Dithienopyrrole-based oligothiophenes for solution-processed organic solar cells

Article abstract of DOI:10.1039/c3cc46066e

Isomeric dicyanovinylene-terminated oligothiophenes 1 and 2 comprising a central dithienopyrrole (DTP) unit have been developed for solution-processed small molecule organic solar cells (SMOSCs) giving the highest power conversion efficiency of 4.8% for DTP-based oligomeric materials. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc46066e

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Dithienopyrrole-based oligothiophenes for
solution-processed organic solar cells†
Cite this: Chem. Commun., 2013,
4
9, 10865
a
b
b
b
Martin Weidelener, Cordula D. Wessendorf, Jonas Hanisch, Erik Ahlswede,
Received 8th August 2013,
a
c
a
a
G u¨ nther G o¨ tz, Mika Lind e´ n, Gisela Schulz, Elena Mena-Osteritz,
Accepted 30th September 2013
a
a
Amaresh Mishra and Peter B a¨ uerle*
DOI: 10.1039/c3cc46066e
www.rsc.org/chemcomm
5
Isomeric dicyanovinylene-terminated oligothiophenes 1 and 2 vacuum-processed SMOSCs and is now transferred to solution-
comprising a central dithienopyrrole (DTP) unit have been devel- processable oligomers. In this respect, oligomers 1 and 2 were
oped for solution-processed small molecule organic solar cells designed such that solubilizing side chains were introduced at the
(SMOSCs) giving the highest power conversion efficiency of 4.8% central nitrogen (2-ethylhexyl) and regioregularly at each residual
for DTP-based oligomeric materials.
thiophene ring (n-hexyl) of the hexamer as a mimic of regioregular
poly(3-hexylthiophene). The two isomeric oligomers now only differ
The last few years have seen great progress in the use of solution- in the positioning of the hexyl side chains, which are directed
ꢀ
1
processable and structurally defined conjugated oligomers, referred ‘‘outward’’ in 1 and ‘‘inward’’ in 2. Good solubilities (Z10 mg mL
)
to as ‘‘small molecules’’, as light harvesting materials in organic have thus been obtained and solution-processed BHJ solar cells
1
2a
solar cells. Power conversion efficiencies (PCE) of up to 9% have based on 1 displayed an excellent PCE of 4.8% and a high fill factor
been reached primarily due to the development of better light- of 0.66. The influence of the positions of the hexyl side chains
2
absorbing donor oligomers. Most of these consist of alternating on device performance is discussed. An A–D–A-type hexamer
donor (D) and acceptor (A) units in the p-conjugated backbone, containing a central DTP unit and terminal cyano acrylic acid
2
b
0
0
2c
such as oligothiophenes, ring-fused dithieno[3,2-b:2 ,3 -d]siloles,
groups was recently used in dye-sensitized solar cells showing a
moderate efficiency of 1.24%.
0
2d
2e
6
or benzo[1,2-b:4,5-b ]dithiophenes for D, and dicyanovinylene,
diketopyrrolopyrroles, pyridyl[2,1,3]thiadiazole, rhodanine, or
cyanoacrylic ester for A moieties. In contrast, the fused and Scheme 1. N-(2-ethylhexyl)dithieno[3,2-b:2 ,3 -d]pyrrole 4 was pre-
electron-rich dithieno[3,2-b:2 ,3 -d]pyrrole (DTP) unit has rarely pared by Pd -catalyzed Buchwald-Hartwig coupling of 4,4 -dibromo-
been used as a building block in p-type semiconducting oligomers 2,2 -bithiophene 3 and 2-ethylhexyl amine in a yield of 92%.
reaching only moderate efficiencies of 0.2–1.3% in SMOSCs.
