Carrier lifetime and charge separation in K⁺-doped CZTS nanocrystals

Cu₂ZnSnS₄ (CZTS) nanocrystals are important materials for next generation solar energy capture and conversion strategies. CZTS contains earth-abundant materials and has optimum band gap energy and high absorption coefficient. However, progress in utilizing CZTS nanocrystals is impeded by the numerous crystal and morphological defects present without high-temperature annealing. These defects commonly result in reduced carrier diffusion length and carrier lifetime. One recent and promising direction for employing CZTS nanocrystals in solar energy capture and conversion strategies is through remediation of defects by doping with alkali earth metal ions such as Li⁺, Na⁺, and/or K⁺. Here we focus on the K⁺ cationic doping since it has demonstrated the highest efficiencies but without a fundamental investigation into the underlying drivers. We first developed a flexible and reproducible synthesis route to prepare K⁺-doped CZTS nanocrystals with low polydispersity. We employed undoped and K⁺-doped nanocrystals to fabricate CZTS photocathodes and evaluated their photoresponsiveness in a photoelectrochemical cell. K⁺-doped CZTS nanocrystals show the previously reported trend of improved photocurrent density. We obtained further insight into the role of K⁺ dopant using Raman spectroscopy, X-ray photoelectron spectroscopy, and transient absorption spectroscopy. K⁺-doping of CZTS nanocrystals boosts charge carrier lifetime and enables better charge extraction efficiency to boost photocurrent. Improved carrier lifetime is attributed to remediation of binary/tertiary impurity phases and surface anion vacancies to yield a higher degree of phase purity and a lower degree of surface electron traps in CZTS.