Spirobifluorene-Based Conjugated Polymers for Polymer Solar Cells

Mar 26, 2012 - PSFDTBT has an optical band gap of 1.97 eV with the low-lying HOMO energy level at −5.4 eV. The hole mobility of the pristine PSFDTBT...
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Spirobifluorene-Based Conjugated Polymers for Polymer Solar Cells with High Open-Circuit Voltage Ming Wang,† Cuihong Li,‡,* Aifeng Lv,† Zhaohui Wang,† and Zhishan Bo‡,†,* †

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China College of Chemistry, Beijing Normal University, Beijing 100875, China

‡

S Supporting Information *

ABSTRACT: A new alternating copolymer (PSFDTBT) based on spirobifluorene, thiophene, and benzothiadiazole units has been synthesized. PSFDTBT has an optical band gap of 1.97 eV with the low-lying HOMO energy level at −5.4 eV. The hole mobility of the pristine PSFDTBT film spin-cast from o-dichlorobenzene (DCB) solution is 7.26 × 10−3 cm2 V−1 s−1 with on/off ratios in the order of 105. Polymer solar cell devices based on the blend films of PSFDTBT and PC71BM show a high open-circuit voltage of 0.94 V and a power conversion efficiency of 4.6% without any post-treatment. All the device measurements were performed in air without encapsulation. This is the first report on spirobifluorene-based conjugated polymers used for polymer solar cells, demonstrating the great potential of spirobifluorene moiety as an electron-donating unit for the construction of main chain donor−acceptor alternating conjugated polymers for high performance polymer solar cells.

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INTRODUCTION Polymer solar cells (PSCs), as the most promising sustainable energy source alternative to inorganic-based solar cells, have received intensive attention in both academia and industry in recent years.1 In comparison with inorganic solar cells, PSCs have the potential advantages of lightweight, flexibility, and manufacturing by low cost roll-to-roll process.2 Up to date, bulk heterojunction (BHJ) structure, in which the active layer consists of an interpenetrating blend of electron donating conjugated polymers and electron accepting fullerene derivatives, is still the most useful active layer structure for high power conversion efficiency (PCE) polymer solar cells.1,3 Intensive researches worldwide on polymer design and device fabrication have led to a rapid development in the field of PSCs.1,4 Recently, power conversion efficiency higher than 7% for BHJ polymer solar cells has been achieved by several groups through the developing of new polymer donor materials with the use of [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]phenyl-C71-butyric acid methyl ester (PC71BM) as electron acceptors.4a−i For the designing high power conversion efficiency donor polymers, the following three points should be considered:5 (1) the polymer should have appropriate highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels for harvesting as more photons as possible from solar radiation;5b (2) the polymer/PCBM blend films should have high and balanced carrier mobility for generating more free charge carriers and efficiently collecting them at corresponding electrodes;6 (3) the polymer should be soluble in common organic solvent for solution processing.7 © 2012 American Chemical Society

Main chain donor−acceptor alternating design is a very efficient way to engineer the band gap of conjugated polymers through the internal charge transfer from donor to acceptor unit.4a−n,8 Main chain donor−acceptor alternating conjugated polymers with 9,9-dialkylfluorene as donor unit and benzothiadiazole as acceptor unit have been extensively studied as donor components in BHJ PSCs.9 PCE of polymer solar cells based on 9,9-dialkylfluorene moieties has been improved to 4.5% with an open circuit voltage (Voc) of almost 1 V.10 Compared with the large number of publications on 9,9dialkylfluorene based D−A alternating copolymers applied in PSCs, less attention was paid to spirobifluorene-based copolymers. In the past few years, spirobifluorene has been used as an ideal building block in construction of stable blue light-emitting materials.11 In the spiro-annulated molecules, the two perpendicular conjugated moieties are connected with the sp3-hybridized carbon atom as the spiro center. This structural feature can minimize the extravagant aggregation of polymers containing spiro-annulated molecules and thus can improve their solubility and processability.11,12 Furthermore, the spirobifluorene structure can prevent the possible photo/ thermal oxidation of the 9-position of the fluorene unit to form the undesired low energy ketonic defect.13 In theory, stable polymer structures are beneficial for the long lifetime of polymer solar cells. Here, we demonstrate for the first time that spirobifluorene can be a very useful building block to construct Received: December 20, 2011 Revised: March 8, 2012 Published: March 26, 2012 3017

