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Efficient bulk heterojunction solar cells based on D–A copolymers as electron donors and PC70BM as electron acceptor
Materials Chemistry and Physics 135 (2012) 25e31
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Efficient bulk heterojunction solar cells based on DeA copolymers as electron donors and PC70BM as electron acceptor G.D. Sharma a, c, *, J.A. Mikroyannidis b, Surya Prakash Singh d a
Physics Department, Molecular Electronic and Optoelectronic Device Laboratory, JNV University, Jodhpur, Rajasthan 342005, India Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR-26500 Patras, Greece c R&D Center for Science and Engineering, Jaipur Engineering College, Kukas, Jaipur, Rajasthan, India d Inorganic & Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500607, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 September 2011 Received in revised form 16 February 2012 Accepted 13 March 2012
Two low band gap conjugated polymers P1 (alternating phenylenevinylene containing thiophene and pyrrole rings) and P2 (alternating phenylenevinylene with dithenyl (thienothiadiazole) segments) having optical band gap 1.65 eV and 1.74 eV, respectively, were used as electron donor along with the PC70BM as electron acceptor for the fabrication of bulk heterojunction solar cells. The power conversion efficiency (PCE) of BHJ devices based on P1:PC70BM and P2:PC70BM cast from THF solvent is about 2.84% and 2.34%, respectively, which is higher than the BHJ based on PCBM as electron acceptor. We have investigated the effect of mixed (1-chloronaphthalene (CN)/THF) solvent, modification of PEDOT:PSS layer and inserting of TiO2 layer, on the photovoltaic performance of polymer solar cell. We have achieved power conversion efficiency of 5.07% for the polymer solar cells having structure ITO/PEDOT:PSS (modified)/P1:PC70BM (CN/THF cast)/TiO2/Al. The effect of solvent used for spin coating, modification of PEDOT:PSS layer and inclusion of TiO2 layer has been discussed in detail. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Phenylenevinylene copolymer Thiophene Pyrrole Low band gap Bulk heterojunction solar cells TiO2 buffer layer
1. Introduction Polymer solar cells (PSCs) recently have attracted a great deal of attention because of the low cost, light weight, and mechanical flexibility [1e6]. The most efficient device structure of PSCs was based on the concept of bulk heterojunction (BHJ) [7,8], which consists of a blend of conjugated polymers and fullerene derivatives as electron donors and acceptors, respectively. Poly(3hexylthiophene) (P3HT) has been used as electron donor along with the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) for the single BHJ polymer solar cells and has reached power conversion efficiency (PCE) between 4 and 5% [9,10]. However, the relatively large band gap and higher position of highest occupied molecular orbital (HOMO) energy level (in between 4.8 and 4.9 eV) of P3HT [11] significantly limit the short circuit current (Jsc) and open circuit voltage (Voc) of the BHJ polymer solar cells, respectively, based on P3HT:PCBM. The mismatch of absorption spectra of P3HT with the solar spectrum significantly limits the photovoltaic performance of the BHJ solar cells. In order to improve the visible
* Corresponding author. Physics Department, Molecular Electronic and Optoelectronic Device Laboratory, JNV University, Jodhpur, Rajasthan 342005, India. E-mail address: [email protected] (G.D. Sharma). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.03.058
absorption and decrease the HOMO energy level of conjugated polymers, design and synthesis of donoreacceptor (DeA) copolymers had been proven to be the most successful strategy to reduce the band gap [12e15]. Research efforts to design this group of conjugated polymers have recently focused on developing low band gap intramolecular charge transfer (ICT) copolymers using donor (D) and acceptor (A) units [16e19]. Significant progress had been made in this field and the PCEs of solution processed polymer solar cells have reached 7e8%, primarily due to the development of new low band gap conjugated polymers [20e23] and the better control of the nanoscale morphology of the interpenetrating networks. To date, the PCEs of conjugated polymer solar cells based on DeA structure have reached high up to 8.13% by Solarmer [24] and 8.3% by Konarka [25]. A literature survey revealed that certain copolymers containing thiophene and pyrrole rings had been recently synthesized and used as donor components for BHJ polymer solar cells and the PCE of these devices ranged from 0.18% to 2.8% [26,27]. Recently, our group has designed two low band gap soluble phenylenevinylene copolymers with cyanovinylene 4-nitrophenyl segments and achieved a PCE up to 4.06% using these copolymers as electron donor along with PCBM as electron acceptor for BHJ solar cells [28]. In this paper, we report the photovoltaic effect of the BHJ devices based on P1:PC70BM and P2:PC70BM blends. P1 is the DeA
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copolymer having optical band gap of 1.65 eV, in which dihexyloxyphenylene, thiophene and pyrrole groups behave as electron donor and the cynovinylene 4-nitrophenyl group behaves as an acceptor unit [29]. P2 is another DeA copolymer having optical band gap about 1.74 eV, in which hexyloxyphenylene and dithenyl (thienothiadiazole) cyanovinylene nitrophenyl units as electron donor and acceptor, respectively [30]. We have already reported that the PCE of BHJ devices based on P1:PCBM and P2:PCBM is about 1.50% and 1.40%, respectively [29,30]. However, the PCE of the photovoltaic devices based on P1:PC70BM and P2:PC70BM cast from THF solvent is about 2.84% and 2.34%, respectively. The enhanced PCE for the device based on PC70BM is attributed to the improvement of both Jsc and Voc as compared to the device based PCBM. The increase in the Jsc is attributed to the strong absorption by the PC70BM as compared to PCBM in the wavelength region below 500 nm. Moreover, the difference in the HOMO level of copolymer and LUMO level of the PC70BM is higher than that for the BHJ active layer based on PCBM, resulted higher value of Voc. The overall PCE of the BHJ active layer processed from a mixed solvent (CN/THF) is about 4.12% and 3.3% for P1:PC70BM and P2:PC70BM BHJ active layer, respectively. This improved PCE has been attributed to the increased crystalline nature of copolymer and hole mobility in the blend, resulting in balanced charge transport. We have used the modified PEDOT:PSS electrode to improve the photovoltaic performance of the devices and achieved overall PCE of 3.54% and 4.4% with P2:PC70BM and P1:PC70BM blends cast from the CN/THF solvents. Further the overall PCE of the PSCs has been improved up to 4.14% and 5.07% for P2:PC70BM and P1:PC70BM blends, respectively, cast from CN/THF solvent, with the incorporation of a TiO2 layer in between the active layer and Al electrode. 2. Experimental details We have used conjugated copolymers P1 and P2 as electron donor and PC70BM as the electron acceptor for the fabrication of BHJ polymer solar cells (chemical structure shown in Fig. 1). The P1 and P2 were synthesized as reported earlier [29,30]. PC70BM and 1chloronaphthalene (CN) were purchased from Aldrich Chemicals. All the reagents and solvents were commercially purchased and were used as supplied. The absorption spectra of the thin films were obtained on a PerkineElmer spectrophotometer. The BHJ PSCs were fabricated having structure of ITO/ PEDOT:PSS/P1 or P2:PC70BM/Al as follow: The indium tin oxide (ITO) coated glass substrates were cleaned by ultrasonication sequentially in de-ionized water, acetone, detergent, and isopropyl alcohol. After drying the substrate, a thin layer of 60 nm thin layer of poly(3, 4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Baytron) was spin coated (3500 rpm for 30 s) on ITO substrate and subsequently baked at 100 C for 20 min in air. The blend of P1 or P2 with PC70BM in THF under weight ratio 1:1, was spin coated on the top of the PEDOT:PSS layer. In order to investigate the effect of mixed solvent on the performance of the PSCs, a small amount of the high boiling point additive 1chloronaphthalene (CN) (2% by volume) was added into the blend (in THF) solution. The active layer thicknesses in all devices were approximately 85 (2 nm) and controlled by changing both the spin coating rate and the concentration of the solution. An aluminum (Al) cathode (100 nm) was then thermally evaporated under vacuum (w105 Torr) through a shadow mask defining the active device area of 16 mm2. We have also fabricated separate hole and electron only devices with ITO/PEDOT:PSS/P1 or P2:PC70BM/ Au and Al/P1 or P2:PC70BM/Al structures, respectively, to measure the hole and electron mobility of the BHJ active layer. The currentevoltage (JeV) characteristics of the devices were measured using a computer controlled Keithley 238 source meter. A xenon
Fig. 1. Chemical structure of P1, P2, PCBM and PC70BM.
