A donor-acceptor-acceptor molecule for vacuum-processed organic solar cells with a power conversion efficiency of 6.4%

Article abstract of DOI:10.1039/c2cc16390j

A D-A-A-type molecular donor (DTDCTP) featuring electron-accepting pyrimidine and dicyanovinylene blocks has been synthesized for vacuum-deposited planar-mixed heterojunction solar cells with C₇₀ as the acceptor, giving a power conversion effic

Full text of DOI:10.1039/c2cc16390j

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Chemical Communications
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Volume 48 | Number 13 | 11 February 2012 | Pages 1825–1932
ISSN 1359-7345
COMMUNICATION
Hao-Wu Lin and Ken-Tsung Wong et al.
A donor-acceptor-acceptor molecule for vacuum-processed organic solar
cells with a power conversion efficiency of 6.4%
1359-7345(2012)48:13;1-E

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A donor–acceptor–acceptor molecule for vacuum-processed organic solar
cells with a power conversion efficiency of 6.4%w
a
b
Shi-Wen Chiu,z Li-Yen Lin,z Hao-Wu Lin,*a Yi-Hong Chen,a Zheng-Yu Huang,a
Yu-Ting Lin,a Francis Lin,b Yi-Hung Liub and Ken-Tsung Wong*b
Received 14th October 2011, Accepted 2nd December 2011
DOI: 10.1039/c2cc16390j
A D–A–A-type molecular donor (DTDCTP) featuring electron-
accepting pyrimidine and dicyanovinylene blocks has been
synthesized for vacuum-deposited planar-mixed heterojunction
solar cells with C70 as the acceptor, giving a power conversion
efficiency as high as 6.4%.
bandgaps of materials, which is beneficial for robust spectral
coverage, and (ii) lowering the highest occupied molecular
orbital (HOMO) of donors to increase the open circuit voltage
(Voc) of the cell. To meet these requirements, small molecules
with acceptor–donor–acceptor (A–D–A)4c,7 architectures have
been successfully applied as donors and exhibited exceptional
performance. On the other hand, the donor–acceptor (D–A)8
molecules incorporating arylamines as the donor moieties also
show promise due to their intrinsically effective photoinduced
intramolecular charge transfer (ICT) characteristics and the
flexibility in structural modification. Notably, the merocyanine
dyes based devices8c,d have demonstrated PCEs surpassing
Organic photovoltaics (OPVs) have attracted considerable
research interest due to their low energy consumption in
fabrication, mechanical flexibility, and low-cost manufacturing.1
Among various OPVs device architectures, polymer bulk
heterojunction (BHJ) solar cells utilizing a self-assembly
phase-separated blend of polymeric donors (D) and fullerene
acceptors (A) as an active layer have been proven to be the
most successful design with state-of-the-art power conversion
efficiencies (PCEs) of 6–8%.2 The BHJ active layers can
provide large exciton dissociation interfaces while maintaining
bicontinuous interpenetrating networks for carrier transport. In
contrast, small-molecule OPVs3 featuring low-molecular-weight
compounds as donors showed relatively lower efficiencies due to
the comparable sizes of donors and acceptors, rendering
spontaneous formation of nanoscale phase-separated domains
difficult. Without the preferred film morphology, there will be
too many isolated traps inside the D/A blending layer, which is
unfavorable for efficient charge transport because of discontinuous
pathways. In this regard, many research endeavors have
adopted several approaches to controlling the favorable D/A
nanomorphology in small-molecule OPVs similar to those
performed in polymer solar cells. For instance, with thermal
annealing,4 solvent annealing,5 or self-assembly formed columnar
structure,6 the PCEs have reached respectable values of over 5%.
Along another line, the molecular design strategies employed
to improve performance of OPVs include (i) reducing the
6%.8d Very recently, we developed
a donor molecule
(DTDCTB, Scheme 1) with donor–acceptor–acceptor
a
(D–A–A) configuration,9 in which an electron-donating ditolyl-
aminothienyl moiety is connected to an electron-withdrawing
dicyanovinylene moiety through another electron-accepting
2,1,3-benzothiadiazole (BT) block. The elaborate combination
of two acceptors enables the resulting molecules to not only
effectively narrow the optical bandgaps but also possess lower
HOMO levels as compared to those of their parent analogs.10
Accordingly, such D–A–A architectures show potential
capabilities of concurrently enhancing the short circuit current
density (Jsc) and Voc as employed in OPVs. Vacuum-processed
devices based on DTDCTB exhibited distinguished light-harvesting
abilities with solar responses extending to the near-IR region
and achieved PCEs of up to 5.81%. However, the moderate
Voc value (B0.79 V) attained in DTDCTB-based devices leaves
room for improvement. In this communication, we report a
a Department of Materials Science and Engineering, National Tsing Hua
University, Hsin Chu 30013, Taiwan. E-mail: hwlin@mx.nthu.edu.tw
b Department of Chemistry, National Taiwan University,
Taipei 10617, Taiwan. E-mail: kenwong@ntu.edu.tw
w Electronic supplementary information (ESI) available: Synthesis,
characterization, absorption spectra of DTDCTP, copies of 1H and
13C NMR spectra, procedures for device fabrication and photovoltaic
characterization, device optimization, mobility diagrams, and a total
absorption spectrum. CCDC 848943 (DTDCTP). For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c2cc16390j
Scheme 1 Synthetic route to DTDCTP and the structure of
DTDCTB.
z Both authors contributed equally to this work.
c
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Chem. Commun., 2012, 48, 1857–1859 1857

