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Polymer solar cells
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Nature Photonics · February 2012
DOI: 10.1038/nphoton.2012.11
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Peking University
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nature photonics
| VOL 6 | MARCH 2012 | www.nature.com/naturephotonics
153
H
arnessing solar energy is one of the most promising ways
to tackle today’s energy issues. Although the present domi-
nant photovoltaic (PV) technology is based on inorganic
materials, high material and manufacturing
costs limit its wide
acceptance
1
. Intensive research has been conducted towards the
development of low-cost PV technologies, of which organic pho-
tovoltaic (OPV) devices are one of the promising. OPV devices
are based on organic semiconductors — carbon-based materials
whose backbones are comprised mainly of alternating C–C and
C=C bonds. Electron delocalization
along the conjugated back-
bone is responsible for the semiconducting properties of OPV
devices
2
. One of the major differences between organic semicon-
ductors and inorganic semiconductors is the presence of tightly
bonded excitons (electron–hole pairs) resulting from their low
dielectric constant (
ε
r
≈ 2–4). The binding energy of the Frenkel
exciton is in the range of 0.3–1 eV (refs 2,3). Such a large bind-
ing energy prevents exciton dissociation by an electrical field (a
non-radiative decay channel) and can
achieve a high electrolu-
minescent efficiency in organic light-emitting devices. The weak
intermolecular van de Waals interaction enables the realization
of low-cost, large-area deposition technologies such as roll-to-
roll printing
3
.
In recent years, organic electronic devices such as
organic light-emitting diodes (OLEDs), organic thin film tran-
sistors, OPVs and organic memory devices have attracted con-
siderable attention, owing to their potential low cost and high
performance characteristics. OLED displays have gained a consid-
erable share in the
portable electronics market, for use in devices
such as smart phones. However, research into OPV cells continues
to lag behind, despite the first patent
4
and the first paper
5
by Tang
appearing ahead of those of OLEDs,
probably owing to the fact
that developing alternative energy sources has been viewed, until
recently, as being relatively unimportant.
OPVs are divided into two different categories according to
whether their constituent molecules are either small or large (pol-
ymers). These two classes of materials are rather different in terms
of their synthesis, purification and device fabrication processes.
Polymer solar cells (PSCs) are processed
from solution in organic
solvents, whereas small-molecule solar cells are processed mainly
using thermal evaporation deposition in a high-vacuum environ-
ment. Using the solution process to fabricate small-molecule solar
cells has recently been gaining momentum
6
, although the film
quality and crystallization is expected to be an issue. PSCs are
attractive owing to a number
of advantageous features
7
, includ-
ing their thin-film architecture and low material consumption
resulting from a high absorption coefficient, their use of organic
