Solar Energy Materials & Solar Cells 95 (2011) 306–309

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

(M.-R. K

journal homepage: www.elsevier.com/locate/solmat

Syntheses and photovoltaic properties of polymeric sensitizers usingthiophene-based copolymer derivatives for dye-sensitized solar cells

Seung-Min Lee a, Su-Bin Lee a, Ki-Hyun Kim a, Sang-Eun Cho a, Young-Keun Kim a,b, Hyun-Woo Park a,b,Jin-Kook Lee a,n, Mi-Ra Kim a,n

a Department of Polymer Science and Engineering, Pusan National University, Busan 609-735, Republic of Koreab Solchem Ltd., Busan 609-735, Republic of Korea

a r t i c l e i n f o

Article history:

Received 2 November 2009

Received in revised form

11 February 2010

Accepted 3 May 2010Available online 20 May 2010

Keywords:

Polythiophene

Dye-sensitized solar cells

Organic sensitizers

48/$ - see front matter & 2010 Elsevier B.V. A

016/j.solmat.2010.05.009

esponding authors.

ail addresses: leejk@pusan.ac.kr (J.-K. Lee), m

im).

a b s t r a c t

We synthesized the thiophene-based copolymers (P(3TAF-co-3TAa)-A-n and P(3TAF-co-3TAa)-B-n)

using two different kinds of thiophene monomers, (N-(3-thienylmethylene)-2-aminofluorene and

3-thiophene acetic acid), as sensitizers on the DSSCs. P(3TAF-co-3TAa)-A-n (n¼1, 2, 3) was

synthesized with different molar ratios (3TAF:3TAa¼1:5, 1:10, 1:20) of monomers at room

temperature, respectively. Also, P(3TAF-co-3TAa)-B-n (n¼1, 2, 3) was synthesized with above molar

ratios of monomers at 0 1C, respectively. The DSSCs devices were fabricated using the thiophene-based

copolymers as sensitizers and their photovoltaic performances were measured by using a solar

simulator under AM 1.5. In the DSSCs devices using polymeric sensitizers, Voc is 0.53–0.60 V, Jsc is

1.9–4.5 mA/cm2, FF is 0.51–0.63 and the power conversion efficiency is 0.63–1.53%, respectively.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Dye-sensitized solar cells (DSSCs) have been attracted muchattention, due to their high light-to-electrical conversion effi-ciency, low cost, and long-term stability [1,2]. The typical dyes atthe present time are Ruthenium complex sensitizers such as theN3, N719, and black dye, due to their intense and wide-rangeabsorption of visible light [3,4]. Recently, one of the issues withDSSCs is to search for alternatives of the Ruthenium complexsensitizers, due to the use of rare metals and the difficulty ofpurification. Besides the Ruthenium complex sensitizers, organicdyes have many advantages as dyes for DSSCs, such as low cost,higher molar absorption coefficients, the wide variety of thestructures, and no resource limitation [5–7]. Among the organicdyes, conjugated polymers such as polythiophenes are promisingcandidates as dyes because of their thermal and environmentalstability, solubility, excellent conductivity, and the reversibletransition between redox and neutral states [8,9].

In order to improve the UV–visible absorbance, copolymerswere designed as the chemical structures having both the fluoreneand thiophene groups. Furthermore, the carboxylic acid group wasintroduced for the adsorption onto the surface of the nanoporousTiO2 layers. Due to the chemical stability and relatively simplesynthesis routes compared to other organic sensitizers, weintroduced the polythiophene derivatives as the dyes on the DSSCs.

ll rights reserved.

rkim2@pusan.ac.kr

In this study, we synthesized the thiophene-based copolymers(P(3TAF-co-3TAa)-A-n and P(3TAF-co-3TAa)-B-n) using twokinds of thiophene monomers (N-(3-thienylmethylene)-2-amino-fluorene and 3-thiophene acetic acid) as dyes on the DSSCs. Thestructures, molecular weights, and optical properties of thesecopolymers were characterized.

2. Experimental

2.1. Measurements

Fourier transform infrared (FT-IR) spectra of the monomer andpolymers in the form of KBr pellet were recorded by a Jasco FT/IR-460 plus spectrometer. 1H NMR spectra were measured on aVarian spectrometer (300 MHz) with CDCl3 and d6-DMSO asa d-solvent. Molecular weights of the polymers were estimated bya gel-permeation chromatography (GPC) using polystyrene as astandard on a Waters (150 GPC) equipped at 40 1C in THF. TheUV–vis spectra were obtained on an Optizen 2120 UV spectro-photometer. Measurement of I–V characteristics of the solar celldevices was carried out by a Solar Simulator (150 W simulator,REC-L11, PECCELL) under simulated solar light with ARC Lamppower supply (AM 1.5, 100 mW/cm2).