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Scheme 1. Synthetic Route to Polymer PSFDTBT and the Chemical Structure of 9,9-Dialkyl-9H-fluorene-Based Polymer P1a

a

Conditions and reagents: (i) Pd(PPh3)4, Na2CO3, toluene−H2O (v/v, 4:1), and tetra(n-butyl)ammonium bromide, reflux.

weight loss up to 340 °C. No clear glass transition was observed from 25 to 300 °C in its DSC curves of the second heating and cooling runs (20 °C/min). Optical and Electrochemical Properties. UV−vis absorption spectra of PSFDTBT in THF solution and in film are shown in Figure 1. In THF solution, PSFDTBT exhibited a

main chain donor−acceptor alternating conjugated polymer, poly[2,7-(3′,6′-dioctyloxy)-9,9′-spirobifluorene-alt-5,5-(4′,7′-di2-thienyl-5′,6′-dioctyloxy-2′,1′,3′-benzothiadiazole)] (PSFDTBT) as shown in Scheme 1, for high efficiency polymer solar cells. The octyloxy chains on spirobifluorene and 2′,1′,3′-benzothiadiazole units can provide PSFDTBT with good solubility in common organic solvents. The transport properties of the polymer are investigated by fabricating organic thin film field-effect transistors (OFETs). The pristine PSFDTBT film spin-cast from DCB solution exhibited hole mobility of 7.26 × 10−3 cm2 V−1 s−1 with on/off ratios in the order of 105. The PSFDTBT/PC71BM blend films show hole mobility of 1.14 × 10−2 cm2 V−1 s−1 and an electron mobility of 2.7 × 10−4cm2 V−1 s−1. Under simulated solar illumination of AM 1.5G (100 mW/cm2), a PCE of 4.6% has been achieved for PSCs composed of PSFDTBT and PC71BM as the active layer in simply processed devices. The devices illustrate a considerably high Voc of 0.94 V, a Jsc of 8.0 mA/cm2, and a comparable fill factor (FF) of 0.60. All device measurements were conducted in air without any encapsulation. Noticeably, PSFDTBT exhibits significantly improved PSC performance when compares with the analogous 9,9-dialkyl-9H-fluorene based polymer P1 (Scheme 1), which showed general PSC performance with PCE of about 3.1% in our previous report.8

Figure 1. UV−vis absorption spectra of PSFDTBT (in THF solution and in film) and PSFDTBT/PC71BM blending film (1:3 by weight).

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broad absorption in the range of ultraviolet to visible region with two peaks located at 395 and 527 nm, respectively. In film, the absorption of PSFDTBT became broader and slightly redshifted with two peaks located at 401 and 562 nm, respectively. In comparison with the absorption spectrum in solution, the absorption spectrum in film became broader and red-shifted for about 35 nm, indicating the aggregation or ordered packing of polymer chains in solid state. Ordered packing of polymer chains is beneficial for improving the charge mobility of resulting films.14 The optical absorption onset of PSFDTBT film is at 629 nm, corresponding to an optical band gap of 1.97 eV. PC71BM was chosen as the acceptor because it has similar electronic properties as PC61BM, but a higher absorption coefficient in the visible region with a broad peak from 440 to 530 nm, which compensates the absorption valley of the polymer.15 As shown in Figure 1, the PSFDTBT/PC71BM blending film (1:3 by weight) has a strong absorption over a broad wavelength range of 400 −700 nm. The electrochemical properties of PSFDTBT were investigated by cyclic voltammetry (CV). The HOMO energy level of PSFDTBT was determined to be −5.40 eV according to the equation EHOMO = −e(Eox + 4.71) (eV), where Eox is the onset oxidation potential in volts vs Ag/Ag+.16 Since the open circuit voltage Voc of BHJ

