In situ growth of perovskite single-crystal thin films (PeSCTFs) on the transport layer is crucial for achieving high-performance perovskite optoelectronic devices, such as solar cells, light emitting diodes, photodetectors, etc. However, in-situ growing PeSCTF on the transport layer with large-area, high-crystal-quality, and low-trap-density simultaneously remains challenging. This work proposes a method for in-situ growing large-area and low-trap-density MAPbBr₃ SCTFs on the PTAA transport layer with the assistance of cesium(I) bis(trifluoromethanesulfonyl)imide (CsTFSI) ionic liquid. After introducing the CsTFSI ionic liquid, the Cs⁺ ions enter the MAPbBr₃ lattice, reducing the formation energy, and the coordination between the lone pair electrons of O in TFSI^- and the empty orbital of Pb in MAPbBr₃ passivates the dangling bond defects of Pb, facilitating the formation of MAPbBr₃ SCTFs with high-crystal-quality. Moreover, the strong interaction between TFSI⁻ and the substrate can enhance wettability and reduce the contact angle, thereby promoting faster solute diffusion and enabling the growth of larger-area MAPbBr₃ SCTFs. Therefore, compared to the sample without CsTFSI addition, the MAPbBr₃ SCTFs with CsTFSI addition exhibit better thermal stability, larger area (increased from 1.79 to 19.68 mm², approximately a 10-fold increase), lower trap density (decreased from 6.86 × 10¹² to 5.39 × 10¹² cm⁻³), and higher carrier mobility (increased from 0.72 to 0.84 cm²∙V⁻¹∙s⁻¹). Moreover, the performance of the photodetector with CsTFSI, including responsivity, EQE, detectivity, and response speed, also increased significantly. This work provides an effective method for the in-situ growth of PeSCTFs with large-area, high-crystal-quality, and low-trap-density simultaneously on the transport layer.