High-Efficiency Polymer Solar Cells with Water-Soluble and Self

Dec 16, 2009 - simple blend of regioregular poly(3-hexylthiophene) (P3HT) as an electron-donating polymer and [6,6]-phenyl-C61-butyric acid methyl est...
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J. Phys. Chem. C 2010, 114, 633–637

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High-Efficiency Polymer Solar Cells with Water-Soluble and Self-Doped Conducting Polyaniline Graft Copolymer as Hole Transport Layer Jae Woong Jung, Jea Uk Lee, and Won Ho Jo* Department of Materials Science and Engineering, Seoul National UniVersity, Seoul 151-742, Korea ReceiVed: August 31, 2009; ReVised Manuscript ReceiVed: October 28, 2009

A graft copolymer, poly(styrene sulfonic acid) grafted with polyaniline (PSSA-g-PANI), is synthesized and used as the hole transport layer in polymer solar cells based on poly(3-hexylthiophene) and [6,6]-phenylC61-butyric acid methyl ester. Electrochemical stability of PSSA-g-PANI is superior to poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), which has widely been used as the hole transport material in polymer solar cells. The unique high transparency in 450-650 nm wavelength and high electrical conductivity of PSSA-g-PANI result in higher short circuit current and higher open circuit voltage of polymer solar cells than those of the device made of PEDOT:PSS. A series of PSSA-g-PANI with different electrical conductivities are synthesized to investigate the effect of conductivity on the performance of polymer solar cells. The device with the most conductive PSSA-g-PANI exhibits the highest power conversion efficiency (∼4%), which is 20% higher than that of the device with PEDOT:PSS. Introduction As demands of renewable energy are increasing, solar cells are becoming one of the most promising candidates as a future energy source. Recently, polymer solar cells (PSCs) have attracted great attention as a potential alternative to conventional Si-based solar cells. Advantages of PSC include low cost, ease of fabrication, and the potentials for flexible, large area solar cells.1,2 A remarkable improvement in the performance of PSCs has been achieved by introducing the donor-acceptor bulk heterojunction (BHJ) structure in the active layer.3-7 To date, a simple blend of regioregular poly(3-hexylthiophene) (P3HT) as an electron-donating polymer and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor has showed high power conversion efficiency of about 3-4% with optimized morphology under AM 1.5 G (AM ) air mass) illumination.8-11 However, still much lower efficiency than Si-based solar cells limits the commercialization of PSCs. Therefore, further improvement in the device performance of PSCs is needed. The conventional PSC device is composed of four layers, indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/P3HT:PCBM/Al. In this BHJ solar cell structure, PEDOT:PSS has most widely been used as the hole transport layer (HTL). As the HTL has been introduced in optoelectronic devices, both stability and performance of the devices have been much improved by smoothing the ITO surface, lowering the work function of the anode, and promoting efficient hole transport. Especially in PSC, the power conversion efficiency (PCE) has been much increased when PEDOT:PSS was introduced as HTL by lowering the contact resistance and leakage current.12-15 Although the performance of PSCs which use PEDOT:PSS as HTL has remarkably been progressed, various problems of PEDOT:PSS in the PSC device have been reported. First, since PEDOT:PSS is dispersed in water with large particle size with ca. 60-80 nm,16 the aggregate of particles may play out as defects of the device, and therefore these defects would induce * To whom correspondence should be addressed. Phone: +82-2-8807192. Fax: +82-2-885-1748. E-mail: whjpoly@ snu.ac.kr.