lamp coupled with AM 1.5 solar spectrum filter was used as light source, and the illumination intensity at the surface of device was around 100 mW cm2. The incident photon to current efficiency (IPCE) spectra of the devices were measured illuminating the devices through halogen lamp coupled with monochromator and the resulting current was recorded on Keithley electrometer, under short circuit condition. The IPCE at a monochromatic wavelength (l) was estimated using following expression
. IPCEðlÞ ¼ 1240 Jsc lPin where Jsc is the photocurrent density under short circuit condition and Pin is the illumination intensity. 3. Results and discussion 3.1. Optical properties of the BHJ thin films The absorption spectra of P1:PC70BM and P2:PC70BM thin films cast from THF and CN/THF mixed solvent are shown in Fig. 2a and b, respectively. The optical absorption spectra of the P1 and P2 had also been already reported in our earlier publications [29,30]. It can be seen from these figures that the absorption spectra of the blend show the combination of individual components i.e. P1 or P2 and PC70BM. The absorption band in the longer wavelength region corresponds to the P1 or P2, which is associated to the interchain pep* transition. The absorption spectra of shoulder at 715 nm and 680 nm for P1 and P2, respectively, are related to the interchain interaction and the height of this shoulder indicates the ordering of the chain packing [31]. It can be seen from these figures that optical
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PC70BM measured from the cyclic voltammetry is around 3.90 eV which is very close to the value reported in literature [34]. The difference between the LUMO level of P1 or P2 and PC70BM is higher than 0.3 eV, which indicates that the blend of P1 or P2 as donor and PC70BM as acceptor can be used as efficient BHJ polymer solar cells. 3.2. Photovoltaic properties
Fig. 2. Optical absorption spectra of the P1:PC70BM and P2:PC70BM blends (a) cast from THF solvent and (b) cast from mixed CN/THF solvents.
absorption spectra of the blend of copolymer (P1 or P2) with PC70BM show broader absorption band as compared to blends with PCBM [29,30], which is attributed to the strong absorption of PC70BM in the lower wavelength region. When the blend film was processed from the mixed solvent (CN/THF), the absorption band in longer wavelength region is red shifted and the intensity also increased (as shown in Fig. 2b). A higher crystallinity degree of copolymer P1 or P2 is indicated by a red shift in absorption band, a clear vibronic shoulder at 715 nm and 680 nm for P1 and P2, which indicates enhanced interchain pep* stacking. The blend film cast from the mixed solvent showed higher crystallinity than cast from THF solvent, which is due to the different vapor pressure and boiling point of the solvents used in mixed solvent. It can be seen from Fig. 1b that the vibronic shoulder in the longer wavelength region is more distinguishable for the film cast from mixed solvent, which indicates that during the film formation cast from the mixed solvent enhanced the crystallinity of the blend. The enhanced shoulder absorption in the film has been observed for many high performance PSC materials, generally indicating a strong intermolecular packing in the solid state caused by their planar and rigid backbones [32,33]. The HOMO level of the P1 and P2 estimated from the cyclic voltammetry data is 5.15 eV and 5.25 eV, respectively, whereas the LUMO level of both P1 and P2 is 3.45 eV. The LUMO level of the
In order to investigate the photovoltaic properties of the copolymers, the BHJ polymer solar cells having structure of ITO/ PEDOT:PSS/P1 or P2:PC70BM/Al, were fabricated, where the copolymers P1 and P2 were used as donors and PC70BM was used as acceptor. We have varied the weight ratio of P1 or P2 and PC70BM and found that optimized performance was achieved with the weight ratio of P1 or P2:PC70BM at 1:1 (w/w). Fig. 3(a) shows the currentevoltage (JeV) characteristics of the devices based on P1 or P2:PC70BM blends cast from THF solvent and the photovoltaic performance data i.e. Jsc, Voc, fill factor (FF) and PCE are complied in Table 1. For comparison the data of the photovoltaic devices based on PCBM as acceptor are also included in Table 1. The BHJ solar cells based on P1:PC70BM and P2:PC70BM cast from THF solvent yield PCE values of 2.56% and 2.34%, respectively. Both P1 and P2 showed better performance using PC70BM as the acceptor than that with the device fabricated with PCBM. The improved PCE can be explained in terms of strong absorption of PC70CM in the visible region, where the both polymers have weak absorption. The increase in the PCE based on the PC70BM as acceptor is due to the increase in both Jsc and Voc. Since the most important factor that determines the value of Voc in the BHJ solar cell is the difference between the HOMO level of donor and the LUMO level of the acceptor, we attribute the increase in the Voc obtained for PC70BM to its higher LUMO value (upward shift toward vacuum level). The increase in the Jsc for the devices based on PC70BM acceptor as compared to PCBM is attributed to the broader absorption of both P1:PC70BM and P2:PC70BM blends, which causes an enhancement in the photogenerated excitons in the blend, resulting in higher photocurrent. The XRD patterns of P1 and P2 show peak at 2q ¼ 8.28 and 7.8, respectively [29,30]. The peak observed in the XRD pattern gives the information about the interlayer spacing of the polymer ordered aggregation. The interlayer spacing estimated from the XRD patterns of P1 and P2 is 13.56 Å and 14.44 Å, respectively. The lower value of interlayer spacing for P1 may lead to improved charge transportation and resulting higher PCE. The PCE of the devices based on P1 or P2:PC70BM blends cast from THF solvent is still low. In solution process, solvent type [35e37], donor and acceptor compositions [38,39], solution concentration [40], incorporation of additive [41e46], and regioregularity of conjugated polymer [47] are considered to develop various morphologies and microstructures of active layer. In general, the morphology of active layer with appropriate phase separation between donor and acceptors and good crystallinity of polymers are favorable for high efficiency solar cells. The above mentioned techniques are also regularly adopted to improve the crystallinity of polymers and phase separation between donor and acceptor. In the active blended layers, polymers often possess both crystalline and amorphous portions. Changes in the crystalline and amorphous portions can give rise to a different morphology and microstructure of active layers. We have investigated the effect of mixed solvent for the thin film processing, on the photovoltaic response of BHJ polymer solar cells. The JeV characteristics of the BHJ devices based on the thin films of blended layer cast from the mixed (CN/THF) solvent are shown in Fig. 3(b) and photovoltaic parameters are complied in Table 1. The overall PCE of the devices based on P1:PC70BM and P2:PC70BM cast from mixed solvent is 4.12% and 3.3%, respectively. The increase in the PCE is mainly
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attributed to the improvement in Jsc and FF. As can be seen from the absorption spectra of the BHJ active layer processed from the mixed solvent that the intensity of the absorption band in longer region is higher and the band also gets broadened as well as red shifted. The broader absorption band implies more excitons are generated in the BHJ layer, resulted more photocurrent. As we have already reported the crystalline nature of the both P1 and P2 has been improved when the film is processed for the mixed solvent, which causes an enhancement in the hole mobility, resulting balanced charge transport [29,30]. The film morphology of the BHJ active layer has been found to be one of the key elements in determining the PCE of the polymer solar cell [48]. To gain better insight into what might be controlling the Jsc and hence lead to enhancement in the PCE of the solar cell based on the films cast from mixed solvent, atomic force microscopy (AFM) was used to examine the surface topography of the blend films cast from THF and mixed solvents. Fig. 4 shows the AFM images of P1:PC70BM blend film cast from THF and mixed CN/THF solvents. As can be seen from these images the domain size becomes finer for the CN/THF cast film as compared to the film cast from THF solvent, which leads to an enhancement in the DeA interfacial area for the exciton dissociation and increases the Jsc and hence PCE. As can be seen from Table 1 that the Voc values for the devices based on the blends THF cast blend are higher than that for the devices based on the blends cast from mixed solvents. This difference is approximately about 0.10 V. The shift in Voc has been attributed to the less negative HOMO position of the copolymers caused by the electronic interaction among the polymer chains in the presence of the higher boiling point solvent, i.e. CN, as reported for other copolymers processed from the mixed solvents [34]. As we have discussed earlier that the optical absorption spectra of the blend cast from the mixed solvents, the both absorption peak and onset corresponds to the copolymers are red shifted. This shift in absorption onset suggests that the copolymer HOMO level experiences an upward energy shift by effect of the solvent used [35]. Such an effect is qualitatively in good agreement with the experimentally observed correlation between the donor HOMO and acceptor LUMO difference (effective band gap of the blend). 3.3. Effect of modified PEDOT:PSS layer
Fig. 3. Currentevoltage (JeV) characteristics of ITO/PEDOT:PSS/P1 or P2:PC70BM/Al BHJ photovoltaic devices, BHJ active layer cast from (a) THF and (b) cast from mixed CN/THF solvents.
