Advances in the Research of the Intermediate Band (IB) Solar Cell
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1031-H01-01
Advances in the Research of the Intermediate Band (IB) Solar Cell Antonio Luque, and Antonio Martà Instituto de Energia Solar, Universidad Politecnica de Madrid, Ciudad Universitaria, Madrid, 28040, Spain ABSTRACT We describe the present state of the intermediate band (IB) solar cell research, a cell concept with very high efficiency potential. A comprehensive presentation of the theory is included followed by a description of its implementation using quantum dots and of the experiments performed to prove the main principles. Presently IB solar cells do not give very high efficiencies; the steps to be taken towards the real achievement of higher efficiencies is described and the use of alloys, instead of nanostructured materials, to fabricate IB cells is also discussed. INTRODUCTION The sun is a huge source of energy but it is relatively diluted. Sustainability arguments require that the sun becomes a major source of electricity. For penetrations above 1/3 of the world demand [1] technologies with high efficiency seem necessary. Furthermore such technologies must present a high learning curve [2]. Because of this, the research of solar cells with very high efficiency potential seems important. Indeed multijunction cells have a very high efficiency potential. Over 40% efficiency has already been achieved [3] under concentrated sunlight with a 3-junction multijunction epitaxial stack of cells. However this stack has about 20 layers and is interconnected by 2 tunnel junctions. Present research in this field is focused in obtaining four junction cells and this is proving to be a hard task. Adding more junctions might be almost impossible and in addition such stacks become highly spectrum sensitive so that it might become unworthy to add many more junctions. Novel cell concepts, and in particular the Intermediate Band (IB) Solar Cell, may come to the support of the stacks of ordinary solar cells to reduce the number of layers and ease the spectral sensitivity problem and reach an efficiency beyond the 45% that is expected achievable with 4 junctions. As matter of fact it is interesting to know that for ideal cells, a stack of two IB cells is equivalent to a stack of six ordinary solar cells, and needs a single tunnel junction and not the five that are required for the latter [4]. In this paper we start with a presentation of some selected theoretical aspects of the IB solar cell and then we describe the demonstrators realized using quantum dots (QD). These demonstrators have permitted us to prove experimentally the principles on which the IB solar cell is based, and also identify the problems needed to be solved to render this cell of practical interest. One of the problems to solve is the weak light absorption by the QD. To overcome this, one of the ways is to develop bulk materials with an IB. This problem is addressed afterwards. Finally conclusions are drawn.
SOME THEORETICAL ASPECTS OF THE IB SOLAR CELL Increasing the current while preserving the voltage An IB solar cell [5] is formed of an IB materia
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