Importance of methylammonium iodide partial pressure and evaporation onset for the growth of co-evaporated methylammonium lead iodide absorbers

Abstract

Vacuum-based co-evaporation promises to bring perovskite solar cells to larger scales, but details of the film formation from the physical vapor phase are still underexplored. In this work, we investigate the growth of methylammonium lead iodide (MAPbI$$_3$$ 3 ) absorbers prepared by co-evaporation of methylammonium iodide (MAI) and lead iodide (PbI$$_2$$ 2 ) using an in situ X-ray diffraction setup. This setup allows us to characterize crystallization and phase evolution of the growing thin film. The total chamber pressure strongly increases during MAI evaporation. We therefore assume the total chamber pressure to be mainly built up by an MAI atmosphere during deposition and use it to control the MAI evaporation. At first, we optimize the MAI to PbI$$_2$$ 2 impingement ratios by varying the MAI pressure at a constant PbI$$_2$$ 2 flux rate. We find a strong dependence of the solar cell device performance on the chamber pressure achieving efficiencies > 14$$\%$$ % in a simple n-i-p structure. On the road to further optimizing the processing conditions we vary the onset time of the PbI$$_2$$ 2 and MAI deposition by delaying the start of the MAI evaporation by t = 0/8/16 min. This way, PbI$$_2$$ 2 nucleates as a seed layer with a thickness of up to approximately 20 nm during this initial stage. Device performance benefits from these PbI$$_2$$ 2 seed layers, which also induce strong preferential thin film orientation as evidenced by grazing incidence wide angle X-ray scattering (GIWAXS) measurements. Our insights into the growth of MAPbI$$_3$$ 3 thin films from the physical vapor phase help to understand the film formation mechanisms and contribute to the further development of MAPbI$$_3$$ 3 and related perovskite absorbers.

Formation of a Secondary Phase in Thermally Evaporated MAPbI3 and Its Effects on Solar Cell Performance

“…Much research has been devoted to controlling the rate of MAI deposition on the basis of its partial pressure in the vacuum chamber and substrate temperature, − allowing for the deposition of stoichiometrically tuned films. However, the optimal deposition conditions do not easily extrapolate from laboratory to laboratory, since they depend on system parameters such as pumping speed, volume of the chamber, temperature of the walls, and others.…”

“…On the other hand, the pressure increases up to 2 orders of magnitude when the MAI source is heated, and it remains at a constant level once the temperature is set. As such, the MAI source temperature can be used as a parameter to control the deposition of MAI in the films in the same way that others have used chamber pressure to control the amount of evaporated MAI. , Differently from PbI 2 , MAI has a low sticking factor and does not stick to the evaporator walls. As a result, MAI vapors fill the whole chamber volume and are present long enough to increase the pressure …”

Highly Anisotropic Organometal Halide Perovskite Nanowalls Grown by Glancing‐Angle Deposition

“…It was also noted that different from the PbI 2 deposition, the MAI sublimation produced an increase in the pressure of the system. Based on these observations and according to the literature, [ 50 ] an optimization process was proceeded and reproducible and stable conditions were found by maintaining the evaporation temperature of the MAI precursors at 160 °C, which produced an increase in the pressure to around (5–8) × 10 −5 mbar. Then, the pumping flux of the turbomolecular pump was reduced using a butterfly valve and fixing the pressure to 2 × 10 −4 mbar.…”

“…It was observed that the deposition of MAI could not be properly controlled by the QCM as referenced in the literature. [ 50 ] The initial growth rate achieved during the first minutes could not be maintained for the whole deposition, and the MAI was finished before the complete transformation to MAPI. It was also noted that different from the PbI 2 deposition, the MAI sublimation produced an increase in the pressure of the system.…”

Efficient Semitransparent Perovskite Solar Cells Based on Thin Compact Vacuum Deposited CH3NH3PbI3 Films

“…This indicates that the excess PbI 2 corresponding to the above PbI 2 reflex may be localized close to the interface between MAPI and TaTm likely due to the low sticking coefficient of MAI at surfaces lacking PbI 2 , suggesting that, first a PbI 2 rich, thin crystalline layer gets deposited on TaTm before the formation of MAPI. [ 21 ] The presence of this excess PbI 2 in the initial 50 nm of the MAPI film is likely responsible for the observed counterintuitive slightly higher absorption in the 50 nm MAPI film than half of that of the 100 nm MAPI film for ≈λ < 565 nm. [ 22 ] Moreover, we observe that the peaks corresponding to the (004) and (220) planes of the tetragonal phase at 2θ ≈ 28.2° and 28.4° are sharper in the case of 100 nm MAPI films (Figure 2b).…”