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International Journal of Photoenergy
Volume 2014, Article ID 786468, 8 pages
http://dx.doi.org/10.1155/2014/786468
Research Article

Effects of Organic Solvents for Composite Active Layer of PCDTBT/PC71BM on Characteristics of Organic Solar Cell Devices

1Department of Electrical Engineering, Inha University, 100 Inha-ro, Nam-gu, Incheon 402-751, Republic of Korea
2Departement of Electrical Engineering, Aichi Institute of Technology, Aichi, Toyota 470-0392, Japan

Received 8 May 2013; Revised 23 November 2013; Accepted 25 November 2013; Published 29 January 2014

Academic Editor: Mahmoud M. El-Nahass

Copyright © 2014 Paik-Kyun Shin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Bulk heterojunction (BHJ) structure based active layers of PCDTBT/PC71BM were prepared by using different organic solvents for fabrication of organic solar cell (OSC) devices. Mixture of precursor solutions of PCDTBT/PC71BM in three different organic solvents was prepared to fabricate composite active layers by spin-coating process: chloroform; chlorobenzene; o-dichlorobenzene. Four different blend ratios (1 : 3–1 : 6) of PCDTBT: PC71BM were adopted for each organic solvent to clarify the effect on the resulting OSC device characteristics. Surface morphology of the active layers was distinctively affected by the blend ratio of PCDTBT/PC71BM in organic solvents. Influence of the blend ratio of PCDTBT/PC71BM on the OSC device parameters was discussed. Performance parameters of the resulting OSC devices with different composite active layers were comparatively investigated. Appropriate blend ratio and organic solvent to achieve better OSC device performance were proposed. Furthermore, from the UV-Vis spectrum of each active layer prepared using the PCDTBT/PC71BM mixed solution dissolved with different organic solvents, a possibility that the nanophase separation structure inside their active layer could appear was suggested.

1. Introduction

The 2007 Nobel Prize for peace was awarded to Al Gore and intergovernmental panel on climate change (IPCC). The IPCC has created an ever-broader informed consensus about the connection between human activities and global warming. Furthermore, the IPCC predicted climate change would significantly impact every aspect of human life in this generation [1]. On replying to the issue, in the 34th G8 Toyako summit, it has been agreed to cut carbon emissions by at least 50 percent by 2050 [2]. To realize that goal, development of new energy sources is inevitable. As for new energy, several candidates have been suggested: wind power; solar energy; biomass energy; geothermal energy. Usage of solar energy would be one of the most promising technologies, because the solar energy has the greatest potential to meet world’s growing electricity needs compared to other renewable energy sources. Production of solar cells has grown drastically with annual growth rate of over 40% in recent years, and the output in 2007 has reached over 3.5 GW, which corresponds to 3.5 nuclear power plants of 100 M kW output [3].

From the beginning of organic solar cell (OSC) having power conversion efficiency (PCE) of ~1% [4], various research works have been devoted to enhance the performance [57], which is still lower than the marginal threshold of 10% and quite inferior to those of the Si based counterparts. Nevertheless, OSC attracts more and more attention due to advantages such as light weight, flexibility, and easy and low-cost fabrication process [810]. Researchers have adopted many ways to achieve high power conversion efficiency (PCE) in OSCs and to cover a large area at low cost [11]. Among the various polymers used, conjugated small-band-gap polymer-based active layers yielded high PCE of 5–9% [1215]. However, PCEs greater than 10% are needed for commercial viability [16]. Power conversion efficiency (PCE) of OSC device could be possibly enhanced by several factors: (1) increase of light absorption area; (2) improvement of charge carrier transport; (3) higher efficiency of exciton generation [17]. To improve performance of OSC devices, bulk heterojunction (BHJ) structure with blended active layer of electron accepter and electron donor has been studied, where interface area between active bilayer could be drastically enlarged and thereby charge separation efficiency could be improved [1820]. Organic polymer of poly [N-9′′-heptadeca-nyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzo-thiadiazole)] (PCDTBT) with deeper highest occupied molecular (HOMO) level and shallow band gap has been focused as a promising material for electron donor layer in BHJ structure [21]. Increase of light absorption area and open circuit voltage () could be achieved by using the PCDTBT as an electron donor material [21]. The extended red absorption of such materials can lead to improved performance via the harvesting of a greater fraction of the sun’s radiation. Solar cells based on PCDTBT : PC71BM blend show high PCE of 5.6% [22]. Park et al. reported a PCE of 6.1% for a solar cell with a titanium suboxide as an optical spacer between the PCTDBT : PC71BM active layer and the cathode [13]. The addition of two dipolar solvents of dimethyl sulfoxide (DMSO) and dimethyl formamide (DMF) to the 1,2-dichlorobenzene solution of PCDTBT led to an improved PCE of 6–7.1% [23]. Wang et al. reported a PCE of 7.1% for devices prepared by spin-coating 1,2-dichlorobenzene : chlorobenzene solution of PCDTBT : PC70BM (3 : 1) with silver nanoparticles used in the active layer [24]. As for an electron accepter material, a fullerene material of -phenyl-C71-butyric acid methyl ester (PC71BM) has been proposed due to superior electron transporting property [25].

