Dopants and additives such as alkali-ions can substantially affect the PV performance of thin-film semiconductor PV 1. For instance, Na has been linked to the passivation of grain boundaries 2, while Sb decreases the crystallization temperature of CIGS 3and. In this work, we shed new light on the role of Na and Sb in the performance of Cu2ZnSnS4 (CZTS) solar cells employing temperature dependent photoluminescence, impedance spectroscopy photoemission microscopy. CZTS films are generated using a single solution precursor from metal salts and thiourea dissolved in DMF-Isopropanol mixture. The precursor is spin-coated onto Mo-coated glass substrates and finally annealed at 550 oC under S atmosphere. The process yields polycrystalline films with a band gap of 1.4 eV consisting of Cu-poor and Zn-rich composition with tetragonal phase. Electron microscopy shows the formation of 1.2 mm films with more crystalline (grain sizes up to 700 nm) and compact morphology upon the inclusion of Sb and Na:Sb. This is due to the lowering of crystallization enthalpy as determined by differential scanning calorimetry. The quantitative analysis of X-ray diffraction reveals decrease in the isotropic thermal parameter, associated with the atomic site disorder, especially for Sn site upon Sb doping.4 Solar cells are completed to have a substrate configuration of: glass/Mo/CZTS/CdS/i-ZnO/ZnO:Al/Ni−Al with individual cells scribed with a total area of 0.5 cm2. Statistical analysis of J-V measurements over 200 devices under AM1.5 G spectrum with 100 mW/cm2 demonstrates an overall improvement in fill-factor (FF) and open-circuit voltage (VOC) with better conformity resulting in a rise of average power conversion efficiency (h) from 3.2 ± 0.6 to 5.2 ± 0.3% on doping. The best performing device with Na:Sb doping yielded an 14.9 mA cm−2 short-circuit current, 610 mV VOC, 63% FF and h of 5.7%, rating amongst top reported for pure sulphide kesterite.5 Recently, a computational study predicted that depending on concentration, Sb doping may either lead to the appearance of states in the gap detrimental to the performance or may boost the performance through lowering the Sn disorder .6 For the first time, we monitor the electronic states due to Sb doping employing low-temperature photoluminescence and surface sensitive energy filtered ultraviolet photoemission spectroscopy which allows the rationalizing the systematic change in device efficiency.
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