Refractive index extraction and thickness optimization of Cu 2 ZnSnSe 4 thin film solar cells Hossam ElAnzeery * ,1,2,3 , Ounsi El Daif 2,4 , Marie Buffi ere 2,5 , Souhaib Oueslati 1,2,6 , Khaled Ben Messaoud 1,2,6 , Dries Agten 2 , Guy Brammertz 7,8 , Rafik Guindi 3 , Bas Kniknie 9 , Marc Meuris 7,8 , and Jef Poortmans 2,5 1 KACST-IntelConsortium Center of Excellence in Nanomanufacturing Applications (CENA), Riyadh, Saudi Arabia 2 Imec–Partner in Solliance, Kapeldreef 75, Leuven 3001, Belgium 3 Department of Microelectronics System Design, Nile University, Cairo, Egypt 4 Qatar Environment and Energy Research Institute (QEERI) Qatar Foundation, Doha, Qatar 5 Department of Electrical Engineering, KU Leuven, Kasteelpark Arenberg 10, Heverlee 3001, Belgium 6 Department of Physics, Faculty of Sciences of Tunis, El Manar, Tunisia 7 Institute for Material Research (IMO), Hasselt University, Wetenschapspark 1, Diepenbeek 3590, Belgium 8 Imec Division IMOMEC–Partner in Solliance, Wetenschapspark 1, Diepenbeek 3590, Belgium 9 TNO, De Rondom 1, Eindhoven 5612 AP, The Netherlands Received 31 October 2014, revised 21 March 2015, accepted 31 March 2015 Published online 21 April 2015 Keywords Cu 2 ZnSnSe 4 , optical absorption, optical properties, refractive index, solar cells, thin films * Corresponding author: e-mail hossam.elanzeery@gmail.com, Phone: þ2 010 900 81 999, Fax: þ97444541528 Cu 2 ZnSnSe 4 (CZTSe) thin film solar cells are promising emergent photovoltaic technologies based on low-bandgap absorber layer with high absorption coefficient. To reduce optical losses in such devices and thus improve their efficiency, numerical simulations of CZTSe solar cells optical character- istics can be performed based on individual optical properties of each layer present in the cell structure. In this contribution, we have first determined the optical coefficients of individual thin films (i.e., (n, k) of the absorber, buffer, and window layers) to build a realistic model simulating the optical behavior of the whole cell stack we propose. Optical characterization was performed using two approaches, one based on ellipsometry measurements for characterizing thin flat cadmium sulfide (CdS) and zinc oxide (ZnO) layers and the other relying on reflectance and transmission (R/T) analysis for the rough CZTSe absorber. Then, we performed numerical simulations using as input experimental optical parameters predicting optimal CZTSe cell structure minimizing optical losses. The impact of each layer’s thickness on the cell’s short- circuit current has been studied. A set of optimal thicknesses of each of the active layers was proposed. Finally, the proposed optical optimization was experimented practically leading to CZTSe cells with 9.7% and 10.4% efficiencies. ß 2015 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction Inorganic thin film solar cells are among the most promising photovoltaic (PV) technologies for cost-effective power generation. This is mainly due to the advantages of thin films for low-cost and high-rate semiconductor deposition over large areas using layers of only few microns. Efficiencies of up to 21% have been demonstrated with Cu(In,Ga)(S,Se) 2 (CIGSSe) at cell level [1]. Recently, kesterite compounds such as Cu 2 ZnSnSe 4 (CZTSe), relatively earth abundant compared to CIGSSe have received significant attention as a potentially inexpensive and reliable absorber material for thin film solar cells [2, 3]. The fundamental properties of CZTSe look promising with a high absorption coefficient of more than 10 4 cm 1 [4] and 12.6% efficient devices have already been reported [5]. Such devices are formed using a multilayer stack of thin films with different optical properties and morphologies. In the case of pure selenide CZTSe phase, a bandgap for CZTSe absorber around 1 eV is commonly reported [6–8] deduced from Raman spectrum and photoluminescence characterization techniques. Opti- cal bandgap versus lattice constant for chalcopyrite I–III– VI 2 and kesterite I 2 –II–IV–VI 4 compounds were also Phys. Status Solidi A, 1–7 (2015) / DOI 10.1002/pssa.201431807 applications and materials science status solidi www.pss-a.com physica a ß 2015 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim