Synthesis and photovoltaic properties of poly(p-phenylenevinylene) derivatives with two triphenylamine and bithiophene conjugated side chains

European Polymer Journal 45 (2009) 2726–2731

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Short communication

Synthesis and photovoltaic properties of poly(p-phenylenevinylene) derivatives with two triphenylamine and bithiophene conjugated side chains Ping Shen a, Bin Zhao a,b,*, Xianwei Huang a, Hui Huang a, Songting Tan a,b,* a b

College of Chemistry and Key Laboratory of Advanced Functional Polymeric Materials, College of Hunan Province, Xiangtan University, Xiangtan 411105, PR China Key Laboratory of Polymeric Materials & Application Technology of Hunan Province, Xiangtan University, Xiangtan 411105, PR China

a r t i c l e

i n f o

Article history: Received 6 February 2009 Received in revised form 19 May 2009 Accepted 28 May 2009 Available online 6 June 2009

Keywords: Bithiophene Conjugated side chains Photovoltaic properties Polymer solar cells Poly(p-phenylenevinylene) derivatives

a b s t r a c t Two soluble poly(p-phenylenevinylene) derivatives (PPVs) with two bithiophenes as conjugated side chains, P1 and P2, were synthesized and characterized for application in polymer solar cells (PSCs). The thermal, photophysical, electrochemical and photovoltaic properties of the PPVs were investigated and compared with those of the PPVs without conjugated side chains. Bulk heterojunction solar cell devices are fabricated using the copolymers as the electron donor and PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) as the electron acceptor. The power conversion efficiencies (g) based on the P1 and P2 are 1.1% and 1.41% under AM 1.5 illumination (100 mW/cm2), respectively. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Among the various applications of p-conjugated polymers, polymer solar cells (PSCs) are attractive because of its advantages of low cost, light weight and possibility to fabricate large area devices. For the applications in PSCs, poly(p-phenylenevinylene) derivatives (PPVs) [1] and polythiophenes (PTs) [2] have attracted much attention due to their good film-forming and optical properties. Up to now, the PSCs based on a blend of PPVs or PTs and PCBM have exhibited power conversion efficiencies (g) over 3% [3] and 6% [4], respectively. However, the g of the PSCs is still low, which is mainly suffered from the narrow absorption band and the low charge carrier mobility of the p-conjugated polymers. To fabricate high efficiency solar cells, the promising conjugated polymers should possess a * Corresponding authors. Address: College of Chemistry, Xiangtan University, Xiangtan 411105, PR China. E-mail addresses: [email protected] (B. Zhao), [email protected] (S. Tan). 0014-3057/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2009.05.031

broader absorption to enlarge absorption spectrum range and higher charge carrier mobility to reduce charge recombination and increase the photocurrent. In order to broaden the absorption spectrum and improve the mobility of charge carries, many endeavors have been done in modifying the chemical structures [5] and optimizing device architectures of p-conjugated polymers [6]. The attachment of conjugated/non-conjugated side chains to the polymer backbone is a common method to tune in optical and electronic properties of p-conjugated polymers [7]. Recently, Li and co-workers have demonstrated that the conjugated side chain [2a and c] could broaden the absorption spectrum and improve the hole mobility of PTs and PPVs efficiently. The published reports have also shown that the hole mobility [8] of PPV neat film and the electrons mobility [9] of PCBM are low and unbalanced. Our previous work [10] has proved that the holeinjecting and transporting ability of PPV derivatives could be improved by introduction of triphenylamine units to PPV backbones as side chains.

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S

S

i ) n-BuLi, -78 C ii ) C8H17Br

C8H17

1 S

NBS

S

Br

DMF C6H13

i ) n-BuLi, -78 C

2, Pd(PPh3)4 DMF,100 C

C8H17

S

OHC

C8H17

2 i ) n-BuLi, -78 C C H ii ) n-Bu3SnCl 8 17

5 CHO S

C8H17

DMF,100 C

S

S

C8H17

S OHC M1

6

C6H13 O CH2P(OC2H5)2

CHO N

SnBu3

C6H13

4

5, Pd(PPh3)4

S

S

THF

C6H13

CHO Br

S C6H13

C6H13 3

Br

S

Bu3Sn

ii ) n-Bu3SnCl THF

N

H3CO

OHC

OC8H17

(C2H5O)2PH2C

M2

O

M3

C8H17 S (m) M1

(n) M2 (m + n) M3

S

OCH3

OCH3

KOC4H9 THF

N

C6H13

C8H17O

m C6H13

C8H17O

S

n

S N

C8H17

P1

m:n=1:0

P2

m:n=3:2

Scheme 1. Synthesis of the monomers and copolymers.

