Band gap and interface engineering of wide gap Cu-containing chalcopyrite absorbers by dry (In,Ga)-S surface treatments
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Band gap and interface engineering of wide gap Cu-containing chalcopyrite absorbers by dry (In,Ga)-S surface treatments David Fuertes Marrón, Sebastian Lehmann, Thomas Schedel-Niedrig, Joachim Klaer, Reiner Klenk, and Martha Ch. Lux-Steiner Solar Energy Department, Hahn-Meitner Institut, Glienicker Str. 100, D-14109 Berlin, Germany ABSTRACT Device-grade CuGaSe2 (CGSe) and CuInS2 (CIS) thin films for photovoltaic applications have been subjected to dry surface treatments based on In-S and Ga-S by means of chemical vapor deposition (CVD), carried out in an open-tube system. Film properties have been monitored from time and temperature processing series. Improved PV performance has been demonstrated from devices based on treated CGSe compared to those based on reference samples. Numerical simulations have been performed, pointing out the conditions such surface treatments should fulfil in order to improve the performance of devices based on wide gap absorbers. INTRODUCTION The appearance of recombination paths at the junction with a reduced energy barrier from that corresponding to the absorber gap hinders so far the achievement of highly efficient devices based on wide gap Cu-containing chalcopyrite absorbers. In this respect, certain design rules have been reported elsewhere [1,2]. It has been found that interface-limitation in wide-gap-based devices is independent of the main chalcogen species (either selenium or sulfur) in the absorber, and that, as far as it has been tested, is also independent of the buffer employed as junction partner. In this contribution we explore the possibility of modifying the near-surface region of wide gap chalcopyrite absorber films made of CuGaSe2 (CGSe) and CuInS2 (CIS), as a means of controlling the structural and electronic properties of the junction with CdS in complete devices. The incorporation of In in the near surface region of CGSe grown by sequential evaporation has demonstrated a positive impact on the PV performance of related devices [3]. The intention of this approach was to overcome interface-related limitations by making the surface to look like that of low-gap high efficiency Cu(In,Ga)Se2-based devices. The amount of In to be used in the treatment seems to be limited due to the band gap reduction associated with In incorporation. On the other hand, simultaneous incorporation of S during the treatment may alleviate constraints in the amount of In offered to the starting CGSe, compensating for gap narrowing. From our results, it turns out that the In-S CVD-treatment leads to an effective gap reduction of the CGSe, resulting from material alloying [4]. However, this gap reduction is not necessarily accompanied with detrimental effects on device performance, as concluded from numerical calculations discussed in the next sections, provided certain design rules are observed. Ga is the natural group-III isovalent substitutional element in CIS, leading to an increase of the energy band gap and the voltage output expected from related devices [5]. Ga grading of CIS in
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