Table 1 Photovoltaic parameters of ITO/PEDOT:PSS/P1 or P2:PC70BM/Al BHJ polymer solar cells. Blend
Jsc (mA cm2)
Voc (V)
FF
PCE (%)
P1:PCBM [29] P1:PC70BMa P2:PCBM [30] P2:PC70BMa P1:PC70BMb P2:PC70BMb
4.6 6.3 4.20 5.25 9.2 7.7
0.78 0.90 0.76 0.93 0.80 0.82
0.42 0.50 0.44 0.48 0.56 0.52
1.50 2.56 1.40 2.34 4.12 3.30
a b
Cast from THF solvent. Cast from CN/THF solvent.
PEDOT:PSS film has high transparency in the visible region, high mechanical flexibility and good thermal stability. It has been used extensively as a modification layer on ITO electrode to improve the hole collection in PSCs. Xia and Quang [49] reported that conductivity of PEDOT:PSS film was significantly enhanced after treatment with a cosolvent of water and a common solvent such as ethanol, dimethyl sulfoxide, acetonitrile or tetrahydrofuran and used these films for the application as transparent electrode in PSC [50,51]. Shinar et al. [52] mixed the original PEDOT:PSS solution with ethylene glycol, the treatment improved the PCE of the PSC based on P3HT:PCBM blend by up to 27%. The authors suggested that these additives may affect the morphology and enhance the conductivity of PEDOT:PSS film, hence improved photovoltaic performance of the device. We have investigated the effect of the modification of PEDOT:PSS layer on the photovoltaic performance of the PSCs based on the P1:PC70BM and P2:PC70BM cast from the mixed solvents. The treatment of the organic solvent on PEDOT:PSS was performed by mixing the aqueous solution of PEDOT:PSS with organic solvent of 2-propanol (1:1 by volume). The thickness of the PEDOT:PSS is 45 nm for the unmodified PEDOT:PSS layer. The JeV characteristics of the ITO/PEDOT:PSS (modified)/P1 or P2:PC70BM (cast from mixed solvent)/Al PSCs are shown in Fig. 5 and the
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Fig. 4. AFM images of the P1:PC70BM thin films cast from THF and CN/THF solvents.
photovoltaic parameters are complied in Table 2. It can be seen from these figures and Table 2 that the Jsc and FF have been improved significantly however the Voc remains almost unchanged. The overall PCE of the devices based on P1:PC70BM and P2:PC70BM is 4.4% and 3.54%, respectively. The increase in the PCE is mainly due to the improvement in Jsc in the devices based on modified PEDOT:PSS electrode. In order to understand the origin of the improved PCE of the devices with organic solvent treated PEDOT:PSS layer, we have investigated the effect of the organic solvent on the transparency, conductivity and surface morphology of the PEDOT:PSS film. We observed that the transmittance of the treated and untreated PEDOT:PSS electrodes in the visible range is similar. Hence, the improved photovoltaic performance of the PSC devices with PEDOT:PSS layer treated with organic solvent is not directly related to the transmission of different PEDOT:PSS layers. The conductivity of PEDOT:PSS films was measured by the four point probe method and it is found that the conductivity of the organic solvent treated PEDOT:PSS (1.18 103 S cm1) layer is higher than that of untreated PEDOT:PSS layer (4.2 104 S cm1). The tendency of the conductivity change is consistent with the tendency of the Jsc and PCE. The improved PV performance with
organic solvent treated PEDOT:PSS resulted from the higher conductivity of the modified layer treated by organic solvent. We have measured the HOMO level of the treated and untreated PEDOT:PSS with the help of the cyclic voltammetry and found that the HOMO levels are about 5.0 eV and 5.15 eV for untreated and treated PEDOT:PSS. We assume that the cosolvent treatment may lower the energy barrier for the charge transport across the PEDOT chains and increases the localization length as reported earlier for the PSC based on P3HT:PCBM blend [53]. We have also recorded the AFM images of the organic solvent treated and untreated PEDOT:PSS layers, and observed that there is a slight increase in the rms value of surface roughness. This increase in the surface roughness may likely increase the contact area between the PEDOT:PSS and active layer, and improve the hole extraction to the anode. On the other hand the rough surface may increase the scattering of incident light back to the active layer and hence leads to the increased absorption. Since the PCE of the PSC depends upon the light harvesting efficiency of the active layer and charge collection efficiency of the electrodes, we conclude that the modified PEDOT:PSS not only improves the light harvesting property of the PSC but also improves the hole collection efficiency and combination of these two effect increases the PCE of PSC. 3.4. Effect of TiO2 buffer layer As the LUMO of the PC70BM is about 3.75 eV, and the work function of the cathode electrode (Al) is about 4.2 eV. This difference between LUMO level of PC70BM and the work function of Al induces a barrier for the electron extraction and leads to lower the PCE of the PSCs. One of the main strategies employed to enhance the PCE is to engineer the cathode electrode to increase the electrode extraction efficiency, by sandwiching a buffer layer between the active layer and cathode electrode [54]. A variety of materials have been deposited through vacuum evaporation or solution
Table 2 Photovoltaic parameters of ITO/PEDOT:PSS(modified)/P1 or P2:PC70BMa/Al and ITO/ PEDOT:PSS (modified)/P1 or P2:PC70BMa/TiO2/Al BHJ polymer solar cells. Device
Fig. 5. Currentevoltage (JeV) characteristics of ITO/PEDOT:PSS (modified)/P1 or P2:PC70BM (mixed solvent cast)/Al BHJ photovoltaic devices.
ITO/PEDOT:PSS(modified)/ P1:PC70BMa/Al ITO/PEDOT:PSS (modified)/ P2:PC70BMa/Al ITO/PEDOT:PSS(modified)/ P1:PC70BMa/TiO2/Al ITO/PEDOT:PSS(modified)/ P2:PC70BMa/TiO2/Al a
Jsc (mA cm2)
Voc (V)
FF
PCE (%)
9.8
0.80
0.56
4.4
8.3
0.82
0.52
3.54
10.3
0.82
0.60
5.07
8.7
0.82
0.56
4.14
Cast from CN/THF mixed solvent.