In this communication, we designed and synthesized a PPV derivative (P1) (Scheme 1) with two alkyl-substituted bithiophene units as the conjugated side chains via WittigHorner reaction. To improve and balance the mobility of charge carries, we also synthesized a novel PPV derivative (P2) containing two bithiophene and triphenylamine conjugated side chains. The effects of the conjugated bithiophene and triphenylamine side chains on the thermal, photophysical, electrochemical and photovoltaic properties of the PPVs were investigated.

already published procedures. All starting materials were purchased from commercial suppliers (Pacific ChemSource and Alfa Aesar) in analytical grade. THF was dried and distilled over sodium and benzophenone. DMF, CHCl3 and CH2Cl2 were dried over by accustomed methods and distilled before use. All chromatographic separations were carried out on silica gel. 2.2. Characterization methods 1

2. Experimental 2.1. Materials and reagents 2-Octylthiophene (1) [2a], 5-octyl-2-(tributylstannyl)thiophene (2) [11], 2-bromo-3-hexyl-thiophene (3) [11], 5-octyl-30 -hexyl-2,20 -bithiophene (4) [11], 5-octyl30 -hexyl-50 -tributylstannyl-2,20 -bithiophene (5) [11], 2,5-dibromoterephthalaldehyde (6) [12], 2,5-bis(4-(N,Ndiphen-ylamino)phenyl)terephthalaldehyde (M2) [10] and 1, 4-bis(diethylphosphonatomethyl)-2-methoxy-5octoxybenzene (M3) [10] were synthesized according to

H NMR and 13C NMR spectra were measured on a Bruker Avance 400 spectrometer. FT-IR, UV–vis absorption and PL spectra were obtained on PE Spectra One, PE Lamada 25 and PE LS-50 luminescence spectrometer, respectively. Thermo-gravimetric analyses (TGA) and differential scanning calorimetric measurements (DSC) were performed under nitrogen at a heating rate of 20 °C/min. The molecular weights and polydispersity index (PDI) were determined using Waters-1515 gel permeation chromatography (GPC) system. Cyclic voltammetry (CV) measurements were done on a EG&G Princeton Applied Research Model 273 Electrochemical Workstation with Pt wires as both working and counter electrode,

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saturated calomel electrode (SCE) as reference electrode in a 0.1 M Bu4NClO4 acetonitrile solution. 2.3. Fabrication and characterization of polymer solar cells The PSCs were constructed in the traditional sandwich structure through several steps. Poly(3,4-ethylene dioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS, from Bayer AG) was spincoated from an aqueous solution on a cleaned indium tin oxide (ITO)/glass substrate giving a thickness of about 30 nm as measured by Ambios Technology XP–2 surface profilometer, and it was dried subsequently at 150 °C for 30 min. The photosensitive blend layer of polymer and PCBM was prepared by spin-coating the chlorobenzene solution of the polymers and PCBM (1:1, w/w) with the polymer concentration of 12 mg/mL on the ITO/PEDOT:PSS electrode, and dried at 80 °C for 30 min, in a nitrogen-filled glovebox. The cathode of devices, consisting of 10 nm of Ca and 200 nm of aluminum, was thermally deposited on the top of polymer film at 5  105 Pa. The active area of the device is 4 mm2. Current density–voltage (J–V) characteristics were measured by a computer controlled Keithley 236 source measurement unit in dark and under AM1.5 illumination conditions, 100 mW/cm2. All these measurements were performed under ambient atmosphere at room temperature.

