Influence of Ge/(Ge+Sn) composition ratio in Cu2Sn1-xGexS3 thin-film solar cells on their physical properties and photovoltaic performances

Abstract

Effect of Ge/(Ge + Sn) composition ratio of Cu2Sn1-xGexS3 (CTGS) films is examined. The CTGS films are fabricated by sulfurization of the 700-nm-thick Cu–SnS2/Ge stacked precursors on Mo-coated soda-lime glass (SLG) substrates. It is revealed that the thickness of Ge precursor (Ge thickness) from 0 to 420 nm enhances the Ge/(Ge + Sn) composition ratio in the CTGS thin films from 0 to about 0.58, thereby increasing bandgap energy (Eg) from approximately 0.97 (Cu2SnS3: CTS) to 1.23 eV (CTGS), respectively. Not only does Ge precursor act as Ge source to form the CTGS films but also it prevents the formation of undesirable MoS2 secondary phase. In addition, the CTGS solar cells with a structure of SLG/Mo/CTGS/CdS/ZnO/ZnO:Al/Ni/Al are fabricated with the Ge thickness from 0 to 420 nm. It is found that the optimized Ge thickness of 200 nm resulting in the Ge/(Ge + Sn) composition ratio of about 0.32 eliminates the excessive MoS2 secondary phase and increases the Eg, thereby increasing conversation efficiency (η) to 5.6% of the CTGS solar cell, higher than that (3.6%) of the CTS solar cell. However, the η is reduced when the Ge thickness over 200 nm because CTGS quality observed by Urbach energy is deteriorated and the conduction band offset at the CdS/CTGS interface demonstrates the large cliff type heterostructure, thus increasing the open-circuit voltage deficit.