*2.2. Device Fabrication*

Firstly, we cleaned patterned indium tin oxide (ITO)-glass substrates sequentially in detergent, acetone, and 2-propanol for 15 min. The hole transport material, PEDOT:PSS, was deposited on the ITO-glass substrates through a spin-coating process and then annealed at 140 °C for 15 min in air. The perovskite precursors were made by mixing Pb(Ac)2 and MAI in 1 M dimethylformamide. Then, the perovskite precursor with *x* mol% cesium, where *x* = 0, 2.5, 5.0, 7.5, or 10.0, was spin-coated on the ITO/PEDOT:PSS substrates at a rotation speed of 4000 rpm, which was followed by annealing at 80 ◦C for 10 min. The electron transport layer (ETL) was formed by spin-coating of PC61BM (30 mg·ml−<sup>1</sup> in Chlorobenzene) on the perovskite layer at 2000 rpm for 30 s. For better charge transport, we deposited a BCP layer onto the ETL by spin-coating BCP solution (0.5 mg·ml−<sup>1</sup> in 2-propanol) at 450 rpm for 30 s. Finally, the deposition of an 80 nm silver electrode was achieved with thermal evaporation under a high vacuum of approximately 10−<sup>6</sup> Torr. The device area was determined by the overlapped rectangle between the ITO and Ag electrode bars, being 0.06 cm<sup>2</sup> (0.2 cm × 0.3 cm).
