Article pubs.acs.org/JPCC
Diarylmethanofullerene: Efficient Polymer Solar Cells with Low-BandGap Copolymer Surya Prakash Singh,*,† CH. Pavan Kumar,† P. Nagarjuna,† G. D. Sharma,*,‡ S. Biswas,§ and J. A. Mikroyannidis∥ †
Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500607, India ‡ R&D Center for Engineering and Science JEC group of Colleges, Jaipur Engineering College, Kukas, Jaipur (Rajasthan) 303101, India § Department of Physics, LNMIT, Jaipur (Rajasthan) India ∥ Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR-26500 Patras, Greece ABSTRACT: The photovoltaic performance of the polymer bulk heterojunction solar cells based on the low-band-gap D− A copolymer P as donor and modified fullerene (modiPC60BM) as acceptor was in. We have achieved power conversion efficiency (PCE) of ∼2.35% for a polymer solar cell with modi-PC 60 BM as an electron acceptor in bulk heterojunction active layer, which is higher than that for PC60BM counterpart (1.50%). The increase in the PCE has been attributed to the increase in absorption in visible and higher LUMO level of modi-PC60BM, resulting in enhancement in Jsc and Voc, respectively. In the device fabrications, we studied the effect of the solvent additive and modified PEDOT:PSS as a hole transport layer. The PCE of the polymer solar cell was improved up to 3.63% when the P:modi-PC60BM active layer was processed with the addition of CN as an additive in the THF solution (CN/THF), which is mainly attributed to more balanced charge transport due to the increased crystallinity of P in the blend. The PCE of polymer solar cell based on the active layer processed from CN/THF has been further improved up to 4.73% when a modified PEDOT:PSS (acetone-treated) was used as the hole transport layer. This increase in the PCE is mainly due to the enhancement in Jsc and FF and may be attributed to the improvement in the conductivity induced by the polar solvent with high dipole moment, leading to more efficient collection of charge carrier by the anode.
1. INTRODUCTION Polymer solar cells are considered to be the potential candidates for solar-energy conversion devices due to their attractive optical and electrical properties, mechanical flexibility, and low cost for large area fabrication.1−6 The most efficient device architecture of polymer solar cells is based on the bulk heterojunction (BHJ) concept,7 in which the active layer, sandwiched between two electrodes with different work functions, consists of a blend of electron-donating material, that is, p type conjugated polymer and an electron-accepting material, that is, n type, such as fullerene derivatives. Regioregular P3HT and PC60BM and PC70BM are most commonly used as donor and acceptor for BHJ active layer and PSC based on P3HT/PC60BM and P3HT:PC70BM blend having yielded PCE of 4 to 5% 8,9 and 7 to 8%, respectively.10−12 But in the case of these blends, further improvement of PCE is limited by the high-lying highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of P3HT and too large energy offset between the LUMO energy levels of P3HT and PCBMs, which results in lower open circuit voltage of devices based on P3HT:PCBMs. Because the overall PCE highly depends on © XXXX American Chemical Society
both short circuit current (Jsc) and open circuit voltage (Voc), extensive research efforts have been devoted to the design and synthesis of new conjugated polymers13−16 and new fullerene derivatives17−23 for BHJ polymer solar cells to improve these parameters. It is well known that the Voc of the BHJ polymer solar cell is an important factor to improve the PCE of the device, and it is proportional to the difference between the LUMO energy level of acceptor and HOMO energy level of the donor used in the BHJ active layer. Therefore, the use of new fullerene derivatives having higher LUMO level is highly desirable to improve the Voc of the polymer solar cells. Li and coworkers recently synthesized an indene-C60 bis-adductor (IC60BA) and a series of PCBM esters with different alkyl chain lengths with higher LUMO energy levels than PCBM.24−28 The higher LUMO energy levels of these fullerene derivatives led to a higher Voc and improved PCE of the PSCs based on these molecules as acceptor and P3HT as donor; the PCE of a PSC based on Received: January 24, 2013 Revised: May 17, 2013
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Scheme 1. Chemical Structure of Copolymer P and modi-PC60BM
photoactive layer under vacuum of 10−5 Torr. The active area of the device was ∼5 mm2. The thin film of the blend was cast from mixed solvents and is described as follows: The blend of the solution of P:modiPC60BM (1:1 w/w) was first prepared using THF solvent and stirred for 4 h. 1-Chloronaphthalene (CN) (0.5 mL) was added to the blend solution prepared from the THF solvent and stirred for another 2 h. For the preparation of modified PEDOT:PSS, the solution was prepared by mixing acetone to PEDOT:PSS (50% by wt) and was stirred for 6 h to obtain a homogeneous solution. After that this solution was spin-coated on ITO-coated glass substrate and then baked at 80 °C for 20 min. The current−voltage (J−V) characteristics of the devices were measured with a computer-controlled Keithley 238 source meter unit. A xenon lamp coupled to an AM1.5 solar spectrum filter was used as the light source, and the optical power at the surface of device was 100 mW/cm2. All measurements were also carried out under ambient conditions.
