The influence of alkyl side chains on molecular packing and solar cell performance of dithienopyrrole-based oligothiophenes

Article abstract of DOI:10.1039/c6ta03453e

The synthesis of a series of dithieno[3,2-b:2′,3′-d]pyrrole (DTP) containing A-D-A oligomers for use in solution-processed bulk heterojunction solar cells is presented. This series allows investigation of the effect of alkyl chains attached to the thiophene moieties or the nitrogen of the DTP unit on the thermal and physical properties of the oligomers. The photoactive layers were probed using absorption spectroscopy, grazing incident X-ray diffraction, and atomic force microscopy. From these experiments, it was found that the hexyl chains attached to the thiophene units are instrumental in obtaining good packing and high fill factors in the solar cell devices. For the fabrication of solar cells, both PEDOT:PSS and V₂O₅ were investigated as hole transport layers and PDMS was employed as a solvent additive. High open-circuit voltages of up to 1.1 V, moderate short-circuit current densities, and high fill factors of up to 0.63 were obtained yielding power conversion efficiencies as high as 5.3%.

Full text of DOI:10.1039/c6ta03453e

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Manuscript for J. Mater. Chem. A
The influence of alkyl side chains on molecular packing and solar cell
performance of dithienopyrrole-based oligothiophenes
Gisela L. Schulz,a,d Prasenjit Kar,a,e Martin Weidelener,a Astrid Vogt,a Marta Urdanpilleta,b Mika Lindén,c
Elena MenaꢀOsteritz,a Amaresh Mishraa,f* and Peter Bäuerlea*
aInstitute of Organic Chemistry II and Advanced Materials, Ulm University, AlbertꢀEinsteinꢀAllee 11, 89081
Ulm, Germany
bDepartment of Applied Physics I, University of the Basque Country (UPV/EHU), Plaza de Europa, 1, 20018
Donostia, San Sebastián, Spain
cInstitute of Inorganic Chemistry II, Ulm University, AlbertꢀEinsteinꢀAllee 11, 89081 Ulm, Germany
dCurrent address: Institute of Polymer Chemistry, University of Stuttgart, Pfaffenwaldring 55,
70569 Stuttgart, Germany
eCurrent address: Department of Chemistry, Indian Institute of Technology, Roorkeeꢀ247667, India
fCurrent address: School of Chemistry, Sambalpur University, Jyoti Viharꢀ768019, Sambalpur, India
*Eꢀmail: amaresh.mishra@suniv.ac.in; peter.baeuerle@uniꢀulm.de
Keywords: dithienopyrrole, solutionꢀprocessing, bulkꢀheterojunction solar cell, metal oxide,
solvent additive, morphology
1

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Abstract
The synthesis of a series of dithieno[3,2ꢀb:2',3'ꢀd]pyrrole (DTP) containing AꢀDꢀA oligomers for
use in solutionꢀprocessed bulk heterojunction solar cells is presented. This series allows investigaꢀ
tion of the effect of alkyl chains attached to the thiophene moieties or the nitrogen of the DTP unit
on the thermal and physical properties of the oligomers. The photoactive layers were probed using
absorption spectroscopy, grazing incident Xꢀray diffraction, and atomic force microscopy. From
these experiments, it was found that the hexyl chains attached to the thiophene units are instrumenꢀ
tal to obtain good packing and high fill factors in the solar cell devices. For the fabrication of solar
cells, both, PEDOT:PSS and V2O5, were investigated as hole transport layers and PDMS was emꢀ
ployed as solvent additive. High openꢀcircuit voltages of up to 1.1 V, moderate shortꢀcircuit current
densities, and high fill factors of up to 0.63 were obtained yielding power conversion efficiencies as
high as 5.3%.
Introduction
The field of bulkꢀheterojunction organic solar cells (BHJSC) made of solutionꢀprocessed (co)oliꢀ
gomers, often referred to as “small molecules”, is progressing rapidly. Recently, using functional
molecular donors and fullereneꢀbased acceptors power conversion efficiencies (PCE) exceeding 9%
have been reported for single junction and 10.1% for homoꢀtandem solutionꢀprocessed OSCs.1ꢀ11
An important part of this progress is the judicious design and synthesis of new photoactive
materials and optimization of activeꢀlayer processing method and several reviews highlighting the
structural development over the last years appeared.12ꢀ16 Various design strategies and building
blocks have been employed for the preparation of novel molecular materials in order to tune their
absorption, packing, and electronic energy levels. An important part of this molecular design is the
length and position of solubilizing alkyl chains, which can strongly affect the selfꢀassembly and
ordering of the molecules in the bulk. To date, molecules with alkyl chains at the αꢀpositions of
terminal thiophene units,9,17ꢀ19 as well as in the βꢀpositions were well investigated.5,11,19ꢀ21 Furꢀ
2

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thermore, optoelectronic properties were tuned by varying the electronꢀdonating or accepting
strength of the building blocks present in the photoactive molecule to modify both, the magnitude
and position of the absorption profile as well as the molecular orbital energy levels.10,22ꢀ24
Combining strong donors with strong acceptors in the conjugated chain results in a redꢀshift of the
main absorption band and an increase of the molar extinction coefficient.