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processes as the buffer layer, such as metals (Ca, Ba) [55,56], transition metal oxides (oxides of Cr, Zn, Ni) [57e59] and organic materials [60]. TiO2 has been synthesized by synthetic routes [60,61], is commonly used for photovoltaic devices as n-type semiconducting material and has been employed as photoanode for dye sensitized solar cells [62]. Recently, solution processed titanium sub-oxide (TiOx) has been applied in both single and tandem organic photovoltaic devices as a multifunctional buffer layer serving as optical spacer and electron transport/hole blocking layer [63e66]. We introduce a buffer layer of solution processed TiO2 nanoparticles (spin coated at 3000 rpm for 60 s) after the deposition of the active polymer:fullerene blended layer and dried at the temperature of 60 C. The JeV characteristics of the ITO/PEDOT:PSS (modified)/P1 or P2:PC70BM (mixed solvent)/TiO2/Al devices shown in Fig. 6 and the photovoltaic parameters are complied in Table 2. Compared to the device without TiO2 layer, the devices we fabricated with TiO2 layer give a PCE of 4.14% and 5.07% for P2:PC70BM and P1:PC70BM blends, respectively. For the devices with TiO2 buffer layer, the PCE improvement is mainly due to the increase in Jsc and FF, whereas the Voc remains almost same. It is well known that the Voc is directly related to difference in the HOMO of donor and LUMO of acceptor material used in the active layer. Although the additional TiO2 layer may help to break the symmetry of electric field in the device, its conduction band edge, which is close to the Fermi level of Al, does not have much contribution to Voc. The energy level diagram of ITO/PEDOT:PSS/P1:PC70BM/TiO2/Al device is shown in Fig. 7. Since the conduction band edge of the TiO2 (4.3 eV) matches with the Fermi level of the Al (4.2 eV) and TiO2 is also a good electron transporter may facilitate the electron transfer from the LUMO of the PC70BM (3.75 eV) to Al [67]. On the other hand, the valence band edge TiO2 (7.5 eV) is deeper than that of HOMO level of P1 (5.15 eV), resulting the prevention of the hole accumulation at the active layereAl interface, which leads to a more reduced interfacial charge recombination. Recently, Lee et al. found that the TiOx layer in organic photovoltaic device decreases the saturation current, resulting in reduced minority carrier density [67]. TiO2 with deeper
Fig. 6. Currentevoltage (JeV) characteristics of ITO/PEDOT:PSS(modified)/P1 or P2:PC70BM (mixed solvent cast)/TiO2/Al BHJ photovoltaic devices.
Fig. 7. Energy level diagram of ITO/PEDOT:PSS/P1:PC70BM/TiO2/Al polymer solar cell.
conduction band edge provides higher energy barrier for holes and therefore reducing the charge carrier injection process and leading to improvement in the FF. Therefore, TiO2 acts as both electron transporting layer as well as hole blocking layer in the device. Besides this, the introduction of the TiO2 buffer layer can redistribute the light intensity in the blend film, i.e. as an optical spacer, which enhances the photon harvesting property. The series resistance (Rs) and shunt resistance (Rsh), calculated from the inverse of the JeV characteristics at V ¼ 0 and V ¼ Voc, respectively, found that the incorporation of TiO2 buffer layer lowers the Rs, while increases the Rsh. The reduction in Rs correlates consistently with the enhancement in Jsc [68]. The incident photon to current efficiency of the devices for the devices based on P1:PC70BM cast from mixed solvent with and without TiO2 layer is shown in Fig. 8. Similar results have been observed for other devices. It can be seen from this figure that the value of IPCE has been increased with the incorporation of TiO2 layer in between the BHJ active layer and cathode Al electrode. The increased value of IPCE attributed to the better collection of electrons by the cathode leading to the higher value of overall PCE.
Fig. 8. Incident photon to current efficiency (IPCE) spectra of (a) ITO/PEDOT:PSS (modified)/P1:PC70BM (mixed solvent)/Al and (b) ITO/PEDOT:PSS (modified)/ P1:PC70BM (mixed solvent)/TiO2/Al devices.
G.D. Sharma et al. / Materials Chemistry and Physics 135 (2012) 25e31
4. Conclusions We have used two low band gap copolymers P1 (alternating phenylenevinylene with thiophene and pyrrole units) and P2 (alternating phenylenevinylene with dithenyl (thienothiadiazole) segments along the backbone) having band gap 1.65 eV and 1.74 eV, respectively, as electron donor along with PC70BM as electron acceptor for the fabrication of BHJ solar cells. The PCEs of BHJ solar cells based on P1:PC70BM and P2:PC70BM cast from the THF solvent are 2.56% and 2.34%, respectively. These values of PCEs are higher than for the devices based on PCBM as electron acceptor which is attributed to the strong absorption of PC70BM in visible region than that for PCBM and also to the higher LUMO level of PC70BM as compared to PCBM. We have also fabricated the PSCs based on P1:PC70BM and P2:PC70BM blends cast from the mixed CN/THF solvents and achieved overall PCE of 4.12% and 3.3%, for P1:PC70BM and P2:PC70BM blends respectively. This increase has been attributed to the effective induced higher copolymer crystallinity and also to the more fine film morphology on nanoscale, of the blend film cast from the mixed solvents, which results higher light harvesting property of efficient exciton dissociation due to the increased interfacial D/A interface. The PCE of the PSCs has been improved up to 4.4% and 3.54% for P1:PC70BM and P2:PC70BM blends cast from mixed solvent, when PEDOT:PSS is modified by the addition on 2proponal in the aqueous PEDOT:PSS solution. This increase is attributed to the enhancement in the conductivity of PEDOT:PSS and larger domain of the PEDOT:PSS, resulted an increase in the hole collection efficiency in the device. Finally, the PCE of the PSC based on modified PEDOT:PSS electrode and P1:PC70BM and P2:PC70BM blends cast from the mixed solvent has been improved up to 5.07% and 4.14%, respectively, when a thin layer of TiO2 processed from sol gel method is inserted in between active layer and Al electrode. We assume that this increase in PCE may be the fact that the TiO2 layer acts as an electron transporting and hole blocking layer, improving the electron extracting efficiency. References [1] F.C. Krebs, T.D. Nielsen, J. Fyenbo, M. Wadstrom, M.S. Pedersen, Energy Environ. Sci. 3 (2010) 512e525. [2] C.J. Brabec, S. Gowrisanker, J.M. Halls, L. Darin, S. Jia, S.P. Williams, Adv. Mater. 22 (2010) 3839e3856. [3] C. Li, M. Liu, N.G. Pschirer, M. Baumgarten, K. Mullen, Chem. Rev. 110 (2010) 6817e6855. [4] D. Gendron, M. Leclerc, Energy Environ. Sci. 4 (2011) 1225e1237. [5] N. Espinosa, R. Garcýa-Valverdea, F.C. Krebs, Energy Environ. Sci. 4 (2011) 1547e1557. [6] B.C. Thompson, P.P. Khlyabich, B. Burkhart, A.E. Aviles, A. Rudenko, G. Shultz, C.F. Ng, L.B. Mangubat, Green 1 (2011) 29e54. [7] B.C. Thompson, J.M.J. Frechet, Angew. Chem. Int. Ed. 47 (2008) 58e77. [8] G. Dennler, M.C. Scharber, C.J. Brabec, Adv. Mater. 21 (2009) 1323e1338. [9] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Adv. Funct. Mater. 15 (2005) 1617e1622. [10] G. Li, V. Shrotriva, J.S. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nat. Mater. 4 (2005) 864e868. [11] J.H. Hou, Z.A. Tan, Y. Yan, Y.J. He, C.H. Yang, Y.F. Li, J. Am. Chem. Soc. 128 (2006) 4911e4916. [12] D. Muhlbacher, M.C. Scharber, M. Morana, Z.G. Zhu, D. Waller, R. Gaudiana, C.J. Brabec, Adv. Mater. 18 (2006) 2884e2889. [13] P.L.T. Boudreault, A. Najari, M. Leclerc, Chem. Mater. 23 (2011) 456e469. [14] J. Peet, J.Y. Kim, N.E. Coates, W.L. Ma, D. Moses, A.J. Heeger, G.C. Bazan, Nat. Mater. 6 (2007) 497e500. [15] Y. Zhang, J.Y. Zou, H.L. Yip, Y. Sun, J.A. Davies, K.S. Chen, O. Acton, A.K.Y. Jen, J. Mater. Chem. 21 (2011) 3895e3902. [16] G.Y. Chen, Y.H. Cheng, Y.J. Chou, M.S. Su, C.M. Chen, K.H. Wei, Chem. Commun. 47 (2011) 5064e5066. [17] M. Wang, X. Hu, P. Liu, W. Li, X. Gong, F. Huang, Y. Cao, J. Am. Chem. Soc. 133 (2011) 9638e9641. [18] E. Wang, Z. Ma, Z. Zhang, P. Henriksson, O. Inganas, F. Zhang, M.R. Andersson, Chem. Commun. (2011) 4908e4910. [19] Y. Lee, Y.M. Nam, W.H. Jo, J. Mater. Chem. 21 (2011) 8583e8590. [20] Y. Liang, Z. Xu, J. Xia, S. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, Adv. Mater. 22 (2010) E135eE138.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Efficient bulk heterojunction solar cells based on DeA copolymers as electron donors and PC70BM as electron acceptor G.D. Sharma a, c, *, J.A. Mikroyannidis b, Surya Prakash Singh d a