P3HT:ICBA reached 6.5%, and the PSCs based on P3HT/ IC70BA showed a PCE of 5.64%.29 Recently a PCE of ∼7.40% has been reported for the P3HT:indene-C70-bisadduct fullerene blend with CN solvent additive.30 In addition to PCBM, several methanofullerene derivatives have so far been studied for the development of efficient acceptors such as spiroannulated methanofullerenes, 31,32diarylmethanofullerenes,33 and alkoxy-substituted PCBM derivatives.34 These derivatives have been shown to have excellent properties as acceptors. Herein we report the fabrication and characterization of the BHJ polymer solar cells based on a low-band-gap copolymer P as electron donor and diarylmethanofullerene (modi-PC60BM) as acceptor. We found that both Voc and Jsc have been increased with the use of modi-PC60BM as compared with PC60BM, which may be due to the shift in the LUMO level and increase in absorption in the visible region for modi-PC60BM, respectively, as compared with PC60BM. We have achieved PCE of 2.35 and 3.63% for the BHJ solar cell based on P:modiPC60BM cast from THF and CN/THF, respectively. The value of PCE is higher than that for solar cells based on PC60BM as an electron acceptor (PCE = 1.50%.)35 When modified PEDOT:PSS (acetone treated) hole transport layer is used, the PCE of the BHJ solar cells (P:modi-PC60BM active layer processed from CN/THF solvent) has been further improved up to 4.73%. This increase in the PCE has been attributed to the increase in conductivity of the modified PEDOT:PSS, which leads to the enhancement of both Jsc and fill factor (FF).
3. RESULTS AND DISCUSSION 3.1. Optical and Electrochemical Properties of modiPC60BM. The absorption spectra of PC60BM and modiPC60BM in the thin film form are shown in Figure 1. It can be seen from this Figure that both fullerene derivatives have the same absorption peak around 376 nm, but the absorption of modi-PC60BM is stronger than its PC60BM counterpart in the visible region. The modi-PC60BM has a weak peak around 376 nm, which may be due to the modification. It should be noted that many modi-PC60BM had displayed a stronger absorption
2. EXPERIMENTAL DETAILS The synthesis and optical and electrochemical properties of the copolymer P34 and modified PC60BM35 have been reported elsewhere. The chemical structure of copolymer P and modiPC60BM are shown in Scheme 1. The BHJ photovoltaic devices were fabricated with indiumtin-oxide-coated (ITO) glass substrate at positive electrode and aluminum as negative electrode. The patterned ITO glass substrate was precleaned in an ultrasonic bath of acetone and isopropanol and then dried at room temperature. A thin layer (60 nm) of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS, Baytron, Germany) was spin-coated onto the ITO glass and baked at 80 °C for 20 min. A tetrahydrofuran (THF) solution of copolymer P:modi-PC60BM (1:1 w/w) was subsequently spin coated on the PEDOT:PSS layer to form a photoactive layer. The thickness of the photoactive layer was about 80−85 nm. The deposition of all organic layers was carried out in an ambient atmosphere. An aluminum (Al) was then evaporated onto the surface of the
Figure 1. Normalized absorption spectra of modified PC60BM and P60CBM. B
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in the visible region (400−600 nm).18 These results support the fact that the modi-PC60BM may be a better electron acceptor than PC60BM. The LUMO level of the modi-PC60BM was estimated from the onset oxidation and reduction potential observed in cyclic voltammetry. The modi-PC60BM showed more negative onset reduction potential (−0.92 V vs NHE) than PC60BM (−0.75 V vs NHE). The LUMO level of modi-PC60BM estimated from the onset reduction potential is −3.78 eV, which is at a higher level than PC60BM (−3.95 eV). The HOMO−LUMO gap estimated from cyclic voltammeter is ∼2.32 eV. The modiPC60BM has two aromatic rings on the cyclopropane, having electron-withdrawing and electron-donating groups. The electron-withdrawing group was expected to increase the electron-accepting ability of fullerene, and the electrondonating group would increase the LUMO energy level toward vacuum level, which leads to an increase in the value of open circuit voltage. The higher energy level of modi-PC60BM is desirable for its application as an electron acceptor in polymer solar cells because it is expected to increase the open circuit voltage of the devices. This diarylmethano-fullerene has a long alkyl chain to increase the solubility and is mixed properly with copolymer, which increases the morphology of the BHJ active layer. The energy band diagram of P and modi-PC60BM is shown in Figure 2. It can be seen that the LUMO offset
Vbi, where V and Vbi are the applied voltage and built-in potential (arises from the difference in the work function of cathode and anode; from the present case it is zero), respectively. β is the electric-field-activation factor of mobility, which accounts for the degree of disorder, particularly the energetic level distribution of the carrier hopping sites in the material. The electron mobility of the pristine PC60BM and modi-PC60BM was (3.5 and 4.8) × 10−4 cm2/(V s), respectively. The higher mobility of modi-PC60BM may be beneficial for the efficient charge transport. 3.3. Optical Properties of Blend. The normalized absorption spectra of the P:PC60BM and P:modi-PC60BM are shown in Figure 3. It can be seen from these Figures that the
Figure 3. Normalized absorption spectra of P:PC60BM (THF cast), P:modi-PC60BM (THF cast), and P:modi-PC60BM (CN/THF cast) thin films.