2f
2c
2b
The new oligomers 1 and 2 were synthesized according to
2b
0
0
0
0
3
0
0
0
7
4
Corresponding bis-stannylated dithienopyrrole 5 was obtained by
In this communication, we now describe synthesis, characteriza- lithiation of 4 using n-BuLi followed by quenching with trimethyltin
tion, and photovoltaic application of two novel A–D–A oligothio- chloride. The synthetic route used to obtain 6 and 7 is described in
phenes 1 and 2 comprising a central DTP unit and dicyanovinylene the ESI† (Schemes S1 and S2). Oligomers 1 and 2 were finally
0
(
DCV) as terminal acceptor groups. The motivation to implement synthesized by Pd -catalyzed Stille-type coupling of 5 with dicyano-
DTP units in A–D–A oligothiophenes is a high charge-transfer vinylene (DCV)-substituted iodobithiophene 6 and bromobithio-
character and thus a red-shift of the longest wavelength absorption, phene 7. After final purification by HPLC, oligomers 1 and 2 were
which should be favourable for solar cells. The structural con- obtained in 84% and 74% yield, respectively.
cept of A–D–A oligothiophenes has been very successfully used in
Electronic absorption spectra of 1 and 2 in dichloromethane
are shown in Fig. 1 and data are summarized in Table 1. DTP-
sexithiophene 1 showed an intensive charge-transfer absorption
band at 593 nm with a molar extinction coefficient e of
a
Institute of Organic Chemistry II and Advanced Materials, Ulm University, Albert-
Einstein-Allee 11, 89081 Ulm, Germany. E-mail: peter.baeuerle@uni-ulm.de
ꢀ1
ꢀ1
b
70 100 L mol
cm
whereas the maximum absorption of
Zentrum f u¨ r Sonnenenergie- und Wasserstoff-Forschung Baden-W u¨ rttemberg,
Industriestraße 6, 70565 Stuttgart, Germany
constitutional isomer 2 is blue-shifted to 562 nm and less
c
ꢀ1
ꢀ1
Institute of Inorganic Chemistry II, Ulm University, Albert-Einstein-Allee 11, 89081
intensive (e = 58 700 L mol cm ) (Table 1). This is due to steric
hindrance of the ‘‘inward’’ hexyl side chains with the DTP moiety.
Compared to the solution spectra, the bands of 1 and 2 as thin films
prepared by spin-coating from chloroform solutions are significantly
red-shifted to 698 and 700 nm (Fig. 1), respectively, indicating
Ulm, Germany
†
Electronic supplementary information (ESI) available: Synthesis of compounds
6
and 7, cyclic voltammetry, additional photovoltaic devices and scanning
electron microscopy images as well as XRD measurements. See DOI: 10.1039/
c3cc46066e
This journal is c The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 10865--10867 10865

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Fig. 2 J–V and EQE curves of devices containing oligomers 1:PC61BM and
2
:PC61BM as photoactive layers.
Table 2 Photovoltaic parameters of solar cells fabricated using the following
device structure: ITO|PEDOT:PSS|Oligomer : PC61BM (1 : 2)|LiF|Al. Active layer
thicknesses of the cells were B75 nm (1) or B60 nm (2) ꢁ 10 nm
Aver. PCE
PCE [%] (ꢁ std. dev.) [%]
ꢀ
2
Oligomer
J
SC [mA cm
]
V
OC [V] FF
Scheme 1 Synthetic pathway developed to yield oligomers 1 and 2.
a
1
2
8.8
3.4
0.83
0.72
0.66 4.8
0.33 0.8
4.39 (ꢁ0.22)
b
0.51 (ꢁ0.19)
a
b
Average over 12 devices. Average over 14 devices.
[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). The current
density–voltage ( J–V) and external quantum efficiency (EQE) plots
are displayed in Fig. 2, whereas Table 2 summarizes the photovoltaic
data as well as the average PCE over at least 12 devices. A more detailed
table showing the effect of varying the oligomer:fullerene content in the
photoactive layer is given in Table S1 (ESI†). An oligomer : PCBM ratio
of 1 : 2 spin-coated from chloroform yielded the best performance for
both oligomers. The data clearly show that for oligomer 1 (‘‘outward’’)
all solar cell parameters are by far better. A higher short circuit current
Fig. 1 Absorbance spectra of oligomers 1 and 2 in solution and as thin films.