In the present work, varieties of organic solar cell devices were fabricated using currently most representative conjugated polymers of PCDTBT and fullerene of PC71BM. Precursor mixture solutions of the PCDTBT and PC71BM were prepared with various compositions in three different organic solvents: (1) 1 wt% of chloroform; (2) 2 wt% of chlorobenzene; (3) 3 wt% of o-dichlorobenzene. The precursor mixture solutions were then used for spin-coating process to fabricate BHJ structure based OSC devices. Critical surface parameters of the resulting organic active layers were extracted from surface morphology by using atomic force microscopy (AFM). Standard performance parameters were then investigated for the OSC devices having different blend ratio of PCDTBT and PC71BM. Effects of the organic solvents on the characteristic of the resulting OSC devices were discussed to suggest an appropriate organic solvent to achieve better performance parameter.

2. Experimental Procedure

2.1. Preparation of Precursor Solutions for Organic Polymer Active Layers

To prepare a bulk heterojunction of organic polymer active layers for organic solar cell device, electron donor layer was prepared with PCDTBT having relatively deeper HOMO level of 5.45 eV. In case of electron acceptor, representative fullerene material of PC71BM with better electron transport property was used. It has been reported that morphology of the active layer could play an important role in the performance of the resulting OSC device, which could be influenced by organic solvent and blend ratio for active layers [2628]. To clarify the influence of the organic solvent on the characteristic and/or the OSC device performance, three different organic solvents were utilized to prepare precursor solution mixtures for blended active layer preparation: (1) chloroform of 1 wt%; (2) chlorobenzene of 2 wt%; (3) o-dichlorobenzene of 3 wt%. Different precursor solution mixtures of PCDTBT and PC71BM with various blend ratios were prepared using the organic solvents: (1) PCDTBT : PC71BM (1 : 3~1 : 6) dissolved in 1 wt% of chloroform; (2) PCDTBT : PC71BM (1 : 3~1 : 6) dissolved in 2 wt% of chlorobenzene; (3) PCDTBT : PC71BM (1 : 3~1 : 6) dissolved in 3 wt% of o-dichlorobenzene.

2.2. Organic Thin Film Solar Cell Device Fabrication

ITO coated glass (Asahi Glass;) was used as substrates. ITO as anode electrode was patterned with 1 cm line width by using a hard mask and exposing them in hydrochloride acid (HCl) vapor ambient. The patterned ITO substrates were cleaned with distilled water to remove residual hydrochloride on the surface. Then the substrates were further cleaned ultrasonically prior to organic layer deposition in neutral detergent, acetone, ethanol, and DI water in each 10 min sequence. As hole transporting buffer layer, a conducting -conjugated polymer of poly (3,4-ethylenedioxythiophene) : poly (styrenesulfonate) [PEDOT : PSS; Heraeus Clevios P AI4083] thin film of 50 nm thickness was deposited by spin-coating technique. Prior to the PEDOT : PSS deposition, the ITO patterned substrates were exposed to UV in vacuum for 10 min to modify its surface into hydrophilic state for better adhesion. The PEDOT : PSS buffer layer was deposited by using the following spin-coating process: (1) rotation with 3000 rpm for 30 s and (2) drying in oven at 120°C for 10 min. Thickness of the PEDOT : PSS layer was 50 nm.

