Studies on Organometal Halide Perovskite Crystal Growth Dynamics during Two-step Process

Abstract

Organic-inorganic hybrid perovskite materials (OIHP) (e.g. CH3NH3PbI3) have become a promising candidate for a photoactive layer of thin-film solar cells. OIHP materials have high absorption coefficients, long carrier diffusion lengths, and ambipolar charge transport capabilities. Moreover, it is possible to apply solution process to fabricate OIHP films, so the OIHP show remarkable performance as a light absorber.

To achieve high efficiency of solar cell, perovskite films should have proper morphological characteristics; sufficiently large grain, full surface coverage, low surface roughness, and good intergrain connection. Various kinds of methods to fabricate perovskite thin films has been developed. Among them, two-step process, also known as sequential deposition method, has been widely studied by various research groups because it is simple and easily reproducible. A two-step process includes sequential deposition of two precursors, PbI2 and CH3NH3I (MAI), it is divided into dipping and spinning processes on how pre-deposited PbI2 film react with MAI solution. The effects of processing conditions of two precursors, engineering factors for dipping and spinning, and environmental effects on perovskite morphology obtained by the two-step process has been researched, but fundamental understanding about formation mechanism of perovskite crystals during two-step process, and the correlation between those variables and morphologies is still insufficient.

In Chapter 2, perovskite crystal growth mechanism during the dipping process was investigated. Time-resolved intermediates of dipping process was prepared by pouring IPA on the converting film, and analyzed. There are two distinct reaction periods on the dipping process; 1st – determining surface morphology, and 2nd – further reaction of buried PbI2 under the first perovskite layer. During the Period I, perovskite crystals nucleated on the PbI2 sources, and experienced conventional nucleation and growth process. Using the kinetics, it was possible to discover the relationship between the reaction rate r and grain size R of obtained perovskite; r∝R^(-1/3). The rate constant can be easily extracted from experiments, so it is possible to predict final morphology of perovskite films obtained by dipping process at a certain processing condition.

In Chapter 3, distinct characteristics of spinning process compared to dipping process were investigated. Usually, the perovskite films obtained by spinning show good interconnected and smooth grains, and the role of spin to form those morphologies was studied. During the spinning, discrete perovskites which resemble that obtained by dipping first form, and they turned into the connected morphologies. We discovered that the critical factor occurring the connection during the spinning is residual MAI solution after spinning on the perovskite film. Highly concentrated residual MAI solution dissolve perovskite crystals already formed, and the dissolved perovskites are recrystallized to connected and smooth morphologies. Additional thermal energy can increase the dissolution of perovskites with combined effects with residual MAI, and more connected and larger perovskites can be achieved. Optoelectrical properties of perovskite films obtained by the two-step process which have representative morphologies were investigated. The film with a large, smooth, and well-interconnected morphology show the highest efficiency of a solar cell and the longest carrier lifetime.