ꢀ
2
ordering of the molecules in the bulk. The improved vibronic density ( JSC, 8.8 vs. 3.4 mA cm ), open-circuit voltage (VOC, 0.83 vs.
resolution of the absorption of oligomer 1 might arise from a better 0.72 V), and fill factor (FF, 0.66 vs. 0.33) resulted in a significantly larger
opt
packing in the thin film. The optical band gaps E
g
for 1 and 2 were PCE of 4.8% for DTP derivative 1 compared to 0.8% for 2. The
1
.61 and 1.55 eV, respectively, which are lower compared to the surprisingly large discrepancy between the devices based on the two
opt
band gaps E
g
(1.82 eV) in solution.
oligomers 1 and 2 will be discussed below upon consideration of the
ꢀ
2
Cyclic voltammetry of oligomers 1 and 2 was performed in morphology of the photoactive layers. The good JSC (8.8 mA cm ) and
dichloromethane (Fig. S1, ESI†) and the detailed data are summarized very high FF (0.66) of 1:PC BM devices are indicative of both high and
in Table 1. Both oligomers showed two reversible 1e oxidation waves, balanced electron and hole transport in the blend.
61
ꢀ
which can be assigned to the stepwise oxidation of the oligomeric
It is noteworthy to mention that we found the performance of
backbone and the formation of stable radical cations and dications, solar cells made of 1 to be quite sensitive to the active layer
respectively. In the negative potential regime, one irreversible reduction thickness and film uniformity over the device area. This is demon-
wave at ꢀ1.50 V is visible, which corresponds to the simultaneous strated in Fig. S3 (ESI†) showing an image of the cross section of
reduction of the acceptor groups (Table 1). HOMO and LUMO energy such a 1:PC61BM blend and in Fig. S2 (ESI†) where the PCE is
levels of compounds 1 and 2 were calculated from the onset of the first plotted against the active layer thickness.
oxidation and reduction wave, respectively. The HOMO energy levels
In Fig. 2b, the EQE plots of 1:PC BM and 2:PC BM based
61 61
are very similar for both oligomers (ꢀ5.30 eV), whereas the LUMO devices are shown. Both oligomers display two maxima at short
energy of 2 (ꢀ3.75 eV) is lower compared to 1 (ꢀ3.68 eV) leading to a (B400 nm) and long wavelengths (B600–700 nm). DTP-oligomer 1
CV
g
smaller electrochemical band gap E
for 2 (1.55 eV vs. 1.60 eV).
showed a maximum EQE of 43%, whereas isomer 2 as expected
The photovoltaic properties of oligomers 1 and 2 were inves- showed a reduced maximum EQE of 24% at around 390 nm due to
ꢀ
2
tigated using a standard device structure based on blends with its much lower JSC (3.4 mA cm ).
Table 1 Summary of optoelectronic properties of oligomers 1 and 2
ꢀ1
ꢀ1
opt a
g
opt a
g
0
0
0
b
c
c
CV d
g
Oligomer lmax [nm] e [L mol cm ] E
(sol) [eV] lmax film [nm] E
(film) [eV] Eox1 [V] Eox2 [V] Ered1 [V] HOMO [eV] LUMO [eV] E
[eV]
1
2
593
562
70 100
58 700
1.80
1.82
(642), 698
663, 700
1.61
1.55
0.25
0.27
0.69
0.63
ꢀ1.49
ꢀ1.50
ꢀ5.28
ꢀ5.30
ꢀ3.68
ꢀ3.75
1.60
1.55
a
opt
g
b
Estimated using the onset of the UV-vis spectra in solution and in thin films E
= 1240/lonset
.
Determined from differential pulse voltammetry
d
c
+
(DPV). Estimated from the onset of the respective redox waves, the Fc/Fc value set to ꢀ5.1 eV vs. vacuum. Calculated from E
g
= ELUMO ꢀ EHOMO
.
1
0866 Chem. Commun., 2013, 49, 10865--10867
This journal is c The Royal Society of Chemistry 2013

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denser packing through a better interdigitation of the hexyl chains,
which seems to be even better in 1 compared to 2. A smaller FWHM
(full width at half maximum) value and higher intensity of the reflection
of the 2:PC BM system show larger crystallites than in the case of the
61
1
:PC BM blend, in agreement with the AFM images (Fig. 3d and inset),
61
where some aggregates can be observed on the surface.