The prepared precursor solutions of PCDTBT : PC71BM mixture were used in a spin-coating process to prepare bulk heterojunction structure active layer of the organic solar cell devices: rotation speed of 1000 rpm and rotation time of 60 s. Then the spin-coated active layers were dried in a vacuum oven at 120°C for 10 min, while the samples based on the organic solvent of o-dichlorobenzene were dried in vacuum for one day. Finally, cathode electrode of the organic solar cell device was deposited by thermal evaporation of Al (100 nm) in vacuum. Effective light absorption surface area of the resulting OSC device was 0.3 cm2. Figure 1 shows a schematic structure of the resulting OSC device.

786468.fig.001
Figure 1: Structure of organic solar cell device with heterojunction active layer structure of PCDTBT : PC71BM and buffer layer of PEDOT : PSS.
2.3. Characterization of Organic Active Layers and Organic Solar Cell Devices

To investigate effect of various blended active layers of PCDTBT : PC71BM on performance of the resulting OSC devices, surface morphology of the various PCDTBT : PC71BM layers was analyzed by using AFM (Seiko Instruments SPA400-SPI4000). The measured surface parameters were discussed in relation to performance parameters of the resulting solar cell device. Absorption spectra of the blended active layer of PCDTBT and PC71BM were investigated by using a UV-Vis spectrometer (Shimadzu UV2450). Standard parameters of the fabricated organic thin film solar cell device were obtained from current density ()–applied voltage () characteristics measured by using an AC meter (Advantest R6441) in an commercial solar simulator: light source from Xenon lamp; air mass filter adjusted for AM1.5; light intensity of 100 mW/cm2. All measurements were carried out in atmosphere at room temperature. Open circuit voltage (), short circuit current density (), fill factor (FF), and power conversion efficiency (PCE) were obtained for the organic solar cell devices, respectively.

3. Results and Discussion

Thickness of the active layer plays decisive role in determining the photovoltaic device characteristics. In order to optimize the active layer morphology and blend ratio of PCDTBT : PC71BM films and analyze their influence on the device characteristics, OSCs with different blend ratios of 1 : 3, 1 : 4, 1 : 5, and 1 : 6 were fabricated with various organic solvents. Blend ratios dependant thickness of PCDTBT : PC71BM active layer prepared with different solvents of chloroform, chlorobenzene, and o-dichlorobenzene was shown in Figure 2. The thickness of the PCDTBT : PC71BM layer gradually decreases while increasing the PC71BM acceptor in the case of chlorobenzene and o-dichlorobenzene, whereas it is random for chloroform. We observed that layer thickness varied between 78 and 94 nm with different blend ratios prepared from different solvents. It has been reported that the highest efficiency was obtained based on PCDTBT : PC71BM solar cells with thickness of active layers between 70 and 90 nm [29].

786468.fig.002
Figure 2: Thickness of PCDTBT : PC71BM active layer films with different composition prepared in solvents of chloroform (1 wt%), chlorobenzene (2 wt%), and o-dichlorobenzene (3 wt%).

