DIFFERENT REACTION PATHWAY FOR THE FORMATION OF Cu_2ZnSnSe_4 THIN FILM FROM DIFFERENT STACKING ORDER OF ELEMENTAL LAYERS

摘要

Most of kesterite based solar cells contain inclusions of the ZnSe phase within the p-type absorber layer. From this fact we hypothesize that the ZnSe becomes a stable structure if Zn and Se are in contact for sufficiently long time and the reaction with other phases is impeded. In this case, the formation of Cu_2ZnSnSe_4 via the following formula is suppressed consequently: Cu_2SnSe_3 + ZnSe → Cu_2ZnSnSe_4. To observe this effect, here three samples were prepared which enable direct-contact (Mo/Cu/Sn/Zn/Se; #1), indirect-contact (Mo/Zn/Sn/Se/Cu; #2), and non-contact (Mo/Zn/Sn/Cu/Se; #3) between Zn and Se layers. Those samples were measured by time resolved in-situ X-ray diffraction to investigate the reaction pathway depending on the elemental stacking order. The experiments showed different temperatures of ZnSe appearance and kesterite crystallization. The samples were also investigtaded by Raman spectroscopy with 488 nm laser light to confirm the presence of ZnSe. Indeed, ZnSe was easily detected in sample #1 and #2, but rarely detected in sample #3. The conclusion is drawn, that delaying the reaction between Zn and Se by avoiding a direct contact between the metals in the precursor stack leads to a reduced amount of ZnSe within the final absorber layer.