Recently, we have reported a series of dithieno[3,2ꢀb:2',3'ꢀd]pyrrole (DTP)ꢀcontaining acceptorꢀ
donorꢀacceptor (AꢀDꢀA)ꢀtype oligomers showing excellent fill factors of up to 0.72 and high PCEs
up to 6.1% in solutionꢀprocessed BHJSCs upon solvent vapor annealing treatment of the active
layer.11 The DTP25 unit has only rarely been used in the development of oligomeric semiconductors
for solar cell applications reaching PCEs in the range of 0.2 to 2.7%.26ꢀ29
Equally important to having wellꢀdesigned photoactive materials, is the selection and optimizaꢀ
tion of an appropriate device structure. Each component must meet specific requirements in order to
produce an efficient solar cell. In this respect, the use of metal oxides as charge transport layers has
been extensively investigated and described in literature.30,31 Vanadium, molybdenum, tungsten,
and nickel oxides (V2O5,32ꢀ35 MoO3,32,36,37 WO3,38,39 NiO40ꢀ42) are typically used as hole transport
layers, whereas zinc oxide (ZnO)43ꢀ46 and titanium dioxide (TiO2)47,48 have been employed as elecꢀ
tron transport layers.49,50 Metal oxides are good candidates for charge transport materials due to
their high optical transparency, tunable work functions, and semiconducting properties. Improveꢀ
ments in solar cell performance have been attributed to increased charge collection, hole or electron
blocking properties, reduced charge recombination, and/or optical spacer effects. In a recent report,
Heeger et al. have demonstrated an improvement of PCE from 6.0% to 8.9% by incorporation of
ZnO as electron extraction layer.1 Another important aspect is the use of metal oxides for Ohmic
contacts to maximize the VOC because the reduction of builtꢀin potential leads to an increase in dark
current as well as carrier recombination.31
In this work, a series of AꢀDꢀA oligomers consisting of terminal dicyanovinylene (DCV) groups
and a central DTP unit linked by thiophene or hexylꢀsubstituted thiophene units were designed and
3

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synthesized. The DCV groups are known to provide balanced acceptor strength and strong intermoꢀ
lecular interactions in the solid state.51 The combination with an electronꢀrich DTP moiety results in
materials with redꢀshifted absorption and very high molar extinction coefficients. Such type of oliꢀ
gomers were incorporated into solutionꢀprocessed BHJSC as electron donor materials and fullerene
[6,6]ꢀphenylꢀC61ꢀbutyric acid methyl ester (PC61BM) as electron acceptor. Device architectures conꢀ
taining either PEDOT:PSS or V2O5 as hole transport layer (HTL) were investigated. The best deviꢀ
ces using V2O5 as HTL and polydimethylsiloxane (PDMS) as solvent additive resulted in PCEs up
to 5.3%. Thinꢀfilm absorption, atomic force microscopy, and grazing incident Xꢀray diffraction
measurements were perform to further understand the morphology of the photoactive layer.
Results and discussion
The four DTPꢀcontaining oligomers were synthesized by Pd0ꢀcatalyzed Stilleꢀtype coupling as
shown in Scheme 1. The synthesis of DCVꢀsubstituted bromothiophene 6 is reported elsewhere.52
DCVꢀsubstituted iodothiophene 7 was prepared in high yield and is shown in detail in Scheme S1
(see ESI†). The Stilleꢀcoupling reaction of either 6 or 7 with bisstannylated DTP 5a,b gave the tarꢀ
get compounds in yields between 73ꢀ83%. The obtained oligomers 1ꢀ4 were purified using high
performance liquid chromatography (HPLC) before they were further characterized.
Scheme 1. Synthesis of dithieno[3,2ꢀb:2',3'ꢀd]pyrroleꢀbased AꢀDꢀA oligomers 1ꢀ4.
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The melting points of all oligomers 1ꢀ4 were determined by differential scanning calorimetry
(DSC). In the DSC curves, they displayed one sharp endothermic peak whereby Tm indicates the
melting temperature. The replacement of the branched ethylhexyl substituent attached to the central
nitrogen in 1 and 2 by a larger octylnonyl in 3 and 4 lowered Tm. All oligomers were found to be
stable up to 330 °C and only showed decomposition on further heating which is indicated by a
broad exothermic peak in the DSC curves (Figure 1).
Figure 1. DSC trace of all AꢀDꢀA oligomers 1ꢀ4 measured under Ar flow at heating rate of 10 °C/min.
UVꢀvis absorption spectroscopy was performed on solutions and thin films spinꢀcoated from
chloroform (Figure 2 and Table 1). High molar extinction coefficients of ~100 000 Mꢀ1 cmꢀ1 with
absorption maxima at approximately 600 nm were observed for oligomers 1 to 4 due to the strong
DꢀA charge transfer character of the DTP donor and the DCV acceptor. Absorption spectra of thin
films revealed a redꢀshifted main absorption band compared to the measurements performed in soꢀ
lution with maxima at 680 nm. The film spectra displayed a shoulder at higher energy at around 610
nm for this band. The structured absorption and sharp onset at longer wavelength is indicative of
good ordering of the molecules which will be discussed later.
The maximum solubility of the oligomers in chloroform was determined to assess their potential
for solutionꢀprocessed solar cells (Table 1). The solubility of oligomer 1 (0.2 mg mLꢀ1) unfortunateꢀ
ly was insufficient for thin film preparation by spinꢀcoating. The solubility of 2 was increased to 2
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mg mLꢀ1 as a result of the additional hexyl chains on the thiophene backbone. Oligomers 3 and 4
displayed even further enhancement of the solubility to 19 and 27 mg mLꢀ1, respectively, upon exꢀ
change of the ethylhexyl by the larger octylnonyl substituent.
