New polymer acceptor for solar cells application

Synthetic Metals 162 (2012) 566–572

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New polymer acceptor for solar cells application Shyambo Chatterjee, Susanta Banerjee ∗ , Pallab Banerji Materials Science Centre, Indian institute of Technology, Kharagpur 721302, West Bengal, India

a r t i c l e

i n f o

Article history: Received 14 December 2011 Received in revised form 31 January 2012 Accepted 3 February 2012 Available online 22 March 2012 Keywords: Fluorinated PPV-derivative Photoluminescence Solar cell

a b s t r a c t New photosensitive fluorine containing PPV derivative (DBTFM-PPV) has been synthesized as an acceptor material via Gilch polymerization reaction. The polymer was soluble in many of the common organic solvents. Optical, electrochemical and photovoltaic properties including hole and electron mobility of this polymer was investigated. Bilayer (ITO/PEDOT:PSS/MEH-PPV/DBTFM-PPV/Al) and bulk hetero junction (ITO/PEDOT:PSS/DBTFM-PPV:MEH-PPV/Al) solar cells were fabricated where DBTFM-PPV act as an acceptor material. Bulk hetero junction (BHJ) solar cell showed higher efficiency than bilayer device. The power conversion efficiency (PCE) of the BHJ solar cells based on DBTFM-PPV:MEH-PPV (2:1) was 0.49% with an open-circuit voltage (Voc ) of 1.16 V, fill factor of 0.49%, and a short-circuit current (Jsc ) of 1.17 mA/cm2 . © 2012 Elsevier B.V. All rights reserved.

1. Introduction Most of the conjugated polymers reported to date have low electron affinity. However, large number of applications like lightemitting diodes (PLEDs), polymer solar cells (PSCs), and field effect transistors (FETs) demands polymers with high electron affinity. For n-type materials, the electron affinity should be high enough for injection of electrons. It is reported that electron affinity should be in the range of 3.0–4.0 eV [1]. For solar cell application, an acceptor polymer should have correctly positioned HOMO and LUMO energy levels with respect to the donor materials [2]. One of the main requirements of enhancement of efficiency of the solar cell is the minimization of energy that is wasted due to mismatch of HOMO and LUMO energy level. Solar cell devices fabricated using donor-acceptor pairs MEH-PPV/PCBM or MDMO-PPV/PCBM has the LUMO–LUMO difference of 1 eV [3]. However, for efficient electron transfer from donor to the acceptor, the required energy is only 0.3–0.4 eV. Hence, 0.6–0.7 eV is wasted energy that has no contribution to the open-circuit voltage (Voc ) of the devices. Therefore, a suitable donor–acceptor pair has to be designed to minimize wasted energy and maximize the Voc value [4–7]. Fullerene-based materials and few small molecules have shown promise in this regard [5]. However, this type of materials has the problem of inadequate absorption in the visible region and required sophisticated processing like vacuum sublimation. The MEH-PPV based polymers have LUMO level of 2.9 eV and HOMO level of 5.4 eV [3,4,8,9]. Thus, a suitable acceptor polymer should have the LUMO and HOMO level of 3.2 eV and 5.7 eV to save

∗ Corresponding author. Tel.: +91 3222 283972; fax: +91 3222 255303. E-mail address: [email protected] (S. Banerjee). 0379-6779/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2012.02.001

