Fabrication of organic solar cell utilizing mixture of solution - Processable phthalocyanine and fullerene derivative with processing additive solvent

VNU Journal of Science: Mathematics – Physics, Vol. 36, No. 1 (2020) 13-19 13 Original Article  Fabrication of Organic Solar Cell Utilizing Mixture of Solution-processable Phthalocyanine and Fullerene Derivative with Processing Additive Solvent Quang-Duy Dao * Faculty of Physics, VNU University of Science, 334 Nguyen Trai,Thanh Xuan, Hanoi, Vietnam Received 08 October 2019 Revised 26 November 2019; Accepted 03 December 2019 Abstract: We demonstrate an efficient bulk heterojunction (BHJ) organic solar cell (OSCs) utilizing a soluble phthalocyanine derivative, 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH2), and a fullerene derivative, 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C71 and roles of processing additive solvent on improvement of the BHJ OSCs. By adding processing additive solvent, filling factor and short-circuit current density are improved to 0.57 and 8.6 mA/cm2, respectively. As a result, the power conversion efficiency of 3.6% is achieved. Otherwise, the effects of processing additive solvent are demonstrated by taking the absorption and photoluminescence spectra of C6PcH2 and composite thin films into account. Keywords: Phthalocyanine, organic solar cells, thin film, small molecule, processing additive. 1. Introduction Organic solar cells (OSCs) utilizing solution-processable small-molecule (SM) donors mixed with fullerene derivative have considered as an alternative to OSCs based on the conventional conjugated polymers [1-6]. Comparing with polymer-based donors, SM-based donors expose the dominant characteristics, such as relatively high charge carrier mobility, well-defined structures without end group contaminants, and simple synthesis and purification. OSCs based on SM donors in both of conventional and inverted structures have exhibited the relatively high photovoltaic performance with the power conversion efficiencies (PCEs) exceeding 10% [1]. ________ Corresponding author. Email address: daoquangduy@hus.edu.vn https//doi.org/ 10.25073/2588-1124/vnumap.4402 Q.-D. Dao / VNU Journal of Science: Mathematics – Physics, Vol. 36, No. 1 (2020) 13-19 14 Discotic liquid crystalline (DLC) materials have been reported as potential SM donors in bulk heterojunction (BHJ) OSC field [7-9]. DLC materials exhibit the appealing characteristics, such as the strong optical absorption at visible-light ranges, the large exciton diffusion length, and the relatively high charge carrier mobility [8]. Furthermore, the high solubility in conventional organic solvents and the strong self-organizing nature which leads to relatively easy fabrication of high quality thin films with large areas of mono-domain have made DLC materials more suitable for BHJ OSC application [10-14]. In this study, we demonstrate the relatively high photovoltaic performance OSCs utilizing non- peripherally substituted octahexyl phthalocyanine (C6PcH2) (Fig. 1(a)) SM donor mixed with a fullerene derivative, 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C71 (PC70BM) and the effects of processing additive on the device photovoltaic performance. 2. Experimental Procedure 2.1. Sample and device fabrication BHJ OSCs, which had the device architecture as shown in Fig. 1(b), were prepared under optimized conditions in accordance with the previous literature [10]. In particular, indium tin oxide (ITO)-coated glass was patterned by wet-etching method using hydrochloric acid at 40 ºC. The patterned ITO-coated glasses were then cleaned with detergent, water, chloroform, acetone, and isopropyl alcohol. In sequence, the ITO substrates were treated with UV-induced ozone to remove the redundant organic solvents. After that, MoOx