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Electronic excitations in thin-film absorbers for photovoltaics

Authors: S. Botti

Ref.: 17th ETSF-workshop on Electronic Excitations, Coimbra, Portugal, 3/10/2012 (2012)

Abstract: I will present some recent developments and results in the study of electronic excitations in complex materials for thin-film solar cells. During the past years, Cu(In,Ga)(Se,S)$_2$ (CIGS) thin-film solar cells has emerged as a technology that could challenge the current hegemony of silicon solar panels. However, there is still a strong need of fundamental knowledge on the electronic properties and defect physics of this family of semiconductor. 1) In collaboration with experimentalists in Luxembourg and Berlin [1], we studied the overall dependence of the band gap of CuInSe$_2$ on the Cu/In ratio of the sample, using a self-consistent GW approach and hybrid functionals, including a feedback mechanism that we recently proposed [2]. Our calculations illustrate consistently the remarkable stability of the band gap of chalcopyrite semiconductors, and explain the experimental observations by a coupled effect of Cu vacancies and lattice distortions within the feedback model. I will also show absorption spectra for perfect crystals and systems with defects [3,4]. 2) I will discuss GW+Bethe Salpeter calculations of the electronic and optical properties of silicon allotropes with interesting characteristics for applications in thin-film solar cells [5]. These new phases were obtained using the minima hopping method for structural prediction. They consist of distorted sp3 silicon networks and have a lower formation energy than other experimentally produced silicon phases. Some of these structures turned out to have quasi-direct and dipole-allowed band gaps in the range 0.8-1.5eV, and to display absorption coefficients comparable with those of chalcopyrites used in thin-film record solar cells. References: [1] "Feedback mechanism for the stability of the band gap of CuInSe2", L. Gutay, D. Regesch, J.K. Larsen, Y. Aida, V. Depredurand, A. Redinger, S. Caneva, S. Schorr, C. Stephan, J. Vidal, S. Botti, S. Siebentritt, accepted in Phys. Rev. B (2012). [2] J. Vidal, S. Botti, P. Olsson, J-F. Guillemoles, and L. Reining, Phys. Rev. Lett., 2010, 104, 056401. [3] I. Aguilera, J. Vidal,P. Wahnón, L. Reining, and S. Botti, Phys. Rev. B, 2011, 84, 085145. [4] "Speeding up the solution of the Bethe-Salpeter equation by a double-grid method and Wannier interpolation", D. Kammerlander, S. Botti, M.A.L Marques, A. Marini, C. Attaccalite, submitted (2012). [5] "Low-energy silicon allotropes with strong absorption in the visible for photovoltaic applications", S. Botti, J.A. Flores-Livas, M. Amsler, S. Goedecker, M.A.L. Marques submitted (2012). http://arxiv.org/abs/1203.5669v1