Abstract:

Inorganic halide CsPbI₃ perovskite colloidal quantum dots (QDs) possess remarkable potential in photovoltaics and light-emitting devices owing to their excellent optoelectronic performance. However, the poor stability of CsPbI₃ limits its practical applications. The ionic radius of SCN⁻ (217 pm) is comparable to that of I⁻ (220 pm), whereas it is marginally larger than that of Br⁻ (196 pm), which increases the Goldschmidt tolerance factor of CsPbI₃ and improves its structural stability. Recent studies have shown that adding SCN⁻ in the precursor solution can enhance the crystallinity and moisture resistance of perovskite film solar cells; however, the photoelectric properties of the material post SCN⁻ doping remain unconfirmed. To date, it has not been clarified whether SCN⁻ doping occurs solely on the perovskite surfaces, or if it advances within their structures. In this study, we synthesized inorganic perovskite CsPbI₃ QDs via a hot-injection method. Pb(SCN)₂ was added to the precursor for obtaining SCN⁻-doped CsPbI₃ (SCN-CsPbI₃). X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were conducted to demonstrate the doping of SCN⁻ ions within the perovskite structures. XRD and TEM indicated a lattice expansion within the perovskite, stemming from the large steric hindrance of the SCN⁻ ions, along with an enhancement in the lattice stability due to the strong bonding forces between SCN⁻ and Pb²⁺. Through XPS, we confirmed the existence of the S peak, and further affirmed that the bonding energy between Pb²⁺ and SCN⁻ was stronger than that between Pb²⁺ and I⁻. The space charge limited current and time-resolved photoluminescence results demonstrated a decrease in the trap density of the perovskite after being doped with SCN⁻; therefore, the doping process mitigated the defects of QDs, thereby increasing their optical performance, and further enhanced the bonding energy of Pb－X and crystal quality of QDs, thereby improving the stability of perovskite structure. Therefore, the photoluminescence quantum yield (PLQY) of the SCN-CsPbI₃ QDs exceeded 90%, which was significantly higher than that of pristine QDs (68%). The high PLQY indicates low trap density of QDs, which is attributed to a decrease in the defects. Furthermore, the SCN-CsPbI₃ QDs exhibited remarkable water-resistance performance, while maintaining 85% of their initial photoluminescence intensity under water for 4 h, whereas the undoped samples suffered complete fluorescence loss due to the phase transformations caused by water molecules. The SCN-CsPbI₃ QDs photodetector measurements demonstrated a broad band range of 400–700 nm, along with a responsivity of 90 mA∙W⁻¹ and detectivity exceeding 10¹¹ Jones, which were considerably higher than the corresponding values of the control device (responsivity: 60 mA∙W⁻¹ and detectivity: 10¹⁰ Jones). Finally, extending the doping of SCN⁻ into CsPbCl₃ and CsPbBr₃ QDs further enhanced their optical properties on a significant scale.

Key words: Inorganic halide perovskite, Quantum dot, Doping, Stability, Photodetector