6533b859fe1ef96bd12b7481

RESEARCH PRODUCT

Development of Kesterite (Cu2ZnSnS4) absorber material for solar cell application.

subject

description

Kesterite-based solar cells still suffer from limited efficiency due to various problems related to the absorber and device interfaces. Absorber defects and non-ideal band alignment at the absorber/buffer hetero-interface, resulting in a cliff-like conduction band offset (CBO), and non-ideal absorber/back contact interfaces are the main factors limiting efficiency.This thesis focuses mainly on the absorber-related problems such as the secondary phases and the band alignment at the absorber-buffer heterojunction as well as the back interface.In the first part, the properties of the absorber layer were studied. Thin layers of Cu2ZnSnS4 (CZTS) were obtained by chemical route. This study revealed the formation of a pure kesterite phase with correct stoichiometry and oxidation state as well as good optical properties.Then, we estimated the band alignment at the CZTS-CdS hetero-interface experimentally by direct X-ray photoelectron spectroscopy (XPS) measurements. XPS spectroscopy was used to determine the valence band offset (VBO) directly by determining the valence band positions at the heterointerface.The conduction band offset (CBO) value was estimated based on UV/Visible spectroscopy band gap and VBO measurements. The results indicate that the CZTS-CdS heterojunction is type II with a negative cliff-like CBO.Next, we developed a new strategy to address the unfavorable CBO issue at the CZTS-CdS heterojunction by partial substitution of zinc. With this strategy, we were able to highlight a favorable spike-like CBO by partial substitution of Zn with Cd and nearly flat band CBO by partial substitution of Zn with Mn.Finally, a "superstrate" cell architecture was developed by reversing the technological process in order to overcome the back interface problem so as to facilitate the diffusion of the Cd from the buffer layer to the absorber with a view to obtaining an ideal band alignment. From our results, solar cells fabricated with this architecture using FTO and ITO as electrodes show interesting and promising performance. An efficiency of 2.44% was obtained for the ITO-based device, which represents the best current efficiency for this design.