freshly distilled THF (30 mL). Potassium tert-butoxide (0.224 g, 2.0 mmol in 10 mL THF) was added into the solution over 30 min using the syringe pump. The reaction mixture was stirred for 36 h at room temperature, and then reacted for additional 12 h at 60 °C. The polymerization solution was poured into 50 mL of methanol and then filtered to obtain a red solid polymer, which was subjected to Soxhlet extraction with methanol, hexane and chloroform, subsequently. The polymer was recovered from the chloroform by rotary evaporation and dried under vacuum. Copolymer P1: M1 (0.428 g, 0.5 mmol) was reacted with M3 (0.268 g, 0.5 mmol) to obtain 0.35 g P1 (yield, 65%) as a red solid. IR (KBr): 967 cm1 (CH@CH, trans-vinylene). 1 H NMR (400 MHz, CDCl3): d = 7.85 (br, 2H, Ar–H), 7.52–7.49 (br, 4H, Th–H), 7.09–6.98 (m, 6H), 6.74 (br, 2H), 3.99–3.92 (br, 2H, –OCH2–), 3.80 (s, 3H, –OCH3), 2.84–2.78 (br, 8H, Th–CH2–), 1.76–1.67 (br, 10H, –CH2–), 1.39–1.28 (br, 42H, –CH2–), 0.88–0.80 (br, 15H, –CH3). Copolymer P2: M1 (0.257 g, 0.3 mmol) and M2 (0.124 g, 0.2 mmol) were reacted with M3 (0.268 g, 0.5 mmol) to obtain 0.4 g P2 (yield, 80%) as a red solid. IR (KBr): 966 cm1 (CH@CH, trans-vinylene). 1 H NMR (400 MHz, CDCl3): d = 7.8 (br), 7.78–7.77 (m), 7.52–7.40 (m), 7.18 (br), 7.04–6.98 (m), 6.74 (b), 4.09– 3.83 (m, –OCH3, –OCH2–), 2.82–2.78 (br, Th–CH2–), 1.71– 1.59 (m), 1.40–1.20 (m), 0.88–0.81 (m).

2.4. Synthesis of monomers and copolymers Synthetic routes of the monomers and polymers are shown in Scheme 1.

3. Results and discussion 3.1. Synthesis and characterization

2.4.1. Synthesis of the monomers 2,5-Bis(5-octyl-30 -hexyl-2,20 -bithiophene-50 -yl)terephthalaldehyde, M1. Under Ar atmosphere, compound 5 (5.2 g, 8 mmol), 6 (1.16 g, 4 mmol), DMF (50 mL) and Pd(PPh3)4 (0.115 g, 0.1 mmol) were added in a 100 mL flask. The mixture was stirred at 100 °C for 24 h and cooled to room temperature, then quenched with 50 mL water and extracted with ether. The combined organic layer was washed with water and brine, dried over anhydrous MgSO4, filtered and evaporated. The crude product was purified using a silica gel column chromatography with petroleum ether/dichloromethane (5:1, v/v) as eluent to obtain 2.1 g M1 (yield, 49%) as a sanguine viscid liquid. IR (KBr): 1 689 cm1 (ms, C@O). 1 H NMR (400 MHz, CDCl3): d = 10.35 (s, 2H), 8.14 (s, 2H), 7.00 (d, 2H, J = 3.44 Hz), 6.93 (s, 2H), 6.77 (d, 2H, J = 3.44 Hz), 2.83 (t, 2H, J = 6.98 Hz), 2.77 (t, 2H, J = 6.88 Hz), 1.68 (m, 4H), 1.39–1.25 (m, 32H), 0.89 (m, 12H). 13 C NMR (100 MHz, CDCl3): d = 191.33, 147.18, 140.08, 136.61, 136.53, 134.77, 134.57, 132.56, 132.47, 130.61, 126.26, 124.60, 31.89, 31.64, 30.65, 30.19, 29.32, 29.25, 29.17, 14.12, 14.09. (C52H70O2S4)n: Calcd. C 73.02%, H 8.25%, S 14.99%; Found: C 76.52%, H 8.26%, S 14.47%. 2.4.2. Synthesis of the copolymer P1 and P2 Under Ar atmosphere, the dialdehyde monomers (M1, M2) (0.5 mmol) and M3 (0.5 mmol) were dissolved in

The general synthetic strategy for the monomers and copolymers is outlined in Scheme 1. The key intermediate, monomer 1 (M1), was synthesized from compound 5 and 6 in 49% yield according to the Suzuki coupling reaction, and purified using a silica gel column with a petroleum ether/ dichloromethane (5:1, v/v) as eluent. The copolymers P1 and P2 were prepared according to the Wittig-Horner polymerization method. The copolymers are soluble in common organic solvents, such as chloroform, dichloromethane, and THF at room temperature. The structures of copolymers were confirmed by 1H NMR and FT-IR. The presence of absorption at around 966 cm1 in the FT-IR spectra of copolymers indicates that the olefin groups are predominantly in the trans-configuration [13]. The disappearance of the characteristic proton peaks –CHO at 10.35 ppm for monomers (M1, M2) and the appearance of new vinylic proton peaks at around 7.1 ppm with aromatic proton peaks in polymers confirm the polymerization reaction. The P2 is a random copolymer as shown in Scheme 1. The actual value (1.47) of m:n was determined by the 1H NMR spectrum of the copolymer P2 according to the ratio of the integral areas of characteristic hydrogen (OCH3, –OCH2– and thiophene–CH2–). The number-average molecular weights (Mn) are 19 and 20 K with a polydispersity index of 1.95 and 1.7 for P1 and P2, respectively. Thermo-gravimetric analysis (TGA) shows 5% weight loss temperatures weights temperature (Td) up to 400 °C. The DSC measurement of copolymers exhibit

