Theoretical and experimental investigation of the recombination reduction at surface and grain boundaries in Cu(In,Ga)Se
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Theoretical and experimental investigation of the recombination reduction at surface and grain boundaries in Cu(In,Ga)Se2 solar cells by valence band control Takahito Nishimura1, Yoshiaki Hirai1, Yasuyoshi Kurokawa1 and Akira Yamada1, 2 1 Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1-NE-16 O-okayama, Meguro-ku, Tokyo 152-8552, Japan 2 Photovoltaics Research Center (PVREC), Tokyo Institute of Technology, 2-12-1-NE-16, Ookayama, Meguro-ku, Tokyo, 152-8552, Japan ABSTRACT We carried out theoretical calculation for Cu(In,Ga)Se2 (CIGS) solar cells with energy bandgap of 1.4 eV assuming formation of a Cu-poor layer on the surface of CIGS films. This calculation result revealed that formation of a thinner Cu-poor layer such as a few nanometers leads to improvement of the solar cells performance. This is because interfacial recombination was suppressed due to repelling holes from the interface by valence band offset (ΔEV). Next, we investigated composition distribution in the cross section of CIGS solar cells with Ga contents of 30% and 70% by transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX). It was revealed that the Cu-poor layer was formed on the surface and at the grain boundary (GB) in the case of conversion efficiency (η) of 17.3%, although it was not formed in the case of lower η of 13.8% for a Ga content of 30%. These results indicate that formation of the Cu-poor layer contributed to improvement of cell performance by suppression of carrier recombination. Moreover, it was also confirmed that although the Cu-poor layer was observed on the surface, it was not observed at the GB in the case of CIGS solar cells with a Ga content of 70% which had η of 12.7%. It is thought that the effect of repelling holes by ΔEV is not obtained at the GB and the solar cell performance in the Ga content of 70% is lower than that in the Ga content of 30%. Thus, we suggest importance of the Cu-poor layer at the GB for high efficiency of CIGS solar cells with high Ga contents. INTRODUCTION So far Cu(In,Ga)Se2 (CIGS) solar cells with energy bandgap (Eg) of 1.15 eV obtained above conversion efficiency (η) of 20% have been achieved by several groups [1,2]. Although generally it has been known that the best solar cell for Eg of 1.4 eV is expected due to matching sunlight spectrum, η deteriorates more than Eg of 1.2 eV [3]. It is thought that some reasons are the cause for the reduction of η [4-6], but these are not an absolute cause. In this study, to reveal the reason, we focused on the surface and at the grain boundary (GB) in CIGS thin film. It has been often reported that a decline of Cu contents has been observed at the surface [7,8] or the GB [9] in CIGS thin films fabricated by three stage methods. If Cu contents of CIGS decreases, valence band maximum fall down although conduction band minimum does not change. Cu(In,Ga)3Se5 is one of the materials which has lower Cu contents more than CIGS and has valence band offset (ΔEV) of about 0.3 eV [8,10]. Our group has revealed that su
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