absorption spectra of the blends show the combination of individual components. The absorption peak in the lower wavelength region (around 380 nm) corresponds to PC60BM and modi-PC60BM in their respective blends, whereas that in longer wavelength region (around 635 nm) corresponds to copolymer P. The overall absorption spectrum for P:modiPC60BM is broader from 350 to 750 nm as compared with P:PC60BM. Therefore, we expect that blend P:modi-PC60BM can harvest more photons than its counterpart P:PC60BM blend and generate more excitons after the photon absorption. 3.4. Photovoltaic Properties BHJ Polymer Solar Cells. Figure 4 shows the current−voltage (J−V) characteristics of the device based on P:modi-PC60BM blend cast from THF solvent, and photovoltaic parameters are listed in Table 1. We have also listed the photovoltaic parameters of the P:PC60BM blend for comparison, as previously reported in our published research paper.34 The device with P:modi-PC60BM blend showed PCE of 2.35% (Jsc = 5.56 mA/cm2, Voc = 0.88 V and FF 0.48), which is higher than that for P:PC60BM blend with PCE of 1.50% (photovoltaic parameters are complied in Table 1). The higher Voc is resulted from the higher LUMO energy level of modiPC60BM as compared with its PC60BM counterpart. Moreover, the value of Jsc = 5.56 mA/cm2 was observed for the device based on modi-PC60BM as an electron acceptor, which is higher than that for the PC60BM-based device (4.60 mA/cm2), ascribed by the broader absorption of modi-PC60BM. The use of modi-PC60BM as an electron acceptor effectively increases both Voc and Jsc of the device, which leads to an enhancement in overall PCE of the BHJ solar cell.
Figure 2. HOMO and LUMO energy levels of copolymer P and modiPC60BM.
between P and modi-PC60BM is sufficiently higher than the exciton binding energy (0.2 to 0.4 eV), indicating the favorable photoinduced charge transfer at the donor/acceptor interface. 3.2. Electron Mobility of modi-PC60BM. We have measured the electron mobility of pristine PCBM and modiPC60BM from the current−voltage characteristics of electrononly devices, in which the pristine fullerene derivate was sandwiched between two Al electrodes, and fit these curves in the space-charge-limited current (SCLC) region. The SCLC current density is given be the modified Mott−Gurney equation36 JSCLC = (9/8)εoεrμ
⎛ 0.89β ⎞ exp⎜ V⎟ ⎝ d ⎠ d
Veff 3
where εo and εr are the permittivity of free-space and the relative dielectric constant of the active layer, respectively, μ is the charge carrier mobility, d is the thickness of the device, and Veff is the voltage dropped across the sample given by Veff = V − C
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charge separation of photogenerated excitons but also percolation pathways for charge carrier transport to respective electrodes, which affects the FF of the devices. We have measured the atomic force microscopy (AFM) images (3 μm × 3 μm scale) of the P:modi-PC60BM active layers with and without CN additives, as shown in Figure 5. The surface roughness of the BHJ active layer with and without CN additive estimated from AFM image is 1.55 and 1.14 nm, respectively. It means the surface roughness of the film becomes much smoother using the CN additive. In contrast with the BHJ thin film without additive, the film with CN additive showed more uniform interpenetrating networks, which is beneficial for the more efficient charge transport, leading to improved Jsc37,38 We have investigated the effect of CN additive on the crystallinity of the P in the P:modi-PC60BM blend film (spincast on quart plate) using the X-ray diffraction (XRD) patterns. Figure 6 compares XRD pattern at 2θ = 8.28° for P:modiPC60BM cast with and without CN additive. It can be seen from this Figure that the XRD peak increased when the blend film was cast with CN additive and indicates the increased crystallinity of P, which agrees with the increased absorption of the blend film processed using CN additive. It is also observed that with the addition of the CN additive the peak position has also been shifted to 2θ = 8.33°, leading to reduction in the interlayer spacing in the blend with the CN additive and