2.2. Syntheses of monomers

2.2.1. N-(3-thienylmethylene)-2-aminofluorene (3TAF)

Equimolar amounts of 2-aminofluorene (0.507 g, 2.80 mmol)and 3-thiophenecarboxaldehyde (0.314 g, 2.80 mmol) in ethanol

S.-M. Lee et al. / Solar Energy Materials & Solar Cells 95 (2011) 306–309 307

at a concentration of 0.1 M, and the reaction mixtures wererefluxed for 18 h under an N2 atmosphere. The solutions werethen cooled to 0 1C overnight. After cooling of the solution, theprecipitates formed from the reaction mixture were filtered andwashed with cold ethanol. The compound was purified byrecrystallization from ethanol. (yield: 36%) 1H NMR (CDCl3) d8.43 (s, 1H, –CH¼N–), 7.67–7.40 (m, 7H, fluorene Ar), 7.28–7.06(m, 3H, thiophene), 3.80 (s, 2H, Ar–CH2–Ar).

2.2.2. 3-Thiophene methyl acetate (3TMA)

3TMA was synthesized by refluxing 3-thiophene acetic acid(3.0 g, 21 mmol) (3TAa) with acidified H2SO4 (five drops)dehydrated methanol (40 ml) for 24 h. The methanol wasevaporated and the residue was extracted by diethyl ether. Theextract was washed with a large quantity of water and dried withan anhydrous MgSO4, and filtered. The 3TMA was obtained afterthe evaporation of diethyl ether. (yield: 89.1%) 1H NMR (CDCl3)d 7.32–6.89 (m, 3H, thiophene), 3.54 (s, 2H, thiophene–CH2–), 3.40(s, 3H, –CH3).

2.3. Syntheses of P(3TAF-co-3TAa)s

P(3TAF-co-3TAa)s were synthesized as the method describedelsewhere [10,11]. An anhydrous FeCl3 was added into a reactionvessel which contained CHCl3. The flask was equipped with astirring bar and an N2 inlet. After stirring for an hour at room

Fig. 1. The synthetic route

temperature, 3-thiophene methyl acetate (3TMA) and anN-(3-thienylmethylene)-2-aminofluorene (3TAF) in CHCl3 wasadded via a syringe to the suspension. The reaction mixture wasstirred for 12 h under a flow of nitrogen. The reaction mixturewas then poured into a large quantity of methanol and the brownprecipitate was collected by filtration. The resulting material waswashed with methanoic hydrochloric acid (1 M). It was collectedand purified by the soxhlet extraction with methanol, ether, andchloroform in turn and then dried in high vacuum. P(3TAF-co-3TMA)-A-n were synthesized with different molar ratios(3TAF:3TMA¼1:5, 1:10, 1:20) of monomers at room temperature,respectively. Also, P(3TAF-co-3TMA)-B-n were synthesized withabove molar ratios of monomers at 0 1C, respectively. The P(3TAF-co-3TAa)s were synthesized by heating of the mixtures ofP(3TAF-co-3TMA)s in 2 M NaOH solutions for 24 h at 100 1C.The resulting solution was filtered to remove the insoluble part.The soluble part was neutralized and precipitated by diluted HClsolution. The precipitate was filtered and dried in vacuum for24 h. The synthetic route of P(3TAF-co-3TAa)s is shown in Fig. 1.

3. Result and discussion

The chemical structures of monomers and thiophene-basedcopolymers were confirmed by 1H NMR and FT-IR spectroscopy.The formations of P(3TAF-co-3TAa)s were confirmed by theappearance of the characteristic absorption band and proton

of P(3TAF-co-3TAa)s.

Fig. 2. FT-IR spectra changes of P(3TAF-co-3TMA)-A-1 after hydrolysis.

Fig. 3. UV–vis absorption spectra of P(3TAF-co-3TAa)s in DMSO solution.

S.-M. Lee et al. / Solar Energy Materials & Solar Cells 95 (2011) 306–309308

signal of the carboxylic acid group. As shown in Section 2.3,we confirmed the proton signals of carboxylic acid at12.58–12.70 ppm.

P(3TAF-co-3TMA)-A-1: 1H NMR (CDCl3) d 7.86 (s, 1H,�CH¼N–), 7.19–7.13 (m, 7H, fluorene Ar), 7.11–7.06 (m, 2H,thiophene), 3.69 (s, 2H, Ar–CH2–Ar), 3.64–3.60 (m, 2H, thiophene–CH2), 3.54–3.52 (m, 3H, –CH3).

