Graphene/Formamidinium Lead Iodide Quantum Dots for UV-visible Photodetectors

Abstract

Formamidinium lead iodide perovskite (FAPbI3) has been introduced as the most promising candidate in solar cells and photodetectors due to its high moisture stability and favorable NIR absorption edge. However, the alpha phase of FAPbI3 with impressive optical properties is stable at above 160 °C. In this research, by taking advantage of a ligand-assisted reprecipitation method (LARP), the alpha phase of FAPbI3 QDs colloidal solution was prepared at room temperature, confirmed by X-ray diffraction (XRD). Although high-coordinating DMF, as the solvent of precursors, failed to synthesize the QDs, the mixture of ACN/GBL, oleic acid, and octylamine was capable of crystallizing ~7.5 nm FAPbI3 QDs. Based on the photoluminescence and UV-vis tests, the emission peak and absorption edge of QDs were measured at 751 nm and 682 nm, respectively. To fabricate an efficient QD-based UV-visible (whole spectrum) photodetector, a monolayer graphene (I2D/IG>2) with high crystallinity (ID/IG~0) was transferred to a Si/SiO2 substrate via the PMMA method. Based on optical microscopy and Raman Spectroscopy results, a remarkable concentration of PMMA contaminations was removed without considerable damage to the graphene crystal structure. After electrode deposition with an interdigitated design, the QDs were deposited on the device with a configuration of Au-Cr/Si/SiO2/Graphene/Au-Cr via drop casting, spin coating, and centrifugation. Among all methods, QD-based photodetectors prepared by centrifugation method showed the highest responsivity at 72 A/W, 104 times higher than that of bare QD photodetector, under blue LED illumination with light intensity of ~2 μW/cm2 at VSD of 0.01 V. This photodetector device responded to UV, green, yellow, and red LEDs, however, the best performance was achieved when photodetectors were exposed to the blue LED. In addition, it was revealed that the performance of devices was dependent on the gate voltage. Therefore, graphene/FAPbI3 QD photodetectors can shed light on the next generation of UV-vis photodetectors.