*2.3. Device Characterization*

The X-ray diffraction (XRD) patterns of the samples and perovskite films were measured with a Bruker-AXS D8 Advance (Malvern Panalytical, Malvern, UK). MAPbI3 perovskite films morphology was measured with scanning electron microscope (SEM, sigma 500, Krefeld, Germany). The GO nanosheets were characterized via transmission electron microscopy (TEM, TecnaiG2 F20, FEI Company, Hillsboro, OR, USA). The photocurrentvoltage (J-V) characteristics of PSCs were analyzed under simulated AM 1.5 G radiation (100 mW/cm2 irradiance) by using a solar simulator (Oriel, model 91192-1000) and a source meter (Keithley 2400, USA). Electrochemical impedance spectroscopy (EIS) was measured with an electrochemical workstation (Zennium, IM6, Kronach, Germany) over the frequency range of 100 mHz−<sup>2</sup> MHz with 10 mV AC amplitude at −1 V bias under simulated AM 1.5 G radiation (100 mW/cm2 irradiance). The steady-state photoluminescence (PL) measurements were acquired using an Edinburgh Instruments FLS920 fluorescence spectrometer (Oxford Instruments, Abingdon, UK). Raman spectroscopy analysis was performed by a micro-Raman instrument (XperRam 200, Nanobase, Seoul, South Korea), using 542 nm excitation with an incident power of 5 mW. The devices were measured under ambient conditions (15% < relative humidity (RH) < 60%) every time (winter, summer). After the measurements, the devices were stored in a humidity-controlled dry room (20% < RH < 40%).