P(3TAF-co-3TMA)-A-2: 1H NMR (CDCl3) d 7.92 (s, 1H,–CH¼N–), 7.20–7.16 (m, 7H, fluorene Ar), 7.12–7.11 (m, 2H,thiophene), 3.75 (s, 2H, Ar–CH2–Ar), 3.70 (s, 2H, thiophene–CH2),3.65–3.42 (m, 3H, –CH3).

P(3TAF-co-3TMA)-A-3: 1H NMR (CDCl3) d 7.92 (s, 1H,–CH¼N–), 7.18–7.14 (m, 7H, fluorene Ar), 7.13–7.11 (m, 2H,thiophene), 3.75 (d, 2H, Ar–CH2–Ar), 3.70 (s, 2H, thiophene–CH2),3.62–3.58 (m, 3H, –CH3).

P(3TAF-co-3TMA)-B-1: 1H NMR (CDCl3) d 7.80 (s, 1H,–CH¼N–), 7.07–7.03 (m, 7H, fluorene Ar), 7.01–7.00 (m, 2H,thiophene), 3.69 (s, 2H, Ar–CH2–Ar), 3.70 (s, 2H, thiophene–CH2),3.65–3.42 (m, 3H, –CH3).

P(3TAF-co-3TMA)-B-2: 1H NMR (CDCl3) d 7.80 (s, 1H,–CH¼N–), 7.07–7.05 (m, 7H, fluorene Ar), 7.03–7.00 (m, 2H,thiophene), 3.63 (s, 2H, Ar–CH2–Ar), 3.59 (s, 2H, thiophene–CH2),3.49–3.46 (m, 3H, –CH3).

P(3TAF-co-3TMA)-B-3: 1H NMR (CDCl3) d 7.83 (s, 1H,–CH¼N–), 7.15–7.13 (m, 7H, fluorene Ar), 7.09–7.06 (m, 2H,thiophene), 3.75 (s, 2H, Ar–CH2–Ar), 3.65 (s, 2H, thiophene–CH2),3.57–3.52 (m, 3H, –CH3).

P(3TAF-co-3TAa)-A-1: 1H NMR (DMSO-d6) d 12.62 (s, 1H,–COOH), 7.85 (s, 1H, –CH¼N–), 7.33–7.29 (m, 7H, fluorene Ar),7.28–7.24 (m, 2H, thiophene), 3.71 (s, 2H, Ar–CH2–Ar), 3.48–3.45(m, 2H, thiophene–CH2).

P(3TAF-co-3TAa)-A-2: 1H NMR (DMSO-d6) d 12.58 (s, 1H,–COOH), 7.87 (s, 1H, –CH¼N–), 7.37–7.33 (m, 7H, fluorene Ar),7.29–7.26 (m, 2H, thiophene), 3.65 (s, 2H, Ar–CH2–Ar), 3.53–3.50(m, 2H, thiophene–CH2).

P(3TAF-co-3TAa)-A-3: 1H NMR (DMSO-d6) d 12.60 (s, 1H,–COOH), 7.85 (s, 1H, –CH¼N–), 7.34–7.31 (m, 7H, fluorene Ar),7.28–7.22 (m, 2H, thiophene), 3.73 (s, 2H, Ar–CH2–Ar), 3.50–3.45(m, 2H, thiophene–CH2).

P(3TAF-co-3TAa)-B-1: 1H NMR (DMSO-d6) d 12.62 (s, 1H,–COOH), 7.88(s, 1H, –CH¼N–), 7.33–7.29 (m, 7H, fluorene Ar),7.28–7.24 (m, 2H, thiophene), 3.71 (s, 2H, Ar–CH2–Ar), 3.48–3.45(m, 2H, thiophene–CH2).

P(3TAF-co-3TAa)-B-2: 1H NMR (DMSO-d6) d 12.62 (s, 1H,–COOH), 7.85 (s, 1H, –CH¼N–), 7.31–7.29 (m, 7H, fluorene Ar),7.33–7.30 (m, 2H, thiophene), 3.79 (s, 2H, Ar–CH2–Ar), 3.58–3.55(m, 2H, thiophene–CH2).

P(3TAF-co-3TAa)-B-3: 1H NMR (DMSO-d6) d 12.70 (s, 1H,–COOH), 7.82 (s, 1H, –CH¼N–), 7.43–7.39 (m, 7H, fluorene Ar),7.32–7.28 (m, 2H, thiophene), 3.70 (s, 2H, Ar–CH2–Ar), 3.49–3.47(m, 2H, thiophene–CH2).