Figure 2. a) UVꢀVis spectra of oligomers 1ꢀ4 measured in chloroform solution. b) Normalized absorption spectra of the
oligomers in thin films prepared by spinꢀcoating onto glass substrates (4 mg/mL in chloroform).
Table 1. UVꢀVis absorption and electrochemical, melting temperature and maximum solubility data of oligomers 1ꢀ4.
Oligomer
ꢁEopt sol
λabs film
[eV]
ꢁEopt film E0ox1 E0ox2 Ep red HOMO LUMO ꢁECV
Tm [°C]e Solubility
λ
abs sol
[nm]a [Mꢀ1 cmꢀ1]
ε
[V]c [V]c [V]c
[eV]d
[eV]d
[eV]
[mg/mL]f
[nm]b
[eV]
1
2
3
4
597
609
605
612
95 000
92 200
1.89
1.88
1.87
1.87
ꢀ
ꢀ
0.51 1.09 ꢀ1.36 ꢀ5.54
0.45 1.03 ꢀ1.43 ꢀ5.49
0.55 1.14 ꢀ1.44 ꢀ5.58
0.53 1.10 ꢀ1.53 ꢀ5.55
ꢀ3.84
ꢀ3.68
ꢀ3.81
ꢀ3.78
1.70
1.81
1.77
1.77
301
309
233
280
0.2
2
681
680
680
1.82
1.83
1.83
105 000
19
107 000
27
aAbsorption spectra were measured in chloroform. bThin films were spinꢀcoated from chloroform solutions (4 mg/mL) on glass substrate at 3000 rpm.
cCyclic voltammetry was performed in dichloromethane/TBAPF6 (0.1 M), scan rate = 100 mV/s, referenced against Fc/Fc+. The oxidation potentials
(E°ox) were calculated by means of cathodic and anodic peak potentials of quasiꢀreversible waves: E° = (Epa+Epc)/2. For the irreversible waves, Ep
values are given. dHOMO/LUMO energy values were calculated by setting Fc/Fc+vac at ꢀ5.1 eV vs vacuum. eMelting temperatures (Tm) were determiꢀ
ned using differential scanning calorimetry. fSolubility was measured in chloroform.
Electrochemical measurements were performed on the series of oligomers 1ꢀ4 in solution in
order to determine the energy levels the frontier orbitals. The data is shown in Figure 3 and summaꢀ
rized in Table 1. The redox properties are nearly similar for all four compounds. Two reversible
oxidation waves at around 0.5V and 1.1 V are observed due to formation of stable radical cations
and dications on the oligomeric backbone. When scanning to negative potentials, an irreversible reꢀ
duction at ~ ꢀ1.4 V, which is typical for DCV acceptor groups, was seen. From the onset of the first
oxidation and reduction potential, HOMO and LUMO energies were calculated and lie around ꢀ5.5
eV (HOMO) and ꢀ3.7 to ꢀ3.8 eV, respectively. These values should be well compatible with the
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energy levels of PC61BM (HOMO = ꢀ6.1 eV, LUMO = ꢀ4.0 eV53) to ensure efficient electron
transfer from the donor to the acceptor.
Figure 3. Cyclic voltammograms of oligomers 1ꢀ4 in (1×10−3 mol L−1 in dichloromethane/nꢀBu4NPF6 as supporting
electrolyte). All scan rates were 100 mV s−1 and the potentials were internally referenced to the Fc/Fc+ couple.
In order to obtain insight into the molecular packing behavior of oligomers 2ꢀ4 in the solid state,
grazing incident Xꢀray diffraction (GIXRD) experiments were performed on neat films. The resulꢀ
ting diffraction patterns are plotted in Figure 4. All oligomers displayed a relatively intense (100)
reflection peak at low angle (2θ = 5.95° for 2, 6.18° for 3, 5.13° for 4) indicating the presence of
crystalline domains. This low angle reflection is assigned to the d100 spacing and correlates to the
separation between the oligothiophene backbones and is directly related to the type and the presence
or absence of the alkyl substituents. Oligomers 2 and 3 containing an ethylhexyl chain attached to
the DTPꢀnitrogen showed a d100ꢀspacing of 14.8 Å and 14.3 Å, respectively, whereas oligomer 4
comprising the larger octylnonyl chain gave an increased d100 spacing of 17.2 Å. The broad reflex
seen between 2θ = 7−15° originates from a combination of donor, acceptor, and the substrate. The
XRDꢀmeasurements on the neat films provide qualitative information regarding the packing behaꢀ
vior of the molecules and as well facilitate peak assignment in the diffraction pattern obtained for
the blend films (vide infra).
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Figure 4. GIXRD plot of neat films consisting of oligomers 2ꢀ4 coated on glass substrate.
Table 2. Solar cell parameters for oligomers 2ꢀ4 using different hole transport layers (HTL). Device structure:
ITO|HTL|Donor:PC61BM (1:2)|LiF|Al. The active D:A layer was processed from chloroform solution at 60 °C.
