Synthesis of TPD–thiophene-based small molecule donor for organic photovoltaic cells

Article abstract of DOI:10.1080/15421406.2016.1200366

The donor–acceptor–donor (D–A–D) type small molecule, TPD2T, was synthesized by the Stille coupling reaction and consisted of the electron-withdrawing thiene[3,4-c]pyrrole-4,6-dione (TPD) unit as an electron-accepting (A) core and electron-rich bithiophene units as electron-donating (D) groups. The absorption maximum of the TPD2T film was observed at 460?nm, and was broader and red-shifted compared to that in solution (λ_max = 450?nm). Organic photovoltaic cells fabricated with the conventional and inverted structures both exhibited similar power conversion efficiencies of 0.14–0.15%. Despite the low short-circuit current values, the TPD2T devices showed high open-circuit voltages of over 1.0?V owing to the relatively low-lying HOMO level of TPD2T (?5.4?eV).

Full text of DOI:10.1080/15421406.2016.1200366

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis of TPD–thiophene-based small molecule
donor for organic photovoltaic cells
Eunhee Lim
To cite this article: Eunhee Lim (2016) Synthesis of TPD–thiophene-based small molecule
donor for organic photovoltaic cells, Molecular Crystals and Liquid Crystals, 635:1, 87-93, DOI:
10.1080/15421406.2016.1200366
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1200366
Published online: 01 Nov 2016.
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Date: 25 November 2016, At: 02:38

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Synthesis of TPD–thiophene-based small molecule donor for
organic photovoltaic cells
Eunhee Lim
Department of Chemistry, Kyonggi University, Gyeonggi, Republic of Korea
ABSTRACT
KEYWORDS
small molecule; organic
photovoltaic cells; OPV
The donor–acceptor–donor (D–A–D) type small molecule, TPD2T, was
synthesized by the Stille coupling reaction and consisted of the
electron-withdrawing thiene[3,4-c]pyrrole-4,6-dione (TPD) unit as an
electron-accepting (A) core and electron-rich bithiophene units as
electron-donating (D) groups. The absorption maximum of the TPD2T
film was observed at 460 nm, and was broader and red-shifted com-
pared to that in solution (λmax = 450 nm). Organic photovoltaic cells
fabricated with the conventional and inverted structures both exhib-
ited similar power conversion efficiencies of 0.14–0.15%. Despite the
low short-circuit current values, the TPD2T devices showed high open-
circuitvoltagesofover1.0Vowingtotherelativelylow-lyingHOMOlevel
of TPD2T (−5.4 eV).
1. Introduction
Organic photovoltaic cells (OPVs) have attracted significant interest and a high power con-
version efficiency (PCE) of 10.6% has been achieved with OPVs based on blends of polymer
donors and fullerene acceptors [1]. However, OPVs based on polymer donors suffer from a
number of limitations such as poor synthetic reproducibility, difficult purification procedures,
and broad molecular weight distributions [2]. Small molecular donors may act as counterparts
for the polymers owing to their well-defined molecular structure and high purity [3]. Various
donor–acceptor (D–A) structures have been used to develop low band gap small molecules
for use in OPVs. Such molecules can efficiently extend the absorption band via intramolecular
charge transfer in the D–A structure to obtain high PCE in OPVs [1].
The representative electron-accepting units are benzothiadiazole (BT) [4] and diketopy-
rrolopyrrole [5]. Recently, the easily-synthesized TPD unit has also been widely used as an
electron-accepting unit. This unit exhibits a symmetric, rigidly fused, planar structure and
strong electron-withdrawing properties induced by the imide moiety [6]. The imide moi-
ety in the TPD unit can be easily modified with alkyl chains to improve the solubility of the
materials. Moreover, the highest occupied molecular orbital (HOMO) level of the materials
could be efficiently lowered by the presence of the electron-withdrawing imide moiety in the
TPD units, while maintaining or reducing the band gap [7]. High PCEs of more than 8%
have been achieved with devices based on the dithienogermole–TPD-based polymer in the
inverted structure [8].
CONTACT Eunhee Lim
ehlim@kyonggi.ac.kr
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