films were thermally evaporated onto the patterned-ITO substrates at a rate of 0.1 Å/s under a vacuum of about 2×10-5 Pa. The thickness of MoOx film was around 6 nm. A solution containing a mixture of C6PcH2 and PC70BM (2:1) in chloroform at a total solids concentration of 22.35 mg/ml was then spin coated on the top of MoOx layers in a N2-filled glove box. The thickness of C6PcH2 and PC70BM BHJ layer was around 150 nm. To improve the nano-scale phase separation of donor and acceptor as well the photovoltaic performance of C6PcH2-based devices, various amount of 1,8- diiodooctane (DIO, as shown in Fig. 1(b)) was added as a processing additive solvent [11]. Finally, 80- nm-thick aluminum and 3-nm-thick LiF films were thermally deposited at a rate of 3 and 0.1 Å/s under a vacuum of about 2 × 10-5 Pa, respectively. The device area was 4 mm2. Figure 1. (a) Chemical structure of materials and (b) device architecture in this study. C6PcH2 was synthesized in accordance with the literature with slight modifications as following procedures [8, 15]. In particular, C6PcH2 SM was fully purified by column chromatography (silica gel with toluene as the eluent) and repeatedly recrystallized from toluene-methanol (3:1) solution. PC70BM (Frontier carbon Ltd.) and DIO (Sigma Aldrich) was used without further purification. Q.-D. Dao / VNU Journal of Science: Mathematics – Physics, Vol. 36, No. 1 (2020) 13-19 15 2.2. Film and device characterization The current density-voltage (J-V) characteristics were measured using a source measurement unit (Keithley 2400) under 1-sun condition (using an XES 301 (AM 1.5 G) full spectrum solar simulator with the irradiation intensity of 100 mW/cm2). The external quantum efficiency (EQE) spectra were measured using a xenon lamp light passing through a mono-chromator as a light source. The measurement system was calibrated using Si reference cells. The measured J-V characteristics show good agreements with the integrated EQE values. While the active-layer thickness was directly measured using atomic force microscopy (AFM, Keyence VN-8000), the thickness of LiF and MoOx thin films was measured using thickness sensor, which inserted in vacuum chamber of with thermal evaporation system. We note that the thickness sensor in this study has been calibrated using AFM. The absorption and photoluminescence (PL) spectra of the C6PcH2:PC70BM BHJ thin films were measured by spectrophotometry (Shimadzu UV-3150) and fluorescence spectrophotometer (Hitachi F-4500), respectively. 3. Results and discussion Figure 2 shows the normalized absorption spectra of C6PcH2, PC70BM, and C6PcH2:PC70BM composites in solid thin films, fabricated on glass substrates. The absorption spectra of the C6PcH2 SM donor exhibited the strong peaks at around 408 and 700 nm, related to allowed B transition and forbidden Q transition of porphyrinoid complexes, respectively [16, 17]. However, the absorption intensity at around 500 nm was quite low. By mixing C6PcH2 SM donor with PC70BM acceptor, the absorption intensity at round 500 nm was improved due to the absorption of PC70BM molecules. As a result, the absorption of C6PcH2: PC70BM composite thin film covered all visible spectral range. Figure 2. Absorption spectra of C6PcH2 (filled-rectangles), PC70BM (empty-rectangles), and C6PcH2:PC70BM composite (filled-circles) thin films. Figures 3(a) shows the EQE spectra of the BHJ OSCs on glass substrates utilizing C6PcH2 SM donor mixed with PC70BM acceptor with various amount of DIO processing additive solvent. The devices utilizing C6PcH2:PC70BM composite thin films without DIO processing additive solvent exhibited broad EQE curves throughout the visible region, with the three predominated peaks at around 700, 400 and 500 nm. The shape of EQE curve was good agreement with the aforementioned absorption spectra Q.