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Title: Exciton size and binding energy limitations in one-dimensional organic materials

In current organic photovoltaic devices, the loss in energy caused by the charge transfer step necessary for exciton dissociation leads to a low open circuit voltage, being one of the main reasons for rather low power conversion efficiencies. A possible approach to avoid these losses is to tune the exciton binding energy to a value of the order of thermal energy, which would lead to free charges upon absorption of a photon, and therefore increase the power conversion efficiency towards the Shockley-Queisser limit. We determine the size of the excitons for different organic molecules and polymers by time dependent density functional theory calculations. For optically relevant transitions, the exciton size saturates around 0.7 nm for one-dimensional molecules with a size longer than about 4 nm. For the ladder-type polymer poly(benzimidazobenzophenanthroline), we obtain an exciton binding energy of about 0.3 eV, serving as a lower limit of the exciton binding energy for the organic materials investigated. Furthermore, we show that charge transfer transitions increase the exciton size and thus identify possible routes towards a further decrease of the exciton binding energy.
Authors:
; ;  [1] ;  [1] ;  [2] ;  [3]
  1. Institut für Angewandte Photophysik, Technische Universität Dresden, Dresden (Germany)
  2. (Germany)
  3. Institute for Theoretical Chemistry, University of Vienna, A-1090 Vienna (Austria)
Publication Date:
OSTI Identifier:
22493435
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 143; Journal Issue: 24; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ABSORPTION; BINDING ENERGY; CONVERSION; DENSITY FUNCTIONAL METHOD; DISSOCIATION; EFFICIENCY; ELECTRIC POTENTIAL; EV RANGE; EXCITONS; MOLECULES; ONE-DIMENSIONAL CALCULATIONS; ORGANIC MATTER; PHOTONS; PHOTOVOLTAIC EFFECT; POLYMERS; TIME DEPENDENCE