9 research outputs found
Efficient Charge Separation of Cold Charge-Transfer States in Organic Solar Cells Through Incoherent Hopping
We demonstrate that efficient and nearly field-independent charge separation of electron hole pairs in organic planar heterojunction solar cells can be described by an incoherent hopping mechanism. Using kinetic Monte Carlo simulations that include the effect of on-chain delocalization as well as entropic contributions, we simulate the dissociation of the charge-transfer state in polymer fullerene bilayer solar cells. The model further explains experimental results of almost field independent charge separation in bilayers of molecular systems with fullerenes and provides important guidelines at the molecular level for maximizing the efficiencies of organic solar cells. Thus, utilizing coherent phenomena is not necessarily required for highly efficient charge separation in organic solar cells.This project has received funding from the Universidad Carlos III de Madrid, the European Union’s Seventh Framework Programme for research, technological development, and demonstration under Grant Agreement No. 600371, el Ministerio de Economı́a, Industria y Competitividad (COFUND2014-51509), el Ministerio de Educación, cultura y Deporte (CEI-15-17), and Banco Santander. We also acknowledge additional funding from the German Research Foundation DFG (GRK1640) and the Bavarian University Centre for Latin America (BAYLAT)
A Combined Theoretical and Experimental Study of Dissociation of Charge Transfer States at the Donor–Acceptor Interface of Organic Solar Cells
The observation that in efficient
organic solar cells almost all
electron–hole pairs generated at the donor–acceptor
interface escape from their mutual coulomb potential remains to be
a conceptual challenge. It has been argued that it is the excess energy
dissipated in the course of electron or hole transfer at the interface
that assists this escape process. The current work demonstrates that
this concept is unnecessary to explain the field dependence of electron–hole
dissociation. It is based upon the formalism developed by Arkhipov
and co-workers as well as Baranovskii and co-workers. The key idea
is that the binding energy of the dissociating “cold”
charge-transfer state is reduced by delocalization of the hole along
the polymer chain, quantified in terms of an “effective mass”,
as well as the fractional strength of dipoles existent at the interface
in the dark. By covering a broad parameter space, we determine the
conditions for efficient electron–hole dissociation. Spectroscopy
of the charge-transfer state on bilayer solar cells as well as measurements
of the field dependence of the dissociation yield over a broad temperature
range support the theoretical predictions
Role of Intrinsic Photogeneration in Single Layer and Bilayer Solar Cells with C-60 and PCBM
In an endeavor to examine how optical excitation of C-60 and PCBM contribute to the photogeneration of charge carriers in organic solar cells, we investigated stationary photogeneration in single-layer C-60 and PCBM films over a broad spectrum as a function, of the electric field. We find that intrinsic photogeneration starts at a photon energy of about 2.25 eV, i.e., about 0.4 eV above the first singlet excited state. It originates from charge transfer type states that can autoionize before relaxing to the lower-energy singlet Si state, in the spirit of Onsager's 1938 theory. We analyze the internal quantum efficiency as a function of electric field and photon energy to determine (1) the Coulombic binding and separation of the electron hole pairs, (2) the value of the electrical gap, and (3) which fraction of photoexcitations can fully separate at a given photon energy. The latter depends on the coupling between the photogenerated charge transfer states and the eventual charge transporting states. It is by a factor of 3 lower in PCBM. Close to the threshold energy for intrinsic photoconduction (2.25 eV), the generating entity is a photo generated electron-hole pair with roughly 2 nm separation. At higher photon energy, more expanded pairs are produced incoherently via thermalization
Facile Synthesis and Chain-Length Dependence of the Optical and Structural Properties of Diketopyrrolopyrrole-Based Oligomers
