Title
Compositional and electric field dependence of the dissociation of charge transfer excitons in alternating polyfluorene copolymer/fullerene blends
Author
Veldman, D.
Ipek, Ö.
Meskers, S.C.J.
Sweelssen, J.
Koetse, M.M.
Veenstra, S.C.
Kroon, J.M.
van Bavel, S.S.
Loos, J.
Janssen, R.A.J.
TNO Industrie en Techniek
Publication year
2008
Abstract
The electro-optical properties of thin films of electron donor-acceptor blends of a fluorene copolymer (PF10TBT) and a fullerene derivative (PCBM) were studied. Transmission electron microscopy shows that in these films nanocrystalline PCBM clusters are formed at high PCBM content. For all concentrations, a charge transfer (CT) transition is observed with absorption spectroscopy, photoluminescence, and electroluminescence. The CT emission is used as a probe to investigate the dissociation of CT excited states at the donor-acceptor interface in photovoltaic devices, as a function of an applied external electric field and PCBM concentration. We find that the maximum of the CT emission shifts to lower energy and decreases in intensity with higher PCBM content. We explain the red shift of the emission and the lowering of the open-circuit voltage (VOC) of photovoltaic devices prepared from these blends with the higher relative permittivity of PCBM (εr = 4.0) compared to that of the polymer (εr = 3.4), stabilizing the energy (ECT) of CT states and of the free charge carriers in blends with higher PCBM concentration. We show that the CT state has a short decay time (τ = ca. 4 ns) that is reduced by the application of an external electric field or with increasing PCBM content. The field-induced quenching can be explained quantitatively with the Onsager-Braun model for the dissociation of the CT states when including a high electron mobility in nanocrystalline PCBM clusters. Furthermore, photoinduced absorption spectroscopy shows that increasing the PCBM concentration reduces the yield of neutral triplet excitons forming via electron-hole recombination, and increases the lifetime of radical cations. The presence of nanocrystalline domains with high local carrier mobility of at least one of the two components in an organic heterojunction may explain efficient dissociation of CT states into free charge carriers. © 2008 American Chemical Society.
Subject
Materials
Industrial Innovation
Absorption
Absorption spectroscopy
Astrophysics
Charge transfer
Charged particles
Dissociation
Electric field effects
Electric field measurement
Electric fields
Electric instrument transformers
Electroacupuncture
Electromagnetic field theory
Electromagnetic fields
Electromagnetism
Electron microscopes
Electron microscopy
Electron optics
Electrons
Emission spectroscopy
Fullerenes
Imaging techniques
Ion exchange
Light emission
Luminescence
Magnetism
Mass transfer
Microscopic examination
Optical properties
Polymers
Semiconducting cadmium telluride
Thick films
Thin films
Transmission electron microscopy
(ethylene vinyl alcohol) copolymers
Alternating polyfluorene (APFO)
Applied (CO)
Charge transfer excitons
Charge-transfer (CT) transitions
Concentration (composition)
Donor-acceptor interfaces
Electric field dependences
Electro optical properties
Electron donor acceptor (EDA)
emission shifts
External electric fields
Fluorene (FI)
fullerene derivatives
Lower energies
Nano crystalline
Open-circuit voltage (OCV)
photovoltaic devices
red shifting
Electric charge
cation
copolymer
fluorene derivative
fullerene derivative
nanocrystal
organic compound
article
chemical composition
chemical model
chemical structure
concentration (parameters)
device
dissociation
electric field
electric potential
electricity
electron
energy
film
intensity modulated radiation therapy
quantitative analysis
transmission electron microscopy
To reference this document use:
http://resolver.tudelft.nl/uuid:96716a5b-ecd3-44eb-a646-4c98368025db
TNO identifier
280041
ISSN
0002-7863
Source
Journal of the American Chemical Society, 130 (24), 7721-7735
Document type
article