could be beneficial for the charge transportation in the device based on the blend processed with CN additive.39 Charge carrier mobility is another important factor that affects the photovoltaic performance of the devices. A high charge carrier mobility and balance between the electron and hole mobility are preferred for the efficient transportation and collection of photogenerated charge carriers. We have measured the hole and electron mobilities of the P:modiPC60BM blend films cast from with and without CN additives by SCLC method38 using the hole only and electron only devices, respectively. We found that the hole mobility has been increased by more than one order of magnitude for blend cast with CN as compared with that processed without additive (increases from 1.6 × 10−5 to 3.4 × 10−4 cm2/(V s)). However, the electron mobility has not been affected much with the additive (in the range of (4.5 to 5.2) × 10−4 cm2/(V s)). The increase in the hole mobility may be due to the more ordered and increased crystallinity of the P in the blend, resulting in more balanced charge transport in the device based on the blend processed with CN additive. The PCE of the P:modi-PC60BM blend cast with processing additive is still low, which significantly hinders their commercial application. The way to increase the PCEs of organic solar cells is to be enhancing both short circuit current as well as FF.40,41 The enhancement in the Jsc can be achieved by absorbing large numbers of photons in the active layer employed in the BHJ solar cells.42 For this, photon absorption in the buffer layer between the electrode and the active layer must be minimized.
Figure 4. Current−voltage characteristics under illumination for the devices (a) ITO/PEDOT:PSS/P:modi-PC60BM (THF cast)/Al, (b) ITO/PEDOT:PSS/P:modi-PC60BM (CN/THF cast)/Al, and (c) ITO/PEDOT:PSS (modified)/P:modi-PC60BM (THF cast)/Al.
To further improve the PCE of the BHJ polymer solar cell based on P:modi-PC60BM, we have used solvent additive in the blend solution of donor and acceptor materials. We have used CN as the solvent additive. Figure 4 also shows the current− voltage characteristics of the device based on P:modi-PC60BM (cast from CN (3% vol)/THF solvent), and the photovoltaic parameters are compiled in Table 1. The photovoltaic performance of the device has been improved significantly using the solvent additive. After the solvent additive, all photovoltaic parameters, that is, Jsc, Voc, and FF, have been improved, which results in the overall PCE of 3.63%. The improvement in the PCE with additive could be ascribed to the higher boiling point of CN, which is beneficial for the selforganization of P and modi-PC60BM to get the nanoscale morphology for exciton dissociation and charge transport. As can be seen from the absorption spectra (Figure 3), the blend cast processed with CN additive shows stronger absorption spectra within the wavelength range 500−750 nm. The stronger absorption could be ascribed to the more ordered structure of P with the treatment of CN additive. The absorption spectra of the blend cast with CN additive shows a more clear and prominent vibronic shoulder around 690 nm, which may be attributed to the more extensive P crystallinity in the blend film. This indicates that adding CN into the blend solution results in a higher ordered crystal structure of P compared with that without additive. The morphology of the BHJ active layer thin film plays an important role in improving the photovoltaic performance of the polymer solar cell. The interpenetrating networks with phase-separated domains between donor and acceptor materials used in BHJ active layer provide not only D/A interfaces for Table 1. Performance of Polymer Solar Cells HTL
Jsc (mA/cm2)
Voc (V)
FF
PCE (%)
PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT:PSS (acetone treated)
5.56 4.60 7.28 8.68
0.88 0.78 0.94 0.94
0.48 0.42 0.54 0.58
2.35 1.50 3.63 4.73
blend a
P:modi-PCBM P:PCBM34 P:modi-PCBMb P:modi-PCBMb a
Cast from THF. bCast from CN/THF. D
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Figure 5. AFM images of P:modi-PC60BM blend thin film cast from THF and CN/THF (image scale 3 μm × 3 μm).