the wasted energy. Lower LUMO energy level makes the polymer good electron acceptor. On contrary, low LUMO energy level of the acceptor materials result in lower open circuit voltage (Voc ) of the PSCs. Thus, to enhance the open circuit voltage of the devices, minimization of wasted energy and for efficient electron transfer, an optimization of acceptor LUMO energy level is required [10–13]. The minimization of the wasted energy in polymers can be done either by modifying the donor or acceptor energy levels. For obtaining acceptor poly(phenylenevinylene) (PPV), the cyano group was generally introduced on the vinylene units of the PPVs such as MEH-CN-PPV [9]. However, the cyano-substituted vinylene units are intrinsically unstable, insoluble, and can be easily photooxidized, while the introduction of cyano group on the phenylene ring of the PPVs results in more stable polymers [10]. Considering the above points it is of interest to develop a soluble and stable PPV derivative that can act as acceptor material. Fluorine has recently attracted attention as an electron-withdrawing group used in high-efficiency photovoltaic polymers. Due to its smaller size, it can be introduced onto the polymer backbone without any deleterious steric effects that a larger electron withdrawing group, such as a nitro or other group would do [7,14]. Beside these polymers containing fluorinated groups are endowed with increased solubility [15] and attachment of CF3 group in phenylene unit increases the photooxidative stability [14]. In consideration of the similarity between the electronegativity of fluorine atoms and cyano groups, it is expected that fluorine containing PPV derivative should behave as an acceptor material. Accordingly we have synthesized fluorine containing PPV derivative as photosensitive material. Throughout electrochemical and photovoltaic studies (bilayer as well as BHJ devices) it was proved that it is an acceptor material. The hole and electron mobility of the donor and acceptor was calculated by space charge limited

S. Chatterjee et al. / Synthetic Metals 162 (2012) 566–572

condition (SCLC) method. This mobility value explained the efficiency of photovoltaic devices.

2. Experiment 2.1. Measurements FTIR spectra were recorded using a NEXUS 870 FTIR (Thermo Nicolet) spectrophotometer at room temperature and humid free atmosphere by making KBr pellets. 1 H NMR was recorded on a Bruker 200 MHz and 500 MHz instrument (Switzerland), respectively. CDCl3 was used as solvent and TMS as reference. Elemental carbon and hydrogen, of the compounds were analyzed by pyrolysis method using Euro EA elemental analyzer. Gel permeation chromatography was performed using a Waters 2414 instrument. THF was used as eluent (flow rate, 0.5 mL/min), polystyrene was used as standard, and RI detector was used to record the signal. Glass transition temperature (Tg ) of the polymer was analyzed by differential scanning calorimetry (DSC) using NETZSCH DSC 200 PC differential scanning calorimeter at a heating rate of 20 ◦ C min−1 under nitrogen atmosphere. Glass transition temperature (Tg ) was taken at the center of the step transition in the second heating run. Thermal decomposition behavior of these polymers was measured on a NETZSCH TG 209 F1 thermal analyzer at a heating rate of 10 ◦ C min−1 under nitrogen atmosphere. The absorption and fluorescence spectra of the polymer solutions were measured using a Perkin Elmer scan UV–vis–NIR spectrophotometer with a 1 cm quartz cuvette. The fluorescence data were measured on a Spex—Fluorolog-3 (model no.: FL3-11) spectrofluorimeter using optically dilute solutions (A < 0.1). Qunine sulfate (0.55, standard: quinine sulfate in 0.1 N H2 SO4 ) was used as the standard for determination of quantum yields. The cyclic voltammetry was performed by a computer control potentiostat and galvanostat system from Gamry instrument. The thickness of the resulting active layer was measured with a Dektak profilometer. The current versus potential (I–V) curves were recorded with a Keithley 2400 digital source meter. A xenon lamp coupled with AM 1.5 solar spectrum filters was used as the light source, and the optical power at the sample was 100 mW/cm2 . The calibration of the incident light was performed with a mono crystalline silicon diode.

567

2.3. Space charge limited current (SCLC) measurements The electronic characteristics of the polymers were investigated with the goal of identifying materials with potential for application in organic electronics In order to investigate the respective hole and electron mobility of the donor and acceptor films, devices have been prepared by following method and the Ca/Al cathode is replaced by evaporated gold (60 nm). Hole-only devices were fabricated on top of a pre patterned ITO substrate. After cleaning the ITO with aqueous detergent, deionized water, 2-propanol, UVozone treatment was applied for 15 min. Polyethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS) was spin-coated from an aqueous dispersion phase to a layer 25 nm thick. The coated substrate was then dried at 120 ◦ C for 45 min. After drying, a solution of the polymer in chloroform (10 mg/mL) was spin-cast on top of the PEDOT:PSS layer to a thickness of 20–30 nm and the samples were dried for 4 h at room temperature and another 2 h at 70 ◦ C under vacuum conditions. The Au electrode was deposited by thermal evaporation under a vacuum of 10−6 Torr. So the hole-only devices in a configuration of ITO/PEDOT:PSS (25 nm)/MEH-PPV (20 nm)/Au were fabricated. For the electron-only devices, PEDOT:PSS layer was replaced with a thin layer of Cs2 CO3 spin coated from solution in 2-ethoxyethanol. Cesium carbonate Cs2 CO3 has been used as an efficient electron-injection layer. The work function of Cs2 CO3 was found to be 2.9 eV. Therefore, it can replace polyethylenedioxythiophene:polystyrenesulfonate as the anodic buffer layer to make electron-only devices ITO/Cs2 CO3 /DBTFM-PPV (30 nm)/Au.

