A study of materials processing and device integration of halide perovskites
Keywords: Materials Science and Engineering.

Hybrid organic-inorganic halide perovskite optoelectronic materials based on MAPbI3 (MA: methylammonium) have attracted tremendous interest in recent years. For example, in the photovoltaic field, power conversion efficiencies of perovskite solar cells (PSCs) have increased from 3% to more than 23% over the past decade due to the remarkable optoelectronic properties and tolerance for disordered microstructures of these hybrid perovskites. However, their poor environmental and thermal stability remains as their Achilles heel from the point of view of perovskites' commercial application. In this work, firstly, I will focus on the stability study of different types of halide perovskites including MAPbI3, Cs0.17FA0.83Pb(I0.83Br0.17)3 and Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3. Replacing the MA+ ion with other cations such as FA+ (FA: formamidinium) and Cs+ while replacing a portion of the iodide anions with Br- or Cl- is a widely-used approach to improve the thermal stability in the presence of water vapor and/or oxygen compared to MAPbI3. We studied the thermal degradation behavior of MAPbI3 using in situ x-ray diffraction (XRD) and found the degradation reaction corresponds to a zeroth order reaction with activation energy 0.92 eV. Then, we studied the thermal degradation of double cation and triple cation perovskites. Properties including the morphology, elemental distribution and optical properties before and after annealing were shown. Activation energies for decomposition for both the double and triple cation perovskite compounds were also extracted from in situ XRD data. For double cation perovskites, we observed zeroth order reaction mechanism and the activation energy is 0.66 eV. For triple cation perovskite, we observed two stage degradation reaction at low temperatures and only one stage at high temperatures. These results are consistent with a water vapor-catalyzed reaction for deprotonation of the organic cations and fomation of volatile HI in these perovskites leading to formation of a lead iodide solid reaction product. Secondly, I will discuss the application of low temperature processed atomic layer deposited (ALD) TiO2 and Al2O3 in perovskite solar cells. We demonstrated that the low temperature processed ALD TiO2 works as both an electron transport material and a protection layer for perovskites. We found that there is potential barrier for electron extraction between the ALD TiO2 and Ag, but no barrier between TiO2 and Al. The interfaces between ALD TiO2 and the metal electrodes were studied using in situ XPS and in situ XRD. We found the potential barrier was about 0.5 eV between Ag and ALD TiO2. Phase change was observed for devices with Ag on top after 8h heating. XPS suggests that Ag could diffuse through the ALD TiO2 and react with the underlying perovskite under heating. With Al as the top electrode, on the other hand, we didn't observe any phase change for devices with Al on top after 8h heating. Nonetheless, in excess of 10% loss in performance was seen after storage in air for one week, which could be due to the oxidation of Al or delamination in the worst case. Finally, we deposited low temperature ALD Al2O3 as an encapsulation layer and the final device showed significant improvement of stability compared to devices without ALD Al2O3. The performance dropped by only 10% after more than 2300 hours stability test in the air. Finally, I will describe the behavior of a new kind of inorganic hole transport layer (HTL) synthesized using atomic layer deposition (ALD). By alloying TiO2 with IrOx in a super-cycle ALD process, we showed that the electron transporting material TiO2 becomes an effective HTL for perovskite solar cells. The composition of this alloy could be easily tuned by changing the cycle ratio of TiO2 and IrOx in each super cycle. The thickness of this alloy could be controlled by changing the super cycle number. With the incorporation of Ir, the work function of the whole system increased a lot from the Scanning Kelvin Probe measurement. The properties of these alloys including composition, UV-visible light transmission, work function and their dependence on Ti--to-Ir ratio were studied. To further explore the utility of this approach, we chose TiO2-IrOxalloy with 15.5% Ir and made perovskite solar cells. The thickness of the alloy film was optimized and the final PSC devices containing this alloy HTL outperforms control devices using conventional NiO as the HTL by more than 10%.