First solar cells based on CdTe nanoparticle/MEH-PPV composites
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Hybrid bulk heterojunction composites are promising material for low-cost organic solar cells. Fundamental measurements with CdTe nanocrystal/MEH-PPV poly [2-methoxy-5-(2⬘-ethyl-hexyloxy)-1,4-phenylene vinylene] composites and the first realization of a solar cell based on this material are presented. Optical and electrochemical properties are discussed as well as the current voltage characteristic of the resulting cell. It was found, that CdTe nanocrystal/MEH-PPV composites are well suited for an organic solar cell, even though the technological realization needs to be improved.
I. INTRODUCTION
The increasing global energy consumption and the limited resources of fossil fuels will force us sooner or later to replace most of the power plants used today by renewable energy sources. Among the renewable sources, solar energy conversion takes a prominent role, because it is globally available, inexhaustible, and the electrical energy can easily be converted to other energy transfer media (e.g., hydrogen). Conventional inorganic solar cells routinely exhibit conversion efficiency of 10% and in the best case up to 30%.1 However, market expansion in the use of conventional solar cells remains limited due to high costs imposed by fabrication procedures involving elevated temperatures, high vacuum, numerous lithographic steps, and discrete processing due to glass carriers. Organic solar cells having conducting polymers as one of the components can serve as a cost-effective alternative to conventional inorganic solar cells because of the processing advantages of polymers. These polymers can be solution processed, and their good mechanical properties open the possibility to design it in flexible shape and sizes. Solar cells based on these ideas were realized, and up to 3% efficiency were produced with [6,6⬘]phenyl C60 butyric acid methylester (PCBM) and MDMO:PPV poly[2-methoxy-5-(3⬘7⬘-dimethyloctyloxy)]-1,4-phenylene vinylene.2 The superior performance of inorganic solar cells is attributed to their better charge mobilities. On the other hand, most of the
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0279 1990
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 7, Jul 2004 Downloaded: 02 Jan 2015
conducting polymers are intrinsic semiconductors, and the primary excitation is a Coulomb bound exciton. Efficient performance of solar cells depends primarily on the charge transport to the corresponding electrodes. Efficient collection of charge carriers requires that the neutral excitons produced by primary excitation be separated into free carriers and that these charge carriers are then transferred to the electrodes without recombining with oppositely charged carriers. Charge separation in conducting polymers has been found to enhance at the interface with materials of high electron affinity where it is energetically favorable for the electron to transfer onto a second material. Solar cells were produced by blending conducting polymers with substituted C60
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