Donor
HTL
JSC
[mA cmꢀ2]
VOC
FF
PCE
[%]
[V]
PEDOT:PSS
PEDOT:PSS
PEDOT:PSS
V2O5
5.2
6.0
7.2
5.1
5.3
7.9
1.07
1.01
1.03
1.10
1.07
1.06
0.55
0.43
0.49
0.62
0.47
0.57
3.1
2.6
3.7
3.5
2.6
4.8
2
3
4
2
3
4
V2O5
V2O5
Solutionꢀprocessed BHJSCs with the standard device structure ITO|PEDOT:PSS|D:A blend|
LiF|Al were fabricated. The obtained photovoltaic parameters are presented in Table 2 and the curꢀ
rent densityꢀvoltage (JꢀV) curve and external quantum efficiency (EQE) spectra are shown in Figure
S1 (see ESI†). All three oligomers 2ꢀ4 displayed openꢀcircuit voltages (VOC) higher than 1 V. Shortꢀ
circuit current densities (JSC) ranged between 5.2 ꢀ 7.2 mA cmꢀ2 and fill factors (FF) between 0.43
and 0.55 were obtained leading to moderate power conversion efficiencies (PCE) of 3.1%, 2.6%,
and 3.7% for 2-, 3- and 4-based devices. Interestingly, upon replacement of the PEDOT:PSS hole
transport layer (HTL) by a solutionꢀprocessed V2O5 layer,35 FFs of the devices increased to 0.47–
0.62 which could be attributed to a combination of different effects. One possibility is improved
hole collection at the anode associated with a better match between the work function of V2O5 and
the HOMO energy level of the oligomer. Another possibility is that the morphology of the D:A
blend is influenced by the different surface energy of V2O5 versus PEDOT:PSS, particularly at the
interface between the hole transport and the photoactive layer. It is significant to note that the openꢀ
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circuit voltages of the photovoltaic cells increased by ~0.03 V upon replacement of PEDOT:PSS
with V2O5, and the corresponding PCEs ranged from 2.6% to 4.8%.
Further optimization of the active layer was then carried out using V2O5 as HTL. The results of
these experiments are shown in Table 3 and Figure 5. It was found that slight improvements in the
FF were observed for 3 and 4 upon incorporation of PDMS as solvent additive.54ꢀ56 Upon close
inspection of the JꢀV characteristics, it appears that 2 and 4ꢀbased devices display high VOCs and
FFs, and low saturation values. A comparatively higher FF and lowering of the saturation values are
an indication of efficient charge separation and extraction.52 The saturation value is calculated by
dividing the current density measured at ꢀ1 V with that at 0 V (JSC). The resulting value describes
the current density dependence on the electric field strength meaning that the lower the saturation
value, the lower the charge recombination process. Oligomer 3 showed the lowest PCE at 2.8% due
to its low FF of 0.51. On the other hand, oligomer 4 demonstrated the highest photovoltaic
parameters within the series, with a JSC of 7.9 mA cmꢀ2, a VOC of 1.07 V, and a FF of 0.63 giving an
excellent PCE of 5.3%. Oligomer 4ꢀbased devices were further tested using different concentration
of PDMS which showed that 0.2 mg mLꢀ1 of additive is optimal for obtaining high performance
BHJSCs (see ESI†, Table S1). The improvement of the PCE for oligomer 4 could be ascribed to the
more favorable morphology due to homogeneous distribution of the donor and acceptor component
as well as controlling the ordering within the individual phases.57,58 However, this improvement was
not significant with blends of 2 and 3; similar behaviour have also been seen in other oligomeric
materials.59 The results also suggest that the hexyl chain on the outer thiophene ring is vital in
influencing the film forming properties and performance improvement of this series of oligomers.
Table 3. Solar cell parameters of oligomers 2ꢀ4 using V2O5 HTLs in a device structure: ITO|V2O5|Donor:PC61BM
(1:2)|LiF|Al. The active layer was cast from chloroform solution using 0.2 mg mLꢀ1 PDMS as an additive. The corresꢀ
ponding dꢀd distances extracted from GIXRD experiments.
Donor
JSC
[mA cmꢀ2]
VOC
FF
PCE
[%]
Jꢀ1 V/JSC
d100 distance
[Å]
[V]
5.3
5.2
7.9
1.10
1.04
1.07
0.61
0.51
0.63
3.6
2.8
5.3
1.08
1.23
1.04
14.6
15.8
17.5
2
3
4
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The external quantum efficiency (EQE) spectra of the three devices are plotted in Figure 5b.
Current is being generated in the solar cells up to 750 nm with a maximum at 680 nm for 2- and 4-
based active layers. These two donor oligomers displayed similar EQE profiles, whereas for the
3:PC61BM device, the EQE profile has a quite different shape. In contrast to 2 and 4, oligomer 3
showed two broad peaks of similar intensity at high and low energies. The high JSC values obtained
for 4:PC61BMꢀbased devices is further reflected in the highest EQE of 62%.
Figure 5. Comparison of JꢀV and EQE curves of the best performing solar cell devices with the optimized structure:
ITO|V2O5|Donor:PC61BM|LiF|Al.
In an attempt to further understand the origin of the different photovoltaic properties of oligoꢀ
mers 2ꢀ4, UVꢀvis spectroscopy and grazing incident Xꢀray diffraction (GIXRD) measurements of
the active layers were performed. In Figure 6, the absorption spectra and diffraction patterns are
compared for the optimized D:Aꢀblend films prepared using V2O5 as HTL and PDMS as solvent adꢀ
ditive. The absorption profiles for the 3:PC61BM blend is less defined and the maximum is blueꢀ
shifted by approximately 50 nm compared to the active layers based on oligomers 2 and 4. On the
other hand, the absorption band of 2:PC61BM and 4:PC61BM are quite similar (Figure 6a). The
sharp onset at longer wavelengths seen in the neat films (Figure 1b), is also observed for the photoꢀ
active blends of 2 and 4 suggesting better ordering of the molecules in the blend.