3. Synthesis 3.1. 1,4-Dimethyl-2,5-bis(4-fluoro-3-trifluoromethyl benzyl)benzene (1)

F 3 2 1 10

9 5

8

2.2. Photovoltaic devices

CF3 4

CF3

6 7

The polymer was used to fabricate bilayer and bulk heterojunction (BHJ) model solar cells. The patterned indium tin oxide (ITO) glass (sheet resistance 30 ) was pretreated in an ultrasonic bath of acetone and isopropanol and treated in an ultraviolet-ozone chamber for 1 h. A thin layer (42 nm) of poly(3,4(PEDOT:PSS) ethylenedioxythiophene):poly(styrenesulfonate) was spin-coated onto the pretreated ITO glass and dried at 100 ◦ C for 1 h. A chlorobenzene solution of a blend of DBTFMPPV:MEH-PPV (2:1, w/w) was subsequently spin-coated on the surface of the PEDOT:PSS layer to form a photosensitive layer and the configuration is ITO/PEDOT:PSS/DBTFM-PPV:MEH-PPV (50 nm)||Al. The bilayer structures consisting of a 25–30 nm thick layer of MEH-PPV spin coated from a chlorobenzene solution (10 mg/mL) on a glass substrate covered with precleaned ITO. After annealing at 70 ◦ C, acceptor layer was spin-coated on top of the first layer from a xylene solution (MEH-PPV is nearly insoluble in xylene at room temperature). An aluminum layer (60 nm) was then evaporated through mask onto the surface of the photosensitive layer under vacuum with the configuration of ITO/PEDOT:PSS/MEH-PPV/DBTFM-PPV (40 nm)/Al.

F 1 2,5-Dibromo-p-xylene (15 g, 56.81 mmol) and 4acid [16] (23.63 g, fluorotrifluoromethylbenzylboronic 113.62 mmol) were mixed in 300 mL toluene and 150 mL of 2 M K2 CO3 solution, Pd(PPh3 )4 (0.4 g, 0.6 mol%) was added as catalyst. The reaction mixture was refluxed for three days under nitrogen atmosphere. The combined organic layers were washed with water (150 mL). The organic layer was extracted with toluene (150 mL). After the organic layer passed through neutral alumina to remove the dissolved catalyst. The organic layer was then dried over anhydrous magnesium sulfate, concentrated under reduced pressure to get yellowish solid product. Further purification by silica gel column chromatography (with n-hexane as an eluent) leads to 19.5 g (80%) of white solid product. 1 H NMR (CDCl , 200 MHz, ı ppm): 7.60–7.58 (m, 2H, H4 , H8 ), 3 7.54–7.51(m, 2H, H2 , H6 ), 7.28–7.24 (t, J = 3.8 Hz, 2H, H3 , H7 ), 7.13 (s, 2H, H1 , H5 ), 2.25 (s, 6H, H9 , H10 ). FTIR (KBr, cm−1 ): 3420, 2970, 2927, 2870, 1617, 1496, 1450, 1382, 1321, 1277, 1240, 1158, 1123, 1053, 916, 884, 832, 791, 663.