degradation of the PSC. Second, because of the strong acidic nature of PSS, it may degrade the ITO surface, and as a result the instability of the interface between ITO and PEDOT:PSS can lead to deterioration of solar cell performance, especially long-term stability and performance.17 Third, PEDOT:PSS (CLEVIOSTM P VP AI 4083) has very high cost and low electrical conductivity (∼10-3 S/cm). Therefore, those problems become obstacles to achievement of highly efficient and largearea PSCs. Consequently, there is a high demand to develop a new hole transport material, which is completely soluble in water (or various organic solvents) and shows better performance than PEDOT:PSS, to fabricate highly efficient and stable PSCs. In our previous reports, we successfully synthesized two novel water-soluble and self-doped conducting copolymers based on polyaniline (poly(styrenesulfonic acid)-graft-polyaniline (PSSAg-PANI)) and polypyrrole (poly(styrenesulfonic acid)-graftpolypyrrole (PSSA-g-PPY)).18,19 Since these copolymers are composed of a water-soluble poly(styrenesulfonic acid) backbone and grafted conducting polymers, they are completely soluble in water. Also these novel graft-type conducting polymers show high chemical stability in a wide range of pH while PEDOT:PSS is dissociated into PEDOT and PSS at highly basic condition. In this regard, it is expected that PSSA-g-PANI can be used as HTL material in PSCs and thus replace PEDOT:PSS. In this work, we synthesized a water-soluble conducting copolymer, PSSA-g-PANI, and compared its solar cell performance with that of PEDOT:PSS, when PSSA-g-PANI is used as a hole transport material of PSCs. To examine the effect of the electrical conductivity of the hole transport material on the performance of PSCs, a series of PSSA-g-PANIs with different conductivity were also synthesized by controlling the molar ratio of aniline (ANI) to styrenesulfonic acid (SSA) during polymerization, and the solar cell performances were measured as a function of the electrical conductivity of PSSA-g-PANI. Experimental Methods Materials. A graft copolymer, PSSA-g-PANI, was synthesized according to the method previously reported.18 Various

10.1021/jp9083844  2010 American Chemical Society Published on Web 12/16/2009

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PSSA-g-PANIs with different molecular weights and different molar ratios of ANI to SSA were synthesized by changing the amount of monomers in a synthetic step. The regioregular P3HT (20 kDa) was synthesized by following the procedure reported in the literature.20,21 PCBM (>99.5%) was obtained from Nano-C and used as received without further purification. PEDOT:PSS (Baytron P VP AI 4083) was purchased from H. C. Stark. Device Preparation. PSSA-g-PANI (2 wt %) was dissolved in deionized water and filtered with a 0.2 µm PES syringe filter before spin-coating. PEDOT:PSS solution was filtered with a 0.45 µm PES syringe filter. Blend solutions of P3HT and PCBM were prepared by dissolving 20 mg of P3HT and 20 mg of PCBM in 1 mL of 1,2-dichlorobenzene. P3HT:PCBM solution was stirred overnight and filtered with a 0.2 µm PES syringe filter before spin-coating. For fabrication of photovoltaic devices, ITO-coated glass (15 Ω/0) was cleaned with acetone and isopropyl alcohol and then dried at 200 °C for 30 min. Before coating of HTL, the ITO-coated glass was treated with UV-ozone. After spin-coating of PSSA-g-PANI (or PEDOT: PSS) on the ITO-coated glass, the devices were dried at 120 °C for 30 min under nitrogen atmosphere. Then, a blend solution of P3HT and PCBM was spin-coated with a thickness of 150 nm. Al (150 nm) were thermally evaporated under vacuum lower than 10-6 Torr on the top of the active layer. The devices were thermally annealed at 150 °C for 15 min under nitrogen atmosphere inside the glovebox. Characterization. The transmittance and UV-visible absorption spectra of HTL films and P3HT:PCBM films coated onto the HTL were measured by an UV-visible spectrophotometer (HP 8452A). Film morphology and thickness were determined by using atomic force microscopy. The cyclic voltammetry (CV) measurements were carried out using Pt working and counter electrodes and Ag/AgCl (3 M NaCl) reference electrode, and PSSA-g-PANI (or PEDOT:PSS) was coated onto the Pt working electrode before the measurments. The acidity of PSSA-g-PANI (or PEDOT:PSS) was measured using the pH meter (Orion 4-Star Plus pH/ISE Meter), where the concentration of PSSA-g-PANI was 1.5 wt %. The electrical conductivity was determined by a four-point sheet resistance measurement system (Model CMT SR2000N, Advanced Instrument Technology). The photovoltaic performance was measured under nitrogen atmosphere inside the glovebox. The current density-voltage (J-V) characteristics were measured with a Keithley 4200 sourcemeter under AM 1.5 G (100 mW/cm2) simulated by a Newport-Oriel solar simulator. The light intensity was calibrated using a NREL certified photodiode and light source meter prior to each measurement. The active area was determined at 0.04 cm2 by attaching a shadow mask onto the solar cell devices. The incident photon-to-current efficiency (IPCE) values were measured using a lock-in amplifier with a current preamplifier under short circuit current state with illumination of monochromatic light. Results and Discussion The chemical structure of PSSA-g-PANI is shown in Figure 1. The details of the synthetic route of PSSA-g-PANI are described in our previous report.18 Molecular characteristics of PSSA-g-PANI used in this study are listed in Table 1. The CV curves of PSSA-g-PANI and PEDOT:PSS are shown in Figure 2. When the highest occupied molecular orbital (HOMO) level was calculated from the onset of oxidation, it was found that PSSA-g-PANI and PEDOT:PSS have nearly equal HOMO levels, 4.80 and 4.79 eV, respectively, and these energy levels of the two polymers are a good match to both the

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Figure 1. Chemical structure of PSSA-g-PANI graft copolymer.

