We have fabricated bulk heterojunction organic solar cells using coumarin 6 (C6) as a small organic dye, for light harvesting and electron donation, with fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), acting as an electron acceptor, by spin-coating technique. We have investigated thermal annealing and blend concentration effects on light harvesting, photocurrent, and performance parameters of the solar cells. In this work, we introduced an experimental method by which someone can easily detect the variation in the contact between active layer and cathode due to thermal annealing after cathode deposition. We have showed, in this work, unusual behavior of solar cell composed of small organic molecules under the influence of thermal annealing at different conditions. This behavior seemed uncommon for polymer solar cells. We try from this work to understand device physics and to locate a relationship between production parameters and performance parameters of the solar cell based on small organic molecules. 1. Introduction Organic solar cells have attracted much attention due to their potential in low-cost solar energy harvesting, as well as applications in flexible, light-weight, colorful, and large-area devices [1]. With the discovery of efficient photo-induced electron transfer from a conjugated polymer to fullerene [2], the bulk heterojunction (BHJ) organic solar cell has become one of the most successful device structures developed in the field to date. By simply blending electron donor with electron acceptor (fullerene) in organic solvents, a self-assembling interpenetrating network can be obtained using various coating technologies ranging from laboratory-scale spin-coating or spray-coating to large-scale fabrication technologies such as inkjet printing, doctor blading, gravure, slot-die coating, and flexographic printing. The BHJ solar cells performed with power conversion efficiency from solar light into electrical energy of up to about 6.1% [3], and with further progress, the efficiency reached about 6.8% [1]. To increase knowledge of material characteristics and behavior for photovoltaic applications, new BHJ systems should be studied. For significant improvements in organic solar cell efficiencies, new donor/acceptor combinations may be required to recognize and identify the photovoltaic properties of materials used in solar cells. To construct a new BHJ solar cell using low-molecular-weight compounds such as coumarin dyes, it is better first to illustrate the advantages of this kind of organic dyes. The coumarin dyes
References
[1]
H. Y. Chen, J. Hou, S. Zhang et al., “Polymer solar cells with enhanced open-circuit voltage and efficiency,” Nature Photonics, vol. 3, no. 11, pp. 649–653, 2009.
[2]
N. S. Sariciftci, L. Smilowitz, A. J. Heeger, and F. Wudl, “Photoinduced electron transfer from a conducting polymer to buckminsterfullerene,” Science, vol. 258, no. 5087, pp. 1474–1476, 1992.
[3]
S. H. Park, A. Roy, S. Beaupré et al., “Bulk heterojunction solar cells with internal quantum efficiency approaching 100%,” Nature Photonics, vol. 3, no. 5, pp. 297–303, 2009.
[4]
X. Jiang Jr., W. F. Burgoyne, and L. M. Robeson, “Sequestration of electroactive materials in a high , insulating polymer matrix for optoelectronic applications—part 2. Photovoltaic devices,” Polymer, vol. 47, no. 11, pp. 4124–4131, 2006.
[5]
Y. A. M. Ismail, T. Soga, and T. Jimbo, “The contribution of coumarin 6 in light harvesting and photocurrent of P3HT:PCBM bulk heterojunction solar cell,” Solar Energy Materials and Solar Cells, vol. 94, no. 8, pp. 1406–1411, 2010.
[6]
K. Hara, T. Sato, R. Katoh et al., “Molecular design of coumarin dyes for efficient dye-sensitized solar cells,” Journal of Physical Chemistry B, vol. 107, no. 2, pp. 597–606, 2003.
[7]
K. Hara, M. Kurashige, Y. Dan-Oh et al., “Design of new coumarin dyes having thiophene moieties for highly efficient organic-dye-sensitized solar cells,” New Journal of Chemistry, vol. 27, no. 5, pp. 783–785, 2003.
[8]
A. Montali, A. R. A. Palmans, J. Bras et al., “Depolarizing energy transfer in photoluminescent polymer blends,” Synthetic Metals, vol. 115, no. 1, pp. 41–45, 2000.
[9]
Y. Takahashi, A. Maeda, K. Kojima, and K. Uchida, “Luminescence of dyes doped in a sol-gel coating film,” Journal of Luminescence, vol. 87, pp. 767–769, 2000.
[10]
A. F. Mansour, H. M. A. Killa, S. Abd El-Wanees, and M. Y. El-Sayed, “Laser dyes doped with poly(ST-Co-MMA) as fluorescent solar collectors and their field performance,” Polymer Testing, vol. 24, no. 4, pp. 519–525, 2005.
[11]
A. F. Mansour, “Photostability and optical parameters of copolymer styrene/MMA as a matrix for the dyes used in fluorescent solar collectors,” Polymer Testing, vol. 23, no. 3, pp. 247–252, 2004.
[12]
V. Kandavelu, H. S. Huang, J. L. Jian, T. C. K. Yang, K. L. Wang, and S. T. Huang, “Novel iminocoumarin dyes as photosensitizers for dye-sensitized solar cell,” Solar Energy, vol. 83, no. 4, pp. 574–581, 2009.
[13]
M. J. Brites, C. Santos, S. Nascimento, B. Gigante, and M. Berberan-Santos, “Synthesis of [60]fullerene-coumarin polyads,” Tetrahedron Letters, vol. 45, no. 37, pp. 6927–6930, 2004.