Physics Department, Molecular Electronic and Optoelectronic Device Laboratory, JNV University, Jodhpur, Rajasthan 342005, India Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR-26500 Patras, Greece c R&D Center for Science and Engineering, Jaipur Engineering College, Kukas, Jaipur, Rajasthan, India d Inorganic & Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500607, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 September 2011 Received in revised form 16 February 2012 Accepted 13 March 2012
Two low band gap conjugated polymers P1 (alternating phenylenevinylene containing thiophene and pyrrole rings) and P2 (alternating phenylenevinylene with dithenyl (thienothiadiazole) segments) having optical band gap 1.65 eV and 1.74 eV, respectively, were used as electron donor along with the PC70BM as electron acceptor for the fabrication of bulk heterojunction solar cells. The power conversion efficiency (PCE) of BHJ devices based on P1:PC70BM and P2:PC70BM cast from THF solvent is about 2.84% and 2.34%, respectively, which is higher than the BHJ based on PCBM as electron acceptor. We have investigated the effect of mixed (1-chloronaphthalene (CN)/THF) solvent, modification of PEDOT:PSS layer and inserting of TiO2 layer, on the photovoltaic performance of polymer solar cell. We have achieved power conversion efficiency of 5.07% for the polymer solar cells having structure ITO/PEDOT:PSS (modified)/P1:PC70BM (CN/THF cast)/TiO2/Al. The effect of solvent used for spin coating, modification of PEDOT:PSS layer and inclusion of TiO2 layer has been discussed in detail. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Phenylenevinylene copolymer Thiophene Pyrrole Low band gap Bulk heterojunction solar cells TiO2 buffer layer
1. Introduction Polymer solar cells (PSCs) recently have attracted a great deal of attention because of the low cost, light weight, and mechanical flexibility [1e6]. The most efficient device structure of PSCs was based on the concept of bulk heterojunction (BHJ) [7,8], which consists of a blend of conjugated polymers and fullerene derivatives as electron donors and acceptors, respectively. Poly(3hexylthiophene) (P3HT) has been used as electron donor along with the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) for the single BHJ polymer solar cells and has reached power conversion efficiency (PCE) between 4 and 5% [9,10]. However, the relatively large band gap and higher position of highest occupied molecular orbital (HOMO) energy level (in between 4.8 and 4.9 eV) of P3HT [11] significantly limit the short circuit current (Jsc) and open circuit voltage (Voc) of the BHJ polymer solar cells, respectively, based on P3HT:PCBM. The mismatch of absorption spectra of P3HT with the solar spectrum significantly limits the photovoltaic performance of the BHJ solar cells. In order to improve the visible
* Corresponding author. Physics Department, Molecular Electronic and Optoelectronic Device Laboratory, JNV University, Jodhpur, Rajasthan 342005, India. E-mail address: [email protected] (G.D. Sharma). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.03.058
absorption and decrease the HOMO energy level of conjugated polymers, design and synthesis of donoreacceptor (DeA) copolymers had been proven to be the most successful strategy to reduce the band gap [12e15]. Research efforts to design this group of conjugated polymers have recently focused on developing low band gap intramolecular charge transfer (ICT) copolymers using donor (D) and acceptor (A) units [16e19]. Significant progress had been made in this field and the PCEs of solution processed polymer solar cells have reached 7e8%, primarily due to the development of new low band gap conjugated polymers [20e23] and the better control of the nanoscale morphology of the interpenetrating networks. To date, the PCEs of conjugated polymer solar cells based on DeA structure have reached high up to 8.13% by Solarmer [24] and 8.3% by Konarka [25]. A literature survey revealed that certain copolymers containing thiophene and pyrrole rings had been recently synthesized and used as donor components for BHJ polymer solar cells and the PCE of these devices ranged from 0.18% to 2.8% [26,27]. Recently, our group has designed two low band gap soluble phenylenevinylene copolymers with cyanovinylene 4-nitrophenyl segments and achieved a PCE up to 4.06% using these copolymers as electron donor along with PCBM as electron acceptor for BHJ solar cells [28]. In this paper, we report the photovoltaic effect of the BHJ devices based on P1:PC70BM and P2:PC70BM blends. P1 is the DeA
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copolymer having optical band gap of 1.65 eV, in which dihexyloxyphenylene, thiophene and pyrrole groups behave as electron donor and the cynovinylene 4-nitrophenyl group behaves as an acceptor unit [29]. P2 is another DeA copolymer having optical band gap about 1.74 eV, in which hexyloxyphenylene and dithenyl (thienothiadiazole) cyanovinylene nitrophenyl units as electron donor and acceptor, respectively [30]. We have already reported that the PCE of BHJ devices based on P1:PCBM and P2:PCBM is about 1.50% and 1.40%, respectively [29,30]. However, the PCE of the photovoltaic devices based on P1:PC70BM and P2:PC70BM cast from THF solvent is about 2.84% and 2.34%, respectively. The enhanced PCE for the device based on PC70BM is attributed to the improvement of both Jsc and Voc as compared to the device based PCBM. The increase in the Jsc is attributed to the strong absorption by the PC70BM as compared to PCBM in the wavelength region below 500 nm. Moreover, the difference in the HOMO level of copolymer and LUMO level of the PC70BM is higher than that for the BHJ active layer based on PCBM, resulted higher value of Voc. The overall PCE of the BHJ active layer processed from a mixed solvent (CN/THF) is about 4.12% and 3.3% for P1:PC70BM and P2:PC70BM BHJ active layer, respectively. This improved PCE has been attributed to the increased crystalline nature of copolymer and hole mobility in the blend, resulting in balanced charge transport. We have used the modified PEDOT:PSS electrode to improve the photovoltaic performance of the devices and achieved overall PCE of 3.54% and 4.4% with P2:PC70BM and P1:PC70BM blends cast from the CN/THF solvents. Further the overall PCE of the PSCs has been improved up to 4.14% and 5.07% for P2:PC70BM and P1:PC70BM blends, respectively, cast from CN/THF solvent, with the incorporation of a TiO2 layer in between the active layer and Al electrode. 2. Experimental details We have used conjugated copolymers P1 and P2 as electron donor and PC70BM as the electron acceptor for the fabrication of BHJ polymer solar cells (chemical structure shown in Fig. 1). The P1 and P2 were synthesized as reported earlier [29,30]. PC70BM and 1chloronaphthalene (CN) were purchased from Aldrich Chemicals. All the reagents and solvents were commercially purchased and were used as supplied. The absorption spectra of the thin films were obtained on a PerkineElmer spectrophotometer. The BHJ PSCs were fabricated having structure of ITO/ PEDOT:PSS/P1 or P2:PC70BM/Al as follow: The indium tin oxide (ITO) coated glass substrates were cleaned by ultrasonication sequentially in de-ionized water, acetone, detergent, and isopropyl alcohol. After drying the substrate, a thin layer of 60 nm thin layer of poly(3, 4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Baytron) was spin coated (3500 rpm for 30 s) on ITO substrate and subsequently baked at 100 C for 20 min in air. The blend of P1 or P2 with PC70BM in THF under weight ratio 1:1, was spin coated on the top of the PEDOT:PSS layer. In order to investigate the effect of mixed solvent on the performance of the PSCs, a small amount of the high boiling point additive 1chloronaphthalene (CN) (2% by volume) was added into the blend (in THF) solution. The active layer thicknesses in all devices were approximately 85 (2 nm) and controlled by changing both the spin coating rate and the concentration of the solution. An aluminum (Al) cathode (100 nm) was then thermally evaporated under vacuum (w105 Torr) through a shadow mask defining the active device area of 16 mm2. We have also fabricated separate hole and electron only devices with ITO/PEDOT:PSS/P1 or P2:PC70BM/ Au and Al/P1 or P2:PC70BM/Al structures, respectively, to measure the hole and electron mobility of the BHJ active layer. The currentevoltage (JeV) characteristics of the devices were measured using a computer controlled Keithley 238 source meter. A xenon
Fig. 1. Chemical structure of P1, P2, PCBM and PC70BM.
