# Synthesis, Structure and Photoluminescence Properties of 2D Organic-Inorganic Hybrid Perovskites
## 3 Results and Discussion
### The Laser Power Effect on the PL Lifetime  
Figure 8a-c shows the lifetime of the two-photon PL emission (pumped at 800 nm) from the perovskite (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\), (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbI\({}_{4}\) and (C\({}_{4}\)H\({}_{9}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\). When the laser power increases from 10 mW to 100 mW, the PL lifetime of (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\) decreases from 0.75 ns to 0.50 ns, (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbI\({}_{4}\) decreases from 0.67 ns to 0.46 ns, and (C\({}_{4}\)H\({}_{9}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\) decreases from 3.02 ns to 2.29 ns.  
It can be seen that the PL lifetime of (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\) is longer than that of (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbI\({}_{4}\) at the same laser power. This is because the atomic radius of Br and I increases in turn, which makes the framework of PbX\({}_{2}\) in the inorganic layer increase in turn, and the forbidden bandwidth of PbBr\({}_{2}\) and PbI\({}_{2}\) in the inorganic part decrease in turn, resulting in the decrease of exciton binding energy. Therefore, the excitons of (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbI\({}_{4}\) are easier to separate and emit compound fluorescence. Thus, the PL lifetime of (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\) is longer than that of (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbI\({}_{4}\) at the same laser power.  
It also can be seen that the PL lifetime of (C\({}_{4}\)H\({}_{9}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\) is longer than that of (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\) at the same laser power. The organic amine cation of (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\) shows disorder, which leads to greater thermal vibration and more electron scattering phonons, leading to non-radiation transition and a sharp decrease of its PL life. Therefore, the PL lifetime of (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\) is less than that of (C\({}_{4}\)H\({}_{9}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\) at the same laser power.  
The relationship of PL intensity and lifetime with laser power can be explained with the energy level diagram [39; 40; 41; 42], as shown in Figure 9. Under the excitation of 800 nm femtosecond laser, the perovskite molecules absorb two photons, exciting the electrons at the top of the valence band to the bottom of the conduction band. Some electrons from the bottom of the conduction band will go back to the top of the valence band, releasing the excess energy in the form of photoluminescence. The electrons at the bottom of the conduction band also relax to the defect state on the electron surface and then return to the top of the valence band and compound photoluminescence with the defect state. The emission intensity is related to the absorption cross section of two photons. As the laser power continues to increase, more and more electrons are excited to the bottom of the conduction band. Some of the electrons at the bottom of the conduction band return to the top of the valence band, and another part relaxes to the defect state of the electron surface and then returns to the top of the valence band. In this process, carrier density increases and the non-geminate recombination plays a dominant role. Therefore, the two-photon PL intensity increases with the increase of laser power, while the PL lifetime decreases with the increase of laser power [43].