Surface morphology of active layer could play an important role for performance improvement of organic thin film solar cell device, which could be influenced by blend ratio of electron donor and accepter polymer. It has been reported that the surface morphology of the active layer strongly influences the device properties [30, 31]. In addition, thickness and morphology of the polymer active layer could be affected by organic solvent due to appropriate solubility of blended polymer combination. In order to obtain the effect of blend ratios and different solvents on the active layer, AFM analysis was carried out to observe the surface morphology. Although the active layers are prepared with the same spin speed (1000 rpm for 60 s) obvious change in surface morphology could be seen in their AFM images due to various blend ratios and solvents. Figure 3 shows AFM images (2D view) of different active layers of PCDTBT : PC71BM prepared with organic solvents of chloroform (1 wt%), chlorobenzene (2 wt%), and o-dichlorobenzene (3 wt%), respectively, where blend ratios of PCDTBT and PC71BM were varied from 1 : 3 to 1 : 6. As can be seen in these images, PCDTBT : PC71BM blend films exhibit large size granular domains with the size of hundreds of nanometers for films prepared from chloroform and chlorobenzene. The size of granular domains increases while increasing the PC71BM ratio in the PCDTBT : PC71BM active layer. Considering the polymer of PCDTBT presented in this report is amorphous phase, increase of PCBM composition might result in formation of aggregates on the blended active layers, and the aggregate observed for chloroform and chlorobenzene solvent could be thought of as aggregates of PC71BM [32]. There was no clear phase separation observed and these types of morphologies with less bicontinuous networks are not suitable for efficient exciton dissociation to the interface of polymer fullerene which was not favorable for charge separation. On the contrary, the PCDTBT : PC71BM samples prepared by using another organic solvent of o-dichlorobenzene (Figure 3) reveal no aggregates of PC71BM, which could be due to higher solubility of PC71BM in o-dichlorobenzene than that in chlorobenzene [23, 33]. When PCDTBT : PC71BM films with different ratios were prepared in o-dichlorobenzene, the surface morphology shows remarkable changes as shown in Figure 3. The films contain more elongated fiber-like nanodomains over the entire area. The AFM images of the PCDTBT : PC71BM in o-dichlorobenzene indicates well miscibility between PCDTBT and PC71BM. With an increase of PC71BM content in the films, the domain size of the PC71BM aggregates is decreased and phase separation takes place compared to films prepared with chloroform and chlorobenzene. This can be attributed to low vapor pressure of o-dichlorobenzene (1.2 mm Hg). As it stays long time in thin film before drying, it renders a better mixing ability between PCDTBT and PC71BM, leading to the formation of interpenetrating network. Therefore, the large size granular domains remain under control in the films prepared with o-dichlorobenzene than those prepared with chloroform and chlorobenzene, despite increasing the PC71BM ratio. In addition, better organization of fiber-like interpenetrating networks of PCDTBT : PC71BM restricts overgrowth of large size PC71BM aggregates.

786468.fig.003
Figure 3: AFM images of the PCDTBT : PC71BM films prepared by using organic solvents of chloroform (1 wt%), chlorobenzene (2 wt%), and o-dichlorobenzene (3 wt%): blend ratio of PCDTBT and PC71BM was varied from 1 : 3 to 1 : 6.

Root-mean-square (RMS) roughness of the different PCDTBT : PC71BM combinations prepared in different solvents was extracted after the AFM analysis and presented in Figure 4. RMS surface roughness values measured for 1 : 3, 1 : 4, 1 : 5, and 1 : 6 PCDTBT : PC71BM blend ratios were ~1.02, 0.79, 1.1, and 1.04 nm for chloroform films; ~1.08, 1.98, 2.03, and 1.5 nm for chlorobenzene; ~0.6, 0.74, 0.56, and 0.59 nm for o-dichlorobenzene, respectively. For chloroform films, initially the roughness decreased until 1 : 4 ratio and increased afterwards, whereas reverse trend was observed for o-dichlorobenzene films. For chlorobenzene films, the roughness increases gradually up to 1 : 5 ratio and suddenly decreased afterward. The highest roughness for PCDTBT : PC71BM blend ratio of (1 : 5), (1 : 5), and (1 : 4) film prepared in chloroform, chlorobenzene, and o-dichlorobenzene was 1.1, 2.03, and 0.74 nm, respectively. It is obvious that the high surface roughness of the film might increase the contact area between the active layer and the contact electrode and thus increase the charge collection. Furthermore, the increased surface roughness may increase internal reflection in the active layer and improve light collection leading to increase in device efficiency [34].

786468.fig.004
Figure 4: Surface roughness values obtained from the AFM images for PCDTBT : PC71BM films prepared with organic solvents of chloroform (1 wt%), chlorobenzene (2 wt%), and o-dichlorobenzene (3 wt%).

