Microstructural features of cadmium telluride photovoltaic thin film devices Venkatesan Manivannan a , Robert A. Enzenroth a , Kurt L. Barth a , Sandeep Kohli b , Patrick R. McCurdy b , Walajabad S. Sampath a, ⁎ a Materials Engineering Laboratory, Department of Mechanical Engineering, USA b Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA Received 15 January 2007; received in revised form 20 April 2007; accepted 18 May 2007 Available online 29 May 2007 Abstract In order to study the microstructure of cadmium telluride (CdTe) photovoltaic thin film solar cells, manufactured by an in-line manufacturing process, Scanning Electron Microscopy characterization (SEM) and X-ray diffraction (XRD) characterization were performed. SEM measurement showed that no substantial changes in the grain structure of CdTe layers occurred during the Cadmium Chloride (CdCl 2 ) treatment. No change in the cubic CdTe lattice parameter “a” was observed for the CdCl 2 treated sample. It is inferred that the primary effect of the CdCl 2 treatment in the devices studied is the passivation of grain boundaries and bulk defects. XRD studies show a loss of preferred orientation (as determined from the peak ratios) of planes during the copper compound treatment indicating recrystallization of the grains due to the Cu treatment. Also the Cu treated sample showed decrease in value of the lattice parameter “a”. © 2007 Elsevier B.V. All rights reserved. Keywords: Cadmium telluride; X-ray diffraction; Scanning electron microscopy 1. Introduction CdTe based Photovoltaic (PV) devices are made up of several thin film layers. These films are deposited by techniques like close-spaced sublimation, high vacuum evaporation, chemical bath deposition, radio frequency sputtering and the like. Each film in the device has a particular function towards device performance. We have recently described an in-line process [1,2] where CdTe PV devices are continuously manufactured. The device structure is shown in Fig. 1. A National Renewable Energy Laboratory certified efficiency of 12. 44% was obtained for such devices manufactured with commercially available soda lime glass. The polycrystalline CdS layer with a band gap of 2.42 eV is largely transparent. The thickness of this layer is important so as to not to shunt the cell but at the same time allowing most of the sunlight to be converted to charge carriers. The p–n junction for the solar cell is formed when a p-CdTe layer is deposited following the n-CdS layer. CdTe devices were manufactured by the patented heated pocket deposition-(HPD) [1] process developed in our labora- tory. The fabrication system is a continuous, all in-line system where all device fabrication steps are performed in one vacuum chamber operating at one pressure. An automated conveyor belt extends from air, through all the processing stations in vacuum and then back to air. The pilot system is being used to determine optimum process conditions (substrate temperature, vapor flux and residual gas compositions) at the film growth interface [3]. Processing conditions including the CdCl 2 flux and substrate temperature were optimized for the stability and performance of the device [4]. Also, controlling process parameters for Cu based contacts is critical to obtain high efficiencies as well as maintaining resistance to degradation [4]. 1.1. Effect of film thickness on device performance In addition to controlling the CdCl 2 treatment and the processing conditions for the back contact, it has been observed that controlling and optimizing CdTe thickness is also critical. Variations in CdTe thickness can alter the effectiveness of the CdCl 2 treatment due to changes in the film volume needed to be Available online at www.sciencedirect.com Thin Solid Films 516 (2008) 1209 – 1213 www.elsevier.com/locate/tsf ⁎ Corresponding author. Tel.: +1 970 491 8619; fax: +1 970 491 8671. E-mail address: sampath@engr.colostate.edu (W.S. Sampath). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.05.043