88
E. LIM
Oligothiophenes have been widely studied and used for organic thin-film tran-
sistors due to their high hole mobilites [9]. Our group also previously reported a
series of oligothiophenes for OPV application, [10] in which the optical and elec-
trochemical properties of the oligothiophenes could be easily controlled by intro-
ducing the electron-accepting BT unit as a core into the conjugation system. The
BT-containing small molecule, 4,7-bis(5ꢀ-hexyl-2,2ꢀ-bithiophene-5-yl)benzothiadiazole
(DH5TB), showed higher OPV performance than all-thiophene-based analogues,
α,αꢀ-dihexylquinquethiophene (DH5T), owing to its low band gap and good film
morphology. The HOMO levels of DH5TB and DH5T were −5.3 eV and −5.1 eV,
respectively [10].
In this study, we synthesized another D–A–D linear-type oligothiophene, 1,3-
di(bithophen-5ꢀ-yl)-5-(2-ethylhexyl)thieno[3,4]pyrrole-4,6-dione (TPD2T), composed
of TPD and bithiophene units as electron-withdrawing and electron-donating units, respec-
tively. OPV devices were fabricated in the conventional and inverted structures by using
TPD2T as a donor material in combination with [6,6]-phenyl-C71 butyric acid methyl ester
(PC71BM) as an acceptor material. The optical, electrochemical, and photovoltaic properties
of the small molecule were investigated and compared with the properties of other linear-type
oligothiophenes, DH5TB and DH5T.
2. Experimental
2.1. Materials
Thiophene-3,4-dicarboxylic acid was purchased from Frontier Scientific. 2-(Tributy-lstannyl)
thiophene, tris(dibenzyllidenacetone)dipalladium(0) (Pd2(dba)3), tri-o-toly-phosphine
(P(o-tol)3), 2-ethylhexylamine and N-bromosuccinimide (NBS) were purchased from Aldr-
ich. 5-(2-Ethylhexyl)thieno[3,4-c]pyrrole-4,6-dione (1), 1,3-dibromo-5-(2-ethylhexyl)
thieno[3,4-c]pyrrole-4,6-dione (2), 1,3-di(thien-2ꢀ-yl)-5-(2-ethylhexyl)thieno[3,4-c]pyrrole-
4,6-dione (3), and 1,3-di(2ꢀ-bromothien-5ꢀ-yl)-5-(2-ethylhexyl)thieno[3,4]pyrrole-4,6-dione
(4) were prepared according to the literature procedures [6].
2.2. Physical measurements
NMR spectra were recorded on a Bruker AVANCE III HD 400 spectrometer. UV-vis spectra
were obtained using a Shimadzu UV/vis spectrometer. Elemental analyses were performed
with a Flash EA 1112 series from Thermo Electron Corporation. The electrochemical prop-
erties of the small molecule were characterized by cyclic voltammetry (CV) with a BAS
100B electrochemical analyzer and calibrated using the ferrocenium/ferrocene redox value
of −4.8 eV as the standard.
2.3.1. Synthesis of compound (1)
A solution of thiophene-3,4-dicarboxylic acid (3.00 g, 17.4 mmol) in acetic anhydride
(62.8 mL) was stirred at 140°C overnight. After removal of the solvent, the resulting brown
solid was dissolved in toluene (160 mL); 2-ethylhexylamine (3.37 g, 26.1 mmol) was then
added and the mixture was refluxed for 24 h. After cooling the mixture and removing the
solvent, the crude solid was dissolved in thionyl chloride (136 mL) and the mixture was
refluxed for 4 h. The white solid product was obtained by column chromatography using

