Novel Growth Methods of Organic-Inorganic Lead Tri-Halide Perovskite Material for Photovoltaic Applications

Whilst different methodologies have been used to grow perovskite materials, still, there are demands to develop commercial approach techniques that could alleviate the problem associated with the various phases formed within the perovskite material, which may cause of its crystallographic instability. Therefore, the research study was aimed to assist in moving towards this objective by developing novel methods for the deposition of perovskite thin films (CH3NH3PbI3 = MAPbI3) that are scalable and industry-acceptable. It was also aimed to focus, along with the scalable process, on an atomic scale of processing to deposit materials with ATM (Atom to Matter), where the defect density within the films are minimised. In view of that, different CVD deposition methods to deposit MAPbI3 thin films were proposed in this study, including Radio Frequency-Plasma Enhanced Chemical Vapour Deposition (RF-PECVD) and Atmospheric-Pressure Chemical Vapour Deposition (APCVD). The first proof-of-concept was demonstrated to obtain perovskite films with a tetragonal crystal structure by a PECVD process that was not reported till date. This achievement is encouraging as the PECVD process is fully scalable and already available technology in the industries. The growth was subsequently successfully achieved by using carbon, nitrogen and hydrogen radicals that were contained a gas such as methane (CH4) and ammonia (NH3). A prior deposition of PbI2 thin films by either spin-coating or thermal evaporating was implemented previously before they were exposed to the organic molecules in the plasma state that was formed by RF power via a PECVD technique. The effect of the variation of PECVD growth parameters (such as substrate temperature, RF power density and chamber pressure) on the properties of the films deposited was critically analysed. The conditions that yielded the best quality material achieved from this study were at substrate temperature of 100 ℃ and under low power density (22 mW/cm2) and high chamber pressure (1000 mtorr). An attempt was made to understand the properties of the resultant films, in terms of the physical, optical and electrical properties, which led to develop and provide appropriate explanations of the possible growth mechanisms. Considering the cost with the use of the scalable process that any an industrial technique would prefer, a modified non-vacuum CVD system was additionally demonstrated in this work to deposit MAPbI3 films at atmospheric pressure, hence the name APCVD. The growth of the MAPbI3 films was controlled based on the concentration of the precursor and the temperature. The lower concentration of the molecules passing over the substrate enhanced the crystallographic stability, where the in-situ cubic structure was achieved. With the use of this novel design of the reactor specifically for the deposition of MAPbI3 perovskite films; excellent stability of these films at room temperature without care of storage for more than two months in an open atmosphere provided an alternative approach to obtain the stable MAPbI3 perovskite film. In addition to the low temperature and lower concentration of the precursor used for the fabrication of a novel design of the reactor to grow films with the bottom-up approach (i.e. atom by atom or molecules), this study was successful in fabricating a proof-of-concept solar cell using this method for the first time. The obtained perovskite material has significantly contributed to understanding of the crystallographic stability issues reported in the photovoltaics which are incorporated with the perovskite materials over the last recent years.