The effect of a high temperature reaction of Cu-In-Ga metallic precursors on the formation of Cu(In,Ga)(Se,S) 2
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The effect of a high temperature reaction of Cu-In-Ga metallic precursors on the formation of Cu(In,Ga)(Se,S)2 Dominik M. Berg, Christopher P. Thompson, William N. Shafarman Institute of Energy Conversion, University of Delaware, Newark, DE 19716, U.S.A. ABSTRACT The influence of higher processing temperatures on the formation reaction of Cu(In,Ga)(Se,S)2 thin films using a three step reactive annealing process and on the device performance has been investigated. High process temperatures generally lead to the formation of larger grains, decrease the amount of void formation and their distribution at the back Mo/Cu(In,Ga)(Se,S)2 interface, and lead to a much faster formation reaction that shortens the overall reaction process. However, high temperature processing also leads to a decrease in device performance. A loss in open circuit voltage and fill factor could be attributed to enhanced interface recombination processes for the samples fabricated at higher process temperatures, which itself may be caused by a lack of Na and subsequent poor passivation of interface defect states. The lack of Na resulted in a decrease in free charge carrier concentration by two orders of magnitude. INTRODUCTION Reactive annealing of Cu-In-Ga precursors in selenium and/or sulfur-containing atmospheres is of high interest for the commercial manufacture of Cu(In,Ga)(Se,S)2 (CIGSS) absorber layers for photovoltaic devices, which have already achieved power conversion efficiencies of 17.5 % for sub-modules [1]. To date, this process has been predominantly conducted using soda-lime glass (SLG) as the substrate, which limits the reaction temperature for industrial purposes to around 520°C due to its low strain point [2]. However, photovoltaic specialty glasses are under development, which permit processing temperatures up to 650°C. Recent results for Cu(In,Ga)Se2 (CIGS) co-evaporation, at temperatures between 600°C and 650°C, have demonstrated more uniform Ga-profiles, enhanced grain size, and improved open circuit voltage (Voc) and fill factor (FF) leading to increased device efficiencies [3, 4]. The reactive annealing of CIGSS has a number of commonly observed issues at the CIGSS/Mo interface, including poor adhesion, Ga accumulation, and void formation [5]. While these issues can be partly controlled at lower temperatures on SLG, increasing processing temperature offers an additional pathway to address these issues. In the present work, we examine whether higher processing temperatures enabled by alkali-containing high temperature specialty glass substrates (HTG), are also beneficial for the reactive annealing process. EXPERIMENT For the conducted experiments, Cu-In-Ga metallic precursors (thickness ≈ 650 nm) were prepared by sputtering multiple alternating layers of Cu0.77Ga0.23 alloy and elemental In onto Mocoated (thickness ≈ 700 nm) HTG substrates (4” x 4”). To form CIGSS, the precursors were
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reacted using a three-step process in H2Se/Ar/H2S atmospheres (see Figure 1) as described by Kim et al. [5]. This process is similar to th
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