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1211-R03-43

First-principles Calculations of Complex Intermediate Band Materials for Photovoltaic Devices P. Wahnón1, I. Aguilera1, P. Palacios1 and K. Sánchez1 1

Instituto de Energía Solar & Dept. Tecnologías Especiales, Universidad Politécnica de Madrid, Ciudad Universitaria s/n, Madrid, 28040, Spain.

ABSTRACT An ab initio study of several compounds candidates to behave as intermediate band materials is presented. The use of these materials as the active element in solar cells is a promising way to enhance the photovoltaic efficiency. Indeed from this point of view, most interesting compounds are those whose host semiconductor presents a band-gap close to the optimum value of 2 eV. Chalcogenide compounds substituted by light transition metals are solid candidates to this end. While they are being further characterized and experimentally synthesized, another approach is being examined. It consists of using Si as host semiconductor. Ti implantation at concentrations several orders of magnitude above equilibrium solubility has shown a probable intermediate band material behavior, the origin of the intermediate band being related to levels of interstitial Ti. Optoelectronic characterization of this material is completed. A novel possibility consists of combining chalcogen S implantation with boron. In this case preliminary results of electronic structure are shown.

INTRODUCTION Intermediate band solar cell concept [1] is enclosed in the so-called third generation of solar cells. It is based on the electronic structure of the absorber material, which presents a narrow, partially-filled intermediate band. This band helps to enhance the photocurrent with regards to a host semiconductor, but leaving unchanged the photovoltage. The additional photocarriers are generated in a double-step process involving the intermediate band due to photons with energies below band-gap can be used to pump electrons from valence band to the intermediate band or from the latter to the conduction band. A number of materials with substitution of atoms by transition metals have been proposed in past years [2-5]. Chalcogenide-derived compounds are particularly interesting today due to their use as thin-film materials in the photovoltaic area. Furthermore, use of thin-film techniques would allow for these materials to reduce the costs of the solar cell. Experimental measurements of defective thiospinel In2S3 with replacement of In by transition metals [6] have shown the characteristic triple absorption spectra predicted by quantum calculations [7]. Growth of larger samples for this compound and other chalcogenide-derived materials are underway. However materials based on Si have been proposed in order to achieve in a short-term an intermediate band solar cell with enhanced efficiency. Si implanted with Ti at concentrations close to 1020 cm-3 was the first candidate [8]. The intermediate band was explained to be formed from the t2g type manifold of 3d electrons of Ti atoms placed in tetrahedral interstitials [9]. While

optical characterizati