lamp coupled with AM 1.5 solar spectrum filter was used as light source, and the illumination intensity at the surface of device was around 100 mW cm2. The incident photon to current efficiency (IPCE) spectra of the devices were measured illuminating the devices through halogen lamp coupled with monochromator and the resulting current was recorded on Keithley electrometer, under short circuit condition. The IPCE at a monochromatic wavelength (l) was estimated using following expression
. IPCEðlÞ ¼ 1240 Jsc lPin where Jsc is the photocurrent density under short circuit condition and Pin is the illumination intensity. 3. Results and discussion 3.1. Optical properties of the BHJ thin films The absorption spectra of P1:PC70BM and P2:PC70BM thin films cast from THF and CN/THF mixed solvent are shown in Fig. 2a and b, respectively. The optical absorption spectra of the P1 and P2 had also been already reported in our earlier publications [29,30]. It can be seen from these figures that the absorption spectra of the blend show the combination of individual components i.e. P1 or P2 and PC70BM. The absorption band in the longer wavelength region corresponds to the P1 or P2, which is associated to the interchain pep* transition. The absorption spectra of shoulder at 715 nm and 680 nm for P1 and P2, respectively, are related to the interchain interaction and the height of this shoulder indicates the ordering of the chain packing [31]. It can be seen from these figures that optical
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27
PC70BM measured from the cyclic voltammetry is around 3.90 eV which is very close to the value reported in literature [34]. The difference between the LUMO level of P1 or P2 and PC70BM is higher than 0.3 eV, which indicates that the blend of P1 or P2 as donor and PC70BM as acceptor can be used as efficient BHJ polymer solar cells. 3.2. Photovoltaic properties
Fig. 2. Optical absorption spectra of the P1:PC70BM and P2:PC70BM blends (a) cast from THF solvent and (b) cast from mixed CN/THF solvents.
absorption spectra of the blend of copolymer (P1 or P2) with PC70BM show broader absorption band as compared to blends with PCBM [29,30], which is attributed to the strong absorption of PC70BM in the lower wavelength region. When the blend film was processed from the mixed solvent (CN/THF), the absorption band in longer wavelength region is red shifted and the intensity also increased (as shown in Fig. 2b). A higher crystallinity degree of copolymer P1 or P2 is indicated by a red shift in absorption band, a clear vibronic shoulder at 715 nm and 680 nm for P1 and P2, which indicates enhanced interchain pep* stacking. The blend film cast from the mixed solvent showed higher crystallinity than cast from THF solvent, which is due to the different vapor pressure and boiling point of the solvents used in mixed solvent. It can be seen from Fig. 1b that the vibronic shoulder in the longer wavelength region is more distinguishable for the film cast from mixed solvent, which indicates that during the film formation cast from the mixed solvent enhanced the crystallinity of the blend. The enhanced shoulder absorption in the film has been observed for many high performance PSC materials, generally indicating a strong intermolecular packing in the solid state caused by their planar and rigid backbones [32,33]. The HOMO level of the P1 and P2 estimated from the cyclic voltammetry data is 5.15 eV and 5.25 eV, respectively, whereas the LUMO level of both P1 and P2 is 3.45 eV. The LUMO level of the
In order to investigate the photovoltaic properties of the copolymers, the BHJ polymer solar cells having structure of ITO/ PEDOT:PSS/P1 or P2:PC70BM/Al, were fabricated, where the copolymers P1 and P2 were used as donors and PC70BM was used as acceptor. We have varied the weight ratio of P1 or P2 and PC70BM and found that optimized performance was achieved with the weight ratio of P1 or P2:PC70BM at 1:1 (w/w). Fig. 3(a) shows the currentevoltage (JeV) characteristics of the devices based on P1 or P2:PC70BM blends cast from THF solvent and the photovoltaic performance data i.e. Jsc, Voc, fill factor (FF) and PCE are complied in Table 1. For comparison the data of the photovoltaic devices based on PCBM as acceptor are also included in Table 1. The BHJ solar cells based on P1:PC70BM and P2:PC70BM cast from THF solvent yield PCE values of 2.56% and 2.34%, respectively. Both P1 and P2 showed better performance using PC70BM as the acceptor than that with the device fabricated with PCBM. The improved PCE can be explained in terms of strong absorption of PC70CM in the visible region, where the both polymers have weak absorption. The increase in the PCE based on the PC70BM as acceptor is due to the increase in both Jsc and Voc. Since the most important factor that determines the value of Voc in the BHJ solar cell is the difference between the HOMO level of donor and the LUMO level of the acceptor, we attribute the increase in the Voc obtained for PC70BM to its higher LUMO value (upward shift toward vacuum level). The increase in the Jsc for the devices based on PC70BM acceptor as compared to PCBM is attributed to the broader absorption of both P1:PC70BM and P2:PC70BM blends, which causes an enhancement in the photogenerated excitons in the blend, resulting in higher photocurrent. The XRD patterns of P1 and P2 show peak at 2q ¼ 8.28 and 7.8, respectively [29,30]. The peak observed in the XRD pattern gives the information about the interlayer spacing of the polymer ordered aggregation. The interlayer spacing estimated from the XRD patterns of P1 and P2 is 13.56 Å and 14.44 Å, respectively. The lower value of interlayer spacing for P1 may lead to improved charge transportation and resulting higher PCE. The PCE of the devices based on P1 or P2:PC70BM blends cast from THF solvent is still low. In solution process, solvent type [35e37], donor and acceptor compositions [38,39], solution concentration [40], incorporation of additive [41e46], and regioregularity of conjugated polymer [47] are considered to develop various morphologies and microstructures of active layer. In general, the morphology of active layer with appropriate phase separation between donor and acceptors and good crystallinity of polymers are favorable for high efficiency solar cells. The above mentioned techniques are also regularly adopted to improve the crystallinity of polymers and phase separation between donor and acceptor. In the active blended layers, polymers often possess both crystalline and amorphous portions. Changes in the crystalline and amorphous portions can give rise to a different morphology and microstructure of active layers. We have investigated the effect of mixed solvent for the thin film processing, on the photovoltaic response of BHJ polymer solar cells. The JeV characteristics of the BHJ devices based on the thin films of blended layer cast from the mixed (CN/THF) solvent are shown in Fig. 3(b) and photovoltaic parameters are complied in Table 1. The overall PCE of the devices based on P1:PC70BM and P2:PC70BM cast from mixed solvent is 4.12% and 3.3%, respectively. The increase in the PCE is mainly
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attributed to the improvement in Jsc and FF. As can be seen from the absorption spectra of the BHJ active layer processed from the mixed solvent that the intensity of the absorption band in longer region is higher and the band also gets broadened as well as red shifted. The broader absorption band implies more excitons are generated in the BHJ layer, resulted more photocurrent. As we have already reported the crystalline nature of the both P1 and P2 has been improved when the film is processed for the mixed solvent, which causes an enhancement in the hole mobility, resulting balanced charge transport [29,30]. The film morphology of the BHJ active layer has been found to be one of the key elements in determining the PCE of the polymer solar cell [48]. To gain better insight into what might be controlling the Jsc and hence lead to enhancement in the PCE of the solar cell based on the films cast from mixed solvent, atomic force microscopy (AFM) was used to examine the surface topography of the blend films cast from THF and mixed solvents. Fig. 4 shows the AFM images of P1:PC70BM blend film cast from THF and mixed CN/THF solvents. As can be seen from these images the domain size becomes finer for the CN/THF cast film as compared to the film cast from THF solvent, which leads to an enhancement in the DeA interfacial area for the exciton dissociation and increases the Jsc and hence PCE. As can be seen from Table 1 that the Voc values for the devices based on the blends THF cast blend are higher than that for the devices based on the blends cast from mixed solvents. This difference is approximately about 0.10 V. The shift in Voc has been attributed to the less negative HOMO position of the copolymers caused by the electronic interaction among the polymer chains in the presence of the higher boiling point solvent, i.e. CN, as reported for other copolymers processed from the mixed solvents [34]. As we have discussed earlier that the optical absorption spectra of the blend cast from the mixed solvents, the both absorption peak and onset corresponds to the copolymers are red shifted. This shift in absorption onset suggests that the copolymer HOMO level experiences an upward energy shift by effect of the solvent used [35]. Such an effect is qualitatively in good agreement with the experimentally observed correlation between the donor HOMO and acceptor LUMO difference (effective band gap of the blend). 3.3. Effect of modified PEDOT:PSS layer
Fig. 3. Currentevoltage (JeV) characteristics of ITO/PEDOT:PSS/P1 or P2:PC70BM/Al BHJ photovoltaic devices, BHJ active layer cast from (a) THF and (b) cast from mixed CN/THF solvents.