TABLE 1: Molecular Characteristics of PSSA-g-PANI and PEDOT:PSS samples PSSA-g-PANI (PANI4) PEDOT:PSS (CLEVIOS P AI 4083)

Mna (g/mol)

HOMOb (eV)

acidityc (pH)

σ (S/cm)

75000

-4.80 -4.79

3.95 1.93

0.10 0.007

a Determined by gel permeation chromatography (GPC) relative to polystyrene standards. b Determined by the following equation: EHOMO ) -4.4 + EOX) for Ag/AgCl reference electrode. c Measured at the concentration of 1.5 wt %.

work function of ITO (4.7-4.8 eV in air) and the HOMO level of P3HT (5.0-5.1 eV) for efficient hole transport. This suggests that PSSA-g-PANI can be used as HTL in PSC instead of the conventional PEDOT:PSS. Furthermore, the oxidation (or reduction) characteristic of PSSA-g-PANI after 5 cycles of CV measurement exhibits behavior almost similar to that after 100 cycles, indicating that the electrochemical stability of PSSAg-PANI is high, while PEDOT:PSS loses complete electrochemical stability after 100 cycles of CV measurement, as shown in Figure 2b. This electrochemical stability of PSSA-gPANI is expected to improve the long-term stability of the PSC device. It should be mentioned here that the lower acidity of PSSA-g-PANI (pH 3.95) compared with PEDOT:PSS (pH 1.93) would contribute to the electrochemical stability (Table 1). In Figure 3, the film surfaces of PEDOT:PSS and PSSA-gPANI are shown by atomic force microscopy. One of the important roles of HTL is to reduce the surface roughness of the anode as a buffer layer, because the most commonly used anode for optoelectronic devices, ITO, has a very rough surface. Especially in a PSC device, the rough surface of ITO can induce a short circuit of the device and as a result reduce the PCE. Therefore, the smooth surface of HTL is very important for fabrication of a high-efficiency PSC device. When the surface roughness of PSSA-g-PANI film is compared with that of PEDOT:PSS film, as shown in Figure 3, it reveals that the surface of the PSSA-g-PANI film is smoother than that of the PEDOT:PSSA film. The root-mean-square (rms) roughness of the surface of the PSSA-g-PANI film is 0.80 nm, while the rms roughness of PEDOT:PSSA is 2.18 nm, which is about three times larger than that of PSSA-g-PANI. This low rms roughness of PSSA-g-PANI film suggests that it can effectively decrease the roughness of the ITO surface as coated on ITO and therefore make the flat interface between HTL and the active layer in a PSC device. The roughness difference between PSSA-g-PANI and PEDOT:PSS may arise from the fact that PSSA-g-PANI is composed of a PSSA backbone and covalently grafted PANI, whereas PEDOT:PSS is a simple complex of PEDOT and PSS. Therefore, the covalently grafted PANI is more miscible with PSSA than PEDOT with PSS. Transmittance of PSSA-g-PANI and PEDOT:PSS films with different thicknesses is shown in Figure 4a. For achieving a

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Figure 2. (a) Cyclic voltammetry measurements of PSSA-g-PANI and PEDOT:PSS and (b) cyclic voltammetry curves after 5 and 100 cycles of measurement.

Figure 3. Height images of (a) PEDOT:PSS and (b) PSSA-g-PANI film coated onto ITO. The thicknesses of both films are 40 nm.

