Full Chalcopyrite Tandem Devices: can we hope?

Nicolas Barreau, Léo Choubrac, Fabien Pineau, Thamer Aloui, Eugène Bertin, Thomas Lepetit, Ludovic Arzel, Sylvie Harel, Lionel Assmann IMN, CNRS - Nantes Université, Nantes, --, France

Cu(In,Ga)(Se,S)2 compounds with chalcopyrite structure are semiconductors with high absorption coefficient and bandgap varying from 1.0 eV up to 2.4 eV depending on their In/Ga and Se/S relative contents. These optical properties make them adapted candidates for full thin film tandem applications, as was imagined in the end of 1990’s. However, the numerous problems encountered to achieve high efficiency from wide gap absorbers led the scientific community to forsake this objective. The recent progress in the development of chalcopyrite thin film-based solar cell onto transparent and conductive back contact led us to try to evaluate the feasibility, the performance and the limitations of wire connected 2T tandems, based exclusively on chalcopyrite technology. This is the purpose of the present contribution, dedicated to the study of devices based on the assembly of CuIn(Ga)Se2 bottom and Cu(In,Ga)S2 top cells.  The performances of the investigated devices appear limited by too low transmission of photons of wavelength above 800 nm by the top cell; this poor transmission is primarily due to the use of heavily doped ITO as back contact. We also investigated what would be the impact of a higher current in the bottom and observed it yields a severe decrease in the fill factor of the tandem, and interpreted it as resulting from the characteristics of the top cell.   As a conclusion of this work, one can suggest that the implementation of full chalcopyrite tandems relies on future progress in the performance of cells based on wide bandgap absorbers and deposited onto transparent back contacts. Our team at the IMN is currently working to overcome these issues, and our last achievements will be presented during the conference.