ORAL

SESSION: SISAMMonAM-R9	3rd Intl. Symp. Surfaces and Interfaces of Sustainable, Advanced Materials (SISAM)
Mon Oct, 23 2017	Room: Condesa III
Session Chairs: Jean-Marie Dubois; Isabelle Braems; Session Monitor: TBA

15:00: [SISAMMonAM06] Keynote
High-efficiency Solar Cells Based on Polycrystalline Cu(In,Ga)Se2 Thin Film: A Discussion on the Influence of Alkali Fluoride Post-deposition Treatments
Nicolas Barreau¹ ; Ludovic Arzel¹ ; Sylvie Harel¹ ; Thomas Lepetit¹ ; ¹Universite de Nantes, IMN, Nantes, France;
Paper Id: 315 [Abstract]

Cu(In,Ga)Se2 thin film solar cell technology has recently achieved the outstanding power conversion efficiency of 22.6 %. Such level of performance is surprising because these devices are based on polycrystalline layers and hetero-junctions; hence implying numerous homo- and hetero-interfaces. Nevertheless, the recent efficiency breakthrough was made possible thanks to specific post- deposition treatments of the Cu(In,Ga)Se2 absorber, which imply both surface and grain boundary modifications. So far, the best solar cells were prepared from absorbers which were treated by heavy alkali fluoride, namely KF or RbF. The present contribution aims at presenting and discussing the various models proposed up to date to explain grains and grain boundary modifications and their influence on the related solar cell performance.

SESSION: SISAMMonAM-R9	3rd Intl. Symp. Surfaces and Interfaces of Sustainable, Advanced Materials (SISAM)
Mon Oct, 23 2017	Room: Condesa III
Session Chairs: Jean-Marie Dubois; Isabelle Braems; Session Monitor: TBA

16:00: [SISAMMonAM08]
Thin-film Solar Cell Based on Cu1-xGaxSe2: The X-dependent Chemical Properties of the CuIn1-xGaxSe2 Interfaces
Isabelle Braems¹ ; Polyxeni Tsoulka² ; Nicolas Barreau³ ; ¹IMN-CNRS, Nantes Cedex, France; ²IMN, Nantes Cedex 3, France; ³Universite de Nantes, IMN, Nantes, France;
Paper Id: 318 [Abstract]

A solar cell is a device that converts a solar energy into an electric current. Studying the properties of the absorber layer is a key point to optimize its conversion efficiency. In this study, we focus on polycrystalline CuIn1-xGaxSe2 (CIGSe), which is one of the most promising absorber layers for solarcells. So far, best labscale energy conversion efficiencies are achieved for x = [Ga]/([Ga]+[In]) ¡O0.3, while the theoretical x-dependent cell efficiency curve predicts better performances for x ¡O0.75. One possible explanation is that Grain Boundaries (GBs) play a specific role as a function of x. We suggest 2 possible phenomena that can occur at the CIGSe GBs: first, The interface and the grain interior compositions differ, and the nature of the predominant species at the interface varies with x. This is consistent with i) recent results obtained by APT (Atom Probe Tomography), and ii) our simple theoretical-based model of segregation driving forces that combine ab initio and statistical thermodynamics. Secondly, a detrimental solid solution can accumulate within the GBs. Our XRD, RAMAN and EDS analyses demonstrate a different behavior of a Cu-rich compound at low and high Ga-ratio. The nature of the accumulated species or compound at the interface can be detrimental or beneficial for the solar cell efficiency. Hence, in this contribution we discuss both experimentally and theoretically these two scenarios.