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Figure 6. (a) Thin film absorption spectra and (b) 1DꢀGIXRD patterns of oligomer:PC61BM blends deposited from
CHCl3:PDMS on V2O5 coated glass substrate.
The diffraction pattern of the GIXRD measurements of the oligomer:PC61BM blend films proꢀ
cessed with PDMS additive on V2O5ꢀcoated glass revealed insight into the organization and ordeꢀ
ring of the oligomers (Figure 6b). The reflexes for all oligomers at low angle (2θ = 6.06° for 2,
5.58° for 3, 5.05° for 4) are assigned to the packing between the adjacent oligomer backbone.11,21
The reflection patterns measured for the neat films helped for the assignment of the reflexes
observed in the blends of 2ꢀ4 with PC61BM. Each oligomer displays characteristic d100ꢀdistances
depending on the size of their alkyl substituents and they are found to continuously increase with
increasing alkyl size in the series: d100 = 14.6 Å for 2, 15.8 Å for 3, and 17.5 Å for 4 (Table 3). The
intensities of the reflections at low angles and their corresponding full width at halfꢀmaximum
(FWHM) revealed information about the relative crystallinity of the photoactive layers. The
2:PC61BMꢀblend had by far the highest crystallinity and was followed by 4:PC61BM and
3:PC61BM. A similar trend is also observed for oligomer:PC61BMꢀblends deposited from CHCl3 on
PEDOT:PSSꢀcoated glass (see ESI†, Figure S2).
Atomic force microscopy (AFM) measurements were conducted in order to gain further insight
into the surface morphology of the photoactive layers. The samples were prepared in the same way
as the corresponding optimized devices, namely by spinꢀcoating the active blends onto ITO/V2O5
from CHCl3/PDMS solutions. The obtained AFMꢀimages are shown in Figure 7. In addition to the
more intuitively understandable topographic images, the phase images exhibited a contrast that is
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produced by a complex combination of factors, such as stiffness, adhesion, or dissipation of the
sample. These factors, which are usually different for the different chemical phases present for the
blends of 2ꢀ4 with PC61BM.
Figure 7. AFM height (aꢀc) and phase (a’ꢀc’) images of samples spinꢀcoated from CHCl3/PDMS solutions on ITO|
V2O5| with (a and a') 2:PC61BM, (b and b') 3:PC61BM, and (c and c') 4:PC61BM. Image size: 1 x 1 ꢂm.
Topography images (Figure 7aꢀc) revealed the surface landscape of the samples; topography
roughness (Rq) was measured to be 7.7 ± 0.7 nm for 2, 1.8 ± 0.1 nm for 3, and 2.5 ± 0.1 nm for 4.
Looking at the phase images, oligomer 2 (Figure 7a') exhibited coarse phaseꢀseparation with microꢀ
meterꢀranged domains, which dramatically exceed the typical exciton diffusion length. Images of 3
and 4 (Figures 7b' and 7c', accordingly), on the contrary, showed finer structures that correlate to
their more moderate roughness. With 3, domainꢀsize ranged from ~50 to 200 nm and domainꢀbounꢀ
daries are decorated with small particles of bright contrast (diameter ~10ꢀ15 nm). On the other
hand, for oligomer 4 elongated and distinctly defined domains with widths of ~ 10ꢀ40 nm are visibꢀ
le. Figure S3 (see ESI†) illustrates the three cases with a 4:PC61BMꢀblend: The active layer was
spinꢀcoated from CHCl3 on either ITO/PEDOT:PSS or ITO/V2O5 and the optimized device from
CHCl3/PDMS on ITO|V2O5|, as analyzed in Figure 7cꢀc', is included for comparison. The finest
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domain separation corresponds to the optimized device. We conclude from the AFM part that
oligomer 2 displays coarse phase separation and high roughness, whereas 4 develops the finest
domains and 3 reveals domains of intermediate size.
Now, we would like to summarize the results on active layer characterization and to correlate
them to the photovoltaic properties of oligomers 2ꢀ4. The absorption and GIXRD experiments reꢀ
vealed that 2 and 4 pack similarly and in an ordered fashion which led to high fill factors (0.61ꢀ
0.62) in the solar cell devices. In the case of the 2:PC61BMꢀblend, AFM revealed a large phase seꢀ
paration, which correlates to the higher degree of crystallinity determined in GIXRD. The resulting
high fill factors but low JSC were obviously due to reduced interfacial area between the donor and
acceptor phases. On the contrary, 4:PC61BM layers displayed a fibrillar structure of much smaller
dimension, thus yielding a finer phase separation, an increased interfacial area, and a larger charge
generation in the solar cell devices. Overall, an increase of JSC from 5.3 to 7.9 mA cmꢀ2 is observed.
The 3:PC61BM blend is however not as well ordered, as evidenced by absorption and XRD specꢀ
troscopy, although the phase separation is finer than that in the 2:PC61BM blend. In the photovoltaic
devices charge transport is limited and both, FF and JSC values, suffer resulting in the lowest PCE
(2.8%) in the series. Correlating the above mentioned results with the chemical structures of the
three oligomers, it seems reasonable to conclude that the hexyl chains attached to the outer thiopheꢀ
ne units are necessary for good packing. It is noteworthy to mention that despite the low solubility
of 2 (2 mg mLꢀ1) it still outperforms 3 (19 mg mLꢀ1) in the solar cell, stressing the importance of
molecular packing over solubility, for this series of molecules.
