Crystallographic and Optical Properties of (Cu1−xLix)2SnS3 Photovoltaic Semiconductor

Abstract

In order to clarify a lack of knowledge on lithium alloying with Cu2SnS3 (CTS), the authors synthesized monoclinic (Cu1–xLix)2SnS3 with 0.0 ≤ x ≤ 0.10 and characterized their crystallographic and optical properties. Their lattice constants, a, b, and c increased and angle β decreased with increasing Li content. Two band‐gaps (Eg1 and Eg2) are observed for the (Cu1–xLix)2SnS3 samples because there is a double onset in the diffuse reflectance spectra. The Eg1 almost linearly increases from 0.91 eV for x = Li/(Li + Cu) = 0.0 to 0.99 eV for x = 0.1 and the Eg2 also linearly increases from 0.99 eV for x = 0.0 to 1.10 eV for x = 0.1. To understand the band diagram of the (Cu1–xLix)2SnS3 solid solution, the energy level of the valence band maximum (VBM) from the vacuum level is determined from the ionization energy measured by photoemission yield spectroscopy (PYS). The energy levels of VBM for the (Cu1−xLix)2SnS3 solid solution monotonically decreases from −5.18 eV for x = 0.0 to −5.33 eV for x = 0.1 with increasing Li content. On the other hand, the energy levels of the conduction band minimum (CBM) is kept almost constant value by changing Li content. Therefore, the authors consider that the increase in the band‐gap energy of the (Cu1–xLix)2SnS3 solid solution is mainly caused by lowering the VBM level.