Table 1 Photovoltaic parameters of ITO/PEDOT:PSS/P1 or P2:PC70BM/Al BHJ polymer solar cells. Blend
Jsc (mA cm2)
Voc (V)
FF
PCE (%)
P1:PCBM [29] P1:PC70BMa P2:PCBM [30] P2:PC70BMa P1:PC70BMb P2:PC70BMb
4.6 6.3 4.20 5.25 9.2 7.7
0.78 0.90 0.76 0.93 0.80 0.82
0.42 0.50 0.44 0.48 0.56 0.52
1.50 2.56 1.40 2.34 4.12 3.30
a b
Cast from THF solvent. Cast from CN/THF solvent.
PEDOT:PSS film has high transparency in the visible region, high mechanical flexibility and good thermal stability. It has been used extensively as a modification layer on ITO electrode to improve the hole collection in PSCs. Xia and Quang [49] reported that conductivity of PEDOT:PSS film was significantly enhanced after treatment with a cosolvent of water and a common solvent such as ethanol, dimethyl sulfoxide, acetonitrile or tetrahydrofuran and used these films for the application as transparent electrode in PSC [50,51]. Shinar et al. [52] mixed the original PEDOT:PSS solution with ethylene glycol, the treatment improved the PCE of the PSC based on P3HT:PCBM blend by up to 27%. The authors suggested that these additives may affect the morphology and enhance the conductivity of PEDOT:PSS film, hence improved photovoltaic performance of the device. We have investigated the effect of the modification of PEDOT:PSS layer on the photovoltaic performance of the PSCs based on the P1:PC70BM and P2:PC70BM cast from the mixed solvents. The treatment of the organic solvent on PEDOT:PSS was performed by mixing the aqueous solution of PEDOT:PSS with organic solvent of 2-propanol (1:1 by volume). The thickness of the PEDOT:PSS is 45 nm for the unmodified PEDOT:PSS layer. The JeV characteristics of the ITO/PEDOT:PSS (modified)/P1 or P2:PC70BM (cast from mixed solvent)/Al PSCs are shown in Fig. 5 and the
G.D. Sharma et al. / Materials Chemistry and Physics 135 (2012) 25e31
29
Fig. 4. AFM images of the P1:PC70BM thin films cast from THF and CN/THF solvents.
photovoltaic parameters are complied in Table 2. It can be seen from these figures and Table 2 that the Jsc and FF have been improved significantly however the Voc remains almost unchanged. The overall PCE of the devices based on P1:PC70BM and P2:PC70BM is 4.4% and 3.54%, respectively. The increase in the PCE is mainly due to the improvement in Jsc in the devices based on modified PEDOT:PSS electrode. In order to understand the origin of the improved PCE of the devices with organic solvent treated PEDOT:PSS layer, we have investigated the effect of the organic solvent on the transparency, conductivity and surface morphology of the PEDOT:PSS film. We observed that the transmittance of the treated and untreated PEDOT:PSS electrodes in the visible range is similar. Hence, the improved photovoltaic performance of the PSC devices with PEDOT:PSS layer treated with organic solvent is not directly related to the transmission of different PEDOT:PSS layers. The conductivity of PEDOT:PSS films was measured by the four point probe method and it is found that the conductivity of the organic solvent treated PEDOT:PSS (1.18 103 S cm1) layer is higher than that of untreated PEDOT:PSS layer (4.2 104 S cm1). The tendency of the conductivity change is consistent with the tendency of the Jsc and PCE. The improved PV performance with
organic solvent treated PEDOT:PSS resulted from the higher conductivity of the modified layer treated by organic solvent. We have measured the HOMO level of the treated and untreated PEDOT:PSS with the help of the cyclic voltammetry and found that the HOMO levels are about 5.0 eV and 5.15 eV for untreated and treated PEDOT:PSS. We assume that the cosolvent treatment may lower the energy barrier for the charge transport across the PEDOT chains and increases the localization length as reported earlier for the PSC based on P3HT:PCBM blend [53]. We have also recorded the AFM images of the organic solvent treated and untreated PEDOT:PSS layers, and observed that there is a slight increase in the rms value of surface roughness. This increase in the surface roughness may likely increase the contact area between the PEDOT:PSS and active layer, and improve the hole extraction to the anode. On the other hand the rough surface may increase the scattering of incident light back to the active layer and hence leads to the increased absorption. Since the PCE of the PSC depends upon the light harvesting efficiency of the active layer and charge collection efficiency of the electrodes, we conclude that the modified PEDOT:PSS not only improves the light harvesting property of the PSC but also improves the hole collection efficiency and combination of these two effect increases the PCE of PSC. 3.4. Effect of TiO2 buffer layer As the LUMO of the PC70BM is about 3.75 eV, and the work function of the cathode electrode (Al) is about 4.2 eV. This difference between LUMO level of PC70BM and the work function of Al induces a barrier for the electron extraction and leads to lower the PCE of the PSCs. One of the main strategies employed to enhance the PCE is to engineer the cathode electrode to increase the electrode extraction efficiency, by sandwiching a buffer layer between the active layer and cathode electrode [54]. A variety of materials have been deposited through vacuum evaporation or solution
Table 2 Photovoltaic parameters of ITO/PEDOT:PSS(modified)/P1 or P2:PC70BMa/Al and ITO/ PEDOT:PSS (modified)/P1 or P2:PC70BMa/TiO2/Al BHJ polymer solar cells. Device
Fig. 5. Currentevoltage (JeV) characteristics of ITO/PEDOT:PSS (modified)/P1 or P2:PC70BM (mixed solvent cast)/Al BHJ photovoltaic devices.
ITO/PEDOT:PSS(modified)/ P1:PC70BMa/Al ITO/PEDOT:PSS (modified)/ P2:PC70BMa/Al ITO/PEDOT:PSS(modified)/ P1:PC70BMa/TiO2/Al ITO/PEDOT:PSS(modified)/ P2:PC70BMa/TiO2/Al a
Jsc (mA cm2)
Voc (V)
FF
PCE (%)
9.8
0.80
0.56
4.4
8.3
0.82
0.52
3.54
10.3
0.82
0.60
5.07
8.7
0.82
0.56
4.14
Cast from CN/THF mixed solvent.
