Boosting the Graded Structure of 2D Perovskite Solar Cell Based on BA₂MA_n–1Pb_nI_3n+1 by Noninteger n Values

In this work, two-dimensional (2D) perovskite absorbers with the chemical formula BA2MAn–1PbnI3n+1 (MA: methylammonium and BA: butylammonium) with different n values and different synthesis conditions are investigated, and their preferential growth and segregation patterns have been thoroughly discerned. For this purpose, 2D perovskites with noninteger n values (1.5, 2.5, and 3.5) are compared to 2D perovskites with integer n values. The optimized preheating and annealing temperatures for preferential growth is found to be 100 °C for both parameters, while the concentration of 1.25 M for n ≤ 2 and 0.75 M for n > 2. 2D perovskites with noninteger n values seem to have optoelectronic properties between that of their integer counterparts with a key difference, which is having a boosted segregation pattern throughout the film, in which the perovskites with smaller n form beside the substrate and the perovskites with larger n forms on top, generating an interface-free graded structure with efficient photon absorption. The devices based on these halide perovskites are fabricated, and perovskite with n = 3.5 shows the best performance with an efficiency of 2.75%. Moreover, stability analyses for 60 days show that these perovskites can be stable after being subjected to humidity and illumination for a long period.