## Materials and Methods  
### Photodetector Fabrication  
* Materials and Reagents  
Indium tin oxide (ITO) substrates, PbBr\({}_{2}\), CsBr, and SnO\({}_{2}\) (12%) hydrocolloid dispersions, and P3 HT (poly[3-hexylthiophene-2,5-diyl]), were purchased from Advance Election Technology Co., Ltd. (Shenyang, China). Dimethyl sulfoxide (DMSO) (>99.7%), Chlorobenzene (CB), and isopropyl alcohol (IPA) (>99.5%) were purchased from Acros (Guangzhou, China). Conductive carbon paste was obtained from Shanghai MaterWin New Materials Co., Ltd. (Shanghai, China). SnO\({}_{2}\) (5%) precursor solution was prepared by diluting SnO\({}_{2}\) (12%) hydrocolloid dispersions with deionized water. For the precursor solution of the hole extraction layer, P3 HT was dissolved in CB with a concentration of 10 mg/mL. An intermediate precursor solution was prepared by mixing 1.5 mmol PbBr\({}_{2}\) and 0.6 mmol CsBr in 1 mL DMSO, which was stirred overnight until its was completely dissolved. A high-concentration CsBr solution was prepared by dissolving 0.9 mmol CsBr with IPA and water mixing solvent (6:4 in volume).
* Device Fabrication  
ITO conductive substrates (2 cm \(\times\) 2 cm) were cleaned by ultrasonic sequentially in deionized water, acetone, and absolute ethanol for 20 min, respectively. The cleaned substrates were dried with nitrogen, followed by a 20 min UV-ozone treatment before being used. In the first step of fabrication, a thin layer of SnO\({}_{2}\) was deposited by spin-coating a 5% diluted SnO\({}_{2}\) precursor solution at 4000 rpm for 30 s, then thermally annealing it at 180 \({}^{\circ}\)C for 30 min in ambient air. After cooling down, the ITO/SnO\({}_{2}\) substrates were transferred to a glovebox with inert gasses. The intermediate solution was spin-coated on the ITO/SnO\({}_{2}\) substrates at 2000 rpm for 100 s, and 150 uL of CB was used to wash the intermediate film in the last 20 s. Then, the perovskite intermediate films were annealed at 100 \({}^{\circ}\)C for 30 min. Next, 100 uL of CsBr solution was dropped onto the intermediate films, which was spun at a speed of 3000 rpm for the last 10 s, and then annealed at 250 \({}^{\circ}\)C for 20 min to obtain CsPbBr\({}_{3}\) perovskite films. Before depositing the top electrode, a thin hole extraction layer of P3 HT was spin-coated on the perovskite. Finally, the top carbon electrode was screen-printed on the CsPbBr\({}_{3}\) films with a conductive carbon paste and annealed at 100 \({}^{\circ}\)C for 15 min, and then a 200-nm copper was thermally evaporated atop carbon electrode.
* Characterizations  
The morphologies of the CsPbBr\({}_{3}\) films were observed using a scanning electron microscope (SEM, CrossBeam 350 FEI) at an operating voltage of 3 kV. The photoluminescence (PL) spectra were measured using the FluoTime 300 system from Pico Quant (Berlin, Germany) and excited by a picosecond pulsed laser with a wavelength of 375 nm. The current-to-voltage (I-V) characteristics were measured using a Keithley 2635B, and the noise spectrum was obtained by fast Fourier transformation of the dark current. Responsivity (\(R\)) was measured using a QE-R quantum efficiency system from Enli Technology Co., Ltd. (Shanghai, China), which was calibrated with a standard Si photodetector. The rise and fall times of the photodetectors were measured using a digital oscilloscope with a semiconductor laser (485 nm) in a square wave triggered by a function generator.