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3.2. Photophysical properties The photophysical characteristics of the copolymers have been investigated by UV–vis absorption and photoluminescence (PL) spectra in dilute chloroform solution as well as in solid films. The results are summarized in Table 1. Fig. 1 shows the UV–vis absorption spectra of P1 and P2 in solution and solid films. There are two maximum absorption peaks: the one in the UV region at around 350 nm (370 nm for solid films) is attributed to the conjugated side chains; another in the visible region at around 480 nm (500 nm for solid films) is ascribed to the p–p* transition of the conjugated PPV main chains. The result of UV–vis absorption spectra for P1 and P2 indicate that the electron-rich alkyl-substituted bithiophene and triphenylamine groups as the side chains can broaden the absorption spectra of the UV region and red-shift the absorption band of the polymer main chains in comparison with that of the PPVs without conjugated side chains [10]. In both solution and solid film, the main-chain absorption peaks of copolymer P2 are red-shifted in comparison with P1 (Fig. 1). As compared with the solution absorption spectra, the copolymers P1 and P2 in solid films are redshifted about 12 and 25 nm of the p–p* transition bands, respectively. This can be explained as the strong interaction between the polymer chains in solid films [14]. Furthermore, from the Fig. 1, we also can see that the copolymers show the almost equivalently strong absorbance in the UV and visible region. The strong absorbance and broad absorption of the copolymers are necessary and favorable for increasing light harvest to improve the power conversion efficiency of the PSCs. The optical band gaps (Eopt g ) estimated from the absorption band-edge of the copolymer films are 2.15 and 2.04 eV for copolymers P1 and P2, respectively, which is lower than that (2.29 eV) of the polymer without conjugated side groups [10]. The PL spectra of P1 show the same maximum emission peaks of 605 nm excitated at the different wavelength corresponding to the two absorption peaks (378 and 491 nm for solid film), which indicates that quick and complete energy transfer occurs from the conjugated side chains (bithiophene) to the PPV main chains after the conjugated side

P1-Solution P2-Solution P1-Film P2-Film

1.0

Normalized absorbance

the glass transition temperatures (Tg) of 144 and 142 °C, respectively. Obviously, the two copolymers shows a better thermal stability than that of the polymer without conjugated side chains [10], which is favorable for applications in PSCs devices.

0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

Wavelength (nm) Fig. 1. UV–vis absorption spectra of P1 and P2 in chloroform solution and solid films.

chains absorb the photons in the wavelength range 350– 400 nm. The same results can be observed for copolymer P2, which agreed with that observed in other PPVs [10] and PTs [2a] bearing the conjugated side chains. This phenomenon ensures that all photons absorbed by the copolymers are useful for the photovoltaic conversion. 3.3. Electrochemical properties Cyclic voltammetry (CV) is often utilized to investigate the information on the charge injection and measure the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of conjugated polymers [15]. Fig. 2 shows the CV curves of copolymers P1 and P2 films on a Pt electrode in 0.1 M Bu4NClO4 acetonitrile solution. The onset oxidation peaks (Eox) for P1 and P2 are 1.03 and 0.91 V (vs SCE), respectively. From the values of Eox, the corresponding HOMO energy levels (EHOMO) of copolymers are calculated by the method reported by Li et al. [16]. Whereas it seems that there are no obvious reduction processes for the two copolymers from the Fig. 2. Therefore, the onset reduction peaks (Ered) and the corresponding LUMO energy levels (ELUMO) of the two copolymers are cannot be obtained directly from the Fig. 2. The ELUMO can be estimated by subtracting the optical band gaps (Eopt g ) from the EHOMO as determined by the CV [17]. The EHOMO and ELUMO levels for P1 and P2 are listed in Table 1.

Table 1 Photophysical and electrochemical properties of the copolymers. Copolymer

P1 P2 a b c d e

Solution kmax[nm]a

Film kmax[nm]b

UV

PL

UV

PL

359,479 349,484

541 542

378,491 358,509

605 599

Measured in chloroform solution. Measured on quarte plate by copolymer cast from chloroform solution. Band gap estimated from the onset wavelength of optical film absorption. Energy levels calculated from the CV. Energy levels calculated from ELUMO = (EHOMO + Eopt g ).

