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4 th Hybrid and Organic Photovoltaic Conference -Uppsala 2012 87 B41 - Spray-coating technique for the realization of Polymer Solar-Cells Gianpaolo Susanna a , Luigi Salamandra a , Thomas Brown a , Andrea Reale a , Francesca Brunetti a , Aldo Di Carlo a a, University of Rome Tor Vergata, viale del politecnico 1,Rome,00133, IT b, CHOSE, Via Giacomo Peroni 400/402 Roma 00131, IT Organic solar cells have gained within the last years a growing interest within the research community. Among all organic solar cells, small molecule and polymer based technology are very promising. In particular, polymer bulk-heterojunction based technology has shown wide margins of improvement in terms of energy conversion efficiency, reaching values above 8% (certified by NREL), and uncertified 9%, actually doubling the performance in the last two years [1-4]. Despite the low efficiency, when compared with the conventional inorganic solar cells, the potential of a Roll-to-Roll (R2R) process and the low-cost large-area definition on flexible substrates, make the BHJ-SCs technology an interesting and economic solution for the production of energy. The solar cell is built up in a multilayer structure in which the deposition of the layers can be performed with different techniques like for example casting, spin-coating, screen and ink-jet printing [5]. Recently conventional spray-coating was introduced as a large area deposition method for the layers of organic solar cells (OSCs) [6]. In this context, we present polymer solar cells that use as active layer P3HT:PCBM, in which we replaced the standard spin-coating deposition of the active layer and of the hole transporting layer (PEDOT:PSS) with the spraycoating technique, reaching power conversion efficiency in the rage of 4%. In these cells a fine tuning of the realization parameters such as the solvent used, the substrate temperature, the film thickness, time of spray, distance between sample and airbrush, substrate temperature has been performed. Furthermore different cells architectures will be presented as function of the required characteristics of the substrate and of the electrode deposition. These results point out the spray-coating as a powerful printing technique to be embedded in a R2R process for the realization of flexible organic photovoltaic devices. References [1] C.J. Brabec et al. Organic Photovoltaics: Materials, Device Physics, and Manufacturing Technologies, Wiley-VCH, Weinheim (2008). [2] Konarka Power Plastic (www.konarka.com) [3] Solarmer (www.solarmer.com) [4] J.Nelson, Materials Today, 14 (2011), pp. 462-470 [5] F.C. Krebs et al. Sol. Energy Mater. Sol. Cells 93 (2009), pp. 1968–1977. [6] G.Susanna et al. Sol. Energy Mater.Sol Cells, 93 (2011), pp. 1775-1778. © SEFIN 2012
4 th Hybrid and Organic Photovoltaic Conference -Uppsala 2012 88 B42 - Mechanism of Mobile Charge Carrier Generation in Blends of Conjugated Polymers and Fullerenes: Significance of Charge Delocalization and Excess Free Energy D. H. K Murthy a , Min Gao a , Martien Vermeulen a , Laurens Siebbeles a , Tom Savenije a a, Optoelectronic Materials Section, Department of Ch, Julianalaan 136, Delft, 2628, NL b, Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, NL Efficient photoinduced generation of charge carriers in blends of soluble conjugated polymers and fullerene derivatives is promising for the development of low-cost bulkheterojunction solar cells. Despite the fact that these cells with efficiencies over 8% have been realised, the mechanism involved in the photogeneration of mobile charge carriers is still unclear. The microwave photoconductance measurements are performed to investigate the importance of the excess free energy involved in the exciton dissociation process for blends of poly(3-hexyl-thiophene) (P3HT) with monoPCBM and bisPCBM. To this end, the effect of the excitation wavelength (visible to NIR) and of the temperature (88 K to 300 K) on the yield of charge carriers is examined. NIR excitation corresponds to the transition of an electron from the HOMO of the P3HT directly to the LUMO of the fullerene derivative forming the charge transfer band (CT). For P3HT:PCBM, the yield of charge carriers was identical on both visible and NIR excitation, even at 88 K temperature. This observation indicates that the excess energy does not affect the yield of charges for P3HT:PCBM even at 88 K. From these results it is inferred that the binding energy between the electron and hole in a CT state is smaller than thermal energy at 88 K that is in large contrast to previously reported values of 0.3 eV. This is ascribed to efficient charge delocalization, which increases the mean distance between the electron and hole at the interface. For P3HT:bisPCBM, the yield of charges decreases by a factor of three on changing the wavelength from the vis to the NIR. Note that the CT state in P3HT:bisPCBM is energetically matches with the triplet level of the P3HT, owing to its smaller electron affinity as compared to monoPCBM. Therefore, a CT state can recombine to triplet level of P3HT resulting in a reduction of the yield on NIR excitation. However, as the yields for P3HT:PCBM and P3HT:bisPCBM are equall on visible excitation, we conclude that for the latter blend formation of mobile charge carrier occurs primarily via the thermally non-relaxed, hot CT state. These observations indicate that the excess energy involved in the exciton dissociation process is indeed important to avoid recombination of the CT states to the triplet level of the polymer in order to achieve higher yields of charge carrier generation. © SEFIN 2012
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