The Role of Sulfur in Solution‐Processed Cu₂ZnSn(S,Se)₄ and its Effect on Defect Properties

Abstract

Understanding the electrically active defects in kesterite Cu2ZnSn(S,Se)4(CZTSSe) is critical for the continued development of solar cells based on this material, but challenging due to the complex nature of this polycrystalline multinary material. A comparative study of CZTSSe alloys with three different bandgaps, made by introducing different fractions of sulfur during the annealing process, is presented. Using admittance spectroscopy, drive level capacitance profiling, and capacitance‐voltage profiling, the dominant defect energy level present in the low sulfur content device is determined to be 0.134 eV above the valence band maximum, with a bulk defect density of 8 × 1014 cm−3, while the high sulfur content device shows a deeper defect energy level of 0.183 eV and a higher bulk defect density, 8.2 × 1015 cm−3. These findings are consistent with the current density–voltage characteristics of the resulting solar cells and their external quantum efficiency. It suggests that as the sulfur content increases, the bandgap of the absorber is enlarged, leading to an increasing open‐circuit voltage (Voc), that is accompanied by stronger recombination due to the higher defect density of the sulfur‐rich absorber. This is reflected in large Voc deficit and poor carrier collection of the high sulfur content device.

Effects of Ge Alloying on Device Characteristics of Kesterite-Based CZTSSe Thin Film Solar Cells

“…Although the deep defect level (0.192 eV) in CZTSSe was decreased to comparatively shallower defect level in CZTSSe­(Ge-3) (0.108 eV, which is as close to Cu Zn antisite as deeper acceptor defect) , the efficiency of CZTSSe­(Ge-3) device is limited in some extent, whereas having a deeper defect level in the CZTSSe device, its device performance is more suffered than that of the CZTSSe­(Ge-3) device. Our results are consistent with the previous reports on the effect of defect levels on the performance of solar cell devices. ,, The deterioration of device parameters must be mainly associated with the defect level coupled with distribution of carriers and non-optimized interface layer in the context of this study. It is expected to have further improvement in the device performance by addressing the issue mainly associated with absorber quality and interface optimization with well-controlled device processing.…”

Cation Substitution of Solution‐Processed Cu2ZnSnS4 Thin Film Solar Cell with over 9% Efficiency

“…With the increase of Cd contents from 0.0 to 1.0, the x d values decrease from 0.28 μm (CZTS) to a minimum of 0.11 μm (Cu 2 Zn 0.6 Cd 0.4 SnS 4 ), and then increase to a maximum of 0.71 μm (CCTS), while the charge density of CZCTS devices achieve the maximum value for Cd content of 0.4. The CZCTS device samples with high charge density generally show small depletion width, consistent with previous reports . The depletion region is an insulating layer where the mobile charge carriers have diffused away, as a result the width of depletion region can exert much influence on the conductivity of charge carriers, consequently affecting the FF of whole device .…”

Benign Solutions and Innovative Sequential Annealing Processes for High Performance Cu2ZnSn(Se,S)4 Photovoltaics

“…The moderate frequency dependence of the capacitance in this high Se content device is consistent with other solution‐processed CZTSSe devices . However, compared to previous cases, the smaller capacitance over the same frequency range could be explained by the carbon‐rich layer in the CZTSSe film. As with other non‐hydrazine processed CZTSSe devices, the use of organic solvents was the main carbon contribution .…”

“…The lowering of temperature leads to reduced capacitance, which indicates that the deep traps and free carriers were gradually frozen out . The moderate frequency dependence of the capacitance in this high Se content device is consistent with other solution‐processed CZTSSe devices . However, compared to previous cases, the smaller capacitance over the same frequency range could be explained by the carbon‐rich layer in the CZTSSe film.…”