Crystal size-controlled growth of Cu2ZnSnS4 films by optimizing the Na doping concentration

Role of Na and Ca as Isovalent Dopants in Cu2ZnSnS4 Solar Cells

“…Indeed, absorption measurements reveal an optical band gap of ∼3.1 eV in NZTS, about twice as high as the 1.4–1.6 eV in pure CZTS . Also, ultraviolet–visible (UV–vis) absorption spectra exhibit a small increase in the onset of absorption, from 1.5 eV at 0% Na to 1.6 eV at 2% Na-doping, qualitatively consistent with our calculations at full Na-doping. Interestingly, the predicted increase in band gap from CZTS to NZTS is larger within the kesterite structure (∼1.18 eV larger) versus the stannite (∼0.66 eV larger), unlike with Ca.…”

“…Doped CZTS samples are typically synthesized by preparing a doped precursor followed by high temperature annealing, where CZTS crystallizes into the kesterite structure. 56 The dopant is incorporated into the CZTS structure only when the kesterite structure forms during annealing, and formation of Cu Zn +Zn Cu complexes also occurs at that time. 24 Thus, as dopant incorporation and Cu Zn +Zn Cu formation occur simultaneously, the formation energy of Na Cu/Zn +Cu Zn +Zn Cu is the relevant quantity to compare against the energy required to form Cu Zn +Zn Cu complexes in undoped CZTS.…”

“…Although the interaction energies of Cu Zn +Zn Cu defect complexes adjacent to Na Cu (∼0.24–0.25 eV) and Na Zn (∼0.09 eV), calculated from the data in Figure a, are similar to or lower than the formation energy of Cu Zn +Zn Cu in undoped CZTS (0.22–0.25 eV), the interaction energy is not the relevant quantity of comparison. Doped CZTS samples are typically synthesized by preparing a doped precursor followed by high temperature annealing, where CZTS crystallizes into the kesterite structure . The dopant is incorporated into the CZTS structure only when the kesterite structure forms during annealing, and formation of Cu Zn +Zn Cu complexes also occurs at that time .…”