In this work, Cs was used as the N-dopant and B₃PyPPM was used as the electron transport layer material to prepare a green phosphorescent organic light-emitting device. Experiments show that N-doping improves device efficiency and alleviates the efficiency roll-off of the device. In order to investigate the reasons for the performance improvement of N-doped devices, tests of Lambertian, open-circuit voltage, and conductivity were conducted. Experiments confirm that Cs-doped devices can improve conductivity and promote the injection and transmission of electrons, so more electrons can recombine with holes to form excitons, the device performance is improved and the efficiency roll-off is alleviated.