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new D–A–A-type molecule, DTDCTP (Scheme 1), where the
pyrimidine acceptor is employed to replace the BT block. We
envisaged that the HOMO of DTDCTP would be lower than
that of DTDCTB and thus an augmented Voc can be anticipated,
although an inevitable loss of photocurrent is foreseeable.
Likewise, DTDCTP was designed to adopt a nearly coplanar
conformation to ensure a compact stacking in the solid state,
thus realizing high extinction coefficients throughout the
spectral coverage.
shows high k across the 470–630 nm wavelength range with
kmax E 1 at 550 nm. The observed kmax value is among the
highest values reported for organic solar-absorbing thin
films,11 implying the existence of compact-packing absorption
dipoles in the DTDCTP thin film that may substantially
increase the absorption efficiency.
Vacuum-processed planar-mixed heterojunction (PMHJ)
devices12 composed of a donor–acceptor blend sandwiched
between a pure donor and acceptor layer as active layers were
adopted in photovoltaic characterization. After fine-tuning the
thicknesses of active layers (Fig. S2–S5 and Tables S2 and S3,
ESIw), the optimized device structures were configured as
follows: (i) device DTDCTP:C60: ITO/MoO3 (20 nm)/DTDCTP
(7 nm)/DTDCTP:C60 (1 : 1, by volume, 40 nm)/C60 (20 nm)/2,9-
dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (10 nm)/Ag,
and (ii) device DTDCTP:C70: ITO/MoO3 (20 nm)/DTDCTP
(7 nm)/DTDCTP:C70 (1: 1, by volume, 40 nm)/C70 (6 nm)/BCP
(10 nm)/Ag. The MoO3 thin film acts as the hole-transporting
layer and the BCP thin film serves as the electron-transporting
layer with exciton blocking characteristics. Fig. 2 shows the current
density–voltage (J–V) characteristics of DTDCTP:fullerene
solar cells. The DTDCTP:C60 device gave a Voc of 0.95 V, a
Jsc of 8.3 mA cmꢀ2, and a fill factor (FF) of 0.54, leading to a
The synthetic pathway of DTDCTP is shown in Scheme 1.
Palladium-catalyzed Negishi coupling reaction of 2-[N,N-di-
(p-tolyl)amino]thiophene with 5-bromo-2-iodopyrimidine
afforded 1, which was then converted to the corresponding
carbaldehyde 2 by lithiation with n-butyllithium and sub-
sequently quenching with ethyl formate. Finally, the aldehyde
2 was condensed with malononitrile to yield the target com-
pound DTDCTP via the Knoevenagel reaction in the presence
of basic aluminium oxide.
The molecular structure of DTDCTP was analyzed by X-ray
crystallography. As shown in Fig. 1(a), DTDCTP displays a
nearly coplanar conformation between thiophene and pyrimidine
with a dihedral angle of 5.81 due to the lack of ortho–ortho steric
interactions. This coplanarity facilitates the electronic coupling
between the electron-donating and electron-withdrawing blocks,
thus enhancing the ICT efficiency to increase the spectral
response range. The highly polar character and coplanar
conformation of the heteroaryl components lead DTDCTP
crystals to pack in an antiparallel manner along both molecular
axis directions. The cofacial arrangement of two pyrimidine
rings of neighboring DTDCTP molecules with a shortest
distance of around 3.3 A indicates significant p–p interactions,
which may facilitate charge carrier transport.
PCE of 4.3% under AM 1.5G, 1 sun (100 mW cmꢀ2
)
simulated solar illumination. Moreover, the DTDCTP:C70
device delivered a markedly high spectra-mismatch-corrected
PCE of 6.4% with a Voc of 0.95 V, Jsc of 12.1 mA cmꢀ2, and
FF of 0.56. The high Voc values for the devices are stemmed
from the low-lying HOMO level (ꢀ5.46 eV) of DTDCTP
acquired by using ultraviolet photoelectron spectroscopy,
which is deeper than that of DTDCTB (ꢀ5.30 eV). It is
noteworthy that this difference in HOMO levels significantly
contributes to the increase in Voc for DTDCTP-based devices.
Apparently, an enhanced Voc can be successfully achieved
through molecular structure modification, and the electron-
withdrawing nature of pyrimidine effectively limits the HOMO-