RESULTS AND DISCUSSION Material Synthesis. As shown in Scheme 1, PSFDTBT is synthesized by Suzuki−Miyaura polycondensation of M1 and M2. The polycondensation was carried out in a biphasic mixture of toluene/aqueous K2CO3 solution (4:1) with Pd(PPh3)4 as the catalyst precursor. After the polymerization proceeded for 72 h, phenylboronic acid and bromobenzene were successively added at a time interval of 4 h to end-cap the polymer. PSFDTBT was obtained as a dark red solid in a yield of 86%. The molecular weight and molecular weight distribution were determined by gel permeation chromatography (GPC) using tetrahydrofuran (THF) as an eluent calibrated with polystyrene standards. PSFDTBT showed a number-average molecular weight (Mn) of 23 kg/mol, a weightaverage molecular weight (Mw) of 78 kg/mol, and a polydispersity index (PDI) of 3.4. Because of the threedimensional structure of spirobifluorene units, PSFDTBT is readily dissolved in common organic solvents such as chloroform, chlorobenzene, toluene, THF, 1,2-dichlorobenzene (DCB), 1,2,4-trichlorobenzene (TCB), etc. at room temperature. Thermogravimetric analysis (TGA) showed that PSFDTBT is of good thermal stability with less than 5% 3018

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Figure 2. (a) Output and (b) transfer characteristics of the spin-coated film of PSFDTBT transistors based on OTS-modified Si/SiO2 substrate.

Table 1. Summary of Photovoltaic Properties of BHJ PSCs Based on PSFDTBT active layer

weight ratio (PSFDTBT:PC71BM)

thickness (nm)

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

PSFDTBT:PC71BM DCB

1:2.5 1:3 1:3.5 1:4 1:2 1:3 1:4 1:3

90 85 70 75 75 70 75 80

0.96 0.95 0.96 0.95 0.94 0.94 0.94 0.97

7.6 7.3 7.6 7.7 7.1 8.0 7.4 6.7

0.56 0.57 0.57 0.56 0.56 0.60 0.60 0.47

4.1 4.0 4.2 4.1 3.7 4.6 4.2 3.1

PSFDTBT:PC71BM TCB

P1:PC71BMa DCB a

Data from ref 8.

Figure 3. J−V curves of PSCs fabricated from PSFDTBT:PC71BM with different weight ratio in DCB (a) and TCB (b) solutions.

PSFDTBT films are shown in Figure 2. The hole mobility of the pristine PSFDTBT film spin-cast from DCB solution is 7.26 × 10−3 cm2 V−1 s−1 with on/off ratios in the order of 105, which is almost two orders higher than that of P1 (the hole mobility of pure P1 film is 2 × 10−5 cm2 V−1 s−1).8 Such comparable high mobility is beneficial for charge transfer in the resulting PSCs.5 For the PSFDTBT/PC71BM blend film (in a weight ratio of 1:3, spin-coated from ODCB solutions), hole mobility (μhole) of 1.14 × 10−2cm2 V−1 s−1 and electron mobility (μe) of 2.7 × 10−4cm2 V−1 s−1 were obtained. Photovoltaic Properties. Broad absorption, appropriate HOMO/LUMO positions, and comparable hole mobility make PSFDTBT a promising donor material in BHJ PSCs. Therefore, the photovoltaic performance of PSFDTBT was investigated in PSCs. Polymer solar cells with a device configuration of ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al were fabricated and characterized under the illumination of AM1.5G (100 mW/cm2). The weight ratio of PSFDTBT to PC71BM and the processing solvent have been carefully optimized. The results are summarized in Table 1. Using DCB as the processing solvent, four polymer solar cells with blends of PSFDTBT and PC71BM in weight ratios of 1:2.5, 1:3,