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Anal. Calcd. for C22 H14 F8 (430.33 g/mol): C, 61.40; H, 3.28; F, 35.32. Found: C, 61.42; H, 3.21; F, 35.36. 3.2. 1,4-Bis(bromomethyl)-2,5-bis(4-fluoro-3-trifluoromethyl benzyl)benzene (2)

F 3 2

CF3 4

Br

1 10

9 5

8

CF3

Br

6 7

F

2

1,4-Dimethyl-2,5-bis(4-fluoro-3-trifluoromethyl benzyl)benzene (1) (5 g, 11.6 mmol), N-bromo succinimide (NBS) (4.48 g, 25.2 mmol) and catalytic amount of benzoyl peroxide (BPO) were taken into dry CCl4 (70 mL). The mixture was refluxed for 8 h under nitrogen. The side product appeared as floating solid was filtered off. The crude product was purified by silica gel column chromatography using n-hexane as eluent to give 1.7 g (24.6%) of white solid 1,4-bis(bromomethyl)-2,5-bis(4-fluoro-3trifluoromethyl benzyl)benzene. 1 H NMR (CDCl , 200 MHz, ı ppm): 7.94 (s, 2H, H4 , H8 ), 7.75–7.69 3 (m, 4H, H3 , H7 ) 7.42 (s, 2H, H1 , H5 ), 7.36–7.31 (m, 2H, H2 , H6 ), 4.36 (s, 4H, H9 , H10 ). FTIR (KBr, cm−1 ): 3424, 2956, 2929, 2870, 1630, 1480, 1475, 1350, 1321, 1268, 1250, 1154, 1060, 916, 890, 838, 668. Anal. Calcd. for C22 H12 Br2 F8 (588.13 g/mol): C, 44.93; H, 2.06; Br, 27.17; F, 25.84. Found: C, 44.95; H, 2.10; Br, 27.19; F, 25.82. 3.3. Preparation of poly(2,5-bis(4-fluoro-3trifluoromethylbenzyl)-1,4-phenylenevinylene)

F CF3

n CF3 F A stirred solution of 4-bis(bromomethyl)-2,5-(4-fluoro-3trifluoromethylbenzyl)benzene (1.5 g, 2.55 mmol) in dry THF (200 mL) was heated to reflux. Then potassium tert-butoxide (0.85 g, 7.65 mmol) in dry THF (1.0 M) was added dropwise through a dropping funnel. The resulting mixture was refluxed for another 6 h and cooled to room temperature. The fibrous solid was obtained after precipitation in large excess of methanol and water (10:1). The polymer was filtered off and washed with methanol after that the polymer was dried under high vacuum to get 1 g of polymer. 1 H NMR (CDCl , 500 MHz, ı ppm): 7.82 (br, 2H), 7.77–7.33 (br, 3 6H). 7.20–7.15 (br, 2H). FTIR (KBr cm−1 ): 3439, 2966, 2926, 1667, 1490, 1417, 1325, 1256, 1130, 1056, 935, 805, 676. Anal. Calcd. for (C22 H10 F8 ): C, 63.16; H, 3.53. Found: C, 62.75; H, 3.93.

Fig. 1. Absorption spectra of the polymer in the THF solution.

4. Results and discussions The synthetic scheme for the preparation of the monomer and the polymer is outlined in Scheme 1. 2,5-Dibromoparaxylene was coupled with 4-fluoro-3-trifluoromethylphenylboronicacid in presence of K2 CO3 and Pd(PPh3 )4 to give compound 1,4-dimethyl2,5-bis(4-fluoro-3-trifluoromethyl benzyl)benzene. The bromination of compound 1,4-dimethyl-2,5-bis(4-fluoro-3-trifluoromethyl benzyl)benzene using N-bromo succinimide (NBS) gave monomer 1,4-bis(bromomethyl)-2,5-bis(4-fluoro-3-trifluoromethyl benzyl) benzene. Finally the polymer (DBTFM-PPV) was synthesized by Gilch process using excess of potassium tertbutoxide in dry THF. To avoid formation of any side product, the solution of potassium tertbutoxide in dry THF was added dropwise in excess amount to the reaction mixture. A yellow powder of polymer was obtained after repeated precipitation in methanol. The polymer is soluble in common organic solvents such as tetrahydrofuran, chloroform, dichloromethane, toluene, and xylene though it dose not contain any alkyl chain. Polymers containing fluorinated groups are endowed with increased solubility [15]. The enhanced solubility of DBTFM-PPV is due to the presence of pendent CF3 group. The number average molecular weight (Mn ) and weight average molecular weight (Mw ) of the DBTFM-PPV were 35,000 and 136,000, respectively, with polydispersity index of 3.8 as determined by gel permeation chromatography (GPC) using THF as eluent and calibrated with polystyrene standards at 30 ◦ C. The polymer was well characterized by 1 H NMR, FTIR, and elemental analysis. In the FTIR spectrum of the polymer sample a peak was found in the 950–970 cm−1 regions which assigned to the out-of-plane vibration of the trans-vinylene groups of the polymers. The benzylic proton peaks at 4.36 ppm disappeared after the polymerization and the new vinylic proton peaks were observed in 7.15–7.20 ppm. Electron withdrawing units results in the downfield shift of the vinylene peak. All other peaks showed good correspondence with the resulting polymer. The polymer showed good thermal stabilities with high glass temperature (Tg ) over 140 ◦ C and high onset decomposition temperatures (Td ) over 230 ◦ C. So the synthesized materials possess good thermal properties which are important advantages in the fabrication electronic devices. 4.1. Optical properties The UV–visible absorption spectrum of the DBTFM-PPV in THF solutions (10−5 M) and thin solid films (Figs. 1 and 2) shows a broad band from 370 to 450 nm. The optical band gap of the polymer