lifting effect that is usually encountered in thiophene-embedded
p-conjugated systems. The larger Jsc obtained in the DTDCTP:C70
device can be ascribed to higher and broader extinction
The absorption band of DTDCTP in CH2Cl2 exhibits a
maximum at 556 nm (e = 53 900 Mꢀ1 cmꢀ1), which broadens
in the vacuum-deposited thin film, possibly due to p–p stacking
in the solid state (Fig. S1 in ESIw). The extinction coefficient (k)
spectrum of the DTDCTP thin film (60 nm) measured by
spectroscopic ellipsometry is depicted in Fig. 1(b). The film
Fig. 2 J–V characteristics of DTDCTP:C60 PMHJ (square) and
DTDCTP:C70 PMHJ (circle) solar cells. Inset: external quantum
efficiency spectrum of DTDCTP:C60 PMHJ (square) solar cells,
external (circle) and internal (solid line) quantum efficiency spectra
of DTDCTP:C70 PMHJ solar cells.
Fig. 1 (a) The X-ray analyzed molecular structure and crystal packing
of DTDCTP. (b) Extinction coefficient (k) spectra of DTDCTP, C60
,
and C70 thin films.
c
1858 Chem. Commun., 2012, 48, 1857–1859
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mainly due to faultless interpenetrating networks of the very thin
(40 nm) mixed layer.
In conclusion, a tailor-made molecule DTDCTP with a D–A–A
molecular architecture has been synthesized and developed as the
donor material in small-molecule OPVs. The adoption of the
pyrimidine acceptor in DTDCTP has successfully improved the
Voc as compared to our recent work,9 and an enhanced PCE of
6.4% has been realized through striking a balance between
photovoltage and photocurrent. This efficiency is among the
highest ever obtained for organic vacuum-deposited single cells.
Further engineering of molecular structures is currently in
progress and will be reported in due course.
Fig. 3 Tapping-mode atomic force microscopy images of the
DTDCTP:C70 mixed layer shown in a 500 nm ꢁ 500 nm surface area.
(a) The topography and (b) the corresponding phase image.
The authors would like to acknowledge the financial support
from National Science Council of Taiwan (NSC 98-2112-M-
007-028-MY3, 98-2119-M-002-007-MY3) and the Low Carbon
Energy Research Center, National Tsing-Hua University.
coefficients of C70 as compared to those of C60 shown in
Fig. 1(b). Therefore, the better light-harvesting capabilities
of C70 significantly contribute to the higher external quantum
efficiencies (EQE) of the DTDCTP:C70 device in comparison
to those of the DTDCTP:C60 counterpart (inset of Fig. 2).
Note that the integrated values of EQE spectra with the
standard AM 1.5G solar spectrum always agree with the Jsc
values with deviation less than 5% in this study. The FF values
were comparable for DTDCTP:C60 and DTDCTP:C70 devices,
suggesting similar blending layer morphologies and charge
carrier percolated networks in both devices. In addition, the
nanoscale morphology of the DTDCTP:C70 mixed thin film
was probed by tapping-mode atomic force microscopy
(AFM). Surface topography and phase images of the thin film
show a smooth morphology with root-mean-square roughness
of B0.86 nm (Fig. 3). Interestingly, the phase image clearly
shows the appearance of phase-separated domains with sizes
of tens of nanometres. This result indicates that a preferred
morphology was formed in the blending layer and thus provided
sufficient D/A interfaces for exciton dissociation and effective
pathways for charge carrier transport. The results together with
balanced electron and hole mobilities in the DTDCTP:C70
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)
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collected at the electrodes, the simulated EQE spectrum matches
well with the experimental data. This result reveals that only
B5% of free charges recombine during the course of transport
towards electrodes. The remarkably high collection efficiency is
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c
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Chem. Commun., 2012, 48, 1857–1859 1859