Enhanced PL intensity and improved PL lifetime indicate that non-radiative recombination is suppressed in FA0.91Cs0.09PbI3, attributed to decreased trap densities. Consistently, the Urbach energy (fig. ) is decreased from 22.2 meV in FAPbI3 to 18.3 meV in FA0.91Cs0.09PbI3. The smaller Urbach energy in FA0.91Cs0.09PbI3 corresponds to a lower density of trap states. These results suggest that, with incorporation of Cs, the FA0.91Cs0.09PbI3 perovskite exhibits significantly reduced non-radiative recombination via defects/traps. Benefiting from these advantages of crystallization decoupling engineering, the resulting FA0.91Cs0.09PbI3 shows much enhanced device performance. Fig. compares the current density-voltage (J-V) characteristics of champion PSCs based on FAPbI3 and FA0.91Cs0.09PbI3 perovskites, respectively. The FA0.91Cs0.09PbI3-based PSC exhibits an impressive PCE of 24.7%, compared with 22.6% for FAPbI3. We also obtained a certified PCE of 23.8% with neglected hysteresis for FA0.91Cs0.09PbI3-based PSC (fig. ). The most striking difference is the Voc, which increases from 1.09 V in FAPbI3 to 1.18V in FA0.91Cs0.09PbI3. The incident photon to electron conversion efficiency (IPCE) (Fig. ) is similar for both devices, with a high value over 90% in the wavelength range of 450~650 nm. The short-circuit current density (Jsc) of the FA0.91Cs0.09PbI3 device is slightly decreased compared with the FAPbI3 device, mainly due to slight increase of the bandgap upon Cs incorporation. Fig. compares the PV parameters of FAPbI3-and FA0.91Cs0.09PbI3-based PSCs for 18 devices respectively, indicating that Cs incorporation also improves the device reproducibility. In addition, the FA0.91Cs0.09PbI3 based PSCs exhibit a smaller hysteresis (fig. and), resulting in a stabilized output power of 24.4% (Fig. ), a new record for the pure iodide FA1-xCsxPbI3 based PSCs. We also fabricated large-area PSCs based on these Cs-incorporated perovskite films. The champion FA0.91Cs0.09PbI3 device, fabricated on 2.5-cm by 2.5-cm substrates with an effective cell area of 1 cm 2 (Fig. ), display a PCE of 22.4%, which is far higher than that of FAPbI3 devices (~19.9%).