-D. Dao / VNU Journal of Science: Mathematics – Physics, Vol. 36, No. 1 (2020) 13-19 16 of the C6PcH2:PC70BM composite thin films. However, the EQE was relatively poor with the intensity of 8% at 700 nm due to the poor nano-scale donor-acceptor separation in the composite thin films [11]. By adding DIO processing additive solvent, the EQE of the OSC utilizing C6PcH2:PC70BM composite thin film was markedly improved. Particularly, the EQE intensity at 730 nm was improved to 43% by adding 0.2 %v/v of DIO processing additive solvent. We suggested that by adding the processing additive solvent, the nano-scale donor-acceptor phase separation of the C6PcH2:PC70BM composite thin film was improved, which caused the improvement in the device photovoltaic performance. However, the EQE intensity of the fabricated devices was reduced by adding 0.3 %v/v of DIO processing additive solvent. Figure 3(b) shows the J-V characteristics of BHJ OSCs on glass substrates utilizing C6PcH2 SM donors mixed with PC70BM acceptors with various amount of the DIO processing additive solvent, under AM 1.5 G illumination at an intensity of 100 mW/cm2. The all device data are summarized in Table 1. The devices based the C6PcH2 SM donor without the DIO processing additive solvent exhibited the relatively poor J-V characteristics with the short-circuit current density (Jsc) of 1.6 mA/cm2 and the open-circuit voltage (Voc) of 0.81 V. With the filling factor (FF) of 0.27, the PCEs of 0.4 % were achieved. By using DIO processing additive solvent, the photovoltaic performance of the BHJ OSC was markedly improved. In particular, the Jsc and FF of the fabricated devices were improved to 8.6 mA/cm2 and 0.57, respectively. As a result, the PCE of 3.6% was achieved. Although the PCE of BHJ OSC utilizing C6PcH2 mixed with PC70BM was still relatively low comparing with those of BHJ OSC utilizing benzodithiophene SM due to the low highest occupied molecular orbital energy levels of C6PcH2 SM and/or the simple architecture of cell in this study, those results indicated that C6PcH2 SM could be a potential donor in BHJ OSC [1]. On the other hand, the PCE of the fabricated devices was reduced to 1.8 % when 0.3 %v/v DIO was added to the mixture of C6PcH2:PC70BM solution. Those results were quite good agreement with the aforementioned EQE spectra of the device utilizing the C6PcH2:PC70BM composite thin films. The PL spectra of C6PcH2 and C6PcH2:PC70BM thin films fabricated on glass substrates with various DIO processing-additive solvent are shown in Fig. 4. The PL spectra of C6PcH2 thin films had a shoulder at around 820 nm and a strong peak at 766 nm, related to the Q-band of the C6PcH2 absorption spectra. The PL intensity of the C6PcH2 SM donors was markedly suppressed by adding the PC70BM acceptors. Those results indicated that the photo-induced electrons transfer from the excited state of C6PcH2 to PC70BM [18-20]. However, the PL intensity of C6PcH2:PC70BM thin films was increased by adding DIO processing-additive solvent. The increases in the PL intensity indicated that the interfacial areas between the donor and acceptor were enlarged or the nano-scale donor-acceptor phase separation was improved by adding DIO processing additive solvent, which was in good agreement with the aforementioned increases in the Jsc and PCE of the fabricated devices [11, 13, 14]. Table 1. OSCs utilizing C6PcH2: PC70BM BHJ thin film with various amounts of DIO processing additive solvent Amount of DIO (%v/v) Voc (V) Jsc (mA/cm2) FF (%) PCE (%) 0 0.81 1.6 0.27 0.4 0.1 0.77 7.0 0.38 2.1 0.2 0.74 8.6 0.57 3.6 0.25 0.73 8.3 0.53 3.2 0.3 0.74 5.5 0.44 1.8 Q.