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Development of Efficient Perovskite Solar Cells with Interfacial and Morphological Manipulation
Organic-inorganic hybrid perovskites (OIHPs) have become promising materials for future low-cost and efficient photovoltage (PV) applications due to their desirable features including optimized and tunable bandgap, large absorption coefficient, long carrier diffusion length and compatibility to solution process. Nowadays great progress has been made on OIHP PVs. OIHP solar cells have the fastest growth on power convert efficiency (PCE) among all other PVs techniques. From the first report of the OIHP materials on PV application at 2009, the PCE increased markedly from 3.8% to a certified value of 22.1% in 6 years. On the other hand, OIHP solar cells suffered instability issues that slow down its commercialization, and thereby it requires a further investigation on the source of instability and technique development to solve this issue. Nevertheless, this dissertation mainly focuses on the improvement of the OIHP solar cell’s efficiency through the enhancement of OIHP film’s morphology and quality by fabrication process, interfacial layer, passivation technique and compositional manipulation. All the techniques introduced in this dissertation solved the major problems for the whole research community, which contributed to the OIHP’s future PV application. The main context of this dissertation is summarized from the publications with C. Bi as first author.^ In Chapter 2, a novel interdiffusion process of the two stacked precursor layers was developed to obtain uniform and pinhole-free OIHP films through low-temperature solution deposition method. This OIHP film fabrication method significantly improved the film morphology, resulting a high device efficiency of 15.4%, with fill factors of ∼80%, under one sun illumination. This is the first time that a planar heterojunction (PHJ) OIHP solar cell had a comparable PCE to mesoporous ones by using solution OIHP film deposition method. Besides the efficiency, the interdiffusion film deposition method resulted in high device yield, that more than 85% of the devices showed PCE larger than 14.5%. To further understand the interdiffusion fabrication process, the formation and evolution of the perovskite film during the thermal annealing was studied. The thermal annealing on interdiffusion process was found to have two functions: 1) driving the interdiffusion of the stacked PbI2 and (methylammonium iodide) MAI precursor layers for the formation of the perovskite, and 2) inducing the recrystallization and grain growth in the formed perovskite films. The thermal annealing was found to have impact on the structural, electrical, optical properties of methylammonium lead iodide (MAPbI3) thin films, which had correlation with the devices’ PCEs. A proper thermal annealing time improved the OIHP film’s crystallinity, doubled crystalline grain size, increased the carrier mobility from 12 to 36 cm2 V-1 s-1, while prolonged the thermal annealing caused partially decomposition and reduced OIHP film’s work function, which eventually lowered the device efficiency.^ In Chapter 3, an efficient wide-bandgap mixed-anion OIHP solar cell was developed with the optimized bandgap of 1.72 eV for future OIHP/Si tandem application. The bandgap of OIHP film was expended by blending methylammonium bromide (MABr) into precursor solution to incorporate halide anion of Br - into perovskite lattice structure. The bandgap of OIHP film was found to have a linear correlation with the amount of blended MABr. By using solvent annealing to increase the grain size and optimizing OIHP film thickness, we obtained a high efficiency of 13.1% with 1.72 eV bandgap on OIHP film. To the best of our knowledge, this was the highest efficiency of wide-bandgap OIHP solar cell at the published date.^ In Chapter 4, the nucleation and grain growth in polycrystalline OIHP film was manipulated for enlarged grains by hydrophobic hole transport layers (HTLs). The hydrophobic surfaces were found to suppress the perovskite heterogeneous nucleation rate and to facilitate the grain boundaries (GBs) migration by imposing less drag force, resulting in enlarged grain size in a same time scale and thermal annealing temperature. The grain size was increased from 300 nm on hydrophilic surface to the maximum value of 5 microns on hydrophobic one, resulting in a large aspect ratio of 14. The enlarged grains significantly reduced the charge recombination in the OIHP film to the level of that in single crystal by reducing the total GB area and better crystallinity on the bottom side of OIHP film. Combined with higher work function of Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) HTL, a high efficiency of 18.3% was obtained.^ In Chapter 5, the spontaneous enhancement on the regular p-i-n OIHP device’s efficiency was revealed. The device was observed to have increase of PCE from an initial value of 18.8% to 20.2% after 24-hour storage in N2, with the spontaneous reduction in trap density of states (tDOS) and prolonged carrier life time. The followed study identified the Na+ ion from soda-lime glass (SLG) was responsible to the observed spontaneous enhancement by investigating the evolution of OIHP film’s photoluminescence (PL) lifetime and device PCE on varied substrates. A further detailed study confirmed the Na+ passivation effect on OIHP films through the spontaneously prolonged carrier PL lifetime observed on the OIHP films grown on Si substrate with intentionally-added Na+ source.^ In Chapter 6, efficient flexible OHIP solar cells were developed on polyethylene terephthalate (PET)/ITO substrate by low-temperature solution fabrication. Firstly, the limitation for flexible OHIP solar cells was identified to be different perovskite film deposition condition than that required on rigid substrate. Then, an improved OIHP film composition was developed from an optimized deposition recipe of precursor ratio, resulting in a significantly enhancement on carrier PL lifetime, purer phase and uniform morphology. By using DMSO solvent annealing that increase the OIHP grain size from 250 nm to 500-2000 nm, we obtained a high device efficiency of 18.1% on flexible PET/ITO substrate. To the best of our knowledge, this is the highest efficiency obtained on flexible OHIP solar cell up to now. Besides the high efficiency, a robust mechanical bending durability was possessed by the flexible devices that retained 85.6% original efficiency after 1000 bending cycles.^
Bi, Cheng, "Development of Efficient Perovskite Solar Cells with Interfacial and Morphological Manipulation" (2016). ETD collection for University of Nebraska - Lincoln. AAI10196380.