30
G.D. Sharma et al. / Materials Chemistry and Physics 135 (2012) 25e31
processes as the buffer layer, such as metals (Ca, Ba) [55,56], transition metal oxides (oxides of Cr, Zn, Ni) [57e59] and organic materials [60]. TiO2 has been synthesized by synthetic routes [60,61], is commonly used for photovoltaic devices as n-type semiconducting material and has been employed as photoanode for dye sensitized solar cells [62]. Recently, solution processed titanium sub-oxide (TiOx) has been applied in both single and tandem organic photovoltaic devices as a multifunctional buffer layer serving as optical spacer and electron transport/hole blocking layer [63e66]. We introduce a buffer layer of solution processed TiO2 nanoparticles (spin coated at 3000 rpm for 60 s) after the deposition of the active polymer:fullerene blended layer and dried at the temperature of 60 C. The JeV characteristics of the ITO/PEDOT:PSS (modified)/P1 or P2:PC70BM (mixed solvent)/TiO2/Al devices shown in Fig. 6 and the photovoltaic parameters are complied in Table 2. Compared to the device without TiO2 layer, the devices we fabricated with TiO2 layer give a PCE of 4.14% and 5.07% for P2:PC70BM and P1:PC70BM blends, respectively. For the devices with TiO2 buffer layer, the PCE improvement is mainly due to the increase in Jsc and FF, whereas the Voc remains almost same. It is well known that the Voc is directly related to difference in the HOMO of donor and LUMO of acceptor material used in the active layer. Although the additional TiO2 layer may help to break the symmetry of electric field in the device, its conduction band edge, which is close to the Fermi level of Al, does not have much contribution to Voc. The energy level diagram of ITO/PEDOT:PSS/P1:PC70BM/TiO2/Al device is shown in Fig. 7. Since the conduction band edge of the TiO2 (4.3 eV) matches with the Fermi level of the Al (4.2 eV) and TiO2 is also a good electron transporter may facilitate the electron transfer from the LUMO of the PC70BM (3.75 eV) to Al [67]. On the other hand, the valence band edge TiO2 (7.5 eV) is deeper than that of HOMO level of P1 (5.15 eV), resulting the prevention of the hole accumulation at the active layereAl interface, which leads to a more reduced interfacial charge recombination. Recently, Lee et al. found that the TiOx layer in organic photovoltaic device decreases the saturation current, resulting in reduced minority carrier density [67]. TiO2 with deeper
Fig. 6. Currentevoltage (JeV) characteristics of ITO/PEDOT:PSS(modified)/P1 or P2:PC70BM (mixed solvent cast)/TiO2/Al BHJ photovoltaic devices.
Fig. 7. Energy level diagram of ITO/PEDOT:PSS/P1:PC70BM/TiO2/Al polymer solar cell.
conduction band edge provides higher energy barrier for holes and therefore reducing the charge carrier injection process and leading to improvement in the FF. Therefore, TiO2 acts as both electron transporting layer as well as hole blocking layer in the device. Besides this, the introduction of the TiO2 buffer layer can redistribute the light intensity in the blend film, i.e. as an optical spacer, which enhances the photon harvesting property. The series resistance (Rs) and shunt resistance (Rsh), calculated from the inverse of the JeV characteristics at V ¼ 0 and V ¼ Voc, respectively, found that the incorporation of TiO2 buffer layer lowers the Rs, while increases the Rsh. The reduction in Rs correlates consistently with the enhancement in Jsc [68]. The incident photon to current efficiency of the devices for the devices based on P1:PC70BM cast from mixed solvent with and without TiO2 layer is shown in Fig. 8. Similar results have been observed for other devices. It can be seen from this figure that the value of IPCE has been increased with the incorporation of TiO2 layer in between the BHJ active layer and cathode Al electrode. The increased value of IPCE attributed to the better collection of electrons by the cathode leading to the higher value of overall PCE.
Fig. 8. Incident photon to current efficiency (IPCE) spectra of (a) ITO/PEDOT:PSS (modified)/P1:PC70BM (mixed solvent)/Al and (b) ITO/PEDOT:PSS (modified)/ P1:PC70BM (mixed solvent)/TiO2/Al devices.
G.D. Sharma et al. / Materials Chemistry and Physics 135 (2012) 25e31
4. Conclusions We have used two low band gap copolymers P1 (alternating phenylenevinylene with thiophene and pyrrole units) and P2 (alternating phenylenevinylene with dithenyl (thienothiadiazole) segments along the backbone) having band gap 1.65 eV and 1.74 eV, respectively, as electron donor along with PC70BM as electron acceptor for the fabrication of BHJ solar cells. The PCEs of BHJ solar cells based on P1:PC70BM and P2:PC70BM cast from the THF solvent are 2.56% and 2.34%, respectively. These values of PCEs are higher than for the devices based on PCBM as electron acceptor which is attributed to the strong absorption of PC70BM in visible region than that for PCBM and also to the higher LUMO level of PC70BM as compared to PCBM. We have also fabricated the PSCs based on P1:PC70BM and P2:PC70BM blends cast from the mixed CN/THF solvents and achieved overall PCE of 4.12% and 3.3%, for P1:PC70BM and P2:PC70BM blends respectively. This increase has been attributed to the effective induced higher copolymer crystallinity and also to the more fine film morphology on nanoscale, of the blend film cast from the mixed solvents, which results higher light harvesting property of efficient exciton dissociation due to the increased interfacial D/A interface. The PCE of the PSCs has been improved up to 4.4% and 3.54% for P1:PC70BM and P2:PC70BM blends cast from mixed solvent, when PEDOT:PSS is modified by the addition on 2proponal in the aqueous PEDOT:PSS solution. This increase is attributed to the enhancement in the conductivity of PEDOT:PSS and larger domain of the PEDOT:PSS, resulted an increase in the hole collection efficiency in the device. Finally, the PCE of the PSC based on modified PEDOT:PSS electrode and P1:PC70BM and P2:PC70BM blends cast from the mixed solvent has been improved up to 5.07% and 4.14%, respectively, when a thin layer of TiO2 processed from sol gel method is inserted in between active layer and Al electrode. We assume that this increase in PCE may be the fact that the TiO2 layer acts as an electron transporting and hole blocking layer, improving the electron extracting efficiency. References [1] F.C. Krebs, T.D. Nielsen, J. Fyenbo, M. Wadstrom, M.S. Pedersen, Energy Environ. Sci. 3 (2010) 512e525. [2] C.J. Brabec, S. Gowrisanker, J.M. Halls, L. Darin, S. Jia, S.P. Williams, Adv. Mater. 22 (2010) 3839e3856. [3] C. Li, M. Liu, N.G. Pschirer, M. Baumgarten, K. Mullen, Chem. Rev. 110 (2010) 6817e6855. [4] D. Gendron, M. Leclerc, Energy Environ. Sci. 4 (2011) 1225e1237. [5] N. Espinosa, R. Garcýa-Valverdea, F.C. Krebs, Energy Environ. Sci. 4 (2011) 1547e1557. [6] B.C. Thompson, P.P. Khlyabich, B. Burkhart, A.E. Aviles, A. Rudenko, G. Shultz, C.F. Ng, L.B. Mangubat, Green 1 (2011) 29e54. [7] B.C. Thompson, J.M.J. Frechet, Angew. Chem. Int. Ed. 47 (2008) 58e77. [8] G. Dennler, M.C. Scharber, C.J. Brabec, Adv. Mater. 21 (2009) 1323e1338. [9] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Adv. Funct. Mater. 15 (2005) 1617e1622. [10] G. Li, V. Shrotriva, J.S. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nat. Mater. 4 (2005) 864e868. [11] J.H. Hou, Z.A. Tan, Y. Yan, Y.J. He, C.H. Yang, Y.F. Li, J. Am. Chem. Soc. 128 (2006) 4911e4916. [12] D. Muhlbacher, M.C. Scharber, M. Morana, Z.G. Zhu, D. Waller, R. Gaudiana, C.J. Brabec, Adv. Mater. 18 (2006) 2884e2889. [13] P.L.T. Boudreault, A. Najari, M. Leclerc, Chem. Mater. 23 (2011) 456e469. [14] J. Peet, J.Y. Kim, N.E. Coates, W.L. Ma, D. Moses, A.J. Heeger, G.C. Bazan, Nat. Mater. 6 (2007) 497e500. [15] Y. Zhang, J.Y. Zou, H.L. Yip, Y. Sun, J.A. Davies, K.S. Chen, O. Acton, A.K.Y. Jen, J. Mater. Chem. 21 (2011) 3895e3902. [16] G.Y. Chen, Y.H. Cheng, Y.J. Chou, M.S. Su, C.M. Chen, K.H. Wei, Chem. Commun. 47 (2011) 5064e5066. [17] M. Wang, X. Hu, P. Liu, W. Li, X. Gong, F. Huang, Y. Cao, J. Am. Chem. Soc. 133 (2011) 9638e9641. [18] E. Wang, Z. Ma, Z. Zhang, P. Henriksson, O. Inganas, F. Zhang, M.R. Andersson, Chem. Commun. (2011) 4908e4910. [19] Y. Lee, Y.M. Nam, W.H. Jo, J. Mater. Chem. 21 (2011) 8583e8590. [20] Y. Liang, Z. Xu, J. Xia, S. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, Adv. Mater. 22 (2010) E135eE138.
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