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
89
dichloromethane:hexane (2:1). Yield: 3.40 g (73.6%). 1H NMR (400 MHz, CDCl3): δ (ppm)
7.80 (s, 2H), 3.60 (d, 2H), 1.37 (m, 9H), 0.90 (m, 6H).
2.3.2. Synthesis of compound (2)
Compound 1 (1.40 g, 5.28 mmol) was dissolved in sulfuric acid (7.8 mL) and trifluoroacetic
acid (26 mL). While stirring, NBS (2.81 g, 15.8 mmol) was added and the reaction mixture
was stirred at room temperature overnight. The mixture was poured into water and extracted
with dichloromethane. The organic phases were combined, washed with brine, and dried
with magnesium sulfate. The product was obtained by column chromatography using chloro-
form:hexane (2:1). Yield: 1.70 g (76.2%). 1H NMR (400 MHz, CDCl3): δ (ppm) 3.60 (d, 2H),
1.37 (m, 9H), 0.90 (m, 6H).
2.3.3. Synthesis of compound (3)
Compound 2 (0.59 g, 1.4 mmol), 2-(tributylstannyl)thiophene (1.2 g, 3.1 mmol), Pd2(dba)3
(0.088 g, 0.096 mmol), and P(o-tol)3 (0.046 g, 0.15 mmol) were dissolved in toluene
(23 mL). The mixture was heated overnight at 90°C. The reaction mixture was extracted
with dichloromethane and water. The organic phases were combined, washed with brine, and
dried with magnesium sulfate. The yellow solid product was obtained by recrystallization
1
from methanol. Yield: 0.35 g (58.2%). H NMR (400 MHz, CDCl3): δ (ppm) 8.21 (d, 2H),
7.95 (d, 2H), 7.33 (t, 2H), 3.60 (d, 2H), 1.37 (m, 9H), 0.9 (m, 6H).
2.3.4. Synthesis of compound (4)
Compound 4 was synthesized in a manner similar to that used for compound 2 by using
compound 3 (0.39 g, 0.91 mmol), chloroform (15 mL), acetic acid (15 mL), and NBS (0.36 g,
2.0 mmol). Yield: 0.47 g (88%). 1H NMR (400 MHz, CDCl3): δ (ppm) 7.67 (d, 2H), 7.09 (d,
2H), 7.33 (t, 2H), 3.6 (d, 2H), 1.37 (m, 9H), 0.9 (m, 6H).
2.3.5. Synthesis of TPD2T
TPD2T was synthesized in a manner similar to that used for compound 3 by using compound
4 (0.30 g, 0.51 mmol), 2-(tributylstannyl)thiophene (0.76 g, 2.0 mmol), Pd2(dba)3 (0.027 g,
0.0291 mmol), P(o-tol)3 (0.017 g, 0.056 mmol), and toluene (10 mL). The orange solid product
was obtained by column chromatography using dichloromethane:hexane (1:1). Yield: 0.13 g
1
(43%). Mp: 135°C. H NMR (400 MHz, CDCl3): δ (ppm) 7.97 (d, 2H), 7.30 (d, 4H), 7.20
(d, 4H), 7.00 (t, 2H), 7.50 (t, 8H), 3.60 (d, 2H), 1.37 (m, 9H), 0.90 (m, 6H). Anal. calcd for
C30H27NO2S5: C, 60.7; H, 4.58; N, 2.36; O, 5,39; S, 27.0, Found: C, 59.7; H, 4.86; N, 1.79; S,
27.9.
2.4. Fabrication of OPV devices
The OPVs were fabricated with the configuration ITO/PEDOT:PSS/TPD2T:PC71BM
(2:1)/LiF/Al. A chloroform solution of small molecule and PC71BM was spin-coated
at 3000 rpm and annealed for 10 min. A LiF/Al layer was then deposited on the
active layer. The OPVs in inverted structure were also fabricated with the configuration
ITO/ZnO/TPD2T:PC71BM (2:1)/MoO3/Ag. A thin layer of sol-gel ZnO layer was spin-coated

