Inducing hybrid perovskite crystal growth on substrates for efficient, ambient stable optoelectronic devices

• Main Author: Riyas Ahmad
• Other Authors: Subodh Mhaisalkar
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials::Photonics and optoelectronics materials; Engineering::Materials::Energy materials
• Online Access: https://hdl.handle.net/10356/148030
• Institution: Nanyang Technological University

Hybrid lead perovskites are at the forefront of the emerging class of optoelectronic materials. With a certified photovoltaic efficiency of 25.2 % for 9 mm2 area devices, it is currently the most efficient emerging photovoltaic technology. The lion’s share of perovskite photovoltaic devices employs polycrystalline thin films of this material, with typical grain sizes around 300 nm. The optoelectronic characteristics like trap density and carrier lifetimes are several orders of magnitude better in perovskite single crystals, suggesting that these materials have the potential to deliver higher power conversion efficiencies than their polycrystalline film counterparts. Even though there are a plethora of reports on polycrystalline devices, works on monocrystalline perovskite photovoltaic cells are scanty, owing to their difficulties in device fabrication procedures. For the fabrication of efficient single crystal-based devices, the crystals need to be sliced into wafers of thickness typically lesser than the diffusion length of charge carriers (of the order of 100 μm). As this material is brittle and has a low shear modulus (of the order 10 GPa), it cannot be sliced down to these thickness regimes. Therefore, the only viable option to fabricate a single crystal/highly-crystalline hybrid perovskite device is to grow the crystals of these materials directly on the device substrates. Photovoltaic devices fabricated using methylammonium lead iodide (MAPbI3) monocrystalline films have yielded efficiencies of 21 % for a device area of 2 mm2; however, scaling beyond a few millimeters in lateral dimensions is yet to be achieved. In this work, two techniques were developed to grow highly crystalline layers of MAPbI3 films over substrates, without any constraint over their lateral dimensions. The mechanism of anti-solvent vapor crystallization (AVC) of lead halide perovskites was meticulously investigated, and sonication modified AVC (S-AVC) was developed. As the nucleation and initial crystal growth were initiated by sonication, this crystal growth technique was independent of the nature of substrates and their surfaces. The grain sizes of these films were around 100 micrometers, three orders of magnitude higher than standard polycrystalline thin films. These highly crystalline films were utilized to fabricate planar photodetectors with a responsivity of 20 AW-1, 1000 times higher than that of polycrystalline thin film-based devices. The thicknesses of S-AVC based films were of the order of hundred micrometers and were crystallographically random-oriented, making it inappropriate to fabricate vertical optoelectronic devices. In order to overcome these shortcomings of the technique, a second method was developed by modifying the standard spin-coating procedure used to deposit perovskite thin films. The perovskite precursor chemistry was tuned, and crystallization kinetics were optimized for inducing heterogeneous nucleation and growth of spherulitic perovskite crystals on substrate surfaces. These films were compact and pinhole-free with grain sizes above 100 micrometers. The film thicknesses were easily controllable to a few hundred nanometers by varying the spin coating parameters and the films were highly oriented along [200], [224] crystallographic planes, thus befitting it for vertical photovoltaic device fabrication. The fabricated photovoltaic cells exhibited superior room ambient stability along with good photoconversion efficiencies above 15 %, owing to the enhanced crystallinity of the perovskite layer. The films of various levels of crystallinity were used to unveil the role of grain boundaries on the ambient stability of the material. It was proved that a reduction in the number of grain boundaries hampers the moisture-induced degradation, thus can be used as an alternative strategy to enhance the ambient stability of perovskite-based optoelectronic devices. Lead halide perovskites are known mixed ionic-electronic conductors and the films of various levels of crystallinity were used to study the effect of grain boundaries on ionic motion through the material. In comparison to standard thin films, the ionic conductance was 100 times higher in S-AVC based films which demonstrated that grain boundaries can reduce ionic and electronic conductance by blocking the charge transport through the material. In a nutshell, two techniques were developed in this thesis to grow highly crystalline layers of hybrid lead halide perovskites over substrates. The grown films were utilized to study the effect of crystallinity on optoelectronic device characteristics, ionic conductance, and moisture stability of the material.