Band Alignment of CdS/Cu 2 ZnSnSe 4 Heterointerface and Solar Cell Performances
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Band Alignment of CdS/Cu2ZnSnSe4 Heterointerface and Solar Cell Performances Takehiko Nagai1, Shinho 2Kim1, Hitoshi Tampo21, Kang Min Kim1, Hajime Shibata1, Shin'ichi 2 2 Takaki , Kenta Kawasaki , Suehiro Kawamura , Takuya Shimamura , Koji Matsubara1, Shigeru 1 2 Niki , Norio Terada 1
Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Central 2 Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan 2 Kagoshima University, Korimoto 1-21-40, Kagoshima 890-0065, Japan ABSTRACT We determined that the conduction band offset (CBO) and the valence band offset (VBO) at the CdS/ Cu2ZnSnSe4 (CZTSe) heterointerface are +0.56 and +0.89eV, respectively, by using X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and inversed photoemission spectroscopy (IPES). A positive CBO value, so-called “spike” structure, means that the position of conduction band becomes higher than that of absorber layer. The evaluated CBO of +0.56 eV suggests that the conduction band alignment at CdS/CZTSe interface is enough to become an electron barrier. Despite such a large spike structure in the conduction band at the interface, a conversion efficiency of 8.7 % could be obtained for the CdS/CZTSe heterojunction solar cells. INTRODUCTION For the recent commercial based thin film solar cells, Cu(In, Ga)Se2 (CIGS) and CdTe are great attractive materials as an absorber layers. However, these solar cells include toxic and rare earth elements. The Cu2ZnSnSe4 (CZTSe) kesterite is one of the candidate absorber materials,[1-5] which can replaced the solar cells based on CIGS and CdTe, because the CZTSe does not include rare earth elements such as In, Ga, and Te. Moreover, CZTSe has a strong optical absorption and proper carrier concentration,[6] and its band gap energy can be finely tuned by adjusting the anion mixing ratio, for example Se/S, from 0.9–1.5 eV. [7, 8] Recently, the efficiency of CZTSSe-based solar cells has reached 12.6% [3] using CdS buffer layer by chemical bath deposition and using the Se-rich CZTSSe with a bandgap energy range of 1.1–1.2 eV. This conversion efficiency is lower than those of CIGS and CdTe heterojunction solar cells. In addition to the quality of CZTSe absorber layer, we consider another main contributor to this poor efficiency as an unfavourable conduction-band offset (CBO) at the interface between CZTSe absorber- and CdS buffer-layer, which can induce a rapid carrier recombination at trap states near the interface. Actually, it is well-known that the band alignment at the heterojunction
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interface plays a crucial role to realize high performance CIGS solar cells.[9-11] Notably, tuning the conduction band offset CBO in a small positive value results in a high open circuit voltage. Despite of several theoretical stud
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