Figure 4. (a) Transmittance of PSSA-g-PANI (blue) and PEDOT:PSS films (red) with different thickness and (b) UV-vis absorption spectra of P3HT:PCBM blend film on PSSA-g-PANI (blue) and PEDOT:PSS films (red) after thermal annealing at 150 °C for 15 min. The thicknesses of the P3HT:PCBM layer and HTL are 150 and 40 nm, respectively. The inset shows the difference of absorption more clearly.

high performance of PSC, a large amount of light should be harvested in the active layer. Since the light is passed through the HTL prior to the active layer, a highly transparent material should be used as the HTL in PSC. PEDOT:PSS with 40 nm thickness exhibits transparency of 93-95% in the range of 450-650 nm, while PSSA-g-PANI with 40 nm thickness shows extremely high transparency over 96% in the range of 450-650 nm wavelength, as shown in Figure 4a. It should be noted here that 450-650 nm is the main absorption range of P3HT. As shown in Figure 4b, the light absorption of P3HT:PCBM

through PSSA-g-PANI is also enhanced compared to the absorption of P3HT:PCBM through PEDOT:PSS in the 450-650 nm range because of the higher transparency of PSSA-g-PANI. Another important parameter of HTL for efficient PSC is electrical conductivity. By measuring the four-point sheet resistance, the conductivity of PSSA-g-PANI is estimated to be ca. 0.10 S/cm, although PEDOT:PSS was 0.007 S/cm at 40 nm thickness. In other reports,22,23 enhanced performance of PSCs was achieved by using highly conductive PEDOT:PSS doped with various polyalchols. Therefore, it is conjectured that

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Figure 5. (a) Typical J-V curves under AM 1.5 G (100 mW/cm2) of polymer solar cell devices with PSSA-g-PANI or PEDOT:PSS as HTL before and after thermal annealing and (b) incident photon-to-current efficiency spectra of polymer solar cell devices with PSSA-g-PANI or PEDOT: PSS as HTL after thermal annealing. All devices were annealed at 150 °C for 15 min.

TABLE 2: Summary of Performance of Polymer Solar Cell Devices with PSSA-g-PANI and PEDOT:PSS Layer before and after Thermal Annealing at 150 °C for 15 min HTL PSSA-g-PANI (PANI4) PEDOT:PSS

annealing VOC FF JSC PCE RSa RPa (min) (V) (%) (mA/cm2) (%) (Ω · cm2) (Ω · cm2) 0 15 0 15

0.51 0.56 0.49 0.56

59 61 58 61

7.2 10.7 5.5 9.7

2.17 3.66 1.56 3.31

4.42 3.71 5.38 5.15

500 1,163 487 1,189

a

RS and RP are the mean series and parallel resistances, respectively, and are measured from the dark current curve.

the higher conductivity of PSSA-g-PANI than PEDOT:PSS would increase the performance of PSCs. Figure 5a compares the J-V curves of PSC when PSSA-gPANI and PEDOT:PSS are used as HTL. Open circuit voltages (VOCs) of both devices are almost the same before and after annealing, because the two materials have almost the same HOMO level. Both devices also show a fill factor (FF) over 60%, indicating that both PSSA-g-PANI and PEDOT:PSS have the balanced charge carrier density and good interfaces between the active layer and HTL and between ITO and HTL. However, a device fabricated with PSSA-g-PANI shows much increased short circuit current (JSC) both before and after annealing than that of PEDOT:PSS. This higher JSC is attributed to higher transparency and higher conductivity of PSSA-g-PANI than PEDOT:PSS: the higher transparency of HTL results in larger light absorption of P3HT and higher JSC. This is manifested by the fact that the more transparent characteristic of PSSA-g-PANI exhibits a high value of incident photon-to-current efficiency in the range of 450-650 nm wavelength, as shown in Figure 5b. Consequently it is concluded that the enhanced light absorption of P3HT through PSSA-g-PANI generates more excitons and thus results in the increase of current density. High electrical conductivity of PSSA-g-PANI is another important reason for high JSC and high PCE. Because the device made of PSSA-g-PANI has lower series resistance and higher parallel resistance compared to the device of PEDOT:PSS (Table 2), it is concluded that PSSA-g-PANI transports holes more efficiently and lowers the leakage current as compared to PEDOT:PSS. Due to these optical and electrochemical characteristics, the PSC with PSSA-g-PANI as HTL exhibits a higher PCE value than that with PEDOT:PSS. To further investigate the effect of the conductivity of hole transport material on the performance of PSC, we synthesized a series of PSSA-g-PANI with different conductivities. The

TABLE 3: Device Performance of Polymer Solar Cell Devices with Various PSSA-g-PANIs as Hole Transport Layer after Thermal Annealing at 150 °C for 15 min HTL PANI1 PANI2 PANI3 PANI4 PANI5 PEDOT:PSS

molar ratio acidity σ VOC FF JSC PCE of ANI/SSA (pH) (S/cm) (V) (%) (mA/cm2) (%) 0.05 0.07 0.20 0.29 0.40