PSCs is correlated to the difference between the LUMO energy level of the acceptor and HOMO energy level of the donor polymer,5,17 therefore a higher Voc for PSFDTBT based polymer solar cells is anticipated due to the low-lying HOMO energy level. According to the equation ELUMO = EHOMO + Eg,opt, the LUMO energy level of PSFDTBT was calculated to be −3.43 eV. The LUMO energy level of PSFDTBT is positioned about 0.80 eV above that of PC71BM (4.2 eV, measured under the same condition), which offers enough driving force for charge separation and transfer without too much energy loss.5,17 Transport Properties. The transport properties of the polymer were investigated by fabricating organic thin film fieldeffect transistors (OFETs). Typical p-channel field-effect transistor behavior was obtained. The hole mobility (μ) was estimated in the saturated regime from the derivative plots of the square root of source-drain current (ISD) versus gate voltage (VG) through equation ISD = (W/2L)Ciμ(VG − VT)2 where W is the channel width, L is the channel length, Ci is the capacitance per unit area of the gate dielectric layer (SiO2, 500 nm, Ci = 7.5 nF/cm2), and VT is the threshold voltage. The output (a) and transfer characteristics (b) of the spin-coated 3019

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surface morphology with fairly small phase separation, and the root-mean-square (rms) values for the blend films from DCB and from TCB are 0.242 and 0.306 nm, respectively. These AFM results are consistent with their photovoltaic device performances (vide supra). The reason for that may be due to both DCB and TCB are good solvents for PSFDTBT. It is well-known that balanced electron and hole mobility of the active layer is required for high performance polymer solar cells.2f The formation of an interpenetrating bicontinuous network between polymer donor and PCBM with an ideal domain size of 10−20 nm is crucial for charge generation and transportation.18 For bulk heterojunction polymer solar cells, appreciate phase separation is a practical way to form bicontinuous donor−acceptor network.19 We believe that the solar cell performance of this type of copolymer can be further improved by the optimization of film morphology and device fabrication conditions.

1:3.5, and 1:4, respectively, as the active layer were fabricated, and their J−V curves are shown in Figure 3a. In all cases, PCEs of about 4.1% were obtained. The performance of PSCs was insensitive to the ratio of donor to acceptor in range of 1:2.5 to 1:4. With a weight ratio of PSFDTBT to PC71BM is 1:3.5, a PCE of 4.2% with a Voc of 0.96 V, a Jsc of 7.6 mA/cm2, and an FF of 0.56 was achieved under illumination from an AM 1.5G solar simulator (100 mW cm−2). Such high Voc of almost 1 V is consistent with the low-lying HOMO energy level of PSFDTBT. In comparison with the device performance of P1 based PSCs,8 devices based on PSFDTBT exhibited almost identical Voc, but higher Jsc and much higher FF. As shown in Table 1, the optimized devices based on P1 showed a PCE of 3.1% with a Voc of 0.97 V, a Jsc of 6.7 mA/cm2, and an FF = 0.47. It is well-known that the processing solvent can dramatically affect the morphology of blend film and thus the photovoltaic performance of devices. When the solvent was changed from DCB to TCB, the photovoltaic performance could be further improved. As shown in Table 1, when the active layer was PSFDTBT/PC71BM in a weight ratio of 1:3, the highest PCE of 4.6% with a Voc of 0.94 V, a Jsc of 8.0 mA/ cm, and an FF of 0.60 was achieved. J−V curves of polymer solar cells fabricated with different weight ratio of PSFDTBT:PC71BM in TCB solutions are shown in Figure 3b. It is worth noting that such performance was achieved from simply fabricated devices without using any additive, posttreatment, or extra layers, which is beneficial for preparation of low cost polymer solar cells. To verify the accuracy of the Jsc measurement results, external quantum efficiencies (EQEs) of solar cells were measured under illumination of monochromatic light. As shown in Figure 4, solar cells, which were fabricated

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CONCLUSION An alternating copolymer PSFDTBT of spirobifluorene and 2′,1′,3′-benzothiadiazole unit was designed and synthesized. PSFDTBT shows a broad absorption, appropriate HOMO/ LUMO positions, and comparable hole mobility. Without using any additive and any post-treatment, polymer solar cells based on PSFDTBT/PC71BM demonstrated a PCE of 4.6% with a high open-circuit voltage of 0.94 V. This is the first report on spirobifluorene-based polymers applied in BHJ PSCs. High open-circuit voltage, simple device fabrication process, and easy synthesis make PSFDTBT a highly promising donor polymers for high efficiency polymer solar cells. We believe that spirobifluorene as a promising building block for conjugated polymer donor materials warrants further exploration and the performance of PSFDTBT-based polymer solar cells can be further improved from the optimization of device fabrication conditions.

