Efficient charge generation and collection in organic solar cells based on low band gap dyad molecules

Article abstract of DOI:10.1039/c1cc11387a

Low band gap dyad molecules were prepared that have absorption spectra matched well with the solar spectrum, and the construction of efficient charge transport pathways was observed. Under AM 1.5 illumination, the devices have achieved the highest J_SC (4.79 mA cm²) and FF (0.46) in dyad-based organic solar cells to date.

Full text of DOI:10.1039/c1cc11387a

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Efficient charge generation and collection in organic solar cells based on
low band gap dyad moleculesw
Seiichiro Izawa,a Kazuhito Hashimoto*ab and Keisuke Tajima*ab
Received 10th March 2011, Accepted 4th April 2011
DOI: 10.1039/c1cc11387a
Low band gap dyad molecules were prepared that have
absorption spectra matched well with the solar spectrum, and
the construction of efficient charge transport pathways was
observed. Under AM 1.5 illumination, the devices have achieved
the highest JSC (4.79 mA cm2) and FF (0.46) in dyad-based
organic solar cells to date.
each other, phase segregation is suppressed, and higher charge
separation, owing to the proximity of the donor and the
acceptor, leads to current generation. These characteristics
of dyads enable single-component OSCs to be constructed,
which is advantageous in terms of nanostructure control and
device reproducibility.9 Many research studies have been
reported on the synthesis of dyads and their application in
OSCs.23 Our group has reported dyad molecules with
p-conjugated oligomers and fullerene derivatives.9,10,13 The
efficiency has been improved from about 0.1% in early studies
to over 1% in recent reports.10 This improvement comes from
changes in the design of the electron donor moiety. Introduction
of a highly crystalline donor part, namely, oligo(p-phenylene-
vinylene) (OPV), resulted in a fill factor (FF) of 0.44, improved
from 0.23 in less crystalline oligothiophene-based dyad
molecules.9,10 Crucial problems that must be addressed in
order to improve the efficiency of dyad OSCs are low FF
and the mismatch between the absorption spectrum of the
donor part and the solar spectrum. So far, these shortcomings
limit the short circuit current (JSC) and FF of the dyad devices
to 3.3 mA cmÀ2 and 0.44, respectively.10
Organic solar cells (OSCs) fabricated by solution processes
using p-conjugated materials have attracted much attention as
a renewable energy source, owing to their low fabrication cost,
light weight, and high flexibility.1,2 To date, the highest
efficiency of bulk heterojunction (BHJ) structures, which are
prepared by mixing electron donor and acceptor materials, has
reached 7–8% when low band gap polymers and fullerene
derivatives are used.3 The large interface between the donor
and the acceptor, and the formation of the nanoscale inter-
penetrating networks that serve as efficient charge transport
pathways are considered to be important for high performance
in OSCs.4 However, such phase-separated nanostructures are
not necessarily well controlled since they are made by simple
mixing of two materials and subsequent thermal annealing
for a short time. Hence, BHJ systems often suffer from undesirable
structural changes such as too much phase segregation,
which causes low reproducibility and poor stability in device
performance.5–7
In this study, low band gap dyad molecules with diketo-
pyrrolopyrrole (DPP) units as the donor and a fullerene group
as the acceptor were synthesized to match the molecules’
absorption spectra with the solar spectrum. The number of
thiophene rings in the donor part was varied in order to tune
the structure of the films. OSC devices were fabricated, and in
this report, the relationship between molecular structure, film
morphology, and device performance is discussed.
Consisting of covalently attached donor and acceptor
moieties, dyad molecules have been widely studied with a
primary aim of gaining fundamental understanding of charge
separation in solution.8–11 Dyads have several advantages over
mixed BHJs when used in OSCs; the use of a molecular based
system can avoid the many problems with polymers such as
molecular weight distribution, batch-to-batch variation, end
group contamination, and difficulty in purification.12 More-
over, since the donor and acceptor molecules are connected to
The chemical structures of the dyads with different lengths
of the oligothiophene unit (n = 1–3) are shown in Fig. 1. DPP
derivatives are known as excellent dyes,14,15 and have been
used as an electron-withdrawing group in so-called donor–
acceptor type low band gap polymers and oligomers.16–19
a Department of Applied Chemistry, Graduate School of Engineering,
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656,
Japan. E-mail: k-tajima@light.t.u-tokyo.ac.jp,
hashimoto@light.t.u-tokyo.ac.jp
b HASHIMOTO Light Energy Conversion Project,
Exploratory Research for Advanced Technology (ERATO), Japan
Science Technology Agency (JST), Japan
w Electronic supplementary information (ESI) available: Materials
and instruments, synthesis, DFT calculations, device fabrication and
measurement, and AFM images and film thickness. See DOI: 10.1039/
c1cc11387a
Fig. 1 Chemical structure of low band gap dyad molecules.
Chem. Commun., 2011, 47, 6365–6367 6365
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Oligothiophene was used as an electron-donating group.
Three oligothiophenes with different lengths were synthesized
to investigate the influence of the connectivity of the donor
part on the film and the charge collection ability. The synthesis
of the low band gap dyad molecules is described in the ESI.w
