ANALYSIS OF CDTE ACTIVATION TREATMENT WITH A NOVEL APPROACH Andrei Salavei^, Ivan Rimmaudo^, Fabio Piccinelli*, Daniele Menossi + , Alessio Bosio + , Nicola Romeo + , and Alessandro Romeo^ ^LAPS-Laboratory for Applied Physics, Department of Computer Science, University of Verona, Ca' Vignal 1, Strada Le Grazie 15, 37134 Verona, Italy, tel.:+39-045-8027974, e-mail address: alessandro.romeo@univr.it * Department of Biotechnology, University of Verona, Strada Le Grazie 15, 37134 Verona, Italy + Physics and Earth Science Department, University of Parma, V.le G.P. Usberti, 7A-43124, Italy ABSTRACT: In this paper we propose a new approach for CdTe activation treatment. Starting from a baseline of CdTe devices made with CdCl 2 activation process we present a double treatment where CdCl 2 and Freon ® , namely difluorochloromethane, treatments are applied in sequence. This new activation process delivered solar cells with efficiencies well above 15%. The mechanisms for these improved efficiencies are studied by morphological (high resolution microscopy), structural (X-ray diffraction analysis) and electrical measurements (capacitance-voltage and current–voltage characteristics) of finished devices. Keywords: CdTe, CdCl 2 , High-Efficiency. 1 INTRODUCTION CdTe thin film solar cells are the most successful thin film photovoltaic devices in terms of production yield, as attested by the latest market performance. Nevertheless this device has also shown to be competitive in terms of efficiency with the latest results from General Electric, where a record efficiency of 19.6% has been obtained [1]. Contrarily to other thin film devices, like CuInGaSe 2 and Cu 2 ZnSnS 4 , CdTe needs in order to perform high conversion efficiencies a morphological and electrical transformation which is generally given by an annealing of the CdTe layer after deposition of CdCl 2 at 400-430 °C in air [2]. Other successful activation treatments have also been applied such as annealing of the CdTe layer in argon and chlorine containing gases (such as Freon ® , namely difluorochloromethane) atmosphere at high temperature [3]. In former publications we have analyzed these different activation treatments on low temperature deposited CdTe device, by tuning the amount of CdCl 2 [4] and by changing the quantity of Freon ® and temperature [5]. In this paper we compare our standard CdCl 2 treated CdTe with our Freon ® treated CdTe and with absorbers that have been treated with both processes (double treatment). This so-called double treatment consists of annealing the CdTe at 400 °C in Freon ® and Argon atmosphere followed by the CdCl 2 deposition in methanol solution and annealing in air. Solar cells with high efficiencies (above 15%) have been obtained with this process. In this paper we present a thorough study of these differently treated devices that are compared with typical CdCl 2 treated devices in order to address the different effects of the new activation treatment. 2 EXPERIMENTAL In our laboratory CdTe devices on superstrate configuration are prepared at temperatures not exceeding 450 °C. As a substrate, a direct current sputtered indium tin oxide (ITO) plus radio frequency sputtered zinc oxide (ZnO) coated glass is used. In the same vacuum chamber 300 nm of CdS and 6 microns of CdTe are deposited at substrate temperatures respectively of 100°C and 340°C. As previously mentioned, the CdTe activation treatment is typically made by depositing a saturated solution of CdCl 2 in methanol and by annealing the stacks in a furnace at 410 °C. Prior to the deposition of the back contact, the CdTe surface is etched by a bromine/methanol (Br-MeOH) solution in order to remove CdCl 2 residues and to make a p + intermediate layer. The back contact is made by a subsequent evaporation of 2nm copper and 50nm gold in the same vacuum chamber at room temperature. Structural analysis of CdTe layers by means of X- Ray diffraction (XRD), Atomic Force Microscopy (AFM) and electronic study of finished devices by means of current density-voltage (JV), capacitance-voltage (CV) and drive level capacitance profiling (DLCP) techniques are applied to clarify the effects of double chlorine treatment on the CdTe layers. AFM was applied with a NT-MDT Solver Pro in semi-contact mode, XRD on the different absorber layers has been performed by a Thermo ARL X´TRA powder diffractometer, operating in Bragg-Brentano geometry equipped with a Cu-anode X-ray source (Kα, λ =1.5418 Å) and using a Peltier Si(Li) cooled solid state detector. JV characteristics are performed with a Keithley SourceMeter 2420 at room temperature with an irradiation of 1000W/m 2 ; DLCP andCV study is made with a HP4284A LCR and with a Janis cryostat with Lakeshore 325 temperature controller in a vacuum of 10 -6 mbar and in a range of temperature between 100 K and 320 K. 3 DISCUSSION 3.1 Introduction In a previous work a thorough study of the effects of different quantity of CdCl 2 -methanol solution with same annealing temperature has been presented [5]. From these studies efficiencies exceeding 14% are routinely obtained [6]. On the other hand a different treatment with chlorine containing gases, such as Freon ® , mixed with Argon has 28th European Photovoltaic Solar Energy Conference and Exhibition 2140