Impact of Li and K co-doping on the optoelectronic properties of CZTS monograin powder

In this work, the impact of Li and K co-doping on the properties of Cu2ZnSnS4 (CZTS) monograin powders used as absorber materials in monograin layer (MGL) solar cells was investigated. CZTS powders were grown in the liquid phase of flux materials with different LiI-KI ratios by synthesis-growth method. According to Atomic Absorption Spectroscopy, the amount of K in the host material did not depend on the molar ratio of LiI to KI in the flux mixtures and remined constant at 0.01 at%. However, the Li concentration depended on the initial amount of LiI in the LiI-KI flux mixture and increased from 0.01 at% to 1.22 at% in the synthesized CZTS. X-ray diffraction, Raman analysis and photoluminescence (PL) analysis confirmed that Li content x = 0.2 and x = 0.4 in the input composition of (Cu1-xLix)1.84Zn1.09Sn0.99S4 resulted in formation of solid solution. The external quantum efficiency measurements showed that the effective bandgap energy of CZTS increased from 1.52 eV to 1.57 eV by increasing the Li content in CZTS from x = 0–0.02 to x = 0.4, respectively. The mean values of VOC of the corresponding monograin layer solar cells increased from 700 to 742 mV. The highest VOC of 784 mV was obtained with device based on solid solution containing 1.22 at% of Li (x = 0.4). The best performing solar cell with power conversion efficiency of 9.4% was obtained with Li and K co-doped CZTS powder (x = 0.002) showing output parameters: VOC = 718 mV, JSC = 20.9 mA/cm2 and FF = 62.5%.