Band gap engineering in Cu2ZnGexSn1-xS4 thin film solar cells

Abstract: Photovoltaics based on kesterite Cu2ZnSnS4 (CZTS) has attracted interest as a sustainable alternative to other thin film technologies due to their tunable material properties and earth-abundance. However, the efficiency is limited to 12.6 % for selenium-containing CZTS, with a large voltage-deficit due to tail states, deep defects, secondary phases etc.In this thesis, Ge incorporation in kesterite solar cells was utilized to reduce Sn-related deep defects and tune the band gap. CZTS, Cu2ZnGeS4 (CZGS), and mixed Cu2ZnGexSn1-xS4 (CZGTS) films, and solar cells were investigated. CZGS showed wurtzite-like phases when sputter-deposited in a sulfur-rich atmosphere using GeS target. The mixture of phases could not recrystallize to kesterite phase after annealing, so, metallic Ge target was utilized for CZGTS absorber fabrication. From varying the Ge content in CZGTS, it could be concluded that CZTS grain growth increased at a low concentration of Ge.Sulfurization of CZTS layers deposited on top of CZGS was done to achieve band gap grading. Glow Discharge Optical Emission Spectroscopy showed smooth grading while Scanning Tunneling Microscopy/Energy Dispersive Spectroscopy showed a separation between larger Sn-rich grains at the front and smaller Ge-rich grains at the back. For longer annealing times, recrystallization of the complete film was seen together with a smeared-out grading.Germanium-rich absorbers often delaminated during the etching of the annealed samples. Adhesive TiN interlayer was used on the Mo-coated Soda-Lime Glass substrate to avoid delamination with partial success. Ge-containing samples showed oxygen-rich grain boundaries and voids. Oxide removal during etching has a possible connection to the issues with adhesion.Solar cell performance was not improved for graded absorbers compared to CZTS. An increasing cliff-like band alignment with the buffer layer could negate the benefit of a band gap gradient since Ge diffused to the front. CZGS solar cells with alternative buffer layer were fabricated to investigate front interface improvement. Zn1-xSnxOy (ZTO) buffer layers were deposited at various temperatures and thicknesses. The open-circuit voltage (Voc) increased to 1.1 V for CZGS/ZTO solar cells; however, Voc was relatively insensitive to ZTO band gap variations. The current was generally low but improved with KCN-etching of the CZGS absorber before deposition of the ZTO buffer layer. A possible explanation for the device behavior is the presence of an oxide interlayer for non-etched devices.The backside interface recombination can be reduced using band gap grading as well with passivation layers. Therefore, ultrathin CZTS with oxide passivation layers of AlxOy or SiOx on the back contact was investigated. The solar cell parameters improved with the addition of a thin oxide layer, but blocking behavior increased with passivation layer thickness.