4H-1,2,6-Thiadiazin-4-one-containing small molecule donors and additive effects on their performance in solution-processed organic solar cells

Article abstract of DOI:10.1039/c4tc02931c

The optical, electrochemical, morphological and transport properties of a series of thiadiazinone (acceptor) and (thienyl)carbazoles (donor) containing π-extended donor-acceptor-donors (D-A-D) are presented. Systematic variations in the number of the thienyl units, the choice of branched or straight alkyl side chains and the use of a processing additive demonstrate their use as electron donors in bulk heterojunction solar cells blended with fullerene acceptors. The best power conversion efficiency (PCE) of 2.7% is achieved by adding to the D-A-D 3:fullerene blend a polydimethylsiloxane (PDMS) additive, that improves the morphology and doubles the hole mobility within the D-A-D:fullerene blend.

Full text of DOI:10.1039/c4tc02931c

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4H-1,2,6-Thiadiazin-4-one-containing small molecule
donors and additive effects on their performance in
solution-processed organic solar cells
Cite this: DOI: 10.1039/x0xx00000x
Felix Hermerschmidt,*a Andreas S. Kalogirou,b Jie Min,c Georgia A. Zissimou,b
Sachetan M. Tuladhar,d Tayebeh Ameri,c Hendrik Faber,d Grigorios Itskos,e
Stelios A. Choulis,a Thomas D. Anthopoulos,d Donal D. C. Bradley,d Jenny
Nelson,*d Christoph J. Brabec,*c Panayiotis A. Koutentis*b
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
The optical, electrochemical, morphological and transport properties of a series of thiadiazinone
(acceptor) and (thienyl)carbazoles (donor) containing π-extended donor-acceptor-donors (D-A-D)
are presented. Systematic variations in the number of the thienyl units, the choice of branched or
straight alkyl side chains and the use of a processing additive demonstrate their use as electron
donors in bulk heterojunction solar cells blended with fullerene acceptors. The best power
conversion efficiency (PCE) of 2.7% is achieved by adding to the D-A-D
3:fullerene blend a
polydimethylsiloxane (PDMS) additive, that improves the morphology and doubles the hole
mobility within the D-A-D:fullerene blend.
facile purification techniques,16 and their reproducible, less
complex syntheses.17 Devices employing solution-processed
1. Introduction
Incident sunlight falling on Earth in 1 hour is sufficient to
satisfy the average yearly human energy consumption;1 not
surprisingly, the interest in solar energy has grown rapidly.
Organic photovoltaics (OPVs), which are free of expensive rare
earth metals, can provide a low-cost, solution-processable and
environmentally friendly alternative to generate electricity.
Bulk-heterojunction (BHJ) OPV devices, which use a bulk
mixture of an electron donating and an electron accepting unit,
to provide a flow of electrons and holes to the relevant
electrodes, have reached certified power conversion efficiencies
(PCEs) for single-junction devices >9%,2,3 and >10% for
tandem cells.4,5 These devices typically contain an active layer
comprising a conjugated polymer as the electron donating unit
such as PTB7, PCDTBT, Si-PCPDTBT or P3HT and a soluble
fullerene as electron acceptor such as PC60BM, PC70BM or
ICBA2,6-8
small molecules as both donor and acceptor components gave
device efficiencies of ~3%,18 which is similar to the average
device performance of the widely studied and extensively
optimized polymer:fullerene P3HT:PC60BM system.19 As a
result of this progress, a wide variety of new small molecules
for solution-processed organic photovoltaics are being
prepared,20-24 including efficient donor-acceptor-donor (D-A-D)
systems.25
A recent electrochemical study of non-oxidised 4H-1,2,6-
thiadiazines indicated electron affinities (LUMO energy values)
of -3.25 to -3.66 eV with reversible oxidations and reductions.26
As such, these heterocycles are promising as acceptor building
blocks for D-A-D small molecule donors and for polymer-
ization with various prominent comonomers to produce solar
harvesting polymers. The presence of an appropriate electron-
deficient 4H-1,2,6-thiadiazine in the donor molecule can be
used to control the energy level of the LUMO while that of the
HOMO can be regulated by the incorporation of electron-rich
substituents such as thiophene and carbazole.