The FT-IR spectra changes after the hydrolysis of P(3TAF-co-3TMA)s exhibited characteristic absorption band at3600–2500 cm�1 due to O–H group of P(3TAF-co-3TAa)s. TheFT-IR spectra changes are shown in Fig. 2. The spectra changes ofother copolymers were similar to Fig. 2, respectively.

The weight–average molecular weight (Mw) of P(3TAF-co-3TAa)s was estimated by gel-permeation chromatography. Mw

and polydispersity were found as 8700 and 1.69 for P(3TAF-co-3TAa)-A-1, 10,400 and 1.41 for P(3TAF-co-3TAa)-A-2, 18,800 and2.76 for P(3TAF-co-3TAa)-A-3, 16,900 and 2.78 for P(3TAF-co-3TAa)-B-1, 19,100 and 2.22 for P(3TAF-co-3TAa)-B-2, 18,400 and2.11 for P(3TAF-co-3TAa)-B-3, respectively. We confirmed thatP(3TAF-co-3TAa)-B-n was relatively high weight-average mole-cular weight to P(3TAF-co-3TAa)-A-n. Above all, molecularweights were influenced by the polymerization temperature,

and we confirmed that copolymers obtained at lower temperaturewere relatively higher molecular weight as compared with thoseproduced at higher temperature.

Fig. 3 shows the UV–vis absorption spectra of P(3TAF-co-3TAa)s in DMSO. While the absorption spectra ofP(3TAF-co-3TAa)-A-n showed broad peaks around 440–460 nm,P(3TAF-co-3TAa)-B-n showed broad peaks around 480–495 nm.A progressive red-shifted band of P(3TAF-co-3TAa)-B-ncompared to P(3TAF-co-3TAa)-A-n is due to the increase of thep-conjugation system according to the increase of the molecularweight. The wide absorption and red shift of absorptionmaximum in visible region is desirable for light harvesting ofsolar energy.

To investigate the photovoltaic performances of DSSC devicesusing P(3TAF-co-3TAa)s as sensitizers, we prepared the DSSCdevices using the liquid state electrolyte with TiO2 adsorbedP(3TAF-co-3TAa)s in DMSO solution for 24 h at room tempera-ture and Pt-coated electrode as two electrodes. The SnO2:F/TiO2/P(3TAF-co-3TAa)s/electrolyte/Pt devices were fabricated accord-ing to procedures described previously [12]. The measurement ofthe I–V characteristics was carried out under the AM 1.5,100 mW/cm2. The active area of DSSC devices was 0.25 cm2. Thepower conversion efficiency of the device was obtained from theperformance parameters, open circuit voltage (Voc), short circuit

Fig. 4. I–V curves of the DSSC devices using P(3TAF-co-3TAa)s as a sensitizer

under an AM 1.5 illumination.

S.-M. Lee et al. / Solar Energy Materials & Solar Cells 95 (2011) 306–309 309

current (Jsc), and fill factor (FF). Pin and Pmax represented theincident light power (100 mW/cm2) and the maximum powerpoint, respectively. The power conversion efficiency (Z) is givenby Z¼Pmax/Pin¼FF� (Jsc�Voc)/Pin with fill factor FF¼(Jmax�

Vmax)/(Jsc�Voc)¼Pmax/(Jsc�Voc). Fig. 4 shows the I–V curves ofthe DSSC devices using P(3TAF-co-3TAa)s as a sensitizer. Asshown in Fig. 4, in case of using P(3TAF-co-3TAa)-A-1, P(3TAF-co-3TAa)-A-2, P(3TAF-co-3TAa)-A-3, Voc is 0.545, 0.543, 0.534(V), Jsc is 1.9, 3.1, 2.6 (mA/cm2), FF is 0.60, 0.58, 0.63, and Z is 0.63,0.99, 0.90 (%), respectively. In case of using P(3TAF-co-3TAa)-B-1,P(3TAF-co-3TAa)-B-2, P(3TAF-co-3TAa)-B-3, Voc is 0.548, 0.548,0.600 (V), Jsc is 4.4, 4.3, 4.5 (mA/cm2), FF is 0.54, 0.51, 0.56, and Z is1.32, 1.22, 1.53 (%), respectively. According to the I–V curves,P(3TAF-co-3TAa)-B-3 showed the highest Voc value among allsynthesized sensitizers. This may be due to the better covering ofthe TiO2 surface than that of any other sensitizers [13]. Thismeans that excellently covered TiO2 decreases the possibility ofredox species to access the TiO2 surface. We also confirmed thesuperior covering of P(3TAF-co-3TAa)-B-3 from the highest Jsc

value. P(3TAF-co-3TAa)-B-n sensitized cells gave higher Jsc valuesas compared with those of P(3TAF-co-3TAa)-A-n due to thebroader absorption band as shown in Fig. 3.

