title: Madelung and Hubbard interactions in polaron band model of doped organic semiconductors.
abstract: The standard polaron band model of doped organic semiconductors predicts that density-of-states shift into the π-π* gap to give a partially filled polaron band that pins the Fermi level. This picture neglects both Madelung and Hubbard interactions. Here we show using ultrahigh workfunction hole-doped model triarylamine-fluorene copolymers that Hubbard interaction strongly splits the singly-occupied molecular orbital from its empty counterpart, while Madelung (Coulomb) interactions with counter-anions and other carriers markedly shift energies of the frontier orbitals. These interactions lower the singly-occupied molecular orbital band below the valence band edge and give rise to an empty low-lying counterpart band. The Fermi level, and hence workfunction, is determined by conjunction of the bottom edge of this empty band and the top edge of the valence band. Calculations are consistent with the observed Fermi-level downshift with counter-anion size and the observed dependence of workfunction on doping level in the strongly doped regime.
authors: Department of Physics, National University of Singapore, Lower Kent Ridge Road, Singapore S117550, Singapore.|Solar Energy Research Institute of Singapore (SERIS), National University of Singapore, 7 Engineering Drive 1, Singapore S117574, Singapore.
authors: Rui-Qi
authors: RQ
authors: Png
authors: Solar Energy Research Institute of Singapore (SERIS), National University of Singapore, 7 Engineering Drive 1, Singapore S117574, Singapore.|Department of Chemistry, National University of Singapore, Lower Kent Ridge Road, Singapore S117552, Singapore.
authors: Mervin C Y
authors: MC
authors: Ang
authors: Department of Physics, National University of Singapore, Lower Kent Ridge Road, Singapore S117550, Singapore.
authors: Meng-How
authors: MH
authors: Teo
authors: Department of Chemistry, National University of Singapore, Lower Kent Ridge Road, Singapore S117552, Singapore.
authors: Kim-Kian
authors: KK
authors: Choo
authors: Department of Physics, National University of Singapore, Lower Kent Ridge Road, Singapore S117550, Singapore.|Solar Energy Research Institute of Singapore (SERIS), National University of Singapore, 7 Engineering Drive 1, Singapore S117574, Singapore.
authors: Cindy Guanyu
authors: CG
authors: Tang
authors: Department of Physics, National University of Singapore, Lower Kent Ridge Road, Singapore S117550, Singapore.
authors: Dagmawi
authors: D
authors: Belaineh
authors: Department of Physics, National University of Singapore, Lower Kent Ridge Road, Singapore S117550, Singapore.|Solar Energy Research Institute of Singapore (SERIS), National University of Singapore, 7 Engineering Drive 1, Singapore S117574, Singapore.|Department of Chemistry, National University of Singapore, Lower Kent Ridge Road, Singapore S117552, Singapore.
authors: Lay-Lay
authors: LL
authors: Chua
authors: Department of Physics, National University of Singapore, Lower Kent Ridge Road, Singapore S117550, Singapore.|Solar Energy Research Institute of Singapore (SERIS), National University of Singapore, 7 Engineering Drive 1, Singapore S117574, Singapore.
authors: Peter K H
authors: PK
authors: Ho