Small molecules have recently been used as donor and
acceptor components in BHJ-OPVs9 and device efficiencies of
7-9% have been achieved for single-junction solar cells,10-15
while a 10.1% PCE has been achieved for a solution-based
small molecule tandem cell.12
Interestingly, while non-oxidised 4H-1,2,6-thiadiazines are
rare heterocycles, several analogues have useful medicinal27
Advantages of small molecules include well-defined
structures and characterisability, higher manufacturing yields,
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Scheme 1 Chemical Structure of small molecule D-A-D’s 2-8 prepared from 3,5-dichloro-4H-1,2,6-thiadiazin-4-one (1).
and agrochemical28 properties. In the material sciences both
Woodward29 and Rees30-32 proposed potentially stable poly(4H-
2.2. Photophysical properties
1,2,6-thiadiazine) analogues of the inorganic superconductor UV-vis absorption spectra of the D-A-Ds 2-8 for both solutions
poly(sulfur nitride). Furthermore, fused analogues were studied (in DCM) and thin films (doctor-bladed from PhCl) (Fig. 1),
as examples of “extreme quinoids” that have ambiguous show that the latter spectra are red-shifted and broader
aromatic character,33 while others displayed unusual liquid compared to the former. This can be attributed to stronger
crystalline properties or behaved as near-infrared dyes.34,35
Herein, we show that 4H-1,2,6-thiadiazinone-containing state.39
small molecule donors can be used in BHJ OPVs. Systematic The least-conjugated 3,5-di(carbazol-3-yl)thiadiazinone
electronic and structural interactions that exist in the solid
2
variations in the structures of the materials and use of a shows an absorption that corresponds to an optical band gap of
processing additive during device fabrication led to power 2.61 eV, while the most conjugated, 3,5-di(terthienylcarbazol-
conversion efficiencies of 2.7%.
yl)thiadiazinone
8, shows an absorption that corresponds to the
smallest optical band gap in the series (1.99 eV). Thiadiazin-
ones 3-7 show absorptions that correspond to optical band gaps
(Egopt) in the range of 2.17-2.02 eV (Table 1).
2. Results and discussion
2.1. Synthesis of small molecule donors 2-8
The absorption profiles depend on the number of thienyls
present in the D-A-Ds. The longest wavelength absorption
A range of thiadiazinone (acceptor) and (thienyl)carbazoles
maxima in the solid state ranks accordingly:
(bithienyl) > (thienyl) >> (no thienyl). In solution, the
2,2'-bithienyl substituted D-A-Ds (534 nm) and (535 nm)
have similar absorption maxima, while the 2,2'-bithienyl
substituted D-A-D (522 nm) that hosts n-hexyl side chains on
the bithienyls, has a slightly shorter wavelength maximum,
which we attribute to increased steric constraints emposed by
the n-hexyl groups on D-A-D 7, that interfere with the
8 (terthienyl) > 7-5
(donor) containing π-extended donor-acceptor-donor (D-A-D)
small molecules (2-8) were prepared and studied as light
harvesting p-type semiconductors (Scheme 1). The synthesis of
the D-A-D small molecules started from 3,5-dichloro-4H-1,2,6-
4
,3
2
5
6
7
thiadiazin-4-one (1), that can be readily prepared in two steps
from dichloromalononitrile and SCl2 in multigram quantities,36
and is a versatile building block that can participate in a range
of palladium catalysed C-C coupling reactions such as Stille
and Suzuki-Miyaura couplings to give various 3,5-di(het)aryl
substituted systems.37,38 The synthesis and their characterisation
appear in the ESI. All the compounds were readily soluble in
tetrahydrofuran (THF) and dichloromethane (DCM).
planarity.