90
E. LIM
Scheme . Synthetic schemes and chemical structures of the small molecules: (i) AcO, °C, overnight;
(ii) n-CHNH, toluene, reflux,  h; (iii) SOCl, reflux,  h; (iv) HSO, CFCOOH, NBS, room temperature,
overnight; (v) -(tributylstannyl)thiophene, Pd(dba), P(o-tol), toluene, °C; (vi) AcOH, NBS, chloroform,
room temperature, overnight; (vii) -(tributylstannyl)thiophene, Pd(dba), P(o-tol), toluene, °C.
onto the ITO surface and then cured at 200°C for 60 min. After the active layer was spin-
coated, MoO3 and Ag layers were formed on the active layer through thermal evaporation.
The current-density versus voltage (J–V) curves were measured using a Keithley 236 source
measure unit in air under AM 1.5 G illumination, 100 mWcm−2. The external quantum effi-
ciency (EQE) was measured using a reflective microscope objective to focus the light output
from a 100 W halogen lamp outfitted with a monochromator and an optical chopper.
3. Results and discussion
3.1. Synthesis and physical properties
The D–A–D type small molecule, TPD2T, was designed to have an electron-deficient TPD
unit as an electron-accepting core (A) and electron-rich bithiophene groups as electron-
donating end groups (D). Initially, the ethylhexyl-substituted TPD (1) was synthesized in
three steps. After bromination by NBS, the thiophene-flanked TPD (3) was synthesized by
the Stille coupling reaction. Compound 3 was brominated by NBS to obtain TPD–thiophene-
based bromide (4). Finally, compound 4 was reacted with 2-(tributylstannyl)thiophene using
the Stille coupling reaction. The synthesized donor molecules were successfully characterized
by 1H NMR and EA. The synthetic routes are shown in Scheme 1.
The UV-vis absorption spectra of the small molecules in chloroform solution and in the
film state are shown in Figure 1. The UV-vis absorption spectrum of TPD2T in solution fea-
tures a single peak with an absorption maximum at 450 nm. The peak in the absorption spec-
trum of the TPD2T film (λmax = 460 nm) was broader and red-shifted compared to that in
solution (ꢀλ = 10 nm) due to increased intermolecular π–π∗ stacking interaction between
the ordered molecules in the film [11]. In comparison, DH5TB showed two absorption bands.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
91
Figure . UV-vis absorption spectra of the small molecules in the solution (S) and film (F) states.
One absorption band was observed for TPD2T, similar to the spectrum of DH5T, indicative
of relatively weaker intramolecular charge transfer of TPD compared to BT [5]. In addition,
the optical band gap (Eg) of TPD2T was determined to be 2.2 eV from the absorption onset
wavelength of the TPD2T film (Eg = 1240/λonset (eV)).
The electrochemical properties of the small molecule were characterized by CV anal-
ysis (Figure 2). The onset of oxidation was observed at 1.0 V for TPD2T in the anodic
scan. The HOMO level of TPD2T was −5.4 eV, according to the empirical relationship pro-
posed by Leeuw et al. (Ip(HOMO) = –(Eonset + 4.39) (eV), where Eonset is the onset poten-
tial of oxidation[12]. Notably, TPD2T has a relatively low-lying HOMO level compared
to those of the previously reported benzothiadiazole (BT)-thiophene-based small molecule
DH5TB (−5.3 eV) and alkyl-substituted quinquethiophene DH5T (−5.1 eV) [10]. TPD-
based molecules commonly have low-lying HOMO levels because TPD has an imide group
that efficiently lowers the HOMO level of the materials [7]. Consequently, TPD2T may exhibit
a higher VOC than DH5TB and DH5T in OPV application. From the optical Eg and HOMO
level, the lowest unoccupied molecular orbital (LUMO) level of TPD2T was calculated to be
−3.2 eV.
Figure . CV curve of TPD2T (inset shows HOMO and LUMO levels of TPD2T and PCBM).

92
E. LIM
Table . Photovoltaic properties of OPV devicesa.
Small molecules
Ta (°C)e
VOC (V)
JSC (mA/cm)
FF (%)
PCE (%)
TPDTb
TPDTc
DHTBd
DHTd


w/o
w/o
. (.)
. (.)
.
. (.)
. (.)
.
 ()
 ()

. (.)
. (.)
.
.
.

