Cu-related recombination in CdS/CdTe solar cells S.H. Demtsu a, ⁎ , D.S. Albin b , J.R. Sites a , W.K. Metzger b , A. Duda b a Department of Physics, Colorado State University, Fort Collins, CO 80523, USA b National Renewable Energy Laboratory, Golden, CO 80401, USA Received 17 July 2006; received in revised form 17 July 2007; accepted 8 August 2007 Available online 16 August 2007 Abstract Cu used in the back contact of CdS/CdTe solar cells is known to improve contact behavior and open-circuit voltage. A study of devices made with varying Cu amounts confirmed these observations. However, Cu was also found to be deleterious to current collection. Time-resolved photoluminescence measurements of CdTe devices show that carrier lifetime decreased with increased Cu concentration. Drive-level-capacitance- profiling and low-temperature photoluminescence suggest this decrease in lifetime was associated with increased recombination center density introduced by Cu in the CdTe layer. The resulting impact of increased Cu on device performance was a voltage-dependent collection of photogenerated carriers that reduced fill-factor. © 2007 Published by Elsevier B.V. Keywords: Cadmium telluride; Solar cells; Photoluminescence; Capacitance profiling 1. Introduction High performance CdTe-based solar cells require the formation of a low-barrier, low-resistance contact, which is frequently accomplished by incorporating Cu at the back contact [1,2]. In general, the back barrier is minimized, and the open-circuit voltage (V oc ) is increased, with the application of an optimal amount of Cu. However, Cu can form both deep donors Cu i and acceptors Cu Cd in CdTe [3]. Substitutional Cu (Cu Cd ) dopes the CdTe more p-type, and interstitial Cu (Cu i ) compensates the p-CdTe. Trap levels due to deep donors caused by formation of Cu complexes in p-CdTe have been reported [3–5], but there has been no direct evidence that Cu increases recombination in CdTe solar cells. Cu diffusion is also reported to be responsible for long-term degradation [6], though recent work suggests that this can be stabilized [7]. In this study, the effect of Cu on CdTe devices was studied by systematically evaporating Cu metal of varying thickness directly onto Te-rich CdTe surfaces prior to application of Pd/Al metal back contact layers. 2. Device fabrication Conventional superstrate solar cell devices were fabricated at NREL by depositing 80-nm CdS and 9-μm CdTe on SnO 2 :F coated glass substrates by chemical-bath deposition and close- spaced sublimation, respectively. Prior to the application of the back contact, the structures were heat treated in a vapor mixture of CdCl 2 and O 2 . After the CdCl 2 treatment, all devices were etched in a nitric–phosphoric acid solution to remove surface oxides, and to create a Te-rich CdTe surface. Cu metal layers with thicknesses varying from 0 to 100 nm were then deposited by electron-beam evaporation onto unheated device structures with a deposition rate of 0.01–0.05 nm/s. The contact was then annealed in helium at 280 °C for 25 min to promote Cu diffusion. X-ray diffraction confirmed the formation of Cu 1.4 Te during this latter step. The control samples, which had no intentional Cu introduced at the back contact, underwent an identical anneal. All devices were completed by evaporating 60 nm of Pd and 300 nm of Al to form the final current carrying electrode. 3. Experimental results and discussion Current density–voltage (J–V) curves for typical devices are shown in Fig. 1. Devices made without intentional Cu were Available online at www.sciencedirect.com Thin Solid Films 516 (2008) 2251 – 2254 www.elsevier.com/locate/tsf ⁎ Corresponding author. E-mail address: sdemtsu@solopower.com (S.H. Demtsu). 0040-6090/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.tsf.2007.08.035