In summary, we have synthesized the novel A–D–A oligothio-
phenes 1 and 2 based on dithieno[3,2-b:2,3-d]pyrrole (DTP) as a
central donor unit and on dicyanovinylene acceptors as end-caps. The
only difference between the two structural isomers showing interest-
ing and nearly the same optoelectronic properties is the positioning
of the regioregular hexyl side chains either ‘‘outward’’ in 1 or
‘‘inward’’ in 2. Surprisingly, this marginal change led to a big
difference in photovoltaic device performance. An excellent PCE of
4.8% has been achieved for DTP-oligomer 1 in SMOSCs which by far
is the highest efficiency obtained for any DTP-based conjugated
oligomeric material. In contrast, DTP-oligomer 2 based SMOSCs
showed rather poor performances (PCE 0.8%), which we attribute
to the very different blend morphology and phase separation, probed
by AFM and XRD-techniques. These investigations show the impor-
tance of the electron-rich DTP unit as a promising building block in
D–A-type conjugated systems giving rise to a larger CT-character and
red-shift of the main absorption band and that subtle fine-tuning of
the solubilizing side chains is necessary for obtaining highly efficient
SMOSCs. Detailed investigations of the device performance by further
careful optimization of the processing conditions are currently in
progress and will be reported in due course.
Fig. 3 AFM images of 1:PC61BM (a, b: topography; c: phase) and 2:PC61BM (d, e:
topography; f: phase) thin films on PEDOT:PSS-coated ITO substrates.
In order to investigate the surface characteristics of the active layer
blend films, AFM investigations were carried out. The samples were
prepared in the same way as the photoactive layers for the solar cell
devices. Fig. 3a depicts the characteristic long range topography
images for both blend films: 1:PC61BM (Fig. 3a) showed a regular
grain-structured surface. The 2:PC BM blend (Fig. 3d and inset),
61
however, showed a flatter topography with few crystallites on top. The
short range images in Fig. 3b and c and Fig. 3e and f underline the
differences in film characteristics: the surface of 1:PC61BM (Fig. 3b) is
composed of fine aligned fibers (B20 nm ꢁ 2 nm) with a 32 nm ꢁ
1
nm height profile. In contrast, the 2:PC61BM (Fig. 3e) blend showed
We gratefully acknowledge the Baden-W u¨ rttemberg Stiftung
for financial support.
an almost flat surface (2.0 nm ꢁ 0.2 nm) on top of which 40–250 nm
size crystallites can be identified. The corresponding phase image
Notes and references
(
Fig. 3f) shows almost no contrast (41 ꢁ 0.21) demonstrating a
completely mixed film. The borders of the crystallites on top show
a higher phase contrast compared with their center and the mixed
phase (251 ꢁ 11, inset Fig. 3f). In the phase images of 1:PC61BM
1
(a) A. Mishra and P. B ¨a uerle, Angew. Chem., Int. Ed., 2012, 51, 2020–2067;
(b) B. Walker, C. Kim and T.-Q. Nguyen, Chem. Mater., 2011, 23, 470–482.