[14]
R. M. Meixner, H. G？bel, F. A. Yildirim, W. Bauhofer, and W. Krautschneider, “Wavelength-selective organic field-effect phototransistors based on dye-doped poly-3-hexylthiophene,” Applied Physics Letters, vol. 89, no. 9, Article ID 092110, 2006.
[15]
C. C. Wu, J. C. Sturm, M. E. Thompson, R. A. Register, E. P. Dana, and J. Tian, “Efficient organic electroluminescent devices using single layer doped polymer thin films with bipolar carrier transport abilities,” IEEE Transactions on Electron Devices, vol. 44, no. 8, pp. 1269–1281, 1997.
[16]
T. Yamanari, T. Taima, J. Sakai, and K. Saito, “Origin of the open-circuit voltage of organic thin-film solar cells based on conjugated polymers,” Solar Energy Materials and Solar Cells, vol. 93, no. 6-7, pp. 759–761, 2009.
[17]
M. T. Lloyd, A. C. Mayer, S. Subramanian et al., “Efficient solution-processed photovoltaic cells based on an anthradithiophene/fullerene blend,” Journal of the American Chemical Society, vol. 129, no. 29, pp. 9144–9149, 2007.
[18]
A. B. Tamayo, X. D. Dang, B. Walker, J. Seo, T. Kent, and T. Q. Nguyen, “A low band gap, solution processable oligothiophene with a dialkylated diketopyrrolopyrrole chromophore for use in bulk heterojunction solar cells,” Applied Physics Letters, vol. 94, no. 10, pp. 103301–103303, 2009.
[19]
J. Sakai, T. Taima, T. Yamanari, and K. Saito, “Annealing effect in the sexithiophene: C70 small molecule bulk heterojunction organic photovoltaic cells,” Solar Energy Materials and Solar Cells, vol. 93, no. 6-7, pp. 1149–1153, 2009.
[20]
Y. Zhao, Z. Xie, Y. Qu, Y. Geng, and L. Wang, “Effects of thermal annealing on polymer photovoltaic cells with buffer layers and in situ formation of interfacial layer for enhancing power conversion efficiency,” Synthetic Metals, vol. 158, no. 21-24, pp. 908–911, 2008.
[21]
G. Li, V. Shrotriya, Y. Yao, and Y. Yang, “Investigation of annealing effects and film thickness dependence of polymer solar cells based on poly(3-hexylthiophene),” Journal of Applied Physics, vol. 98, no. 4, pp. 043704–043708, 2005.
[22]
P. Vanlaeke, A. Swinnen, I. Haeldermans et al., “P3HT/PCBM bulk heterojunction solar cells: relation between morphology and electro-optical characteristics,” Solar Energy Materials and Solar Cells, vol. 90, no. 14, pp. 2150–2158, 2006.
[23]
H. Kim, W. W. So, and S. J. Moon, “Effect of thermal annealing on the performance of P3HT/PCBM polymer photovoltaic cells,” Journal of the Korean Physical Society, vol. 48, no. 3, pp. 441–445, 2006.
[24]
Y. A. M. Ismail, T. Soga, and T. Jimbo, “Photovoltaic properties of bulk heterojunction organic solar cell composed of coumarin 6 dye as light harvester and donor material,” Japanese Journal of Applied Physics, vol. 49, no. 5, Article ID 052301, pp. 1–7, 2010.
[25]
W. J. Yoon, K. Y. Jung, J. Liu et al., “Plasmon-enhanced optical absorption and photocurrent in organic bulk heterojunction photovoltaic devices using self-assembled layer of silver nanoparticles,” Solar Energy Materials and Solar Cells, vol. 94, no. 2, pp. 128–132, 2010.
[26]
Y. M. Chang, W. Su, and L. Wang, “Influence of photo-induced degradation on the optoelectronic properties of regioregular poly(3-hexylthiophene),” Solar Energy Materials and Solar Cells, vol. 92, no. 7, pp. 761–765, 2008.
[27]
S. Kazim, M. Zulfequar, M. M. Haq, P. K. Bhatnagar, and M. Husain, “Electrical and optical properties of thin films based on poly [2-methoxy-5 (2'-ethyl hexyloxy)-1,4-phenylene vinylene] doped with acridine orange dye with possible photovoltaic applications,” Solar Energy Materials and Solar Cells, vol. 91, no. 15-16, pp. 1462–1466, 2007.
[28]
J. J. Dittmer, R. Lazzaroni, P. Leclère et al., “Crystal network formation in organic solar cells,” Solar Energy Materials and Solar Cells, vol. 61, no. 1, pp. 53–61, 2000.
[29]
X. M. Jiang, R. Osterbacka, O. J. Korovyanko et al., “Spectroscopic studies of photoexcitations in regioregular and regiorandom polythiophene films,” Advanced Functional Materials, vol. 12, no. 9, pp. 587–597, 2002.
[30]
H. Hoppe, M. Niggemann, C. Winder et al., “Nanoscale morphology of conjugated polymer/fullerene based bulk-heterojunction solar cells,” Advanced Functional Materials, vol. 14, no. 10, pp. 1005–1011, 2004.
[31]
C. D. Dimitrakopoulos and D. J. Mascaro, “Organic thin-film transistors: a review of recent advances,” IBM Journal of Research and Development, vol. 45, no. 1, pp. 11–27, 2001.
[32]
H. Hoppe and N. S. Sariciftci, “Organic solar cells: an overview,” Journal of Materials Research, vol. 19, no. 7, pp. 1924–1945, 2004.
