2 research outputs found
Impact of Molecular Orientation and Spontaneous Interfacial Mixing on the Performance of Organic Solar Cells
A critically important question that
must be answered to understand
how organic solar cells operate and should be improved is how the
orientation of the donor and acceptor molecules at the interface influences
exciton diffusion, exciton dissociation by electron transfer, and
recombination. It is exceedingly difficult to probe the orientation
in bulk heterojunctions because there are many interfaces and they
are arranged with varying angles with respect to the substrate. One
of the best ways to study the interface is to make bilayer solar cells
with just one donor–acceptor interface. Zinc phthalocyanine
is particularly interesting to study because its orientation can be
adjusted by using a 2 nm-thick copper iodide seed layer before it
is deposited. Previous studies have claimed that solar cells in which
fullerene acceptor molecules touch the face of zinc phthalocyanine
have more current than ones in which the fullerenes touch the edge
of zinc phthalocyanine because of suppressed recombination. We have
more thoroughly characterized the system using in situ X-ray photoelectron
spectroscopy and X-ray scattering and found that the interfaces are
not as sharp as previous studies claimed when formed at room temperature
or above. Fullerenes have a much stronger tendency to mix into the
face-on films than into the edge-on films. Moreover we show that almost
all of the increase in the current with face-on films can be attributed
to improved exciton diffusion and to the formation of a spontaneously
mixed interface, not suppressed recombination. This work highlights
the importance of spontaneous interfacial molecular mixing in organic
solar cells, the extent of which depends on molecular orientation
of frontier molecules in donor domains
Importance of the Donor:Fullerene Intermolecular Arrangement for High-Efficiency Organic Photovoltaics
The performance of organic photovoltaic
(OPV) material systems
are hypothesized to depend strongly on the intermolecular arrangements
at the donor:fullerene interfaces. A review of some of the most efficient
polymers utilized in polymer:fullerene PV devices, combined with an
analysis of reported polymer donor materials wherein the same conjugated
backbone was used with varying alkyl substituents, supports this hypothesis.
Specifically, the literature shows that higher-performing donor–acceptor
type polymers generally have acceptor moieties that are sterically
accessible for interactions with the fullerene derivative, whereas
the corresponding donor moieties tend to have branched alkyl substituents
that sterically hinder interactions with the fullerene. To further
explore the idea that the most beneficial polymer:fullerene arrangement
involves the fullerene docking with the acceptor moiety, a family
of benzoÂ[1,2-b:4,5-b′]Âdithiophene–thienoÂ[3,4-<i>c</i>]Âpyrrole-4,6-dione polymers (PBDTTPD derivatives) was synthesized
and tested in a variety of PV device types with vastly different aggregation
states of the polymer. In agreement with our hypothesis, the PBDTTPD
derivative with a more sterically accessible acceptor moiety and a
more sterically hindered donor moiety shows the highest performance
in bulk-heterojunction, bilayer, and low-polymer concentration PV
devices where fullerene derivatives serve as the electron-accepting
materials. Furthermore, external quantum efficiency measurements of
the charge-transfer state and solid-state two-dimensional (2D) <sup>13</sup>CÂ{<sup>1</sup>H} heteronuclear correlation (HETCOR) NMR analyses
support that a specific polymer:fullerene arrangement is present for
the highest performing PBDTTPD derivative, in which the fullerene
is in closer proximity to the acceptor moiety of the polymer. This
work demonstrates that the polymer:fullerene arrangement and resulting
intermolecular interactions may be key factors in determining the
performance of OPV material systems