Observation of Sodium Diffusion in CIGS Solar Cells with Mo/TCO/Mo Hybrid Back Contacts
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Observation of Sodium Diffusion in CIGS Solar Cells with Mo/TCO/Mo Hybrid Back Contacts Yukiko Kamikawa1, Hironori Komaki1, Shigenori Furue1, Akimasa Yamada1, Shogo Ishizuka1, Koji Matsubara1, Hajime Shibata1, Shigeru Niki1 1
Research Center for Photovoltaic Technologies, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan ABSTRACT CIGS solar cells were fabricated on a hybrid back contact comprised of a TCO layer (ZnO:Ga (GZO)) and Mo layers. It was discovered that an additional Mo layer introduced underneath the TCO layer promotes sodium diffusion through the TCO back contact into the upper CIGS absorber layer. Improvement in VOC and JSC values relative to those of sodium-free solar cells was achieved with the Mo/GZO/Mo hybrid back contact as a result of the enhanced sodium diffusion. INTRODUCTION Chalcopyrite Cu(In,Ga)Se2 (CIGS) and related multinary compounds are attracting much attention due to their applicability in highly efficient and cost-effective solar cell modules1-3. In these systems, alkali metals, especially sodium (Na), are widely known to play an important role in improving photovoltaic performance4, 5. Alkali doping via diffusion from a soda-lime glass (SLG) substrate is one of the easiest ways to incorporate alkali metals into the CIGS absorber, and high efficiencies (>20%) have been achieved using the doping methods1,6.Transparent conductive oxides (TCOs), such as In2O3, SnO2, and ZnO, enable formation of bifacial or tandem transparent structures. In addition, they enable light trapping with textured structures that can be readily formed by optimization of the deposition conditions or through chemical etching. However, when a TCO is introduced as the back contact on an SLG substrate, the preferable diffusion of alkali metals is blocked by the oxide layer7, 8. In this study, it was discovered that sodium diffusion through the TCO back contact into the upper CIGS absorber layer was remarkably promoted when an additional Mo layer was introduced underneath the TCO back contact (ZnO:Ga (GZO)). The concentration of diffused sodium into CIGS absorbers deposited on variously structured back contacts (GZO, Mo, Mo/GZO, GZO/Mo, Mo/GZO/Mo) on SLG substrates were studied using secondary ion mass spectrometry (SIMS) and electron probe microanalyzer (EPMA). The photovoltaic (PV) properties of CIGS solar cells fabricated on variously structured back contacts (Mo/GZO, Mo/GZO/Mo, and Mo) on SLG substrates were also evaluated. EXPERIMENT In this study, back contacts comprised of Mo, GZO, Mo/GZO, GZO/Mo, and Mo/GZO/Mo structures were used to fabricate CIGS solar cells. Mo and GZO layers were fabricated using radio-frequency (13.56 MHz) magnetron sputtering and reactive plasma
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deposition, respectively. Each electrode was fabricated on an SLG substrate. The structures of the typical CIGS solar cells fabricated in this study are shown in Fig. 1. The CIGS films were deposited on the back contact/SLG structures using a three-stage process4, 9, 10. The hi
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