Optimal Morphology for a Composite Solar Cell
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Optimal Morphology for a Composite Solar Cell V. M. Burlakov, G. A. D. Briggs, And A. P. Sutton Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK ABSTRACT Charge transport in a composite solar cell (CSC) is analysed by considering independent and random migration of the photo-generated electrons/holes over interpenetrating conducting networks. It is shown that besides an interface area and the materials parameters the efficiency (average number of the carriers reaching the electrode per time unit) of the CSC depends on the effective dimensionality of the conducting channels. Our analysis shows that the 1-d network is ~20% more effective than the 3-d one, and therefore the morphology of the 1-d type for the organic phase within a 3-d inorganic one is preferable for the CSC. It is shown that the CSC with bulk generation of excitons is potentially more efficient than a dye-sensitised solar cell. The highest efficiency of collection of photo-generated carriers for the CSC at highest possible current for the given material properties cannot exceed ∼40% and 34% for 1-d and 3-d networks respectively. INTRODUCTION Though the efficiency of organic-inorganic composite solar cells (CSC) as photovoltaic devices is well below that of their inorganic counterparts, they are potentially cheaper and might be produced in vast quantities to cover large areas. This makes CSC very promising for future applications. An ordinary organic-inorganic composite solar cell consists of a mixture of organic hole conducting media, usually a conjugated polymer such as MEH-PPV, and an inorganic electronconducting media, usually TiO2. The charge carriers are produced at the organic-inorganic interface due to dissociation of excitons, which in turn are photo-generated within the organic phase, in contrast to the Grätzel cell, in which the excitons are generated directly at the interface within the dye molecules [1]. For the charge carriers to be able to reach the electrodes each phase (organic or inorganic) must form continuous conducting channels ending up at appropriate electrodes. The efficiency of the CSC is determined by the generation rate of excitons, i.e excitonic absorption of light α, exciton diffusion length λ, organic-inorganic interface area S, and diffusion coefficients of carriers De and Dh in the appropriate conducting channels. There is one more parameter affecting the CSC efficiency, and describing the way the materials are organised in the composite, i.e., the composite morphology. One may wonder if the structure and the way the conducting channels are organised can affect the CSC efficiency for a given interface area. An analysis of the morphology maximizing the efficiency of the CSC with given materials properties is the subject of the current paper. MODEL OF A COMPOSITE SOLAR CELL Consider a model system consisting of a number of sites arranged in a simple cubic lattice with lattice constant a. Some sites are assigned to the organic phase, the other to the inorganic one, so that each family of sites fo
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