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EXPERIMENTAL SECTION

Materials and Instruments. Unless otherwise noted, all chemicals were purchased from commercial suppliers and used without further purification. Solvents were dried using standard procedures. M14j and M211g were synthesized according to literature procedures. The catalyst precursor Pd(PPh3)4 was prepared according to the literature and stored in a Schlenk tube under nitrogen.20 Tetrahydrofuran (THF) and diethyl ether (Et2O) were distilled from sodium with benzophenone as an indicator under nitrogen atmosphere. Hexane and dichloromethane (CH2Cl2) were distilled from CaH2. Chloroform was distilled before use. All reactions were performed under an atmosphere of nitrogen and monitored by thin layer chromatography (TLC) on silica gel 60 F254 (Merck, 0.2 mm). Column chromatography was carried out on silica gel (200−300 mesh). 1H and 13C NMR spectra were recorded on a Bruker DM 300 or AV 400 spectrometer in CDCl3. UV−vis absorption spectra were obtained on a SHIMADZU UV−visible spectrometer model UV1601PC. Elemental analyses were performed on a Flash EA 1112 analyzer. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were performed on TA2100 and Perkin-Elmer Diamond DSC instrument, respectively, under a nitrogen atmosphere at a heating rate of 10 °C/min to record TGA and DSC curves. Elemental analysis was performed on a Vario EL elemental analysis instrument. Number- (Mn) and weight-average (M w ) molecular weights were measured by gel permeation chromatography (GPC) on a PL-GPC 50 with THF as an eluent and calibrated with polystyrene standards. Atomic force microscopy (AFM) images of blend films were obtained on a Nanoscope IIIa Dimension 3100 operating in the tapping mode. The film thickness

Figure 4. EQE curves of PSCs fabricated from PSFDTBT:PC71BM = 1:3.5 (w/w) in DCB and PSFDTBT:PC71BM = 1:3 (w/w) in TCB.

from PSFDTBT:PC71BM in a ratio of 1:3.5 (w/w) in DCB solutions and PSFDTBT:PC71BM in a ratio of 1:3 (w/w) in TCB solutions, both exhibited a broad photoresponse extending from 350 to 700 nm. Especially, the device based on a ratio of PSFDTBT to PC71BM of 1:3 (w/w) in TCB solutions exhibited the maximum EQE of 72% at 479 nm. The Jsc calculated from integration of the EQE with an AM 1.5G reference spectrum roughly agreed with the Jsc obtained from the J-V measurements. To understand the relationship between device performances and active layer morphology, atomic force microscopy (AFM) measurements were conducted to investigate the surface morphology of PSFDTBT/PC71BM blend films. Figure 5 shows the AMF height images of PSFDTBT:PC71BM (1:3.5, w/w) blend film spin coated from DCB and PSFDTBT:PC71BM (1:3, w/w) blend film spin coated from TCB. In these two cases, AFM height images showed similar 3020

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Figure 5. AFM height images (2 × 2 μm2) of PSFDTBT/PC71BM (1:3.5, w/w) blend films spin coated from DCB (a) and PSFDTBT/PC71BM (1:3, w/w) blend films spin coated from TCB (b). solar simulator (Model SS-50A, photo Emission Tech., Inc.). The J−V curves were recorded at the temperature of about 25 °C. Fabrication and Characterization of Organic Field-Effect Transistors (OFETs). The thin film organic field-effect transistors (OFET) of all three polymers were investigated by spin-coating method. Each polymer with the concentration of 10 mg/mL in odichlorobenzene (DCB) was spun on OTS-treated Si/SiO2 substrates. Au electrodes were thermally deposited through a multiple finger configuration mask with channel length 50 μm and width 2500 μm separately. Measurements were performed in air using a Keithley 4200SCS semiconductor parameter analyzer and a Micromanipulator 6150 probe station in a clean and shield box.

