authors.forename: Vinich
authors.initials: V
authors.lastname: Promarak
authors.affiliation: School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Wangchan, Rayong, 21210, Thailand.
authors.forename: Taweesak
authors.initials: T
authors.lastname: Sudyoadsuk
authors.affiliation: Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai, 50200, Thailand.
authors.forename: Nawee
authors.initials: N
authors.lastname: Kungwan
authors.affiliation: Department of Chemistry, Advanced Materials Science and Engineering Center, and Institute for Energy Studies, Western Washington University, Bellingham, Washington, 98225.
authors.forename: Tim
authors.initials: T
authors.lastname: Kowalczyk
authors.affiliation: Department of Chemistry, Graduate School of Science, Nagoya University, Institute of Transformational Biomolecules (WPI-ITbM) and, Nagoya, 464-8602, Japan.
authors.forename: Stephan
authors.initials: S
authors.lastname: Irle
authors.affiliation: Center for Organic Electronic and Alternative Energy, Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Ubon Ratchathani University, Ubon Ratchathani, 34190, Thailand.
authors.forename: Siriporn
authors.identifier: 0000-0001-5943-6878
authors.initials: S
authors.lastname: Jungsuttiwong
keywords: DFTB; TD-DFT; charge recombination; molecular dynamics; solar cell