S. Chatterjee et al. / Synthetic Metals 162 (2012) 566–572

F3C

Br

F

F3C

F

F3C Toluene/water

H3C

569

CH3

+

F

B(OH)2

K2CO3/Pd(Ph3P)4

H3C

CH3

CCl4

Br

NBS/BPO

Br

Br C4H9OK/THF F

F

CF3

CF3

F3C

F

n

F

CF3

Scheme 1. Synthesis of the polymer.

was 2.38 eV which calculated from absorption band edge (520 nm). The emission spectrum displays a large Stokes-shift with a peak maximum at 420 nm in solution (10−5 M) and 510 nm (thin film; Fig. 3). The magnitude of the Stokes shift suggests large differences in the conformation of the polymer ground and excited states and energy migration to minority segments having greater conjugation lengths [17]. DBTFM-PPV showed relatively high fluorescence quantum yields (0.79) in solution (0.55, standard: quinine sulfate in 0.1 N H2 SO4 ). The band gap of the polymer was determined both from the UV–vis absorption spectra and from the cyclic voltammogram. Optical band gap energy can be calculated by using equation: E (eV) = h = hc/ = 1240/ (nm) = 1240/520 = 2.38 eV. Where h is Plank’s constant,  is wavelength in nm and c is the speed of light, E denotes the band gap energy of the polymer. The optical value (2.38 eV) corresponds to the pure band gap between the valence band and the conduction band. While the electrochemical band gap (2.7 eV) may be the results of the optical band gap coupled with the interface barrier for charge injection, which makes it larger [18]. A

wider band gap is a disadvantage, in that less light is harvested from the solar spectrum though the larger gap between the HOMO and the LUMO on the polymer provides an opportunity to increase the open circuit voltage [6]. It is very important to observe that whether the MEH-PPV emission is quenched by DBTFM-PPV in BHJ devices (Fig. 4). It has reported that more than 99% of the MEH-PPV emission is quenched by PCBM, suggesting efficient dissociation of excitons in MEH-PPV. Strong quenching is also reported for devices with CN-ether-PPV as electron acceptor [3]. To demonstrate potential applications of these polymers in solar cell, we have used MEH-PPV as an electron donor and DBTFM-PPV as an electron acceptor. The DBTFM-PPV selected as acceptors because it shows distinct differences in the position of the LUMO levels with respect to MEH-PPV, which should strongly influence the exciton dissociation probability and the open-circuit voltage. Polymer blends of DBTFM-PPV:MEH-PPV in various compositions (2:1, 1:1, and 1:2), were prepared, and their photoluminescence spectra are presented in Fig. 4. Complete photoluminescence quenching is observed for 2:1 compositions

Fig. 2. Absorption spectra of the polymer in the solid thin film.

Fig. 3. Emission spectra of polymer in the solid thin film.

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Fig. 4. Photoluminescence spectra of a neat MEH-PPV film and Photoluminescence quenching with different ratio of electron acceptors on MEH-PPV.