4. Conclusion

In this work, we showed that thiophene-based copolymers(P(3TAF-co-3TAa)s) can act as a sensitizer of DSSC device, due to

the adsorption onto TiO2 layer by a carboxylic acid group. Theoverall power conversion efficiency of DSSC devices usingP(3TAF-co-3TAa)-B-n showed higher values comparing to thatof the devices using P(3TAF-co-3TAa)-A-n, due to the higher Jsc

values. These devices showed the possibility of the replacement ofthe Ruthenium complex dyes using polymeric sensitizers in thedye-sensitized solar cells.

Acknowledgement

This research was supported by the Converging ResearchCenter Program through the National Research Foundation ofKorea (NRF) funded by the Ministry of Education, Science andTechnology (20090082141).

References

[1] A. Hagffeldt, M. Gratzel, Light-induced redox reactions in nanocrystallinesystems, Chem. Rev. 95 (1995) 49–68.

[2] A. Goetzberger, J. Luther, G. Willeke, Solar cells: past, present, future,Sol. Energy Mater. Sol. Cells 74 (2002) 1–11.

[3] M.K. Nazeeruddin, S.M. Zakeeruddin, R. Humphry-Baker, M. Jirousek, P. Liska,N. Vlachopoulos, V. Shklover, C.H. Fisher, M. Gratzel, Acid-base equilibria of(2,2’-bipyridyl-4,4’-dicarboxylic acid)ruthenium(II) complexes and the effectof protonation on charge-transfer sensitization of nanocrystalline titania,Inorg. Chem. 38 (1999) 6298–6305.

[4] M.K. Nazeeruddin, P. Pechy, Efficient panchromatic sensitization of nano-crystalline TiO2 films by a black dye based on a trithiocyanato–rutheniumcomplex, Chem. Commun. (1997) 1705–1706.

[5] G. Zhou, N. Pschirer, J.C. Shoneboom, F. Eickemeyer, M. Baumgarten,K. Mullen, Ladder-type pentaphenylene dyes for dye-sensitized solar cells,Chem. Mater. 20 (2008) 1808–1815.

[6] Z.S. Wang, F.Y. Li, C.H. Huang, Highly efficient sensitization of nanocrystallineTiO2 films with styryl benzothiazolium propylsulfonate, Chem. Commun.(2000) 2063–2064.

[7] J.K. Park, H.R. Lee, J. Chen, H. Shinokubo, A. Osuka, D. Kim, Photoelec-trochemical properties of doubly b-functionalized porphyrin sensitizers fordye-sensitized nanocrystalline-TiO2 solar cells, J. Phys. Chem. C 112 (2008)16691–16699.

[8] S. Roth, W. Graupner, Conductive polymers: evaluation of industrialapplications, Synth. Met. 57 (1993) 3623–3631.

[9] S. Roth, M. Filzmoser, Conducting polymers - thirteen years of polyacetylenedoping, Adv. Mater. 2 (1990) 356–360.

[10] S. Yanagida, G.K.R. Senadeera, K. Nakamura, T. Kitamura, Y. Wada,Polythiophene-sensitized TiO2 solar cells, J. Photochem. Photobiol. A Chem.166 (2004) 75–80.

[11] B.S. Kim, L. Chen, J. Gong, Y. Osada, Titration behavior and spectral transitionsof water-soluble polythiophene carboxylic acids, Macromolecules 32 (1999)3964–3969.

[12] H.J. Lee, W.S. Kim, S.H. Park, W.S. Shin, S.H. Jin, J.K. Lee, S.M. Han, K.S. Jung,M.R. Kim, Effects of nanocrystalline porous TiO2 films on interface adsorptionof phthalocyanines and polymer electrolytes in dye-sensitized solar cells,Macromol. Symp. 235 (2006) 230–236.

[13] J.R. Mann, M.K. Gannon, T.C. Fitzgibbons, M.R. Detty, D.F. Watson, Optimizingthe photocurrent efficiency of dye-sensitized solar cells through thecontrolled aggregation of chalcogenoxanthylium dyes on nanocrystallinetitania films, J. Phys. Chem. C 112 (2008) 13057–13061.