Incorporation of Sb, Bi, and Te Interlayers at the Mo/Cu-In-Ga Interface for the Reaction of Cu(In,Ga)(Se,S) 2
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Incorporation of Sb, Bi, and Te Interlayers at the Mo/Cu-In-Ga Interface for the Reaction of Cu(In,Ga)(Se,S)2 Kihwan Kim1, Jaesung Han1,2 and William N. Shafarman1 1 Institute of Energy Conversion, University of Delaware, Newark, DE 19716, USA 2 Yeungnam University, Gyeongsan, Gyeongbuk 712-749, Republic of Korea ABSTRACT In this work, we investigate the effects of Sb, Bi, or Te interlayers at the Mo/Cu-In-Ga interface on the reaction to form Cu(In,Ga)(Se,S)2 in order to control void formation and improve adhesion. Interlayers with 10 nm thickness were evaporated onto the Mo back contact prior to sputtering the metal precursors. CIGSS absorber layers were formed by a three-step H2Se/Ar/H2S reaction and solar cells were fabricated. The influences of each interlayer were characterized in the precursor and reacted films in terms of the density of the void formation, film structure and morphology, adhesion, and device performance. INTRODUCTION Selenization/sulfization of Cu-In-Ga metal precursors is being developed as a commercialscale method of fabricating CuInSe2-based solar cells. Reaction in H2Se and H2S produced 30 x 30 cm2 sub-modules with efficiency > 17% [1]. Critical issues in the reaction of metal precursors in Se- and/or S- containing atmospheres to form Cu(In,Ga)Se2 (CIGS, or when S is included, CIGSS) include void formation and poor adhesion at the CIGSS/Mo interface. These may result in reduced device performance yield and a narrower process window compared to co-evaporated CIGS cells. It has been proposed that the voids result from the agglomeration of slow reacting Cu-Ga intermetallic phases during selenization [2]. The group Va elements, Sb and Bi, have been reported to enhance recrystallization of CIGSS via a surfactant effect [3,4] and Te has been shown to induce wetting of metallic species on Mo [5]. In this work, the incorporation of Sb, Bi or Te interlayers before the metal precursor was investigated to promote greater intermixing/dispersing of the Cu-Ga intermetallic, induce more uniform selenization and minimize void formation by reducing the slow-reacting CuxGa intermetallic. With each interlayer the density of the void formation, film structure and morphology, adhesion, and device performance were characterized. EXPERIMENT Three types of interlayers were deposited onto Mo-coated soda-lime glass prior to metal precursor deposition. Using electron-beam evaporation, 10 nm-thick Sb, Bi, and Te thin layers were deposited onto the Mo back contact. Then, CuInGa metal precursors were deposited onto SLG/Mo substrate by sputtering of Cu0.77Ga0.23 and In targets. With a rotating substrate platen, ~700 alternating Cu0.77Ga0.23 and In layers yield 650-nm thick precursors with Ga/(Ga+In) § 0.26 and Cu/(Ga+In) § 0.91, as measured by X-ray fluorescence (XRF).
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The metal precursors with and without interlayers were reacted to form CIGSS absorbers by a three-step H2Se/Ar/H2S reaction which consists of selenization at 400 °C for 50 min (1st step), Ar annealing at 550 °C for 20 min (2nd step) and sulfization
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