-D. Dao / VNU Journal of Science: Mathematics – Physics, Vol. 36, No. 1 (2020) 13-19 17 Figure 3. (a) EQE spectra and (b) J-V characteristics of OSCs utilizing C6PcH2:PC70BM BHJ thin film with various amounts of DIO processing additive solvent: 0 (rectangles), 0.1 (circles), 0.2 (up-triangles), 0.25 (down-triangles), and 3 (stars) %v/v. Figure 4. PL spectra of C6PcH2 (rectangles) and C6PcH2:PC70BM BHJ thin film with various amounts of DIO processing additive solvent: 0 (circles), 0.25 (stars), and 3 (up-triangles) %v/v. Figure 5(a) shows the absorption spectra at the Q band of the C6PcH2 and C6PcH2:PC70BM composite in solution and solid thin films. It indicated that the width of the Q band was changed by adding PC70BM and/or DIO processing additive solvent. Figure 5(b) shows the the Davydov splitting at the Q band or the width of Q band of the C6PcH2 and C6PcH2:PC70BM composite in solution and solid thin films. We note that the width of the Davydov splitting at Q band of phthalocyanine molecules is related to the interaction energy between discotic C6PcH2 with different site symmetries. In particular, the increases in the Davydov splitting at Q band suggest that the molecular interaction of C6PcH2 molecules was reinforced. As shown in Fig. 5(b), the Davydov splitting at Q band increased and molecular interaction of C6PcH2 molecules was reinforced after removing the solvent. On the other hand, the Davydov splitting at Q band decreased by adding PC70BM acceptor. Those results suggest that the dispersion of PC70BM molecules into the C6PcH2 domains was occurred and weakened the molecular interaction of discotic C6PcH2. However, by adding the DIO processing additive solvent, the Davydov splitting at Q band increased again. We suggest that the doping DIO processing additive solvent separated the phase of C6PcH2 donors and PC70BM acceptors, which was in good agreement with the aforementioned improvement of the BHJ OSCs utilizing the C6PcH2:PC70BM composites thin films [11, 13, 14]. Q.-D. Dao / VNU Journal of Science: Mathematics – Physics, Vol. 36, No. 1 (2020) 13-19 18 Figure 5. (a) Absorption spectra C6PcH2 in chloroform solution (empty-rectangles) and solid thin films (filled- circles) and absorption spectra of C6PcH2:PC70BM BHJ thin film with various amounts of DIO processing additive solvent: 0 (filled-rectangles) and 2 (up-triangles) %v/v and (b) dependence of size of the Davydov splitting of C6PcH2 and C6PcH2:PC70BM BHJ thin film. 4. Conclusions In summary, an efficient BHJ OSC in ITO/MoOx/C6PcH2:PC70BM /Al structure was fabricated with the relatively high photovoltaic performance. Furthermore, the roles of the DIO processing additive solvent on the photovoltaic performance of BHJ OSCs utilizing C6PcH2: PC70BM composite thin films was taken into account. Particularly, by adding 0.2 %v/v of DIO processing additive solvent, the nano- scale donor-acceptor phase separation was improved and the EQE at the Q band of BHJ OSC utilizing C6PcH2:PC70BM composite thin films increased from 8 to 43%. Hence, the Jsc and FF were markedly improved to 8.6 mA/cm2 and 0.57, respectively. Finally, the PCE of 3.6% was achieved. Acknowledgments This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2018.320. Author also thanks Prof. Akihiko Fujii (Osaka University), Prof. Masanori Ozaki (Osaka University), and Dr. Yo Shimizu (National Institute of Advanced Industrial Science and Technology) for material and equipment supports. References [1] R. Ilmi, A. Haque, M.S. Khan, High efficiency small molecule-based donor materials for organic solar cells, Org. Electron. 58 (2018) 53-62. [2] L. Schmidt-Mende, A. Fechtenkotter, K. Mullen, E. Moons, RH. Friend, J.D. MacKenzie, Self-organized discotic liquid crystals for high-efficiency organic photovoltaics, Science 293 (2001) 1119-1122. [3] Y. Wang, M. Chang, B. Kan, X. Wan, C. Li, Y. Chen, All-Small-Molecule Organic Solar Cells Based on Pentathiophene Donor and Alkylated Indacenodithiophene-Based Acceptors with Efficiency over 8% ACS Appl. Energy Mater. 