Georgia Institute of Technology Materials Science and Engineering

Committee Members:   

Prof. Zhiqun Lin, Advisor, MSE 

Prof. Vladimir Tsukruk, MSE 

Prof. Juan-Pablo Correa-Baena, MSE

Prof. Naresh Thadhani, MSE

Prof. Zhitao Kang, GTRI

Continuous Production and Improved Stabilities of Luminescent Colloidal Perovskite Nanocrystals as Next-Generation Emitters Abstract:

Due to their tremendous optoelectronic properties, lead halide perovskite materials have recently been extensively studied for their use in numerous applications, including as solar cells, photodetectors, scintillators, and light emitting diodes. Perovskite nanocrystals, in particular possess tremendous potential as emitters due to their room temperature solution processability, defect tolerance, widely tunable emission wavelength, narrow full width at half maximum of emission, and high photoluminescence quantum yield. Unfortunately, perovskite nanocrystals do have some notable weaknesses, such as poor stability in many ambient conditions (moisture, heat, ultraviolet light) and a reliance on batch processing techniques. Many of these strengths and weaknesses originate from the ionic nature of the ABX3 metal halide perovskite crystal structure. For the many advantageous properties of perovskite nanocrystals to be realized in at a commercial scale, the stabilities and production rate of perovskite nanocrystals must be significantly increased. To that end, this dissertation provides three unique approaches to either increase the stability or production rate of metal halide perovskite nanocrystals. These strategies include using unique organic chemistries to rationally craft molecules that will boost stability. Additionally, the use of a flow reactor for continuous synthesis of perovskite nanocrystals is explored. Specifically, three different research projects employ different strategies to enhance stability and production, as summarized below:

First, nonlinear block copolymer nanoreactors were integrated into flow reactors to continuously manufacture highly stable perovskite nanocrystals. Star-like poly(acrylic acid)-b-polystyrene copolymer nanoreactors were first synthesized via sequential atom transfer radical polymerization. They were then integrated in a house-built flow reactor to template the growth of perovskite nanocrystals. A variety of parameters were tuned to optimize synthesis, including antisolvent composition, antisolvent flowrate, and precursor solution flowrate. Due to the permanent ligation from the polymer nanoreactor, perovskite nanocrystals manufactured with this strategy display significantly enhanced colloidal, UV, and thermal stabilities over those synthesized with conventional ligands. Such scaling up of highly stable perovskite nanocrystals represents an important step towards their eventual use in many practical applications in optoelectronic materials and devices.

Second, Ruddlesden-Popper perovskite nanoplatelets were continuously manufactured via a flow reactor. Because of their enhanced quantum confinement, colloidal two-dimensional Ruddlesden-Popper (RP) perovskite nanosheets with a general formula L2[ABX3]n-1BX4 stand as a promising narrow-wavelength blue-emitting nanomaterial. A flow reactor was designed and optimized to continuously produce high-quality n=1 RP perovskite nanoplatelets. The effects of antisolvent composition, reactor tube length, precursor solution injection rate, and antisolvent injection rate on the morphology and optical properties of the nanoplatelets was systematically examined. The investigation suggests that flow reactors can be employed to synthesize high-quality L2PbX4 perovskite nanoplatelets (i.e., n =1) at rates greater than 8 times that of batch synthesis. Mass-produced perovskite nanoplatelets promise a variety of potential applications in optoelectronics, including light emitting diodes, photodetectors, and solar cells.

Third, thiol-ene chemistry was used to rationally engineer alkylammonium cations for highly stable Ruddlesden-Popper perovskite nanoplatelets. The hydrophobic nature of the bulky alkylammonium cations on Ruddlesden-Popper perovskite has been shown to increase their stability relative to conventional ABX3 perovskite. Additional chemical alteration to these organic molecules should thus further enhance stability. In this dissertation, thiol-ene click chemistry was used in two different methods to engineer the alkylammonium cations. First, in-situ crosslinking of unsaturated cations to craft organic shells around individual L2PbBr4 nanoplatelets was attempted. Second, ex-situ­ synthesis of various superhydrophobic perfluorinated alkylammonium cations was performed. The properties and stabilities of nanoplatelets made with increasing degrees of perfluorination were studied, and it was found that increasing the number of fluorinated carbons in the alkylammonium cations resulted in greater water stability. This study demonstrates how unique chemistries can be used to craft perovskite materials that can meet the stability demands of broad applications.

Through these three projects, this dissertation shows that the stability and production rate of perovskite nanocrystals can be increased towards that needed for  widespread adoption. Encouragingly, the strategies presented are widely applicable and do not necessitate difficult experimental conditions. Thus, future studies can build on the fundamental science explored in this dissertation to further push perovskite nanocrystals to reach their tremendous potential.