Germanium substitution in CZTS solar cells

• Main Author: Goh, Daniel Yong Yi
• Other Authors: Lydia Helena Wong
• Format: Final Year Project
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Materials
• Online Access: http://hdl.handle.net/10356/76715
• Institution: Nanyang Technological University

Copper zinc tin sulfide (Cu2ZnSnS4, CZTS) is a promising non-toxic and earth-abundant material for thin-film solar cells. However, progress in the efficiency of CZTS solar cells is hindered by a large VOC deficit. An effective way to improve the efficiency of CZTS devices is cation substitution, which is the partial substitution of Cu, Zn and Sn with other elements to alleviate efficiency losses from antisite disorder and impurity phases. Recently, one focus has been on the substitution of Sn with Ge, but research has largely focused on the selenide and sulfoselenide, CZTSe and CZTSSe. In this study, the effect of the substitution of Sn with Ge on the crystal structure, morphology, EQE and J-V device characteristics of the pure sulfide CZTS is investigated. XRD results show that the 2θ values of the peaks shift to higher values, indicating incorporation of Ge into the crystal lattice of CZTS. SEM images suggest the formation of secondary phases for Ge substitution beyond 10%. EQE measurements show slightly improved charge collection at 5% Ge substitution, while J-V measurements of devices made from Ge-substituted CZTS show an increased VOC, JSC and fill factor (FF) at 5% Ge substitution which coincides with the peak efficiency for Ge substitution. The improved VOC is postulated to be due to the reduction of Sn-related antisite defects in the crystal lattice. The EQE and J-V device characteristics are then compared against Ge-substituted sulfoselenide CZTSSe devices. The highest efficiency of 6% for the sulfide CZTS is found to occur at 5% Ge, while the peak performance occurs at a higher 10% Ge for the sulfoselenide. Recommendations are made to correct the Ge loss found from EDS results and double cation substitution of Cd and Ge is proposed as a possible future study.