To investigate effect of surface morphology on optical absorbance of active layer of PCDTBT : PC71BM, UV-Vis spectra of the active layers prepared by using different organic solvents were investigated. Optical absorbance of organic active layer is directly related to overall performance of the resulting OSC device. Figure 5 shows the UV-vis absorption spectra of PCDTBT : PC71BM films prepared from chloroform, chlorobenzene, and o-dichlorobenzene. It is important to note that all these films were spin-coated from the solution with the same rotation speed of 1000 rpm for 60 s. The spectra are not normalized. From Figure 5, the blend layer prepared from chloroform absorbs the less amount of light, whereas layer prepared from chlorobenzene solvent absorbs more light, and o-dichlorobenzene solvent prepared films absorb less amount of light than chlorobenzene solvent, but high absorption takes place at red wavelength region as seen from Figure 5. It is to be noted that the change in film thickness was observed for these solvents. This may be attributed to less viscous nature of the blend solution prepared from chloroform and o-dichlorobenzene; therefore the blend film is thin under the same spin-coating rotation, which causes less light absorption. At the same time, the solution is more viscous when chlorobenzene used as solvent for active layer. Consequently, the film becomes thick and absorbs more light than chloroform and o-dichlorobenzene. It implies that choice of organic solvent and blend ratio of PCDTBT and PC71BM could have decisive influence on performance of the resulting OSC device.

786468.fig.005
Figure 5: Absorption spectra of active layer of PCDTBT : PC71BM prepared by using organic solvents of chloroform, chlorobenzene, and o-dichlorobenzene with PCDTBT : PC71BM ratios of (1 : 5), (1 : 4), and (1 : 4), respectively.

The OSC devices were optimized by varying the ratios of the PCDTBT : PC71BM and the solvents of blend film. The photovoltaic parameters obtained for various devices are presented in Table 1. Figure 6 shows the current density ()–voltage () curves under AM 1.5 conditions (100 mW/cm2) of PCDTBT : PC71BM blend layer prepared from chloroform, chlorobenzene, and o-dichlorobenzene with blend ratio of 1 : 5, 1 : 4, and 1 : 4, respectively. of all the OSC devices reveals approximately 0.9 V, which corresponds to the difference between the LUMO level of PC71BM and the HOMO level of PCDTBT [35]. From the results we observed similar open-circuit voltage () with small random variation with different blend ratios prepared in different solvents of chloroform, chlorobenzene, and o-dichlorobenzene indicating similar characteristics of all solar cells. It is well known that the is influenced by density of defect states at the interface in solar cells [36]. PCDTBT : PC71BM devices with active layer thicknesses between 70 and 90 nm have single state defect trap-assisted centre whereas multilevel recombination centers play major role infor the layer thickness beyond 110 nm [37]. In our study, blend layer thickness lies between 78 and 93 nm; the change in may be attributed to possible recombination process via single-defect trap-assisted centers. Moreover, a small reduction in was observed in other solar cells with increase of PCBM concentration [38, 39]. It might be caused by leakage current, dissociated charge loss at the electrodes due to band bending, and shift of charge transfer state while increasing the PC71BM ratio in blend layer [3941]. Though small variations are seen in , the , the FF, and the PCE of the devices vary significantly with blend ratios as seen from Table 1.

tab1
Table 1: Photovoltaic parameters of the OSC devices with different solvents and different PCDTBT : PC71BM blend ratios.
786468.fig.006
Figure 6: Current density (J)–voltage (V) characteristics of ITO/PEDOT : PSS/PCDTBT : PC71BM/Al devices with an active layer prepared by using organic solvents of chloroform (D/A; 1 : 5), chlorobenzene (D/A; 1 : 4), and o-dichlorobenzene (D/A; 1 : 4).