Conclusion
In summary, we have designed and synthesized a series of dithieno[3,2ꢀb:2',3'ꢀd]pyrroleꢀbased oliꢀ
gomers 1ꢀ4 for their application as donor material in bulkꢀheterojunction solar cells. The oligomers
showed broad absorption from 500 to 700 nm with high molar absorption coefficients peaking at
around 600 nm. The HOMOꢀLUMO energy levels estimated from electrochemical measurements
13

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were found to be suitable for efficient charge transport to the PC61BM acceptor. In BHJSCs, all oliꢀ
gomers blend with PC61BM showed PCEs in the range of 2.6ꢀ3.7% when using PEDOT:PSS as hoꢀ
le transport layer. The performance was further improved by replacing PEDOT:PSS by a V2O5
layer. In order to further improve the performance, PDMS was used as solvent additive giving in
particular a positive effect for oligomer 4 resulting in the highest PCE of 5.3% for this series of
oligomers. The results presented here demonstrate that subtle changes in the type and size of the soꢀ
lubilizing alkyl substituents may have a larger impact on the photovoltaic performance. The influenꢀ
ce of the hole transport layer and solvent additive on the morphology of the photoactive layer was
investigated by absorption spectroscopy, AFM, and GIXRD. These experiments indicated that the
use of V2O5 improves the hole transport and/or collection and the solvent additive allows for a morꢀ
phological reꢀorganization of the blend leading to higher order in the BHJ.
Experimental part
Materials and Methods: NMR spectra were recorded on a Bruker DRX 400 spectrometer (1H
NMR: 400 MHz, 13C NMR: 100 MHz). Chemical shift values (δ) are expressed in parts per million
using residual solvent protons (1H NMR, δH = 7.26 for CDCl3; 13C NMR, δC = 77.0 for CDCl3) as
internal standard. The splitting patterns are designated as follows: s (singlet), d (doublet), t (triplet)
and m (multiplet). Elemental analyses were performed on an Elementar Vario EL. Melting points
were determined using a Mettler Toledo DSC 823. Thin layer chromatography was carried out on
aluminum plates, preꢀcoated with silica gel, Merck Si60 F254. Preparative column chromatography
was performed on glass columns packed with silica gel, Merck Silica 60, particle size 40 − 63 ꢂm.
High performance liquid chromatography was performed on a Hitachi instrument equipped with a
UVꢀvis detector Lꢀ7420, columns (Nucleosil 100ꢀ5 NO2 with a pore size of 100 Å) from Macheryꢀ
Nagel using a dichloromethane/nꢀhexane mixture (40:60) as eluent. EI mass spectra were recorded
on a Varian Saturn 2000 GCꢀMS, MALDIꢀMS experiments were performed using a MS Bruker Reꢀ
flex 2 (Bruker Daltonik GmbH, Bremen, Germany) using transꢀ2ꢀ[3ꢀ(4ꢀtertꢀbutylphenyl)ꢀ2ꢀmethylꢀ
14

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2ꢀproꢀpenylidene] malononitrile (DCTB) as matrix. UV/Vis absorption spectra were recorded in 1
cm cuvettes with Merck Uvasol grade solvents on a Perkin Elmer Lambda 19 spectrometer. Cyclic
voltammetry experiments were performed with a computerꢀcontrolled Autolab PGSTAT30 potenꢀ
tiostat in a threeꢀelectrode singleꢀcompartment cell with a platinum working electrode, a platinum
wire counter electrode, and a Ag/AgCl reference electrode. All potentials were internally referenced
to the ferrocene/ferricenium couple. Tetrahydrofuran and N,Nꢀdimethylformamide (Merck) were
dried using a MB SPSꢀ800 solvent purifying system (MBraun). All synthetic steps were carried out
under argon atmosphere. Tetrakis(triphenylphosphine)palladium(0) was prepared according to a
literature known procedure. XRD measurements were carried out using a Bruker D8 diffractometer
with a fixed incoming angle of 0.2°. Surface images of photoactive layers were recorded with the
help of an atomic force microscope (MultiMode V AFM, Bruker). The microscope was operated in
air at room temperature in the tapping mode using commercial silicon tips with a resonance freꢀ
quency of 200–400 kHz and spring constants of ≈50 N m –1. The images were analyzed with the
help of the WSxM software.60
Device fabrication: Photovoltaic devices were made by spinꢀcoating PEDOT:PSS (Clevios P,
VP Al4083) onto preꢀcleaned, patterned indium tin oxide (ITO) substrates (15 Ω per square)
(Kintec). The photoactive layer (~80 nm) was deposited by spinꢀcoating from of a mixed solution of
donor with PC61BM (total concentration of 15 mg/mL in CHCl3) at 60°C. For the devices made
with poly(dimethylsiloxane) (PDMS) as additive, a stock solution of 0.2 mg/mL PDMS in CHCl3
was used. PC61BM was purchased from Solenne BV, Netherlands. The counter electrode of LiF (1
nm) and aluminum (100 nm) was deposited by vacuum evaporation at 2 x 10ꢀ6 Torr. Solꢀgel method
involves making a solution of vanadium(V) oxytriisopropoxide [VO(OC3H7)3] in isopropyl alcohol.