D-A-D 7, however, shows a 22 nm red shift of the
absorption maximum between solution and solid state, to give
similar absorption maxima to the solid state films of the 2,2'-
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bithienyls analogues
respectively); in the solid state, π-π and lipophilic interactions small emission from PC70BM at ~720 nm41 were observed.
must enable a structural reorganisation of D-A-D to overcome
The PL quenching can be quantified by a quenching ratio,42
these steric constraints. A similar magnitude red shift can be yielding values >90% for each D-A-D blend (Fig. 2 inset).
expected for the D-A-D that has the same number of alkyl These high quenching ratios attest to efficient exciton
side chains as D-A-D but this was not observed: presumably, dissociation and charge separation at the heterointerfaces of the
D-A-D
, which is significantly larger, requires more energy to D-A-D:PC70BM blends.43
reorganize in the solid state, although the position of the
n-hexyl substituted thienyl also differs from that of D-A-D
and this can also play a role. The additional n-hexyl side chains
on the D-A-D also influence the overall absorption range,
5 and 6 (Table 1, 544 vs 545 and 539 nm, quenched; only traces of the D-A-Ds’ emission together with a
7
8
7
8
7
,
7
resulting in broad absorption between 300-600 nm, which
includes the absorption regime of PC70BM (350-500 nm),40
this broadening effect was also not observed with D-A-D 8.
6
Fig. 2 Photoluminescence measurements of pristine small molecule D-A-Ds 2-8
and blends with PC70BM. The inset shows an enlarged area of the blend PL.
2.3. Electrochemical properties
Cyclic voltammetry (CV) of the D-A-Ds show that the 3,5-di-
(carbazolyl)thiadiazinone has the lowest HOMO energy level
(5.92 eV), as well as the largest electrochemical band gap
2
echem
(Eg
2.56 eV), leading to poor hole transport between
acceptor and donor HOMOs. The inclusion of one or two
thienyls between the central thiadiazinone and the peripheral
carbazoles significantly raised the HOMO (-5.54 to -5.40 eV)
but only marginally raised the LUMO (-3.59 to -3.52 eV)
energy levels leading to a net reduction of the Egechem (Table 1).
echem
The Eg
values of D-A-Ds 2-8 (2.56-1.74 eV) were in
close agreement with the optical band gaps calculated from the
onset of the UV-vis absorption. Modifying the N-alkyl chain on
the carbazole from a 2-ethylhexyl to a n-decyl chain did not
affect the electrochemical characteristics of the D-A-Ds; the
thiadiazinone pairs
HOMO/LUMO energy levels as well as electrochemical band
gaps. A slightly larger deviation was observed with analogue
and the pair owing to the incorporation of the n-hexyl side
chains on the two thienyls. The 3,5-di(terthienylcarbazolyl)-
thiadiazinone , owing to an extended π system and inclusion
3/4 and 5/6 gave similar values of
7
5/6
Fig. 1 Comparison of all the normalised UV-vis spectra of the D-A-Ds 2-8 in a)
solution (DCM) and b) film (doctor-bladed from PhCl).
8
The influence of the carbazole N-alkyl side chain (n-decyl
or 2-ethylhexyl) is negligible: D-A-D pairs and gave
similar absorptions. This cannot be said for the n-hexyl side
of more electron-rich terthienyls, has a higher HOMO energy
level (-5.34 eV) while its LUMO energy (-3.60 eV) remains
comparable with the other thiadiazinones, giving a more
desirable Egechem of 1.74 eV (Table 1).
3
/4
5/6
chains on the interior thienyls, which, in at least one example
(D-A-D 8), affect the absorption maximum in solution.
On moving from D-A-D
2 to 8, cyclic voltammetry shows a
Photoluminescence (PL) spectra of the pristine D-A-Ds
show a single broad emission, Stoke-shifted by 150-200 meV
from the respective molecule’s absorption peak (Fig. 2). In
contrast, the emissions from the blends with PC70BM are
small but gradual increase in their HOMO energies while their
LUMO energies, with the exception of
changes (Fig. 3).
2, show only minor
As will be described below, this shift in energy leads to a
decrease in the measured values of VOC from 1.00 to 0.74 V
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Fig. 3 Experimentally obtained HOMO and LUMO energy level values (rounded to
nearest 0.1 eV) of the donor small molecule compounds in relation to the
acceptor fullerene PC70BM.
when moving from D-A-D
3 to 8, as the value of the VOC is
proportional to the difference between HOMO energy level of
the donor material and the LUMO energy level of the acceptor
compound.44
Table 1 Overview of all optical and electrochemical characteristics of the studied D-A-Ds in solution and in the solid state
echem
λmax sol.