The best optimized device for PCDTBT : PC71BM with ratio of 1 : 5 prepared from chloroform as solvent demonstrated the of 0.90 V, theof 5.09 mA/cm2, the FF of 0.56, and the PCE of 2.55%. The device with 1 : 4 blend ratio prepared using chlorobenzene as solvent exhibits best performance with the of 0.93 V, of 5.55 mA/cm2, FF of 0.56, and PCE of 2.91%. For chloroform solvent films the varied from 4.39 to 5.78 mA/cm2 when the ratio was increased from 1 : 3 to 1 : 4. Further increasing the PC71BM concentration drops suddenly. For chlorobenzene solvent films random variation of was observed with increasing PC71BM amount. This can be attributed to poor active layer morphology with large size domains and no interpenetrating networks, which limits charge carrier transport, resulting in lower . One of the effective ways to improve light absorption is increasing the active layer thickness and thus increasing the . However, the device efficiency is dramatically decreased with increasing active layer thickness due to the increased charge recombination in these cells [42]. The device with 1 : 4 ratio prepared from o-dichlorobenzene as solvent demonstrates the best performance with the of 0.90 V, the of 8.41 mA/cm2, the FF of 0.55, and the PCE of 4.15%. Relatively higher boiling point of o-dichlorobenzene could possibly be effective to control the nanoscale morphology of blended PCDTBT : PC71BM. Effective formation of those interfaces might result in such higher PCE. The same trend of variations in chloroform solvent film was observed too for o-dichlorobenzene solvent film. The values for chloroform and chlorobenzene solvent films are low when compared with o-dichlorobenzene films due to absence of nanoscale interpenetrating network in the active layer.

The highest(8.41 mA/cm2) and the PCE (4.15%) were observed (Figure 6 and Table 1) for the PCDTBT : PC71BM (1 : 4) blend film prepared from o-dichlorobenzene solvent, and this implies that the nanoscale fiber-like phase separation morphology (Figure 3) forms effective individual pathways for holes and electrons, which enhance the charge collection efficiency. From Table 1, it is clear that the values continuously decrease when increasing PC71BM concentration in the blend layer. It indicates that concentration of PC71BM in the blend film has strong impact on and FF, which leads to lower PCE [43]. Regarding the results presented in this study, the blend ratios of (1 : 5) and (1 : 4), (1 : 4) would be preferable for the organic solvents of chloroform, chlorobenzene, and o-dichlorobenzene, respectively. Maximum PCE value observed in the present study is moderate, when compared with highest reported values [44] and this can be improved by further optimization of the devices via adding transport layers, mixture of solvents, and solvent additives.

4. Summary and Conclusions

In summary, varieties of OSC devices were fabricated based on representative blended organic polymer active layers of PCDTBT : PC71BM in combination with a conducting -conjugated polymer of PEDOT : PSS as hole transporting buffer layer. The blended active layers of PCDTBT : PC71BM were prepared by spin-coating technique, with different blend ratios of 1 : 3 to 1 : 6 being adopted for three different organic solvents of chloroform (1 wt%), chlorobenzene (2 wt%), and o-dichlorobenzene (3 wt%), respectively. Surface morphology of the blended active layers was investigated by AFM, through which surface roughness parameters were studied in relation to blend ratio and organic solvent. It was confirmed that the blend ratio and organic solvent influenced directly the surface morphology of the blended active layers, which in turn could be decisive to performance parameters (, , FF) of the resulting OSC devices. Formation of fine nanoscale morphology structure of blended PCDTBT : PC71BM could be influenced by the combination of organic solvent and blend ratio. O-dichlorobenzene (3 wt%) was proved to be the most appropriate choice among the three different organic solvents presented in this study to achieve better OSC performance. The device with (1 : 4) blend ratio prepared from o-dichlorobenzene as solvent demonstrates the best performance with the of 0.90 V, the of 8.41 mA/cm2, the FF of 0.55, and the PCE of 4.15%. Effective formation of fiber-like nanostructure in the active layer might result in higher PCE () for the case of PCDTBT : PC71BM blend prepared from o-dichlorobenzene.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Authors’ Contribution

Paik-Kyun Shin and Palanisamy Kumar contributed equally to this work.

Acknowledgments

This research was partly supported by the MEXT Private University Project Grant under Contract no. S1001033, JST Adaptable and Seamless Technology Transfer Program (R&D no. AS232Z02610B), and Joint research between Aichi Institute of Technology and NDS.

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