Eur. Phys. J. Appl. Phys. 42, 187–191 (2008) DOI: 10.1051/epjap:2008073 T HE EUROPEAN P HYSICAL JOURNAL APPLIED PHYSICS Synthesis and characterization of In 2 S 3 : Na thin films prepared by vacuum thermal evaporation technique for photovoltaic applications A. Timoumi a , H. Bouzouita, and B. Rezig Photovoltaic and semiconductor materials laboratory National Engineering School of Tunis, Belvedere PO Box 37, 1002 Tunis, Tunisia Received: 19 January 2008 / Accepted: 11 February 2008 Published online: 30 April 2008 – c  EDP Sciences Abstract. In2S3 thin films containing different quantities of sodium have been synthesized by co- evaporation of sodium and In2S3 powder from separate sources using vacuum thermal evaporation method. Films were deposited on ordinary glass at 240 ◦ C. The process of incorporation of sodium was studied as function of at.% Na. Films have been characterised by means of X-ray diffraction, SEM, EDAX and spectrophotometry. X-ray diffraction analysis confirmed the initial amorphous nature of deposited lay- ers and revealed the formation of In2S3 as function of annealing layers containing sodium in nitrogen at 300 ◦ C for 2 h. Energy Dispersive X-ray Analysis (EDAX) revealed the composition of the films as a function of the sodium incorporation. Surface Electron Microscopy showed that these films were granular and homogenous. The films have an n-type electrical conductivity and their optical direct band gap can be managed between 2.20 and 2.45 eV by controlling their sodium content. The variation of parameters for as-deposited and annealed films has been studied within x ≤ 4 at.% solid solution composition. Thin layers with homogeneous surfaces, direct band gap energy (Eg) of about 2.45 eV for 4 at.% Na and 0.9 μm-thick have been achieved. PACS. 61.82.Fk Semiconductors – 81.15.Ef Vacuum deposition – 68.55.-a Thin film structure and morphology 1 Introduction Thin film solar cells based on Cu(In,Ga)Se 2 , and CuInS 2 absorber layers showed promising solar energy conversion efficiencies higher than 19.5% [1] using CdS as buffer layer. But, presently efforts are there for replacing CdS layer by indium sulfide thin films to avoid toxic cadmium in order to make more ecofriendly photovoltaic technology. The substitution of the cadmium sulfide (CdS) buffer layer is an important issue for the Cu(In,Ga)(S, Se) 2 -based so- lar cell community [2,3]. Until now, solar cells with in- dium sulfide [4] thin films as alternative buffer layers have demonstrated efficiencies similar to those reached with CdS. So, there has been significant increase in research on III 2 - VI 3 materials. These materials have found great use in the electronic industry for optoelectronic or pho- tovoltaic applications [5–7]. There are many reports on different techniques used for the preparation [8] as well as the characterization of indium sulfide thin films [9–11]. Recently, Naghavi et al. [12] did a systematic study on the influence of different deposition parameters on growth of In 2 S 3 thin films by Atomic Layer Deposition (ALD). a e-mail: timoumiabdelmajid@yahoo.fr Ernits et al. [13] studied the effect of substrate temper- atures of In x S y thin film prepared by ultrasonic spray pyrolysis; their objective is to replace chemical bath de- posited (CBD) CdS buffer layers in Cu(In,Ga)Se 2 (CIGS) solar cells. Earlier we studied effect of the temperature on the growth of In 2 S 3 by thermal evaporation tech- nique [14]. Indeed, indium sulfide have exhibited a direct optical transition with a band gap energy (E g ) around 2.0–2.3 eV [15,16], which increases up to 2.9 eV by adding oxygen [17,18]. Although an indirect optical transition (E g =2.2 eV) has also been reported [19]. In this pa- per, we investigate the possibility to synthesize In 2 S 3 thin films containing a controlled rate of sodium and study their effect on the structural, morphological and optical properties. 2 Experiments 2.1 Thin films preparation Indium sulfide thin films containing Na were prepared by co-evaporation of the In 2 S 3 powder and the Na element in a high-vacuum system with a law pressure of 13 × 10 -4 Pa Article published by EDP Sciences