(nm)
414
504
501
534
535
522
538
λmax film
(nm)
428
512
514
545
539
544
566
Egopt sol.
(eV)
2.61
2.17
2.12
2.04
2.02
2.05
1.99
Egopt film
(eV)
2.49
2.07
2.06
1.88
1.87
1.91
1.82
Eox
(V)
EHOMO
(eV)
Ered
(V)
ELUMO
(eV)
Eg
D-A-D
(eV)
2.56
2.01
2.02
1.83
1.84
1.85
1.74
2
3
4
5
6
7
8
0.82
0.44
0.44
0.32
0.30
0.25
0.24
-5.92
-5.54
-5.54
-5.42
-5.40
-5.35
-5.34
-1.72
-1.57
-1.58
-1.51
-1.54
-1.60
-1.50
-3.38
-3.53
-3.52
-3.59
-3.56
-3.50
-3.60
As expected from the CV data, the D-A-D
useful photovoltaic performance. While the LUMO/LUMO
2
does not yield
2.4. Photovoltaic properties
OPV-BHJ devices based on an optimal blend ratio of donor/acceptor offset is not unfavourable since it approximates
D-A-D:PC70BM 1:2 wt%, showed PCE with high VOC values the offset for the well-studied P3HT:PCBM system,40 the lack
(~1 V) (Table 2, Fig. 4).
of a favourable HOMO/HOMO offset between donor and
acceptor (see Fig. 3) presumably leads to an unfavourable hole
Table 2 Photovoltaic properties of D-A-D:PC70BM (1:2, wt%) devices,
transfer. The VOC values of the devices based on the D-A-Ds
3-
under the illumination of AM 1.5, 100 mW/cm2.
8
, however, do agree with the HOMO energy levels of the
D-A-D:
PC70BM
VOC
(V)
JSC
(mA/cm2)
0.06
FF
(%)
PCE
(%)
relevant molecules.
The highest performing D-A-Ds 3-5 also have the highest
2
3
3a)
4
5
6
7
8
0.70
1.00
1.00
0.95
0.90
0.90
0.90
0.85
24.26
32.30
33.08
33.10
36.43
40.35
31.51
31.50
0.01
2.09
2.71
1.91
2.23
2.52
1.20
1.17
produced JSC (Table 2), and a trend exists between measured
JSC and device performance, which also holds true for the
obtained external quantum efficiency (EQE).
The fill factors (FF) obtained for the series of
D-A-D:PC70BM blends also roughly show this same trend,
however the low values limit the overall device performance
obtained. As will be shown below, this can primarily be
attributed to the charge carrier mobility and morphology within
the system.
6.47
8.20
6.07
6.82
7.50
4.24
4.35
a) with PDMS, 0.1 mg/mL
To improve the device performance, an additive was
introduced into the D-A-D:PC70BM active blend. This
approach has been successfully applied in a variety of polymer
and small molecule-based systems.47-51 Adding a small amount
of polydimethylsiloxane (PDMS; 2000 g/mol) to the D-A-D
The VOC values reduce approximately with the reduction in
donor HOMO/acceptor LUMO energy level offset (from CV),
which can be regarded as proportional to VOC
shown in the empirical equation by Sharber et al.,46 relating the
band gap with VOC
.
44,45 This has been
:
3
:PC70BM blend led to an increase (~30%) in JSC and PCE.
onor
However, this increase was not seen with other blends; similar
behaviour occurs in other small molecule material systems.52
oc
(
- cceptor)-
V
(1)
U
with e being the charge of an electron and E the energy level of
the donor HOMO and acceptor LUMO, respectively.
Tentative
reasons
for
the
improvement
in
the
D-A-D :PC70BM blend are given below.
