Photovoltaic Properties of Organic Heterojunctions with Ordered Nanostructures Prepared By Anodic Porous Alumina

Organic thin film solar cells have attracted much attention as a next generation energy conversion devices due to their unique properties [1-3].  Control of the nanostructures of organic heterojunctions is essential to improve the conversion efficiency of the organic solar cells.  In the present report, we describe the fabrication of the organic solar cells with controlled nanostructures and their photovoltaic properties.  Fabrication of organic heterojunctions with controlled geometrical nanostructures was carried out by using the substrates with ordered textured patterns of nanometer scale, which were prepared by nanoimprinting using anodic porous alumina molds.  Anodic porous alumina, which is formed by anodization of Al in acidic electrolyte, is a typical self-ordered material and is promising candidate for the starting material for making nanostructures [4].  Use of anodic porous alumina as a mold for nanoimprinting allows the high throughput formation of the substrates with ordered nanostructures of polymers [5,6].  The heterojunctions of organic thin layers were formed on the substrates with ITO layer using a vacuum deposition apparatus.  The ordered nanostructure of the substrates could be maintained even at the organic layer of heterojunctions.  Optical properties of the obtained samples and the photovoltaic properties were examined.  In the samples with controlled geometrical structures prepared by the present processes, improvement of conversion efficiencies was confirmed compared to the samples without controlled geometrical structures.  This improvement was thought to be caused by the increase of absorption of light at the heterojunctions due to the antireflection effect caused by the periodic nanostructures.  The controlled nanostructure of the heterojunction also contributed to the improvement of the conversion efficiency due to the effective charge separation at the junction.  The present process, which allows high throughput formation of the organic heterojunctions with controlled geometrical structures, will be used for the formation of organic solar cells with high conversion efficiencies.

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