Briefly, a methyl group and hexaethylene glycol were attached
to the DPP core. The modified DPP core was then connected
to the oligothiophenes via Stille coupling.20 Finally, the dyad
molecules 4T (n = 1), 6T (n = 2) and 8T (n = 3) were
synthesized via the esterification reaction between the DPP–
oligothiophene donor part and a fullerene derivative with a
carboxylic acid chloride functional group.9 The p-oligomers
are attached to the fullerene derivative through a long flexible
spacer of hexaethylene glycol, which enables moderate
segregation of the donor and acceptor groups, and leads to
efficient charge separation and construction of efficient charge
transport pathways at the same time.10 After purification by
column chromatography and preparative HPLC, the product
was identified by 1H-NMR spectroscopy and MALDI-TOF-MS.
The spectral data are available in the ESI.w
The absorption spectra of 4T, 6T, and 8T in chloroform
solution are shown in Fig. 2(a). Absorption peaks at around
330 nm correspond to fullerene, and those at around 600 nm
correspond to the oligomers. As the number of thiophene units
was increased from 4T to 8T, continuous shifts of absorption
edges were observed due to the increased p-conjugation length
in the donor part. Absorption spectra of the dyads in thin films
are shown in Fig. 2(b). Compared with the absorption spectra
in the solution in Fig. 2(a), red shifts of the absorption edges
and broadening of the absorption width were observed.
The absorption spectra of all dyads matched well with the
solar spectrum, as shown in Fig. 2(b), owing to the low band
gap properties.
The first oxidation (Eox) and reduction potentials (Ered) of
the dyad molecules in solution were determined by cyclic
voltammetry (CV). The energies of the highest occupied
molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) were estimated by assuming the
Fc/Fc+ potential to be À4.8 eV relative to the vacuum level.17
The results are summarized in Table 1. The LUMO energy
levels were almost the same as in PCBM because the acceptor
groups have analogous structures. In contrast, the energy
levels of the HOMO in the donor became higher-lying as the
number of thiophenes increased, owing to the conjugation
length in the donor part.
Solar cell devices were fabricated with a simple configuration of
ITO/dyad/Ca (20 nm)/Al (30 nm). The solutions of 4T, 6T,
and 8T had concentrations of 20, 20, and 15 g LÀ1
,
respectively, and the active layers were spin-coated at a
spinning rate of 2500 rpm. The active layers were, respectively,
90 nm, 70 nm, and 100 nm in thickness, as measured by
surface profilometry. After Ca and Al evaporation, the devices
with 6T and 8T dyads were thermally annealed at 110 1C for
5 min under an N2 atmosphere, while the device with 4T dyad
was not annealed because the device performance was found in
preliminary studies to be degraded by annealing.21 The OSC
devices were evaluated under irradiation of simulated solar
light (AM 1.5, 100 mW cmÀ2) through a photomask with an
aperture size of 0.06 cmÀ2
.
J–V characteristics of the dyad devices under AM 1.5
illumination are shown in Fig. 3(a), and the performance of
each device is summarized in Table 1. As the number of
thiophene units was increased in the p-conjugated oligomers,
the photocurrent increased from 0.69 to 4.79 mA cmÀ2
;
furthermore, FF also increased from 0.25 to 0.46. In contrast,
OC decreased from 0.6 to 0.51 V when the thiophene number
V
was increased. This is because the energy level of the HOMO
in the donor part became higher, as determined by CV. Due
to low band gap properties and favorable hole transport
properties, the device of 8T dyad showed power conversion
efficiency of over 1%. In particular, JSC and FF were the
highest among dyad-based solar cells.
In the external quantum efficiency (EQE) spectra shown in
Fig. 3(b), photocurrent response at longer wavelengths can be
seen, owing to the low band gap properties, which led to high
JSC. Especially in the 8T dyad device, the photocurrent
response was extended to 850 nm. Furthermore, when the
number of thiophene units was increased from 4T to 8T, the
EQE value increased gradually to their highest values, from
0.08 to 0.27. The increase in JSC induced by the change of the
absorption spectra was estimated from the normalized UV-vis
spectra and the solar spectrum. The results indicate that the
broader absorption of 8T compared with 4T should increase
JSC by only about 25%. However, the estimated increase was
much smaller than the observed enhancement of JSC under
solar illumination for 8T (about 7-fold higher than JSC for 4T).
Therefore, the major factor in the improved device
performance could be a more effective construction of hole
transport pathways with a greater number of thiophene units.
Fig. 2 (a) Absorption spectra of dyad molecules in CHCl3 solution.
(b) Solar spectrum and absorption spectra of dyad molecules in thin
films.
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Table 1 Optical and electrochemical properties and devise performances
Material
Eoxa/V
Ereda/V
Egb/eV
VOC/V
ISC/mA cmÀ2
FF
PCE (%)
4T dyad
6T dyad
8T dyad
PCBM
0.44
0.33
0.25
À1.11
À1.13
À1.11
À1.16
1.86
1.77
1.73
0.60
0.56
0.51
0.69
3.17
4.79
0.25
0.33
0.46
0.10
0.56
1.11
Half potentials determined by cyclic voltammetry vs. Fc/Fc+
.
Optical band gaps.
a
b
should be improved by introduction of a larger hole transport
part, and this study demonstrates the high potential of
donor–acceptor dyads as photovoltaic materials. Further
optimization of energy levels could enable realization of higher
3,22
VOC and JSC
,
and highly efficient solar cells based on a
single component.
Notes and references
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Chem. Commun., 2011, 47, 6365–6367 6367