3
4 | J. Name., 2012, 00, 1-3
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To understand the reasons behind the trends in PCE, JSC and
The improvement in morphology on adding PDMS has been
EQE in our series of D-A-D:PC70BM systems, we performed attributed to the smaller feature sizes visible in AFM images,
morphological and charge transport characterisations (see resulting in higher JSC values.48 In our case, adding PDMS to
below).
the D-A-D
as well contrasted as noted previously.48 Since the D-A-D
+PDMS blend showed an increase in JSC and device
3:PC70BM led to similar small feature sizes but not
2.5. Morphology
3
The D-A-D:PC70BM blends were investigated using atomic performance compared to the absence of additive (Table 2), we
force microscopy (AFM) to probe the surface topography and investigated the charge carrier mobility in pristine materials and
in general, showed similar roughness parameters (rms 1-5 nm). blends (see below) to identify if transport related phenomena
Films of blends containing D-A-Ds
straight alkyl side chains, showed similar but distinctly
different topologies than those containing D-A-Ds +PDMS
and , which all host branched alkyl side chains; blends of the For efficient photovoltaic performance values, good transport
4, 6-8, which host only were the reason for the increase in device performance.
2.6. Charge carrier mobility
3, 3
5
latter had smaller feature sizes than their straight alkyl chain properties within the active layer are important. Hole mobilities
analogues. This was particularly clear when comparing the ≥10-3 cm2/Vs are desired to avoid recombination of separated
AFM images of D-A-D pairs 3/4 as well as 5/6
, which differ carriers before they reach their respective electrode.53 To
only by their side chains. As such, the nature of the side chains provide insight into the hole mobilities of the series of D-A-Ds,
influences the structural blending between the D-A-Ds and hole-only space-charge limited current (SCLC) devices were
PC70BM. No film forming correlation was noted regarding the prepared (Table 3, Fig. 6).
number of thienyls present in the D-A-Ds.
Fig. 4 Current density-voltage (J-V) curves (a) and EQE spectra (b) of solar cells
based on D-A-D:PC70BM (1:2 wt%), respectively, under the illumination of AM
1.5, 100 mW/cm2.
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Fig. 5 Atomic force microscopy images showing D-A-D:PC70BM blends (1:2 wt%).
All image sizes are 5 × 5 μm rms stands for root mean square and indicates the
roughness of the surface.
Table 3 Hole mobilities determined from the SCLC measurements
D-A-D
Hole mobilityb)
Blend based
hole mobilityb)
(μh) (cm2/Vs)
8.23×10-7
(μh) (cm2/Vs)
The hole mobilities obtained through SCLC measurements
correlate well with the respective small molecule blend OPV
performance, indicating that the hole mobility is responsible for
the differences seen in the device performance. The same trend
can be seen in the JSC values obtained in the photovoltaic
devices (Table 2), confirming the finding that the carriers in the
blends with the higher mobilities can reach the electrodes
before recombining and therefore contribute to the overall
obtained current.
2
3
3a)
4
5
6
7
8
3.34×10-6
1.49×10-4
4.55×10-5
9.40×10-5
9.03×10-6
2.65×10-5
1.08×10-4
5.62×10-6
4.49×10-6
6.24×10-6
2.24×10-5
8.08×10-5
2.82×10-6
3.36×10-6
This
D-A-D
is
particularly
the
case
for
the
a) Blended film based on D-A-D 6:PC70BM with PDMS, 0.1 mg/mL. b) The
reported mobility data are average values of each pristine and blended films
for six diodes.
3
:PC70BM:PDMS blend film, which shows a doubling
in hole mobility upon inclusion of the additive (Table 3) as
opposed to without. On the other hand, the poor hole mobility
Having ascertained this trend in hole-mobility with device
performance in SCLC-based devices, representative field effect
transistors were fabricated to confirm the SCLC findings and
also to examine electron mobilities under lateral transport
conditions (Table 4).
seen in the D-A-D 2:PC70BM blend film supports the
observations made from electrochemistry, where there is no
favourable driving force to convey holes to the relevant
electrode.
Table 4 Representative FET linear mobility values for holes (h+) and
electrons (e-).