1.86 2.23 3.77 3.95 4.90 1.93

0.0005 0.005 0.85 0.10 0.05 0.007

0.52 0.54 0.59 0.56 0.57 0.56

51 60 62 61 60 61

9.2 9.6 10.9 10.7 10.1 9.7

2.44 3.11 3.99 3.66 3.45 3.31

conductivity of PSSA-g-PANI was controlled by changing the molar ratio of ANI to SSA during polymerization. The optical and electrochemical characteristics of all the PSSA-g-PANIs are almost the same, and the molecular weights of the copolymers are also similar (ca. 75-90 kDa). The only difference of these copolymers is the ratio of ANI to SSA. The ratio of ANI to SSA in PSSA-g-PANI, the conductivity of polymer thin films (40 nm of thickness), and the performance of PSCs are listed in Table 3. When the conductivities of the copolymers are compared to each other, it is realized that the conductivity is increased first and then decreased to exhibit a maximum as the ratio of ANI to SSA is increased. The copolymers with a low ratio of ANI to SSA exhibit very low conductivity because of a small amount of conductive PANI part. In other words, a much larger part of the PSSA backbone lowers the charge transport, because PSSA is not a conjugated polymer and acts as an insulator, and therefore the copolymers with a low ratio of ANI to SSA (PANI1 and PANI2) exhibit poor electrical conductivity. However, as the ratio of ANI to SSA is further increased, the electrical conductivity starts to decrease. This can be explained by considering the doping effect of PSSA to PANI. Since it is well-known that highly doped PANI exhibits enhanced electrical conductivity,24-26 further increase of PANI (further decrease of PSSA, which acts as a dopant) causes the low degree of doping, which results in low conductivity. The J-V curves of PSC devices with different PSSA-gPANIs are shown in Figure 6. The use of highly conductive HTL exhibits enhanced performance, whereas the use of low conductive HTL exhibits poor performance. When higher conductive copolymers than PEDOT:PSS are used as HTL, both JSC and VOC are slightly increased while FF is almost not changed, resulting in higher PCE than that of PEDOT:PSS. Especially, the device fabricated with the most conductive copolymer, PANI3, exhibits JSC of about 11 mA/cm2, which is

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J. Phys. Chem. C, Vol. 114, No. 1, 2010 637 be easily prepared by simply changing the molar ratio of ANI to SSA during polymerization. Furthermore, perfect solubility in water and the highly stable nature of the electrochemical property are also important features of PSSA-g-PANI. Acknowledgment. The authors thank the Ministry of Education, Science and Technology (MEST), Korea for financial support through the Global Research Laboratory (GRL) program. References and Notes

Figure 6. J-V curves under AM 1.5 G (100 mW/cm2) of polymer solar cells with different PSSA-g-PANI copolymer as HTL. All devices were annealed at 150 °C for 15 min.

an enhanced value over 10% more than the device with PEDOT: PSS. This improvement of JSC is the result of increased light absorption of P3HT and efficient transport of holes from P3HT to anode. The decreased serial resistance of a device with PSSAg-PANI also explains the effect of highly conductive HTL on the performance of PSC. When a PSC device is fabricated with PANI2, which has conductivity similar to PEDOT:PSS, VOC, JSC, FF, and PCE are similar to those of the device with PEDOT:PSS. PANI1, which exhibits the lowest conductivity, shows the most inferior performance. This performance variation, especially JSC, shows the same trend as the conductivity variation of PSSA-g-PANI. These results clearly demonstrate that higher conductive and more transparent hole transport material improves the performance of PSC. Conclusions We have synthesized a novel hole transport material, PSSAg-PANI, and studied the effects of optical transparency and electrical conductivity of this material on the performance of PSC. The device with PSSA-g-PANI exhibits about 4% PCE, which is 20% higher than that of the device with PEDOT:PSS due to unique high transparency in the UV-vis region (especially 450-650 nm) and high conductivity of PSSA-g-PANI, when the blend of P3HT and PCBM is used as the active layer of PSCs. To the best of our knowledge, this is the first attempt to use PSSA-g-PANI as a HTL material in PSCs. A more promising feature of PSSA-g-PANI as HTL in optoelectronic device is the easiness of conductivity tuning. The copolymers with different optical, electrical, and chemical properties can

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