improve the PCE of the organic solar cells based on P:modiPCBM processed from the CN/THF solvent. We have measured the J−V characteristics of the device based on the acetone-treated PEDOT:PSS hole transport layer and P:modi-PC60BM (processed from CN/THF solvent) under illumination. The polymer solar cell made with PEDOT:PSS film treated with acetone had a PCE of 4.73% with Jsc of 8.68 mA/cm2, Voc of 0.94 V, and FF of 0.58, which is greater than that for pristine PEDOT:PSS thin film. The increase in the PCE is mainly due to the enhancement in Jsc and FF. However, the value of Voc remains same. This may be attributed to the increase in the conductivity of PEDOT:PSS film due to the rearrangement of PEDOT segment with polar solvent. The PEDOT:PSS consists of conductive and hydrophobic PEDOT and insulating and hydrophilic PSS. The PEDOT is equally distributed inside the PEDOT:PSS grains, and PSS surrounds the outside.52 In this type of structure, the insulating PSS reduces the conductivity of the thin film because the outer insulating region prevents the carrier transport. When the acetone (polar solvent with high dipole moment of 2.88 D) is added inside the grains of PEDOT:PSS, the PEDOT segments separate from the inner PSS chains. The PEDOT, which is concentrated inside the grain, distributed radially from the grain center after the addition of polar solvent, which provides a continuous conductive pathway for charge carriers.53 This leads to an increase in the conductivity of PEDOT:PSS with the addition of acetone. The holes created in the active layer after the exciton dissociation at the D/A interface easily reach to the electrode without being recombined and trapped in the hole transport layer due to the distributed conductive PEDOT pathways with the addition of acetone in pristine PEDOT:PSS. This effect increases both FF and Jsc and results an improvement in overall PCE.38 The optical absorption spectra of modified PEDOT:PSS thin film showed higher transparency in the visible region and indicates that an increased number of photons are absorbed by the BHJ active layer, thereby enhancing the light-harvesting property. This effect also contributed to the increased PCE of solar cell.
Figure 6. XRD pattern of the P:modi-PC60BM blend spin-cast from THF with and without CN additives.
The Jsc can also be increased by enhancing the charge transport efficiency so that the charge carrier generated in the active layer can be easily transported to the electrode without loss within the BHJ active layer.43,44 In organic solar cells, the absorption of incident photons by the active layer generates excitons and generally dissociated at the D/A interface in the active layer. Electrons and holes in the active layer diffused through the active layer and finally collected by the cathode and anode, respectively.45 The total number of photogenerated charge carriers collected by the electrodes determines the FF of the device. However, carriers may be lost due to the recombination and trapping within the interface or during the transportation of charge carrier through the active layer and fail to reach the electrode, thus leading to a reduction of the PCE.46−48 The recombination and trapping of the charge carrier can be reduced by using the buffer layer with high conductivity. PEDOT:PSS is composed of PEDOT, which is hydrophobic and conductive, and PSS, which is hydrophilic and insulating; therefore, conductivity of its thin film can be enhanced by organic solvent treatment.48−50 It is well known that the PCE of the organic solar cells can be improved using the solventtreated PEDOT:PSS by increasing its conductivity.49−51 We have used modified PEDOT:PSS (treated with acetone) to
4. CONCLUSIONS In conclusion, we have used a low-band-gap D−A copolymer P and modi-PC60BM as electron donor and electron acceptor for E
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the fabrication of BHJ polymer solar cells and showed higher PCE (2.35%) than that for PC60BM as electron acceptor (1.50%). The improved PCE was due to increase in both Voc and Jsc. The higher Voc value is due to the higher LUMO level of modi-PC60BM than PC60BM, and Jsc is related to the strong absorption of modi-PC60BM than PC60BM in visible region. The further improvement in PCE up to 3.63% has been achieved with the P:modi-PC60BM blend processed from CN/ THF solvent and attributed to the more balance charge transport in the BHJ active layer due to the increased crystallinity and smoother film formation, as evidenced from the XRD, optical absorption and AFM data. We have also investigated the effect of added solvent (acetone) in the PEDOT:PSS and found that the PCE has been increased up to 4.73%. The acetone-treated PEDOT:PSS showed higher conductivity and transparency in the visible region, which increases both the photon harvesting efficiency of the BHJ layer and the conductive pathways for the carriers. These two effects increase the Jsc and FF, resulting in higher PCE.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. M. Lakshmi Kantam, Director of CSIR-IICT for her constant encouragement. We also thank Dr. V. Jayathirtha Rao for helpful discussions. CH. Pavan Kumar thanks UGC, New Delhi for junior research fellowship.
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