Here, we report the synthesis, optical properties, and solid-state packing of monodisperse oligomers of diketopyrrolopyrrole (DPP) up to five repeating units. The optical properties of DPP oligomers in solution and the solid state were investigated by a combination of steady-state and transient spectroscopy. Transient absorption spectroscopy and time-correlated single photon counting (TCSPC) measurements show that the fluorescence lifetime decreases with an increase in the oligomer size from monomer to trimer, thereby reaching saturation for pentameric DPP oligomers. The solid-state packing and crystallinity were probed by using advanced techniques, which included grazing incidence small-angle X-ray scattering (GISAXS) and X-ray diffraction (XRD) to elucidate the structure-property trend. Collectively, our chain-length dependent studies establish the fundamental correlation between the structure and property and provide a comprehensive understanding of the solid-state properties in DPP-DPP based conjugated systems
Facile Method for the Investigation of Temperature-Dependent C<sub>60</sub> Diffusion in Conjugated Polymers
We developed a novel
all-optical method for monitoring the diffusion of a small quencher
molecule through a polymer layer in a bilayer architecture. Experimentally,
we injected C<sub>60</sub> molecules from a C<sub>60</sub> layer into
the adjacent donor layer by stepwise heating, and we measured how
the photoluminescence (PL) of the donor layer becomes gradually quenched
by the incoming C<sub>60</sub> molecules. By analyzing the temporal
evolution of the PL, the diffusion coefficient of C<sub>60</sub> can
be extracted, as well as its activation energy and an approximate
concentration profile in the film. We applied this technique to three
carbazole-based low-bandgap polymers with different glass temperatures
with a view to study the impact of structural changes of the polymer
matrix on the diffusion process. We find that C<sub>60</sub> diffusion
is thermally activated and not driven by WFL-type collective motion
above <i>T</i><sub>g</sub> but rather by local motions mediated
by the sidechains. The results are useful as guidance for material
design and device engineering, and the approach can be adapted to
a wide range of donor and acceptor materials
Spectroscopic Study of Thiophene–Pyrrole-Containing S,N-Heteroheptacenes Compared to Acenes and Phenacenes
In
this study, we report a detailed spectroscopic study concerning
the energy levels and vibrational structure of thiophene–pyrrole-containing
S,N-heteroacenes. The aim of the study is first, to understand the
differences in the photoluminescence (PL) efficiencies in this structurally
similar series and second, to compare the electronic structure of
S,N-heteroacenes to that of linear acenes and phenacenes, with a view
to derive guidelines for the design of singlet fission materials.
For S,N-heteroacenes comprising seven fused heterocyclic rings, we
observe a higher PL quantum yield for derivatives with terminal thienothiophene
units than for thienopyrrole-capped ones. This is assigned to a stronger
tendency of the thienopyrrole-capped derivatives to form nonemissive
associates in dilute solution, producing emissive excimers at higher
concentration. By conducting time-resolved PL studies at 77 K, we
further determine the lowest singlet and triplet energies for the
S,N-heteroacenes with three, five, and seven fused rings. We show
that their energies evolve with oligomer length analogously to those
of phenacenes, yet in a fundamentally different way from that of linear
acenes. This difference in evolution is attributed to the increasingly
biradical character in acenes with increasing chain length in contrast
to the S,N-heteroacenes and phenacenes
Role of Intrinsic Photogeneration in Single Layer and Bilayer Solar Cells with C<sub>60</sub> and PCBM
In
an endeavor to examine how optical excitation of C<sub>60</sub> and
PCBM contribute to the photogeneration of charge carriers in
organic solar cells, we investigated stationary photogeneration in
single-layer C<sub>60</sub> and PCBM films over a broad spectrum as
a function of the electric field. We find that intrinsic photogeneration
starts at a photon energy of about 2.25 eV, i.e., about 0.4 eV above
the first singlet excited state. It originates from charge transfer
type states that can autoionize before relaxing to the lower-energy
singlet S<sub>1</sub> state, in the spirit of Onsager’s 1938
theory. We analyze the internal quantum efficiency as a function of
electric field and photon energy to determine (1) the Coulombic binding
and separation of the electron–hole pairs, (2) the value of
the electrical gap, and (3) which fraction of photoexcitations can
fully separate at a given photon energy. The latter depends on the
coupling between the photogenerated charge transfer states and the
eventual charge transporting states. It is by a factor of 3 lower
in PCBM. Close to the threshold energy for intrinsic photoconduction
(2.25 eV), the generating entity is a photogenerated electron–hole
pair with roughly 2 nm separation. At higher photon energy, more expanded
pairs are produced incoherently via thermalization