Hydrolysis begins upon spinꢀcoating of VO(OC3H7)3 at 5750 rpm and is complete after exposure to
ambient conditions with a relative humidity of 60% for 60 minutes without any heating to form thin
layer of V2O5. The work function of V2O5 was reported to be 5.6 eV.35 The active areas of the cells
were 0.1 cm2. Film thicknesses were measured using a Dektak profilometer. JꢀV characteristics
15

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were measured under AM 1.5G conditions at 100 mW cmꢀ2 with a AAA solar simulator from Oriel
Instruments using a Keithley 2400 source meter. Spectral response was measured under
monochromatic light from a 300 W Xenon lamp in combination with a monochromator (Oriel,
Cornerstone 260) modulated with a mechanical chopper. The response was recorded as the voltage
over a 220 Ω resistance, using a lockꢀin amplifier (Merlin 70104). A calibrated Siꢀcell was used as
reference. The devices were kept behind a quartz window in a nitrogen filled container.
Synthesis: 2,2'-(5,5'-[N-{2-Ethylhexyl}dithieno[3,2-b:2',3'-d]pyrrole-2,6-diyl]-bis(methane-
1-yl-1-ylidine)dimalononitrile (1). A mixture of 4ꢀ(2ꢀethylhexyl)ꢀ2,6ꢀbis(trimethylstannyl)dithieꢀ
no[3,2ꢀb:2',3'ꢀd]pyrrol 5a (0.218 g, 0.353 mmol), 2ꢀbromoꢀ5ꢀ(2,2ꢀdicyanovinyl)thiophene
6
(0.177 g, 0.742 mmol) and Pd(PPh3)4 (0.02 g, 17.7 ꢂmol) was combined in degassed DMF
(100 mL) and stirred at 80 °C for 20 hours. After cooling, the resulting precipitate was filtered off
and washed several times with methanol. After drying, Oligomer 1 (0.190 g, 0.313 mmol, 83%) was
obtained as a dark green solid. M.p. 301 oC (DSC). 1H NMR (tetrachloroethaneꢀd2, 100 °C, δ ppm)
7.71 (s, 2H, C=CH), 7.61 (d, 2H, J = 4.2 Hz, ThH), 7.30 (d, 2H, J = 4.1 Hz, ThH), 7.28 (s, 2H,
ThH); 4.11ꢀ4.05 (m, 2H, NꢀCH2), 1.98 (m, 1H, ꢀCHꢀ), 1.35 (m, 8H, ꢀCH2ꢀ), 0.92 (t, 3H, J = 7.4 Hz,
ꢀCH3), 0.87 (t, 3H, J = 7.0 Hz, ꢀCH3). HRMS (MALDIꢀTOF) m/z: calcd. for C32H25N5S4
607.09928; found 607.09866 [M+] (δm/m = 1.0 ppm).
2,2'-(5,5'-[N-{2-Ethylhexyl}dithieno[3,2-b:2',3'-d]pyrrole-2,6-diyl]-bis[3-hexylthien-5,2-diyl])-
bis(methane-1-yl-1-ylidine)dimalononitrile (2). 4ꢀ(2ꢀEthylhexyl)ꢀ2,6ꢀbis(trimethylstannyl)dithieꢀ
no[3,2ꢀb:2',3'ꢀd]pyrrol 5a (58.9 mg, 95.4 µmol), 2ꢀiodoꢀ4ꢀhexylꢀ5ꢀ(2,2ꢀdicyanovinyl)thiophene 7
(89.8 mg, 243 µmol) and Pd(PPh3)4 (6.8 mg, 5.91 µmol) were added in a Schlenkꢀtube and evacuaꢀ
ted for about 10 minutes. After addition of DMF (5 mL), the reaction mixture was carefully degasꢀ
sed and heated to 60 °C for 3.5 h. Then the reaction mixture was poured into saturated aqueous
NH4Cl solution. The organic compounds were extracted with DCM. The combined organic phases
were dried with Na2SO4 and the solvents were removed by rotary evaporation. The crude product
was purified via digesting with ethyl acetate and subsequent column chromatography using silica
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1
(DCM) to obtain oligomer 2 (60.0 mg, 77.3 µmol, 81%) as a dark solid. M.p.: 309 °C. H NMR
(400 MHz, CDCl3, δ) 7.80 (s, 2H, C=CH), 7.28 (s, 2H, ThH), 7.13 (s, 2H, ThH), 4.13ꢀ4.03 (m, 2H,
NꢀCH2), 2.74 (t, J = 7.8 Hz, 4H, αꢀCH2), 2.04ꢀ1.95 (m, 1H, ꢀCHꢀ), 1.69ꢀ1.61 (m, 4H, βꢀCH2), 1.44ꢀ
1.27 (m, 20H, ꢀCH2ꢀ), 0.94ꢀ0.89 (m, 12H, ꢀCH3). 13C NMR (100 MHz, CDCl3, δ ppm) 157.06,
148.74, 147.22, 146.91, 134.78, 128.74, 125.46, 117.51, 115.06, 114.03, 110.53, 73.99, 52.13,
40.35, 31.54, 31.21, 30.75, 29.32, 29.06, 28.59, 24.26, 23.02, 22.51, 13.92, 13.89, 10.74. HRMS
(MALDIꢀTOF) m/z: calcd for C44H49N5S4: 775.28708; found [M]+: 775.28628 (δm/m = 1.0 ppm).