.
aThe values in parentheses were obtained from devices without annealing.
bITO/PEDOT:PSS/small molecule:PCBM (:)/LiF/Al
cITO/ZnO/small molecule:PCBM (:)/MoOX/Ag
dData from ref. []
eTa: annealing temperature (w/o = without annealing)
3.2. Organic photovoltaic properties
Finally, OPV devices were fabricated with both conventional and inverted structures. The
photovoltaic performances were 0.14–0.15% and thermal annealing did not cause any
improvement in the device performance. The relatively low device performance of TPD2T
is due to the low JSC of 0.68 mA cm−2 and fill factor (FF) of 22%. In general, the low FF
of solution-processed small molecule-based OPVs is attributed to their poor film quality and
inferior charge transport to the electrodes [13]. The photovoltaic properties of the blend films
are summarized in Table 1. The device fabricated with TPD2T showed a high VOC of up to
1.04 V, which is higher than the values for other oligothiophenes having similar linear-type
structures (0.55 V for DH5TB and 0.51 V for DH5T) [10]. The high VOC is derived from the
relatively low-lying HOMO level (−5.4 eV). In general, the HOMO levels are strongly depen-
dent on the energy difference between the HOMO of the donor and the lowest unoccupied
molecular orbital of the acceptor [14].
Figure 3 shows the EQE curve for the inverted structure device. The curve of the EQE
response and the UV absorption of the TPD2T film are well matched, indicative of photocur-
rent generation by TPD2T. The maximum EQE response of 7.5% was observed at 460 nm,
which is the absorption maximum of the TPD2T film. In this study, introduction of the
electron-withdrawing TPD unit proved to be a good strategy for improving the VOC of the
OPV device due to the low HOMO level induced by the electron-withdrawing imide moiety
Figure . Comparison of EQE curve of the TPD2T:PCBM device with the UV-vis absorption spectrum of the
TPD2T film.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
93
in the TPD unit. Future studies may focus on modification of TPD2T by introducing other
alkyl groups or changing the electron-rich end groups.
4. Conclusions
The D–A–D type small molecule TPD2T composed of electron-withdrawing TPD and
electron-donating thiophene units was successfully synthesized. The OPV device based on
TPD2T and PC71BM exhibited a higher VOC of 1.04 V owing to the low-lying HOMO level
than the previously reported linear-type oligothiophenes (DH5TB and DH5T).
Acknowledgments
This work was supported by Kyonggi University Research Grant 2014.
References
[1] You, J., Dou, L., Yoshimura, K., Kato, T., Ohya, K., Moriarty, T., Emery, K., Chen, C.-C., Gao, J.,
Li, G., & Yang, Y. (2013). Nat. Commun., 4, 1446.
[2] Umeyama, T., & Imahori, H. (2014). J. Mater. Chem. A., 2, 11545.
[3] Walker, B., Kim, C., & Nguyen, T.-Q. (2010). Chem. Mater., 23, 470.
[4] Chen, C.-C., Chang, W.-H., Yoshimura, K., Ohya, K., You, J., Gao, J., Hong, Z., & Yang, Y. (2014).
Adv. Mater., 26, 5670.
[5] Gao, H., Li, Y., Wang, L., Ji, C., Wang, Y., Tian, W., Yang, X., & Yin, L. (2014). Chem. Commun.,
50, 10251.
[6] Zou, Y., Najari, A., Berrouard, P., Beaupre, S., Reda Aich, B., Tao, Y., & Leclerc, M. (2010). J. Am.
Chem. Soc., 132, 5330.
[7] Fu, L., Pan, H., Larsen-Olsen, T. T., Andersen, T. R., Bundgaard, E., Krebs, F. C., & Chen, H.-Z.
(2013). Dyes Pigments, 97, 141.
[8] Small, C. E., Chen, S., Subbiah, J., Amb, C. M., Tsang, S.-W., Lai, T.-H., Reynolds, J. R., & So, F.
(2012). Nat. Photonics, 6, 115.
[9] Lim, E., Jung, B. J., Shim, H.-K., Taguchi, T., Noda, B., Kambayashi, T., Mori, T., Ishikawa, K.,
Takezoe, H., & Do, L.-M. (2006). Org. Electron., 7, 121.
[10] Kong, J. A., Lim, E., Lee, K. K., Lee, S., & Kim, H. S. (2010). Sol. Energ. Mater. Sol. C., 94, 2057.
[11] Steinberger, S., Mishra, A., Reinold, E., Levichkov, J., Uhrich, C., Pfeiffer, M., & Bauerle, P. (2011).
Chem. Commun., 47, 1982.
[12] De Leeuw, D. M., Simenon, M. M. J., Brown, A. R., & Einerhand, R. E. F. (1997). Synthetic Met.,
87, 53.
[13] Zhou, J., Wan, X., Liu, Y., Long, G., Wang, F., Li, Z., Zuo, Y., Li, C., & Chen, Y. (2011). Chem. Mater.,
23, 4666.
[14] Scharber, M. C., Mühlbacher, D., Koppe, M., Denk, P., Waldauf, C., Heeger, A. J., & Brabec, C. J.
(2006). Adv. Mater., 18, 789.