Fig. 6. Cyclic voltammograms recorded during electrochemical doping of polymer film electrode in 0.1 M nBu4 NClO4 solution at a scan rate of 50 mV s−1 .

[19,20]. This behavior suggests that the LUMO of DBTFM-PPV is below the LUMO of MEH-PPV (2.7–2.9 eV) to provide the driving force for exciton dissociation via charge transfer. For the 1:1 and 1:2 blends, however, only incomplete photoluminescence quenching was observed indicating that charge transfer is not efficiently carried out in this system [17]. The significant photoluminescence quenching is a strong indication that the 2:1 blending ratio of acceptor to donor may be useful for solar cell applications. Empirically, the overall energetic driving force for a forward electron transfer from the donor to the acceptor is represented by the energy difference (offset) between the LUMO energy level of the donor and acceptor (Fig. 5). It appears that a minimum LUMO energy difference required to exciton splitting and charge dissociation is 0.3 eV [5,21].

be 4.4 eV below vacuum, and the data are summarized in Table 1. The reduction peak potential of DBTFM-PPV is −1.42 V and corresponding n-dedoping peak appears at −1.2 V. The reproducibility of the cyclic volammograms in the potential range of 0 to −2.5 V is very good, indicating a good electrochemical stability for the ndoping/dedoping processes of DBTFM-PPV [22]. In contrast with the n-doping/dedoping process, the current associated with the p-doping in the oxidation of DBTFM-PPV is very small. The result suggests that DBTFM-PPV is a very good electron acceptor and an n-type conjugated polymer. The LUMO energy level of the polymer is close to other reported n-type polymeric material since it is generally assumed that a high electron affinity (>3 eV) is necessary for n-type materials [1,4,23]. 4.3. Photovoltaic properties

4.2. Electrochemical properties The redox behavior of the polymer was investigated by cyclic voltammetry (Fig. 6). This was performed in an acetonitrile solution with a concentration of 0.1 M. The polymer showed a quasi reversible reduction wave and an irreversible oxidation wave. The HOMO and LUMO energy levels were 5.9 eV and 3.2 eV estimated from the onset oxidation (1.5 eV) and reduction potentials (1.2 eV) assuming the absolute energy level of ferrocene/ferrocenium to

For photovoltaic materials, high charge carrier mobility is preferred to efficient transporting of the photo induced separated charge carriers, which consequently contributes to higher photocurrent collection. The space-charge limited current (SCLC) method has been often used to determine the hole as well as electron mobility of conjugated polymers. Here we measure the hole mobility of polymer only, bilayer and BHJ devices and electron mobility of polymer only device by the SCLC method. The SCLC

Fig. 5. Energetic diagram of the bilayer polymer solar cell and bulk-hererojuction polymer solar cell. The energetic levels given for the donor and the acceptor correspond to MEH-PPV and DBTFM-PPV respectively.

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Table 1 Absorption, emission maxima and redox potential of the polymer. Polymer

P a b c d

In solutiona

In solid filmb

Oxidation

Reduction

Band gap

abs (nm)

PL (nm)

ϕPL

abs (nm)

PL (nm)

Eg c (eV)

Eonset (V)

EHOMO (eV)

Eonset (V)

ELUMO (eV)

Eg d (eV)

410

420

0.79

420

510

2.38

1.5

5.9

1.2

3.2

2.7

−5

Measured with THF solution at a concentration of 10 M. 60 (±10) nm thickness. Band gap estimated from the wavelength of optical absorption of solid film. Band gap estimated from the onset oxidation and reduction potential.

mobility is calculated from the intercept of the plot of J0.5 vs V and Eq. (1) J = 9ε0 εr h V 2 /8L3

