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.

Bifacial sodium-incorporated treatments: Tailoring deep traps and enhancing carrier transport properties in Cu2ZnSnS4 solar cells

“…Such capacitance responses are associated with the capture and emission rates of the active defect traps at certain temperature [32]. Following the model proposed by Kimerling et al [32], the capacitance at high frequency mainly represents the response of free carriers, whereas the capacitance at low frequency is associated with the response originated from free carriers and deep traps [33]. Figure 4 shows the admittance spectra of CZTS:Na devices with a NaF layer of 0, 2, and 5 nm in thickness.…”

Comparison of chalcopyrite and kesterite thin-film solar cells

“…Accordingly, 257 we performed AS measurements to determine the energy levels of 258 the defects inside the band gap of the absorber layer. With AS, the 259 characteristics of the majority carrier trapping defects can be 260 determined for solar cell materials and devices [36,37]. Fig.…”

“…From previous studies, we know that E A 270 corresponds to the bulk defects (i.e., hole traps) within an absorber 271 layer in a relatively high-temperature region (i.e., greater than 272 180 K) [36,37,39]. As E A increases, hole carriers can be trapped 273 more easily, and therefore the concentration of hole carriers 274 decreases.…”

Formation and impact of secondary phases in Cu-poor Zn-rich Cu2ZnSn(S1−Se )4 (0≤y≤1) based solar cells

“…Thus, it is highly relevant to understand the inner limitation mechanism concerning performance in kesterites based devices. Some preliminary works addressing these issues have demonstrated that Se-rich CZTSSe solar cells have lower defect energy level and bulk defect density, better charge separation and carrier collection efficiency than S-rich CZTSSe solar cells [5,6]. In addition, it was found that the Urbach energy is lower for CZTSSe devices with E g r1.2 eV, when compared to devices based on absorbers with higher band gap [7].…”