To summarize, the current study reveals that the spacer cations, especially their functional group, play an important role in the optoelectronic properties, thermostability, and photo- exhibit excellent thermal stability under the extreme operating conditions. Moreover, the delocalized orbitals of the conjugated benzene ring in PEA + and its relatively high dielectric constants were found to weaken the quantum well confinement and potential barrier well, thus allowing the effective electron-hole separations and photogenerated carrier out-of-plane transport. The functional group, such as the conjugated benzene ring, thence not only plays a critical role in the modulation of optoelectronic properties of 2D monolayer hybrid halide perovskites (e. g. the electronic bandgap and carrier mobility) but also in the long-term stability and photogenerated carrier out-of-plane transports.