An original flexible structure for Organic Photovoltaic Devices
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An original flexible structure for Organic Photovoltaic Devices Ornella Sannaa, Mario Cossua, Tomas Piliaa, Annalisa Bonfiglioa,b a Dept. of Electrical and Electronic Engineering, University of Cagliari, and INFM, Cagliari, Italy b INFM – S3 Centre “nanoStructures and bioSystems at Surfaces”, Modena, Italy ABSTRACT A simple idea is proposed for the realization of organic photovoltaic devices on flexible substrates. According to this, a poly(ethylene terephtalate) layer (Mylar), transparent, a few micrometers thick and completely flexible, works as mechanical support. It is an insulating material, which has good mechanical and dielectric properties and, most important, it is completely transparent to ultraviolet light. For this reason it could be suitable for being used as transparent supporting layer for large area photovoltaic devices. Furthermore, its mechanical properties allow to employ it in a roll-to-roll lamination procedure that could give rise to low cost extended films carrying solar cells on the surface. Starting from this flexible, transparent layer an organic semiconductor layer is sandwiched between two electrodes, being one of them transparent and directly deposited on the flexible substrate. Different organic semiconductors have been used in order to test the device behaviour. In spite of the non-ideal working conditions (no nitrogen atmosphere, no protection from oxygen), the dark I-V characteristics are in agreement with the results already reported in literature for the employed organic materials. A comparison between dark and light measurement is presented, reporting fairly good results with respect to the photocurrent. Due to the extreme mechanical flexibility of their structure, it could be possible to build working devices, able to be transferred on unusual substrates as, for example, textiles or 3D surfaces, paying the way to very innovative applications. Work is in progress in this direction. INTRODUCTION The study of organic solar cells has achieved a great importance in the last years, with the aim to obtain more efficient architectures that could improve the performances of the devices and fully exploit the opto-electronic properties of the polymers used for their realisation [1]. In its simplest form, an organic solar cell consists of a single polymer layer sandwiched between two different electrode materials. Photons can enter the cell through the transparent contact (ITO and/or PEDOT:PSS) and create excitons upon absorption in the organic film. Single layer devices have very low efficiencies due to very small photoactive region limited to the only employed active material, which could cause dramatic recombination losses. An important breakthrough was realised by Tang [2] with the thin-film two-layer organic photovoltaic cell based on a planar junction of copper phthalocyanine and a perylene tetra carboxylic derivative. However, the active region of these types of cells only extends over the thin interface between the donor and acceptor materials. Small diffusion lengths only
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