(1)

where ε0 is the permittivity of free space, εr is the dielectric constant of the polymer which is assumed to be around 3 for the conjugated polymers, h is the hole mobility, V is the voltage drop across the device, and L is the film thickness of active layer [24,25]. The hole-only devices of (ITO/PEDOT:PSS/MEH-PPV(20 nm)/Au) and electron only devices of (ITO/Cs2 CO3 /DBTFM-PPV (30 nm)/Au) were fabricated in order to estimate the hole mobility as well as electron mobility of the polymers via space-charge limit current (SCLC) theory (ESI). The calculated hole and electron mobility is 3.11 × 10−2 , 1.35 × 10−4 cm2 V−1 s−1 . Absence of insulating alkyl chain increases the mobility of the polymer. The calculated hole mobility of BHJ and bilayer devices in SCLC condition is 1.29 × 10−4 and 1.71 × 10−5 respectively (ESI). The ratio of electron to hole for polymer only, BHJ and bilayer devices is 0.004, 1.04 and 7.89. Thus unbalanced charge transport is anticipated for the PSCs based on the DBTFM-PPV. From photoluminescence quenching and lower HOMO and LUMO energies we have fabricated bulk heterojunction PSCs as well as bilayer device with a structure of ITO/PEDOT:PSS/DBTFMPPV:MEH-PPV (50 nm) (2:1, w/w)/Al and ITO/PEDOT:PSS/MEHPPV/DBTFM-PPV (40 nm)/Al. Fig. 7 shows the J–V curves of the devices for bilayer and BHJ cells. The bilayer consisting of donor and the acceptor polymer provides information on the exciton dissociation probability for the different donor–acceptor combinations. The bulk heterojunction device has power conversion efficiency (PCE) of 0.49% where as bilayer device is 0.05%. When the DBTFMPPV:MEH-PPV weight ratio is 2:1, the PSCs device exhibited the best performance. In bilayer devices the loss of absorbed photons further away from the interface (the typical exciton diffusion

Table 2 Photovoltaic performances of the cells. PSCs

Voc

Jsc

FF

PCE (%)

Bilayer BHJ

0.28 1.16

0.55 1.17

0.33 0.49

0.05 0.49

length in organic semiconductors) and results in low quantum efficiencies. The bulk heterojunction cell (basically 2:1) has heavily increased (orders of magnitude) interfacial area between the donor and acceptor phases and resulted in improved efficiency of the solar cell. The open-circuit voltage (Voc ), short circuit current density (Jsc ), fill factor (FF), and the power conversion efficiency (PCE) are shown in Table 2. The higher Voc (1.16 V) of the BHJ based devices could be explained by the larger interfacial area for exciton charge separation due to the good dispersion of DBTFM-PPV in the MEHPPV matrix, smaller LUMO offsets of the both polymers and large difference of accepter LUMO and donor HOMO energy level. So, the energy loss was less when the photo excited electron transfers from MEH-PPV to the DBTFM-PPV (MEH-PPV:HOMO, −5.3 eV; LUMO, −2.9 eV. DBTFM-PPV:HOMO, −5.9 eV; LUMO, −3.2 eV) [8,26]. However, the wider band gap of donor and the ratios between the hole and electron mobility indicate that unbalanced charge transport is anticipated for the PSCs based on this polymer [27]. This is the cause of lower values of short circuit current density (Jsc ) and lower efficiencies [28]. 5. Conclusions We have succeeded in synthesizing a high molecular weight, soluble semifluorinated PPV derivative. The polymer showed high thermal stability, high glass transition temperature and high electron affinity. This new polymer was used as an acceptor material for fabrication of BHJ as well as bilayer solar cells. The BHJ device has power conversion efficiency (PCE) of 0.49% where as bilayer device is 0.05%. These values are comparable with many other cyano substituted PPVs with better solubility and thermal stability. Hence, this new polymer may find application in BHJ solar cells. Further tuning of the power conversion efficiency could be attempted by structural modification of DBTFM-PPV by substitution of the -F connected to the aromatic ring. Acknowledgment The author acknowledges the financial support provided by the CSIR (Grant No. 01(2110)/07/EMR-II) of India. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.synthmet.2012.02.001. References

Fig. 7. Characteristic J–V curves of the optimized devices of polymer based bilayer and BHJ solar cells under 1 Sun condition (100 mW/cm2 ).

[1] C.R. Newman, C.D. Frisbie, D.A. da Silva Filho, J.L. Brédas, P.C. Ewbank, K.R. Mann, Chem. Mater. 16 (2004) 4436–4451.

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