was determined by a Dektak 6 M surface profilometer. Electrochemical measurements were performed on a CHI 630A Electrochemical Analyzer with a standard three-electrode electrochemical cell in a 0.1 M tetrabutylammonium tetrafluoroborate solution in CH3CN at room temperature with a scanning rate of 0.1 V/s−1. A glassy carbon working electrode, a Pt wire counter electrode, and an Ag/AgNO3 (0.01 M in CH3CN) reference electrode were used. The experiments were calibrated with the standard ferrocene/ferrocenium (Fc) redox system and assumption that the energy level of Fc is 4.8 eV below vacuum. Synthesis of Poly[2,7-(3′,6′-dioctyloxy)-9,9′-spirobifluorenealt-5,5-(4′,7′-di-2-thienyl-5′,6′-dioctyloxy-2′,1′,3′-benzothiadiazole)] (PSFDTBT). A mixture of M1 (142 mg, 0.2 mmol), M2 (165 mg, 0.2 mmol), K2CO3 (1.35 g, 10 mmol), tetra(n-butyl)ammonium bromide (200 mg, 0.62 mmol), toluene (20 mL), and H2O (5 mL) was carefully degassed before Pd(PPh3)4 (2 mg, 2 μmol) was added. The mixture was refluxed under nitrogen atmosphere with vigorous stirring for 72 h. Then phenylboronic acid and bromobenzene were added to end-cap the polymer. Chloroform and water were added; the organic layer was separated and concentrated; and the residue was poured into methanol. The resulted solid was collected by filtration and washed with acetone to remove oligomers and catalyst residues to afford PSFDTBT as a red solid (194 mg, 86%). The resulted polymer was fully soluble in common organic solvents such as THF, chloroform, and chlorobenzene. 1H NMR (400 MHz, CDCl3): δ 8.34 (s, 2H), 7.38 (m, 6H), 7.06 (m, 10H), 6.71 (m, 10H), 4.05 (s, 8H), 1.84 (m, 16H), 1.31 (m, 32H), 0.90 (s, 12H); 13C NMR (100 MHz, CDCl3): δ 159.6, 151.7, 150.8, 145.7, 143.1, 140.9, 133.6, 131.7, 124.8, 123.0, 120.3, 117.4, 114.6, 106.0, 74.4, 68.4, 64.8, 31.9, 30.4, 29.5, 29.3, 26.2, 26.0, 22.7, 14.1. Anal. Calcd for C71H84N2O4S3: C, 75.76; H, 7.52; N, 2.49. Found: C, 75.77; H, 7.56; N, 2.53. Polymer Solar Cell Fabrication and Characterization. PSCs were fabricated with the device configuration of ITO/PEDOT:PSS/ polymer:PC71BM/LiF/Al. The conductivity of ITO is 20 Ω/□. PEDOT:PSS is Baytron Al 4083 from H. C. Starck and was filtered with a 0.45 μm PVDF film before use. A thin layer of PEDOT:PSS was spin-coated on top of cleaned ITO substrate at 3000 rpm/s for 60 s and dried subsequently at 130 °C for 15 min on a hot plate before transferred into a glovebox. The active layer was prepared by spincoating the solution of polymers and PC71BM on the top of ITO/ PEDOT:PSS. The top electrode was thermally evaporated, with a 0.6 nm LiF layer, followed by 80 nm of aluminum at a pressure of 10−6 Torr through a shadow mask. Four cells were fabricated on one substrate with an effective area between 0.04 and 0.05 cm2. The measurement of devices was conducted in air. Current−voltage characteristics were recorded using a Keithley 2400 Source Meter under AM 1.5 illumination with an intensity of 100 mW/cm2 from a

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ASSOCIATED CONTENT

S Supporting Information *

1

H and 13C NMR spectra and plots of hole and electron transport performance. This material is available free of charge via Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (C.L.) [email protected]; (Z.B.) [email protected] or [email protected].. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We express thanks for the financial support by the NSF of China (20834006, 51003006, and 21161160443), the 973 Programs (2011CB935702 and 2009CB623603), and the Fundamental Research Funds for the Central Universities.

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REFERENCES

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dx.doi.org/10.1021/ma202752h | Macromolecules 2012, 45, 3017−3022