Eopt [eV]c g

EHOMO [eV]d

ELUMO [eV]e

2.15 2.04

5.43 5.31

3.28 3.27

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0.5 mA

Current (mA)

P1

P2

-2

-1

0

1

2

Potential (V vs SCE) Fig. 2. Cyclic voltammograms of the copolymer films on a platinum electrode in Bu4NClO4/CH3CN at 50 mV/s.

The EHOMO of copolymer P2 (5.31 eV) increased more than that of P1 (5.43 eV) in comparison with the polymer without conjugated side groups (5.47 eV) [10], which is attributed to the introduction of triphenylamine moieties in the copolymer P2. This indicates that the triphenylamine moieties attached to the PPVs main chains could heighten the EHOMO level, which makes the potential barrier for the hole-injection at the P2/ITO interface slightly lower than that at P1/ITO, and thereby improved the hole-injection and transporting property of P2. 3.4. Photovoltaic properties In order to check whether the conjugated side chains of two copolymers do make a contribution to the photoelectronic conversion in the PSCs, a bulk heterojunction PSC device based on the blends of P1 or P2 (as electron donor) and PCBM (as electron acceptor) has been fabricated and investigated. The employed device structure was ITO/PEDOT–PSS/polymer:PCBM(1:1, w/w)/LiF/Al. Fig. 3 shows the

Current Density mA/cm2

P1 P2

4

4. Conclusions Two new PPV derivatives with bithiophene and triphenylamine conjugated side chains, P1 and P2, were synthesized via Wittig-Horner reaction. The research results show that the conjugated side chains can broaden and red-shift the absorption spectrum as well as heighten the HOMO energy levels of the copolymers, and as a result of improvement in the power conversion efficiencies. The best PSC performance is obtained from the device based on P2/PCBM (1:1, w/w) with a power conversion efficiency of 1.41% (Voc = 0.84 V, Jsc = 4.74 mA/cm2, FF = 0.35). The result indicates that the copolymers are attractive polymer materials for the development of PSCs.

8 6

current–voltage (J–V) curves of the PSCs under the illumination of AM1.5 G (100 mW/cm2). The cell based on P1/ PCBM shows an open-circuit voltage (Voc) of 0.76 V, a short-circuit current density (Jsc) of 3.92 mA/cm2, a calculated fill factor (FF) and g value of 0.37 and 1.10%, respectively. The corresponding device parameters found using P2/PCBM are 0.84 V, 4.74 mA/cm2, 0.35 and 1.41%. The PSC based on P2/PCBM shows higher Voc than that for the cell based on P1/PCBM, although the EHOMO of P2 (5.31 eV) is higher than that of P1 (5.43 eV) (Table 1). Generally the Voc of bulk heterojunction solar cells is determined by the energy difference between the acceptor (PCBM) ELUMO and the donor (polymer) EHOMO [18]. In this stage, the slight increase of the Voc for the cell P2/PCBM may be explained by the larger interfacial area for exciton charge separation due to the good dispersion of PCBM in polymer P2 matrix [19]. Since the Jsc may depend upon the copolymer through its light-harvesting and hole transport properties [20]. The higher Jsc (4.74 mA/ cm2) of the P2/PCBM cell compared with P1/PCBM (3.92 mA/cm2) should be a result of the broader and red-shifted absorption spectrum as well as the lower potential barrier for the hole-injection at the polymer/ITO interface of P2. The g value of the cell P2/PCBM (1.41%) is increased by 29% in comparison with that of the cell P1/PCBM (1.10%) due to its relatively higher Voc and Jsc. Compared with the polymer (0.09%) [10] without the conjugated side chains, the g value of P1 and P2 increased markedly, which indicates that the significant improvement of g for P1 and P2 benefit from the extended conjugated side chain with the introduction of the bithiophene and triphenylamine moieties and the enhancement of hole-transporting properties. Considering the low Jsc and FF values of the devices, there is a big room for future improvement in performance of PSCs based on the copolymers and PCBM. Further studies are underway to optimize devices based on these promising materials for photovoltaic cell.

2 0 -2 -4 -6 -8 -0.50

-0.25

0.00

0.25

0.50

0.75

1.00

1.25

Voltage (V) Fig. 3. J–V characteristic of the cells made of polymer/PCBM (1:1, w/w) under the illumination of AM 1.5 G (100 mW/cm2).

Acknowledgements This work was supported by the key project (2008FJ 2004) and Nature Science Foundation (09JJ3020, 09JJ 4005) of Hunan Province of China.

P. Shen et al. / European Polymer Journal 45 (2009) 2726–2731

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