1 (2018) 2150−2156. [4] Y. Huo, H.-L. Zhang, X. Zhan, Nonfullerene All-Small-Molecule Organic Solar Cells, ACS Energy Lett. 4 (2019) 1241−1250 Q.-D. Dao / VNU Journal of Science: Mathematics – Physics, Vol. 36, No. 1 (2020) 13-19 19 [5] A.K.K. Kyaw, D.H. Wang, D. Wynands, J. Zhang, T.-Q. Nguyen, G.C. Bazan, A.J. Heeger, Improved light harvesting and improved efficiency by insertion of an optical spacer (ZnO) in solution-processed small-molecule solar cells, Nano Lett. 13 (2013) 3796-3801. [6] T.S. van der Poll, J.A. Love, T.-Q. Nguyen, G.C. Bazan, Non-basic high-performance molecules for solution- processed organic solar cells, Adv. Mater. 24 (2012) 3646-3649. [7] S. Sergeyev, W. Pisulab, Y.H. Geerts, Discotic liquid crystals: a new generation of organic semiconductors, Chem. Soc. Rev. 36 (2007) 1902-1929. [8] Y. Miyake, Y. Shiraiwa, K. Okada, H. Monobe, T. Hori, N. Yamasaki, H. Yoshida, M.J. Cook, A. Fujii, M. Ozaki, Y. Shimizu, High carrier mobility up to 1.4 cm2V-1s-1 in non-peripheral phthalocyanine, Appl. Phys. Express 4, (2011) 021604-1-021604-3. [9] Z. An, J. Yu, S.C. Jones, S. Barlow, S. Yoo, B. Domercq, P. Prins, L.D.A. Siebbeles, B. Kippelen, High electron mobility in room-temperature discotic liquid-crystalline perylene diimides, S. R. Marder, Adv. Mater. 17 (2005) 2580-2583. [10] Q.-D. Dao, T. Hori, T. Masuda, K. Fukumura, T. Kamikado, F. Nekelson, A. Fujii, Y. Shimizu, M. Ozaki Mechanism of Degradation and Improvement of Stability on Mesogenic-Phthalocyanine-Based Bulk Heterojunction Solar Cell, Jpn. J. Appl. Phys. 52 (2013) 012301-1-012301-5. [11] Q.-D. Dao, T. Hori, K. Fukumura, T. Masuda, T. Kamikado, A. Fujii, Y. Shimizu, M. Ozaki, Efficiency enhancement in mesogenic-phthalocyanine-based solar cells with processing additives, Appl. Phys. Lett. 101 (2012) 263301-1-263301-4. [12] Q.-D. Dao, A. Fujii, M. Ozaki, Fabrication of tandem solar cells with all-solution processed multilayer structure using non-peripherally substituted octahexyl tetrabenzotriazaporphyrins, Jpn. J. Appl. Phys. 55 (2016) 03DB01-1- 03DB01-6. [13] Q.-D. Dao, T. Uno, M. Ohmori, K. Watanabe, H. Itani, A. Fujii, Y. Shimizu, M. Ozaki, Effects of thermal-annealing and processing-additive treatment on crystallization-induced phase separation in organic solar cells utilizing octapentyl tetrabenzotriazaporphyrins, J. Phys. D: Appl. Phys. 48 (2015) 385103-1-385103-5. [14] Q.-D. Dao, T. Hori, K. Fukumura, T. Masuda, T. Kamikado, A. Fujii, Y. Shimizu, M. Ozaki, Effects of processing additives on nanoscale phase separation, crystallization and photovoltaic performance of solar cells based on mesogenic phthalocyanine, Org. Electron. 14 (2013) 2628–2634. [15] J.C. Swarts, E.H.G. Langner, N. Krokeide-Hove, M.J. Cook, Synthesis and electrochemical characterisation of some long chain 1,4,8,11,15,18,22,25-octa-alkylated metal-free and zinc phthalocyanines possessing discotic liquid crystalline properties, J. Mater. Chem.11 (2001) 434-443. [16] S.M. Critchley, M.R. Willis, M.J. Cook, J. McMurdo, Y. Maruyama, Deposition of Ordered Phthalocyanine Films by Spin Coating, J. Mater. chem., 2(2) (1992) 157-159. [17] J. Mack, M.J. Stillman, N. Kobayashi, Application of MCD spectroscopy to porphyrinoids, Coordination Chemistry Reviews 251 (2007) 429-453. [18] S. Morita, A.A. Zakhidov, K. Yoshino, Doping effect of buckminsterfullerence in conducting polymer: change of absorption spectrum and quenching of luminescence, Solid State Commun. 82 (1992) 249-252. [19] N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene, Science 258 (1992) 1474-1476. [20] K. Yoshino, X.H. Yin, S. Morita, T. Kawai, A.A. Zakhidov, Enhanced photoconductivity of C60 doped poly(3- alkylthiophene), Solid State Commun. 85 (1993) 85-88.