(a) V. Gupta, A. K. K. Kyaw, D. H. Wang, S. Chand, G. C. Bazan and
A. J. Heeger, Sci. Rep., 2013, 3, 1965; (b) G. Long, X. Wan, B. Kan,
Y. Liu, G. He, Z. Li, Y. Zhang, Y. Zhang, Q. Zhang, M. Zhang and
Y. Chen, Adv. Energy Mater., 2013, 3, 639–646; (c) T. S. van der Poll, J. A.
Love, T.-Q. Nguyen and G. C. Bazan, Adv. Mater., 2012, 24, 3646–3649;
2
(Fig. 3c), fiber domains with an averaged contrast of 181 can be
identified. It is possible to assign the lighter regions (higher phase
shift) to areas with mostly donor oligomer 1, whereas the darker
regions (lower phase shift) contain mostly acceptor PC61BM. The
analysis of the phase images reveals domains B20 nm in size for the
higher phase shift. We conclude that the photoactive blends contain-
ing DTP-oligomer 1 phase-separate into domains of approximately
(d) J. Zhou, Y. Zuo, X. Wan, G. Long, Q. Zhang, W. Ni, Y. Liu, Z. Li,
8
G. He, C. Li, B. Kan, M. Li and Y. Chen, J. Am. Chem. Soc., 2013, 135,
8484–8487; (e) B. Yin, L. Yang, Y. Liu, Y. Chen, Q. Qi, F. Zhang and
S. Yin, Appl. Phys. Lett., 2010, 97, 023303; ( f ) J. Liu, B. Walker, A. Tamayo,
Y. Zhang and T.-Q. Nguyen, Adv. Funct. Mater., 2013, 23, 47–56.
S. C. Rasmussen and S. J. Evenson, Prog. Polym. Sci., 2013, DOI:
j.progpolymsci.2013.04.004.
(a) A. Yassin, G. Savitha, P. Leriche, P. Frere and J. Roncali, New J.
Chem., 2012, 36, 2412–2416; (b) A. Yassin, P. Leriche, M. Allain and
J. Roncali, New J. Chem., 2013, 37, 502–507; (c) R. Grisorio,
G. Allegretta, G. P. Suranna, P. Mastrorilli, A. Loiudice, A. Rizzo,
M. Mazzeo and G. Gigli, J. Mater. Chem., 2012, 22, 19752–19760;
3
4
2
0 nm in size, whereas on the surface of oligomer 2-based blends, no
phase separation of the two components can be observed. In the
latter very finely mixed blend, recombination of the generated charges
during solar cell operation should occur as a result of the lack of
percolation pathways. A significant amount of recombination in
(
d) B. H. Wunsch, M. Rumi, N. R. Tummala, C. Risko, D.-Y. Kang,
2
:PC BM devices is consistent with the rather low fill factor (0.33)
61
K. X. Steirer, J. Gantz, M. Said, N. R. Armstrong, J.-L. Br ´e das,
D. Bucknall and S. R. Marder, J. Mater. Chem. C, 2013, 1, 5250–5260.
(a) K. Schulze, C. Uhrich, R. Sch u¨ ppel, K. Leo, M. Pfeiffer, E. Brier,
E. Reinold and P. B ¨a uerle, Adv. Mater., 2006, 18, 2872–2875;
ꢀ2
and short-circuit current density (3.4 mA cm ) as well as the reduced
open-circuit voltage (0.72 V, see Table 2).
5
X-ray diffraction (XRD) patterns were measured for films of
(
b) R. Fitzner, E. Mena-Osteritz, A. Mishra, G. Schulz, E. Reinold,
1
:PC61BM and 2:PC61BM on PEDOT:PSS-coated ITO substrates. In both
M. Weil, C. K o¨ rner, H. Ziehlke, C. Elschner, K. Leo, M. Riede,
M. Pfeiffer, C. Uhrich and P. B ¨a uerle, J. Am. Chem. Soc., 2012, 134,
cases, the low-angle reflections can be ascribed to the presence of some
crystallites of the oligomer (Fig. S4, ESI†). The d-spacing, which
correlates to the separation of the thiophene backbones and depends
on the length and orientation of the side chains, was determined to be
11064–11067.
6
7
D. Sahu, H. Padhy, D. Patra, J.-F. Yin, Y.-C. Hsu, J.-T. S. Lin, K.-L. Lu,
K.-H. Wei and H.-C. Lin, Tetrahedron, 2011, 67, 303–311.
X. Hu, M. Shi, L. Zuo, Y. Nan, Y. Liu, L. Fu and H. Chen, Polymer,
2
011, 52, 2559–2564.
8 V. Shrotriya, Y. Yao, G. Li and Y. Yang, Appl. Phys. Lett., 2006, 89, 063505.
observed for regioregular poly(3-hexylthiophene) (B16 Å) indicating a 9 X. Xiao, Z. Wang, Z. Hu and T. He, J. Phys. Chem. B, 2011, 114, 7452–7460.
15.0 Å for 1:PC BM and 15.6 Å for 2. This is lower than typically
61
9
This journal is c The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 10865--10867 10867