Charge carrier mobility μ (cm2/Vs)
Material
2
3
6
h+
h+
h+
e-
8.6×10-6
1.2×10-6
<1.0×10-6
1.2×10-5
<1.0×10-6
1.9×10-5
5.6×10-5
3.4×10-5
4.8×10-6
2.3×10-5
9.1×10-6
2.3×10-5
6.2×10-5
3.2×10-5
3.9×10-6
3.3×10-5
4.9×10-6
3.1×10-5
+PDMS
+PC70BM
+PC70BM
+PDMS
h+
e-
The representative FET measurements show roughly the
same trend in hole mobilities with device performance but,
average one order of magnitude lower values than those
obtained using SCLC (Table 3). When comparing pristine
D-A-D
unaffected. In the D A D
reduced, indicating inhibition of hole mobility by the fullerene.
However, the addition of PDMS to the D-A-D :PC70BM
3
and D-A-D
3
+PDMS, the hole mobility is largely
3
:PC70BM blend, the hole mobility is
6
blend leads to an approximate doubling in hole mobility. This
same trend is also observed in the SCLC mobility values
obtained, but at an order of magnitude higher. This may be
attributed to the fact that lateral and not vertical transport
characteristics are measured in FETs.
Overall, the increase in hole mobility and relative stability
of electron mobility upon addition of an additive are consistent
with other observations, specifically diiodooctane50 and
1-chloronaphthalene,51 both of which behave similarly to
PDMS.54 The addition of PDMS to the D-A-D:PC70BM blends
Fig. 6 Hole-only mobilities of pristine (a) and blended films (b) with PC70BM as
acceptor.
of
increases, but did lead to the significant increase in hole
mobility for the D-A-D :PC70BM blend above.
2 and 6 did not lead to significant hole or electron mobility
3
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The morphology of the D-A-D
therefore not optimum and addition of PC70BM disturbs the and characterisation of organic solar cells. See DOI: 10.1039/b000000x/
hole mobility within the D-A-D :PC70BM blend. The addition
of PDMS then, has a beneficial effect on the D-A-D
3:PC70BM blend is D-A-D molecules, electrochemistry, cyclic voltammograms, fabrication
3
3
:PC70BM 1. United States Department of Energy, Basic Research Needs for Solar
blend morphology, leading to the observed increases in hole
only mobility.
Energy Utilization, Report on the Basic Energy Sciences Workshop
on Solar Energy Utilization April 18-21, 2005, Argonne National
Laboratory, Lemont, IL, 2005.
2. Z. He, C. Zhong, S. Su, M. Xu, H. Wu and Y. Cao, Nat. Photon.,
3. Conclusions
2012, 6, 593.
A series of small molecule donors containing 4H-1,2,6-
thiadiazin-4-one were studied as active layer materials in
organic solar cells. High VOC values were observed but the JSC
and FF device performance parameters limited the PCE values.
3. S.-H. Liao, H.-J. Jhuo, Y.-S. Cheng and S.-A. Chen, Adv. Mater.,
2013, 25, 4766.
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a Molecular Electronics and Photonics Research Unit, Department of
Mechanical Engineering and Materials Science and Engineering, Cyprus
University of Technology, 3041 Limassol, Cyprus
b Department of Chemistry, University of Cyprus, 1678 Nicosia, Cyprus
c Institute of Materials for Electronics and Energy Technology, Friedrich-
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Page 9 of 10
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02931C
Journal Name
RSCPublishing
ARTICLE
4H-1,2,6-Thiadiazin-4-one-containing small molecule
donors and additive effects on their performance in
solution-processed organic solar cells
Cite this: DOI: 10.1039/x0xx00000x
Felix Hermerschmidt,*a Andreas S. Kalogirou,b Jie Min,c Georgia A. Zissimou,b
Sachetan M. Tuladhar,d Tayebeh Ameri,c Hendrik Faber,d Grigorios Itskos,e
Stelios A. Choulis,a Thomas D. Anthopoulos,d Donal D. C. Bradley,d Jenny
Nelson,*d Christoph J. Brabec,*c Panayiotis A. Koutentis*b
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Textual Table of Contents Entry
Donor-acceptor-donors containing 4H-1,2,6-thiadiazinone and
(thienyl)carbazoles are presented, achieving a best power
conversion efficiency of 2.7% when blended with fullerene
acceptors.
This journal is © The Royal Society of Chemistry 2013
J. Name., 2013, 00, 1-3 | 1

Journal of Materials Chemistry C
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DOI: 10.1039/C4TC02931C
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