2,2'-(5,5'-[N-{1-Octylnonyl}dithieno[3,2-b:2',3'-d]pyrrole-2,6-diyl]-bis(methane-1-yl-1-ylidi-
ne)dimalononitrile (3). In a Schlenk tube, 2,6ꢀbis(trimethylstannyl)ꢀ4ꢀ(1ꢀoctylnonyl)dithieno[3,2ꢀ
b:2′,3′ꢀd]pyrrole 5b (0.2 g, 0.27 mmol), 2ꢀ[(5ꢀbromothienꢀ2ꢀyl)methylene]malononitrile 6 (0.15 g,
0.64 mmol) and Pd(PPh3)4 (0.015 g, 13.4 ꢂmol) were dissolved in dry DMF (5 mL) and the solution
was purged with argon for 10–15 minutes. The solution was then stirred at 60 °C for 19 h. After
completion the reaction mixture was poured into saturated aqueous NH4Cl solution. The organic
phase was extracted with DCM. The combined organic phases were dried over Na2SO4 and the solꢀ
vents were removed by rotary evaporation. The crude product was purified by column chromategraꢀ
phy on silica gel (eluent DCM) to yield pure oligomer 3 as a dark solid (144 mg, 0.196 mmol,
1
73%). M.p.: 233 °C. H NMR (400 MHz, CDCl3, δ) 7.76 (s, 2H, C=CH), 7.60 (d, 2H, J = 4.2 Hz,
ThH), 7.35 (s, 2H, ThH), 7.30 (d, 2H, J = 4.2 Hz, ThH), 4.30ꢀ4.17 (m, 1H, NꢀCH), 2.10ꢀ1.85 (m,
4H, ꢀCH2ꢀ), 1.36ꢀ1.0 (m, 24H, ꢀCH2ꢀ), 0.81 (t, J = 6.9 Hz, 6H, ꢀCH3). 13C NMR (100 MHz, CDCl3,
δ ppm) 157.38, 148.93, 147.09, 134.53, 128.50, 125.43, 115.29, 114.26, 111.23, 73.13, 34.97,
31.78, 29.33, 29.09, 26.73, 22.65, 22.59, 14.10. HRMS (MALDIꢀTOF) m/z: calcd for C41H43N5S4
733.24013; found [M]+: 733.23935 (δm/m = 2.2 ppm).
2,2'-(5,5'-[N-{1-Octylnonyl}dithieno[3,2-b:2',3'-d]pyrrole-2,6-diyl]-bis[3-hexylthien-5,2-diyl])-
bis(methane-1-yl-1-ylidine)dimalononitrile (4). In a Schlenk tube, 2,6ꢀbis(trimethylstannyl)ꢀ4ꢀ(1ꢀ
octylnonyl)dithieno[3,2ꢀb:2′,3′ꢀd]pyrrole 5b (0.2 g, 0.27 mmol), 2ꢀ((3ꢀhexylꢀ5ꢀiodothienꢀ2ꢀyl)methꢀ
ylene)malononitrile 7 (0.24 g, 0.64 mmol) and Pd(PPh3)4 (0.015 g, 13.4 ꢂmol) were dissolved in
17

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dry DMF (5 mL) and the solution was carefully degassed with argon and stirred at 60 °C for 18 h.
Subsequently, the reaction mixture was cooled down to room temperature and poured into saturated
ammonium chloride solution. The organic phase was extracted with dichloromethane followed by
drying over Na2SO4 and removal of the solvents. After column chromatography using SiO2 (DCM)
the pure product 4 (0.186 g, 0.20 mmol) was isolated as a dark solid in 77% yield. M.p.: 280 °C. 1H
NMR (400 MHz, CDCl3, δ) 7.80 (s, 2H, C=CH), 7.34 (s, 2H, ThH), 7.14 (s, 2H, ThH), 4.27ꢀ4.17
(m, 1H, NꢀCH), 2.74 (t, J = 7.8 Hz, 4H, αꢀCH2), 2.10ꢀ1.85 (m, 4H, ꢀCH2ꢀ), 1.70ꢀ1.60 (m, 4H, βꢀ
13
CH2), 1.45ꢀ1.0 (m, 36H, ꢀCH2ꢀ), 0.91 (t, J = 6.9 Hz, 6H, ꢀCH3), 0.81 (t, J = 6.9 Hz, 6H, ꢀCH3). C
NMR (100 MHz, CDCl3, δ ppm) 157.37, 148.95, 147.10, 134.51, 128.49, 125.42, 115.29, 114.24,
111.22, 73.14, 60.94, 34.99, 31.79, 31.56, 31.33, 29.35, 29.30, 29.25, 29.15, 29.10, 26.72, 22.64,
22.58, 14.10. HRMS (MALDIꢀTOF) m/z: calcd for C53H67N5S4: 901.42793; found [M]+: 901.42703
(δm/m = 0.99 ppm).
Acknowledgements
We thank the BadenꢀWürttembergꢀStiftung for funding this work as part of the research program
“Organic Photovoltaics & Dye Solar Cells”. PK would like to acknowledge the Alexander von
Humboldt Foundation for financial support. MU thank the Spanish MINECO (MAT2010ꢀ21156ꢀ
C03ꢀ03) and the University of the Basque Country (EHUA12/05) for their support of this work. We
are grateful to Kirill Zilberberg and Prof. Thomas Riedl from University of Wuppertal for their
support in processing the metal oxide layers. We would like to thank Samuel Blessing (Institute of
Inorganic Chemistry I, University of Ulm) for carrying out the XRD measurements.
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Journal of Materials Chemistry A
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DOI: 10.1039/C6TA03453E
Table of Content
Dithienopyrrole-based small molecular materials were developed achieving PCEs up to 5.3% in
bulk-heterojunction organic solar cells by the tuning of the alkyl substitution pattern and use of
solvent additive.